A Treatise on Chemistry: The
non-metallic elements. New ...
Henry Enfield Roscoe, Carl Schorlemmer,
Harold Govett Colman, Arthur Harden


1837
ARTES
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A TREATISE ON CHEMISTRY

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1895

A
TREATISE ON CHEMISTRY
BY
SIR H. E. ROSCOE, F.R.S. AND C. SCHORLEMMER, F.R.S.
VOLUME I
THE NON-METALLIC ELEMENTS
"Chymia, alias Alchemia et Spagirica, est ars corpora vel mixta, vel composita, vel
aggregata etiam in principia sua resolvendi, aut ex principiis in talia com-
binandi."-STAHL, 1723
NEW EDITION COMPLETELY REVISED BY SIR H. E. ROSCOE
ASSISTED BY DRS. H. G. COLMAN AND A. HARDEN
WITH THREE HUNDRED AND SEVENTY-FOUR ILLUSTRATIONS AND
A PORTRAIT OF DALTON ENGRAVED BY C. H. JEENS
NEW YORK
D. APPLETON AND COMPANY
72 FIFTH AVENUE
1895
Authorized Edition.
50
PREFACE TO THE FIRST EDITION
IT has been the aim of the authors, in writing the
present treatise, to place before the reader a fairly
complete, and yet a clear and succinct, statement
of the facts of Modern Chemistry, whilst at the same
time entering so far into a discussion of Chemical
Theory as the size of the work and the present
transition state of the science permit. Special atten-
tion has been paid to the accurate description of the
more important processes in technical chemistry, and
to the careful representation of the most approved
forms of apparatus employed. As an instance of this,
the authors may refer to the chapter on the Manu-
facture of Sulphuric Acid. For valuable information
on these points they are indebted to many friends
both in this country and on the Continent.
The volume commences with a short historical
sketch of the rise and progress of chemical science,
and a few words relative to the history of each
element and its more important compounds prefaces
the systematic discussion of their chemical properties.
For this portion of their work, the authors wish here
to acknowledge their indebtedness to Hermann Kopp's
classical works on the History of Chemistry.
In the part of the volume devoted to the description
of the non-metallic elements, care has been taken to
vi
PREFACE
select the most recent and exact experimental data,
and to give references in all important instances, as
it is mainly by consulting the original memoirs that
a student can obtain a full grasp of his subject.
Much attention has likewise been given to the
representation of apparatus adapted for lecture-room
experiment, and the numerous new illustrations re-
quired for this purpose have all been taken from
photographs of apparatus actually in use. The fine
portrait which adorns the title-page is a copy, by the
skilful hands of Mr. Jeens, of a daguerreotype taken
shortly before Dalton's death.
MANCHESTER, July, 1877.
PREFACE TO THE THIRD EDITION
In this new, completely revised and reprinted edition I
have endeavoured to carry out the aims which were
put forward in the preceding preface seventeen years
ago. Deprived of the aid of my late friend and
colleague, I have been fortunate in securing the help
of two of the ablest of my former students, and to
them I tender my thanks. How far we have succeeded
in bringing this edition up to the level of the science
of the present day, it will be for the public to judge.
All I can say is that no pains have been spared to
do so.
LONDON, September 29th, 1894.
H. E. RosCOE.
CONTENTS
xi
Coal-Gas
Wood-Gas
Oil-Gas
Water-Gas.
Nature of Flame.
Silicon or Silicium
Silicon and Hydrogen.
Silicon and the Halogens
Silicon and Oxygen
Silicon and Sulphur
Silicon and Nitrogen
Silicon and Carbon
CRYSTALLOGRAPHY
COMPARISON OF METRICAL WITH ENGLISH MEASURES
ADDENDUM
INDEX
PAGE
769
787
788
790
792
800
804
806
814
823
824
825
827
874
876
877
CONTRACTIONS EMPLOYED IN THIS PART
Am. Chem. Journ.
Annalen =
Liebig's Annalen der Chemie.
Ann. Chim. Phys. Annales de Chimie et de Physique.
Ber. Berichte der deutschen chemischen Gesellschaft.
Bull. Soc. Chim. Bulletin de la Société Chimique de Paris.
Chem. Centr. = Chemisches Centralblatt.
Chem. Zeit. Chemiker Zeitung.
Compt. Rend. = Comptes Rendues de l'Académie des Sciences.
Gazzetta Gazzetta Chimica Italiana.
Journ. Chem. Soc. Journal of the Chemical Society.
J. Pr. Chem. Journal für praktische Chemie.
Journ. Soc. Chem. Ind. Journal of the Society of Chemical Industry.
Monatsh. Monatshefte für Chemie.
Phil. Mag.
Philosophical Magazine.
Phil. Trans. = Philosophical Transactions of the Royal Society.
Pogg. Ann.
Annalen der Physik und Chemie (Poggendorf).
Proc. Chem. Soc. Proceedings of the Chemical Society.
Proc. Roy. Soc.
Rec. Trav. Chim.
Proceedings of the Royal Society.
Recueil des Travaux Chimiques des Pays-Bas.
Wied. Ann. = Annalen der Physik (Wiedemann).
Zeit. analyt. Chem. = Zeitschrift für analytische Chemie.
Zeit. angew. Chem. = Zeitschrift für angewandte Chemie.
Zeit. anorg. Chem.
Zeit. phys. Chem.
Zeit. physiol. Chem.
Zeitschrift für anorganische Chemie.
Zeitschrift für physikalische Chemie.
Zeitschrift für physiologische Chemie.
=
=
=
=
1=3
=
American Chemical Journal.
=
=
l
CHEMISTRY
HISTORICAL INTRODUCTION
In looking back at the history of our Science, we find that
although the ancient world possessed a certain empirical know-
ledge of chemical facts derived chiefly from an acquaintance
with pharmaceutical and manufacturing art, the power of con-
necting or systematizing these facts was altogether wanting.
The idea of experimental investigation was scarcely understood,
and most of those amongst the ancients who desired to promote
a knowledge of Nature attempted to do so rather by pursuing
the treacherous paths of speculation, than the safe though tedious
route of observation and experiment. They had no idea of the
essential differences which we now perceive between elements
and compound substances, nor did they understand the meaning
of chemical combination. The so-called Aristotelian doctrine
of the four elements, Earth, Water, Air or Steam, and Fire,
bore no analogy to our present views as to the nature and pro-
perties of the chemical elements, for with Aristotle these names
rather implied certain characteristic and fundamental properties
of matter than the ideas which we now express by the term
chemical composition. Thus " Earth" implied the properties
of dryness and coldness; "Water," those of coldness and wet-
ness; "Air or Steam," wetness and heat; "Fire," dryness and
heat. All matter was supposed to be of one kind, the variety
which we observe being accounted for by the greater or less
abundance of these four conditions which were supposed to be
essential to every substance, that which was present in the
greatest degree giving to the substance its characteristic pro-
perties. To men holding such views, a change of one kind of
4
HISTORICAL INTRODUCTION
matter into a totally different kind appeared probable and
natural. Thus, the formation of water from air or vice versa
is described by Pliny as a usual phenomenon seen in the
formation and disappearance of clouds, whilst the ordinary
experience, that cold acts as a solidifying and hardening
agent, bears out Pliny's view, that rock crystal is produced
from moisture, not by the action of heat, but by that of cold,
so that it is, in fact, a kind of ice. A transformation of one
sort of substance into another quite different thus appears
not only possible but probable, and we are not surprised to
learn that, under the influence of the Aristotelian philosophy,
which throughout the middle ages was acknowledged to be the
highest expression of scientific truth, the question of the trans-
mutability of the base into the noble metals was considered
to be perfectly open.
Much light of a very interesting and remarkable character
has been thrown upon the origin of alchemy, the artificial pro-
duction of the noble metals, by the discovery of an Egyptian
papyrus, which contains more than a hundred metallurgical
recipes written in Greek, many of which consist of elaborate
directions for the falsification of the precious metals. The con-
nection between these working notes of a fraudulent Egyptian
goldsmith and the dreams of the later alchemists is attested
by the reappearance of several of these very recipes in the
writings of the following century under the guise of formula of
transmutation.¹
The oldest works of a strictly chemical character date from
about the beginning of the third century of our era, and are due
to Greek authors resident in Egypt, where our science appears
to have had its birth. Among these writers, the most ancient
and distinguished of whom is known as Zosimus of Panopolis,
the possibility of the transmutation of the metals is fully
accepted, and their works consist mainly of directions for the
achievement of this object, couched however in language so
deeply tinged with religious mysticism, symbolism and metaphor
as to be almost unintelligible. They allude to their subject as
the "divine art," and it is not until the fourth century that we
find in the works of the Byzantine writers the term Chemia
applied to the art which treats of the production of gold and
Berthelot, Les Origines de l'Alchimie, p. 3. Introduction à l'Etude de la
Chimie des Anciens et du Moyen Age, pp. 21, 59.
2 Berthelot, Collection des Anciens Alchimistes Grecs. Part I.
THE ARABIAN ALCHEMISTS
5
silver. The fact that all these authors were closely connected
with the celebrated schools of Alexandria, the last resting-place
of the proscribed secrets of the Egyptian priests, adds to the
probability that our science was first practised in Egypt.
Plutarch, indeed, states that the old name for Egypt was
Chemia, and that this name was given to it on account of the
black colour of its soil. The same word was used to designate
the black of the eye, as the symbol of the dark and mysterious.
It is therefore possible that chemistry originally meant simply
the Egyptian-or secret-knowledge, whilst others identify the
name with the Greek xvuós, sap or liquid, the name of the
agent of transmutation being applied to the art.
The Aristotelian philosophy became known to and was ex-
tended by the Arabians, who, in the year 640, overran Egypt,
and thence, through Northern Africa, penetrated into Spain.
They first became acquainted with chemistry in Egypt, and pre-
fixed the Arabic article to the original name, so that the word
alchemy has from that time been used to signify the art of
making gold and silver.
The works of Geber, the most celebrated of Arabian alchem-
ists, are handed down to us through Latin translations. In
these books we learn that the aim of the science of which
Geber treats is the transmutation of the base into the noble
metals. He describes many chemical operations, such as filtra-
tion, distillation, crystallization, and sublimation; and by these
he prepares new substances or purifies the old ones. Bodies
such as alum, green vitriol, saltpetre, and sal-ammoniac are em-
ployed; and we find that he was able to prepare nitric acid, or
aqua fortis, and from it the valuable solvent for gold, aqua regia.
It is probable that even sulphuric acid was known to Geber, and
certainly a number of metallic compounds, amongst which were
mercuric oxide and corrosive sublimate, the preparation of
which he describes, were known. Geber was the first pro-
pounder of a chemical theory. He asserts that the essential
differences between the metals are due to the preponderance of
one of two principles, mercury and sulphur-of which all the
metals are composed. The first principle is characteristic of
the truly metallic qualities, whilst the latter causes the peculiar
changes noticed when the metals are exposed to heat. The
noble metals were supposed to contain a very pure mercury, and
are, therefore, unalterable by heat, whilst the base metals contain
1
Kopp, Beiträge zur Geschichte der Chemie. 1 Stück. p. 40.
6
HISTORICAL INTRODUCTION
so much sulphur that they lose their metallic qualities in the fire.
These constituents may, however, not only be present in different
proportions, but also in different degrees of purity or in different
states of division; and thus it might naturally be supposed that, if
not by a variation in their relative quantity, at any rate by a
change in their condition, such an alteration in the properties of
one metal may be brought about as would produce from it some
other known metal. Thus gold and silver contain a very pure
mercury, which in the one instance is combined with a red and
in the other with a white sulphur; and he explains that the
reason why these two metals amalgamate so easily is because
they already contain a large quantity of mercury, and are
therefore quickly attracted by the liquid metal.
Whilst Greece and Italy sank deeper and deeper into bar-
barism, arts and science flourished under Arabian dominion,
and the academies of Spain were thronged with students
from all parts of the Christian world. The knowledge of
alchemy spread from this source over Western Europe, and in
the thirteenth century we find alchemists of the Arabian school
in all the chief countries of Europe. In Christian Spain lived
the celebrated Raymund Lully; in France we hear of Arnold
Villanovanus; Albertus Magnus flourished in Germany. Then
Thomas Aquinas, pupil of Albertus, was also an alchemist,
as was our own Roger Bacon (1214-1294), who was tried at
Oxford for sorcery, and who, to disprove these charges, wrote
the celebrated treatise, in which he shows that appearances
then attributed to supernatural agency were due to common and
natural causes. It was Roger Bacon, from his rare accomplish-
ments and learning termed Doctor Mirabilis, who first pointed
out the possible distinction between theoretical alchemy, or
chemistry studied for its own sake, on the one hand, and prac-
tical alchemy, or the striving after certain immediately useful
ends, on the other.
1
Although all these men agreed that the transmutation of
metals was not only possible but that it was an acknowledged
fact, and that for the preparation of gold and silver the philoso-
pher's stone was needed, it is difficult, not to say impossible, now
to understand their methods or processes, inasmuch as all that
they have written on this subject is expressed in the ambiguous
and inflated diction of the Byzantine and Arabian authors.
The fourteenth century finds the study of alchemy widely
1 Epistola de secretis operibus artis et naturæ, et nullitate magia.
TRANSMUTATION
7
spread over the civilized world, and the general attention which
the subject attracted gave rise to the discovery of a large number
of chemical substances. By the end of the fifteenth century,
although the knowledge of chemical facts had continued to in-
crease, the old views respecting the ultimate composition of
matter were still accepted. In addition, however, to the sulphur
and mercury, supposed by Geber to be the universal constitu-
ents of matter, we find a third constituent, viz. salt, introduced.
We must bear in mind however that these three principles, like
the four Aristotelian elements, were not supposed to be identical
with the common substances which bear their names.
That men of such wide experience and great powers, as the
chemists of this period proved themselves to be, could bring
themselves to believe in the possibility of the discovery of the
philosopher's stone, a substance of such potency that when
thrown on the base metals in a state of fusion (moment of pro-
jection) it transmutes them into gold and silver, appears to us
very remarkable. No one doubted the possibility of such a
transmutation, and the explanation may be found in the fact,
at that time well known, that the colour of certain metals can
be altered by the addition of other bodies. Thus Geber knew
that when red copper is melted with tutty (an impure oxide of
zinc), the golden-yellow brass is obtained; and also that other
minerals (those which we now know to contain arsenic) give to
copper a silver-white colour. Still the difference between these
alloys and the noble metals must soon have been discovered,
and the possibility of the transmutation lay rather in the notion
already alluded to, that the different metals contained the same
constituents arranged either in different quantities or in different
states of purity. Nor were experimental proofs of this view
wanting. Thus Geber believed that by adding mercury to lead
the metal tin was formed, and the solid amalgam does closely
resemble tin in its appearance. Then again the metallurgical
processes were in those days very imperfect, and the alchemists
saw proof of their theory in the formation of a bead of pure
silver from a mass of galena, or in the extraction of a few
grains of gold out of a quantity of pyrites. It was not until
the beginning of the seventeenth century that it was proved
that galena frequently contains silver, and that traces of gold
are often found in iron pyrites. Even so late as 1709 we find
Homberg stating that pure silver after melting with pyrites is
found to contain gold, and it was only after several chemists had
8
HISTORICAL INTRODUCTION
performed the experiment with a like result that the mineral
itself was acknowledged to contain traces of gold.
Again, it was not at this time recognised that some salts are
metallic compounds, and the precipitation of copper from a solu-
tion of blue-stone by metallic iron was supposed to be a transmu-
tation of iron into copper. These apparent experimental proofs
of the truth of the alchemical doctrine were accompanied by a
mass of traditional evidence; that is, of stories handed down
from generation to generation, in which cases of the transmuta-
tion of metals are circumstantially narrated. Thus the belief in
the fundamental principle of alchemy became firmly established.¹
A satisfactory explanation of the belief in the power of the
philosopher's stone to heal disease and to act as the elixir vitæ,
the grand panacea for human ills, is more difficult to find. It
may possibly have at first arisen from a too literal interpretation.
of the oriental imagery found in the early Arabian writers, where,
although the peculiar doctrine of elixir vitæ is unknown, we
find such passages as the following:-"If thou carriest out my
prescription with due care thou shalt heal the bad disease of
poverty." The Arabians called the base metals "diseased."
Thus Geber says, " Bring me the six lepers, so that I may heal
them; "that is, transmute the other six known metals into
gold. The belief in the healing power of the philosopher's stone
was also much strengthened by the discovery, about this time,
of many substances which produce remarkable effects on the
human frame, and of these the alchemists of the thirteenth
century write in the most exaggerated and exalted terms.
The work known by the fantastic title of the Triumph-Wagen
des Antimonii, which contains a large amount of accurate
information concerning the preparation and the medicinal pro-
perties of many of the compounds of antimony, and is ascribed
to the authorship of a monk, Basil Valentine by name, who was
supposed to have lived at the beginning of the fifteenth century,
has been shown by the late Prof. Schorlemmer to be an undoubted
forgery dating from about 1600, the information being culled
from the works of other writers and thrown into the mystical,
semi-religious style suitable to the earlier period. The same
appears to be true of the other writings attributed to this author.
The man who effected the inestimable union between
1 For further information on this subject Kopp's classical work, Die Geschichte
der Chemie, or Thomson's History of Chemistry, may be consulted.
2 See also Kopp, Beiträge III. 110.
I
THE MEDICAL CHEMISTS
9
chemistry and medicine was Paracelsus (1493-1541). He
not only assumed the existence of three components of all
inorganic substances, but he was the first who included animal
and vegetable bodies in the same classification, and held that
the health of the organism depends on the continuance of the
true proportions between these ingredients, whilst disease is due
to a disturbance of this proper relation.
The era thus inaugurated by Paracelsus continued up to the
end of the seventeenth century. Chemistry was the handmaid
of medicine, and questions respecting the ultimate composition
of matter were considered of secondary importance to those
relating to the preparation of drugs. Of the contemporaries
of Paracelsus, Agricola (1490-1555) was one of the most dis-
tinguished, and his remarkable work De Re Metallica contains
a complete treatise on metallurgy and mining, which did much
to advance the processes of technical chemistry, many of the
methods which he describes being in use even at the present
day. Whilst Agricola devoted himself to the study of metallurgy,
his countryman, Libavius, greatly assisted the general progress
of science, inasmuch as he collected together in writings which
are characterised by a clear and vigorous style, all the main facts
of chemistry; so that his Alchemia, published in 1595, may be
regarded as the first handbook of chemistry. His chief object
was the preparation of medicines, but he still maintained the
science in its old direction and distinctly believed in the trans-
mutation of metals.
The first who formally declined to accept the Aristotelian
doctrine of the four elements, or that of Paracelsus of the three
constituents of matter, was Van Helmont (1577-1644). He
denied that fire had any material existence, or that earth can be
considered as an element, for it can, he says, be produced from
water, but he admitted the elementary nature of air and water,
and gave great prominence to the latter in its general dis-
tribution throughout animate and inanimate nature. Van
Helmont's acknowledgment of air as an element is the more
remarkable, as he was the first to recognise the existence of
different kinds of air and to use the term gas. Thus, his "gas
sylvestre," which he clearly distinguished from common air, is
carbonic acid gas, for he states that it is given off in the process
of fermentation, and also formed during combustion, and that
it is found in the "Grotto del Cane," near Naples. He also
mentions a "gas pingue" which is evolved from dung, and is
10
HISTORICAL INTRODUCTION
inflammable. It was Van Helmont who first showed that if a
metal be dissolved in an acid it is not destroyed, as was formerly
believed, but can again be obtained from solution as metal by
suitable means; and he considered the highest aim of the
science to be the discovery of a general solvent which would at
the same time serve as a universal medicine, and to which the
name of "alkahest" was given.
Although Van Helmont accomplished much towards the
overthrow of the Paracelsian doctrine, his discoveries of the
different gases were forgotten, and even up to the middle
of the seventeenth century much divergence in opinion on
fundamental questions prevailed. Those who were interested
in the connection of chemistry with medicine still believed in
the dreams of the alchemist, and held to the old opinions; whilst
those who, advancing with the times, sought to further the
science for its own sake, or for the sake of its important technical
applications, often upheld views more in accordance with those
which we now know to be the true ones. Among the names of
the men who, during this period, laboured successfully to pro-
mote the knowledge of chemistry, that of Glauber (1603-1668)
must be first mentioned. He was both alchemist and medicinal
chemist, and discovered many valuable medicines. Another name
of importance at this epoch is that of N. Lemery (1645-1715).
He, as well as Lefebre and Willis, believed in the existence of
five elements; mercury or spirit, sulphur or oil, and salt are the
active principles; water or phlegm, and earth are the passive
ones. Lemery's ideas and teachings became well known through
the publication of his Cours de Chymie (1675) which was trans-
lated into Latin, as also into most modern languages, and exerted
a great influence on the progress of science. In this work the
distinction between mineral and vegetable bodies was first clearly
pointed out, and thus for the first time the distinction between
Inorganic and Organic chemistry was realized.
Pre-eminent amongst the far-seeing philosophers of his time.
stands Robert Boyle (1627-1691). It is to Boyle that we owe the
complete overthrow of the Aristotelian as well as the Paracelsian
doctrine of the elements, so that, with him we begin a new
chapter in the history of our science. In his Sceptical Chymist,¹
The Sceptical Chymist or Chemico-physical Doubts and Paradoxes, touching the
Experiments whereby vulgar Spagyrists are wont to endeavour to evince their Salt,
Sulphur, Mercury, to be the true Principles of Things. First published in 1661
(Boyle's Works, 1772, 1, 458).
"THE SCEPTICAL CHYMIST”
11
he upholds the view that it is not possible, as had hitherto been
supposed, to state at once the exact number of the elements; that
on the contrary all bodies are to be considered as elements which
are themselves not capable of further separation, but which can
be obtained from a combined body, and out of which the com-
pound can be again prepared. Thus he states, "That it may
as yet be doubted whether or no there be any determinate
number of elements; or, if you please, whether or no all com-
pound bodies do consist of the same number of elementary in-
gredients or material principles."1 Boyle, it is clear, was the first
to grasp the idea of the distinction between an elementary and a
compound body, the latter being a more complicated substance
produced by the union of two or more simple bodies and differing
altogether from these in its properties. He also held that chemical
combination consists in an approximation of the smallest particles
of matter, and that a decomposition takes place when a third
body is present, capable of exerting on the particles of the one
element a greater attraction than the particles of the other
element with which it is combined. More, however, than for his
views on the nature of the elements, is science indebted to Boyle
for his clear statement of the value of scientific investigation for
its own sake, altogether independent of any application for
the purposes either of the alchemist or of the physician. It
was Boyle who first felt and taught that chemistry was not to be
the handmaid of any art or profession, but that it formed an essen-
tial part of the great study of Nature, and who showed that from
this independent point of view alone could the science attain to
vigorous growth. He was, in fact, the first true scientific chemist,
and with him we may date the commencement of a new era for
our science when the highest aim of chemical research was
acknowledged to be that which it is still upheld to be, viz., the
simple advancement of natural knowledge.
In special directions Boyle did much to advance chemical
science (his published writings and experiments fill six thick
quarto volumes), particularly in the border land of chemistry and
physics; thus in the investigations on the "Spring of the Air,"
he discovered the great law of the relation existing between
volumes of gases and the pressures to which they are subjected,
which still bears his name.
Although Boyle was aware of the fact that many metals when
heated in the air form calces which weigh more than the metals
1 Boyle's Works, 1772, 1, 560.
12
HISTORICAL INTRODUCTION
themselves, and although he examined the subject experimentally
with great care, his mind was so much biassed by the views he held
respecting the material nature of flame and fire that he ignored
the true explanation of the increase of weight, namely, that it is
due to the absorption of a ponderable constituent of the atmo-
sphere, and looked upon the gain as a proof of the ponderable
nature of fire and flame, giving many experiments having for
their object the "arresting and weighing of igneous corpuscles." ¹
Similar views are found expressed in his essay "On the
mechanical origin and production of fixedness," written in
1675, where Boyle, speaking of the formation of mercuric oxide
from the metal by exposure to the air at a high temperature,
says, "chemists and physicians who agree in supposing this pre-
cipitate to be made without any additament, will, perchance,
scarce be able to give a more likely account of the consistency
and degree of fixity, that is obtained in the mercury; in which,
since no body is added to it, there appears not to be wrought
any but a mechanical change, and though I confess I have
not been without suspicions that in philosophical strictness
this precipitate may not be made per se, but that some
penetrating igneous particles, especially saline, may have asso-
ciated themselves with the mercurial corpuscles.'
We owe the next advances in chemistry to the remarkable
views and experiments of Hooke (Micrographia, 1665), and of
John Mayow (Opera Omnia Medico-physica, 1681). The former
announced a theory of combustion, which although it attracted
but little notice, more nearly approached the true explanation than
many of the subsequent attempts. He pointed out the similarity
of the actions produced by air and by nitre or saltpetre, and he
concluded that combustion is affected by that constituent of the
air which is fixed or combined in the nitre.3 Hooke did not
complete his theory or give the detail of his experiments, but
similar conclusions seem to have been independently arrived at
by Mayow, who in 1669 published a paper, De Sal-Nitro et Spiritu
Nitro-aëreo, in which he points out that combustion is carried on
by means of this "spiritus nitro-aereus" (another, and not an in-
appropriate name for what we now call oxygen), and he also
distinctly states that when metals are calcined, the increase of
weight observed is due to the combination of the metal with this
"spiritus." Mayow was one of the first to describe experiments
2 Boyle's Works, 4, 309.
1 Boyle's Works, 3, 706-718.
3 Micrographia, pp. 103-5.
1
I
THE THEORY OF PHLOGISTON
13
made with gases collected over water, in which he showed that
air is diminished in bulk by combustion, and that the respiration
of animals produces the same effect. He proved that it is the
nitre-air which is absorbed in both these processes, and that an
inactive gas remains, and he drew the conclusion that respiration
and combustion are strictly analogous phenomena. There is,
therefore, no doubt that Mayow clearly demonstrated the
heterogeneous nature of air, although his conclusions were not
admitted by his contemporaries.
Another theory which was destined greatly to influence and
benefit chemical discovery, was advanced about this time by
J. J. Becher (1635-1682), and subsequently much developed.
and altered by G. E. Stahl (1660-1734). It made special
reference to the alterability of bodies by fire, and to the
explanation of the facts of combustion. Becher assumed that
all combustible bodies are compounds, so that they must
contain at least two constituents, one of which escapes during
combustion, whilst the other remains behind. Thus when
metals are calcined, an earthy residue or a metallic calx remains;
metals are therefore compounds of this calx with a combustible
principle, whilst sulphur or phosphorus are compounds contain-
ing a principle which causes their combustion. Bodies unalter-
able by fire are considered to have already undergone combus-
tion; to this class of bodies quicklime was supposed to belong,
and it was assumed that if the substance which it had lost in
the fire were again added a metallic body would result. The
question as to whether there be only one or several principles of
combustibility was freely discussed, and Stahl decided in favour
of the former of these alternatives, and gave to this combustible
principle the name Phlogiston (4λoyorós, burnt, combustible).
An example may serve to illustrate the reasoning of the
upholders of the Phlogistic theory. Stahl knew that oil
of vitriol is a product of the combustion of sulphur; hence
sulphur is a combination of oil of vitriol and phlogiston.
But this latter is also contained in charcoal, so that if we can
take the phlogiston out of the charcoal and add it to the oil of
vitriol, sulphur must result. In order that this change may be
brought about, the oil of vitriol must be fixed (ie. rendered
non-volatile) by combining it with potash; if then, the salt thus
obtained is heated with charcoal, a hepar sulphuris (a compound
also produced by fusing potash with sulphur) is obtained. The
argument shows that when charcoal is heated with oil of vitriol
14
HISTORICAL INTRODUCTION
the phlogiston of the charcoal combines with the oil of vitriol,
and sulphur is the result. The phlogiston contained in sulphur
is not only identical with that contained in charcoal, but also
with that existing in the metals, and in all organic bodies, for
these are obtained by heating their calces with charcoal, or with
oil or other combustible organic bodies.
The amount of phlogiston contained in bodies was, according
to Stahl, very small, and the greatest quantity was contained in
the soot deposited from burning oil. It was likewise considered
that the phlogiston given off by combustion is taken up again
from the air by plants; and the phenomena of fermentation and
decay were believed to depend upon a loss of phlogiston which,
however, in this case only escapes slowly. Stahl explains
why combustion can only occur in presence of a good supply
of air, because in this case the phlogiston assumes a very
rapid whirling motion, and this cannot take place in a closed
space.
However false from our present position we see the phlogistic
theory in certain directions to be; and although we may now
believe that the extension and corroboration of the positive views
enunciated by Hooke and Mayow might have led to a recognition
of a true theory of chemistry more speedily than the adoption
of the theory of phlogiston, we must admit that its rapid
general adoption showed that it supplied a real want. It was
this theory which for the first time established a common
point of view from which all chemical changes could be observed
enabling chemists to introduce something like a system by class-
ing together phenomena which are analogous and are probably
produced by the same cause, for the first time making it pos-
sible for them to obtain a general view of the whole range of
chemical science as then known.
It may appear singular that the meaning of the fact of the
increase of weight which the metals undergo on heating, which
had been proved by Boyle and others, should have been wholly
ignored by Stahl, but we must remember that he considered their
form rather than their weight to be the important and character-
istic property of bodies.
Stahl also, perhaps independently, arrived at the same con-
clusion which Boyle had reached, concerning the truth of the
existence of a variety of elementary bodies, as opposed to the
Aristotelian or Paracelsian doctrine, and the influence which a
clear statement of this great fact by Stahl and his pupils
¡
I
JOSEPH BLACK
15
-amongst whom must be mentioned Pott (1692-1777)
and Marggraf (1709–1782)—exerted on the progress of the
science was immense. It is only after Stahl's labours that a
scientific chemistry becomes, for the first time, possible. The
essential difference between the teaching of the science then
and now being that the phenomena of combustion were then
believed to be due to a chemical decomposition, phlogiston being
supposed to escape, while we account for the same phenomena
now by a chemical combination, oxygen or some element being
taken up.
Thus Stahl prepared the way for the birth of modern chemis-
try. It was on August 1st, 1774, that Joseph Priestley dis-
covered oxygen gas.
Between the date of the establishment of the phlogistic theory
by Stahl, and of its complete overthrow by Lavoisier, many
distinguished men helped to build up the new science-Black,
Priestley, and Cavendish in our own country, Scheele in Sweden,
and Macquer in France. The classical researches of Black on the
fixed alkalis (1754)¹ not only did much to shake the foundation
of the phlogistic theory, but they may be described with truth
as the first beginnings of a quantitative chemistry, for it was by
means of the balance, the essential instrument of all chemical
research, that Black established his conclusions. Up to this time
the mild (or carbonated) alkali was believed to be a more simple
compound than the caustic alkali. When mild alkali (potashes)
was brought into contact with burnt (caustic) lime, the mild alkali
took up the principle of combustibility, obtained by the lime-
stone in the fire, and it became caustic. Black showed that in
the cases of magnesia-alba and chalk the disappearance of the
effervescence on treatment with an acid after heating, was
accompanied by a loss of weight. Moreover, as Van Helmont's
older observations were quite forgotten, he was the first clearly
to establish the existence of a kind of air or gas, termed fixed
air (1752) totally distinct both from common atmospheric air
and from modifications of it, by impurity or otherwise, such
as the various gases hitherto prepared were believed to be.
This fixed air, then, is given off when mild alkalis become
caustic, and is taken up when the reverse change occurs.
This clear statement of a fact which of itself is a powerful
argument against the truth of the theory in which he had been
1 "Experiments upon Magnesia-alba, Quicklime, and other Alkaline sub.
stances."-Edin. Phys, and Literary Essays, 1755.
16
HISTORICAL INTRODUCTION
brought up, was sufficient to make the name of Black illustrious,
but he became immortal by his discoveries of latent and specific
heats, the principles of which he taught in his classes at Glasgow
and Edinburgh from 1763. The singularly unbiassed character
of Black's mind is shown in the fact that he was the only
chemist of his age who completely and openly avowed his
conversion to the new Lavoisierian doctrine of combustion.
From an interesting correspondence between Black and
Lavoisier, it is clear that the great French chemist looked on
Black as his master and teacher, speaking of Black's having
first thrown light upon the doctrines which he afterwards more
fully carried out.¹
This period of the history of our science has been called that
of pneumatic chemistry, because, following in the wake of
Black's discovery of fixed air, chemists were now chiefly engaged
in the examination of the properties and modes of preparation
of the different kinds of airs or gases, the striking and very
different natures of which naturally attracted interest and stimu-
lated research.
No one obtained more important results or threw more light
upon the existence of a number of chemically different gases than
Joseph Priestley. In 1772 Priestley was engaged in the examin-
ation of the chemical effect produced by the burning of com-
bustible bodies (candles) and the respiration of animals upon
ordinary air. He proved that both these deteriorated the air
and diminished its volume, and to the residual air he gave the
name of phlogisticated air. Priestley next investigated the
action of living plants on the air and found to his astonishment
that they possess the power of rendering the air deteriorated
by animals again capable of supporting the combustion of a
candle.
Fig. 1, a reduced fascimile of the frontispiece to Priestley's
celebrated Observations on Different Kinds of Air, shows the
primitive kind of apparatus with which this father of pneumatic
chemistry obtained his results. The mode adopted for generating
and collecting gases is seen; hydrogen is being prepared in the
phial by the action of oil of vitriol on iron filings, and the gas is
being collected in the large cylinder standing over water in the
pneumatic trough; round this trough are arranged various other
pieces of apparatus, as, for instance, the bent iron rod holding
a small crucible to contain the substances which Priestley desired
1 Brit. Assoc. Reports, 1871, p. 189.
JOSEPH PRIESTLEY
17

FIG. 1.
3
18
HISTORICAL INTRODUCTION
to expose to the action of the gas. In the front is seen a large
cylinder in which he preserved the mice, which he used for
ascertaining how far an air was impure or unfit for respiration,
and standing in a smaller trough is a cylinder containing living
plants, the action of which on air had to be ascertained.
On August 1st, 1774, Priestley obtained oxygen gas by
heating red precipitate by means of the sun's rays concen-
trated with a burning glass, and termed it dephlogisticated air,
because he found it to be so pure, or so free from phlogiston,
that in comparison with it common air appeared to be impure.
Priestley also first prepared nitric oxide (nitrous air or gas),
nitrous oxide (dephlogisticated nitrous air), and carbonic oxide;
he likewise collected many gases for the first time over mercury,
such as ammoniacal gas (alkaline air), hydrochloric acid gas
(marine acid air), sulphurous acid gas (vitriolic acid air), and
silicon tetrafluoride (fluor acid air).' He also observed that when a
series of electric sparks is allowed to pass through ammoniacal
gas, an increase of volume occurs, and a combustible gas is formed,
whilst on heating ammonia with calx of lead phlogisticated air
(nitrogen gas) is evolved.
Priestley's was a mind of rare quickness and perceptive
powers, which led him to the rapid discovery of numerous new
chemical substances, but it was not of a philosophic or delibera-
tive cast. Hence, although he had first prepared oxygen, and
had observed (1781) the formation of water, when inflammable
air (hydrogen) and atmospheric air are mixed and burnt together
in a copper vessel, he was unable to grasp the true explanation
of the phenomenon, and he remained to the end of his days a
firm believer in the truth of the phlogistic theory, which he had
done more than any one else to destroy.
Priestley's notion of original research, which seems quite.
foreign to our present ideas, may be excused, perhaps justified, by
the state of the science in his day. He believed that all dis-
coveries are made by chance, and he compares the investigation
of nature to a hound, wildly running after, and here and there
chancing on game (or as James Watt called it, "his random
haphazarding"), whilst we should rather be disposed to compare
the man of science to the sportsman, who having, after persistent
effort, laid out a distinct plan of operations, makes reasonably
sure of his quarry.
In some respects the scientific labours of Henry Cavendish
1 Priestley's Observations on Different Kinds of Air, 1, 328.
HENRY CAVENDISH
19
(1731-1810) present a strong contrast to those of Priestley; the
work of the latter was quick and brilliant, that of the former was
slow and thorough. Priestley passed too rapidly from subject to
subject even to notice the great truths which lay under the
surface; Cavendish made but few discoveries, but his researches
were exhaustive, and for the most part quantitative. His
investigation on the inflammable air ¹ evolved from dilute acid
and zinc, tin, or iron, is a most remarkable one. In this memoir
we find that he first determined the specific gravity of gases,
and used materials for drying gases, taking note of
alterations of volume due to changes of pressure and tempera-
ture. He likewise proved that by the use of a given weight of
each one of these metals, the same volume of inflammable gas
can always be obtained no matter which of the acids be employed,
whilst equal weights of the metals gave unequal volumes of the
gas. Cavendish also found that when the above metals are
dissolved in nitric acid, an incombustible air is evolved, whilst if
they are heated with strong sulphuric acid sulphurous air is
formed. He concluded that when these metals are dissolved in
hydrochloric or in dilute sulphuric acid their phlogiston flies
off, whilst when heated with nitric or strong sulphuric acids,
the phlogiston goes off in combination with an acid. This is
the first occasion in which we find the view expressed that
inflammable air is phlogiston-a view which was generally held,
although Cavendish himself subsequently changed his opinion,
regarding inflammable air as a compound of phlogiston and
water.
The discovery of oxygen by Priestley, and of nitrogen by
Rutherford, naturally directed the attention of chemists to the
study of the atmosphere, and to the various methods for ascer-
taining its composition.
Although Priestley's method of estimating the dephlogisti-
cated air by means of nitric oxide was usually employed,
the results obtained in this respect by different observers
were very different. Hence it was believed that the composition.
of the air varies at different places, and in different seasons, and
this opinion was so generally adopted, that the instrument used
for such measurements was termed a eudiometer (evdía, fine
weather, and μéтρov, a measure). Cavendish investigated this
subject with his accustomed skill in the year 1781, and found
that when every possible precaution is taken in the analysis,
1 On Factitious Air. Hon. Henry Cavendish. Phil. Trans. 1766, 141.
20
HISTORICAL INTRODUCTION
"the quantity of pure air in common air is 19," or 100 volumes
of air always contain 2018 volumes of dephlogisticated, and 79′2
volumes of phlogisticated air, and that, therefore, atmospheric air
had an unvarying composition. But the discovery which more
than any other is for ever connected with the name of Cavendish
is that of the composition of water (1781). In making this dis-
covery Cavendish was led by some previous observations of
Priestley, and his friend Warltire. They employed a detonating
closed glass or copper globe holding about three pints, so
arranged that an electric spark could be passed through a
mixture of inflammable air (hydrogen) and common air, but
though they had observed the production of water, they not
only overlooked its meaning, but believed that the change was
accompanied by a loss of weight.
a loss of weight. Cavendish saw the full
importance of the phenomenon and set to work with care and
deliberation to answer the question as to the cause of the
formation of the water. Not only did he determine the
volumes of air and hydrogen, and of dephlogisticated air
(oxygen) and inflammable air (hydrogen) which must be mixed
to form the maximum quantity of water, but he first showed
that no loss of weight occurred in this experiment and that the
formation of acid was not an invariable accompaniment of the
explosion.
On this important subject it is interesting to hear Caven-
dish's own words; in the Philosophical Transactions for 1784,
page 128, we read :—
"From the fourth experiment it appears that 423 measures
of inflammable air are nearly sufficient to phlogisticate 1,000
of common air; and that the bulk of the air remaining after
the explosion is then very little more than four-fifths of the
common air employed; so that, as common air cannot be reduced
to a much less bulk than that by any method of phlogistication,
we may safely conclude when they are mixed in this proportion,
and exploded, almost all the inflammable air and about one-fifth
part of the common air, lose their elasticity and are condensed
into a dew which lines the glass." Since 1,000 volumes of air
contain 210 volumes of oxygen and these require 420 volumes of
hydrogen to combine with them, we see how exact Cavendish's
1 Phil. Trans. 1784, 119, and 1785, 372, Mr. Cavendish's experiments on air.
2 A similar apparatus (originally due to Volta) was used by Cavendish. The
pear-shaped glass bottle with stopcock, usually called Cavendish's eudiometer,
would not be recognised by the great experimenter.
HENRY CAVENDISH
21
66
experiments were. The better," he continues, "to examine
the nature of the dew, 500,000 grain measures of inflammable
air were burnt with about 2 times that quantity of common air
and the burnt air made to pass through a glass cylinder eight
feet long and three-quarters of an inch in diameter, in order to
deposit the dew. . . . By this means upwards of 135 grains
of water were condensed in the cylinder, which had no taste or
smell, and which left no sensible sediment when evaporated
to dryness; neither did it yield any pungent smell during the
evaporation; in short, it seemed pure water." Cavendish then
sums up his conclusions from these two sets of experiments
as follows:-" By the experiments with the globe it appeared
that when inflammable and common air are exploded in a
proper proportion, almost all the inflammable air, and near one-
fifth of the common air, lose their elasticity and are condensed
into dew. And by this experiment, it appears that this dew is
plain water, and consequently that almost all the inflammable
air and about one-fifth of the common air are turned into pure
water."
Still more conclusive was the experiment in which Cavendish
introduced a mixture of dephlogisticated air and inflammable
air nearly in the proportions of one to two into a vacuous
glass globe, furnished with a stopcock and means of firing by
electricity. "The stopcock was then shut and the included
air fired by electricity, by which means almost all of it lost
its elasticity. By repeating the operation the whole of the
mixture was let into the globe and exploded, without any fresh
exhaustion of the globe."
Priestley had previously been much led astray by the fact that
he found nitric acid in the water obtained by the union of the
gases. Cavendish, by a careful series of experiments, explained
the occurrence of this acid, for he showed that it did not form
unless an excess of dephlogisticated air was used, and he traced
its production to the presence in the globe of a small quantity of
phlogisticated air (nitrogen) derived from admixture of common
air. He likewise proved that the artificial addition of phlogisti-
cated air increased the quantity of acid formed in presence of
dephlogisticated air (oxygen), whilst if the latter air were re-
placed by atmospheric air no acid was formed, in spite of the
large amount of phlogisticated air (nitrogen) present. In this
way he showed that the only product of the explosion of pure
dephlogisticated with pure inflammable air is pure water.
22
HISTORICAL INTRODUCTION
Although Cavendish thus distinctly proved the fact of the com-
position of water, it does not appear from his writings that he
held clear views as to the fact that water is a chemical compound
of its two elementary constituents. On the contrary, he seems
to have rather inclined to the opinion that the water formed
was already contained in the inflammable air, notwithstanding
the fact that in 1783 the celebrated James Watt had already
expressed the opinion that "water is composed of dephlogis-
ticated and inflammable air."1 Cavendish's general conclusions
in this matter may be briefly summed up in his own words as
follows: From what has been said there seems the utmost
reason to think that dephlogisticated air is only water deprived
of its phlogiston, and that inflammable air, as was before said,
is either phlogisticated water, or else pure phlogiston; but in
all probability the former." To the end of his days Cavendish
remained a firm supporter of the phlogistic view of chemical
phenomena, but after the overthrow of this theory by Lavoisier's
experiments the English philosopher withdrew from any active
participation in scientific research.
"6
Whilst Priestley and Cavendish were pursuing their great
discoveries in England, a poor apothecary in Sweden was
actively engaged in investigations which were to make the name
of Scheele (1742-1786) honoured throughout Europe. These
investigations, whilst they did not bring to light so many new
chemical substances as those of Priestley, and did not possess
the quantitative exactitude which is characteristic of the labours
of Cavendish, opened out ground which had been entirely
neglected, and was perhaps unapproachable by the English
chemists. Scheele's discoveries covered the whole range of
chemical science. A strong supporter of the phlogistic theory,
he held peculiar views (see his celebrated treatise Ueber die
Luft und das Feuer) as to the material nature of heat and light,
and their power of combining with phlogiston, and, like Stahl,
he considered modification in the forms of matter to be of much
greater importance than alteration in its weight. In experiment-
ing upon the nature of common air he discovered oxygen gas
independently of, but probably somewhat later than, Priestley.
The investigations which led Scheele to this discovery are of
interest as a remarkable example of exact observations leading
to erroneous conclusions. His object was to explain the part
played by the air in the phenomenon of combustion; and for
1 Letter from Watt to Black, 21st April, 1783.
KARL WILHELM SCHEELE
23
this purpose he examined the action exerted by bodies sup-
posed to contain phlogiston upon a confined volume of air.
Thus he found that when a solution of hepar sulphuris (an alka-
line sulphide) was brought into contact with a given volume
of air, that volume gradually diminished, the residual air being
incapable of supporting the combustion of a taper. The same
result was observed when moist iron filings or the precipitate
formed by the action of potash on a solution of green vitriol was
employed. Scheele argued that if the effect of the combination
of phlogiston with air is simply to cause a contraction, the
remaining air must be heavier than common air. He found,
however, that it was in fact lighter, and hence inferred that a
portion of the common air must have disappeared, and that
common air must consist of two gases, one of which has the
power of uniting with phlogiston. In order to find out what
had become of the portion of air which disappeared, Scheele
heated phosphorus, metals, and other bodies in closed volumes
of air, and found that these act just as the former kind of sub-
stances had done. Hence he concluded that the compound
formed by the union of the phlogiston with one of the constituents
of the air is nothing more nor less than heat or fire which escapes
through the glass. In confirmation of the truth of this
hypothesis, Scheele believed that he had experimentally realised
the decomposition of heat into phlogiston and fire-air. Nitric
acid had, in his belief, a great power of combining with
phlogiston, forming with it red fumes; he found that when he
heated nitre in a retort, over a charcoal fire, with oil of vitriol,
he obtained, in addition to a fuming acid, a colourless air,
which supported combustion much better than common air.
This he explained by assuming that when charcoal burns, the
phlogiston combines with the fire-air to form heat, which passes
into the retort, and is there decomposed into phlogiston,
which by combining with the acid gave rise to the red nitrous
fumes, and pure fire-air. He conceived that he had brought
about the same chemical decomposition of heat by warming
black oxide of manganese with sulphuric acid, or, still more
simply, by heating calx of mercury; for here it was clear enough
that by bringing heat and calx of mercury together, the phlo-
giston combined with the latter, and fire-air was liberated,
thus:-
Heat.
ཟ
Mercury.
Phlogiston + Fire-air + Calx of Mercury Calx of Mercury + Phlogiston + Fire air.
24
HISTORICAL INTRODUCTION
In the year 1774 Scheele made his great discovery of chlorine
gas, which he termed dephlogisticated muriatic acid; in the same
year he showed that baryta was a peculiar earth; shortly after-
wards he proved the separate existence of molybdic and tungs-
tic acids, whilst his investigations of prussian blue led to the
isolation of hydrocyanic acid, of which he ascertained the
properties. It was, however, especially in the domain of
animal and vegetable chemistry that Scheele's most numerous
discoveries lay, as will be seen by the following list of organic
acids first prepared or distinctly identified by him-tartaric,
oxalic (by the action of nitric acid on sugar), citric, malic, gallic,
uric, lactic, and mucic. In addition to the identification of each
of these as distinct substances, Scheele discovered glycerin, and
we may regard him not only as having given the first indication.
of the rich harvest to be reaped by the investigation of the
compounds of organic chemistry, but as having been the first
to discover and make use of characteristic reactions by which
closely allied substances can be detected and separated, so
that he must be considered one of the chief founders of an-
alytical chemistry.
We have now brought the history of our science to the point
at which Lavoisier placed it in the path which it has ever since
followed. Before describing the overthrow of the phlogistic
theory it may be well shortly to review the position of the
science before the great chemist began his labours about one
hundred years ago. Chemistry had long ceased to be the slave
of the alchemist or the doctor; all scientific chemists had
adopted Boyle's definition, and the science was valued for its
own sake as a part of the great study of nature. Stahl had well
defined chemistry to be the science which was concerned with
the resolution of compound bodies into their simpler consti-
tuents, and with the building up of compounds from their
elements; so that the distinction between pure and applied
chemistry was perfectly understood. Geber's definition of a
metal as a fusible, malleable substance, capable of mixing
with other metals, was still accepted; gold and silver were con-
sidered to be pure or noble metals, whilst the other malleable
metals, copper, tin, iron, and lead, were called the base metals.
Mercury, on the other hand, was thought to be only a metal-like
body until it was frozen in 1759. After that date it was con-
sidered to be a true metal in a molten state at the ordinary
temperature Arsenic, antimony, bismuth, and zinc, from being
LAVOISIER
25
:
brittle, were classed as semi-metals, and to these well-known
bodies were added cobalt in 1735, nickel in 1751, and man-
ganese in 1774, whilst platinum was recognised as a peculiar
metal in 1750, and molybdenum and tungsten were discovered
about 1780. The several metals were supposed to be compounds
of phlogiston with metallic calces, whilst sulphur, phosphorus,
and carbon were looked upon as compounds of phlogiston with
the acids of these elements. Of the simple gases the following
were known inflammable air (hydrogen), supposed to be either
pure phlogiston or phlogisticated water; dephlogisticated or fire-
air (oxygen); phlogisticated air (nitrogen); and dephlogisticated
muriatic acid (chlorine). When the metals dissolve in acids the
phlogiston was thought to escape (as inflammable air) either in
the pure state or combined with water. It was also known that
when a metal is calxed, an increase of weight occurs, but this
was explained either by the metal becoming more dense, which,
in the opinion of some, would produce an increase of weight, or
by the absorption of fiery particles, or again by the escape of
phlogiston, a substance which instead of being attracted is re-
pelled by the earth. In short, confusion and difference of
opinion in the quantitative relations of chemistry reigned su-
preme, and it was not until Lavoisier brought his great powers
to bear on the subject that light was evoked from the darkness
and the true and simple nature of the phenomena was rendered
evident.
In the year 1743 Lavoisier was born. Carefully educated,
endowed with ample means, Lavoisier, despising the usual occu-
pations of the French youth of his time, devoted himself to
science, his genius, aided by a careful mathematical and physical
training, rendering it possible for him to bring about a complete
revolution in the science of chemistry. Before his time quanti-
tative methods and processes were considered to be purely
physical, though they now are acknowledged to be chemical,
and of all these, the determination of the weights of bodies
taking part in chemical change, as ascertained by the balance,
is the most important. Others, indeed, before him, had made
quantitative investigations. Black and Cavendish almost ex-
ceeded Lavoisier in the exactitude of their experiments, but it
is to the French philosopher that the glory of having first
distinctly asserted the great principle of the indestructibility
of matter belongs. Every chemical change, according to him,
consists in a transference or an exchange of a portion of the
26
HISTORICAL INTRODUCTION
material constituents of two or more bodies; the sum of the
weights of the substances undergoing chemical change always
remains constant, and the balance is the instrument by which
this fundamental fact is made known.
In his first important research (1770) Lavoisier employs
the balance to investigate the question, much discussed at the
time, as to whether water on being heated becomes converted
into earth. For one hundred and one days¹ he heated water in
a closed and weighed vessel; at the end of the experiment the
weight of the closed vessel remained unaltered, but on pouring
out the water he found that the vessel had lost 174 grains,
whilst on evaporating the water, he ascertained that it had dis-
solved 204 grains of solid matter. Taking the excess of 3:0
grains as due to unavoidable experimental errors, he concludes
that water when heated is not converted into earth. Shortly
after this, the same question was examined independently by
Scheele, who obtained the same results by help of qualitative
analysis, which showed that the water had taken up a constituent
of the glass, viz., the alkaline silicates.
When he became acquainted with the novel and unexpected
discoveries of Black, Priestley, and Cavendish, a new light burst
upon the mind of Lavoisier, and he threw himself instantly
with fresh ardour into the study of specially chemical
phenomena. He saw at once that the old theory was in-
capable of explaining the facts of combustion, and by help
of his own experiments, as well as by making use of the
experiments of others, he succeeded in finding the correct
explanation, destroying for ever the theory of phlogiston, and
rendering his name illustrious as having placed the science of
chemistry on its true basis. On looking back in the history of
our science we find indeed that others had made experiments
which could only be explained by this new theory, and in cer-
tain isolated instances the true explanation may have previously
occurred to the minds of others. Thus in 1774 Bayen showed
that calx of mercury loses weight, evolving a gas equal in weight
to what is lost, and he concludes that either the theory of phlo-
giston is incorrect, or this calx can be reduced without addition
of phlogiston. This, however, in no way detracts from Lavoisier's
glory as having been the first to carry out the true ideas consist-
ently and deliberately through the whole science. It is the
systematic application of a truth to every part of a science which
1 Euvres de Lavoisier, 2, 22.
1
I
·
I
LAVOISIER
27
constitutes a theory, and this it was that Lavoisier and no one
else accomplished for chemistry.
When a man has done so much for science as Lavoisier, it
seems almost pitiful to discuss his shortcomings and failings.
But it is impossible in any sketch of the history of chemistry to
ignore the question how far Lavoisier's great conclusions, the
authorship of which no one questions, were drawn from his
own discoveries, or how far he was indebted to the original
investigations of his contemporaries for the facts upon which
his conclusions are based. The dispute has recently assumed
fresh interest. Certain chemists consider that to him alone the
foundation of modern chemistry is to be ascribed, both as
regards material and deduction, whilst others, affirming that
Lavoisier made use of the discoveries of his predecessors,
and especially of the discovery of oxygen by Priestley,
without acknowledgment, assert that he went so far as to
claim for himself a participation in this discovery to which
he had no right whatever, and insist that until he had thus
obtained, from another, the key to the problem, his views upon
the question of combustion were almost as vague as those of
the phlogistonists themselves. To enter into a full discussion
of the subject would lead us into a historical criticism which
would outrun our space. Suffice it to say that many of the
charges which have been brought against Lavoisier's good
faith unfortunately turn out upon investigation to be well
founded, so that whilst we must greatly admire the clear sight
of the philosopher, we cannot feel the same degree of respect for
the moral character of the man.
His investigations on the phenomena of combustion began in
the year 1772. In a first memoir¹ Lavoisier finds not only that
when sulphur and phosphorus are burnt no loss of weight
occurs, but that an increase of weight is observed. Hence he
concludes that a large quantity of air becomes fixed. This dis-
covery leads him to the conclusion that a similar absorption of
air takes place whenever a body increases in weight by combus-
tion or calcination. In order to confirm this view, he reduces
litharge with charcoal, and finds that a considerable quantity
of air is liberated. This, he asserts, appears to him to be
one of the most interesting experiments made since the time
of Stahl.
Lavoisier's next publication was his Opuscles physiques et
1 Sur la Cause de l'Augmentation des Poids. Euvres, 2, 99.
28
HISTORICAL INTRODUCTION
chymiques, commenced in 1774. In these memoirs he first
examines the kind of air given off in the processes of breathing,
combustion, and fermentation. The views which he expresses are
similar to those put forward long before by Black, to whom
he repeatedly refers as the originator of them, this acknowledg-
ment of his indebtedness to the Scottish philosopher being
repeated in the letters from Lavoisier to Black which have
been already referred to,¹ in one of which the following passage
occurs :-" Plus confiant dans vos idées que dans les miennes
propres, accoutumé à vous regarder comme mon maître," &c.
In the year 1774 he describes experiments on the calcination
of lead and tin, which he, like Boyle, heats in closed glass
globes so long as the vessel is closed it does not change in
weight, but when the neck of the flask is broken, air rushes in,
and the weight increases. He further shows that only a por-
tion of the air is taken up by the molten metal, and that the
residual air is different from common air, and also from fixed air.
From these statements it is clear that Lavoisier considered that
the air consists of two different elastic fluids, but that he was
not acquainted with Priestley's discovery of oxygen. Nor were
his views at this time so precise or well defined as we should
gather from reading his papers published in the memoirs
of the French Academy for 1774. The explanation is
simple enough, inasmuch as owing to the careless and tardy
manner in which the memoirs of the French Academy were
at that time edited, changes in the original communications
were frequently made by the writers before publication, so that
the papers printed in the memoirs were corrected to suit
alteration in view or in fact which had become known to the
authors between the times of reading and of publication.
Thus, for instance, it is clear that the paper 2 detailing the
results of his experiments on the calcination of the metals
above referred to, which was read before the Academy in Nov.
1774, does not express the same views which we find given
in the extended description of his experiments contained.
in the volume of the memoirs for 1774, which however
was not published till 1778. So that although Lavoisier in
1774 considered air to be made up of several different elastic
fluids, it is certain that he was not then acquainted with the
kind of air which was absorbed in calcination, that his views
1 British Association Reports, 1871, 190.
2 Journal de Physique for Dec. 1774.
THE DISCOVERY OF OXYGEN
29
on the subject were in reality very similar to those expressed
a century before by Jean Rey (1630), Mayow (1669), and later,
by Pott (1750), and that they were far from being as precise
and true as we should gather them to have been from the perusal
of his extended memoir, printed in 1778 and corrected so as
to harmonise with the position of the science at that date.
It is not until we come to a paper, Sur le nature du
principe qui se combine avec les métaux pendant leur calcina-
tion, first read in 1775 and re-read on Aug. 8, 1778, that
we find a distinct mention of oxygen gas, which he first termed
“l'air éminemment respirable," or "l'air pur," or "l'air vital,"
and that we see that the whole theory of combustion is clear to
Lavoisier. He shows that this gas is necessary for the cal-
cination of metals, he prepares it from precipitatum per se,
as Priestley had previously done, and in the year 1778 we
find the first mention of oxygen or the acidifiant principle. The
name was given to it because he observed that combined with
carbon this substance forms carbonic acid, with sulphur vitriolic
acid, with nitrous air nitric acid, with phosphorus phosphoric
acid, although with the metals in general it produces the
metallic calces. In his Eléments de Chimie, published in 1782,
we find the following words under oxygen gas :-" Cet air que
nous avons découvert presque en même temps, Dr. Priestley, M.
Scheele et moi,” 1 Now there is no doubt whatever that in
October, 1774, Dr. Priestley informed Lavoisier, in Paris, of the
discovery he had lately made, and that Lavoisier was at that
time unacquainted with the fact that precipitatum per se yields
this new gas on heating. Hence we cannot admit Lavoisier's
claim to the joint discovery of oxygen, a claim, it is to be
remembered, not made until eight years after the event had
occurred. In corroboration of this conclusion we find in
Priestley's last work, published in 1800, and singularly enough
entitled The Doctrine of Phlogiston Established, the following
succinct account of the matter. "Now that I am on the subject of
the right of discoveries," he says, "I will as the Spaniards say,
leave no ink of this kind behind in my ink-horn, hoping it will be
the last time I shall have any occasion to trouble the public about
it. M. Lavoisier says (Elements of Chemistry, English edition,
p. 36) This species of air (meaning dephlogisticated) was
discovered almost at the same time by Mr. Priestley, M. Scheele,
and myself.' The case was this: having made the discovery
¹ Euvres, 1, 38.
30
HISTORICAL INTRODUCTION
some time before I was in Paris in 1774, I mentioned it at the
table of M. Lavoisier, when most of the philosophical people in
the city were present; saying that it was a kind of air in which
a candle burned much better than in common air, but I had
not then given it any name. At this all the company, and Mr. and
Mrs. Lavoisier as much as any, expressed great surprise; I told
them then I had gotten it from precipitatum per se and also from
red lead. Speaking French very imperfectly, and being little
acquainted with the terms of chemistry, I said plomb rouge and
was not understood till M. Macquer said, 'I must mean minium.'
M. Scheele's discovery was certainly independent of mine, though
I believe not made quite so early."
The two memoirs in which Lavoisier clearly puts forward his
views on the nature of combustion and respiration are, first, one
read before the Academy in 1775, Sur la combustion en général,
and second, one entitled Réflexions sur la Phlogistique, published
by the Academy in 1783. In the first of these memoirs he
does not attempt to substitute for Stahl's doctrine a rigorously
demonstrated theory, but only an hypothesis which appears to
him more conformable to the laws of nature, and less to con-
tradict known facts. In the second memoir he develops his
theory, denying the existence of any "principle of combust-
ibility," as upheld by Stahl, stating that the metals, and such
substances as carbon, sulphur, &c., are simple bodies which on
combustion enter into combination with oxygen, and concluding
that Stahl's supposition of the existence of phlogiston in the
metals, &c., is entirely gratuitous, and more likely to retard than
to advance the progress of science.
The triumph of the antiphlogistic (Lavoisierian) doctrines
was, however, not complete until the discovery of the compound
nature of water by Cavendish in 1783 became fully known.
The experiment concerning the combination of hydrogen (phlo-
giston) and oxygen to form water was at once repeated and
confirmed by Lavoisier and Laplace on the 24th June, 1783,
and then Lavoisier was able satisfactorily to explain the changes
which take place when metals dissolve in acids, and to show
that the metals are simple bodies which take up oxygen on
combustion, or on solution in acid, the oxygen being derived
in the latter case either from the acid or from the water
present.
Here, again, if we investigate the position occupied by Lavoisier
respecting the discovery of the composition of water we shall see
CAVENDISH, WATT, AND LAVOISIER
31
that, not content with the glory of having been the first to
give the true explanation of the phenomena, he appears to claim.
for himself the first quantitative determination of the fact,¹
although it is clear that he had been previously informed by
Blagden of Cavendish's experiments.2
The verdict concerning the much-vexed question as to the
rival claims of Cavendish, Watt, and Lavoisier, cannot be more
forcibly or more concisely given than in the following words of
Professor Kopp-Cavendish first ascertained the facts upon
which the discovery of the composition of water was based,
although we are unable to prove that he first deduced from these
facts the compound nature of water, or that he was the first
rightly to recognise its constituent parts. Watt was the first to
argue from these facts the compound nature of water, although
he did not arrive at a satisfactory conclusion respecting the
nature of the components; whilst Lavoisier, also from these
facts, first clearly recognised and stated the true nature of the
components of water.
Although at this period the experimental basis of the true
theory of combustion was complete, it was some time before the
clear statements of Lavoisier were accepted by chemists. Many
of those who were most distinguished by their discoveries
remained to the last wedded to the old ideas, but by degrees, as
fresh and unprejudiced minds came to study the subject, the
new views were universally adopted.
In considering this great discussion from our present point
of view, we cannot but recognise in the phlogistic theory the
expression of an important fact, of which, however, the true in-
terpretation was unknown to the exponents of the theory. The
phlogistonists assert that something which they term phlogiston
escapes when a body burns; the antiphlogistonists prove, on the
other hand, that no escape of material substance then occurs, but
that on the contrary, an addition of oxygen (or some other
element) always takes place. In thus correcting from one aspect
the false statement of the followers of Stahl, Lavoisier and his
disciples to some extent overlooked an interpretation which may
1 Euvres, 2, 338.
2 For an exhaustive discussion of this subject we must refer the reader to
George Wilson's Life of Cavendish, 1849, as well as to Prof. H. Kopp's Beiträge
zur Geschichte der Chemie. Die Entdeckung der Zusammensetzung des Wassers.
Vieweg und Sohn, 1875. See also Grimaux, Lavoisier, F. Alcan, Paris. Thorpe,
Brit. Ass. Reports, 1890, p. 761., and Berthelot, La Révolution Chimique,
F. Alcan, Paris, 1890.
32
HISTORICAL INTRODUCTION
truly be placed upon the statements of the phlogistonists, for if in
place of the word "phlogiston," we read "energy," this old theory
becomes the expression of the latest development of scientific
investigation. We now know that when two elements combine,
Energy, generally in the form of heat, is usually evolved,
whilst in order to resolve the compound into its constituent
elements an expenditure or absorption of an equal amount
of energy is requisite.
The fact that every distinct chemical compound possesses a
fixed and unalterable composition, was first proved by the
endeavour to fix the composition of certain neutral salts.
Bergman from the year 1775, and Kirwan from 1780, were
occupied with this experimental inquiry, but their results did
not agree sufficiently well to enable chemists to come to a satis-
factory conclusion, and it was to Cavendish that we owe the first
proof that the combining proportion between base and acid follows
a distinct law, whilst to him we also owe the introduction of the
word "equivalent" into the science. It is, however, to Richter
(1762-1807) that we are indebted for the full explanation of the
fact, that when two neutral salts undergo mutual decomposition,
the two newly-formed salts are also neutral. He shows in his
"Stöchiometrie," that the proportions by weight of different bases
which saturate the same weight of a given acid will also saturate
a different but a constant weight of a second acid. So that if
we have determined what weight of a given base is required to
saturate a given weight of several different acids, and also if we
know the weights of the different bases which are needed for the
neutralization of a given weight of any one of these acids, we
can calculate in what proportion each of these bases will unite
with any one of these acids. Richter also showed that when the
different metals are separately dissolved in the same quantity
of sulphuric acid, each one takes up the same quantity of
oxygen; or, as we may now express it, the varying quantities of
these different oxides which neutralize one and the same
quantity of any acid, all contain the same quantity of oxygen.
These important observations attracted but little attention or
consideration from Richter's contemporaries, all of whom were
busily engaged in carrying on the phlogistic war in which he
himself took an active part in defence of the older doctrine.
The investigations of Richter and his predecessors had
reference mainly to the proportions by weight in which acids
and bases unite, which, according to Lavoisier's theory, are not
"ESSAI DE STATIQUE CHIMIQUE"
33
simple substances, whilst Lavoisier recognised the fact that the
elements themselves combine in definite proportions by weight.
In opposition to this view of combination in definite unalterable
quantities, L. Claude Berthollet published in 1803 his cele-
brated Essai de statique Chimique, in which he refers the
phenomena of chemistry to certain fundamental properties of
matter, endeavouring to explain chemical changes by the motions
of the particles of matter on the same principles as Newton's
theory of gravitation accounts for the simpler motions of the
heavenly bodies. Considering chemical change from this
mechanical point of view, Berthollet pointed out the cir-
cumstances under which we can accomplish the highest
development of the science, namely, prediction of phenomena;
and, if, in his assumed identity of the laws of gravitation
and chemical action, he was mistaken, the aim which he set
before himself is that which has remained, and will ever
remain, the highest ideal of the science. The influence which
Berthollet's views exercised on the progress of the science was
less powerful than it otherwise would have been owing to the
fact that he, considering chemical combination to be based upon
purely mechanical laws, was obliged to admit that an alter-
ation of the conditions, such as mass and temperature, must
generally produce an alteration in the composition of the
chemical compound, and was, therefore, forced to the conclusion
that combination may take place, as a rule, between variable
proportions of the elements, fixity of proportion being the
exception, due to some special physical property of the com-
pound containing those proportions, such as insolubility or
elasticity. The opposite view that combination only takes
place in a small number of definite fixed proportions was
defended by his countryman Proust, and this led to a keen
debate between the two French philosophers which lasted from
the year 1801 to the year 1808. In the end, however, Proust
proved conclusively that Berthollet's views were incorrect
inasmuch as he showed that when one metal gives rise to two
oxides, the weight of the metal which combines with the same
quantity of oxygen to form the various oxides is a different
but a fixed quantity, so that combination does not take place by
the gradual addition of one element, but by sudden increments.
This observation ought in fact to have led to the recognition by
Proust of the law of combining proportions, but his analyses
4
34
HISTORICAL INTRODUCTION
were not sufficiently accurate for this purpose,¹ so that neither
Proust nor Richter arrived at the true expression of the facts
of chemical combination, and it was reserved for John Dalton,
(1766-1844) clearly to state the great law of chemical com-
bination in multiple proportions, and to found upon this a theory
which fully explains the observed facts.
Democritus, and after him Epicurus and Lucretius, had long
ago taught that matter is made up of small indivisible particles,
and the idea of the atomic constitution of matter, and even the
belief that chemical combination consists in the approximation
of the unlike particles, had been already expressed by Kirwan in
1783, as well as by Higgins in 1789. Dalton was, however, the
first to propound a truly chemical atomic theory, the only one
hitherto proposed which explains the facts of chemical combina-
tion in a satisfactory manner. The cardinal point upon which
Dalton's atomic theory rests, and in which it differs from all
previous suggestions, is that it is a quantitative theory respecting
the constitution of matter, whereas all others are simply quali-
tative views. For whilst all previous upholders of an atomic
theory, including even Higgins, had supposed that the relative
weights of the atoms of the various elements are the same,
Dalton at once declared that the atoms of the different elements
are not of the same weight; and that the relative atomic weights
of the elements are the proportions by weight in which the elements
combine, or some multiple or submultiple of these.
In 1803 Dalton published his first table of atomic weights of
certain elements and their compounds, as an appendix to a paper
read before the Manchester Literary and Philosophical Society,
Oct. 23, 1803, on the absorption of gases by water and other
liquids. As a reason for introducing these numbers, Dalton states
that the different solubility of gases in water depends upon the
weight and number of the ultimate particles of the several gases.
"The inquiry," he continues, "into the relative weights of the
ultimate particles of bodies is a subject, as far as I know, entirely
new; I have lately been prosecuting this inquiry with remark-
able success. The principle cannot be entered upon in this
paper, but I shall subjoin the results as far as they appear
ascertained by my experiments."
1 Journal de Physique, 59, 260 and 321.
JOHN DALTON
35
Hydrogen.
Azot
Carbon.
Ammonia.
Oxygen
Water.
Dalton's First Table of the Relative Weights of the Ultimate
Particles of Gaseous and other Bodies.
•
•
·
•
1
4.2
4.3
5.2
5.5
6.5
7.2
Phosphorus
Phosphuretted hydrogen 8.2
9.3
·
Nitrous gas
Ether.
9.6
Gaseous oxide of carbon 9.8
Nitrous oxide
Sulphur
13.7
14.4
Hypo-nitric acid. . 15.2
Sulphuretted hydrogen 15:4
15.3
15.1
19.9
25.4
•
Carbonic acid
Alcohol
·
•
•
·
•
·
·
•
Sulphureous acid
Sulphuric acid
Carburetted hydrogen,
•
.
·
•
from stagnant water.
Olefiant gas
•
·
6.3
5.3
Thus then, at the end of a paper on a physical subject, does
Dalton make known a principle the discovery of which at once
placed the science of chemistry upon its true basis, and has
rendered the name of its discoverer second only to that of
Lavoisier amongst the founders of the science.
It is not easy to follow in detail the mental or experimental
processes by which Dalton arrived at this great theory. Certain
it is, however, that the idea which lay at its foundation had long
been in his mind, which was essentially of a mathematical and
mechanical turn, and that it was by his own experimental deter-
minations, and not by combining any train of reasoning derived
from the previous conclusions of other philosophers, that he was
able to prove the correctness of his theory. Singularly self-reliant,
accustomed from childhood to depend on his own exertions,
Dalton was a man to whom original work was a necessity.¹ In
the preface to the second part of his New System of Chemical Phil-
osophy, published in 1810, he clearly shows his independence
and even disregard of the labours of others, for he says-" Having
been in my progress so often misled by taking for granted the
results of others, I have determined to write as little as possible
but what I can attest by my own experience."
As early as 1802, in an experimental inquiry into the propor-
tions in which the several gases constituting the atmosphere
occur, Dalton clearly points out" that the elements of oxygen
may combine with a certain portion of nitrous gas" (our nitric
1 Lonsdale's Life of Dalton. Longmans, 1874.
36
HISTORICAL INTRODUCTION
oxide) "or with twice that portion, but with no intermediate
quantity," and this observation was clearly the first which led to
the possibility of drawing up the table already given.¹ In that
table, it will be seen that the relative weights of the smallest
particle of nitrous gas is given as 93, that of Azot (nitrogen)
being 4.2, and that of oxygen 5·5. Dalton clearly intending by
this to express that the gas is a compound of one atom of nitro-
gen with one atom of oxygen, whilst the substance to which
he gives the name of hypo-nitric acid (now called nitrogen
peroxide), is a compound in which one atom of nitrogen is
combined with two of oxygen, and therefore having the rela-
tive atomic weight of 15.2.2
The first public announcement of the atomic theory, and of the
law of combination in multiple proportions upon which it was
founded, was, singularly enough, not made by Dalton himself, but
by his friend, Professor Thomas Thomson, of Glasgow, who pub-
lished in 1807 an account of Dalton's discovery in the third edition
of his System of Chemistry. In the following year (1808) Dal-
ton made known his own views in the remarkable book entitled
A New System of Chemical Philosophy, in which (Part i. p. 213) he
says " It is one great object of this work to show the importance
and advantage of ascertaining the relative weights of the
ultimate particles, both of simple and compound bodies, the
number of simple elementary particles which constitute one
compound particle, and the number of less compound particles
which enter into the formation of one more compound particle."
Thomson states that during the years 1803 and 1804 Dalton
was occupied with the examination of the composition of the
two gaseous hydro-carbons, marsh gas and olefiant gas, and the
results of this examination led him to the adoption of the
atomic theory. He found that both these bodies consist solely
of carbon and hydrogen, and that the first of these gases contains
twice as much hydrogen to a given quantity of carbon as the
second. Hence he concluded that olefiant gas contains one
atom of carbon combined with one of hydrogen, whereas marsh
gas consists of one atom of carbon combined with two atoms
1 Manchester Memoirs, 2nd Series, 1, 250.
2 Certain inaccuracies in the values of the weights of some of the compounds
occur in this table; thus, 42 +5.5 97, whilst 93 appears opposite nitrous
oxide. Whether these are merely printer's errors or are to be explained in some
other way can now only be conjectured. See Roscoe on Dalton's First Table of
Atomic Weights. Manchester Lit. and Phil. Soc. Mem. 3rd Series, 5, 269,
1874-5.
-
DALTON'S ATOMIC THEORY
37
of hydrogen. The same idea and method of investigation he
then applied to the oxides of carbon, oxides of sulphur, oxides
of nitrogen, to ammonia and other bodies, and he showed that the
composition of these might be most simply explained by the
assumption that one atom of one element is attached to 1, 2, 3,
&c. atoms of another. The novelty and importance of his view
of the composition of chemical compounds induced Dalton to
introduce a method of graphic representation of the atoms of
the elements, and the system he adopted was as follows:-
Oxygen
Hydrogen
Nitrogen
Carbon
Water
·
·
·
Acetic acid
•
Ammonia
Carbonic oxide.
Carbon dioxide.
Olefiant gas
Marsh gas
Nitrous oxide .
Nitric oxide
Nitrous acid
•
·
·
Symbol.
Relative Weight
of the atom.
7
1
5
5
8
6
12
19
6
7
17
12
26
26
These atomic weights, it is evident, are far from being those
which we now accept as correct, indeed they are different from
those given in his first table, for Dalton not only frequently
altered and amended these numbers, according as his experi-
ments showed them to be faulty, but even distinctly asserts
the doubtful accuracy of some. Chemists at that time did not
possess the means of making accurate determinations, and when
we become acquainted with the rough methods which Dalton
adopted, and the imperfect apparatus he had to employ, we can-
not but be struck with the clearness of his vision and the
boldness of grasp which enabled him, thus poorly equipped, to
establish a doctrine which further investigation has only more
firmly established, and which, from that time forward, has served
38
HISTORICAL INTRODUCTION
as the pole star round which all other chemical theories
revolve.
Amongst those to whose labours we are indebted for advancing
Dalton's atomic theory are Thomas Thomson and Wollaston, but
before all, the great Swedish chemist Berzelius, to whom we
owe the first really exact values for these primary chemical
constants. With remarkable perseverance he ascertained the
exact composition of a large number of compounds, and was, there-
fore, able to calculate the combining weights of many elements,
thus laying the foundation-stones of the science as it at present
exists. In 1818 Berzelius published his theory of chemical pro-
portions, and of the chemical action of electricity, and in
these remarkable works he made use of the chemical symbols and
formulæ such as we now employ, to denote not only the qualita-
tive, but also the quantitative composition of chemical compounds.
From this time forward it was satisfactorily proved and generally
acknowledged that the elementary bodies combine together
either in certain given proportions by weight, or in simple
multiples of these proportions; and, through the researches of
Berzelius and others, the list of elements, which at the time.
of Lavoisier amounted to twenty-three in number, was now con-
siderably increased.
Next in order comes Humphry Davy's discovery of the com-
pound nature of the alkalis (1808), proving that they are not
simple substances but oxides of peculiar metals, and thus entirely
revolutionizing the views of chemists as to the constitution of a
large and important class of compounds, including the salts of
the alkaline earths. The discussion in 1810 as to the constitu-
tion of chlorine-then termed oxygenated muriatic acid-decided
by Davy and Gay-Lussac in favour of its elementary nature, was
likewise a step of the greatest importance and of wide applica-
tion. In 1811 iodine was discovered by Courtois, and most
carefully investigated by Gay-Lussac, who proved the close
analogy existing between this element and chlorine. The dis-
covery of many other elements now opened out fresh fields for
investigation, and gave the means of classifying those already
known. The names and properties of these will be found in
the portions of this book specially devoted to their description.
If Dalton, as we have seen, succeeded in placing the laws of
chemical combination by weight on a firm basis, to Gay-Lussac
belongs the great honour of having discovered the law of
the combination of gaseous bodies by volume. In the year 1805
HUMBOLDT AND GAY-LUSSAC
39
Gay-Lussac and Alexander von Humboldt found that one volume
of oxygen combines with exactly two volumes of hydrogen to
form water, and that these exact proportions hold good at what-
ever temperature the gases are brought into contact. This
observation was extended by Gay-Lussac, who in 1808 published
his celebrated memoir on the combination of gaseous bodies,¹ in
which he proves that gases not only combine in very simple
relations by volume, but also that the alteration of volume which
these gases undergo in the act of combination may be expressed by
a very simple law. Hence it follows that the densities of gases
must bear a simple relation to their combining weights. The true
explanation of these facts was first given by Avogadro in 1811,
and his hypothesis is now universally admitted both by
chemists and physicists. According to the Italian philo-
sopher the number of smallest particles or molecules con-
tained in the same volume of every kind of gas is the same,
similar circumstances of pressure and temperature being of
course presupposed.
The discovery by Gay-Lussac of the laws of volume-
combination, together with Avogadro's explanation of the law,
served no doubt as most valuable supports of Dalton's atomic
theory, but the truth of this latter theory was still further
asserted by a discovery made by Dulong and Petit in 1819.
These French chemists determined the specific heat of thirteen
elementary bodies, and found that the numbers thus obtained,
when compared with the atomic weights of the same bodies,
showed that the specific heats of the several elements are
inversely proportional to their atomic weights, or in other
words, the atom of each of these elements possesses the same
capacity for heat. Although subsequent research has shown
that this law does not apply in every case, it still remains a
valuable means of controlling the atomic-weight determina-
tions of many elements.
In the same year a discovery of equal importance was an-
nounced by Mitscherlich-that of the law of Isomorphism.
According to this law, chemically analogous elements can re-
place each other in many crystalline compounds, either wholly
or in part, without any change occurring in the crystalline form
of the compound. This law, like that of atomic heats, has proved
of great value in the determination of atomic weights.
Gradually the new basis given by Dalton to our science was
1 Mémoires d'Arcueil, 2, 207.
40
HISTORICAL INTRODUCTION
widely extended by these discoveries and by the researches of
other chemists, and a noble structure arose, towards the com-
pletion of which a numerous band of men devoted the whole
energies of their lives.
Especially striking was the progress made during these years
in the domain of Organic Chemistry, or the chemistry of the
substances found in, or obtained from, vegetable or animal
bodies. Dalton had in vain endeavoured to obtain analytical
results to prove that the complicated Organic bodies followed the
same laws as the more simple Inorganic compounds. It is to
Berzelius that we owe the proof that this is really the case, and
his exact analyses placed organic chemistry in this respect on a
firm and satisfactory basis. There still remained, however, much
doubt as to the strict identity of the laws according to which
organic and inorganic compounds were severally formed. Most
of the compounds met with in mineral chemistry could be easily
prepared by the juxtaposition of their constituents; they were
of comparatively simple constitution, and could as a rule be pre-
pared by synthesis from their constituent elements. Not so
with organic bodies; they appeared to be produced under cir-
cumstances wholly different from those giving rise to mineral
compounds; the mysterious phenomena of life seemed in some
way to influence the production of these substances and to
preclude the possibility of their artificial preparation. A great
step was therefore made in our science when, in 1828, Wöhler
artificially prepared urea, a body which up to that time had been
thought to be a product peculiar to animal life. This discovery
broke down at once the supposed impassable barrier between
organic and mineral chemistry, pointed out the rich harvest of
discovery since so largely developed, especially by Liebig, in the
synthesis of organic substances, and paved the way to the know-
ledge which we have gained, chiefly through the labours of the
last-named chemist, that the science of Physiology consists
simply in the Chemistry and Physics of the body.
GENERAL PRINCIPLES OF THE SCIENCE
I MATTER is capable of assuming three different states or con-
ditions-the solid, the liquid, and the gaseous. Of these, the
first two have, for obvious reasons, been recognised from the
earliest ages as accompanying very different kinds of substances,
It is, however, only within a comparatively short time that men
have come to understand that just as there are many distinct
kinds of solids and liquids, so there are many distinct kinds of
gases (Van Helmont). These may, indeed, be colourless and in-
visible, but, nevertheless, they can readily be shown to differ one
from another. Thus, Black, in 1752, collected a peculiar gas,
which we now know as carbonic acid gas, or carbon dioxide, obtained
by the action of dilute acids on marble; to this gas he gave the
name of "fixed air," because it is fixed in the alkaline carbonates,
which at that time were called the mild alkalis, in contradis-
tinction to the caustic alkalis. This invisible gas does not, like
air, support the combustion of a taper, and, unlike air, it renders
clear lime-water turbid; it is also much heavier than air, as can
be shown by pouring it downwards from one vessel to another,
by drawing it out of a vessel by means of a syphon, or by pouring
it into a beaker glass previously equipoised at one end of the
beam of a balance (see Fig. 2). That the gas has actually been
poured out is seen either by a burning taper being extinguished
when dipped into the beaker glass, or by adding some clear
lime-water, which then turns milky.
In 1766, Cavendish showed that the gas termed by him in-
flammable air, and obtained by the action of dilute acids on
metallic zinc or iron, is also a peculiar and distinct substance,
to which we now give the name of hydrogen gas. It is so much
lighter than air that it may be poured upwards, and takes fire
when a light is brought in contact with it, burning with a pale
blue flame. Soap-bubbles blown with hydrogen ascend in the
42
GENERAL PRINCIPLES OF THE SCIENCE
air, and if hydrogen be poured upwards into the equipoised bell-
jar hung mouth downwards on the arm of the balance (Fig. 3),
the equilibrium will be disturbed, and the arm with the bell-jar
will rise.
2 On August 1st, 1774, Priestley heated some red precipi-
tate (oxide of mercury) and obtained from it a new colour-
less gas called oxygen, and this, although invisible, possesses

FIG. 2.
Gaid
properties quite different from those of air, carbonic acid gas, or
hydrogen gas. A red hot chip of wood is at once rekindled
when plunged into this gas, and bodies such as iron wire or steel
watch-spring, which do not burn in the air, burn with brilliancy
in oxygen.
These examples suffice to show that invisible gases exist
which differ in the widest degree from each other, though many
more illustrations of the same principle might be given.
THE EXPERIMENTAL METHOD
43
3 The method which we have had to adopt in order thus to
discern differences between these invisible gases, is termed the
Experimental Method. Experiments may be said to be ques-
tions put to nature, and a science is termed experimental, as
opposed to observational, when we are able so to control and
modify the conditions under which the phenomena occur as to
produce results which are different from those which are

FIG. 3.
Dimm
otherwise met with. Chemistry is, therefore, one of several
experimental sciences, each of which has the study of natural
phenomena for its aim. These sciences are most intimately con-
nected, or, rather, the division into separate sciences is quite
arbitrary, so that it is not possible exactly to say where the
phenomena belonging to one science begin and those appertaining
to another science end. Nature is a connected whole, and the
divisions which we are accustomed to make of natural phenomena
44
GENERAL PRINCIPLES OF THE SCIENCE
into separate sciences serve only to aid the human mind in its
efforts to arrange a subject which is too vast in its complete
extent for the individual to grasp. Although it may not be
possible exactly to define the nature of the phenomena which
we class as chemical, as distinguished from those termed physical,
it is not difficult, by means of examples, to obtain a clear idea
of the kind of observations with which the chemist has to do.
Thus, for instance, it is found that when two or more given
substances are brought together, under certain conditions they
may change their properties, and a new substance, differing
altogether from the original ones, may make its appearance.
Or, again, a given substance may, when placed under certain
conditions, yield two or more substances differing entirely from
the original one in their essential properties. In both these
cases the change which occurs is termed a chemical change; if
several distinct substances have coalesced to form one new
substance, an act of chemical combination is said to occur; if one
substance is made to yield two or more distinct new bodies, a
chemical decomposition has taken place. These acts of chemical
union and disruption occur alike amongst solid, liquid, and
gaseous bodies; they are regulated in the first place by the
essential nature of the substances, and secondly, by the circum-
stances or conditions under which they are brought together.
It is also to be observed that these actions of chemical union
in the first place do not occur when the component materials
are situated at a distance from each other, close contact
being necessary in order that such changes should take
place; whilst secondly, we almost invariably notice that such
a combination is attended with an evolution of heat and,
sometimes, of light.
4 Some simple illustrations of chemical action may here be
cited :-If powdered sulphur and fine copper-filings be well
mixed together, a green-coloured powder will result, in which,
however, a powerful microscope will show the particles of sulphur
lying by the side of the particles of copper. On heating this green
powder in a test tube, the mass suddenly becomes red-hot, and,
on cooling, a uniform black powder is found. This is neither
copper nor sulphur, but a chemical compound of the two, in
which no particle of either of the substances can be seen, how-
ever high a magnifying power be employed, but from which, by
the employment of certain chemical means, both copper and
sulphur can again be extracted. Here then we have a case
CHEMICAL ACTION
45
of chemical combination. An experiment similar to that made by
Priestley when he discovered oxygen, may serve as an illustra-
tion of a chemical decomposition. 216 grams¹ of red oxide of
mercury are heated in a small retort provided with a receiver,
and a gas delivery tube passing to the top of a graduated cylinder
filled with water to the beginning of the graduations, and standing
in the pneumatic trough over water (Fig. 4). On heating the red
oxide by means of the Bunsen gas flame, it first becomes dark-
coloured and then soon begins to decompose into metallic
mercury, which collects in small bright drops in the neck of the
retort gradually running down in the receiver, and into oxygen
gas, which passes through the delivery tube and collects in the
graduated cylinder. After the heat has been continued for
some time, the whole of the red powder will have disappeared,

FIG. 4.
having been changed by heat into metallic mercury and oxygen.
On allowing the retort to cool, a volume of 1120 cubic centi-
metres of gas has been collected, and this, on application of the
red-hot chip test, is shown to be oxygen. If this experiment
is properly conducted, the volume of oxygen obtained is always
found to be the same from the same weight of oxide (provided
the temperature and pressure at which the gas is measured are
the same in the different experiments), viz., 1120 cubic centi-
metres from 21.6 grams of oxide.
Another interesting case of chemical change is the decompo-
1 For a table of equivalent values of the common English weights and measures
with those of the metrical system, see Appendix to this volume.
46
GENERAL PRINCIPLES OF THE SCIENCE
sition of water by galvanic electricity as discovered by Nicholson
and Carlisle in 1800. For the purpose of exhibiting this we
only need to pass a current of electricity from four or six
Grove's or Bunsen's elements by means of two platinum
poles through some water acidulated with sulphuric acid
(Fig. 5). The instant contact is made, bubbles of gas begin to
ascend from each platinum plate and collect in the graduated
tubes, which at first are filled with the acidulated water. After

FIG. 5.
10
00
a little time it will be seen that the plate which is in con-
nection with the zinc of the battery evolves more gas than the
one which is in contact with the platinum or carbon of the
battery; and after the evolution has continued for a few minutes
one tube will be seen to contain twice as much gas as the
other. On examination, the larger volume of gas will be found
to be hydrogen, and will take fire and burn when a light
INDESTRUCTIBILITY OF MATTER
47
is brought to the end of the tube in which it was collected
whilst the smaller volume of gas is seen to be oxygen,
a glowing chip of wood being rekindled when plunged
into it.
5 In many cases of chemical action, the products are gaseous,
whilst one or more of the materials acted upon are solid or liquid.
Hence, in these cases, a disappearance or apparent loss of matter
occurs. It has, however, been shown by many accurate experi-
ments that in these cases the loss of matter is only apparent, so
that chemists have come to the conclusion that matter is in-
destructible, and that in all cases of chemical action in which
matter disappears, the loss is apparent only, the solid or liquid
being changed into an invisible gas, the weight of which is, how-
ever, exactly identical with that of its component parts. We
only require to allow a candle to burn for a few minutes in
a clean flask filled with air in order to show that the materials
of the candle, hydrogen and carbon, unite with the oxygen of
the air to form, in the first place, water, which is seen in small
drops bedewing the bright sides of the flask, and in the second,
carbon dioxide or carbonic acid gas, whose presence is revealed
to us by lime-water being turned milky. The fact that the sum
of the weights of the products of combustion (water and carbon
dioxide) is greater than the loss of weight sustained by the candle
is clearly shown by an experiment made by means of the appa-
ratus (Fig. 6), which consists of a tube equipoised on the arm of
a balance. In the long vertical tube a taper is placed, the other
end of the system being attached to a gasholder filled with
water, which, on being allowed to run out, causes a current of
air to pass through the tube, and thus maintains the combustion
of the taper.
The water and carbonic acid gas which are formed
are absorbed by the bent tube, which contains caustic potash.
After the taper has burnt for a few minutes, the apparatus is
disconnected from the gasholder and allowed to vibrate freely,
when it will be found to be appreciably heavier than it was
before the taper had burnt, the explanation being that the excess
of weight is due to the combination of the carbon and hydrogen
of the wax with the oxygen of the air.
6 Another series of experiments which also show plainly the
fact of the indestructibility of matter, and are of historical in-
terest, are those by which it has been clearly demonstrated that
the air consists of two different gases, oxygen and nitrogen.
The fact of the composite nature of air was proved by Priestley
48
GENERAL PRINCIPLES OF THE SCIENCE
in 1772 by setting fire, by means of a burning glass, to
charcoal contained in a vessel of air. He showed that fixed air
(carbonic acid gas) was produced, and that on the absorption of
this fixed air by lime-water, one-fifth of the original bulk of
the air disappeared, and a colourless gas remained, which did
not support combustion or respiration. It was not, however,
till the year 1775, after he had discovered oxygen, that Priest-

FIG. 6.
ley distinctly stated that this gas was contained in common
air, and about the same time Scheele came to an identical
conclusion from independent experiments. But the method,
by which the existence of oxygen in the air was first demon-
strated in the clearest way, is that adopted by Lavoisier, and
described in his Traité de Chimie. Into a glass balloon (Fig. 7)
having a long straight neck, Lavoisier brought 4 ounces of pure
mercury; he then bent the neck so that when the balloon rested
1 Part i., chap. iii.
LAVOISIER'S EXPERIMENTS
49
on the top of the furnace, the end of the bent neck appeared
above the surface of the mercury contained in the trough, thus
placing the air in the bell-jar in communication with that in
the balloon. The volume of the air (reduced to 28 inches of
mercury and a temperature of 10°) contained in the bell-jar and
balloon amounted to 50 cubic inches. The mercury in the balloon
was now heated, by a fire placed in the furnace, to near its boil-
ing point. For the first few hours no change occurred, but then
red-coloured specks and scales began to make their appearance.
Up to a certain point these increased in number, but after a
while no further formation of this red substance could be noticed.

FIG. 7.
After heating for twelve days the fire was removed, and the
volume of the air was seen to have undergone a remarkable
diminution: the volume, measured under the same conditions as
before, having been reduced from 50 to between 42 and 43 cubic
inches. The red particles were next carefully collected, and on
weighing, were found to amount to 45 grains. These 45 grains
were next introduced into a small retort connected with a
graduated glass cylinder (Fig. 8), and on heating they yielded
41 grains of metallic mercury and from 7 to 8 cubic inches of
a gas which was found to be pure oxygen. Thus, the whole of
the oxygen, whether measured by volume or by weight, which
was withdrawn from the air by the mercury, was obtained again
when the oxide formed was decomposed by heat.
5
50
GENERAL PRINCIPLES OF THE SCIENCE
7 It is not merely to the investigation of changes occurring in
the essential properties of inorganic or mineral matter that the
chemist has to direct his attention. The study of many of the
phenomena observed in the vegetable or animal world also claim
his notice. So much so, indeed, is this the case that the science
of physiology has been defined as the physics and chemistry
of the body. The simplest as well as the most complicated
changes which accompany life are, to a great extent, dependent
upon chemical laws, and, although we are still unable fully to
explain many of these changes, yet each year brings us addi-
tional aid, so that we may expect some day to possess an exact

FIG. 8.
knowledge of the chemistry of life. In order to be convinced
that vital actions are closely connected with chemical
phenomena, we only need to blow the air from our lungs through
clear lime-water to see from the ensuing turbidity of the water
that carbonic acid gas is evolved in large quantities during the
process of the respiration of animals, and when we further
observe that the higher animals are all warmer than surrounding
objects, we come to the conclusion that the process of respiration
is accompanied by oxidation, and that the breathing animal
resembles the burning candle, not only in the products of this
combustion, viz., water and carbon dioxide, but in the heat
ELEMENTARY AND COMPOUND BODIES
51
which that combustion evolves, the difference being that in the
one case the oxidation goes on quickly and is confined to one
spot (the wick of the candle) whereas in the other it goes on
slowly and takes place throughout the body. In like manner
the living plant is constantly undergoing changes, which are as
necessary for its existence as the act of breathing is for animals.
One of the most fundamental of these changes is readily seen if
we place some fresh green leaves in a bell-jar filled with spring
water and expose the whole to sunlight. Bubbles of gas are
observed to rise from the leaves, and these, when collected, prove
to be oxygen. In presence of the sunlight the green leaf has
decomposed the carbonic acid gas held in solution in the spring
water, assimilating the carbon for the growth of its body and
liberating the oxygen as a gas. Nor, indeed, are the investiga-
tions of the chemist now confined to the organic and inorganic
materials of the earth which we inhabit. Recent research has
enabled him, in conjunction with his colleague the physicist, to
obtain a knowledge of the chemistry as well as of the physics
of the sun and far distant stars, and thus to found a truly
cosmical science.
8 It is the aim of the chemist to examine the properties of
all the different substances which occur in nature, so far as they
act upon each other, or can be made to act so as to produce
something different from the substances themselves; to
ascertain the circumstances under which such chemical changes
occur, and to discover the laws upon which they are based.
In thus investigating terrestrial matter it is found that all the
various forms of matter with which we are surrounded, or
which have been examined, can be divided into two great
classes.
I. ELEMENTARY BODIES.-Elements, or simple substances, out
of which no other two or more essentially differing substances
have been obtained.
II. COMPOUND BODIES, or compounds, out of which two or
more essentially differing substances have been obtained.
Only twenty-three elements were known during the lifetime
of Lavoisier; now we are acquainted with not less than sixty-
nine. Of these, and their compounds with each other, the whole.
mass of our globe, solid, liquid, and gaseous, is composed, and
these elements contribute the material out of which the fabric
of our science is built. The science of chemistry has for its
aim the experimental examination of the elements and their
52
GENERAL PRINCIPLES OF THE SCIENCE
compounds, and the
the investigation of the laws of their
combination one with another.
The following is an alphabetical list of the elementary bodies
known at present (1893):-
Aluminium
Antimony.
Arsenic
Barium
Beryllium
Bismuth
Boron
Bromine
Cadmium.
·
·
•
Cæsium
Calcium
Carbon
Chlorine
Cerium
Chromium
·
Lead.
Lithium
•
•
•
•
•
•
·
Iron .
Lanthanum
Magnesium
Manganese
Mercury
•
Cobalt
Copper.
Didymium
Erbium
Fluorine
Gallium
Germanium .
Gold . .
Hydrogen
Indium
Iodine
Iridium
•
•
•
•
•
•
·
•
•
•
•
LIST OF ELEMENTS.¹
Atomic weight.
26.9
119.4
74.4
135.97
8.99
•
·
.
·
•
•
•
•
•
•
139.0
51.7
58.6
62.8
Pr. 142.5
Ne.139.7
}
·
.
.
0
•
•
•
•
•
.
-
•
·
206.4
10.74
79.36
111.3
131.9
39.7
11.91
35.19
165.0
18.9
69.4
71.8
195.7
1.0
112.8
125.91
191.7
55.6
137.5
205'4
6.98
Molybdenum
Nickel
Niobium
Nitrogen
Osmium
•
Oxygen
Palladium
Phosphorus
Platinum
Potassium
Rhodium
Rubidium
Ruthenium
Samarium
Scandium
Selenium
·
Silver.
Silicon
Sodium
Strontium
Sulphur
Tantalum
Tellurium
Thallium
Thorium
Thulium
Tin . .
Titanium
24.2
54.6
198.9
1 See also Appendix.
·
•
•
•
•
•
·
·
·
•
•
•
•
•
•
Tungsten
Uranium
Vanadium
Ytterbium
Yttrium . .
Zinc
Zirconium
•
•
·
·
•
•
•
•
•
•
•
•
•
•
•
•
•
0
0
•
•
·
0
•
·
·
Atomic weight.
95.2
58.6
93.5
13.94
•
·
•
. 189.3
•
. 104.7
. 30'S
193.3
•
•
•
. 102.0
84.8
. 103.0
149.0
43.8
. 78.5
•
·
•
15.88
•
38.85
•
107.13
28.2
22.87
86.8
31.82
181.0
. 124.0
.202.6
. 230.7
.169.7
. 117.2
47.7
182.6
. 237.6
. 50.8
. 172.0
88.0
65.0
90.0
LIST OF ELEMENTS
53
In addition to the above the existence of many other elements,
discovered in certain rare Norwegian and American minerals,
has been announced, among which are Decipium, Holmium,
Iduminium, Norwegium, and Yttrium-a. These, however, have
not as yet been very perfectly investigated, and their atomic
weights and chemical relationships remain undetermined.
For the sake of convenience it is customary to divide the
elements into two classes-the Metals and the Non-Metals, or
Metalloids, a distinction which was first made about the time of
Lavoisier, when only a few elements were known. Now,
however, the division is a purely arbitrary one, as it is not
possible to draw an exact line of demarcation between these two
groups, so that there are cases in which an element has been
considered as a metal by some chemists and as a non-metal by
others. To the first class belong such substances as gold, silver,
mercury, and tin; to the second substances which are gaseous at
the ordinary temperature, such as hydrogen, nitrogen, and
oxygen, together with certain solid bodies, as carbon and sulphur.
The number of metals is much larger than that of the non-
metals: we are acquainted with fifty-four metals, and with
only fifteen non-metals.
9 In this treatise the elements will be considered in the follow-
ing order, although another system of classification, termed the
natural system, based upon the chemical properties of the
elements, will be discussed and explained in a later volume.
The natural system is, however, as yet imperfect, and its value
can only be properly appreciated when a fuller knowledge of the
properties of the elements has been gained.
ARRANGEMENT OF THE ELEMENTS.
I.-NON-METALS.
Hydrogen
Fluorine.
Chlorine
Bromine
Iodine
Oxygen
Sulphur
Selenium
Tellurium
.
·
·
·
·
•
Symbol.
. H
•
•
•
·
•
F
Cl
Br
I
O
S
Se
Te
·
Nitrogen
Phosphorus
Arsenic
Boron
Carbon
Silicon
•
.
•
•
Symbol.
N
•
•
•
·
·
•
P
As
B
C
Si
54
GENERAL PRINCIPLES OF THE SCIENCE
Potassium
Sodium .
Lithium.
Rubidium
Cæsium.
Calcium.
Strontium
Barium
Beryllium
Magnesium
Zinc
Cadmium
Lead
Thallium
·
Copper
Silver
Mercury
·
·
•
•
Aluminium
Scandium
Indium
Gallium
Yttrium
Cerium
Lanthanum
Didymium
•
·
•
·
Erbium.
Ytterbium.
Thulium
Samarium
·
•
•
•
.
•
·
·
•
Symbol.
. K
. Na
·
•
•
•
•
•
. Ba
•
.
·
•
II.-METALS.
· Zn
Cd
ᏞᎥ
Rb
C's
•
8*£ 1953 2=
•
Ca
Sr
·
Y
Ce
La
. Di
Be
Mg
Pb
TI
Er
. Yb
. Tu
Cu
Ag
Hg
€.
Sa
Al
Sc
· In
Ga
Manganese
Iron
Cobalt
Nickel
•
•
Chromium
Molybdenum.
Tungsten
Uranium
Tin
Titanium
Zirconium
Thorium
Bismuth
Tantalum
Niobium
Vanadium.
Antimony
Germanium
Gold
Platinum
•
..
·
•
Ruthenium
Osmium.
•
•
Palladium.
Rhodium
Iridium
•
•
·
•
•
Symbol.
Mn
. Fe
Co
Ni
·
·
3
•
•
·
•
•
·
·
· Bi
Ta
Nb
·
Cr
Mo
W
U
Sn
Ti
Zr
Th
•
V
Sb
Ge
Au
Pt
Pd
I Rh
· Ir.
Ru
Os
Of these elements only four occur largely in the air, about
thirty have been detected in the sea, whilst all the sixty-nine
are found irregularly distributed throughout the solid mass of
our planet.
Some are very abundant, and are widely distributed, whilst
ARRANGEMENT OF THE ELEMENTS
55
others have hitherto been found only in such minute quantities
and so seldom, that even their properties have not yet been
satisfactorily examined. Thus oxygen is found throughout the
air, sea, and solid earth in such quantities as to make up nearly
half the total weight of the crust of our planet, whilst the com-
pounds of cæsium, although tolerably widely distributed, occur
only in very minute quantity, and those of erbium have as yet
been met with only in very small quantities, and in very few
localities.
In order to obtain an idea as to which elements form the main
portion of the solid crust of the earth, we may examine the
composition of all the different kinds of granitic or eruptive
rocks which constitute by far the greater part of the earth's
crust. From analyses made by Bunsen we find that all granitic
rocks possess a composition varying between the limits given in
the following table, so that these numbers give a fair idea of
what is known of the average chemical composition of the solid
globe. All the other elements occur in quantities less than
any of those mentioned in the table.
The Composition of the Earth's Solid Crust in 100 parts
by weight.
Oxygen
Silicon
Aluminium.
Iron .
•
•
•
44.0 to 48.7
22.8 36.2
""
9.9 6.1
9.9 2.4
""
""
Calcium.
Magnesium.
Sodium.
Potassium
•
•
.
•
•
•
6.6 to 0.9
2.7, 0·1
2.4 2.5
1.7 3.1
""
23'
10 In considering for the first time the subject of the
elements, the question will at once suggest itself—Are these
sixty-nine all the elements which make up our earth, or
is it likely that other hitherto undiscovered elements exist?
Judging from analogy, remembering what has previously
occurred, and looking to the incomplete state of our know-
ledge concerning the composition of the earth's crust, we
may fairly conclude that it is all but certain that other
elementary bodies remain to be discovered. Every improve-
ment in our methods of examination leads to the detec-
tion either of new elements or of old ones in substances
in which they had previously been overlooked. Thus by
the methods of Spectrum Analysis no less than five
(cæsium, rubidium, thallium, indium, and gallium) new
elements have been discovered, and the existence of several
56
GENERAL PRINCIPLES OF THE SCIENCE
others rendered highly probable especially among the rare
earths. By help of this method we are also enabled to come
to certain conclusions respecting the distribution and occur-
rence of these same elements in some of the heavenly bodies,
and we learn that many of the metals, and even non-metals,
which are well known to us on the earth, are found in the sun
and the fixed stars. The conclusion that the terrestrial ele-
ments exist beyond the bounds of our planet is borne out by the
chemical examination of the meteoric stones which are constantly
falling upon the surface of the earth. In hundreds of these which
have been examined, no single case of the discovery of an un-
known element has occurred. The substances of which meteorites
have been found to consist are iron, nickel, oxygen, calcium,
silicon, carbon, and other well-known terrestrial elements.
Another question which may here be asked is-Are these
elements really undecomposable substances? and to this it may
be replied, that so far as our chemical knowledge enables us to
judge, we may assume, with a considerable degree of probability,
that by the application of more powerful means than are at
present known, chemists will succeed in obtaining still more
simple bodies from the so-called elements. Indeed, if we examine
the history of our science, we find frequent examples occurring of
bodies which only a short time ago were considered to be elemen-
tary which, upon more careful examination, have been shown to
be compounds.
II A very remarkable fact observed in the case of many
elements is that they are capable of existing in more than one
distinct condition, presenting totally different physical qualities.
One of the most striking examples of these allotropic modifica-
tions or conditions of matter (äλλos, another—τроπоs, a way or
mode) occurs with carbon, which exists as Diamond, Graphite,
and Charcoal, bodies which as regards colour, hardness, specific
gravity, &c., bear certainly but a slight resemblance to each
other, but which, when they are burnt in oxygen, all give the
same relative weight of the same product, viz., carbonic acid,
thereby proving their chemical identity.
12 The Balance.-As it is the aim of the chemist to examine
the properties of the elements and their compounds, and as the
weight-determination of a substance is of the greatest import-
ance, it becomes necessary for him to ascertain with great pre-
cision the proportion by weight in which these several elements
combine, as well as that in which any one of them occurs in a
THE BALANCE
57
given compound, and for this purpose the Balance is employed.
By means of this instrument the weight of a given substance is
compared with the unit of weight. It consists essentially of a
light but rigid brass beam (Fig. 9), suspended on a fixed hori-
zontal axis situated at its centre; and this beam is so hung as to
assume a horizontal position when unloaded. At each end of the
beam scale-pans are hung, one to receive the body to be weighed
and the other for the weights. When each pan is equally
weighted the beam must still retain its horizontal position, but
when one pan is more heavily weighted than the other, the beam
will incline on the side of the heavier pan. The balance is, there-
fore, a lever with equal arms, and it is evident that the weight of
the substance relative to the unit weight employed is the sum of

M
FIG. 9.
the weights necessary to bring the balance into equilibrium. The
two important requisites in a balance are (1) accuracy, (2) sen-
sibility, and these can only be gained by careful construction.
It needs but little consideration to see that in a delicate balance
the friction of the various parts must be reduced to a minimum.
This is usually accomplished by suspending the beam by means of
an agate knife-edge, working on an agate plane, whilst the pans are
attached to each end of the beam by a somewhat similar arrange-
ment shown in Fig. 10. The position of the axis of suspension
relatively to the centre of gravity of the beam is likewise a matter
58
GENERAL PRINCIPLES OF THE SCIENCE
of consequence. If the axis of suspension and the centre of
gravity in a balance were coincident, the beam would remain
stationary in all positions in which it might be placed. If the
axis of suspension be placed below the centre of gravity the
beam would be in a condition of unstable equilibrium. Hence
the only case in which the balance can be used is that in which
the point or axis of suspension is above the centre of gravity,
for in this case alone will the beam return to a horizontal
position after making an oscillation, and in this case the balance.
may be considered as a pendulum, the whole weight of the
beam and pans being regarded as concentrated at the centre
of gravity. In order that the weight of the substance and the
sum of the measuring weights in the scale-pan may be equal,
it is evident that the axis of suspension must be exactly in the
centre of the beam, or in other words, that the balance must
have arms of equal length. It is also necessary that the
balance should have great sensibility; that is, that it may be
9
FIG. 10.
hk
H
po
moved by the smallest possible weight; for this end it is like-
wise requisite that the vertical distance of the centre of gravity
below the axis of suspension should be as small as possible. As
the whole weight of the instrument may be regarded as con-
centrated at the centre of gravity, it evidently requires a less
force to act at the end of the beam to move the instrument
when the distance of the centre of gravity from the point of
suspension of the balance is small, than when that distance is
greater, inasmuch as in the latter case the weight has to be
lifted through a longer arc. The sensibility of the balance is
also increased, both by increasing the length of the beam and
by diminishing the weight of the beam and of the load. When,
however, the beam is made either too long or too light it ceases
PROPERTIES OF GASES
59
1
to be rigid, and a serious source of error is introduced. If great
accuracy in the weighing is desired, it is advisable to have
recourse to the method of weighing by vibration by which
the excursions of the moving beam are accurately observed
instead of its approach to the horizontal position. By combining
this method of vibrations with that of double weighing, which
consists in reversing the position of the weights in the two
pans, it is possible for a good balance which is loaded with a
kilogram in each pan, to turn with 0-0007 grm. or about
the both of the weight in either pan.
PROPERTIES OF GASES.
13 The chemist has to deal with matter in all its various
states; solids, liquids, and gases alike being the objects of his
examination. The study of gases in particular has led to most
important results in the theoretical branch of the science, the
system of formulæ now employed being in fact founded upon
observations made upon matter in the state of gas.
RELATION OF VOLUME TO PRESSURE. BOYLE'S LAW.
14 The gaseous condition of matter is well defined to be that
in which it is capable of indefinite expansion. If a quantity
of gas as small as we please is placed in a closed vacuous
space, however large, the gas will distribute itself uniformly
throughout that space. The relation between the volume and
pressure of a gas, the temperature remaining constant, is
expressed by the well-known law of Boyle (1662), viz., that
the volume of the gas varies inversely as the pressure, or, in
other words, the pressure of a gas is proportional to its density;
so that the pressure exerted on the containing vessel by two
portions of any given gas is the sum of the pressures which
each portion would exert if present by itself. Dalton extended
this law, inasmuch as he showed that if different gases, which
do not act chemically on each other, are mixed together, the
pressure exerted is likewise the sum of the separate pressures
of the different gases.
1 See Prof. W. H. Miller, Phil. Trans. 1856, 763; and article "Balance,"
Watts' Dictionary, 1st Edition.
60
GENERAL PRINCIPLES OF THE SCIENCE
RELATION OF VOLUME TO TEMPERATURE. DALTON'S LAW.
15 Another simple numerical law, which characterises the
gaseous condition, is known as the law of Dalton, but often
referred to as the law of Charles or of Gay-Lussac.2 This
states that all gases heated under constant pressure expand by
an equal fraction of their volume at 0° centigrade for equal
increments of temperature, one volume at 0° becoming 1:3665
at 100°; so that the coefficient of expansion of gases is 0.003665
or nearly for an increase from 0° to 1° centigrade. If the
temperature of the gas be lowered, the volume contracts in the
same proportion. When, on the other hand, the volume of a
gas is kept constant and the temperature raised, the pressure
of the gas increases in this same ratio; so that a mass of gås
which at 0° has a pressure of 1, has at 100° a pressure of
1.3665 if it be not allowed to increase in volume.
273
The behaviour of substances in the gaseous state as regards
pressure and temperature is distinguished by its simplicity and
uniformity from that of the solid and liquid forms of matter.
For in the case of solids and liquids the effect on the volume
of alteration of pressure as well as of temperature is different
for every substance, whilst gases are all uniformly affected.
Hence we are led to conclude that the gaseous form of matter
is that in which the constitution is most simple, and this result
is borne out by many other considerations.
THE KINETIC THEORY OF GASES.
16 The doctrine that heat is only a kind of motion is one which
is now generally admitted, so that a hot body may be regarded as
possessing a store of energy, some portion of which at any rate
may be made use of to accomplish actual work. The energy of
motion is termed Kinetic (from kivéw, I move), and this energy
is communicated when the body possessing it comes to rest by
contact with some other body.
The other form of energy
1 Dalton, Manchester Memoirs, v. 535 (1801).
2 The experiments of Gay-Lussac were published at a later date than those of
Dalton. The results obtained by Charles were never published, but were
verbally communicated to Gay-Lussac. Gay-Lussac, Ann. Chim. [1], 43, 137.
Dixon, Memoirs Lit, and Phil. Soc. of Manchester [4], 4, 36.
THE KINETIC THEORY OF GASES
61
depending on position with respect to other bodies and not
upon the condition of matter is termed Potential energy. It
has been shown that in a hot body a very considerable portion
of the energy arises from a motion of the parts of the body; so
that every hot body is in motion, but this motion is not one
affecting the motion of the mass as a whole but only that of the
molecules or small portions of the body. These molecules may
consist of a collection or system of smaller parts or atoms
which partake as a whole of this general motion of the molecule.
The subject of the motion of the smallest particles of matter
attracted the attention of the ancients, and Lucretius held that
the different properties of matter depended upon such a motion,
Daniel Bernoulli was the first to conceive the idea that the pres-
sure of the air could be explained by the impact of its par-
ticles on the walls of the containing vessel, whilst in the year
1848, Joule showed that these views were correct, and cal-
culated the mean velocity which the molecules must possess
in order to bring about the observed pressure. Since the
above date, Clausius, Maxwell, and other physicists, have
extended and completed the dynamical theory of gases.
Many of the phenomena observed in gases and also in liquids,
especially diffusion, prove that the large number of small
particles or molecules of which these forms of matter are
made up are in a constant condition of change or agitation,
and the hotter a body is the greater is the amount of this
agitation. According to the kinetic theory, these molecules
are supposed to move with great velocity amongst one another,
and, when not otherwise acted on by external forces, the direc-
tion of this motion is a rectilinear one, and the velocity uniform.
The molecules, however, come into frequent contact with one
another, or as Maxwell describes it, encounters between two
molecules occur. In these encounters, and also when the mole-
cules strike the surface of the containing vessel, no loss of
energy takes place, provided of course that everything is at
the same temperature, so that the total energy of the enclosed
system remains unaltered.
17 From these principles, assuming simply that the molecules
have weight and are in motion, and applying the usual laws of
masses in motion, the experimental laws of gases, already
alluded to, as well as others, may be deduced. The pressure of
a gas is thus due to the impacts of its molecules upon the walls
1 Brit. Assoc. Reports, 1848, 2nd Part, p. 21.
62
GENERAL PRINCIPLES OF THE SCIENCE
of the containing vessel; hence, when twice as many molecules
are crowded into a given space, by the compression of the gas
to half of its original volume, the frequency of the impacts, and,
therefore, also the resulting pressure will be doubled, or, in other
words, the pressure is inversely proportional to the volume
(Boyle's Law). The temperature is measured by the kinetic
energy of the molecules, which is equal to the product of half
their mass into the square of their velocity i.e., mv². Increase
of temperature, therefore, means increase of the velocities of the
molecules, and hence if the volume of a gas be kept constant
and its temperature raised, both the force of impact of each
molecule against the wall of the vessel and the number of
impacts per second will be increased, the pressure rising in pro-
portion to the square of the velocity, or in other words in direct
proportion to the rise of temperature (Dalton's Law). The tem-
perature at which the velocity of the molecules, and, therefore,
also the kinetic energy, would become equal to nothing, is called
the absolute zero of temperature and is found by calculation
to be -273° centigrade, temperatures reckoned from this zero
being termed absolute temperatures.
When two gases are at the same pressure, the total
kinetic energy of the molecules in equal volumes must be
the same; if their temperatures are also equal, the kinetic
energy of each molecule must also be equal, and it hence
follows that the number of molecules in equal volumes of the
two gases must be the same. Assuming then that the tem-
perature of a gas represents the kinetic energy of its molecules
it follows that equal volumes of all gases under similar conditions
of temperature and pressure contain equal numbers of mole-
cules. This is the statement known to chemists as Avogadro's
theory (p. 98), which is thus shown to rest upon a sound
physical foundation.
18 The velocities of the molecules can be calculated for any
given temperature when we know the density of the gas and its
pressure at that temperature. As will be seen from the table
given below their magnitude is very considerable, and this
accounts for the great velocity with which disturbances, such as
sound waves or explosions, are propagated through gases.
VELOCITY OF MOLECULES OF GASES
63
Hydrogen
Ammonia
Oxygen
Carbon dioxide
Chlorine.
Hydriodic acid
Velocity at 0°
in
metres per second.
1843
628
461
392
310
230
These numbers represent the mean velocity of the molecules,
since it is supposed that owing to encounters the velocity is
not absolutely uniform but varies about an average value;
in hydrogen for example at 0° C. and 760mm. some of the
molecules are moving at a much slower rate than 1843 metres
per second, whilst others are moving much more rapidly. In
addition to the energy of translation which the molecule as a
whole possesses, the atoms of which it is composed and which
are capable of motion relatively to each other have also a
certain amount of kinetic energy. When a gas is heated, there-
fore, both of these kinds of energy are increased, or in other
words the motion of the atoms within the molecule is increased
at the same time as the velocity of the molecule itself.
Clerk-Maxwell and others have calculated that the actual num-
ber of molecules which we must conclude to be present in one
cubic centimetre of a gas at the standard temperature and pres-
sure is no less than 21 trillions (21,000,000,000,000,000,000
or 21 x 1018); the weight of a single molecule of hydrogen
being, therefore, about 0.000 000 000 000 000 000 04 milli-
grams or (0.04 × 10−18).
DIFFUSION OF GASES.
19 Early in the history of the chemistry of gases it was
observed that when gases of different specific gravities, which
exert no mutual chemical action, are once thoroughly mixed,
they do not of themselves separate in the order of their several
densities by long standing. On the contrary, they remain
uniformly distributed throughout the mass. Priestley¹ proved
this by very satisfactory experiments; but he believed that if
the different gases were very carefully brought together, the
1 Observations on Air, 2, 441.
64
GENERAL PRINCIPLES OF THE SCIENCE
heavier one being placed beneath and the lighter one being
brought on to the top without being mixed with the other, they
would then, on being allowed to stand, not mix, but continue
separate one above the other. Dalton, in 1803 proceeded to
investigate this point, and he came to the conclusion that a
lighter gas cannot rest upon a heavier, as oil upon water, but
that the particles of the two gases are constantly diffusing
through each other until an equilibrium is reached, and this
without any regard to their specific gravities. This conclusion
Dalton regarded as a necessary consequence of his theory of the
constitution of matter, according to which the particles of all
gaseous bodies exert a repulsive influence on each other, and
each gas expands into the space occupied by the other as it
would into a vacuum. In fact, however, it does not so expand,
for the rate at which a gas diffuses into another gas is many
thousand times slower than that at which it rushes into a
vacuum. As was usual with him, the apparatus used by Dalton
in these experiments was of the simplest kind. It consisted of
a few phials and tubes with perforated corks. "The tube
mostly used was one 10 inches long and of inch bore; in
some cases a tube of 30 inches in length and inch bore was
used; the phials held the gases which were the subject of
experiment and the tube formed the connection. In all cases the
heavier gas was in the lower phial and the two were placed in
a perpendicular position, and suffered to remain so during the
experiment in a state of rest; thus circumstanced it is evident
that the effect of agitation was sufficiently guarded against; for
a tube almost capillary and 10 inches long, could not be instru-
mental in propagating an intermixture from a momentary com-
motion at the commencement of each experiment." The gases
experimented on were atmospheric air, oxygen, hydrogen, nitro-
gen, nitrous oxide and carbonic acid; and after the gases had
remained in contact for a certain length of time the composition
of that contained in each phial was determined, and invariably
showed that a passage of the heavier gas upwards and the lighter
gas downwards, had occurred. Similar experimental results
were also obtained by Berthollet in 1809.3
The passage of gases through fine pores was likewise observed
by Priestley in the case of unglazed earthenware retorts which
although perfectly air-tight so as not to allow of any escape by
1 Manch. Memoirs, 1805, p. 259.
3 Mém. d'Arcueil, 2, 463.
2 Phil. Mag. 4, 26, 409.
4 Observations, &c., 2, 414.
DIFFUSION OF GASES
65
blowing in, allowed the vapour of water to pass out whilst air
came in, even where the gas in the retort was under a greater
pressure than that outside. Dalton was the first to explain this
fact as being due to precisely the same cause as that which
brings about the exchange of gases in the phials connected with
the long tubes, only that here we have a large number of small
pores instead of one (the bore of the tube) of sensible magnitude.
20 In the year 1823 Döbereiner¹ made the remarkable observa-
tion that hydrogen gas collected over water in a large flask which
happened to have a fine crack in the glass, escaped through the
crack into the air, whilst the level of the water rose in the flask
to a height of nearly three inches above its level in the trough.
Air placed in the same flask did not produce a similar effect, nor
was this rise of the water observed with the flask full of hydro-
gen when it was surrounded with a bell-jar filled with the same
gas.
As in the former instance, the discoverer of the fact was un-
able to explain the phenomenon, and it was not until 1832 that
Thomas Graham 2 in repeating Döbereiner's experiments showed
that no hydrogen could escape by the crack without some air
coming in, and enunciated the law of gaseous diffusion founded
on the results of his experiments, viz., that the rate at which gases
diffuse is not the same for all gases but that their relative rates
of diffusion are inversely proportional to the square roots of their
densities, so that hydrogen and oxygen having the relation of
their densities as 1 to 16 the relative rates of diffusion are as
4 to 1.
Instead of using cracked vessels Graham employed a diffusion
tube consisting of a glass tube open at each end and about six to
fourteen inches in length and half an inch in diameter; a wooden
cylinder is introduced into the tube so as to fill it with the exception
of half an inch at one end, and this unoccupied space is filled with
a plug of plaster of Paris; the cylinder being withdrawn after
the paste of plaster has set. With such a tube divided into
volumes of capacity, filled with gas and placed over water, the
rate of the rise or depression of the water could be easily
observed and the composition of the gas both before and
after the experiment ascertained. In this way the relative
diffusibility of various gases was determined, the results of
Graham's experiments being shown in the following table.
1 Ann. Chim. Phys. 1823, 24, 332.
Elin. Phil. Trans. 12, 1834, 222. Phil. Mag. 1833, 2, 175.
6
66
GENERAL PRINCIPLES OF THE SCIENCE
Gas.
Hydrogen
Marsh gas
Steam
Carbonic oxide
Nitrogen
Ethylene
Nitric oxide
DIFFUSION OF GASES.
Nitrous oxide.
Carbon dioxide
Sulphurous acid
Density.
Square
root of
density.
3.7935
0-06949 0.2636
3.83
0.559 0.7476 1.3375 1.344
0.6235 0.7896 1.2664
0.9678 0.9837 1.0165 1.1149
0.9713 0.9856 1.0147 1.0143
0.9889 1.0112 1:0191
1.0196 0.9808
1.0515 0.9510
1.0914 0.9162
0.978
1.039
1.1056
Oxygen.
Sulphuretted hydrogen. 11912
0.9487
0.95
1.527 1.2357 0.8092 0.82
1-52901 1.2365 0.8087 0.812
2.247 1.4991 0.6671 0.68
1
/ density.
1.00
Oxygen
Hydrogen
0.44
Carbon dioxide. 0.72
Velocity of
diffusion
of air = 1.
The observed velocities of diffusion agree very closely with
those obtained by calculation. This is, however, only the case
when the porous plate through which the diffusion takes place
is very thin. If the plate be thick the gases have to pass
through a series of capillary tubes, and the rate of diffusion is
considerably diminished by the friction.
•
The passage of the gases through capillary tubes has been
termed transpiration of gases, and this proceeds according to
other laws than those of diffusion, as in transpiration we have to
do with a motion of the mass of the gas, whereas in diffusion the
motion is purely molecular. Thus when allowed to pass through
capillary tubes the rate of transpiration of equal volumes of the
following gases was found by Graham to be represented by the
numbers :--
•
-
The numbers bear no relation to the square roots of the
densities of the gases.
Of all substances, that which is best adapted for exhibiting the
laws of diffusion is a thin plate of artificial graphite. With a
porous plate of graphite 0.5 mm, in thickness Graham ¹ obtained
1 Phil. Trans. 1863, 392.
DIFFUSION OF GASES
67
the following times of diffusion into air under a pressure of
100 mm. of mercury.
Hydrogen.
Oxygen
Carbon dioxide.
Hydrogen
Air
Oxygen.
Carbon dioxide
.
Time of molecular
passage.
0.2472
1.0000
1.1886
•
·
•
•
When the same gases were allowed to diffuse into a vacuum
the following were the results :—
Time of molecular
passage.
0.2505
0.9501
1.0000
1.1860
.
•
·
.
·
•
Square root of
density O=1.
0.2509
1.0000
1.1760
.
·
·
Square root of
density O=1.
•
•
•
0.2509
0.9507
1.0000
1.1760
Hence it appears that a plate of artificial graphite is practically
impermeable to gas by transpiration but is readily penetrated by
gases when in molecular or diffusive movement, whether the gases
pass under
pressure into air or into a vacuum, and this sub-
stance, therefore, serves as a kind of "pneumatic sieve" which
permits the passage of the molecules but not the masses of the
gas.
21 The phenomena of diffusion can be strikingly demonstrated
by the following experiments:-
First. To one end of a glass tube about 1 metre in length and
1 cm. in diameter, having a bulb blown on to it, a cylindrical
porous cell (such as those used for galvanic batteries) is fixed by
means of a caoutchouc cork. The other end of the tube is drawn.
out to a fine point and bent round as shown in Fig. 11. If now
a vessel filled with hydrogen be held over the porous jar this
gas will enter more quickly than the air can issue, viz., in the pro-
portion of the inverse square roots of their densities, that is, as
√144 to 1, or as 3.8 volumes to one volume, so that the pressure
in the porous cell will increase and the coloured water placed in
the bulb will be driven out in the form of a fountain through the
narrow jet.
A second experiment showing the mode in which one gas may
be separated from another by diffusion (termed atmolysis from
aTuós vapour and Auw I loosen) is the following. A slow current
of the detonating gas obtained by the electrolysis of water, and
68
GENERAL PRINCIPLES OF THE SCIENCE
consisting of 2 volumes of hydrogen to one volume of oxygen, is
allowed to pass through a common long clay tobacco-pipe, the
gas on issuing from the pipe being collected over water in a
pneumatic trough. On bringing the gas, thus collected, in
FIG. 11.
contact with a flame it no longer detonates. On the contrary,
it will rekindle a glowing chip of wood, thus showing that in
its passage through the porous pipe the greater portion of the
lighter hydrogen has escaped by diffusion through the pores of
the clay, whilst the heavier oxygen has not passed through.
DIFFUSION OF GASES
69
A third experiment to illustrate the law of diffusion is one
which possesses interest from another point of view, inasmuch
as it has been proposed to employ the arrangement for giving
warning of the outbreak of the dangerous and explosive
gas termed fire-damp by the coal-miners. Fire-damp or
marsh gas is lighter than air, and 134 volumes of this
gas will diffuse through a porous medium in the same time
as 100 volumes of air will do. Hence if a quantity of
fire-damp surround the porous plate, the volume within the
vessel will become larger, and this increase of volume may be

Mutte
C
FIG. 12.
made available either to drive out water as in the first experi-
ment or to alter the level of a column of mercury so as to
make contact with a connected battery and then to ring a
warning bell. The latter form of apparatus is seen in Fig. 12.
Holding a beaker-glass (A) filled with hydrogen or common
coal-gas over the plate of porous stucco fastened into the tube-
funnel an increase of volume occurs inside the glass tube and
a consequent depression of the mercury takes place in the bend of
the tube which is sufficient to make metallic contact with a
second platinum wire fused through the glass and to bring the
70
GENERAL PRINCIPLES OF THE SCIENCE
current to act on the magnet of the electric-bell (B). The other
tube is arranged for showing that a dense gas, such as
carbonic acid, does not diffuse through the porous septum so
quickly as air escapes. By immersing the porous plate in a
jar (c) filled with the heavy gas, the volume inside the tube
becomes less, the level of the mercury in the bend is altered,
contact is again made with the battery, and the ringing of the
bell gives notice of the change.
22 Effusion of Gases is the name given by Graham to the
flow of gases under pressure through a minute aperture in a
metallic plate. The law of diffusion is found to hold good
with regard to this molecular motion of gases, the times
required for equal volumes of different gases to flow through
an aperture of a diameter of 3 of an inch having been found
to be very nearly proportional to the square roots of their
densities, and the velocity of flow to be inversely as the square
roots of their densities.
This law, which is true for the flow of all fluids through a
small aperture in a thin plate has been applied by Bunsen ¹
for the purpose of determining the specific gravity of gases,
the method serving admirably when only small quantities of
the gas can be obtained.
DEVIATIONS FROM THE LAWS OF BOYLE AND DALTON.
23 These laws are only approximately true, for it is found that
no gas exactly follows them under all conditions of temperature
and pressure, although under moderate variations of these
conditions the deviations are but slight.
The ideal gas
which would agree with these laws under all conditions is called
a perfect gas, and this term is often also applied to gases which
approach this state. The effect of high pressures upon certain
gases is shown in the following table exhibiting the experi-
mental results obtained by Natterer.2
1 Gasometry, p. 121.
2 Pogg. Ann. 94, 436.
DEVIATIONS FROM BOYLE'S LAW
71
Pressure in
Atmospheres. Hydrogen. Oxygen. Nitrogen.
1
1
1
1
50
50
50
100
98
100
500
396
439
1,000
623
595
1,500
776
2,000
899
3.600 1,040
40
30
P. V.
Table of Deviations from the Law of Boyle.
20
Number of volumes of gas compressed into one volume.
Two factors seem to be involved in producing deviations of
this character. In the first place, as the density of the gas
40
P. in Atmospheres
A Hydrogen.
80
120
160
50
99
381
519
590
641
710
A
B Ethylene.
FIG. 13.
200
B
Air. Nitric Oxide.
1
1
50
100
396
527
607
661
800
240
280
50
100
412
544
617
669
C Carbon dioxide.
320
increases, the volume occupied by the molecules themselves,
which we must suppose to be unalterable by pressure, bears a
greater proportion to the distance between them, which can be
lessened by compression, and consequently the volume diminishes
less rapidly than when the density is smaller. Secondly, as the
72
GENERAL PRINCIPLES OF THE SCIENCE
I
molecules approach one another more closely, they tend to cohere,
and the volume is thereby diminished more rapidly than the
pressure increases. The actual effect produced is generally due
to a combination of these two causes; thus the volume of carbon
dioxide at a temperature of 100° decreases more rapidly than we
should expect from Boyle's law until a pressure of 160 atmo-
spheres is reached, after which it decreases less rapidly. The
behaviour of several gases is shown in Fig. 13, on the preceding
page. In every case the value of the product of pressure and
volume should remain invariable, according to the law of Boyle,
whilst the actual values are seen to deviate considerably from
this theoretical constancy.
Deviations of a similar character are observed when the density
of a gas is increased by lowering its temperature.
If any gas be subjected to a greatly increased pressure and its
temperature simultaneously greatly reduced, a point is at last
reached at which the gas undergoes sudden contraction and
becomes converted into a liquid. Just before liquefaction the
gas may be considered as the vapour of a liquid,
a liquid, so that carbon
dioxide at a high pressure and a low temperature may be
compared to steam just about to condense.
THE CONTINUITY OF THE GASEOUS AND LIQUID STATES OF
MATTER.
24 It is matter of everyday experience that the tension of the
vapour of water and of other liquids heated with excess of the
liquid in closed vessels, increases in a very rapid ratio with
increase of temperature, and that the density of the steam or
vapour in such a case undergoes a similar rapid increase. Thus
at 231° the weight of a cubic metre of steam is part of the
weight of the same bulk of water at 4°, the point of maximum
density, the weight of steam at 100°, being only 100 of that of
the same bulk of water, so that at a temperature not very far
above 230° the weight of the vapour will become equal to that
of the liquid. The result of this must be that under these circum-
stances a change from the gaseous to the liquid state is not
accompanied by any condensation, and in such a case the distinc-
tions we have been in the habit of drawing between these
conditions of matter cease to have any meaning. So long ago as
1822, Cagniard de la Tour ¹ made experiments upon the action of
1 Ann. Chim. Phys. [2], 21, 127, 22, 410.
THE CRITICAL TEMPERATURE
73
liquids sealed up in glass tubes of a capacity but little greater
than that of the liquid. When a tube one-fourth filled with
water was heated up to about 360° the water entirely disap-
peared, the tube appearing empty, and as the vapour cooled a
point was reached at which a kind of cloud made its appearance,
and a few moments later the liquid was again visible. Cagniard
de la Tour considered that the substance when thus heated
assumes the gaseous condition; but Dr. Andrews has shown
that in such an experiment the properties of the liquid
and those of the vapour constantly approach one another, so that
above a given temperature the properties of the two states can-
not be distinguished. Hence it follows that at all temperatures
above this particular one no increase of pressure can bring
about the change by condensation which we term liquefaction.
This temperature is called by Andrews the critical point. For
many gases, the critical points are situated at temperatures
which render the observation of this fact easy. Thus, for
example, the critical temperature for carbon dioxide (or carbonic
acid gas) is 30°92, and at all temperatures above this point no
condensation from gas to liquid occurs; so that if the pressure
on the gas be gradually increased up to 150 atmospheres a
steady diminution of volume occurs as the pressure augments,
and no
sudden diminution of volume will Occur in
any stage of it. The temperature may then be gradually
allowed to fall until the carbon dioxide has reached a temperature
below the critical one when it is found to be a liquid; thus
beginning as a gas and by a series of gradual changes, pre-
senting nowhere any abrupt alteration of volume or sudden
evolution of heat, it ends by being an undoubted liquid. This
clearly shows that the properties of a gas can be continually and
imperceptibly changed into those of a liquid.
The tension of the substance at the critical temperature is
known as the critical pressure.
The critical temperatures of a few gases and liquids are given
in the following table :-
1 Phil. Trans. 1869, Part 2, 575.
74
GENERAL PRINCIPLES OF THE SCIENCE
Oxygen
Ethylene
Carbon dioxide.
Nitrous oxide
Chlorine
Alcohol
Benzene
Sulphur dioxide.
Cyanogen
Hydriodic acid
Ammonia.
Chlorine
•
·
LIQUEFACTION OF GASES.
2
25 The first instance of a substance, which, under ordinary
conditions, is known as a gas, being transformed by pressure into
a liquid is chlorine gas. This gas was first liquefied under pres-
sure by Northmore ¹ in 1806. Faraday investigated the subject
fully shortly afterwards showing that many other gases such as
sulphur dioxide, carbonic acid, euchlorine, nitrous oxide,
cyanogen, ammonia, and hydrochloric acid can also be
reduced to the liquid state. In these experiments Faraday em-
ployed bent tubes made of strong glass, in the one limb of
which, being closed at the end, materials were placed which on
being heated will yield the gas; the open limb of the tube was
then hermetically sealed and the gas evolved by heating the
other end. The pressure or tension exerted by the gas itself,
when thus generated in a closed space is sufficient to condense
a portion into the liquid state. The following table shows the
maximum tensions of some of these more readily liquefiable
gases at 0°:-
•
•
•
•
Atmospheres.
1.53
2:37
3.97
4:40
8.95
•
·
-113°
+13°
+30°-9
+36°
+141°
+234°
+281°
Table of Tensions.
Atmospheres.
Sulphuretted hydrogen 10:00
Hydrochloric acid. . 26-20
32:00
38.50
Nitrous oxide
Carbon dioxide.
•
•
If, therefore, any of the above gases at 0° be exposed to pres-
sures exceeding those given in the table they will condense to
liquids, their critical temperatures being all above 0°.
The liquefaction of gases can be brought about not merely by
1 Northmore, Nicholson's Journal, 12, 368; 13, 232.
Phil. Trans. 1823, 160. Ibid, 1823, 189.
LIQUEFACTION OF GASES
75
simple exposure to high pressure but also to low temperature;
thus, if we reduce the temperature of sulphur dioxide, under
the ordinary atmospheric pressure, to -10° it liquefies, and when
the temperature sinks to 76°, the liquid freezes to an ice-like
mass.
26 Until the important researches of Pictet 2 and Cailletet 3
certain gases, such as oxygen, hydrogen, and nitrogen, had
resisted all attempts to reduce them to the liquid or solid state,
because their critical temperatures had never been reached, and
to these the name of the permanent gases was given.
The meeting of the French Academy of the 24th December,
1877, was a memorable one. On that day the Academicians.
were told that Cailletet had succeeded in liquefying both oxygen.
and carbon monoxide at his works at Chatillon-sur-Seine, and
that the former gas had also been liquefied by Raoul Pictet at
Geneva.
-
These experimenters soon succeeded in condensing the other
gases already named, with the exception of hydrogen, which
has, however, since yielded to the lower temperatures produced
by later workers, and thus we are able to give experimental
proof of the view which has been frequently expressed that all
bodies without exception possess the power of cohesive attraction.
These important results were arrived at independently by both
observers, each having made the question the subject of many
years' study and experiment. It is difficult, on reading the
description of these experiments, to know which to admire most,
the ingenious and well-adapted arrangement of the apparatus
employed by Pictet, or the singular simplicity of that used by
Cailletet. The latter gentleman is one of the greatest of French
ironmasters, whilst the former is largely engaged as a manu-
facturer of ice-making machinery, and the experience and
practical knowledge gained by each in his own business have
materially assisted to bring about one of the most interesting
results in the annals of scientific discovery.
The process successfully adopted in each case consisted in
simultaneously exposing the gas to a very high pressure and to
a very low temperature.
The increase of pressure was effected by Pictet by evolving
the gas in a wrought-iron vessel strong enough to withstand an
1 Faraday, Phil. Trans. 1845, 155.
* Compt. Rend. 85, 1214, 1220.
3 Ibid. 815.
76
GENERAL PRINCIPLES OF THE SCIENCE
enormous tension; whilst in Cailletet's arrangement the same
end was brought about by a hydraulic press. For the purpose of
obtaining a low temperature the first experimenter made use of
the rapid evaporation of liquid carbon dioxide, thus producing
a constant temperature of - 130°. Cailletet, on the other hand,
effected the same end by suddenly diminishing the pressure
upon the compressed gas. This sudden expansion gives rise to
a rapid diminution of temperature caused by the transference
of heat into the motion of the particles of the expanding gas
(chaleur de detente). So great is the amount of heat thus
absorbed that the temperature of the particles sinks below the
critical point of the gases, and a condensation occurs, the finely
divided liquid oxygen or nitrogen appearing as a mist in the
tube.
PICTET'S METHOD OF LIQUEFYING GASES.
27 A vertical section and a ground plan of Pictet's apparatus
are shown in Figs. 14 and 15. p and p' are two pumps,
p' being an exhausting, and p a compressing pump, such as
are used in the ice-making machines. These are employed
respectively for the volatilization and condensation of liquid
sulphur dioxide, contained in the outer inclined double-
jacketed tube (R) Fig. 15; and both pumps are so arranged
that there is the largest possible amount of difference of pres-
sure always kept up between the two cylinders. In the tube
(R) the pressure is so regulated that the liquid sulphur dioxide
evaporates at a temperature of - 65°. The gaseous sulphur dioxide
passes through the pumps and is condensed to a liquid by a stream
of cold water which surrounds the reservoir (c) at a tempera-
ture of + 25°, and under a pressure of 275 atmospheres. The
liquid then flows back through the small tube (z) into the
tube (R). () and (o) are two smaller pumps which act upon
liquid carbon dioxide which is contained in the tube (s). These
pumps are so arranged that the evaporation of the liquid takes
place from the tube (s) at a temperature of 140°, this being
surrounded by the liquid sulphur dioxide, and flowing under
a pressure of five atmospheres through the tube (s) into the
tube or cylinder surrounding the tube (A). All these portions
of the apparatus are made of strong cold-drawn copper tubes
able to resist a high pressure. (B) is a strong wrought iron
PICTET'S EXPERIMENTS
77

X
Sulphur dioxide
m
502
G
Gasometer for coz
JOUE
C
Reservoir
for Sulphur Dioxide
Cold water
m
FIG. 14
Sura
V
SOA
SHOP 40
Sulphur
Dioxide
4
Carbon Dioxide
22U544
VIGASIDE
poxyaca
D
A
Chlorate
of Potash
78
GENERAL PRINCIPLES OF THE SCIENCE

Pump for
Co2
TOHH
Pump for
So2
Cold
water
Carbon Dioxide
す
​Gasometer
s Carbon Dioxide
Carbon Dioxide
mp
VERDOT
Oxygen T
m
Sulphur Diocide
Sulphur Dioride
Rebel
כפלין
& Carbon Dioxcute sulphur Dearde
7767
Carbon Dioxide
Cold water
༡
FIG. 15.
U
Sulphur Dioxide
Carbon Doxides
D
152097
Oxygen
A
B
PICTET'S EXPERIMENTS
79
retort employed for the evolution of the gas about to be con-
densed. The gas thus generated passes into the long thin
copper condensation-tube (A), four metres in length, which is
surrounded by a bath of liquid carbon dioxide at a tem-
perature varying from 120° to 140°. The end of this
condensation-tube is provided with a well-fitted stopcock (v)
and a Bourdon's manometer at (m), capable of indicating a
pressure up to 800 atmospheres. With this apparatus oxygen
was first condensed on the 22nd December, 1877.
The following description of the experiment will render
intelligible the working of the process :
(1.) 9 A.M.—The pumps for condensing and rarefying the
sulphur dioxide were set to work.
-
(2.) 9.30.-The temperature of the upper tube was - 55°.
The pumps for condensing and rarefying the carbon dioxide
were started.
(3.) 10.40.-Temperature inside the tubes - 60°; pressure,
5 atmospheres. 800 litres of carbon dioxide have been
liquefied.
(4.) 11.0-The shell containing the chlorate of potash is
now heated.
(8.) 12.37
(9.) 12.38
--
(5.) 11.15.-The temperature of the carbon dioxide sinks
to 130°. The pressure of oxygen in the copper tube=5
atmospheres. The pressure then began gradually to rise,
and at last it remained constant, as is seen in the following
table:
(10.) 12.40.
(11.) 12.42.
(12.) 12.44
-
(13.)
(14.)
(6.) 12.10 P.M.-Pressure of oxygen 50 ats.; temp. as before.
(7.) 12.36
100
200
. 460
. 525
. 526
525
471
475
1.0
1.5 .
•
•
•
39
د.
""
""
""
""
"9
99
""
""
39
""
༦༦ུ
""
The whole of the
The pressure now remained constant.
interior of the glass tube was filled with liquid oxygen. On
opening the stopcock at the end of the oxygen tube, a lustrous
jet of liquid oxygen issued with great violence, whilst around
it was a haze of particles of what was taken to be solid oxygen.
80
GENERAL PRINCIPLES OF THE SCIENCE

WAPARERERPRO
X
S
FIG. 16.
CAILLETET'S EXPERIMENTS
81
The general arrangement and appearance of Pictet's apparatus
is shown in Fig. 16 in which the end of the oxygen condensa-
tion-tube (A), the stopcock (v), and the manometer (m) are
seen.
CAILLETET'S PROCESS FOR LIQUEFYING GASES.
28 The apparatus employed by M. Cailletet¹ for the lique-
faction of oxygen is shown in Fig. 17. The first part of
this apparatus consists of a powerful hydraulic press, the
soft steel cylinder (A) of which is fixed by the bands (BB)
on to a horizontal cast-iron bed. A steel piston works into

FM
lille
m
elege feele
Inumi
FIG. 17.
60
G
N
Bor
eeeee
this cylinder through a stuffing-box, and to the end of the
piston is attached the screw (F), which can be carried backwards
or forwards by turning the wheel (M) either in one direction or
the other. The hydraulic cylinder is filled with water from the
vessel (G), and this communicates with the interior of the
cylinder by a fine opening, which can be perfectly closed at
pleasure by means of a conical valve attached to a piston worked
by the wheel (0). On withdrawing this piston water flows into
the cylinder.
The second part of the apparatus is the receiver (Fig. 19).
1 Compt. Rend., 10, 85, 815; and Ann. Chim. Phys. [5], 15, 132.
7
82
GENERAL PRINCIPLES OF THE SCIENCE
This consists of a glass tube with reservoir at the lower end
firmly bedded into a steel head (B, Fig. 19), sufficiently strong
This receiver is
to resist a pressure of 1,000 atmospheres.
placed in direct connection with the hydraulic pump by means
P
T
FIG. 18.
of a flexible metallic tube (TU) of small diameter. A steel
head is firmly screwed on to the upper part of the receiver by
the screw (E¹) and this head carries the glass tube (T), which
B
FIG. 19.
RE
ΓΙΑ
H

TU
contains the gas to be experimented upon. The shape and
mode of fixing this tube with its reservoir of gas is seen in
Fig. 18; whilst in Fig. 19 the same is shown placed in position
with the lower portion dipping into the mercury which fills the
CAILLETET'S EXPERIMENTS
8883
lower part of the steel receiver. As the glass reservoir is exposed
to the same pressure on both its inside and outside surfaces,
its dimensions may be made large in spite of the extremely
high pressures to which it is subjected in the course of the
experiments. The thin tube, on the other hand, which passes
out above the steel head of the condenser has of course to
support the pressure necessary for the condensation of the gases,
and hence it must be made of strong glass with a capillary
bore. A glass cylinder (M) resting on the iron flange (s) serves
to enable the experimenter to surround the tube either with
a freezing-mixture or with a warm liquid. When the reservoir

FIG. 20.
has been filled with the pure dry gas under examination the
end of the tube is carefully hermetically sealed, and the whole
screwed into position. Water is then forced into the receiver
from the hydraulic cylinder; this forces the mercury into the
reservoir, and the compressed gas condenses in the capillary
tube, where the changes which occur can be readily observed.
The position of the receiver and capillary tube is shown at
a and m, Fig. 17. The pressure is measured by two manometers
(N and N¹, Fig. 17).
With this apparatus Cailletet liquefied ethylene at + 4° under
a pressure of 46 atmospheres; acetylene under the ordinary
84
GENERAL PRINCIPLES OF THE SCIENCE
temperature at a pressure of 86 atmospheres; nitric oxide and
marsh gas required to be cooled to - 11°, and these became
liquid at the respective pressures of 104 and 108 atmospheres.
Oxygen and carbon monoxide remained gaseous at a temperature
of 29° under a pressure of 300 atmospheres, as did nitrogen
at a temperature of 13° and under a pressure of 200
atmospheres. When, however, this pressure was suddenly
reduced a thick mist was formed in the tubes, and this con-
densed, forming small drops. Hydrogen, on the other hand,
appeared, when the pressure from 300 atmospheres was suddenly
removed, in the form of a slight mist, but dried air liquefied
under a pressure of 200 atmospheres after it had been well
cooled with liquid nitrous oxide. Air has also recently been.
solidified (Dewar).
1
29 An apparatus for exhibiting the liquefaction of the
difficultly condensible gases has been constructed by Messrs.
Ducretet et Cie. of Paris. The condensing arrangements of this
apparatus are seen in Fig. 20, the apparatus of the receiver and
glass tube and reservoir being identical with that just described
(Fig. 20). The piston of the hydraulic cylinder is worked by
the lever (L), and by this means a pressure of 200 atmospheres
can be reached. In order to increase this pressure up to 300
atmospheres a steel plunger can be slowly forced into the
cylinder by means of the first wheel. The second wheel works
a second plunger, by the withdrawal of which the pressure can
be suddenly diminished, and thus the temperature so reduced
that the gas is liquefied by the intense cold produced.
The method now adopted for the liquefaction of oxygen, air,
nitrogen, &c., is a modification of that of Pictet, the gas being
strongly compressed by a powerful pump and simultaneously
cooled by liquid ethylene boiling under reduced pressure, by
means of which temperatures of 100° to -150° can be
maintained.2
1 Rue des Feuillantines, 89.
2 Wroblewski, Compt. Rend. 98, 304; Olszewski, Compt. Rend. 98, 365, 101,
338; Dewar, Proc. Roy. Institution, 13, part 3, p. 695.
BOILING-POINTS OF LIQUIDS
85
BOILING-POINTS OF LIQUIDS.
30 It is evident from what has been said that the distinction
between gas and vapour is only one of degree, for a vapour is
simply a gas below its critical temperature. The same laws
according to which the volumes of gases vary under change of
temperature and pressure apply also to vapours, at any rate
when they are examined at temperatures considerably above their
points of condensation. When a gas or vapour is near this point
its density increases more quickly than the pressure, and as soon
as the point is reached the least increase of the pressure brings
about a condensation of the whole to a liquid. The tempera-
ture at which the liquid again assumes the gaseous form is
termed the boiling-point of the liquid, and at this tempera-
ture the tension of the vapour is equal to the superincumbent
pressure. The following table gives the boiling-points of
some well-known bodies under a pressure of 760 mm. of
mercury:
Oxygen
Nitrous oxide
Carbonic acid
Cyanogen
Sulphurous acid
Ethyl chloride
Prussic acid
Water
Acetic acid
"
.
Table of Boiling-Points.
- 181°4
- 105°
78°
21°
10°
+ 12°5
26°5
-
Ether.
Carbon disulphide
Chloroform
Bromine .
Alcohol.
Benzene.
-
345
433
61°
63°
78°-4
80°.3
100°
118°
Propionic acid
Butyric acid
Aniline.
Phenol
-
.
Napthalene
Phosphorus
Sulphuric acid
Mercury.
Sulphur.
Stannous chloride
Zine bromide
Zinc chloride .
Cadmium
Zinc
+141°
162°
182°
183°
218°
290°
332°
350°
440°
606°
650°
730°
860°
1049°
31 Liquids possess a notable tension below their boiling-
points; thus water gives off vapour at all temperatures, and even
slowly evaporates when in the solid state, for the tension of the
vapour coming from ice at
10° is 0·208 mm. According to the
86
GENERAL PRINCIPLES OF THE SCIENCE
experiments of Faraday, there is, however, a limit to evapora-
tion;
thus he found that mercury, which gives out a perceptible
amount of vapour during the summer, emits none in the winter ;
and that certain compounds which can be volatilized at 150°
undergo no evaporation when kept for years at the ordinary
temperature. The tension of the vapour of a liquid is constant
for a given temperature, and this amount, which is always
reached when an excess of the liquid is present, is termed the
maximum tension of the vapour. Dalton¹ in 1801 discovered
that this maximum tension or density of a vapour is not altered
by the presence of other gases, or, in other words, that the
quantity of a liquid which will evaporate into a given space is
the same whether the space is a vacuum or is filled with
another gas.
The same philosopher also believed that the
vapours of all liquids possessed an equal tension at tempera-
tures equally distant from their boiling-points. Regnault 2
has, however, shown by exact experiments that the above
conclusions can only be considered as approximately true, inas-
much as he found that about 2 per cent. more vapour ascends
into a space filled with gas than into a vacuum, whilst at
considerable but equal distances from the boiling-point the
tensions of volatile liquids are by no means equal.
LAWS OF CHEMICAL COMBINATION.
32 The composition of a chemical compound can be ascertained
in two ways: (1) By separating it into its component elements,
an operation termed analysis (ávaλów, I unloose), and (2) by
bringing the component elements under conditions favourable to
combination, an operation termed synthesis (ovvτionμi, I place
together). In both of these operations the balance is employed;
the weight of the compound and of the components in each
instance must be ascertained, except indeed in the case of
certain gases of known specific gravity, when a measurement of
the volume occupied by the gas may be substituted for a deter-
mination of its weight.
It is one of the aims of analytical chemistry to ascertain with
great precision the percentage composition of all chemical sub-
1 Manch. Memoirs, 1st series, 5, 535.
2 Mémoires de l'Acad. des Sciences, 21, 465.
LAWS OF CHEMICAL COMBINATION
87
stances, and this branch of inquiry is termed quantitative
analysis, as contradistinguished from that which has only to
investigate the kind of material of which substances are com-
posed, and which is hence termed qualitative analysis.
COMBINATION BY WEIGHT.
33 The first great law discovered by the use of the balance, is
that the elements combine with one another in a limited number
of definite proportions, this number being almost invariably found
by experiment to be a small one. When two elements are
brought together under such conditions that they can combine,
it is always found that one or more of a small number of com-
pounds is produced, the particular substance or substances
formed depending upon the special circumstances of the experi-
ment. Thus carbon is found to be capable of uniting with
oxygen in two different proportions, producing two distinct sub-
stances, carbonic acid gas and carbon monoxide, these being the
only compounds of carbon with oxygen which are known. Some
elements, on the other hand, only form one compound with each
other, whilst others again form a larger number. Each one of
these compounds is found to have a fixed composition, contain-
ing the elements of which it is made up in a definite proportion
by weight, and this fixity of composition is used as a character-
istic of a chemical compound as opposed to a mere mechanical
mixture, the constituents of which may be present in any vari-
able proportions. In whatever way the conditions under which
the elements are made to combine may be varied, it is always
found that they unite in exactly the same ratio, unless, as some-
times happens, the changed conditions are favourable to the
production of one of the small number of other compounds
which can be formed by the same elements. Thus, for instance,
the combination of silver with chlorine has been brought about
in no less than four different ways, but in every case it was found
that the resulting compound contained 107-13 parts of silver for
35 19 of chlorine.¹ The combination of chlorine with phosphorus,
on the other hand, takes place in two distinct ratios, so that when
an excess of phosphorus is present, the resulting compound con-
tains 10:27 parts of this element for 35 19 parts of chlorine, whilst
if the latter be kept in excess, this weight of it only combines
1 Stas, Recherches, etc. pp. 108, 210. 1865.
88
GENERAL PRINCIPLES OF THE SCIENCE
with 6:16 parts of phosphorus. These are, however, the only two
compounds of these elements which are known. In like manner
hydrogen combines with oxygen to yield water, a substance
which contains 88:81 parts of oxygen to 11:19 of hydrogen. If
these elements are brought together in proportions differing
from those in which they are present in water, the excess of one
element remains in the free state; thus, if 98.81 parts of oxygen
by weight be brought together with 11:19 parts of hydrogen
under circumstances in which they can combine, 88.81 parts of
the oxygen will combine with all the hydrogen to form 100
parts of water, whilst 10 parts of oxygen remain in the free
state.
It will therefore be seen that the chemical combination of
two or more elements does not result in the production of a
series of compounds varying gradually in composition, according
to the conditions of the experiment, but yields one or more
compounds, each of which contains its constituents in a perfectly
fixed and definite ratio.
34 As has been said, the case frequently occurs of two elements
uniting to form several compounds, for each of which the law of
definite proportion holds good. The special relations which
exist between the weights of the two elements entering into
combination were first discovered by John Dalton, and indeed
form the basis of his atomic theory. Thus the two elements,
carbon and oxygen, unite to form two distinct compounds,
carbonic oxide gas and carbonic acid gas, and 100 parts of each
of these bodies are found by analysis to contain the following
weights of the elements :-
Carbon
Oxygen
•
Carbonic Oxide Gas.
42.86
57.14
100.00
.
Carbonic Acid Gas.
27.27
72.73
100.00
Knowing these facts Dalton asked himself what was the
relation of one element (say of the oxygen) in both compounds
when the other element remains constant? He thus found that,
in proportion to the carbon, the one compound contained.
exactly double the quantity of oxygen which the other contained;
thus:-
LAWS OF CHEMICAL COMBINATION
89
Carbon
Oxygen
Carbon
Hydrogen
Carbonic Oxide Gas.
10.0
13.3
23.3
Thus again, analysis showed that two compounds which carbon
forms with hydrogen, viz., marsh gas and olefiant gas, have the
following percentage composition :—
.
Marsh Gas.
74.95
25.05
•
100.00
Carbonic Acid Gas.
10.0
26.6
36.6
(1) (2)
(3)
Nitrogen. 63.71 46.75 36.91
Oxygen
36.29 53.25 63.09
Olefiant Gas.
85:68
14.32
100.00
Dalton then calculated how much hydrogen is combined in
each compound with 10 parts by weight of carbon, and he
found that in olefiant gas there are 167 parts by weight of
hydrogen to 10 of carbon, whilst marsh gas contains 3:34 parts
of hydrogen to the same quantity of carbon, or exactly double
as much.
As another example we may take the compounds of nitrogen
and oxygen, of which no less than five are known to exist. The
percentage composition of these five bodies is found by experi-
ment to be as follows:-
(5)
(4)
30.51
25.99
69.49 74.01
100.00 100.00 100.00 100.00 100.00
If then, like Dalton, we inquire how much oxygen is con-
tained in each of these five compounds, combined with a fixed
weight, say 10 parts of nitrogen, we find that this is represented
by the numbers 5·7, 114, 17·1, 22·8, and 28·5. In other words,
the relative quantities of oxygen are in the ratio of the simple
numbers 1, 2, 3, 4, and 5.
35 The above examples illustrate the relations exhibited in the
combination of two or more of the elements to form compounds,
but a careful examination of the quantitative composition of a
90
GENERAL PRINCIPLES OF THE SCIENCE
whole series of chemical compounds leads to a further conclusion
respecting the nature of the laws of chemical combination.
which is of the highest importance. Let us examine the com-
position of any given series of compounds as determined by
analysis, such as the following:
Hydrogen Chloride.
Chlorine
Hydrogen.
Sodium Chloride.
Chlorine
Sodium
·
·
Bromine
Hydrogen.
Bromine
Sodium
Hydrogen Bromide.
Sodium Bromide.
.
Hydrogen Iodide.
Iodine.
Hydrogen.
CHLORIDES.
97.24
2.76
100.00
60.61
39.39
100.00
98.76
1.24
BROMIDES.
100.00
77.62
22.38
100.00
99.21
0.79
Chlorine
Potassium
100.00
Potassium Chloride.
Chlorine
Silver.
IODIDES.
Silver Chloride.
·
Bromine .
Potassium
Bromine .
Silver.
•
·
Potassium Bromide.
Silver Bromide.
Iodine.
Potassium
.
100.00
47.53
52:47
Potassium Iodide.
100.00
24.73
75.27
67.13
32.87
100.00
42.55
57.45
100.00
76.42
23.58
100.00
LAWS OF CHEMICAL COMBINATION
91
Sodium Iodide.
Iodine.
Sodium
Hydrogen Chloride.
Chlorine. . 35.19
Hydrogen 1.00
.
·
"
36.19
Potassium Chloride.
Chlorine
35.19
Potassium 38.85
84.62
15.38
Arranged in this way we do not notice any simple relation
existing between the components of this series, except that the
quantity of hydrogen is always smaller than that of the chlorine,
bromine, or iodine, whilst the quantity of sodium is always
smaller than that of potassium, and this again is less than the
quantity of silver.
36 If, however, instead of examining a constant weight of
the several compounds we ask ourselves, how much of the
one constituent in each compound combines with a constant
weight of that constituent which is common to several, we
shall obtain at once a clear insight into the law of the
formation of the compound. In the series of hydrogen com-
pounds for instance, let us calculate (by simple proportion) how
much chlorine, bromine, and iodine combine with the unit.
weight of hydrogen. We then obtain for the composition of these
compounds :-
100.00
74.04
Iodine.
Silver .
Hydrogen Bromide.
Bromine 79.36
Hydrogen. 1:00
•
80.36
CHLORIDES.
Silver Iodide.
Sodium Chloride.
Chlorine 35.19
Sodium
22.87
58.06
54:03
45.97
Iodine
Hydrogen
Continuing our calculation, let us next ask how much of the
metals, potassium, sodium, and silver, unite with 35 19 parts by
weight of chlorine to form chlorides; with 7936 parts of bro-
mine to form bromides, and with 125 91 parts of iodine to form
iodides. The result is as follows:-
100.00
Hydrogen Iodide.
"
125.91
1.00
•
126.91
Silver Chloride.
Chlorine
35.19
Silver. 107.13
143.03
92
GENERAL PRINCIPLES OF THE SCIENCE
Potassium Bromide.
Bromine .
Potassium
.
79:36
38 85
118.21
""
164.76
Sodium Bromide.
Bromine. 79.36
Sodium
22.87
Potassium Iodide.
Iodine. . 125·91 Iodine.
Potassium.
Sodium.
38.85
BROMIDES.
""
IODIDES.
39
Sodium Iodide.
38.85 of potassium combine with
22.87 sodium
107.13,,
silver
100,, hydrogen
102.23
39
. 125.91
22.87
29
148.78
Now for the first time a remarkable relation becomes apparent,
for it is clear that the SAME weights of the metals, potassium,
sodium, and silver, which combine with 35:19 parts of chlorine
to form chlorides, also combine with 79:36 parts of bromine to
form the bromides, and with 125'91 parts of iodine to form the
iodides. In other words, if we replace the 3519 parts by
weight of chlorine in each of these compounds by 79:36 parts
of bromine, we get the bromides, and if by 12591 parts of iodine
we obtain the iodides of the metals. Hence one and the same
weight of metals (38.85 of potassium, 22.87 of sodium, and 107.13
of silver) has the power of forming compounds with the precise
quantities of chlorine, bromine, and iodine respectively, which
unite with 1 part by weight of hydrogen, to form the hydrides
of these elements. These quantities of the elements in question
are called equivalent quantities, because they are the amounts of
them which will combine with the same weight of some other
element.
Silver Bromide.
Bromine . 79:36
Silver.. 107·13
Silver Iodide.
Iodine . . 125.91
Silver . 107:13
35 19 of chlorine
bromine
79-36,,
125.91 „,
iodine
29
•
23
186:49
29
233:04
Į
respec-
tively.
""
99
""
Similar results are obtained from the examination of the
compounds of all the other elements, so that a number may be
LAWS OF CHEMICAL COMBINATION
93
333
Water.
assigned to each element which is termed the combining weight
or equivalent weight of the element.
37 Taking an example from another group of chemical com-
pounds we find that the well-known oxides of hydrogen, lead,
copper, mercury, and cadmium possess the following percentage
composition:-
Hydrogen 11.19
Oxygen
.88.81
•
Mercury
Oxygen
•
Mercury Oxide.
.
Mercury
Sulphur
100.00
Sulphuretted Hydrogen.
Hydrogen . 6.01
Sulphur . 93.99
100.00
•
•
Mercury Sulphide.
OXIDES.
Lead Oxide.
Lead
Oxygen
100.00
86.20
13.80
•
92.60
7:40 Oxygen
100.00
92.82
7.18
·
100.00
SULPHIDES.
Lead Sulphide.
Lead.
Sulphur
·
Cadmium
Cadmium Oxide.
Whilst the corresponding sulphides exhibit the following
composition:
86.58
13 42
100.00
Copper Oxide.
Copper 79.82
Oxygen 20.18
.
·
Cadmium
Sulphur.
100.00
87:49
12:51
.
Cadmium Sulphide.
100.00
Copper Sulphide.
Copper . 66.76
Sulphur
33.24
77.73
2227
100.00
100.00
38 If, as before, we now compare the quantity of each element
united with one and the same weight of oxygen, taking 7.94
parts of this element, because this is the amount of it which
combines with one part of hydrogen and is, therefore, the
equivalent weight of oxygen, we get the following numbers :-
94
GENERAL PRINCIPLES OF THE SCIENCE
Water.
Hydrogen 1.00
Oxygen. 7.94
8.94
Mercury
Oxygen
•
Mercury Oxide.
·
•
Mercury
Sulphur
•
Sulphuretted Hydrogen.
Hydrogen 1.00
Sulphur. . 1564
16.64
·
•
•
Mercury Sulphide.
""
""
""
Lead
Oxygen
""
33
And, if we investigate the sulphides, we find that one and the
same weight of sulphur, viz. 1564 parts by weight unites with
weights of these elements to form sulphides, which are identi-
cal with the amounts that combined with 7.94 parts by weight
of oxygen to form oxides. Thus we have:-
""
Lead Oxide.
23
99.45
7.94
Lead
Sulphur
29
.
107.39
""
99.45
15.64
115.09
•
Lead Sulphide.
•
102.68
7.94
110.62
·
·
Cadmium
Oxygen
1 part by weight of hydrogen combines with
102.68 parts by weight of lead combine with
31.41
,, copper
99.45
55.63
102.68
1564
118.32
Cadmium Oxide.
Copper Oxide.
Copper . 31.41
Oxygen.
7.94
39.35
Hence we see again that the amounts of these elements which
unite with an equivalent of oxygen also combine with an
equivalent of sulphur, so that
Cadmium
Sulphur
29
mercury
,, cadmium
Copper Sulphide.
Copper . 31.41
Sulphur 15.64
47.05
Cadmium Sulphide.
""
99
•
23
•
""
55.63
7.94
63.57
93
•
""
(7.94 of oxygen
1564 of sulphur
1564 of sulphur
55.63
15.64
71-27
and these are, therefore, the equivalent weights of these
elements.
DALTON'S ATOMIC THEORY
395
39 When one element combines with another in more than one
proportion it is said to have more than one equivalent, and
since the amounts of one element which combine with a fixed
weight of a second are in a simple ratio to one another, it
follows that the several equivalents of an element must also
stand in a simple ratio to one another. Iron for example
forms several different compounds with oxygen, two of which
have the following composition as determined by analysis:-
Iron
Oxygen
·
Iron
Oxygen
Ferrous Oxide.
77.78
22.22
.
•
100.00
Ferric Oxide.
70.00
30.00
Calculating the amount of iron combined with the equivalent
(7.94 parts) of oxygen we find
(1)
27.78
7.94
35.72
100.00
(2)
18.52
7.94
26.46
These amounts of iron are, however, in the simple ratio of
2:3, 27-78 being the equivalent of iron in ferrous oxide and
18.52 in ferric oxide.
40 It will be seen, therefore, that combination always takes
place between certain definite and constant proportions of the
elements or between multiples of these.
41 At a time when the question of combination in a limited
number of definite proportions was still under discussion, John
Dalton's speculative mind conceived an hypothesis which clearly
explained the law of combination in constant proportions, and
solved the question as to the nature of the compounds formed
by the union of two or more elements in several different pro-
portions. The hypothesis known as Dalton's Atomic Theory
may be said to have become one of the most important
foundation stones of the science, and to have exerted an
influence on its progress greater than that of any other
generalization, with, perhaps the single exception of Lavoisier's
explanation of the phenomena of combustion, and the discovery
of the indestructibility of matter.
96
GENERAL PRINCIPLES OF THE SCIENCE
The Atomic Theory, then, follows the doctrines of the Greek
philosophers so far as it supposes that matter is not continuous,
but made up of extremely small individual particles termed
atoms (a privative and Téμvo I cut); but differs from that of
the ancients and becomes truly a chemical atomic theory inas-
much as it supposes the atoms of different elements not to
possess the same weights, but to be characterised by different
weights. Thus the atom of oxygen is 15 88 times as heavy as
the atom of hydrogen, and the weights of the atoms of oxygen
and chlorine are as 15 88 to 3519. Dalton assumed, in the
second place, that chemical combination consists in the
approximation of the individual atoms to each other.
Having
made these assumptions he was able to explain why combination
always takes place between certain amounts of the elements or
between multiples of these amounts; since combination being
supposed to take place between some number of atoms of each
of the elements which unite, it follows that the amounts which
combine must be some finite multiple of the weights of the
atoms.
It is thus clear that the atomic theory explains the formation
of all compounds which are found to exist, but it is equally
evident that it in no way decides how many compounds can be
formed by any two or more elements. This at present can only
be learned by experiment, but we are not without indications
that a time approaches when this further problem will receive a
theoretical solution.
42 Although the atomic theory satisfactorily explains all the
known laws of chemical combination, the actual existence of
atoms is far from being thereby positively proved; indeed,
from purely chemical considerations it appears unlikely that the
question will ever be solved. Nevertheless, there is evidence,
gradually becoming more cogent, connected with certain physical
phenomena, which compels us to admit a limit to the divisi-
bility of matter. The phenomena in question belong to the science
of molecular physics, and have reference to such subjects as the
capillary attraction of liquids, the diffusion of gases, and the pro-
duction of electricity by the contact of metals. Reasoning from
facts observed in the study of these subjects, physicists have not
only come to the conclusion that matter is discontinuous, and,
therefore, that indivisible particles or molecules (molecula, a small
¹ Williamson, "On the Atomic Theory," Chem. Soc. Journ., 22, (1869) 328 and
433.
COMBINATION BY VOLUME
97
mass) exist, but they have even gone so far as to indicate the order
of magnitude which these molecules attain. Thus Sir William
Thomson states that in any ordinary liquid or transparent or
seemingly opaque solid, the mean distance between the centres
of contiguous molecules is less than one hundred-millionth, and
greater than the two thousand-millionth of a centimetre. Or, in
order to form a conception of this coarse-grainedness, we may
imagine a rain-drop or a globe of glass as large as a pea to be
magnified up to the size of the earth, each constituent molecule
being magnified in the same proportion; the magnified structure
would be coarser-grained than a heap of small shot, but probably
less coarse-grained than a heap of cricket-balls.¹
The molecular constitution of matter is likewise an essential
condition of the mechanical theory of gases, by means of which
nearly every known mechanical property of the gases can be
explained on dynamical principles, so that in this direction
again, we have a confirmation of the real existence of molecules
(p.60).
43 It will be seen that the atomic theory as proposed by Dal-
ton does not provide any means for ascertaining what the relative
weights of the atoms really are. We have seen, for instance,
that 7.94 parts by weight of oxygen combine with one part of
hydrogen to form water, but we cannot draw any conclusion from
this fact as to the atomic weight of oxygen, (that of hydrogen
being taken as equal to one), until we know how many atoms of
each of these elements have taken part in the combination. If
the combination is between an equal number of atoms of each
element, then the relative weights of their atoms must be as
1:7-94, whilst if two atoms of hydrogen have combined with one
of oxygen the relation will be as 1: 15·88. The answer to this
question-as to the number of atoms of each element between
which combination has taken place-has been supplied by a
study of the laws of combination of gaseous substances.
COMBINATION BY VOLUME.
44 The discovery by Gay-Lussac and Humboldt in 1805 of
the simple relation existing between the combining volumes of
oxygen and hydrogen gases, followed by that of the general law
of gaseous volumes enunciated in 1808 by Gay-Lussac alone,
1 Nature, March 31, 1870.
8
98
GENERAL PRINCIPLES OF THE SCIENCE
serves as a powerful argument in favour of Dalton's Atomic
Theory. This law states that the volumes in which gaseous sub-
stances combine bear a simple relation to one another and to the
volume of the resulting product. This is true both for elementary
and compound gases, the simple relations which exist being
illustrated by the following table:
1 vol. of chlorine and 1 vol. of hydrogen form 2 vols. of hydrochloric acid gas.
1 vol. of oxygen and 2 vols. of hydrogen form 2 vols. of steam.
1 vol. of nitrogen and 3 vols. of hydrogen form 2 vols. of ammonia.
1 vol. of oxygen and 2 vols. of carbonic oxide form 2 vols. of carbon dioxide.
According to the atomic theory, however, combination takes
place between the atoms of which substances are made up, and
it hence follows, if we accept this theory, that the number of
atoms which is contained in a given volume of any gaseous body,
must stand in a simple relation to that contained in the same
volume of any other gas (measured under equal circumstances
of temperature and pressure). The simplest as well as the
most probable supposition respecting this question is that put
forward by Avogadro in 1811,1 who assumed that equal volumes
of all the different gases, both elementary and compound, contain
the same number of particles or integrant molecules, and this
theory is now generally accepted by physicists, who have arrived
at the same conclusion as the chemists have reached by an inde-
pendent train of reasoning (p. 61). If we take the simplest case
of volume combination, that of one volume (one molecule) of
chlorine and one volume (one molecule) of hydrogen uniting to
form two volumes (two molecules) of hydrochloric acid gas, it is
clear that, since each molecule of hydrochloric acid contains at
least one atom of chlorine and one of hydrogen, there are at least
twice as many atoms as molecules of these elements present.
Hence, to conform to Avogadro's theory, the integrant molecule
of free chlorine and of free hydrogen must consist of at least two
atoms combined together, and we shall represent the combination
as taking place between one volume (one molecule of two atoms)
of chlorine, and one volume (one molecule of two atoms) of hydro-
gen, forming two volumes (two molecules) of the compound
hydrochloric acid gas. Again, two volumes of steam are formed
from two volumes of hydrogen and one volume of oxygen, hence
if there are the same number of molecules of steam, of hydrogen
1 Journ. de Phys., par De la Métheric, 73, Juillet, 1811, 58-76. Ibid., Feb,
1814.
1
AVOGADRO'S THEORY
99
and of oxygen in the same volume of each gas, it is clear that in
the formation of water from its elements, each molecule of
oxygen must be split up into two similar parts. We are thus
led to distinguish between the atom and the molecule, the latter
term being applied to the smallest particle of an element or com-
pound which can exist in the free state. Avogadro's theory refers
exclusively to these molecules and states that equal volumes of
all gases, measured under the same physical conditions, contain
equal numbers of molecules.
An immediate consequence of this theory is of the utmost
importance. If we weigh equal volumes of two gases, we are
obviously weighing equal numbers of their molecules, and the
ratio of the weights of the gases will also be the ratio of the
weights of their molecules, so that we are thus enabled to deter-
mine the relative weights of the molecules of all gases, by simply
finding their relative densities. Hydrogen gas is taken as the
standard of comparison because it has a lower density than any
other gas, and, since it has been shown that its molecule can be
divided into at least two parts (p. 98), the weight of its molecule
is taken as equal to two. The molecular weight of any gas is
therefore equal to twice its density comparedth hydrogen;
nitrogen, for example, is about 14 times as heavy as hydrogen
and hence its molecular weight is about 28, whilst carbon dioxide
has a density of 22 and therefore a molecular-weight of 44.
45 When the molecular weight of a gas and also its composi-
tion, as
determined by analysis, are both known, it is possible to
calculate what proportion of each of the component elements is
present in the molecule. Water for instance contains 88-81 per
cent, of oxygen and 11:19 of hydrogen, whilst the density of
steam is about 8.94, its molecular weight being, therefore, equal
to 17-88. If now we calculate how much oxygen and hydrogen.
are present in 17.88 parts of water, we find that this amount is
of 15.88 of oxygen and 2 of hydrogen. Carbon dioxide
again has a molecular weight of 437, and contains
made
up
Carbon ..
Oxygen
•
.
. 27.27
. 72.73
100.00
Hence 437 parts of this gas contain 11-91 of carbon and 31-76
of oxygen.
100
GENERAL PRINCIPLES OF THE SCIENCE
A repetition of this process for all the known compounds of some
particular element enables us to ascertain the least amount of that
element which is ever found in a molecule of one of its compounds,
and to this amount the name of atom is given. A comparison
of all the compounds of oxygen, for example, teaches us that the
least amount of it ever found in a molecule is 15 88 parts,
and this is, therefore, taken as the atomic weight of oxygen.
This having been ascertained, we are in a position to say
that the molecule of water contains one atom of oxygen,
whilst that of carbon dioxide contains two. All the non-metallic
elements form compounds which can exist in the state of gas,
and hence the atomic weights of all these elements have been
found by this method. Many of the metals on the other
hand do not form volatile compounds, and the atomic weights
of these have, therefore, to be determined by different methods.
A discussion of these methods will be found in a later volume
(Vol II. Part I. p. 24). In any case it must be remembered that
the method described above is not generally capable of great
accuracy and only yields an approximate number for the atomic
weight, the exact value being found by determining the equiva-
lent of the element by an accurate analysis of one of its
compounds, and then taking as the exact atomic weight the
multiple of this number which approaches most closely to the
approximate number obtained from the molecular weights.
46 It will be seen that the determination of the atomic
weight is quite distinct from that of the molecular weight.
This is well shown in the case of carbon; a comparison of the
gaseous compounds of carbon shows that the atomic weight of
this element is about 12, this being the least amount of it
which is found in the molecule of one of its compounds; we are,
however, quite ignorant of the molecular weight of carbon itself,
since its density in the state of gas has never been determined.
The molecules of some elements contain two atoms, this being
the case with hydrogen, oxygen, nitrogen, chlorine and others,
whilst the molecules of mercury vapour and of the vapours
of some of the other metals consist of single atoms, the molecular
and atomic weights being, therefore, identical.
47 For the first time we may now employ chemical symbols,
a kind of shorthand, by which we can conveniently express
the various chemical changes. To each element we give a
symbol, usually the first letter of the Latin, which is generally
also that of the English name. Thus O stands for oxygen;
EMPIRICAL AND MOLECULAR FORMULÆ
101
H for hydrogen; S for sulphur; Au for gold (aurum); Ag for
silver (argentum). These letters, however, signify more than
that a particular substance takes part in the reaction. They
serve also to give the quantity by weight in which it is present.
Thus O does not stand for any quantity, but for 15.88 parts by
weight (the atomic weight) of oxygen; H always stands for
one part by weight of hydrogen; and in like manner S, Au,
and Ag stands invariably for 31-28, 1957, and 107-13 parts by
weight of the several elements respectively. By placing
symbols of any elements side by side, a combination of the
elements is signified, thus:-
HCl Hydrochloric acid
HBr Hydrobromic acid
Carbon
Hydrogen
HI Hydriodic acid
HgO Mercuric oxide.
If the molecule contains more than one atom of any element,
this is indicated by placing a small number below the symbol
of the atom of the element, thus H₂O signifies 17.88 parts by
weight of a compound (water) containing two atoms or 2
parts by weight of hydrogen and one atom or 15.88 parts by
weight of oxygen. In such a case as this, where the molecular
weight and the number of atoms in the molecule are known
and expressed in the formula, the latter is said to be a molecular
formula and consequently represents such a weight of the
substance as will in the state of gas occupy the same volume as
two parts by weight of hydrogen. When the molecular weight
of the compound to be represented by a formula is not known,
it is only possible to express the relative number of the atoms
of the constituent elements which are present. A formula of
this kind is known as an empirical formula and may be
calculated for any substance of which the composition has been
determined by analysis.
The gas
known as ethylene has the following composition as
determined by analysis:
•
85.62
14.38
10000
In order to find the empirical formula of this substance it is
only necessary to divide the percentage of each element by the
102
GENERAL PRINCIPLES OF THE SCIENCE
atomic weight of the element, which gives us the ratio of the
number of atoms of each of the two elements, and if we express
this ratio in the smallest possible whole numbers we have
at once the relative numbers of atoms present in the molecule,
without, however, having any information as to the absolute
number.
Carbon
Hydrogen
Percentage.
85.62
14.38
Percentage
Atomic Weight.
7.19
14:38
Simplest
Ratio.
1
2.
The simplest or empirical formula of ethylene is therefore
CH, The density of this gas however is found to be equal to
13.91 and its molecular weight is therefore 2782, its molecular
formula being consequently C₂H..
________________
It is usual to represent chemical changes in the form of
equations; the materials taking part in the change being
placed on one side and the products formed, which are always
equal to them in weight (p. 48), being placed on the other.
If we heat potassium chlorate, a substance which has the
empirical formula KClO,, it is decomposed into oxygen and
potassium chloride and this decomposition is represented by the
equation
2KCIO₂ = 2KC1 + 302
in which the sign + connects the two products and signifies
"together with." This equation is an expression of the fact,
ascertained by experiment, that 121.68 (38.85+35·19+3 × 15.88)
parts of this salt by weight leave behind on heating 74-04
parts (35.19+38-85) of potassium chloride and liberate 47.64
parts of oxygen. Hence it is clear that the quantity of oxygen
which is obtained from any other weight of the salt and vice
versa can be found by a simple calculation when the equation
representing the chemical change is known.
To take a more complicated case, when we know that the
equation representing the change which occurs when we heat
potassium ferrocyanide, the empirical formula of which is
KCN Fe, with strong sulphuric acid H₂SO, and water, is the
following:-
4
K_C.NFe+6H,SO,+6H,O=6CO+2K,SO,+3(NH,),SO,+FeSO4
yielding carbon monoxide gas, CO, potassium sulphate K₂SO,
ammonium sulphate (NH4)2SO4, and iron sulphate FeSO4, we
CALCULATION OF CHEMICAL QUANTITIES
103
can easily calculate how many grams of carbon monoxide gas,
CO, can be obtained from any given weight of the ferrocyanide,
KCN Fe, inasmuch as analysis proves that the amounts
represented by these formulæ are made up as follows:
Carbon Monoxide.
Carbon C 11.91
Oxygen O 15.88
27.79
Ferrocyanide of Potassium.
Potassium K 155:40
Carbon C 71-46
Nitrogen N. 83.64
Iron Fe 55.58
366.08
The foregoing equation then shows that 366-08 parts by
weight of the ferrocyanide yields 166 74 parts by weight of car-
bon monoxide, and hence a simple proportion gives the quantity
yielded by any other weight. The illustration is, however, not
yet complete; commercial potassium ferrocyanide contains, as do
many crystalline compounds, a certain quantity of water of
crystallization, which is given off when the salt is heated, in
consequence of which the crystals fall to a powder. But, as
the equation shows, a certain quantity of water takes part in
the reaction, and it is, therefore, unnecessary to dry the salt
previously if only we know how much water of crystallization
it contains. Analysis has shown that the commercial salt
has the composition KCN Fe+3H₂O; hence if we add
3x 17.88, the weight of 3 molecules of water, to 366'08, we
obtain the number 41972 as the weight of the hydrated salt,
which must be taken in order to obtain 166-74 parts by weight
of carbon monoxide.
As, however, the quantity of a gas is almost always estimated
by measuring its volume, and from this volume calculating its
weight, it becomes of the greatest importance to know how to
calculate the volume of a gas from its weight, or vice versa.
This can readily be done, for we know that the density of every
compound in the gaseous state is half its molecular weight; or
that every molecule in the gaseous state occupies the volume filled
by two parts by weight of hydrogen. One litre of hydrogen gas
at 0° and under 760mm of mercury weighs 0.089872 gram.;
hence the weight of a litre of any other gas measured under the
same circumstances of temperature and pressure is obtained by
multiplying the density of the gas, or half its molecular weight,
104
GENERAL PRINCIPLES OF THE SCIENCE
by the above number, or by 0-0899, when absolute accuracy
is not required. Thus one litre of carbon monoxide weighs
27.79
×0.0899-1.249 grms., and 166-74 grms. of this substance
2
occupy a volume of
litres at 0° and 760mm.
It is now easy to calculate what volume this weight will
occupy at any other temperature or pressure, for we know
that all gases expand by 3 of their volume at 0°C. when
their temperature is raised 1°C. (at constant pressure, Law
of Dalton), and that their volume is inversely proportional
to the pressure to which they are subjected (Law of Boyle).
Hence if the temperature at which the gas was collected
were 17°C., and if the barometer then stood at 750mm
the volume (v) in litres of the carbon monoxide collected
would be
166.74
1.249
V =
O
166 74 x (273 +17) x 760
1.249 x 273 × 750.
EXPERIMENTAL METHODS FOR THE DETERMINA-
TION OF MOLECULAR WEIGHTS.
48 We have seen above (p. 99) that the density of every com-
pound in the gaseous state is half its molecular weight, from
which it follows that in order to determine the molecular weight
of a substance it is simply necessary to determine its density in
the state of gas. This process can be readily applied to such
substances as are gases under ordinary conditions of temperature
and pressure, and also to such as can be rendered gaseous by
moderate increase of temperature, but is of course inapplicable
to bodies which decompose on heating or which cannot easily
be vapourised. In such cases, however, several methods are
available which depend upon the properties of solutions, and
hence, since nearly all chemical compounds are soluble in
some liquid, the molecular weight of a body can almost always
be determined.
Special experimental methods have to be adopted for each of
these classes of substances.
DETERMINATION OF MOLECULAR WEIGHTS
105
I. DETERMINATION OF THE MOLECULAR WEIGHTS OF
PERMANENT GASES.
49 For this purpose it is only necessary to determine the specific
gravity of the gas, air being usually taken as the practical unit
of comparison. A large glass balloon capable of holding from 1 to
10 litres of the gas, is employed and is first of all freed from air
as far as possible by the vacuum pump, and weighed. In
weighing a body of such large volume it is essential to make
allowance for the buoyancy of the air, since the body to be
weighed appears to be lighter than it really is by an amount
equal to the weight of the air which it displaces. This is best
accomplished by suspending a vessel of similar size and shape to
the other arm of the balance, by which arrangement the effect
of buoyancy is neutralized, each of the vessels being affected in
the same manner, and at the same time the uncertainties of
calculation, due to the varying temperature, pressure, and
moisture of the atmosphere are avoided, as well as any inaccuracy
due to condensation on the surface of the glass. As soon as the
weight of the empty vessel has been ascertained, it is removed
from the balance and filled with the pure dry gas, the tempera-
ture and pressure being carefully observed. The globe is then
reweighed with the same precautions as before. One additional
correction must be here noticed as it gives rise to a considerable
error especially when light gases are being weighed. The
capacity of a vacuous globe of glass is found to be perceptibly
less than that of the same globe when filled with gas at the
pressure of the atmosphere, and hence the globe displaces less
air in the former condition than in the latter. The difference
between the corrected weights of the globe empty and filled
with the gas is equal to the weight (W) of the given volume
of gas at the observed temperature and pressure. The same
globe is then filled with dry air freed from carbonic acid gas,
or else with pure hydrogen, and the weight of an equal volume
of the latter (A) under the same conditions thus ascertained.
The specific gravity of the gas compared with air is then equal
W
to Since air has been found to be 14:39 times as heavy as
Α'
•
hydrogen and the molecular weight of a gas is twice its density
with respect to hydrogen, it is only necessary to multiply the
106
GENERAL PRINCIPLES OF THE SCIENCE
specific gravity by 14:39×2 to obtain the molecular weight of the
gas in question.
The most accurate determinations of this kind have been
made by Regnault, and more recently by Rayleigh, Leduc &c.
II. DETERMINATION
OF THE MOLECULAR WEIGHTS OF
VOLATILE LIQUIDS AND SOLIDS.
50 A detailed account of the various methods proposed for this
purpose is to be found in a later volume (Vol. III. Pt. I.
pp. 84-112); only the two most frequently employed will be here
briefly described.
The specific gravity of the vapour of a liquid or solid can be
determined in two ways; either by weighing the vapour which
occupies a known volume under given conditions of temperature
and pressure (Dumas' method) or by measuring the volume occu-
pied under given conditions by the vapour of a known weight
of the liquid or solid (Gay-Lussac, Hofmann, Victor Meyer).
I. METHOD OF DUMAS.
51 For this purpose a thin glass globe is employed of 150—200
cubic centimetres in capacity, having a finely drawn out neck;
the exact weight of the globe, weighed in air and filled with
dry air at a certain temperature and pressure, having been
found, a small portion of the substance of which the vapour
density is to be determined is brought inside, and the globe
then heated by plunging it into a water- or oil-bath raised
to a temperature at least 30° above the boiling point of the
substance; as soon as the vapour has ceased to issue from the
end of the neck, this end is hermetically sealed before a blow-
pipe, and the exact temperature of the bath as well as the
barometric pressure observed. The bulb thus filled with
vapour is carefully cleaned, allowed to cool, and accurately
weighed. The point of the neck is next broken under water,
which rushes into the globe, the vapour having condensed, and,
if the experiment has been well conducted, completely fills it.
The bulb is then weighed full of water, and its capacity calcu-
lated from the weight of water which has entered.
DETERMINATION OF VAPOUR DENSITY
107
We have now all the data necessary for the determination.
In the first place we have to find the weight of the given
volume of the vapour under certain circumstances of tempera-
FIG. 21.
-
ture and pressure, and we then have to compare this with the
weight of an equal volume of hydrogen gas measured under
the same circumstances. The following example of the deter-
mination of the vapour density of water may serve to illustrate
the method:-

FIG. 22.
Weight of globe filled with dry air at 15.5°
Weight of globe filled with vapour at 140°
Capacity of the globe
•
23-449 grams.
23-326
178 cc.
""
108
GENERAL PRINCIPLES OF THE SCIENCE
As the barometric column (760 mm.) underwent no change
from the beginning to the end of the experiment, no correction
for pressure is necessary. In order to get at the weight of
the vacuous globe the weight of air contained must be deducted
from the weight of the globe in air.
Now 1 cc. of air at 0° and 760 mm. weighs 0·001293
178 x 273
288.5
grams, and 178 cc. of air at 15.5° would occupy
1684 cc. at 0°, so that the weight of this air is 1684 ×
⚫001293 0.218 gram; hence the weight of the vacuous globe
is 23-231 (23449 - 0.218), and the weight of the vapour
23:326 23-231 = 0·095 gram. We must now find what
178 cc. of hydrogen at 140° will weigh. One thousand cc. of
hydrogen at 0° weigh 00899 gram. ; 178 cc. at 140° will con-
178 x 273
117.6 × 0.0899
tract to
= 117.6 at 0°, which weigh
413
1000
=
-
= 896 is the density of the
0.095
= 0·0106 gram. Hence
0.0106
vapour as found by experiment.
9
The molecular weight of water is therefore about 17-82. In
this example many minor corrections, such as the expansion of
the glass globe, the error of the mercurial thermometer, &c., are
not considered, but the above method carried out as described
gives results which are sufficiently accurate when the object,
as in this case, is to control the molecular weight of a
compound.
II. METHOD OF VICTOR MEYER.
52 The glass vessel () filled with air is heated by the vapour
of water or other liquid placed in the bulb tube (c), which may
if necessary be replaced by an air-bath, until no more air is
observed to pass out of the gas delivery tube (a). The cork (d)
is then removed and the weighed quantity of the substance of
which the vapour density is required, contained in a small glass
bulb, is dropped into the tube (b) and the cork then quickly
inserted; the substance rapidly evaporates and displaces a
portion of the air of the apparatus which is collected in a
graduated tube over water and carefully measured. This
method has the great advantage of dispensing with a knowledge
of the temperature to which the tube is heated. It is to be
DISSOCIATION
109
=
borne in mind that what we require to know is the weight of
air (or hydrogen) equal in bulk to the vapour. Whether
this volume of air be measured at the temperature of the
vapour or at that of the atmosphere, it has of course the same
weight.
An example will make this clear. In a determination of
the molecular weight of chloroform, CHCl,,
heated by water vapour, it was found that
01008 gram. of substance displaced 20 cc. of
air, measured over water at a temperature
of 15°C. and a barometric pressure of 770
The corrected volume of dry air is
therefore 189 cc., the weight of which is
1890-01293
0.0244 gram. The va-
pour density of chloroform is then equal to
0-1008
mm.
14:39 =
0-024-4 = 413 compared with air, or 413 ×
594 compared with hydrogen, the
molecular weight being accordingly 1188, which
agrees closely with 118.5, the number calculated
from the formula, CHCl3.
d
DISSOCIATION.
53 In many cases the vapour density found
by experiment alters with the temperature at
which the determination is carried out. Iodine
vapour, for example, is found to have a con-
stant density of about 126 between the tem-
peratures of 400-700° C., and its molecular
weight then corresponds to the formula I.
Above this temperature, however, the density is
found to gradually diminish, until at about 1,500°
it again becomes constant at about 63, almost
exactly half of its previous value. We must,
therefore, assume that the molecule of iodine (I,) is decomposed
at temperatures above 700°, a gradually increasing number of its
molecules being broken up as the temperature rises, until finally
at 1,500 nearly all the molecules have been broken up into free
atoms, each of which must now be considered as a separate
molecule, the molecular weight of iodine at these high
-19
FIG. 23.
110
GENERAL PRINCIPLES OF THE SCIENCE
1
temperatures being 126, and its formula I, the change being
represented by the equation:
I₂
I + I.
2 vols. 2 vols. 2 vols.
=
A decomposition of this kind is known as dissociation. It
must be remembered that in order to prove that a substance
exists in the state of vapour with a definite molecular weight,
the vapour density must be found to be constant throughout a
considerable range of temperature. Thus, iodine vapour has a
constant density between the temperatures 400--700˚, and only
begins to dissociate above the latter temperature. Sulphur
vapour on the other hand at a temperature near its boiling-
point has a density of 96, corresponding to the formula S; but
this density is not constant for any definite range of temperature
but gradually decreases until it reaches the value of 32 (S₂) at a
temperature of 600°, above which it remains constant. In the
case of iodine vapour, therefore, we have good evidence that
molecules of the formula I, exist, whereas this is wanting for
the existence of sulphur molecules containing six atoms,
although it was formerly believed that these existed at low
temperatures.
54 Frequently a compound which exists in the solid or liquid
state cannot be converted into vapour without undergoing dis-
sociation. Thus ammonia and hydrochloric acid unite directly
to form ammonium chloride:
NH3 + HCl = NHẠC.
Phosphorus trichloride absorbs two atoms of chlorine, and is
converted into the pentachloride, thus:-
PC13 + Cl₂ = PC15.
These compounds, however, only exist in the solid or liquid
state; when they are heated they decompose into the two mole-
cules from which they have been formed. In some cases this
decomposition can be readily seen; thus antimony pentachloride
SbCl, decomposes into the trichloride, SbCl, and free chlorine.
Other compounds, such as pentachloride of phosphorus, PCI,
appear to volatilize without decomposition, but in this case it can
be proved that the vapour is a mixture, and contains the mole-
cules of two gases, phosphorus trichloride, PC13, and free chlorine.
OSMOTIC PRESSURE
111
=
The vapour densities of these bodies accordingly do not follow
the usual law; thus the vapour of chloride of ammonium, if
it consisted of similar molecules, must possess the density of
35-1913-94 +4
26.56. In fact, however, its density is only
half this number, for four volumes contain one molecule of
ammonia and one of hydrochloric acid; hence its density (or the
weight of one volume) is half the above or 13.28.
2
In the same way iodine forms both a monochloride ICl and a
trichloride ICl; the first of these bodies is volatile without
decomposition, the second, however, decomposes on distillation
into the molecules ICI and Cl2.
III. DETERMINATION OF THE MOLECULAR WEIGHTS OF
SUBSTANCES IN SOLUTION.
55 Until recently the molecular condition of substances in solu-
quite unknown, but much light has been thrown upon
the subject by the researches of Raoult, Van't Hoff, Arrhenius,
Ostwald, and others, who have shown that a remarkable
analogy exists between the properties of substances in dilute
solution and those of gases. This conclusion has been chiefly
derived from the study of the interesting phenomena now to
be described. When a solution of a crystalloid substance in
water
is placed in a vessel closed by a porous membrane,
such as a
piece of parchment, and the whole immersed in
pure water, it is found that the dissolved substance gradually
passes outwards through the film of parchment, whilst water
passes inwards, until, after a sufficient time has elapsed, equi-
librium is established and the liquid has the same composition
both inside and outside the membrane, this process being known
as osmosis.
Diaphragms of other substances can, however, be obtained
which allow the water to pass freely through them in the same
way as the parchment, but prevent the outward passage of the
dissolved substance, and are therefore said to be "semi-
permeable." Such a diaphragm can be prepared by filling an
ordinary porous cell with a dilute solution of potassium ferro-
1 For the literature of this subject the volumes of the Zeitschrift für Physika-
lische Chemie (Leipzig, Engelmann), edited by Van't Hoff and Ostwald, may be
consulted.
112
GENERAL PRINCIPLES OF THE SCIENCE
cyanide and simultaneously immersing it in one of copper
sulphate. These two substances gradually diffuse into the
porous walls of the cell and produce an insoluble layer of copper
ferrocyanide, which is found to be semi-permeable for solutions
of many salts and other bodies. If now such a cell, containing
a dilute solution of sugar, be placed in a vessel containing pure
water, the latter is found to pass inwards through the film,
whilst the sugar does not pass out, and consequently the level
of the liquid within the cell rises. If, however, the cell be com-
pletely filled and connected with an arrangement for measuring
the pressure it will be found that the latter gradually increases,
but after a time becomes constant. The pressure thus observed
is termed the osmotic pressure of the solution, and is found
to depend only upon (1) the nature of the dissolved substance,
(2) the concentration of the solution, and (3) the temperature.
This osmotic pressure plays the same part in the theory
of dilute solutions as the gaseous pressure in that of gases,
and is found to follow the same laws. Thus when the
strength of the solution is doubled the osmotic pressure be-
comes twice as great; when it is halved the pressure falls to
one half, and so on. Now doubling the strength of a solu-
tion is in reality halving the volume occupied by the unit
weight of the dissolved substance, so that the law that the
osmotic pressure of a dilute solution varies directly as its
concentration corresponds exactly with Boyle's law of gases
(p. 59). This is well seen in the following table, which
illustrates the variation of osmotic pressure with the concen-
tration, for solutions of cane sugar:1
Strength of sugar solution.
1%
2%
4%
6°。
Osmotic Pressure.
53.5 cm.
101.6
208.2
307.5
Pressure per cent. of sugar.
53.5 cm.
50.8
52.1
51.2
The osmotic pressure is also found to vary with the tem-
perature in the same way as does the pressure of gases.
Thus a solution of cane sugar was observed to have an osmotic
pressure of 544 cm. at 32°, whilst the same solution at 14·1° had
a pressure of 512 cm. Calculating the pressure at the lower
1 See Pfeffer, "Osmotische Untersuchungen," 71 quoted in Zeit. Phys. Chem.,
1, 484.
DEPRESSION OF THE FREEZING-POINT
113
•
temperature from that at the higher according to Dalton's law
544 x 287.1
for gases (p. 60) we obtain the number
= 51.0,
305
or almost exactly that observed.
56 Another remarkable fact which these researches have estab-
lished is that solutions of substances which contain quantities
of the compounds proportional to their molecular weights dis-
solved in equal amounts of the solvent possess the same osmotic
pressure; if we consider cane sugar (molecular weight 340)
and alcohol (molecular weight 46), for example, we find that a
solution of sugar in water containing 340 grams in 100 cc. has
the same osmotic pressure as one of 0.46 grams of alcohol in
the same volume. Moreover it appears that the osmotic pres-
sure of a solution is numerically equal to the pressure of a gas
containing the same number of molecules per unit of volume
as the solution does of molecules of the dissolved substance. A
one per cent. solution of sugar, for example, has at 15° the
specific gravity of 1.006, so that 1 gram of sugar is contained
in 100-6 cc. of the solution. Now the molecular weight of
sugar is 340, and if we calculate what volume of solution con-
tains 340 grams of sugar, we obtain the number 342 litres.
Now we know that two grams of hydrogen (the molecular
weight of which is 2) at a temperature of 15° occupy a volume of
= 23.53 litres when the pressure is equal to one
atmosphere. If this volume of the gas be expanded to 34-2
litres the pressure of the expanded gas will, by Boyle's law, be
22.26 288
273
equal to
760 x 23:53
34.2
that is to say 52.3 cm. The
The gas there-
fore which contains the same number of molecules per litre as
there are sugar
molecules in a one per cent. solution of cane
sugar, would, at the temperature of 15°, have a pressure of
523 cm., whilst the osmotic pressure of the solution itself has
been found to be 520 cm.
As a consequence of these remarkable relations it will be
seen that the molecular weight of a dissolved substance can be
determined by measuring the osmotic pressure of the solution,
but the experimental difficulties are so great as prevent the
general use of the method for this purpose.
to
57 The same result can, however, be attained by several allied
methods, the chief of which depends upon the alteration pro-
duced in the freezing-point of a liquid by dissolving some other
9
114
GENERAL PRINCIPLES OF THE SCIENCE
substance in it. When a dilute solution, such as one of sugar
in water, is cooled, the solvent, in this case the water, begins to
separate out, and if the solution be originally sufficiently
dilute, the solid matter which deposits is quite free from the
dissolved substance. The temperature at which the separation
of solid matter commences is, however, lower than the freezing-
point of the pure solvent, the amount of the depression being
proportional to the concentration of the solution (Blagden).¹
Raoult has further found that the extent of the depression
depends upon the molecular weight of the dissolved substance,
to which it is inversely proportional, or in other words, that if
equal weights of a series of compounds be each dissolved
in a liquid so as to produce equal volumes of solution, the
depressions of the freezing-point thus caused are inversely pro-
portional to the molecular weights of the compounds. If these
depressions be then multiplied by the molecular weights of the
compounds a constant number is obtained, which is known as
the molecular depression for the solvent in question. This is
illustrated by the following table of results obtained by Raoult 2
with substances dissolved in acetic acid:
Substance
Carbon tetrachloride
Carbon disulphide
Sulphur chloride
Arsenious chloride
Formula
CCI
CS2
S₂Cl₂
AsCl3
SnCl
Stannic chloride
SO₂
Sulphur dioxide
Sulphuretted hydrogen H₂S
Molecular Weight
153
76
134
180
258
64
34
D
MD
0.252
38.6
0.505
38.4
0.286
38.3
0.234
42.1
0.159
41.0
0.601 38.5
1.047
35.6
In this table D signifies the depression in degrees Centigrade
produced by 1 grm. of substance dissolved in 100 grms. of glacial
acetic acid, whilst MD is the product of this number with the
molecular weight of the compound. It will be seen that
the molecular depression for this solvent is nearly constant,
the numbers varying on either side of an average value of
about 39.
Upon these facts a very simple method for determining the
molecular weight of a soluble substance has been based. A
weighed quantity of a suitable solvent is placed in a glass tube,
1 Phil. Trans., 1788, 78, 277.
2 Ann. Chim. Phys. [5], 2, 93.
DEPRESSION OF THE FREEZING-POINT
115
the latter surrounded by a freezing mixture, and the freezing-
point of the liquid determined by means of an accurate thermo-
meter, graduated to of a degree. An exactly weighed
amount of the substance is then added, allowed to dissolve
completely, and the freezing point of the solution then carefully
ascertained in the same way as before. The difference observed
between these two temperatures (d) is the depression of the
freezing point produced by (b) grms. of the substance dissolved
in (a) grms. of the solvent. From this is calculated the depression
which would be produced by 1 grm. of the substance dissolved
in 100 grms. of the solvent (D), which is equal to
d x a
b × 100
If
now, the molecular depression is MD, it follows that the mole-
cular weight of the substance in question (M) is given by the
equation :
M
=
MD MD x b x 100
D
d ха
=
Thus 1.35 grm. of carbon tetrachloride dissolved in 55 grms.
of acetic acid lowered the melting point of the latter from
16°750 to 16°132, the depression being therefore equal to
0°618.
Since the molecular depression, MD, for acetic acid is
39, it follows that the molecular weight of carbon tetrachloride
must be
39 × 1:35 × 100
55 x 0.618
=
154.9.
This number agrees satisfactorily with that obtained by the
vapour-density method, viz., 153.
58 The addition of a soluble substance to a liquid not only
lowers the freezing-point of the latter, but also diminishes its
vapour-pressure.
Like the depression of the freezing-point,
this diminution of vapour-pressure depends upon the molecular
weight of the dissolved substance and upon the strength of the
solution.
Several methods of determining the molecular weight
of dissolved substances have been based upon these facts, for
the details of which the original papers must be consulted.¹
These three classes of phenomena are intimately connected
with one another, inasmuch as it has been experimentally found
that dilute solutions, the solvent being the same in all, which
have the same osmotic pressure have also the same freezing-
1 Beckmann, Zeit. Phys. Chem. 4, 532, 6, 437, 8, 223; Will und Bredig, Ber.
22,1084.
116
GENERAL PRINCIPLES OF THE SCIENCE
point and the same vapour pressure, such solutions being termed
isotonic. This relation has also been deduced theoretically, so
that if any one of these three facts be known about a solution
the other two can be calculated.
It is important to remember that these statements only hold
good in the case of dilute solutions, and cannot be applied to
concentrated solutions, just as the laws of Boyle and Dalton do
not apply without modifications to gases at high pressures and
low temperatures.
AQUEOUS SOLUTIONS.
59 The behaviour of aqueous solutions when examined by the
methods just described is somewhat anomalous, but an explana-
tion of the irregularities observed has been arrived at from a
study of the phenomena which occur when an electric current
is passed through such solutions. Pure water is practically a
non-conductor of electricity, and substances which are soluble
in it may be divided into two classes according as they do or do
not produce solutions which conduct electricity. Those of the
former class are termed electrolytes, and the passage of the
current through their solutions is accompanied by their
decomposition, whilst those of the second class are known as
non-electrolytes. The laws of osmotic pressure, &c., as stated
above are true of all non-electrolytes, but require some
modification before they can be applied to electrolytes, a class
of bodies which includes acids, alkalis, and almost all metallic
salts. When the molecular weight of one of these sub-
stances is determined from an aqueous solution by any of the
methods described, the number obtained is found to be some
fraction of that which was to be expected, generally about or }.
This result is comparable with that obtained by the vapour
density method with such vapours as undergo dissociation,
and it seems probable that something of an analogous
nature also occurs in dilute aqueous solutions of electro-
lytes. When a current of electricity is passed through a
solution of hydrochloric acid, HCl, hydrogen is given off at
the negative pole and chlorine at the positive, and the acid.
is said to have been decomposed into the two "ions" hydrogen
and chlorine. Many facts make it appear probable that this
decomposition is not actually brought about by the electric
ELECTROLYTIC DISSOCIATION
117
current but that the ions exist already separated in the solution.
According to this view, then, a dilute solution of hydrochloric
acid does not only contain molecules of HCl but a large propor-
tion of separated ions H and Cl. Each one of these ions behaves
like a molecule of a non-electrolyte and, therefore, produces its
own effect in lowering the freezing point, etc., the total depres-
sion being due to the sum of the ions, each considered as an in-
dependent molecule, together with the unaltered molecules.
Since the depression is inversely proportional to the molecular
weight, the result obtained by this method seems to show that
hydrochloric acid has about one half of the molecular weight
corresponding to the formula HCI. In general, salts, such as
common salt, NaCl, or silver nitrate, AgNO3, which are broken
up into two ions (Na and Cl, Ag and NO3), appear to have about
half the calculated molecular weight, whilst the salts of a dibasic
or a divalent metal of the types represented by sodium
sulphate, Na,SO,, and calcium chloride, CaCl,, which give three
ions (Na, Na and SO; Ca, Cl and Cl) appear to have a
molecular weight approaching one third of that calculated.
acid
According to this theory of "electrolytic dissociation" which
is due to the Swedish physicist Arrhenius, the greater number of
salt molecules in a strong solution of an electrolyte are unaltered,
but some of them have been dissociated into their ions; when
the solution is diluted the number of dissociated molecules
increases rapidly, and in a very dilute solution nearly all the
salt is present in the form of ions. The following table shows
the percentage of dissociation which would account for the results
obtained by the freezing point method for a few common salts :¹
Grms. in 100 cc. water. Percentage of Molecules dissociated.
NaCl 0.682
87
3.155
79
into 2 ions.
2.381
85
2.206
75
1.583
67
AgNO3
CaCl2
KSÓ
into 3 ions.
Each ion is supposed to bear an equal charge of electricity,
electropositive ions having a positive, electronegative a neg-
ative charge. When, therefore, it is said that a dilute solution
hydrochloric acid contains free ions of hydrogen and chlorine,
it must not be supposed that what we are familiar with as free
hydrogen and chlorine are meant; what are actually present are
of
1 Arrhenius, Zeit. Phys. Chem. 2, 491.
118
GENERAL PRINCIPLES OF THE SCIENCE
1
positively charged atoms of hydrogen, and negatively charged
atoms of chlorine. These charged atoms cannot escape from the
solution without giving up their charges of electricity to some
oppositely charged body and this is what happens at the poles
of the cell during electrolysis. The hydrogen ions give up
their positive charges to the negatively charged pole and escape
as free hydrogen, the atoms uniting to form molecules, whilst the
chlorine ions behave in a similar manner at the positively charged
pole. In many cases the liberated atoms enter into reaction
with the water surrounding the pole and thus give rise to
secondary products which either escape or remain in the solu-
tion (p. 252). Concerning the exact nature of the separation of
the ions our knowledge is still incomplete, but many facts in
addition to those already adduced (see p. 283, where some
of these are discussed) point to the conclusion that such a
separation does actually take place. The fact that electrolytes
in aqueous solution show this anomalous behaviour, renders it
impossible to employ the methods described above for determin-
ing their molecular weights, unless a complete study of the
behaviour of each substance is made. Substances which give
abnormal numbers when examined in aqueous solution, usually
yield normal results when their solutions in other solvents are
examined.
CHEMICAL NOMENCLATURE.
60 Nomenclature is the spoken language of chemistry, as no-
tation is the symbolic written language of the science. With
the progress of discovery chemical nomenclature has naturally
undergone great and frequent changes. The ancients were
acquainted with only seven metals, viz. :—gold, silver, copper,
tin, iron, lead, and mercury. Of these the first six are men-
tioned by Homer; mercury was not known in his time, but
mention is made of the liquid metal by authors living one cen-
tury before Christ. These seven metals were originally supposed
to be in some way connected with the seven heavenly bodies
then known to belong to our system. To bright yellow gold
the name of Sol was given; whilst white silver was termed
Luna; copper, which had chiefly been obtained from the
island of Cyprus and received its common name (cuprum)
from this source, was likewise called Venus, after the pro-
I
·
I
}
CHEMICAL NOMENCLATURE
119
tectress of the island. Tin was specially dedicated to Jupiter;
iron to Mars, the god of war; whilst heavy dull lead was con-
nected with Saturn; and the mobile quicksilver was called
Mercury, after the active messenger of the gods. The alchemists
not only invariably used these names, but employed the signs
of the heavenly bodies as symbols for the metals, and many
remnants of this practice are found to this day in all
languages. Thus we still speak of "lunar caustic" for silver
nitrate, “saturnine poisoning" for poisoning by lead, whilst the
name mercury has become the common one of the metal. To
come to later times we find that the language of the alche-
mists was always and designedly obscure and enigmatical, so
that their names for chemical compounds were not based on
any principle but even chosen for the sake of secrecy or de-
ception, and, therefore, bore no relation to the substances
themselves. From these fanciful terms the progress to a
better state of things has been slow, and the changes which
the names have undergone have been numerous, whilst the
same substance has at one time frequently been designated
by many distinct names, several of which are still in use.
Bodies were generally named and classed by the alchemists
by virtue of certain real or fancied resemblances existing be-
tween their physical properties. Thus, bodies which can be
obtained by distillation and are, therefore, easily volatile, were
all termed spirits, so that alcohol (spirits of wine) was classed
together with hydrochloric acid (spirits of salt), and these
again with spirits of turpentine, although these three sub-
stances are chemically as different as any three substances
well can be. In the same way, all viscid, thick liquids were
termed oils, and thus sulphuric acid, or oil of vitriol, came to
be placed in the same class as olive oil; whilst semi-solid
bodies, such as antimony trichloride, were termed butters, and
considered to be analogous to common butter.
As soon as chemistry became a science, the nomenclature
assumed a more scientific character. Some of the terms which
came into use during the growth of the science have been
mentioned in the Historical Introduction. These terms have
by degrees been much changed, and, such revolutions have
accompanied the progress of the science, that at present the
same compound is not unfrequently designated by different
Thus it is clear that our nomenclature has not yet
attained a permanent form; the names of chemical substances
names.
120
GENERAL PRINCIPLES OF THE SCIENCE
are not identical in different languages, and even in the same
language, difference of practice in naming compounds is found
among chemists.
Nevertheless, we are guided by certain
specific rules, and the science no longer suffers from the
arbitrary nomenclature which the descriptive natural sciences
have to endure.
61 The foundation of the modern system of chemical names
was laid by Lavoisier and his colleagues,' and the plan proposed
by them has been maintained, with slight modifications,
up to the present time. The principle upon which our system
(for Inorganic Chemistry at least) is founded is, that every
compound being made up of two or more elementary bodies
united in different proportions, the name of that compound
shall signify the nature of its elementary constituents, and as
nearly as possible the relative proportions in which they are
believed to be present. In the case of the Carbon compounds
(Organic Chemistry) it was soon found impossible, from the
large number of closely-allied substances, uniformly to apply
this system, and names suggested by the origin of the bodies
have been in many cases adopted.
No special rule has been applied to the nomenclature of the
elements. The old common names of those which have long
been known have in most cases been retained, and when new
elements have been discovered they have been named according
to no pre-arranged plan. Some are named from the locality in
which they have first been found; some from a characteristic
property or from the mode of their discovery, whilst the names
of others, such as Gallium, Scandium, and Germanium, bear
witness to the patriotism of their discoverers. By common
consent the names of all recently discovered metals end
in "-ium," as sodium, barium, vanadium. The names of a
group of allied non-metallic elements end in "-ine," thus
we have fluorine, chlorine, bromine, and iodine; those of
another group of somewhat analogous non-metallic elements
end in "-on," as boron, carbon, silicon, whilst those of
two other non-metals, more nearly resembling the metals,
end like the latter in "-ium," thus we have selenium and
tellurium.
Lavoisier introduced the term "oxyde" to signify the com-
1 Méthode de Nomenclature Chimique, proposé par MM. de Morveau, Lavoisier,
Berthollet, et de Fourcroy. Paris, 1787. Translated into English by Pearson.
Second edition, 1799.
CHEMICAL NOMENCLATURE
121
The compounds of
Hydrogen
Fluorine
Chlorine
Bromine
Iodine
binations of oxygen with the other elements, and words with the
same ending have been since employed to denote the simple
combinations of two elements or groups of elements, thus:-
Oxygen
Sulphur
Selenium
Phosphorus
Carbon
form
Hydrides
Fluorides
Chlorides
Bromides
Iodides
Oxides
Sulphides
Selenides
Phosphides
Carbides
-
such as
Phosphorus hydrides.
Calcium fluoride.
Sodium chloride.
Magnesium bromide.
Lead iodide.
Mercury oxide.
Zinc sulphide.
Potassium selenide.
Calcium phosphide.
Iron carbide.
It frequently happens that a metal forms several distinct
oxides or chlorides, in which the constituents are present in
simple multiple proportions of their combining weights. In
these cases it is usual to give to each compound a name indi-
cating either the number of atoms of oxygen which we believe
to be combined with one atom of metal, or the simplest relation
which we suppose it possible to exist between the number
of atoms of metal and oxygen in the molecule: thus the oxide
believed to contain one atom of oxygen is termed the monoxide;
that containing two atoms is the dioxide; whilst oxides containing
three, four, or five atoms of oxygen are called trioxides, tetrox-
and pentoxides respectively. Sometimes the first oxide is termed
the protoxide (p@Tos, first), the second deutoxide (deútepos,
second), the third trioxide (Tpíros, third), and the highest
peroxide.
When the relation of metal to oxygen is that of 2 to 3, as
in red hæmatite, Fe,O,, the Latin prefix sesqui, meaning one and
a half, is used, and the oxide is termed a sesquioxide. The
same mode of designation applies to the compounds of metal
with sulphur, chlorine, &c.: thus we speak of iron sesqui-
sulphide,
or if we please, sesqui-sulphide of iron FeS, of
antimony trichloride, or, if we prefer it, the trichloride of
antimony, SbCl..
In the case of metals, such for instance, as iron and mercury
which form two distinct series of compounds, one corresponding
to a lower oxide, and another to a higher one, it is customary to
122
GENERAL PRINCIPLES OF THE SCIENCE
use the endings "-ous" and "-ic" (introduced by Berzelius ¹)
to denote the difference between the two sets of compounds.
Thus we have the mercurous and the mercuric salts. Among
others, mercurous oxide, Hg,O, mercurous chloride, HgCl (com-
monly called calomel), mercurous nitrate, HgNO3; and, on the
other hand, mercuric oxide, HgO, mercuric chloride, HgCl, (com-
monly called corrosive sublimate), mercuric nitrate, Hg(NO3)2
In the same way we have the ferrous salts (from fer-
rum, iron) corresponding to ferrous oxide, FeO (also termed
the monoxide), and the ferric salts corresponding to ferric
oxide, Fe2O3.
The endings -ous and -ic are applied not only in the case
of oxides and chlorides but also in that of acids. Thus
sulphurous acid, H,SO,, contains less oxygen than sulphuric
acid, H,SO; nitrous acid, HNO,, less than nitric acid, HNO3;
and carrying this distinction still further, the names of salts of
acids ending in -ous terminate in -ite, whilst those derived from
acids in -ic end in -ate; thus for example-
Nitrous acid forms salts termed nitrites.
Sulphurous acid
sulphites.
nitrates.
sulphates.
Nitric acid
Sulphuric acid
23
""
3
در
93
29
""
With respect to the nomenclature of acids and salts some
difference of opinion has been expressed by chemists, and
hence a certain amount of confusion exists in chemical writings.
Lavoisier, when he devised the present scheme of chemical
nomenclature, believed that it is oxygen (oğús, acid, yevváw, I
produce) which gives to the bodies formed by combustion in the
gas their acid characters, and hence the highest oxides of the
metals and non-metals were termed acids, thus P₂O, was called
phosphoric acid,, CO₂ carbonic acid, CrO, chromic acid, &c.
The ordinary well-known substances possessing acid properties,
such as nitric acid, HNO3, and sulphuric acid, H₂SO4, were
looked upon as hydrates of the anhydrous oxides, N₂O and
SO3, from which they may be obtained by the action of water,
thus:-
5
N₂O+H₂O=2HNO。
and SO₂+H₂O=H₂SO4.
1 Journ. de Physique, Oct. 1811.
NOMENCLATURE OF ACIDS AND SALTS
123
Acid bodies were, however, next discovered, such as hydrochloric
acid, HCl, hydrofluoric acid, HF, and hydrocyanic acid, HCN,
which contain no oxygen; and thus it appears that Lavoisier's
notion that the presence of oxygen is alone necessary to form an
acid is incomplete, and a more correct definition of an acid is that
it is a hydrogen compound, in which the whole or a part of the
hydrogen is capable of being replaced by a metal; in other words
an acid is an hydrogen salt. Nitric acid, therefore, is hydrogen
nitrate or hydric nitrate, HNO3, and by replacing the hydrogen
by the metal potassium we obtain potassium nitrate or nitrate
of potassium, KNO. Sulphuric acid is hydrogen sulphate or
hydric sulphate, H,SO,, and either one or both the atoms of
hydrogen can be replaced by potassium, giving rise, in the first
instance to a salt termed hydrogen potassium sulphate (or com-
monly bisulphate of potash) HKSO4, and in the second case to
potassium sulphate (commonly termed sulphate of potash),
KSO Acids which contain one atom of hydrogen replaceable
by metals are called monobasic, those containing two, dibasic and
those containing three tribasic. Nitric acid, HNO3, is a mono-
basic acid, sulphuric acid, H₂SO,, a dibasic, and phosphoric acid,
H.PO,, in which all the atoms of hydrogen can be replaced, a
tribasic acid.
The anhydrous oxides (such as NO, and SO), from which
the acids are derived, may be best termed anhydrides or acid-
forming oxides, whilst the oxides which have the power of acting
as bases and of forming salts when brought into contact with
acids, are termed basic oxides.
62 At the time when our nomenclature was invented all
salts were supposed to be compounds of an acid and a base;
and names were given which indicated the fact that when
the acid and the base are brought together a neutral salt is
produced; thus, if we add potash (the base) to sulphuric acid
(the acid) a salt is formed to which the name sulphate of potash
was given, and this view of the formation of salts being still
held, the name indicating this view is still commonly used.
Where the acid is combined with a heavy metallic oxide, as for
instance when oxide of lead is dissolved in an acid such as
nitric acid, the common name nitrate of lead, or more simply
lead nitrate, does not exhibit the analogy between this salt and
that obtained by adding nitric acid to potash and called nitrate
of potash. In order to assimilate these names some chemists
have termed the first nitrate of oxide of lead, corresponding to
124
GENERAL PRINCIPLES OF THE SCIENCE
nitrate of potash (potash being oxide of potassium); whilst
others, to avoid the recurrence of the word " of," and to shorten
the names, prefer to mention in the name of the salt not the
base but the metal or basylous group, so that the similar names
of lead nitrate and potassium nitrate become the designation of
both compounds. Other chemists prefer to modify the termina-
tion of the name of the metal, making it an adjective, thus:-
potassic nitrate, and, as the common word lead does not lend
itself to such adjective forms, we are compelled to use the Latin
word and term the salt plumbic nitrate.
In this work no special system of nomenclature will be
adopted to the exclusion of every other system. As a rule,
however, the ordinary name of the metal will be retained for
the salts, thus :-lead nitrate, zinc sulphate, potassium chloride.
But this will not preclude the occasional use of the com-
mon terms, as nitrate or carbonate of soda, whilst such names
as ferrous- and ferric-, mercurous- and mercuric- salts, will of
course be employed.
We define an acid to be a hydrogen salt and, therefore, HNO3
will be, as a rule, termed nitric acid: the names hydrogen
nitrate, or hydric nitrate, may sometimes be used. Bodies such
as N2O5, SO3, CrO3, will not be termed acids but are referred to
as anhydrides or acid-forming oxides. In some few instances the
body CO₂ may be mentioned as carbonic acid or carbonic
acid gas, owing to the fact that it has for a long time.
been so called; but the systematic name by which it will be
designated in these pages is carbon dioxide.
The following comparison of some of the older and common
and the scientific and more modern names of important acids
and salts may prove useful.
Older and Common Name.
Nitric acid
Nitrous acid
Sulphuric acid
Sulphurous acid
Chloric acid
Chlorous acid
Hypochlorous acid
ACIDS.
Formula.
HNO3
HNO2
H₂SO4
H.SO
HClO3
HClO2
HCIO
Modern and Scientific Name.
Hydrogen nitrate.
Hydrogen nitrite.
Hydrogen sulphate.
Hydrogen sulphite.
Hydrogen chlorate.
Hydrogen chlorite.
Hydrogen hypochlorite.
NOMENCLATURE OF ACIDS AND SALTS
125
¦
Older and Common Name.
Nitrate of potash
Nitrate of silver
Sulphate of lime
Sulphite of lead
Chlorate of potash
Chlorite of soda
Hypochlorite of potash
Protosulphate of iron
Perchloride of iron
SALTS.
Formula.
KNO3
AgNOg
CaSO4
PbSO3
KCIO3
NaClO,
KCIO
FeSO
FeCl
Modern and Scientific Name.
Potassium nitrate.
Silver nitrate.
Calcium sulphate.
Lead sulphite.
Potassium chlorate.
Sodium chlorite.
Potassium hypochlorite.
Ferrous sulphate.
Ferric chloride.
This system of nomenclature is, however, by no means
perfect, nor is it universally carried out. Were we to do so
long and inconvenient names would have to be used. Thus
instead of the common name alum, we should have to use the
words potassium aluminium sulphate, and for bitter-spar the
name calcium magnesium carbonate. Hence we shall often use
the common instead of the strictly scientific names, as common
salt for sodium chloride, caustic potash for potassium hydroxide.
sulphuric acid for hydrogen sulphate, and nitre or saltpetre for
potassium nitrate.
63 All the non-metallic elements form volatile compounds with
hydrogen, and if these be compared it is found that they fall
into four classes; this is apparent in the following table, which
contains the molecular formulæ of these compounds :
THE NON-METALLIC ELEMENTS.
I. Hydrogen. Hydrofluoric Hydrochloric Hydrobromic
acid.
acid.
acid.
II.
IV.
Hì
H
Water.
H}O
III. Ammonia.
H
HN
H
HI
FS
Marsh Gas.
H
H
H
H
Hì
}}}
Sulphuretted
Hydrogen.
HIS
HS
Phosphuretted
Hydrogen.
H
HP
H
C
Cl
H
HS
H
Seleniuretted
Hydrogen.
Br S
H
H
H
H
Se
Arseniuretted
Hydrogen.
H
HAs
H
}
Siliciuretted
Hydrogen.
Si
Hydriodic
acid.
HI
Н
I
Telluretted
Hydrogen.
HI
HS
Te
Hydride of
Boron.
H
HB
H
It appears from this that the elements differ in their be-
haviour, one atom of some of them combining with one atom of
hydrogen, whilst an atom of others combines with two atoms of
hydrogen, and others again with three and four. Each element
therefore possesses a certain combining power or valency as it is
called.
VALENCY.
127
The elements of the first group in the preceding table are
said to be monovalent, those of the following groups being
divalent, trivalent and tetravalent. This is also sometimes
expressed by saying that the elements of the first group are
monads, and those of the succeeding ones dyads, triads and
tetrads. So long as we only examine the compounds of the non-
metals with hydrogen, the relations are simple and definite; but
when the comparison is extended to their compounds with other
elements, it is found that the valency does not possess a constant
value. Phosphorus, for example, combines with chlorine in two
different proportions, producing the compounds phosphorus tri-
chloride, PCI, and phosphorus pentachloride, PCI,, in the first
of which one atom of phosphorus is united with three atoms of
chlorine, whilst in the second it is combined with five. It is true
that when the latter of these substances, PC,, is heated, it is
decomposed into the simpler molecules PC1, and Cl, and hence
the conclusion was at one time drawn that the combination
between the molecules PCl, and Cl, to form PCI, differs in
some way from that between the atoms of phosphorus and
chlorine to form PC1. Substances of this kind, which cannot
be vapourised without decomposing into simpler molecules were
then called molecular compounds, and the valency of an element
measured by the number of atoms of monad elements combining
with it to form a compound vapourising without decomposition.
The whole question turns upon the molecular weights of the
compounds under discussion, and although phosphorus penta-
chloride cannot be vapourised without decomposition, it is found
that in solution its molecular weight does correspond to the
formula PCI. In the light of our present knowledge, there-
fore, these distinctions cannot be maintained, and the valency
of the elements must be looked upon as a variable quantity. A
more definite value appears to attach to the maximum valency
displayed by the elements in particular classes of compounds,
which can be ascertained from the molecular formulæ of their
compounds, but even this is subject to exceptions.
Elements which are of equal valency combine with and replace
one another atom for atom, whilst one atom of a divalent element
can replace or combine with two monovalent atoms, and a trivalent
atom either three monovalent or one mono- and one divalent
atom, as is seen in the following equations:
2 HI+Cl₂ = 2 HCl + I2
128
THE NON-METALLIC ELEMENTS
с
H
H
H
+2 28}
CI
P Cl +
Cl
=
Nitric acid..
Sulphuric acid
Phosphoric acid
Iodic acid
•
H}O = P{2}
+2
.
64 A careful study of the chemical properties of substances
teaches that in many cases special relations exist between the
different atoms of which the molecule is made up, and these may
be expressed in formulæ which are known as constitutional
formulæ. The properties of many oxy-acids for instance show
that their hydrogen atoms stand in a special relation to some
of their oxygen atoms. This is expressed in the following
constitutional formulæ for some of the commoner acids:
.
cfo
10+2
.
·
H
HS
Η
·
O
HO.NO.
(HO) 2.SO2.
(HO),PO.
HO.IO.
The group or radical (HO) which appears in all these formula
is known as the hydroxyl group and behaves as a monovalent
radical, since, although it is not known in the free state, it is
found in water and other compounds combined with or re-
placing a monovalent atom.
It is only after the constitution of a compound has been
ascertained that it is possible to determine the valency of the
atoms of which it is composed. Thus the formula of iodic acid,
HIO,, might be either that given above or
H.0.0.0.I.
In the former case, which seems to be the more probable (p. 333),
the atom of iodine, being directly combined with two divalent
atoms and a monovalent group, would be pentavalent, whilst in
the latter it would be monovalent.
HYDROGEN
129
H = 1.
HYDROGEN.
65 It has already been stated (see Historical Introduction)
that water was long supposed to be an elementary or simple
substance, and it was not until the year 1781 that Cavendish
proved that water was produced by the union of oxygen and
hydrogen gases, whilst Humboldt and Gay-Lussac first showed
in 1805 that these gases combine by volume in the simple
relation of one to two. Turquet de Mayerne at the commence-
ment of the seventeenth century had indeed obtained an
inflammable gas by the action of dilute oil of vitriol on
iron, but the true nature of this gas was first ascertained
by Cavendish in 1766,1 when he showed that hydrogen was a
peculiar gas to which he gave the name of " inflammable air."
Hydrogen occurs almost solely in a state of combination in
nature, although it has been found to exist in the free state
mixed in small quantities with other gases in certain volcanic
emanations. It has also been found by Graham as occluded
gas in the meteoric iron from Lenarto, and by Mallet in a
meteorite from Virginia. It is produced in the decay and
decomposition of various organic bodies, being found in the
intestinal gases of many animals, as also, according to Sadtler,
in the gases given off by the oil-wells of Pennsylvania.
In a state of combination hydrogen occurs in water, of which
it constitutes very nearly one ninth part by weight (exactly
11:18 per cent.), and from this it derives its name (vdwp, water;
and yevváw, I give rise to). Hydrogen likewise occurs in
nature, though in smaller quantities, combined with sulphur,
phosphorus, chlorine, bromine, iodine, and nitrogen, whilst it
forms an essential portion of nearly all organic substances.
of one
66 Preparation.—(1) Pure hydrogen is best prepared by the
electrolysis of acidulated water. For this purpose a mixture
part by weight of pure sulphuric acid with ten parts
of water is placed in the glass decomposing cell (Fig. 24).
The positive pole consists of a platinum wire (a) melted through
the glass and placed in contact with mercury amalgamated with
1 66
Experiments
on Factitious Air." Phil. Trans. 1766, p. 144.
Bunsen, Pogg. Ann. 83, 197. Ch. St. Claire Deville, Compt. Rend. 55, 75.
3 Proc. Roy. Soc. 15, 502.
Proc. Roy. Soc. 20, 365.
10
130
THE NON-METALLIC ELEMENTS
zinc (b), whilst the negative pole (c) is composed of a platinum
plate. When the current from two or three of Bunsen's
elements is passed through the apparatus a constant stream of
pure hydrogen is evolved, and after being washed by the small
quantity of sulphuric acid contained in the bulbs (d), the gas
may be collected for analytical purposes. The oxygen of the
water is all absorbed by the zinc amalgam, oxide of zinc, and
ultimately zinc sulphate being formed, whilst the whole of the
hydrogen is evolved in the pure state. Instead of dilute

X
g
n
FIG. 24.
sulphuric acid, dilute solutions of caustic potash or soda are
frequently employed.
(2) By acting on water with the alkali metals, or with an
amalgam of sodium or potassium. In this case the metal re-
places an equivalent quantity of hydrogen in the water, hydrogen
gas and the soluble hydroxide of the metal being formed,
thus:
K₂+2H20=2KHO+H2.
When a small piece of potassium is thrown into a basin of
water, it swims about on the surface, and with a hissing noise
PREPARATION OF HYDROGEN
131
bursts into flame; this is due to the fact that the metal in
uniting with the oxygen of the water evolves heat enough to
melt the metal and to ignite the liberated hydrogen, which
then burns with a flame coloured violet by the presence of
the vapours of the metal. Sodium, likewise, decomposes water,
but the hydrogen in this case does not take fire spontaneously
unless the water be hot, or the motion of the bead of metal
be stopped, as when the metal is thrown on to a viscid starch-
paste or on to a moistened sheet of blotting-paper, in which
cases the globule of melted metal remaining in one place be-
comes hot enough to cause the ignition of the hydrogen, which
then burns with the yellow flame characteristic of the sodium.

1
LALLING
FIG. 25.
compounds. If the blotting-paper be previously stretched upon
an inclined wooden tray and moistened with a red solution.
of litmus, the track of the molten potassium or sodium, as it
runs over the paper, will be seen by a blue line showing the
formation of an alkaline product. In order to collect the
hydrogen thus evolved, the small clean globule of sodium may
be caught and depressed below the surface of the water by
means of a little sieve of wire-gauze under the open end of a
cylinder 1; the bubbles of gas then rise and may be collected, as
shown in Fig. 25.
Explosions may ensue if the sodium adheres to the glass.
132
THE NON-METALLIC ELEMENTS
(3) By passing steam over red-hot iron wire or iron borings
placed in an iron tube and heated in a furnace as shown in
Fig. 26, (a) being a retort in which water is boiled. The iron
is converted into the black or ferrosoferric oxide Fe3O4, and
hydrogen is evolved, thus:
3Fe+4H,O=Fe3O4+4H2.
(4) The most convenient mode of preparing hydrogen gas for
ordinary use where absolute purity is not requisite, is by the
action of sulphuric acid, diluted with six to eight times its
weight of cold water, upon metallic zinc; the water must be
added because if none be present the zinc sulphate ZnSO
formed in the reaction coats the surface of the metal, which is
thus protected from the action of the acid. Hydrochloric acid

FIG. 26.
diluted with twice its weight of water may also be employed,
and poured upon clippings of metallic zinc contained in a gas-
generating bottle. Other metals, such as iron, may be used
instead of zinc, and magnesium is sometimes employed where
a very pure gas is required. The acid is gradually poured
upon the metal by means of the tube funnel, and the evolved
gas can be collected in cylinders over the pneumatic trough as
shown in Fig. 27. The above reactions are represented as
follows:
4
H₂SO₁+Zn = ZnSO4+H2.
2HCl+ZnZnCl2 + H.
Care must be taken that all the air is expelled from the flask
before the gas is collected, and in order to ensure freedom from
air the gas is first allowed to fill an inverted test-tube, which is
PREPARATION OF HYDROGEN
133
then brought mouth downwards to a flame; if the hydrogen
burns quietly all air has been expelled, if it burns with a slight
explosion the evolution must be allowed to continue before the
gas is collected.
Hydrogen thus prepared always contains small quantities of
impurities derived from the materials used; these can be got
rid of by passing the gas though various absorbents, Of these
impurities the most common are arseniuretted hydrogen, when
the zinc, iron, or acid contains arsenic; phosphuretted hydrogen,
when they contain phosphorus; nitrous fumes when the acid
contains nitric acid or nitrates; sulphur dioxide and sulphuretted
FIG. 27.
hydrogen when these gases are contained in the acid or when
diluted, sulphuric acid is allowed to come in contact
with the metal.
In order to purify the gas, the best method is to pass it
through two U-tubes, each one metre in length, filled with
broken glass; in the first tube the glass is moistened with an
aqueous solution of lead nitrate, which absorbs the sulphuretted
hydrogen; the second tube contains an aqueous solution of
silver sulphate, by which the arseniuretted and phosphuretted
hydrogen gases are arrested. After this the gas is passed
through a third tube containing pumice moistened with a strong
134
THE NON-METALLIC ELEMENTS
solution of caustic potash; then through two others, one con-
taining pumice moistened with strong sulphuric acid, and the
other phosphorus pentoxide, by means of which the gas is
thoroughly dried. When absolute purity is aimed at the use
of sulphuric acid for drying the gas has to be discontinued,
since sulphur dioxide is formed when hydrogen is dried in
this way.
When the hydrogen is evolved from metallic iron, or even
from impure zinc, the gas possesses a very unpleasant smell,
due to the presence of small quantities of volatile hydrocarbons
derived from the carbon contained in the metal. The best way
of removing this odour is to pass the hydrogen through a tube
filled with small pieces of charcoal which absorbs the hydro-
carbon. Another impurity which it is much more difficult to re-
move from hydrogen is atmospheric air. This is partly contained
dissolved in the liquids used in the preparation of the gas, but its
presence may also be due to the high diffusive power of hydrogen,
which causes it to escape through the pores of the cork and
caoutchouc, whilst at the same time a certain quantity of air
diffuses into the apparatus. In order to free the hydrogen from
traces of oxygen, the gas must be passed through a red-hot
tube filled with metallic copper, and then the water, produced
by the combination of the oxygen and hydrogen, absorbed by
passing the gas over phosphorus pentoxide. The nitrogen of
the air cannot be got rid of, so that its presence must be
prevented by a careful air-tight construction of the apparatus.
(5) Strong aqueous solution of potash dissolves metallic zinc
in presence of iron, hydrogen being liberated, and a compound
of zinc oxide and potash K,ZnO, being formed. This process
yields an inodorous gas:
2KHO + Zn = H₂ + K₂ZnO2.
Aluminium may also be used in place of zinc.
(6) When zinc is immersed in an aqueous solution of any
ammoniacal salt (except the nitrate), such as sal-ammoniac, at
40° in contact with metallic iron, hydrogen is also rapidly
evolved.¹
67 Properties.--Hydrogen is a colourless, tasteless, inodorous
gas it is the lightest substance known, being 14:391 times as
light as atmospheric air, and has therefore a density of 006949. By
1 Lorin, Compt. Rend. 60, 745.
PROPERTIES OF HYDROGEN
135
carefully weighing a glass globe, first empty, and then filled with
air and hydrogen, Regnault found that 1 litre of hydrogen
at 0° and under a pressure of 760 mm. of mercury, weighs at
the latitude of Paris, 0-089578 grams; he omitted, however,¹ to
make a correction for the compression of the glass vessel when
vacuous by the external atmospheric pressure, and if this
correction be introduced his results show that the litre of
hydrogen weighs 0-89894 grams under the above conditions, or
that 1 gram of hydrogen occupies 11.126 litres at the normal
temperature and pressure.
Hydrogen has been liquefied by Wroblewski2 and Olszewski ³;
the former cooled hydrogen under a pressure of 190 atmospheres
by means of boiling nitrogen, then quickly lowered the pressure
to 1 atmosphere, and obtained a grey foam-like mass, the tem-
perature being 208 211°. Olszewski compressed the gas to
150 atmospheres, cooled it by surrounding it with liquid air
boiling in a vacuum, and suddenly reduced the pressure to 40
atmospheres; it then forms a colourless liquid which is trans-
parent, and not grey as stated by Wroblewski. The liquid
obtained in a similar manner from electrolytic gas is likewise
colourless. The statement of Pictet, that hydrogen condenses
to a steel-blue liquid which by rapid evaporation yields solid
particles of the same colour, has since proved to be erroneous.
Hydrogen is an inflammable gas taking fire when brought
in contact with a flame, and combining with the oxygen of the
air to form water; it does not support ordinary combustion or
animal life; when pure it may be breathed without danger for
a short time, but it produces a singular effect upon the voice,
weakening it and rendering it of higher pitch. On combining
oxygen to form water, one gram of hydrogen evolves
heat sufficient to raise 34,462 grams of water from 0° to 1°
Centigrade, and this is termed the calorific power of hydrogen,
which is, therefore, equal to 34,462 thermal units or calories.
with
Hydrogen gas is very slightly soluble in water, 1 cc. of the
latter dissolving only 0-021 cc. at 0.5°; the numbers obtained
by Bunsen appeared to show that the solubility of hydrogen
remained constant between 0° and 20° but this has proved on
further investigation to be incorrect. The absorption coefficient
3
-
1 Rayleigh, Proc. Roy. Soc. 43, 356; Crafts, Compt. Rend. 106, 1662.
Compt. Rend. 100, 979.
Compt. Rend. 99, 133; 101, 238.
4 Ann. Chim. Phys. [5], 13, 145.
136
THE NON-METALLIC ELEMENTS
between 0° and 20° is given by the following interpolation
formula: 1
C = 0·02148 0.0002215t + 0.00000285t2.
Hydrogen is somewhat more soluble in alcohol than in water,
and its solubility diminishes with the temperature.
The
--

FIG. 28.
following interpolation formula gives the absorption coefficient
(C) in alcohol for temperatures, from 0° to 25°:-
C = 0.06925
0.0001487t + 0·000001t2.
68 Absorption of Hydrogen by Metals.-Graham 2 and Deville
and Troost have shown that hydrogen gas possesses the
peculiar capability of diffusing through the pores of certain
red-hot metals, such as iron, platinum, or palladium. When
—
1 Winkler, Ber. 24, 98; Timofejew, Zeit. Physik. Chem. 6, 141.
2 Graham, Proc. Roy. Soc. 15, 223; 16, 422; 17, 212 and 500.
8 Deville and Troost, Compt. Rend. 57, 894.
ABSORPTION OF HYDROGEN BY METALS
137
hydrogen gas is passed through a red-hot palladium tube, the
rate at which the hydrogen permeates the metal is such that
through a surface of one square metre, 3992-22 cc. of the gas
pass each minute, whereas the rate of permeability through the
same surface of platinum is 489.2 cc. and that through
a sheet of caoutchouc of the same thickness and area is
represented by the passage of 127.2 cc. of gas in the same
time.
The power of hydrogen to pass through hot iron, palladium, and
platinum, whilst it cannot pass through when the metals are cold,
probably depends on the fact that this gas is absorbed at a high
temperature, and does not require the assumption of anything
like porosity in the structure of the metals.
This property,
which has been termed by Graham. "occlusion," can be ex-
amined as follows: A known weight of the metal palladium
in foil or wire is placed in a small porcelain tube, glazed inside
and out, and connected by one end to a Sprengel's mercury pump,
which by the flow of mercury down the long tube (Fig. 28)
yields an almost perfect vacuum. The tube, having been ex-
hausted, is now heated to redness, and a stream of hydrogen
passed over the red-hot metal for some time, after which the
tube is allowed to cool; the current of gas is then stopped,
and the tube again rendered vacuous. Next, the tube is again
heated, and the gas, which is thus evolved and driven out by
means of the falling mercury, is collected and measured in the
divided jar placed at the lower end of the barometric tube
of the pump.
Of all metals palladium possesses this power of absorbing
hydrogen in by far the highest degree. A palladium wire was
found by Graham to absorb at a red heat 935 times its volume
of hydrogen and increased in length from 609-14 mm. to
618-91 mm., or 1.6 per cent. In another experiment the metal
showed an increase in bulk of 9827 per cent.
the ordinary temperature palladium absorbs 376 volumes of
Even at
the gas.
If palladium be employed as negative electrode in the electro-
lysis of water, it likewise very readily absorbs hydrogen, occluding
935 times its volume of hydrogen, the expansion being pro-
portional in all directions to the amount of hydrogen absorbed.
If the electrolysis be continued after the above point is reached,
palladium becomes supersaturated with hydrogen, the limit
supersaturation varying with the strength of the current;
the
of
138
THE NON-METALLIC ELEMENTS
the excess of hydrogen is, however, evolved immediately the
current ceases.¹
The appearance of the metal does not undergo any change
after this absorption of hydrogen, but its specific gravity and its
conducting power for heat and electricity as well as its tenacity.
are somewhat diminished, though to a much less degree than
would probably be the case by the similar admixture of any
non-metallic substance. For various reasons Graham concluded
that the hydrogen is not chemically combined with the palla-
dium, but rather that the hydrogen assumes the solid form
and acts as a quasi-metal, giving rise to a kind of alloy, such,
for instance, as is obtained when sodium and mercury are
brought together. The name Hydrogenium has been given
to this absorbed form of hydrogen, and its specific gravity has
been calculated from the expansion of alloys of palladium
with platinum, gold, and silver, when charged with hydro-
gen to be 0733, whereas the number obtained from experi-
ments with pure palladium (in which the wire, after heating,
does not return exactly to its original volume) is 0.863.
these two numbers, the former is probably the most trust-
worthy, but subsequent determinations by Dewar 2 give a specific
gravity of 0.620 to hydrogenium, which is equal to the con-
densation of 7 litres of gas into the space of 1 cc. According
to Graham, hydrogenium is distinctly magnetic (more so than
palladium); and has an electric conductivity of 599, that of
palladium being 810, and that of copper 100. Troost and
Hautefeuille have shown that when hydrogenized palladium is
heated in vacuo, the tension of the hydrogen given off rapidly
diminishes until the mass contains 600 vols. of H, this com-
position corresponding to the formula Pd,H, after which the
tension remains constant until the decomposition is complete.
It is therefore probable that a definite compound of this formula
exists, which has the power of absorbing further quantities of
hydrogen.
Of
Similar compounds of hydrogen with the alkaline metals
such as Na,H and K,H have been prepared by the last-
mentioned chemists, and from the observed densities of these
compounds as compared with those of the metals themselves,
1 Thoma, Zeit. Physik. Chem. 3, 69.
2 Dewar, Phil. Mag. [4], 47, 324 and 342.
3 Compt. Rend. 78, 686—690; Journ. Chem. Soc. 27, 660.
Compt. Rend. 78, 968.
EXPERIMENTS WITH HYDROGEN
139
the density of the combined hydrogen has been calculated to be
0-62, a number exactly agreeing with Dewar's observations.
Platinum at a red heat absorbs 38 times, and at 100° 0.76
times, its volume of hydrogen; and red-hot iron only 0.46 of
its volume.
The meteoric iron of Lenarto, containing 90.88 per cent. of
iron, yields when heated in vacuo 2.85 times its volume of a
gas consisting almost entirely (85.68 per cent.) of hydrogen.
This, coupled with the fact that under the ordinary pressure
iron absorbs only about half its volume of hydrogen, would
appear to show that the Lenarto meteorite has come from an
atmosphere containing hydrogen under a pressure much greater
than that of our own atmosphere, and thus we obtain an un-
expected confirmation of the conclusions drawn from spectro-
scopic observations by Huggins, Lockyer, and Secchi respecting
the existence of dense and heated hydrogen atmospheres in the
sun and fixed stars.
=
The spectrum of hydrogen consists essentially of four bright
lines-one in the red, identical with Fraunhofer's dark line c,
and one in the greenish blue coincident with the dark line F.
The wave-lengths of these four lines, according to Angström's
measurements are, C=6562, F 4861, Blue = 4340, and Indigo
=4101 (in 10 millionths of a millimetre).
68 Experiments with Hydrogen.-The following experiments
show that hydrogen is a very inflammable gas, burning with
a nearly colourless flame, but incapable of supporting ordinary
combustion.
(1) When a lighted taper is brought to the open end of a
cylinder filled with hydrogen, the gas will burn slowly and
quietly if the open end be held downwards; but quickly and
with a sudden rush of flame if the gas be allowed to escape by
holding the mouth of the jar upwards.
(2) That hydrogen does not support the combustion of a
taper may be shown by thrusting a burning taper into a jar of
hydrogen held with its mouth downwards; the gas inflames and
burns round the open end of the cylinder, but the taper goes
be rekindled on withdrawal at the flame of burning
may
out and
hydrogen.
(3) Or the stream of gas issuing from the drawn-out end of
a tube and furnished with a platinum nozzle attached to the
generating flask may be ignited, care being taken that all the
1 Graham, Proc. Roy. Soc. 15, 502.
140
THE NON-METALLIC ELEMENTS
air has previously been expelled, when the flame will burn with
a quiet and almost colourless flame.
(4) Owing to the lightness of hydrogen it may be collected by
upward displacement. A jar filled with air is placed over the
tube by which the gas escapes from the generating flask; in
a short time the lighter gas will have displaced (Fig. 29) the
heavier air, and the jar is found to be full of hydrogen.
FIG. 29.
J
(5) Another striking mode of showing the relative weight of
air and hydrogen has already been described in Fig. 3, page 43.
The suspended beaker-glass is equipoised by weights placed in
the pan at the other end of the beam of the balance, and the air
is then displaced by pouring upwards the hydrogen contained
in a large cylinder. The beam will no longer be horizontal,
and weights must be placed on the beaker-glass to restore the
equilibrium.
(6) Another experiment illustrating the same property of
EXPERIMENTS WITH HYDROGEN
141
hydrogen, is to fill a cylinder with the gas and to bring its mouth
downwards, together with another cylinder filled with air, also
mouth downwards; by gradually lowering the end of the hydro-
gen cylinder until the two cylinders come mouth to mouth, the
hydrogen will be found in the upper cylinder, whilst on stand-
ing for a moment or two the lower one will be found to be full
of air.

FIG. 30.
(7) Soap bubbles or small collodion balloons ascend when
filled with hydrogen gas; the caoutchouc balloons, now so com-
mon, are filled and expanded by forcing hydrogen in with a
syringe, as seen in Fig. 30. In consequence of its low specific
gravity, hydrogen used to be frequently employed for inflating
balloons, but at present coal gas is used for this purpose.
142
THE NON-METALLIC ELEMENTS
THE HALOGENS
FLUORINE.
69. FLUORINE occurs not uncommonly combined with calcium,
forming the mineral fluor-spar, or calcium fluoride, CaF2, crys-
tallizing in cubes and octahedra, and found in Derbyshire, the
Harz, Bohemia, and elsewhere. It is likewise contained in
other minerals, such as cryolite, a fluoride of aluminium and
sodium (3 NaF+AIF), found in Greenland, and occurs in smaller
quantities in fluor-apatite, yttrocerite, topaz, lepidolite, &c.
Fluorine has been detected in minute traces in sea-water and in
the water of many mineral springs. Nor is its presence con-
fined to the mineral kingdom, for it has been found in the
enamel of the teeth as well as in the bones of mammalia, both
fossil and recent, and it is said to have been detected in the
blood, in the brain, and in milk.
F-18.9
The fact that glass can be etched when it is exposed to the
fumes arising from fluor-spar heated with sulphuric acid, was
known towards the latter part of the seventeenth century.
Scheele first stated that fluor-spar was the calcium salt of a
peculiar acid, which he obtained in an impure state by distilling
a mixture of sulphuric acid and fluor-spar in a tin retort. Scheele
also prepared the gaseous tetrafluoride of silicon, SiF, by the
action of the acid thus produced upon silica. It is, however, to
the researches of Gay-Lussac, and Thénard,¹ that we are indebted
for the first reliable information concerning hydrofluoric acid.
The views then held concerning this compound were incorrect,
inasmuch as it was supposed to contain oxygen, and termed
fluoric acid, until Ampère in 1810, and subsequently Davy,
showed that this acid is analogous to hydrochloric acid, and
that fluor-spar, formerly termed fluate of lime, is, in fact, a
compound analogous to calcium chloride, containing the metal
calcium combined with an element similar to chlorine, termed
fluorine (from fluo, I flow, because of the use of fluor-spar as
a flux in smelting operations). Even up to recent years the
nature and constitution of the fluorine compounds has been
discussed; and it is only within the last two or three decades that
1 Ann. Chim. Phys. [1], 69, 204.
FLUORINE
143 ·
Gore's researches taken together with the preparation of organic
fluorides have definitely proved the true analogy of the hydrogen
compounds of fluorine and chlorine, while quite recently Moissan
has succeeded in isolating fluorine, and thus has solved one
of the most difficult problems of modern chemistry. The
reason why fluorine has for so long resisted the innumerable
attempts which have been made to isolate it, will be easily
understood from its properties.
70 Moissan obtained fluorine by the electrolysis of pure anhydrous
hydrofluoric acid in which some potassium hydrogen fluoride was
dissolved in order to enable the liquid to conduct the electric
E
p-
LONGH
TANICHIDAD
ERMARINHOG
FIG. 31.
E
current which hydrofluoric acid by itself does not do. The
latest form of apparatus employed by Moissan consists of a U-
shaped tube of iridioplatinum with two small platinum side
tubes attached, and possessing a capacity of about 160 cc., in
which a mixture of about 100 grams of anhydrous hydrofluoric
acid and twenty grams of potassium hydrogen fluoride is placed.
The construction of the vessel is seen in Fig. 31; the open ends
are closed by stoppers (F) of fluor-spar, ground so as nearly to
fit the tube and wrapped round by thin platinum foil. The
144
THE NON-METALLIC ELEMENTS
electrodes of iridio-platinum (bb) pass through the stoppers
which are held in position by brass caps and screws (E) the joints
being rendered air-tight by placing leaden washers at p, and

Jay
лесе
00
006
weee
FIG. 32.
coating all the surfaces with shellac. During the electrolysis,
for which twenty-five Bunsen cells arranged in series are required,
the platinum U-tube filled with the mixture of hydrofluoric acid
and potassium fluoride is placed in a glass cylinder as shown in
FLUORINE
145
Fig. 32, into which liquid methyl chloride is passed from the
steel cylinder. This liquid at once boils and the temperature is
reduced to -23° at which the electrolysis is carried on. A second
glass cylinder surrounds that in which the methyl chloride is
evaporating, and contains fragments of calcium chloride to dry
the air and thus prevent the formation of hoar frost on the inner
cold cylinder.¹
Pure hydrogen is evolved from the negative pole and is carried
off by the platinum exit-tube to the left of the figure. Fluorine
is evolved at the positive pole and passes from the U-tube to a
spiral tube of platinum also placed in a glass cylinder containing
rapidly evaporating methyl chloride, so that the temperature is
kept at about-50°. This serves to retain hydrofluoric acid
FIG. 33.
vapours which are carried over with the gaseous fluorine, whilst
the latter passes on through two platinum tubes containing
lumps of sodium fluoride which salt absorbs the last traces of
hydrofluoric acid. The decomposition is not merely the simple
one represented by the equations
(1) 2KF = 2K + F₂
(2) 2K+ 2HF
=
2KF + H₂
inasmuch as the platinum electrode at which the fluorine is
liberated is much corroded with formation of a certain quantity
of a black powder consisting of platinum fluoride.
The volume
1 Ann. Chim. Phys. [6], 12, 473; [6], 24, 226.
11
146
THE NON-METALLIC ELEMENTS
of fluorine obtained with this apparatus is from three to four
litres per hour.
71 Properties of Fluorine.-Fluorine is a light greenish-yellow
gas, paler and more yellow in colour than chlorine, possessing a
penetrating odour resembling that of hypochlorous acid. It has
a sp gr. (air-1) of 1·265 and does not liquefy under atmospheric
pressure at a temperature of -93°. It does not fume in dry air
but in presence of moisture hydrofluoric acid is formed and
ozone set free, whilst the gas, even in small quantity exerts a
most irritating effect on the eyes and mucous membrane.
Fluorine is the most intensely active element with which we are
acquainted; with hydrogen it combines explosively even in the
dark and at temperatures as low as -23°. In order to observe
the action of fluorine on hydrogen and other gases a tube of
platinum (Fig. 33) closed at each end by plates of transparent
fluor-spar is employed. The fluorine is passed into the observa-
tion tube by one of the thin platinum side tubes, the other gas
by the second, whilst the resultant of the action passes out by
the platinum delivery tube. The direct combination of fluorine
and hydrogen may however be more simply shown by inverting
a jar filled with hydrogen over the positive exit tube of the
electrolyte apparatus. As soon as the gas comes in contact with
the hydrogen a blue red-bordered flame appears at the end of the
platinum tube, hydrofluoric acid being formed, which slowly at-
tacks the glass jar.
Fluorine is distinguished from all the other elements by its
inability to combine with oxygen, no combination taking place
when these two gases are brought together even at 500°, nor
does it combine directly with nitrogen.
To examine the action of fluorine on liquids and solids. it
suffices to place the substance to be examined in a test tube, and
to allow the fluorine to pass into the latter from the electrolytic
apparatus, the gas having no action on dry glass. In this way it
can be shown that fluorine at once liberates chlorine from potas-
sium chloride and carbon tetrachloride at the ordinary tempera-
ture; sulphur and selenium at once melt and take fire in the
gas, and tellurium like the former elements also combines
directly with formation of a fluoride. With iodine, bromine,
phosphorus, arsenic and antimony it combines with incandescence;
crystallised silicon, amorphous boron and finely divided carbon,
such as lamp-black and charcoal, when thrown into fluorine take
fire and burn with formation of fluorides. The action of fluorine
HYDROFLUORIC ACID
147
on the metals is even more decided than that which it has on
the non-metallic elements; the alkali-metals and those of the
alkaline earths ignite in the gas; lead is slowly transformed into
the fluoride, and finely divided iron becomes red hot on exposure
to the gas. Magnesium, aluminum, manganese, nickel and silver
when slightly warmed burn brightly in fluorine; gold is not at-
tacked at the ordinary temperature, but between 300° and 400°
becomes covered with a yellow coating of gold fluoride, and
platinum under similar conditions yields two fluorides, which
like the gold compound readily decompose into the metal and
fluorine at a dull red heat.
In order to prove that the gas evolved by electrolysis is in
reality fluorine and not a higher hydrogen compound of that
element, Moissan attached to the fluorine delivery tube a weighed
platinum tube containing iron wire, whilst to the negative
delivery tube an arrangement for collecting and measuring the
hydrogen was attached. On starting the electrolysis, and heating
the platinum tube containing the iron wire, the whole of the
fluorine was absorbed with formation of iron fluoride, whilst the
hydrogen simultaneously evolved was collected and measured.
As the mean of two experiments it was found that 79 cc. of
hydrogen weighing 0-00703 gram were obtained; this corre-
sponds to 0.1335 of fluorine whereas the mean increase of weight
of the iron was 0.135 gram.
The atomic weight of fluorine has been determined by several
chemists by converting either calcium fluoride, potassium fluoride,
or sodium fluoride into the corresponding sulphate. The mean
of fairly agreeing experiments gives the number 18.9.
FLUORINE AND HYDROGEN.
HYDROFLUORIC ACID. HF 19.9.
72 Anhydrous hydrofluoric acid, HF, is a volatile colourless
liquid, best obtained, according to Fremy¹ and Gore,2 by heating
to redness in a platinum retort the double fluoride of hydrogen
and potassium HF+KF, which has been previously fused. A
description of the process employed for preparing pure hydro-
Ann. Chim. Phys. [3], 47, 5.
2 Phil. Trans. 1869, 173.
148
THE NON-METALLIC ELEMENTS
·
fluoric acid may give an idea of the difficulty and danger of
chemical investigations on fluorine and fluorides, as well as of
the precautions which must be taken.
(1) For this purpose about 200 grammes of the fused salt was
placed by Gore in a platinum bottle, or retort (a, Fig. 34). No
vessels of glass, porcelain, or other substance containing silica
can be used in the preparation of this acid, as the silica is at
once attacked by hydrofluoric acid unless it is absolutely anhy-
drous, a volatile tetrafluoride of silicon and water being formed,
thus:-
4HF+SiO2=2H₂O+SiF.
---
The platinum bottle was then gently heated so as to fuse the
salt, and thus completely drive off any traces of water. The
long platinum tube was then connected by means of a lute of
fused sulphur to the neck of the bottle, the condenser surrounding
this tube being filled with a freezing mixture poured through
FIG. 34.
the open tube b, whilst the platinum bottle c, immersed in a
freezing mixture, was employed to receive the distillate. This
bottle was provided with an exit-tube of platinum, upon the
upper end of which a short angle tube g of platinum, turned
downwards, was fixed to prevent condensed moisture from running
down into the bottle. On gradually raising the temperature, the
fused salt begins to decompose, hydrofluoric acid is given off
as a gas, which condenses in the platinum tube and runs into
the platinum bottle. Great care must be taken to have all the
apparatus free from moisture, and the acid must be re-distilled
in order to remove traces of saline matter which are apt to be
HYDROFLUORIC ACID
149
carried over. According to Moissan, the acid thus obtained still
contains traces of moisture, which can only be removed by
subjecting the liquid to electrolysis, the water present being
then decomposed by the fluorine evolved with formation of
hydrofluoric acid and ozone.
The acid thus obtained is a highly dangerous substance, and
requires the most extreme care in its manipulation, the inhalation.
of its vapour having produced fatal effects. A drop on the
skin gives rise to blisters and sores which only heal after a very
long period. From its great volatility the anhydrous acid can
only be safely preserved in platinum bottles having a flanged
mouth, a platinum plate coated with paraffin being tightly
secured to the flanged mouth by clamp screws. The acid must
be kept in a cool place not above a temperature of 15°, other-
wise it is very likely to burst the bottle, and a freezing mixture
should always be at hand when experimenting with it (Gore.)
กาลก
FIGS. 35, 36.

g
Anhydrous hydrofluoric acid can also be obtained by acting on
dry silver fluoride with hydrogen.
(2) If the hydrofluoric acid is not required to be perfectly
anhydrous a much easier process than the foregoing can be
adopted. This consists in the decomposition of fluor-spar by
strong sulphuric acid, when hydrofluoric acid and calcium.
sulphate
are formed, thus:-
CaF2+H2SO4=2HF+CaSO4.
For this preparation vessels of platinum, or, on the large scale,
vessels of lead, can be employed. On heating the mixture, the
nearly anhydrous acid which 'distils over can either be condensed
1
Professor Nicklés, of Nancy, died in 1869 from accidentally breathing the
Vapour of this acid whilst endeavouring to isolate fluorine.
150
THE NON-METALLIC ELEMENTS
by passing through a tube placed in a freezing mixture, or into
a small quantity of water contained in a platinum dish if a dilute
acid be needed. The dilute acid may be preserved in gutta-
percha bottles, but this substance is at once acted upon by the
anhydrous acid.
One form of platinum apparatus used for preparing the gas is
shown in Fig. 35. The U-tube is placed in a freezing mixture
when the gas has to be condensed. If an aqueous solution of
the acid is needed, the arrangement shown in Fig. 36 may be
employed. It consists of a leaden retort, a, on to which a leaden
head, c, can be cemented at bb'. The neck of the retort fits into
a leaden receiver at e, in which is placed a platinum basin con-
taining water. The acid vapours are absorbed by the water, and
thus a solution of the acid is obtained free from lead, which
would not be the case if the water had been simply placed in
the leaden vessel. The tube g serves to allow the escape of air
and of excess of hydrofluoric acid gas.
73 Properties. The specific gravity of liquid anhydrous hydro-
fluoric acid at 15° is 0·9879 (Gore), or it is a little lighter than
water. It boils at 19° 4, and solidifies at -102°-5, and melts again
at 92°3. The tension of its vapour at 15° is 390mm. If it is
perfectly dry it does not act on glass; the slightest trace of
moisture, however, renders it capable of doing so. The acid
scarcely acts upon the metalloids or on the noble metals,
and the other metals do not decompose the acid below 20°.
Potassium and sodium dissolve in it as in water with
evolution of hydrogen and formation of a fluoride; it decom-
poses the carbonates with effervescence and with formation of
fluorides.
-
The composition by volume of the anhydrous acid was ascer-
tained by Gore by measuring the volume of hydrogen needed to
combine with the fluorine contained in a given weight of silver
fluoride. From this and other experiments he arrived at the
conclusion that one volume of hydrogen necessarily yields two
volumes of hydrofluoric acid gas, and that this contains for every
one part by weight of hydrogen 191 parts by weight of
fluorine.
Thorpe and Hambly¹ have shown that the vapour density of
hydrofluoric acid varies rapidly with variation of temperature
1 Journ. Chem. Soc. 1889, 1, 163.
HYDROFLUORIC ACID
151
and pressure. The following table gives the results of their
experiments at temperatures between 26°4 and 88°1 :—
Pressure
Temp. of Vapour V.D. air 1
in mm.
26°.4
745
27°.8
746
29°.2 750
32°0
743
33°.1 750
33°.8 758
36°.3 739
38°.7 751
39°2
743
42°.8
741
4723
745
57°5
750
69° 4
746
8821
741
1.773
1.712
1.578
1.377
1.321
1.270
1.115
1.021
1.002
0.910
0.823
0737
0.726
0.713
Molecular
weight.
51.18
49.42
45.54
39.74
38.12
36.66
32.20
29.46
28.94
26.26
23.76
21.28
20.96
20.58
These numbers show that the process of dissociation of the
vapour of hydrogen fluoride is quite continuous, and that there-
fore there is no evidence of the existence of molecules H,F,
as was formerly supposed. It was also found that the vapour
density is lowered by diminishing the pressure of the gas at
a constant temperature of about 32°.
Hydrofluoric acid is very soluble in water, the specific gravity
of the solution rising to 125. The concentrated aqueous acid
becomes weaker on boiling until at 120° it attains a constant
composition of from 36 to 38 per cent. of the anhydrous acid,
but it does not thus form a definite hydrate; when allowed to
evaporate over caustic lime in the air, the aqueous acid attains
a constant composition containing 32 6 per cent. of the anhydrous
acid 1
74 Qualitative Detection of Fluorine.-In order to test for the
presence of hydrofluoric acid, its power of etching on glass is
made use of. For this purpose a small flat piece of glass is
covered with a thin and even film of melted bees'-wax, and,
after cooling, some lines or marks are made by removing the wax
by a sharp but not a hard point. The dry substance to be tested
¹ Roscoe, Journ. Chem. Soc. 13, 162.
152
THE NON-METALLIC ELEMENTS
is placed in a platinum crucible or small leaden cup, and covered
with strong sulphuric acid, the crucible being gently warmed;
after the lamp has been removed, the slip of covered glass is
placed on the crucible and allowed to remain for ten minutes.
The wax can then be re-melted and wiped off with blotting
paper, when the etching, indicating the presence of fluorine,
will be seen. In performing this experiment it is well to re-
member, on the one hand, that if the quantity of fluorine present
be very small, the etching may not at once be visible but may
become so by breathing on the surface of the glass, whilst, on
the other hand, if the point employed to remove the wax be a
hard one, a mark or scratch may sometimes thus be seen on the
glass when no fluorine is present
75 Fluorides. The compounds of fluorine with the metals are
best formed by acting on the metal or on its oxide, hydrate, or
carbonate, with hydrofluoric acid. The fluorides of the alkalis
of silver as well as those of most heavy metals dissolve in water;
those of the alkaline earths are insoluble; and those of the
earths, with the exception of fluoride of yttrium, are soluble in
water. Most of the fluorides unite with hydrofluoric acid to form
crystalline compounds, which are termed the acid fluorides. They
also have a remarkable facility of union among themselves,
giving rise to double salts, which frequently crystallise well.
They are all decomposed by treatment with sulphuric acid,
yielding hydrofluoric acid and a sulphate, whilst some, as the
silver salt, even undergo the same decomposition in presence of
hydrogen alone.
The divisions on the glass of eudiometers and thermometers
are etched by hydrofluoric acid, which is evolved from a mixture
of fluor-spar and strong sulphuric acid in a long leaden trough,
over which are placed the glass tubes covered with wax, and
having the divisions marked upon them by scratching off the wax
The etching is best effected in the cold, and with anhydrous
hydrofluoric acid; the tube must in this case be exposed for
some hours to the action of the gas, and the trough covered with
several folds of thick paper.
CHLORINE
153
1
CHLORINE. Cl. = 35°19.
76 Chlorine gas was first obtained and its properties first
examined by Scheele in 1774; he prepared it by the action.
of hydrochloric acid on manganese ore, and termed it "dephlo-
gisticated marine acid gas." Berthollet, in 1785,2 showed that,
according to the then prevailing antiphlogistic theory, chlorine
could be regarded as a compound of hydrochloric acid gas
with oxygen, and this view of its constitution was held until
the year 1810, when Davy satisfactorily proved the elementary.
nature of the gas and gave it the name which it now bears
Após, greenish-yellow), Gay-Lussac and Thénard having,
in the year 1809 thrown out the suggestion that it might be
considered to be a simple body.
Chlorine does not occur in the free state in nature, but is
found in large quantities combined with the alkali metals,
forming the chlorides of sodium, potassium, and magnesium,
which constitute the largest proportion of the solid components
of sea-water. Sodium chloride, NaCl, also occurs as rock-salt,
in large deposits in the tertiary formation in various localities,
whilst the chloride of potassium, although occurring less
frequently, is found in certain localities, as in the salt-beds of
Stassfurt, in Germany, both in the pure state, as sylvine KCl, and
in combination with chloride of magnesium and water, as
carnallite KCIMgCl₂+6H₂O. The chlorides and oxychlorides of
several other metals also occur in nature, although in small
quantities; thus we have lead oxychloride PbCl,PbO, known
as matlockite; ferric chloride FeCl3, found in the craters
of volcanoes; silver chloride, or horn silver, AgCl; copper
oxychloride, or atacamite, Cu,CI(OH),, and many others.
The chlorides of the alkalis occur in the bodies of plants and
animals, and play an essential part in the economy of the
animal and vegetable worlds. Chlorine likewise occurs combined
with hydrogen, forming hydrochloric acid, a substance which
is found in nature in small quantities in certain volcanic gases.
Opusc. Tome 1, 247.
2 Mém. de l'Acad des Sciences, Paris, 1785, p. 276.
3 Phil. Trans. 1811, pp. 1 and 32; Bakerian Lecture for 1810, read Nov. 10th,
+ Memoires d'Arcueil. Tome 2, 357.
1810.
154
THE NON-METALLIC ELEMENTS
77 Preparation.-(1) Chlorine gas is prepared by the action
of the black oxide of manganese or manganese dioxide MnO2, on
strong hydrochloric acid, HCl, thus:-
4HC+MnO₂=Cl₂+ MnCl₂+2H₂O.
This reaction consists in the removal of two atoms of hydrogen
in two molecules of hydrochloric acid by union with one atom
of oxygen of the manganese dioxide to form water, the two
atoms of chlorine being set free; whilst the manganese mon-
oxide MnO, which may be considered as also being formed,
dissolves in the two remaining molecules of hydrochloric acid
to produce manganese chloride, MnCl2, and another molecule of
water, H₂O. When manganese dioxide and cold concentrated

FIG. 37.
hydrochloric acid are brought together a dark brownish-green
solution is formed, and this, on heating, evolves chlorine gas,
whilst manganese chloride, MnCl,, is formed. There can be
little doubt that this dark coloured solution contains a higher
and unstable chloride of manganese, probably MnCl, corre-
sponding to MnO2, which, on heating, decomposes into
MnCl₂+Cl₂.
For this preparation the oxide of manganese should be used
in the form of small lumps free from powder, and the hydro-
chloric acid poured on, so as about to cover the solid; on gently
heating, the gas is copiously evolved.
PREPARATION OF CHLORINE
155
(2) It is often more convenient for laboratory uses to evolve
the hydrochloric acid in the same vessel in which it is acted.
upon by the manganese dioxide, and to place a mixture of one
part of this substance and one part of common salt in a large
flask (Fig. 37) containing a cold mixture of two parts of strong
sulphuric acid and two of water; on very slightly warming the
mixture, a regular evolution of gas takes place. The change
which here occurs was formerly supposed to be represented by
the equation:-
2NaCl+3H₂SO¸+MnO₂=Cl₂+2NaHSO¸+ MnSO¸+2H₂O.
according to which the whole of the chlorine is evolved in the
free state. Klason has shown, however,¹ that this is not the case,
the correct equation being as follows:-
4NaCl+MnO₂+2H₂SO,=2NaHSO₁+Na,SO,+MnCl₂+2H₂O+Cl₂
In order to purify and dry the gas prepared by either of the
above methods, it is necessary to pass it, first through a wash
bottle (b, Fig. 38) containing water, to free it from any hydrochloric
acid gas which may be carried over, then through a second wash
bottle (a) containing strong sulphuric acid, to free it from the
larger quantity of the aqueous vapour which it takes up from
the water, and lastly through a long inclined tube (c) containing
pieces of pumice-stone, moistened with strong and boiled sul-
phuric acid. The tube (d) which dips under water serves as a
safety-tube in case the evolution of gas becomes too rapid, when
the excess of gas can thus escape. In order to expel the air
which fills the apparatus, the evolution of the chlorine must
be allowed to go on until the gas is almost entirely absorbed
by a solution of caustic soda. As the crude black oxide of
manganese frequently contains carbonate of lime, the presence
of which will cause the admixture of small quantities of carbon
dioxide, CO, with the chlorine, it is advisable to moisten the
ore before using it with warm dilute nitric acid, which will
dissolve out the carbonate of lime, leaving the manganese
dioxide unacted upon; after well washing, the latter may be
used without danger of this impurity.
(3.) By heating a mixture of bichromate of potash (potassium
dichromate) and hydrochloric acid, chlorine gas can also be
¹ Ber. 23, 334.
156
THE NON-METALLIC ELEMENTS
obtained, chromium chloride and potassium chloride being
formed; thus:-
14HC1+K,Cr₂0,=3Cl₂+2CrCl₂+7H₂O+2KCl.
Chlorine gas is also evolved when an acid is added to an

FIG. 38.
alkaline hypochlorite or to bleaching powder; to prepare it from
the latter, the bleaching powder is made coherent by mixing
with plaster of Paris or by compressing it to form a solid cake
which is afterwards broken into lumps of suitable size.¹ As the
Winkler, Ber. 20, 184; Thiele, Annalen, 253, 239.
PREPARATION OF CHLORINE
157
1
evolution of the gas takes place at the ordinary temperature, the
Kipp's apparatus shown later under sulphuretted hydrogen may
be employed, by which a current of the gas can be obtained as
required.
(4) By passing a mixture of air and hydrochloric acid over
heated bricks, a portion of the hydrogen of the hydrochloric
acid is oxidised, water and chlorine gas being formed (Oxland,
1847). If the mixture of gases be allowed to pass over a heated
surface impregnated with certain metallic salts, especially sul-
phate of copper, the oxidation of the hydrogen of the hydro-
chloric acid goes on to a greater extent, and by the absorption of
the unaltered hydrochloric acid, a mixture of chlorine and nitrogen
gases can be obtained. This process, patented by the late
Mr. Henry Deacon, of Widnes, is used on a large scale for the eco-
nomic production of chlorine and bleaching powder. The singular
and but imperfectly understood decomposition which takes place
may be shown on a small scale by the following arrangement :-
The hydrochloric acid gas is evolved from the common salt and
sulphuric acid in the large flask a, Fig. 39, this gas passes into
the tube, b, in which are placed pieces of tobacco-pipe moistened
with a saturated solution of copper sulphate, the tube being
exposed to a gentle heat. As the hydrochloric acid enters
the tube containing the sulphate of copper it mixes with
atmospheric air which is driven in by the tube c, from the gas-
holder. On passing over the heated copper sulphate, the hydro-
chloric acid and the oxygen of the air act upon one another,
water and chlorine gas being formed according to the equation.
4HCl + O2 = 2H₂O + 2C12.
During the process the sulphate of copper remains unchanged
and may be used for a great length of time. The mixture of
chlorine, nitrogen, steam, and any undecomposed hydrochloric
acid pass by a bent tube into a bottle, e, containing water, by
which the last-named substance is arrested, together with a
portion of the steam which is condensed; the mixed gases, still
containing some aqueous vapour, are then passed through a
tube, d, containing calcium chloride, by which the gases are
completely dried, after which the chlorine mixed with the
nitrogen may be collected by displacement in a cylinder.
(5.) Chlorine gas is prepared for manufacturing purposes on
a large scale by means of reaction (1); the mixture of black
oxide of manganese and hydrochloric acid being placed in large
158
THE NON-METALLIC ELEMENTS

ESSAN
TOULO
FIG. 39.
square tanks, made of Yorkshire flags clamped together by iron
rods, and the joints made tight by a rope of vulcanised caout-
chouc. On heating the mixture by a steam-pipe the chlorine
PROPERTIES OF CHLORINE
159
gas is evolved. For a description of the details of this mode of
manufacture, see the paragraph on Bleaching Powder.
(6) A large number of processes have been patented during
the past few years for preparing chlorine from the calcium or
magnesium chlorides obtained in large quantities as a bye-
product in the manufacture of soda from common salt by the
ammonia-soda process (Vol. II., Part 1, p. 153), the most suc-
cessful of which is known as the Weldon-Pechiney process, and
is worked directly in connection with the soda manufacture.
The mother-liquors containing ammonium chloride are heated
with magnesia in order to recover the ammonia, and the residual
solution of magnesium chloride evaporated until it has the com-
position MgCl₂+6H,O, and then thoroughly mixed with 15 parts
of magnesia for every 1 part of magnesium chloride present. The
mass quickly hardens with evolution of considerable quantities
of heat, and is broken up into pieces of the size of a walnut, the
dust formed at the same time being added to a fresh portion of
magnesium chloride solution. The magnesium oxychloride.
MgOCI, thus formed is heated in air first to 250-300°, whereby
the water is evolved, together with at most 80% of the chlorine
in the form of hydrochloric acid. The temperature, is then
gradually raised until it reaches more than 1000°, the mixture
of chlorine, hydrochloric acid, and air being continuously drawn
off, the hydrochloric acid removed by washing with water and
the chlorine converted into chloride of lime or potassium
chlorate. 45% of the chlorine is evolved in the free state,
40%, as
hydrochloric acid, and 15° remains in the residue. The
latter is sifted, and the coarser portions, which contain all the
undecomposed chlorides, again heated, whilst the finely divided
portion, consisting of magnesia, is again employed for decom-
posing the ammonium chloride, or for converting the mag-
nesium chloride into the oxychloride.¹
78 Properties.-Chlorine at the ordinary atmospheric tempera-
ture and pressure is a transparent gas of a greenish-yellow
colour, possessing a most disagreeable and powerfully suffocating
smell, which, when the gas is present in small quantities only, re-
sembles that of seaweed, but when it is present in large quantities
acts as a violent irritant, producing coughing, inflammation of the
mucous membranes of the throat and nose, and when inhaled
in the
pure state even causing death.
1 See Dewar, Journ. Soc. Chem. Ind. 6, 775; Kingzett, Journ. Soc. Chem. Ind.
7,286.
160
THE NON-METALLIC ELEMENTS
The density of chlorine has been determined by Ludwig,' who
has shown that at the ordinary temperature it is equal to 2:48
(air-1), but that it gradually decreases with increase of tempera-
ture til at 200° it has the constant value of 2:45 (air = 1) or
35.26 (H1). The molecule of the gas therefore consists of
two atoms, and its molecular formula is Cl₂, the theoretical
density being 35:19. The density remains constant up to about
1200°, but above this temperature it begins to decrease, and at
1400 is only 2:02, the molecules Cl, being partly split into the
atoms at this temperature.2
One litre of the gas under the normal conditions weighs 3.208
grams.

MARVE
H
FIG. 40.
CI
When subjected to pressure at the ordinary temperature, or to
a temperature below - 34° under atmospheric pressure, chlorine
condenses to a liquid which solidifies at-102°,3 boils at -33.6°,+
and at 0° has a sp. gr. of 14405 and a vapour pressure of 366
atmospheres. The critical point of the liquid is 146°.5
Liquid chlorine has a yellow colour, with a tinge of orange in
thick layers, is not miscible with water, does not conduct
¹ Ber. 1, 232.
2 Langer and Meyer, Pyrochem. Untersuch, p. 46 (Vieweg, 1885).
3 Olzewski, Monatsh. 5, 127.
4 Faraday, Phil. Trans. 1823, 160.
Knietsch, Annalen, 253, 100.
COMBUSTIONS IN CHLORINE
161
electricity, and has a refractive index lower than that of
water; it is now prepared commercially and brought into the
market in cylinders containing 5 kilos. of the liquid.
Chlorine gas dissolves in about half its volume of cold water,
and as the gas instantly attacks mercury, it must either be col-
lected in the pneumatic trough over hot water, or by displacing
the air from a dry cylinder, as shown in Fig. 38, care being
H
CI
FIG. 41.
taken that the excess of chlorine is allowed to escape into a
draught cupboard, as represented in the drawing.
with
19 Combustions in Chlorine.-Chlorine is not inflammable,
and does not directly combine with oxygen; it unites, however,
great energy with hydrogen, forming hydrochloric acid
HC1, and to this property it owes its peculiar and valuable
bleaching, power. It also combines with many metals, giving
rise to a class of compounds termed the metallic chlorides.
In each case of combination with chlorine a definite quantity
of heat is given out, whilst sometimes light is also emitted, so
FIG. 42.
12
162
THE NON-METALLIC ELEMENTS
that the essential phenomena of combustion are observed. Thus
if we plunge a jet from which a flame of hydrogen burns into
a cylinder of chlorine gas (Fig. 40), the hydrogen continues to
burn, but instead of water being produced, hydrochloric acid is
formed by the combustion. In like manner, if we bring a light
to the mouth (held downwards) of a cylinder of hydrogen and
then bring this over a jet from which chlorine gas is issuing
(Fig. 41), a flame of chlorine burning in hydrogen will be seen.
If two equal sized cylinders, filled, one with dry chlorine
and the other with dry hydrogen, are brought mouth to mouth,
the two glass plates closing them withdrawn and the gases
allowed to mix, and if then a flame is brought near the mouths
of the cylinders, the mixed gases combine with a peculiar noise,
and dense fumes of hydrochloric acid gas are seen. This experi-
ment must, however, be made in a room partially darkened, or
performed by gas- or candle-light, as the two gases combine
with explosion in sunlight or strong daylight.
The following experiments are cited as showing the power
with which chlorine unites with hydrogen, even when the latter
is combined with some other element.
(1.) If four volumes of chlorine, Cl, be mixed with two
volumes of olefiant gas, CH, and a light applied to the
mixture, the chlorine immediately combines with the hydrogen
of the latter to form hydrochloric acid, HCl, while the carbon
is set free in the form of a black smoke thus :—C₂H + 2Cl₂
= 4HCl + 2C.
(2.) If a piece of filter-paper be dipped in oil of turpentine,
C10H16, and plunged into a jar of chlorine gas, the paper bursts
into flame; the chlorine combining with the hydrogen of the
turpentine, while the carbon is deposited.
(3.) When sulphuretted hydrogen gas, HS, is passed into
chlorine water, hydrochloric acid is formed by the union of the
hydrogen of the former with the chlorine of the latter, and the
sulphur is set free in the form of a light yellow precipitate.
(4) When a lighted taper is plunged into a jar of chlorine, it
continues to burn with a dull red light, and dense fumes as well
as a cloud of black smoke are emitted, arising from the com-
bination of the hydrogen of the wax with the chlorine, and the
liberation of the carbon.
In order to exhibit the combination of certain elements with
chlorine, whereby heat and light are evolved, the following
experiments may be made :-
EXPERIMENTS WITH CHLORINE
163
(1) Place some leaves of Dutch metal (copper in thin
leaves) in a flask provided with a stopcock (Fig. 42) and exhaust
the flask with the air-or water-pump; attach the outer end
of the stopcock to the neck of another flask containing moist
chlorine gas.
On opening the stopcock the chlorine will rush.
into the vacuous flask, and the copper leaf will take fire, dense
yellow fumes of copper chloride being formed. (2.) Finely
powdered metallic antimony thrown into a jar of chlorine gives
rise to a shower of brilliant sparks, chloride of antimony being
produced. If the jar be placed on the table over a powerful
down-draught, all risk of escaping fumes will be avoided. (3.)
A small piece of phosphorus placed in a deflagrating spoon and
plunged into a jar of chlorine first melts, and, after a few minutes,
bursts into flame with formation of the chlorides of phosphorus.
(4) Metallic sodium melted in a spoon also takes fire on im-
mersion in the moist gas, burning brightly, with the production
of common salt-sodium chloride; but it is a singular fact, first
observed by Wanklyn' that sodium may be melted in dry chlorine.
without any combination occurring, the surface of the molten
metal remaining bright and lustrous. Cowper has shown that
other metals are also not attacked by dry chlorine.
80 Alleged Allotropic Condition of Chlorine.-It has been stated
by Draper³ that chlorine which has been exposed to light, com-
bines with hydrogen more easily than that which has been kept in
the dark, and the conclusion has been drawn that chlorine exists
in two allotropic modifications. Subsequent careful experiments
made on this subject by Bunsen and Roscoe,' failed to detect the
slightest difference between insolated and non-insolated chlorine.
According to the observations of Budde 5 dry chlorine gas,
when exposed to the chemically active rays of the sun increases
in volume, and the expansion is not due to any action of the
heating rays.
This subject has been recently investigated by
Richardson, who has confirmed
Budde's results, and has
further shown that the expansion is proportional to the actinic
intensity of the light. If the light be previously passed through
cobalt
glass, expansion still takes place, though to a lesser
extent, but if faintly yellow or ruby glass be used no expansion
is observed.
1 Journ. Chem. Soc. 1883, i. 153.
3 Phil. Mag. for 1845, 27, 327; ditto for 1857, 14, 161.
A Phil. Trans. 1857, Part ii. p. 378.
Thil. Mag. 1871 [4] 42, 290.
2 Chem. News, 20, 271.
6 Phil. Mag. 1891 [5] 32, 277.
164
THE NON-METALLIC ELEMENTS
Richardson has availed himself of this property of chlorine
to construct a continuous recording actinometer. The apparatus
consists of two bulbs connected by a narrow tube, one of which is
filled with dry air and the other with dry chlorine, sulphuric acid
being employed to separate the two gases. The bulbs are fixed on
the beam of a balance in such a manner that the flow of acid from
one arm to the other produces a movement of the beam, which is
communicated by means of a lever to à pen and is recorded on a
rotating drum. By means of an ingenious compensating arrange-
ment the expansion caused by the heat rays is eliminated.¹
The cause of the expansion which occurs when chlorine is
acted upon by the actinic rays is as yet unknown. It may
possibly be due to the fact that under these conditions some of
the diatomic molecules Cl, are dissociated into monatomic mole-
cules, the change being similar to that which takes place when
chlorine is exposed to temperatures above 1200°. It has also
been suggested by Tyndall that the expansion is due to the
great absorptive power of chlorine for the more refrangible rays,
and its low emission power for the less refrangible rays.
81 Bleaching Power of Chlorine.-The characteristic bleaching
action which chlorine exerts upon organic colouring matters,
and which has become of such enormous importance in the
cotton and paper trades, depends upon its power of combining
with hydrogen. This bleaching action takes place only in
presence of water, the colouring matter being oxidised and
destroyed by the liberated oxygen of the water, whilst the
hydrogen and chlorine combine together.
That dry chlorine does not act upon colouring matters may be
readily shown by immersing a piece of litmus paper, or, better
still, a small piece of turkey-red cloth, previously, well dried,
in a jar of dry chlorine, when the colour will remain for hours
unaltered, whereas the addition of a small quantity of water
causes its immediate disappearance.
Chlorine cannot as a rule destroy mineral colours, nor can it
bleach black tints produced by carbon; this is well shown by
rendering illegible the ordinary print (printer's ink is made with
lamp-black or carbon) on a piece of card or paper, by covering
the whole with common writing ink (which generally consists
of the iron salts of organic acids). On immersing the blackened
card in moist chlorine gas, or in a solution of chlorine water, the
printed letters will gradually make their appearance.
1 Phil. Mag. loc. cit. p. 283.
2 Pogg. Ann. 144, 213.
BLEACHING ACTION OF CHLORINE
165
Chlorine also possesses powerful disinfecting properties, and
the gas is largely used for the destruction of bad odours and of
the poisonous germs of infectious disease floating either in the
air or in water. It is probable that this valuable property also
depends upon the oxidation, and consequently the destruction of
these poisonous emanations and miasmata.
82 Chlorine and Water. Hydrate of Chlorine, Cl+5H₂O.—
When chlorine gas is passed into water a few degrees above
L
FIG. 43.
the freezing-point, a solid crystalline compound of the gas and
water, termed chlorine hydrate, is formed. By quickly pressing
the crystals between blotting-paper, they may be freed from
adhering water and analysed. Faraday found that they con-

A
B
C
FIG. 44.
E
tained 27.70 per cent. of chlorine, showing that they are
composed of one atom of chlorine to five molecules of water, the
hydrate having, therefore, the composition Cl+5H2O. Accord-
ing to Bakhuis Roozeboom they have the composition Cl+4H₂O
or Cl₂+8H₂O.¹ This hydrate forms beautiful, apparently regular
1 Rec. Trav. Chim. 3, 59.
166
THE NON-METALLIC ELEMENTS
octohedra, and decomposes readily, the critical point of decom-
position being 9°6 in open and 28°7 in closed vessels; in the
latter case it forms two layers, one of liquid chlorine, and the
other of the aqueous solution of the gas. This decomposition
of the hydrate may be made use of as the most ready way
of preparing liquid chlorine. For this purpose the dried hydrate
is placed in the limb (a b) of a strong bent glass tube (Fig 43).
The open limb is then sealed whilst the hydrate is kept cool
by dipping the other limb into a freezing mixture; after the tube
is closed, the limb (a b) is placed in a vessel of lukewarm water
and the crystals then resolve themselves into two distinct layers
of yellow liquid, the lower of which is liquid chlorine. By placing
the limb (be) in a freezing mixture the liquid chlorine distils over,
leaving the less volatile aqueous solution of chlorine behind.

FIG. 45.
Aqueous solution of Chlorine possesses a greenish-yellow
colour and smells strongly of the gas. Chlorine is most soluble
in water at 10°, as below this temperature the formation of the
hydrate commences, and as the temperature increases above 10°
the solubility diminishes, until at 100° no gas dissolves. It is
prepared by passing washed chlorine gas through water as shown
in Fig. 44, chlorine being evolved in the flask A, and the
solution of the gas obtained in the bottles C, D, and E.
If we wish to absorb in water the whole of a small quantity
of chlorine gas evolved in a given reaction, the apparatus
represented in Fig. 45 may be used. Chlorine is led by a gas
delivery-tube into an inverted retort having a wide neck and
filled with water; the gas displaces some of the water, collects in
the upper portion of the retort, and may there be absorbed and
its quantity estimated.
CHLORINE AND WATER
167
83 The absorption co-efficient of chlorine in water between.
10° and 41° 5, is given by the equation
C = 3·0361 – 0·046196t + 0·0001107ť²
Temperatures
Centigrade. Coefficient.
2.5852
2.5413
2.4977
2.4543
2.4111
2.3681
2.3253
2.2828
from which the following values are obtained.
One volume of water absorbs the following volumes of chlorine
gas calculated at 0° and 760 mm.¹
10
11
12
13
14
15
16
17
18
19
20
21
24
25
·
·
1
"
•
•
.
•
•
.
·
·
·
•
·
•
·
•
•
•
•
2.2405
2.1984
2.1565
2.1148
2.0734
2:0322
1.9912 . .
1.9504
·
·
•
•
•
•
•
•
•
•
Difference.
Temperatures
Centigrade. Coefficient.
1.9099
1.8695
1.8295
1.7895
1.7499
1.7104
1.6712 .
1.6322 .
1.5934
1.5550
1.5166
1:4785
1·4406
1.4029
1.3655
26
0.0439 27
0.0436 28
0.0434 29
0.0432
30
0.0430 31
0.0428 32
0.0425 33
0.0423 34
0.0421 35
0.0419
36
0 0417
37
0.0414 38
0.0412 39
0.0410 40
0.0408
2H2O + 2Cl₂
·Schönfeld, Annalen, 93, 26; 96, 8.
•
=
•
.
•
•
•
•
•
·
·
•
•
•
•
·
.
·
•
·
.
·
·
•
•
•
•
·
•
•
·
•
•
•
.
·
.
.
·
•
•
.
•
If chlorine mixed with another gas, such as hydrogen or
carbon dioxide CO2, be passed into water at temperatures
between 11° and 38°, the volume of absorbed chlorine is found
to be greater than that calculated from the law of Dalton and
Henry for partial pressures, and this excess of dissolved chlorine
varies in amount with the temperature of the water, and
with the nature of the other gas present.
2
Saturated chlorine water gives off chlorine freely on exposure
to the air; it bleaches organic colouring matters, and if free
from hydrochloric acid, it does not redden a piece of blue
litmus paper before it bleaches it.
exposed to direct sunlight, it undergoes decomposition if suf-
ficiently dilute, with formation of hydrochloric acid and oxygen:
When chlorine water is
Difference.
0.0405
0.0404
0.0400
0.0400
0.0396
0.0395
0.0392
0.0390
0.0388
0.0384
0:0384
0 0381
0.0379
0.0377
0:0374
4HCl + 02.
2 Roscoe, Journ. Chem. Soc. 1856, 14.
168
THE NON-METALLIC ELEMENTS
1
It has been proposed to employ this reaction in measuring the
chemical action of light, but the decomposition is not sufficiently
regular for this purpose; thus Pedler ¹ has shown that a solution
containing 1 molecule of chlorine to 64 of water undergoes no
appreciable alteration during two months' exposure to tropical
sunlight, whilst more dilute solutions undergo more or less
decomposition, as shown in the following table:
Mols. HO for
1 mol. Cl.
64
88
130 .
140
412 .
•
Percentage of CI
acting on water.
no action
29
46
29
78
In the case of more dilute solutions, the reaction appears to
take place almost completely in accordance with the above
equation, except in so far as small quantities of chloric acid are
formed. In diffused daylight, however, a considerable quantity
of hypochlorous acid is formed, which is in turn decomposed by
light with formation of chloric acid, so that in this case the
reactions are probably those put forward by Popper.2
4Cl₂ + 4H,O = 4HCl + 4HCIO
8HCIO = 2HClO3 + 6HCl + O2
Under certain conditions, however, sunlight brings about the
reverse change, causing the formation of free chlorine from a
mixture of hydrogen chloride and oxygen (see p. 175).
CHLORINE AND HYDROGEN.
HYDROCHLORIC ACID, CHLORHYDRIC ACID, HYDROGEN
CHLORIDE, OR MURIATIC ACID. HCl = 36·19.
84 The Arabian alchemists were acquainted with hydro-
chloric acid in its mixture with nitric acid to form aqua regia,
which they obtained by distilling nitre, sal-ammoniac, and vitriol
together, but the first mention of the pure acid under the
2 Annalen, 227, 161.
Journ. Chem. Soc. 1890, i, 613
CHLORINE AND HYDROGEN
169
1
name of "spiritus salis," prepared from "guter vitriol" and "sal
commune," occurs in the supposed works of Basil Valentine.
Glauber first obtained this acid by the action of sulphuric acid
on common salt about the year 1648, and Stephen Hales, in
his work on Vegetable Staticks, published in 1727, observed that
a large quantity of a gas which was soluble in water was evolved
when sal ammoniac and oil of vitriol were heated together. It
was not, however, until Priestley¹ collected the gas thus evolved
over mercury, by using this metal instead of water in a pneu-
matic trough, that the gaseous hydrochloric acid was first pre-
pared, and to this gas Priestley gave the name of marine-acid
air, as calling attention to its production from sea-salt. Lastly,
Davy in 1810 proved that the gas, which had been considered
to be an oxygen compound, was entirely composed of chlorine
and hydrogen.
Hydrochloric acid gas, the only known compound of chlorine
and hydrogen, occurs in the exhalations from active volcanoes,²
especially in Vesuvius, and in the fumeroles on Hecla. In
aqueous solution, the acid has been found in the waters of
several of the South American rivers rising in the volcanic
districts of the Andes. It is also found in small quantities in
the gastric juice of man and other animals.
85 Hydrochloric acid can be formed by the direct union of its
constituent elements. If equal volumes of chlorine and hydro-
gen be mixed together, no combination occurs so long as the
mixture remains in the dark and at the ordinary atmospheric
temperature; but if the mixed gases be exposed to a strong
light,
or if a flame be brought to the mouth of the jar, or an
electric spark passed through the gases, a sudden combination
takes place, the heat suddenly evolved by the union of the
chlorine and hydrogen being sufficient to produce a violent
explosion. In order to exhibit this singular action of light,
inducing the combination of chlorine and hydrogen, a small thin
flask may be filled, in a darkened room, half with chlorine gas
by displacement over hot water) and half with hydrogen.
The flask, corked and covered up, may then be exposed
either to sunlight, or to the bright light of burning magnesium
ribbon, when a sharp explosion will instantly occur, the
3
Observations on Different Kinds of Air, 1772, vol. iii. 208.
Pseudo-Volcanic Phenomena of Iceland, Cav. Soc. Mem, p. 327.
Palmieri, The Late Eruption of Vesuvius, 1872, p. 136.
Bunsen, Annalen, 62, 1.
170
THE NON-METALLIC ELEMENTS
flask will be shattered, and fumes of hydrochloric acid will
be seen.
A better method of showing this combination is to obtain a
mixture of exactly equivalent volumes of chlorine and hydrogen
by the electrolysis of aqueous hydrochloric acid itself. For this
purpose an apparatus shown in Fig. 46 is employed; this con-
sists of an upright glass tube filled with about 120 cc. of pure
fuming aqueous hydrochloric acid, containing about 30 per
cent. of HCl. Two poles of dense carbon, as used for the electric
lamp, pass through tubulures in the sides of the glass, being
fastened in their place by means of caoutchouc stoppers. The

FIG. 46.
apparatus having been brought into a room lighted only by a
candle or small gas flame, the carbon poles are connected with
three or four Bunsen's elements, the current from which is
allowed to pass through the liquid. At first gas is evolved from
the negative pole only, and this consists of hydrogen, whilst
all the chlorine, which is evolved at the positive pole, is ab-
sorbed by the liquid. After the evolution has gone on for two or
three hours the liquid becomes saturated with chlorine, and the
gases are given off at each pole in exactly equivalent volumes,
and consist of hydrogen and chlorine, uncontaminated with oxy-
gen, or oxides of chlorine.¹ The gaseous mixture thus obtained
is washed by passing through a few drops of water contained
1 Roscoe, Journ. Chem. Soc. 1856, 16.
HYDROCHLORIC ACID
171
egg
in the bulb-tube ground into the neck of the evolution vessel,
and then passes into a thin glass bulb of about the size of a hen's
blown on a piece of easily fusible tubing. At each end the
tube is drawn out so as to be very thin in the glass, and to have
the internal diameter not greater than 1 mm., whilst at the
extremities the tube is wider, so as to fit ordinary caoutchouc
joinings. In order to absorb the excess of chlorine the further
end of the bulb is placed in connection with a condenser
containing slaked lime and charcoal placed in alternate layers.
When the gas has passed through the bulb-tube (at the rate of
about two bubbles every second) for about ten minutes, the join-
ings are loosened and each end stopped by a piece of glass rod.
In order to preserve the gaseous mixture, which is unalterable in
the dark for any length of time, the bulbs are hermetically sealed.
For this purpose the thinnest part of the tube is brought some little

FIG. 47.
distance above a very small flame from a Bunsen's gas-burner:
the glass softens below a red heat, and the ends may be drawn
out and sealed with safety. It is, however, advisable to hold the
bulb in a cloth during the operation of sealing, as not unfre-
quently the gas explodes. As soon as one bulb is removed a
second is introduced, and placed in connection with the evolution
flask, and after ten minutes sealed as described.
thus obtained should be numbered, and the first and last tested
by exposing them to a strong light, and if these explode, all the
intermediate bulbs may be considered good. Sixty such bulbs
The bulbs
172
THE NON-METALLIC ELEMENTS
may be prepared with the above quantity of acid, and may be
kept in the dark for an unlimited time without change.¹ On
exposing one of these bulbs to the light emitted by burning
magnesium ribbon, or to bright daylight, a sharp explosion occurs
and hydrochloric acid is formed (Fig. 47).
3
When the mixture of hydrogen and chlorine is exposed to
diffused daylight combination gradually takes place, the rate of
formation of hydrochloric acid increasing with an increasing
proportion of the more refrangible rays of light. Bunsen and
Roscoe 2 found, however, that the action of light is at first very
slow and only attains its full activity after a certain length of
time. Thus, for example, when a mixture of the gases was
exposed to the light of a small petroleum lamp burning at a
constant rate it was found that the amount of hydrochloric acid
formed in each minute increased for the first nine minutes and
then became constant. To this phenomenon they have given
the name "photo-chemical induction." From the recent in-
vestigations of Pringsheim it seems probable that the phe-
nomenon is due to the formation of an intermediate product,
for, just as in the case of carbon monoxide and oxygen the
mixture of gases undergoes combination far more easily when
moist than dry, even a very bright light bringing about an
explosion in the latter case only with great difficulty, and
hence it appears unlikely that in the combination we have a
simple reaction between hydrogen and chlorine, but that inter-
mediate compounds are formed by the combined influence of
light and moisture. The amount of hydrochloric acid formed
during the first period of the action of light would then be too
small, because the formation of the acid could only take place
after a certain quantity of the intermediate compound had been
formed; after a time, however, the quantity of the intermediate
compound formed would be equal to the quantity decomposed,
and the amount of the latter in the mixture and of the hydro-
chloric acid formed in a given time would remain constant. The
first period corresponds to the period during which "induction"
takes place, and the second to that in which the light brings
about the formation of the product of the reaction at a
constant rate.
Concerning the nature of this intermediate compound, which
is probably present in only extremely minute quantities, nothing
1 Roscoe, Proc. Manch. Lit. and Phil. Soc. Feb. 1865.
2 Phil. Trans. 1857.
3 Ann. Phys. Chem. [2], 32, 384.
PREPARATION OF HYDROCHLORIC ACID
173
can be definitely said; Pringsheim has, however, suggested that
the two reactions may be represented by the following equations:
(1) H₂O + Cl₂ Cl₂O + H₂
(2) 2H₂ + Cl₂O = H₂O + 2HCI.
=
Combination of hydrogen and chlorine also takes place when
the mixed gases are heated to a sufficiently high temperature :
in closed vessels explosion occurs between 240° and 270°, whereas
if the mixture is passed in a stream through the heated vessel
the explosion does not take place till the temperature reaches
430° to 440°.1
86 Hydrochloric acid is also formed by the action of chlorine
upon almost all hydrogen compounds, which are decomposed by
it either in the dark or in presence of light; thus sulphuretted
hydrogen, olefiant gas, turpentine (see p. 162), and water, are
all decomposed by chlorine, hydrochloric acid being formed.
When hydrogen is passed over many metallic chlorides, such as
silver chloride, hydrochloric acid is evolved, and the metal
produced, thus :—
2AgCl + H₂ = 2Ag + 2HC1.
By these and other reactions hydrochloric acid is frequently
formed; but none of them serve for the preparation of the gas
on a large scale.
or
87 Preparation.-For this purpose six parts by weight of com-
mon salt are introduced into a capacious flask, and eleven parts
of strong sulphuric acid slowly poured on it through a bent tube-
funnel; the gas, which is at once rapidly evolved, is purified
any sulphuric acid or salt which may be carried over, by
passing through a small quantity of water contained in a wash
bottle, and it may then either be collected by displacement (like
chlorine),
or over mercury, or passed into water, as shown in Fig.
from
53, if an aqueous solution of the acid is needed.
The reaction which here occurs is represented by the equation :
NaCl + H₂SO₁ = HCl + NaHSO.
1
Hydrochloric acid comes off, and a readily soluble acid sulphate
of soda,
hydrogen sodium sulphate, NaHSO,, is left. If two
molecules of salt be taken to one of sulphuric acid, a less easily
soluble salt, normal sodium sulphate Na,SO4, is formed, a greater
Freyer and V. Meyer, Zeit. Phys. Chem. 11, 28.
174
THE NON-METALLIC ELEMENTS
heat being needed to complete the decomposition than when an
excess of acid is employed, thus :-
NaCl + NaHSO₁ = Na„SO4 + HCl.
The pure aqueous acid is best prepared for laboratory use
from pure salt and pure sulphuric acid.
88 Properties.-Hydrochloric acid is a colourless gas, which
was first liquefied by Davy and Faraday,' who estimated the
tension of the gas to be 20 atmospheres at 16°, 25 atmos-
pheres at - 4, and 40 atmospheres at + 10°. The method
adopted by Davy for the liquefaction of the gas is shown in Fig. 48.
d
ん
​FIG. 48.
b
-
Into a tube, three times bent at a right angle and closed at one
end, are placed, at a, a few small pieces of sal-ammoniac (a com-
pound of hydrochloric acid and ammonia); some strong sulphuric
acid is then poured by means of a bent tube-funnel (d), into the
second bend at b. The open end of the tube is next carefully
drawn out, thickened, and closed before the blow-pipe, and when
the tube is cold, it is so inclined as to allow the acid to flow on to
the sal-ammoniac. Hydrochloric acid gas is at once disengaged
according to the equation:-
2 NHẠC1 + H_SO = 2 HC1 + (NH,),SO,
and after a time the pressure becomes sufficiently great to
liquefy the further portions of the gas which are evolved, and
1 Davy and Faraday, Phil. Trans. 1823, p. 164.
PROPERTIES OF HYDROCHLORIC ACID
175
ピン​錠
​IT
which
eilee
Obe
urie
the
+M
by gentle heat the liquid may be distilled over into the empty
limb of the tube. It is a colourless liquid, has a specific gravity
of 0.854 at 105°, and solidifies at 115° to a crystalline
mass which melts at 112°5 (Olszewski), and does not conduct
electricity. The action of liquid hydrochloric acid upon various
substances has been carefully examined by Gore.¹ These experi-
ments show that the liquid acid has but a feeble solvent power
for bodies in general, and, with the exception of aluminium, the
metals are not attacked by it.
Hydrochloric acid gas is heavier than air. Its specific gravity,
according to the most accurate experiments of Biot and Gay-
Lussac, is 1.278 (air = 1) whilst according to Leduc 2 it is 1.2696
or its density is 18:39 (H= 1), the calculated density being
18095. At 1500° the gas appears to have undergone no change,
but at 1700 a considerable amount of dissociation has taken
place. The gas fumes strongly in the air, uniting with atmo-
spheric moisture, and it is instantly absorbed by water or ice,
yielding the aqueous acid. It possesses a strongly acid reaction
and suffocating odour, and is not inflammable. A burning candle
is extinguished when plunged into the gas, the outer mantle of
the flame, before extinction, exhibiting a characteristic green
coloration.
-
--
3
-
Direct sunlight has no action on a dry mixture of hydrogen
chloride and oxygen, but if more moisture than is necessary for
the complete saturation of the gas be present, decomposition
gradually takes place, free chlorine and water being formed.
The amount of chlorine liberated depends upon the proportion.
of oxygen present; thus a mixture of 4 volumes HCl and
1 volume O only gave 0.34 per cent. of free chlorine after 24
exposure, whilst with 8 volumes of oxygen 73-81 per cent.
of the chlorine was liberated. It appears that in some cases
hypochlorous acid is also formed.*
days'
89 The composition of hydrochloric acid gas can be best
ascertained
as follows:-
Metallic sodium decomposes the gas into chlorine, which
combines with the metal to form sodium chloride, and into
hydrogen, which is liberated. If a small piece of sodium be
heated in a deflagrating spoon until it begins to burn, and
then plunged into a jar of hydrochloric acid gas, the combus-
1 Proc. Roy. Soc. 14, 204.
Comptes Rend. 116, 968.
4 McLeod, Journ. Chem. Soc. 1886, i. 591; Richardson, Journ. Chem. Soc.
1887, i. 802.
Langer and V. Meyer, Pyrochem. Unters. (Vieweg), p. 67.
176
THE NON-METALLIC ELEMENTS
tion of the metal (union with chlorine) will go in the gas.
In order to show what volume of hydrogen is evolved from
a given volume of hydrochloric acid gas by this reaction,
the following experiment may be made with the eudiometer
tube, the construction of which is clearly seen in Fig. 49. To
begin with, both limbs are filled completely with dry mercury,
then the end of the tube carrying the stop-cock is connected by
a piece of caoutchouc tubing with an evolution flask, from which
pure hydrochloric acid gas is being slowly evolved from a mix-
ture of dry salt and strong sulphuric acid, care being taken that
the air has been driven out. On turning the stop-cock at the top
of the tube, and opening the screw-tap on the caoutchouc in the
U-tube, the mercury will run out, and dry hydrochloric acid gas
FIG. 49.
will enter the one limb, whilst air fills the other to the same
level. As soon as the gas reaches a mark on the tube indicating
that it is two-thirds full of gas the stopcock is closed. A
small quantity of sodium amalgam is now prepared by pressing
six or eight small pieces of clean cut sodium, one by one,
under the surface of a few ounces of mercury contained in a
porcelain mortar. The amalgam is then poured into the open
limb of the U-tube so as to fill it, and the end firmly closed with
the thumb; the hydrochloric acid gas is now transferred to the
limb containing the amalgam, and well shaken so as to bring
the gas and amalgam into contact. The gas is next passed back
into the closed limb, and the pressure equalised by bringing the
mercury in both limbs to the same level, which is easily done
by allowing some mercury to flow out by loosening the screw-tap
COMPOSITION OF HYDROCHLORIC ACID
177
at the bottom of the U-tube. The hydrochloric acid gas will
be completely decomposed by contact with sodium amalgam,
chloride of sodium being formed, whilst the hydrogen is left
in the gaseous state. This will be found to occupy exactly
half the volume of the original gas, the level of the mercury
having risen to a mark previously made and indicating exactly
one-third of the capacity of the tube. As the closed limb is
provided with a stopcock, the residual gas may be inflamed,
and thus shown to be hydrogen.
90 It still, however, remains to ascertain the volume of the chlo-
rine which has disappeared. This is done as follows:-Two glass
tubes about 50 cm. long and 1.5 cm. in diameter, drawn out at each
end to fine threads, are filled with the gaseous mixture evolved
by the electrolysis of the aqueous acid (see p. 170). The process
is conducted exactly as if a bulb were being filled, and the tubes
are then sealed up and kept in the dark. When it is desired to
exhibit the composition of the gas, one of the tubes thus filled
is brought into a dimly-lighted room, and one of the drawn-
out ends broken under mercury. No alteration in the bulk of
the gas will be noticed. The mercury in which the tube dips is
now replaced by a colourless solution of iodide of potassium,
and by giving the tube a slight longitudinal shaking, a little
of this solution is brought in contact with the gas. No sooner
does the liquid enter the tube than it becomes of a dark brown
colour, due to the liberation of the iodine, the chlorine uniting
with the potassium to form the chloride of that metal. Ă
consequent diminution of bulk occurs which corresponds
precisely to the volume of chlorine contained in the tube,
and in a few moments the column of liquid fills half the
tube, proving that half the volume of the mixed gas con-
sists of chlorine, and the other half of hydrogen. We have,
however, learnt from the previous experiment that hydrochloric
acid gas contains half its own volume of hydrogen, so that
we have now ascertained (1) that hydrochloric acid gas is
entirely made up of equal volumes of chlorine and hydrogen,
and (2) that these elementary components combine together
without change of volume to produce the compound hydrochloric
acid gas.
This fact may be further illustrated by exposing a second
sealed-up tube, containing the electrolytic gas, for a few minutes,
first to a dim, and then to a stronger daylight. The greenish
colour of the chlorine will soon disappear, a gradual combination
13
178
THE NON-METALLIC ELEMENTS
of the gases having occurred. On breaking one end of the tube
under mercury, no alteration of bulk will be observed, whilst on
raising the open end into some water poured on the top of the

FIG. 50.
mercury, an immediate and complete absorption will be noticed,
and the tube will become filled with water.
In order to determine with a greater degree of exactitude
than is possible by the above methods, the relation existing
between the two gases, a quantitative analysis of the chlorine
contained in a given volume of the electrolytic gas must be
MANUFACTURE OF HYDROCHLORIC ACID
179
made. Two experiments thus conducted gave the following
results:-
I.
49.85
Chlorine
Hydrogen ... 50-15
100.00
...
...
29
II.
50.02
49.98
100.00
vol. of chlorine weighing
hydrogen
...
...
"3
Calculated.
50.00 volumes.
50.00
Showing that the gas obtained by decomposing aqueous hydro-
chloric acid consists exactly of equal volumes of chlorine and
hydrogen, or
35.19
2
100.00
= 17.595
1
144
2
1 vol. of hydrochloric acid weighing 18.095
91 Hydrochloric acid gas is very soluble in water, and the solu-
tion is largely used for laboratory and for commercial purposes,
and frequently termed muriatic acid. In order to exhibit the
solubility of the gas in water, a large glass globe (Fig. 50) placed
on a stand is filled, by displacement, with the gas; a tube,
reaching to the centre of the globe and dipping to the bottom of
an equal-sized globe placed beneath, being fixed in a caoutchouc
stopper placed into the neck of the upper globe. Between the two
globes the tube is joined by a piece of caoutchouc tubing, closed
by a screw-tap. When it is desired to show the absorption, the
lower globe is filled with water coloured blue by infusion of
litmus, the screw-top is opened and a little of the water forced
into the upper globe (so as to begin the absorption) by blowing
through the side tube into the space above the surface of liquid
in the lower globe. As soon as the water makes its appearance
at the top of the tube, a rapid absorption occurs, the liquid rushes
up in a fountain, and at the same time becomes coloured red.
=
""
...
0.500
92 Manufacture of Hydrochloric Acid. This acid is obtained on
the large scale as a bye-product in the manufacture of soda-ash,
(Cf. Vol. II. Part I. p. 132.) In the alkali works 10 cwt. of
salt is introduced into a large hemispherical iron pan, 9 feet
in diameter, heated by a fireplace underneath, and covered
by a brickwork dome; upon this mass of salt the requisite
180
THE NON-METALLIC ELEMENTS
quantity (10 cwt.) of sulphuric acid (sp. gr. 1·7) is allowed
to run from a leaden cistern placed above the decomposing

FIG. 51.
B
pan.
Torrents of hydrochloric acid gas are evolved, which
collect in the space between the pan and the brickwork dome,
whence they pass by a brickwork or earthenware flue into
MANUFACTURE OF HYDROCHLORIC ACID
181
upright towers or condensers, built of bricks soaked in tar, or of
Yorkshire flags fitted and clamped together. These towers, shown.
in vertical section in Fig. 51, and in ground plan in Fig. 52, are
filled with bricks or coke, down which a small stream of water,
from a reservoir at the top of the tower, is allowed to trickle.
The gas passing upwards, as shown in the figures by the arrow,
meets the water and is dissolved by it; and as the acid-liquor
approaches the bottom of the tower it becomes more and more
nearly saturated with the gas.
The aqueous commercial acid thus obtained from impure
materials is generally far from pure; it is usually of a yellow
O
ATH
ARA
10
15
QAÑAD
20
FIG. 52.
AAD
25 30 Feet.
colour, due to organic matter, and may also contain sulphur
dioxide, sulphuric acid, chlorine, iron, and arsenic: this last is
often present in large quantities, being derived from the pyrites
used in making the sulphuric acid.
The presence of arsenic may be detected by Marsh's reaction;
or by the addition of stannous chloride, which produces a brown
precipitate of impure arsenic. To remove traces of arsenic,
solution of stannous chloride may be added, the precipitate
allowed to settle, and the clear liquid re-distilled. Chlorine may
be detected by the addition to the diluted acid of pure iodide.
of potassium and starch solution, when if chlorine be present
the blue iodide of starch will be formed. The presence of
182
THE NON-METALLIC ELEMENTS
sulphuric acid can be easily ascertained by adding chloride of
barium solution to the diluted acid, whilst that of sulphurous
acid may be shown by adding zinc to the diluted acid, when
sulphuretted hydrogen will be given off and its presence readily

FIG. 53.
ascertained by its blackening action on lead paper. It is,
however, not easy to separate these substances so as to obtain
a strong pure acid from one originally impure, and by far the
simplest plan is to exclude the foreign matters by employing
pure materials to begin with.
PROPERTIES OF HYDROCHLORIC ACID
183
Temp.
93 The pure saturated aqueous acid is a colourless liquid
fuming strongly in the air, and freezing when cooled below
- 40° to a butter-like mass having the composition HCl+2H₂O.
It is prepared for laboratory use by means of the apparatus
shown in Fig. 53. One volume of water at 0° absorbs 503
times its volume of hydrochloric acid gas. The weight and
volume of the gas absorbed under the pressure of 760 mm. by
one gram of water at different temperatures is given in the
following table.¹
0°
4
8
12
16
20
24
28
•
Grms. HCl.
0.825
0.804
0.783
0.762
0.742
0.721
0.700
0.682
Temp.
32°
36
40
44
48
52
56
60
Grms. HCl.
0.665
0.649
0.633
0.618
0.603
0.589
0.575
0.561.
The weight of gas dissolved under changing pressure (the tempe-
rature remaining constant) does not vary proportionally to the
pressure, and, therefore, this gas does not follow Dalton and
Henry's law. Thus, for instance, under the pressure of 1 metre
of mercury 1 grm. of water dissolves 0856 grm. of the gas;
according to Dalton and Henry's law the weight of gas absorbed
under a pressure of 1 decimetre of mercury should be 0.0856
grm., whereas it is found to be 0.657 grm.
On heating a saturated solution of the gas in water having a
specific gravity of 1.22, hydrochloric acid gas is given off, and the
liquid becomes weaker. On the other hand, a weak acid on being
boiled loses water and becomes stronger, so that at last both
the strong acid and the weak acid reach the same strength, and
both when boiled distil over unchanged, provided the pressure
does not vary. The aqueous acid, which boils unchanged at
110° under the normal pressure contains 20-24 per cent. of
hydrochloric acid HC1.2 If the distillation proceeds under a
greater or less pressure than the normal, distillates of constant
composition are obtained, but each one contains a different
quantity of hydrochloric acid. This is clearly seen from the
following table.
1 Roscoe and Dittmar, Quart. Journ. Chem. 1860, 128.
2 Roscoe and Dittmar loc. cit.
184
THE NON-METALLIC ELEMENTS
Column I. gives the pressure in metres of mercury under
which distillation was conducted; Column II. the percentage
of hydrochloric acid (HCl) found in the residual acid.
I.
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.76
Temp.
0°
10
20
30
40
50
II.
23.2
22.9
22.3
21.8
21.4
21.1
20.7
20.4
20.24
.
I.
0.8
* 0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
Here the percentage of the acid and the constant composition
obtained by distillation under a pressure of 0·5 decimetre is seen
to be 23-2 HC1; whereas when the pressure is increased to 2:5
metres the percentage of the acid of constant boiling point is 18.0
HCI. From this it is clear that definite hydrates of hydro-
chloric acid (i.e., compounds of hydrochloric acid and water in
simple atomic proportions) are not formed on distillation, al-
though it happens that by chance the liquid distilling under a
pressure of 760 mm. corresponds to HCl + 8H,O. In the same
way, if dry air is passed through aqueous hydrochloric acid a
part of the acid is vaporized and a residue is obtained which
for each given temperature remains of constant composition.
An acid weaker or stronger than this ultimately attains this
composition.
II.
20.2
19.9
19.7
19.5
19.4
19.3
19.1
19.0
18.9
The following table shows the composition of the constant
aqueous hydrochloric acids obtained by leading air at given
temperatures through the liquid.
Per Cent. of HCl,
25.0
24.7
24.4
241
23.8
23.4
Temp.
60°
I.
1.7
1.8
1.9
2.0
2.1
2.2
70
80
90
100
2.3
2.4
2.5
•
II.
18.8
18.7
18.6
18.5
18.4
18.3
18.2
18.1
18.0
Per Cent. of HCl.
23.0
22.6
22.0
21.4
20.7
Hence it is seen that an aqueous acid which boils unaltered
under a given pressure, and, therefore, at a constant tempera-
ture, contains the same percentage of HCl as the constant acid
THE CHLORIDES
185
100 of Aqueous
Acid contain
obtained by passing dry air through the aqueous acid. Thus
the boiling point of the acid under 01 metre of pressure, con-
taining 22.9 per cent. of HCl, is from 61° to 62°; and if dry air
be passed through an aqueous acid at 62° the constant point is
attained when the liquid contains 22.9 per cent. of HCl.
94 The following table gives the specific gravity of solutions
of aqueous hydrochloric acids of varying strengths, according
to the experiments of Kolb.¹
HCl.
2.22
3.80
6.26
11.02
15.20
18.67
20-91
23.72
25.96
29.72
31.50
34.24
36.63
38.67
40.51
41.72
43.09
0°
Specific Gravity at
1.0116
1.0202
1.0335
1.0581
1.0802
1.0988
1.1101
1.1258
1.1370
1.1569
1.1666
1.1806
1.1931
1.2026
1.2110
1.2165
1.2216
15°
1.0103
1.0189
1.0310
1.0557
1.0751
1.0942
1.1048
1.1196
1.1308
1·1504
1.1588
1.1730
1.1844
1.1938
1.2021
1.2074
1.2124
From these numbers the percentage of any acid of known
specific gravity can easily be found by interpolation.
95 The Chlorides.-The compounds of chlorine with the metals
are formed either by the direct union of chlorine with a metal
or by the replacement of the hydrogen in hydrochloric acid by
a metal. Certain metals enter very readily into combination
with chlorine, heat being always evolved, and the phenomena
of combustion frequently observed. Other metals again do
not combine so easily. Most of the metallic chlorides are
soluble in water; amongst those insoluble are silver chloride
AgCl, mercurous chloride (calomel) HgCl, and cuprous chloride
¹ Compt. Rend. 74, 737.
186
THE NON-METALLIC ELEMENTS
Cu,Cl. Many metals combine in more than one proportion with
chlorine, thus we find :
Cuprous Chloride, Cu,Cl2,
Mercurous Chloride, HgCl,
Tin Dichloride, SnCl2,
Platinum Dichloride, PtCl2,
Ferrous Chloride, FeCl2,
The chlorides of the metals are usually prepared by one of
the following processes. (1) By acting on the metal with
chlorine gas, especially when the anhydrous chloride is required.
(2) By the action of chlorine upon metallic oxides, when it
drives off the oxygen and unites with the metal to form a
chloride. (3) By acting on the metal with hydrochloric acid.
(4) By dissolving the oxide, hydrate, or carbonate of the metal
in hydrochloric acid. (5) In certain cases, by adding a soluble
chloride to a solution of a salt of the metal, when the metallic
chloride is obtained as an insoluble precipitate.
Chlorine also unites with all the non-metallic elements, and
with certain groups of atoms termed radicals, to form chlorides
of these elements and radicals respectively, some examples of
which are as follows:-
Non-Metallic Chlorides.
Hydrochloric Acid.
Chloride of Sulphur
Trichloride of Boron
Tetrachloride of Silicon
Pentachloride of Phosphorus
·
Chloride of Sulphuryl
Chloride of Phosphoryl
Chlorides of Inorganic Radicals.
Chloride of Ethyl .
Chloride of Ethylene
Chloride of Acetyl.
Chloride of Cyanogen
•
Cupric Chloride, CuCl.
Mercuric Chloride, HgCl.
Tin Tetrachloride, SnCl.
Platinum Tetrachloride, PtCl.
Ferric Chloride, FeCl3.
•
.
Chlorides of Organic Radicals.
•
•
•
•
·
•
•
HCI.
SCI.
BClg.
SiCl
PC15.
SO Cla
POCI3.
C₂H,Cl.
C₂H4Cl₂.
. C₂HOCI.
CNCI.
96 Detection and Estimation of Chlorine.-In the free state
chlorine gas is recognized by its peculiar colour, its suffocating
DETECTION OF CHLORINE
187
smell, and by its bleaching action on organic colouring matters.
When present in smaller quantities, its presence may be de-
tected by the blue colour which it causes on a paper moistened
with a solution of iodide of potassium, KI, and starch paste,
owing to the fact that chlorine liberates iodine from its com-
pound with potassium, combining with the metal to form the
chloride, KCl, whilst the liberated iodine forms a deep blue
compound with starch. This reaction is very delicate, but it
must be remembered that an excess of chlorine again removes
the blue colour, and also that the same effect is produced by
bromine, nitrous fumes, ozone, and other oxidising substances.
When combined with metals to form chlorides soluble in
water, the element is usually detected by the formation of the
curdy white precipitate of silver chloride AgCl, on addition of
a solution of silver nitrate, AgNO3, to that of a soluble chloride,
such as KCl, thus:—
KCl + AgNO3 = AgCl + KNO。.
One part of chlorine in one million parts of water can thus be
detected a faint opalescence occurring. The precipitated silver
chloride becomes violet-coloured on exposure to light, and is in-
soluble in water and dilute acids, especially nitric acid, but
readily soluble in ammonia and in solutions of potassium cyanide,
and sodium thiosulphate (the so-called hyposulphite of soda).
Mercurous nitrate likewise produces in solutions of a chloride a
white precipitate of mercurous chloride (calomel), which does
not dissolve, but turns black on addition of ammonia.
In order to detect a chloride in presence of an iodide and
bromide, the dried salt is distilled with potassium chromate and
strong sulphuric acid, when chromium oxychloride, CrO2Cl2,
distils over as a dark red liquid, decomposed by addition of
water or ammonia, and yielding a yellow solution which, on
addition of hot acetic acid and a soluble lead salt, gives a
yellow precipitate of lead chromate; neither bromine nor iodine
forms a similar compound with chromium. Chlorine, when
combined to form a chloride, is always estimated as silver
chloride, AgCl, and according to Stas 142:32 of silver chloride
contain 35 19 of chlorine. If the chlorine is present in the free
state it can be determined by volumetric analysis (see Chlorimetry,
under Bleaching Powder), or it may be reduced by sulphur di-
oxide, SO, to hydrochloric acid, and then precipitated as silver
chloride and weighed.
188
THE NON-METALLIC ELEMENTS
4
The atomic weight of chlorine was first determined by
Berzelius,¹ together with that of silver and potassium. Penny 2
and Marignac have also made similar determinations; the
latter obtaining the numbers 35-372 and 35-462 by a method
similar to that employed by Berzelius. It is to Stas that we
owe the most exact determinations. He converted pure silver into
silver chloride by four different methods, and found as the mean
of closely agreeing results that the relation of the weight of
silver to that of the silver chloride it yields is 1:1-3285. It
is further known that the relation of the atomic weights of
silver and oxygen is 67456:1, this ratio having been also
obtained from a large number of experiments, such for example,
as the ratios of equivalent quantities of the following:-
Ag₂S: Ag₂SO, and AgCl: AgClO3.
4
Hence if the atomic weight of oxygen is taken as 15.88 that of
chlorine is 35.19.
BROMINE. Br = 79°36
5
97 BROMINE does not occur in the free state in nature; it
was discovered in the year 1826 by Balard, who prepared it
from the liquor called bittern, remaining after the common
salt has crystallised out from concentrated sea-water, in which
it occurs, combined with metals to form bromides; he gave it
the name from Bpwμos, a bad smell.
Bromine occurs in combination with silver in certain ores,
from Mexico, Chili, and Bretagne; but is found in large
quantities (combined with sodium, potassium, magnesium, or
calcium, forming bromides) in the water of many mineral springs,
some of which contain enough to serve as a source of this
element. It is also found, though in very small quantity, in
all sea-water, and has been detected in sea-weed from many
1 Pogg. Ann. 8, 1.
3 Annalen, 44, 11; Bibl. Univ. de Genève, 43, 350.
2 Phil. Trans. 1839, p. 20.
4 Recherches sur les Lois des Proportions Chimiques, Bruxelles, 1865.
5 Ann. Chim. Phys. [2], 32, 337.
6 The water of the Dead Sea is said to contain large quantities, of no less than
0.42 gram. in the litre (Lartet).
BROMINE
189
localities, and even in certain marine animals, as well as in
English rock-salt. The mineral springs at Kreuznach, Kis-
singen, and Schonebeck, and the potash beds of Stassfurt, as
well as certain American springs in Ohio and elsewhere, contain
considerable quantities of bromine, the commercial article being
obtained almost entirely from the two last-named sources.
Preparation. In order to detect bromine in a mineral water,
or to prepare it in small quantities, the following method is
employed. The mother-liquor remaining after the brine from
any of the above sources has been well crystallised is treated
with a stream of chlorine gas, so long as the yellow colour of
the liquid continues to increase in depth. Chlorine has the
power of liberating bromine from bromides, itself uniting with
the metal, and the bromine being set free, thus :-
MgBr, + Cl₂ = MgCl2 + Br₂.
The addition of excess of chlorine is to be avoided, as a com-
pound of chlorine and bromine is then formed. The yellow
liquid is then well shaken with chloroform, which dissolves the
bromine, forming, on standing, a brown solution below the
aqueous liquid. On adding caustic potash to this solution the
colour at once disappears, the bromine combining to form the
bromide KBr, and bromate of potassium, KBrO, thus :—
3Br₂+ 6KHO = KBrO3 + 5KBr + 3H₂O.
2
On concentrating the solution a mixture of these salts
remains, and from these the bromine is again liberated by dis-
tilling the liquid with black oxide of manganese and sulphuric
acid in a tubulated retort. The decomposition which here
occurs is similar to that which takes place in the preparation of
chlorine, thus:-
2KBr+ 3H2SO4 + MnO₂ = Br₂+ 2KHSO4 + MnSO4 + 2H₂O.
2
2
Dark red fumes of bromine are liberated, and a black liquid
condenses in the well-cooled receiver.
If the bromine is required to be anhydrous it must be
redistilled over concentrated sulphuric acid; and if iodine is
present this must be got rid of previously by precipitation as
subiodide of copper.
By far the greater quantity of the bromine brought into
"
"
190
THE NON-METALLIC ELEMENTS
commerce is now manufactured at Stassfurt from the mother-
liquor remaining after the separation of the potassium salts
contained in the salt deposits, the process adopted consisting in
the treatment of the liquors with chlorine under suitable con-
ditions. The most recent apparatus for this purpose is shown
in Fig. 54, and is so arranged that the process is continuous.
The mother-liquor enters the apparatus by the hydraulically
sealed pipe a, and by means of the sandstone drum b, and
perforated plate e, is distributed equally over the whole area of

A
w
FIG. 54.
D
MISORDER
m2
the tower A. The latter is filled with balls over which the
liquor flows, thus exposing a large surface to the action of
the chlorine gas passing through the tower in the opposite
direction; the waste liquor passes away by the pipe d, which is
sufficiently large to allow of the simultaneous passage of the
chlorine gas to the tower from the generator D. In order to
free the waste liquor completely from all traces of chlorine and
bromine it is run into the vessel B, which is kept full to the
bottom of the pipe d; to pass away from B the liquor must pass
over the sandstone shelves, in the direction shown by the
PROPERTIES OF BROMINE
191
arrows, in doing which it is subjected to the action of a current
of high pressure steam which is introduced into the apparatus
by the pipe g. All the chlorine and bromine are thus driven
out of the liquor and rise to the top of B, where they mingle
with the chlorine gas passing from the generator to the tower,
whilst the now quite innocuous liquor passes away by the pipe i.
The bromine vapour and excess of chlorine pass out from the
top of the tower by the pipe o, through the condenser p, where
most of the bromine condenses, the last traces of the bromine
and chlorine being removed by the vessel c, which contains iron
filings kept moist by a small stream of water.
The bromine thus obtained is purified by redistillation, the
small quantities of chlorine present being removed by the
addition of potassium- or ferrous-bromide, or by collecting
separately the more volatile portions of the distillate, which
contain all the chlorine in the form of chloride of bromine.
The manufacture of bromine was commenced in Stassfurt in
1865, in which year the quantity obtained only amounted to
25 cwt.; in 1885 the amount had risen to 260 tons, whilst the
quantity manufactured in America during the same year was
estimated at 120 tons, and elsewhere 20 tons, giving a total
production of about 400 tons.
98 Properties.-Bromine is a heavy mobile liquid, so dark
as to be opaque except in thin layers. It is the only liquid.
element at the ordinary temperature except mercury.
Its
specific gravity at 0° is 3·1883: it freezes at -7° to a dark brown
solid, evaporates quickly in the air, and boils at 59° (Thorpe).
Bromine possesses a very strong unpleasant smell, the vapours
when inhaled produce great irritation, and affect the eyes very
painfully. When swallowed, it acts as an irritant poison, and
when dropped on the skin it produces a corrosive sore, which is
very difficult to heal.
In its general properties, as well as in those of its compounds,
bromine closely resembles chlorine, although they are not so
strongly marked. Thus it bleaches organic colouring matters, but
much less quickly than chlorine does, and it combines directly
with metals to form bromides, though its action is less energetic
than that of chlorine. It does not combine at all at ordinary
temperatures with metallic sodium; indeed these two substances
may be heated together to 200° before any perceptible action
commences, whereas bromine and potassium cannot be brought
together without combination occurring, sometimes with almost
192
THE NON-METALLIC ELEMENTS
explosive violence. ¹ The addition, however, of a drop of water
to bromine and clear sodium sets up a lively reaction. If
brought into contact with free bromine, starch-paste is coloured
orange yellow. The vapour density of bromine at temperatures
slightly above its boiling point is somewhat higher than the
normal-namely, 5.8691 (air = 1), but at 228° the density is
5.5247, showing that at that temperature the bromine molecules
contain 2 atoms. 2 Moreover, if bromine vapour be mixed with
10 volumes of air, its vapour density, calculated from that of
the mixture, corresponds to the formula Br, even at the ordinary
temperature. At about 1570° the observed density is about
3
of that at 228°, so that, as in the case of chlorine, the
molecules Br, are at this temperature partially dissociated into
molecules consisting of simple atoms.*
Bromine is largely employed in the colour industry and also
in medicine, whilst smaller quantities are used in analytical and
synthetical chemistry, and as a disinfectant. To employ it for
the latter purpose, advantage is taken of the fact that the
siliceous earth known as "kieselguhr" absorbs as much as
75 per cent. of its weight of bromine, and still retains its
solid form; the product is sold under the name of "bromum
solidificatum."
Bromine and Water.-A definite crystalline compound of
bromine and water is obtained by exposing a mixture of the
two substances to a temperature near the freezing point. This
hydrate consists of Br+ 5H,O, and undergoes decomposition
into bromine and water at 15°. The analysis of the compound
gave the following results:—
Bromine.
5H₂O
•
•
Calculated.
79.36
89.40
168.76
47.00
53.00
100.00
Found (Löwig).
45.5
54.5
100.0
From the dissociation pressure, Roozeboom 5 believes that it
has the composition Br + 4H,O or Br₂+8H₂O.
The solubility of bromine in water between the tem-
Jahn, Ber. 15, 1238.
1 Merz und Weith, Ber. 6, 1518.
3 Langer and v. Meyer, Ber. 15, 2773.
4 V. Meyer and Züblin, Ber. 13, 405; Crafts, Compt. Rend. 90, 183.
5 Rec. Trav. Chim. 3,
73.
BROMINE AND WATER
193
·
At 5°
10
15
20
peratures of 5° and 30°,1 is shown in the following table, 100
grams of saturated bromine water containing by weight—
27
"9
39
:;
3853
25
30
3600 grams of bromine.
3.327
3.226
3.208
3.167
3.126
"3
99
29
""
99
99
39
29
99
1 Dancer, Journ. Chem. Soc. 1862, ii, 477.
"Merz and Holzman, Ber. 22, 867.
99
The solution of bromine in water has an orange red colour;
it soon loses bromine in contact with the air, and bleaches
organic colouring matter. Bromine water is permanent in the
dark, but on exposure to sunlight it becomes acid from the form-
ation of hydrobromic acid and evolution of oxygen. Bromine
also dissolves readily in chloroform, carbon disulphide, alcohol,
ether, and acetic acid.
The atomic weight of bromine has been determined by
Marignac and by Stas; the latter obtained as a mean of a
large number of experiments 79-36 as the atomic weight of
bromine when that of oxygen is 15.88.
BROMINE AND HYDROGEN.
HYDROBROMIC ACID. HBr = 80·36.
99 Bromine, like chlorine, forms only one compound with
hydrogen, containing one atom of bromine and one of hydrogen,
but, unlike chlorine, these two bodies do not unite to form
hydrobromic acid when brought together in sunlight. If,
however, hydrogen and the vapour of bromine are passed
through a red-hot tube containing finely-divided metallic plati-
num or pieces of charcoal, a combination occurs of equal
volumes of bromine and hydrogen and formation of hydro-
bromic acid gas. The combination of these two bodies may be
easily shown by passing hydrogen over bromine vapour and
lighting the escaping gas, when dense fumes of hydrobromic
acid will be noticed.
Preparation.-Hydrobromic acid gas cannot well be prepared
by the action of the ordinary acids on the bromides, as in the
8
14
194
THE NON-METALLIC ELEMENTS
case of hydrochloric acid, owing to the facility with which
hydrobromic acid splits up, with formation of free bromine. If,
however, phosphoric acid be used, free hydrobromic acid is
obtained.
Sulphuric acid of sp. gr. 1:41 liberates hydrobromic acid from
potassium bromide without simultaneous formation of bromine,
and in this manner a dilute solution of the acid may be obtained,
which may be concentrated by distillation."
One of the best methods of preparing hydrogen bromide is
to bring bromine and phosphorus together in presence of a little
water, when a violent action occurs, hydrobromic acid gas and
phosphoric acid being formed :-
:-
P+5Br+ 4H₂O = 5HBr + H3PO4.

FIG. 55.
In order to prepare the gas a flask provided with a doubly
bored caoutchouc cork, Fig. 55, is made use of; through one of the
holes a gas delivery-tube is fixed, whilst through the other a stop-
pered funnel tube is passed. A mixture of one part by weight
of amorphous phosphorus and two parts of water is introduced
into the flask, and ten parts of bromine are allowed to fall drop
by drop through the stoppered funnel-tube on to the mixture
in the flask. As each drop falls in a sudden evolution of gas
occurs, accompanied in the first part of the operation by a flash
of light, and as soon as a certain amount of hydrobromic acid
has been formed the bromine dissolves quietly, and on gently
warming the flask hydrobromic acid gas is given off. This is
then allowed to pass through a U-tube containing amorphous
1 Feit and Kubierschky, J. Pharm. [5], 24, 159.
HYDROBROMIC ACID
195
phosphorus, to free it from any vapour of bromine, and may
be collected in dry stoppered cylinders by displacement or over
mercury. The same apparatus serves for preparing a saturated
aqueous solution of the gas; for this purpose the gas delivery-
tube is removed and a short tube substituted for it. This passes
through a cork fitting in the tubulus of a retort placed in the
position shown in Fig. 56; the neck of the retort dips under
water, and the retort itself serves as a safety tube in case the
gas be absorbed so quickly that the liquid rushes back, as then
the solution is not sucked back into the flask, but simply rushes
into the upper part of the retort.
A second method is to pass a current of hydrogen sulphide
evolved from one of the continuous apparatus through a layer of
bromine contained in a tall cylindrical vessel and covered by a
layer of water or hydrobromic acid, when the following reaction
takes place :
Br₂ + H2S
2HBr + S.
The liberated sulphur partially combines with the excess of
bromine, forming bromides of sulphur. The gas evolved is
washed by passing through a solution of potassium bromide in
hydrobromic acid containing a little amorphous phosphorus in
suspension, and is thus obtained free from bromine and sulphur-
etted hydrogen; with this method the evolution of the gas can
be regulated very exactly.¹
1
=
A third method is to pass a mixture of hydrogen and bromine
vapour through a tube in which a platinum spiral is heated to
bright redness. For this purpose a glass tube about 18 cm. in.
length and 15 mm. in width is fitted at each end with a cork
carrying a small tube and a piece of stout copper wire; the ends
of the stout wires are joined within the tube by a platinum spiral
1 in. in length, which is maintained at a bright red heat by
means of an electric current; a stream of hydrogen is then
bubbled through bromine heated to 60° and passed through the
wide tube, the small tube being plugged with glass wool to
prevent possible explosions. So long as the hydrogen is in
slight excess, the hydrogen bromide is quite free from bromine,
and the presence of a small quantity of hydrogen is of no
importance for most purposes, especially when the aqueous
solution of the acid is required.2
Recoura, Compt. Rend. 110, 784. 2 Newth, Chem. News, 1891, ii. 215.
196
THE NON-METALLIC ELEMENTS
100 Properties.-Hydrobromic acid is a colourless gas, having a
strong irritating smell, with an acid taste and reaction. It fumes
strongly in the air, and on exposure to a temperature of -73°
(obtained by the evaporation of a mixture of ether and solid
carbonic acid), it condenses to a colourless liquid, and afterwards
freezes at 87° to a colourless ice-like solid.¹
-
Like hydrogen chloride, hydrogen bromide is decomposed by
sunlight in presence of oxygen and moisture, with liberation of
bromine, but the dry mixture of the gases is unaffected.2
FIG. 56.
Fy
Pure aqueous hydrobromic acid is colourless, and remains so
even when exposed to air; it fumes when saturated at 0°, and
then possesses a specific gravity of 178. The weak aqueous acid
becomes stronger, and the concentrated acid weaker on distilla-.
tion, until an acid containing from 47-38 to 47.86 per cent. of
HBr, distils over under pressures varying from 0.752 to 0.762
1 Faraday, Phil. Trans. 1845, p. 155.
2 Richardson, Journ. Chem. Soc. 1887, i. 804.
HYDROBROMIC ACID
197
metres. When the pressure under which the distillation occurs
varies, the composition of the constant acid changes as that of
hydrochloric acid does, and if a stream of dry air be passed
through the aqueous acid a point is reached, different for each
temperature, at which the acid no longer undergoes change. Thus
the acid which evaporates unchanged in air at 100° contains
49-35 per cent. HBr, whilst that obtained at 16° contains
5165 per cent of HBr.
Per Cent. HBr.
10.4
The variation of the specific gravity of the aqueous acid with
the percentage of hydrobromic acid dissolved has been determined
by Topsöe, as also by C. R. A. Wright," who obtained the
following numbers:—
23.5
30.0
40.8
48.5
49.8
Spec. Grav. at 15°.
1.080
1.190
1.248
1.385
1.475
1.515
The composition of this gas is analogous to that of hydro-
chloric acid, and can be ascertained in a similar way by bringing
a given volume of the dry gas in contact with sodium amalgam,
when sodium bromide is formed and hydrogen liberated, the
volume of which is found to be exactly half that of the original
hydrobromic acid gas.
101 The Bromides.-These compounds are formed in a similar
manner to the corresponding chlorides. They possess an analogous
composition with these, and exhibit similar properties. Bromine
unites with nearly all the metals, forming bromides, which are
also produced by the action of metals on hydrobromic acid, or
by the action of vapour of bromine on the metallic oxides,
oxygen being liberated.
The metallic bromides are nearly all soluble in water, the
most insoluble being silver bromide, AgBr, mercurous bromide,
HgBr, and lead bromide, PbBrą, which latter is slightly soluble.
All the bromides are solid at the ordinary temperature, but when
heated they fuse and volatilize, some undergoing decomposition
others remaining unchanged. They are, however, all decomposed
by chlorine, either in the cold or upon heating, a metallic chloride
being formed and bromine liberated; they are also decomposed
2 Chem. News, 23, 242.
1 Ber. 3, 404.
198
THE NON-METALLIC ELEMENTS
by sulphuric and nitric acids, with evolution of hydrobromic
acid, which again is partly oxidized, bromine being set free.
102 Detection and Estimation of Bromine.-Bromine when in the
free state may be recognized by the red colour of its vapour, and
by its exceedingly disagreeable odour, and by imparting to starch
paste an orange-yellow colour. When present in small quantities
it may be detected by shaking up with chloroform or ether,
which dissolves it, and acquires thereby a red or brownish colour.
Bromine in the state of a soluble bromide may be detected by
giving with silver nitrate a yellowish white precipitate of silver
bromide, which is insoluble in nitric acid, and dissolves only with
difficulty in ammonia, but readily in cyanide of potassium. Also
by giving with nitrate (but not chloride) of palladium a reddish-
brown precipitate of the bromide; by its tinging carbon di-
sulphide yellow in presence of hydrochloric acid and a drop of
sodium hypochlorite; and by the liberation of bromine on heat-
ing with sulphuric acid, with sulphuric acid and manganese
dioxide, or with sulphuric acid and potassium bichromate.
When the bromine is present as a soluble bromide it is usually
estimated by precipitation as bromide of silver, which contains
42.42 per cent. of bromine. In presence of chlorine the two
elements are precipitated together by nitrate of silver, the pre-
cipitate is then fused and weighed. A portion of it is next
ignited in a current of chlorine, when the whole of the bromine
is expelled, the residue of silver chloride weighed, and from the
weight thus obtained and that of the mixed silver salts, the
quantities of chlorine and bromine are calculated. For every
79.36 parts of bromine expelled, 35·19 parts of chlorine have been
substituted; or, if a difference of 44:17 is observed, 79:36 parts
of bromine must have been present. Hence, for any other
difference the weight of bromine is found by multiplying that
difference by
79.36
44.17
= 1.797.
CHLORINE AND BROMINE.
103 Bromine monochloride, BrCl.-When chlorine gas is passed
into liquid bromine cooled below 10° it is largely absorbed, a red-
dish brown volatile mobile compound of the two elements being
formed.¹ It dissolves in water yielding a yellow solution, from
¹ Balard, Ann. Chim. Phys. 32 337: Bornemann, Annalen, 189, 183.
IODINE
199
which, on cooling below 0°, a crystalline hydrate BrCl+10H₂O
separates out, which melts at + 7°. This is possibly simply a
mixture of chlorine and bromine hydrates.
IODINE. I=125'91.
104 Iodine was discovered in 1812 by Courtois,¹ of Paris, in
the mother-liquors of the soda salts which are prepared from
kelp or burnt seaweed. It was afterwards examined by Davy,2
and much more completely by Gay-Lussac.³ Iodine derives its
name from iocidns, violet-coloured, owing to the peculiar colour
of its vapour, by means of which it was first discovered.
Like chlorine and bromine, iodine does not occur in the free
state in nature, but is found combined with metals to form
iodides, which occur in small quantities, but widely diffused,
both in the organic and inorganic kingdoms, having been
detected in sea-water, in sea-plants and animals, and in many
mineral springs. The quantity of iodine present in sea-water
is extremely small, but certain plants and even animals have
the power of absorbing and storing up the iodine. The ash of
the deep-sea-weed (Fucus palmatus especially) contains more
iodine than that which grows in shallow water, and it is
from the weed collected on exposed and rocky coasts, as the
north and west coasts of Ireland, Scotland, and France, that a
large portion of the iodine of commerce is obtained. Of late
years, however, the quantity manufactured from kelp has
considerably decreased, owing to the discovery of iodine in the
crude Chili saltpetre or Caliche NaNO3, from the mother
liquors of which it is now chiefly obtained; it is likewise found
in combination with silver in a Mexican silver ore, in some
specimens of South American lead ore, in certain dolomites,
and in small quantities in almost every deposit of rock salt.
Iodine has also been found in coal, and it has been detected in
some few land and fresh-water plants, and in many sea animals,
as in sponges and oysters, and also in cod liver oil.
105 Preparation from Seaweed.-The stormy months of the
spring are those in which the deep-sea tangle is thrown up on
¹ Courtois, Clement, and Desormes, Ann. Chim. 88, 304.
2 Phil. Trans. 1814, 2, 74 and 487.
Ann. Chim. 88, 311, 319, and 91, 5.
200
THE NON-METALLIC ELEMENTS
the north coasts of France and Ireland and the western coasts
of Scotland. The inhabitants collect the weed, allow it to dry
during the summer, and then burn it in large heaps. The ash
thus obtained is termed kelp in Scotland and varec in Normandy;
it contains from 01 to 03 per cent. of iodine. When the
seaweed is completely burnt, and when the ash is fused, a con-
siderable fraction of the iodine is lost from volatilization; hence
it is preferable to carbonize the weed in closed retorts, the whole
of the iodine remaining in the ash, whilst the tar and ammonia-
cal liquor are also recovered; the carbon remaining after the

FIG. 57
lixiviation of the ash resembles animal charcoal, and is used as a
disinfectant.
In the most recent process for obtaining the iodine from sea-
weed the latter is directly lixiviated without previous carboniza-
tion, the residual apparently unaltered plants being converted
into algin, a substance which resembles gelatin and is employed
as a substitute for bladder skins, &c. On lixiviating systematic-
ally either the kelp or the carbonized weed, a concentrated
solution of the alkaline carbonates, chlorides, sulphates, and a
small quantity of sulphites and sulphides, together with the
MANUFACTURE OF IODINE
201
iodides and bromides of the alkali metals, is obtained, and from
this solution the carbonates, chlorides, and sulphates are allowed
to crystallize, leaving the bromides and the iodides in the
mother-liquor. This liquor is then treated in several ways in
order to obtain the iodine.
(1) An excess of sulphuric acid is added to the liquor, when the
sulphides and sulphites which it contains are decomposed and the
iodine and bromine liberated as hydriodic and hydrobromic
acid. In this process the liquor, after any separated crystals
of sodium sulphate which may have formed have been taken
out, is placed in iron boilers, Fig. 57, surrounded with brick-
work, each gently heated by a separate fire to a temperature of
60°, and fitted with leaden hoods, which can be lifted off by
means of a chain and winch. Each cover is fitted with a leaden
pipe (a), and this is connected with a series of glass or earthen-
ware condensers, termed udells, fitting one into the other. After
the introduction of the liquor, the covers are luted on with clay,
the pipes (a) fixed in their receptacles and connected with the
condensers. Manganese dioxide is then thrown little by little into
the still through the hole (b), which can be closed by a stopper.
The iodine thus liberated condenses in the receivers, and the
accompanying water escapes through a tubulus at the bottom of
each receiver and runs away along the channel (c).
When no
more iodine distils over, the leaden pipes are dismounted, and
the stills are connected with a second receiver (D). More
manganese dioxide is then added, and the bromine, which
hitherto has not been liberated, is now disengaged and collects
in (D) and in the Woulff's bottles (E).
The decomposition occurring during the formation of the
iodine is represented by the following equation:-
2NaI + 3H2SO4+MnO₂=I2+2NaHSO4 + MnSO4+2H₂O.
The iodine thus obtained may then be partially purified by
resublimation, but even then invariably contains traces of
chloride, bromide, and cyanide of iodine.
(2) The iodine contained in the liquors, after separation of
the crystallizable alkaline salts, may also be liberated by the
addition of sulphuric acid containing a considerable quantity of
nitric acid. The acidified liquor is then agitated with the most
volatile portion of petroleum (petroleum-naphtha, kerosine),
which dissolves the iodine. The petroleum solution of iodine
1 One ton of kelp usually yields 12 lbs. of iodine.
202
THE NON-METALLIC ELEMENTS
is next drawn off from the aqueous liquor and shaken up with
an aqueous solution of caustic soda, whereby the iodine is
withdrawn from the hydrocarbon and converted into iodide
and iodate of sodium. The iodine is then liberated from these
salts by the addition of hydrochloric acid, thus :—
5 NaI+NaIO3+6HCl=6NaCl+3H₂O+312.
(3) In France the mother liquor is treated with a slight
excess of sulphuric acid, filtered from sulphur, diluted to 40°
Tw., and saturated with chlorine, care being taken to avoid
adding an excess of the latter, which would cause loss owing to
the formation of chloride of iodine. The iodine separates out
in the solid form and is filtered off, dried, and resublimed.
Preparation from Caliche.-The crude Chili saltpetre, known
as Caliche, now forms the chief source of iodine; to obtain the
a
FIG. 58.
b
iodine the last mother-liquor obtained in the preparation of the
sodium nitrate, which contains about 20 per cent. of sodium
iodate, is treated with a solution of sodium bisulphite, obtained
by burning native sulphur and passing the products into a solu-
tion of sodium carbonate. The iodine separates out in the solid
form, and is filtered off and purified by resublimation, the
vapours being condensed in a series of udells similar to those
shown in Fig. 57.
In order to purify the commercial iodine it is washed with a
small quantity of water, dried on porous plates, and resublimed.
According to Stas,¹ the only mode of obtaining chemically-pure
iodine (free from every trace of chlorine and bromine) is to
dissolve the commercial resublimed substance in iodide of
potassium solution, and then to precipitate the iodine by water.
The precipitate is well washed with water and then distilled
1 Recherches, p. 136.
PROPERTIES OF IODINE
203
with steam, the solid iodine in the distillate collected and
dried in vacuo over solid nitrate of calcium, which is frequently
changed, and distilled afterwards over solid caustic baryta to
remove the last traces of water and of hydriodic acid.
106 Properties.-Iodine is a bright, shining, crystalline, opaque,
blackish-grey solid. The crystals when large possess almost a
metallic lustre; it crystallizes by sublimation in the rhombic
system, in the form of prisms or pyramids. Finer crystals are
obtained from solution, as by exposing to the air a solution
of iodine in ether, or in an aqueous solution of hydriodic acid.
The crystals thus obtained have the ratio of their axes represent-
ed by the numbers 4: 3: 2. The crystal represented in Fig.
4:3:
58 a was obtained by Marignac from solution in hydriodic
acid.
Iodine is a heavy substance, having a specific gravity of 4.948
at 17°, melting at 114°2 and solidifying at 113°6 (Stas). It
boils at 184-35 under 760 mm. pressure, giving rise to a vapour
which, seen by transmitted white light, possesses, when chemi-
cally pure, a splendid deep blue colour, but when mixed with
air a reddish-violet colour (Stas). The specific gravity of
iodine vapour was found by Deville and Troost to be 8.72
(air=1), which corresponds to the density, 1259, proving
that the molecule, or two volumes of iodine gas, weighs
125.91 x 2 = 251-82 and that the molecular formula is I.
When iodine vapour is heated above 700° its specific gravity
begins to diminish until at 1700° it becomes constant, and is
half that at 700°, the vapour consisting of free atoms. At the
ordinary temperature it volatilizes slowly, shining crystals
being deposited on the sides of a bottle on the bottom of which
a little iodine has been placed. It is a bad conductor of
electricity, and possesses a peculiar smell less penetrating than,
though similiar to that of, chlorine and bromine. The specific
heat of solid iodine is, according to the experiments of
Regnault, 0.05412, and that of the liquid 0-10882. The latent
heat of fluidity of iodine is 117 thermal units, its heat of
vaporization 23.95 thermal units.
When an electric discharge is passed through a heated
Geissler's vacuum-tube containing a trace of iodine vapour,
a spectrum of bright lines is obtained, characteristic of this
1
Ramsay and Young, Journ. Chem. Soc. 1886, 1, 453.
2 V. Meyer, Ber. 13, 394, 1010, 1103; v. Meyer and Biltz, Ber. 22, 725;
Meier and Crafts, Compt. Rend. 90, 690; 92, 39.
204
THE NON-METALLIC ELEMENTS
element. This emission spectrum is, however, not identical
with the characteristic absorption-spectrum of iodine, so care-
fully mapped by Thalén, and seen when white light is passed
through iodine vapour. Salét 3 has recently shown that when
an electric current of feeble tension is passed through a Geissler's
tube containing iodine, another set of bright bands is obtained,
which are identical in position with the dark bands of Thalen's
absorption-spectrum, each bright band being replaced by a black
band when the vapour is illuminated from behind.
107 In its chemical properties iodine resembles chlorine and
bromine; the two latter elements have the power of displacing
iodine from its combination with metals (or electro-positive
elements), thus:-
2KI + Cl₂ = 2KCl + I2.
The compounds of iodine with oxygen (or with electro-
negative elements) are, on the other hand, more stable than
those of the other two elements. Thus iodine expels chlorine
from the chlorates with formation of iodate and free chlorine :-
2KCIO3 + I2 = 2KIO3 + Cl2.
These differences are explained when we examine the amount
of heat evolved by the several decompositions in question. This
heat may be taken as a measure of the relative affinity or power
of combination which the elements exhibit towards one another.
Thus the heat evolved on the combination of chlorine, bromine,
and iodine with hydrogen, or the heat modulus of the reactions
according to Julius Thomsen's experiments is :-
4
H + Cl.
H+ Br.
H+ I
•
·
•
·
.
Cl + 3 0 + H .
Br+ 3 0 + H .
I +30 + H.
21836 heat units
8337
- 6060
•
When these same elements unite with oxygen and hydrogen
to form the oxyacids (HClO3, HBrO3, HIO3) the heat evolved
is as follows:-
29
:
. 43211
-
""
1 Plücker and Hittorf, Phil. Trans. 1865, 28.
2 Kon Svenska Acad. Handb. 1869.
4 Journ. Chem. Soc. 1873, 1188.
23761 heat units
5344
99
""
3 Phil. Mag. [4], 44, 156.
PROPERTIES OF IODINE
205
=
Hence we see that as regards affinity for oxygen chlorine
stands nearly midway between bromine and iodine, for
43711 + 5344
24277
2
Iodine dissolves but very sparingly in water, one part being
soluble in 5524 parts of water at 10°; but it dissolves freely in
an aqueous solution of potassium iodide, and in alcohol, yielding
brown solutions. Tincture of iodine of the pharmacopoeia
contains oz. of iodine, oz. of iodide of potassium, rectified
spirit 1 pint. It is also soluble in chloroform, carbon disulphide,
and many liquid hydrocarbons, imparting to these liquids a
fine violet or dark-red colour when present in small quantities,
and in larger quantities forming a black opaque solution, which
in the case of carbon disulphide is diathermous, allowing the
invisible heating rays of low refrangibility to pass, though it is
opaque to the visible rays.
Although iodine, unlike chlorine and bromine, does not combine
readily with hydrogen, it unites with many of the metals and
non-metals with evolution of light and heat. Thus solid
phosphorus, when brought into contact with iodine, first melts
and then bursts into flame owing to the heat evolved in the act
of combination: and powdered antimony takes fire when thrown
into iodine vapour, antimony iodide being produced, whilst if
the vapour of mercury be passed over heated iodine, immediate
action occurs, the iodides of mercury being formed. When
iodine is brought into contact with water and filings of iron or
zinc, a violent reaction occurs, colourless solutions of the
respective iodides resulting. The action of iodine upon the
alkali-metals is analogous to that of chlorine and bromine.
Sodium and iodine can be heated together without any alteration,
whilst if potassium be employed an explosive combination
occurs.
Potash at once decolorises a solution of iodine, iodide and
iodate of potassium being produced, thus:-
312 + 6KHO=5KI + KIO3 + 3H₂O.
When acted upon by strong nitric acid, iodine is completely
oxidized to iodic acid HIO.
The most characteristic property of free iodine is its power
of forming a splendid blue colour with starch-paste. This is
formed when starch granules are brought into contact with the
vapour of iodine, or, better, when a solution of iodine is added
206
THE NON-METALLIC ELEMENTS
to starch paste. The blue colour disappears on warming the
solution, but reappears on cooling, and its formation serves
as a most delicate test for the presence of iodine.¹ In order
to exhibit this property, a few grains of iodide of potassium
may be dissolved in three or four litres of water placed
large glass cylinder, and some clear, dilute, well-boiled
starch-paste added. As the iodine is here combined with the
metal, no coloration will be seen, but if a few drops of chlorine
water be added, or, better, if a little of the air (containing free
chlorine) from a bottle of chlorine water be poured on to the
surface of the liquid, a blue film will be formed, which on
stirring will impart a blue tint to the whole mass. Iodine
both free and in combination is largely used in medicine. The
atomic weight of iodine has been very accurately determined
in several ways by Marignac and Stas, the mean value obtained
from a large number of closely-agreeing experiments being
125.91 (0 = 15.88).
(O
IODINE AND HYDROGEN.
HYDRIODIC ACID. HI=126.91.
108 Iodine and hydrogen undergo partial combination when
they are passed over finely-divided platinum heated to redness,
or over charcoal at a bright red heat, forming a strongly acid
gas, having properties very similar to hydrochloric and hydro-
bromic acid.
Hydriodic acid can also be obtained by heating iodide of
potassium with phosphoric, but not with sulphuric, acid; for
when this latter acid is used, sulphur dioxide SO, and free
iodine are formed at the same time, thus :—
3H, SO₁ + 2KI = 2KHSO4 + I2½ + SO2 + 2H₂O.
On the other hand, hydriodic acid is easily prepared by allow-
ing iodine and phosphorus to act on one another in presence of
water, thus:-
:-
P+51 + 4H₂O = 5HI + H₂PО.
Preparation. For this purpose 1 part by weight of amorphous
phosphorus and 15 parts of water are brought together in a
1 Collin and Gaultier de Claubry, Ann. Chim. 90, 87.
2 Merz and Holzmann, Ber. 22, 867.
HYDRIODIC ACID
207
tubulated retort or flask, provided with a caoutchouc cork and
gas delivery-tube, and to these 20 parts of iodine are gradually
added, the contents of the flask during this operation being kept
cool by immersing the flask in cold water. When all the iodine
has been added, and as soon as no further evolution of gas can
be noticed, the flask may be gently warmed. The gas thus
obtained may either be received in dry bottles filled with
FIG. 59.
mercury in the mercurial trough, or it may be collected by
displacement, as it is more than four times as heavy as air.
If we possess a concentrated solution of hydriodic acid, the
gas may be obtained in a still more simple manner.
Two parts
of iodine are dissolved in aqueous hydriodic acid of specific
gravity 17, and this solution is allowed to fall, drop by drop, by
means of a stoppered funnel-tube, into a flask containing amor-
phous phosphorus covered with a thin layer of water. The evo-
lution of gas occurs at first without any application of heat being
208
THE NON-METALLIC ELEMENTS
necessary, but after a time the flask may be slightly warmed.
The apparatus, Fig. 59, is used for the purpose of preparing,
according to the above method, a saturated aqueous solution of
hydriodic acid.
When hydriodic acid is prepared by the foregoing methods
it frequently contains phosphuretted hydrogen; the forma-
tion of this impurity may, however, be avoided if the
iodine is never allowed to come in contact with an excess of
phosphorus. According to Lothar Meyer, this may be readily
carried out as follows:-100 parts of iodine moistened with
ten parts of water are placed in a tubulated retort, the neck of
which is inclined upwards; a thin paste of 5 parts of amorphous
phosphorus and 10 parts of water is then allowed gradually to
drop in through the tubulus of the retort. For this purpose a
dropping funnel is employed, which, in place of a stopcock, is
furnished with a glass rod, ground at the lower end to fit into
the funnel tube; by raising the latter, small quantities of the
paste are allowed to pass into the retort. The first few drops
must be added cautiously, waiting each time for the reaction to
moderate, as otherwise an explosion may occur; after a short
time, however, larger quantities may be added at once, and the
mixture may be completed in a quarter of an hour. The iodine.
carried over mechanically settles for the most part in the neck
of the retort, and may be removed completely in most cases by
washing with a little water. The collection of the gas or the
preparation of the solution may be carried out in the manner
already described.
Properties.-Hydriodic acid exists at the ordinary temperature
and pressure as a colourless gas, having a strongly acid reaction
and suffocating odour, and fuming strongly in the air. It can
be condensed to a colourless liquid by a pressure of four
atmospheres at 0°, or by exposure, under the ordinary atmos-
pheric pressure, to the low temperature of a bath of ether and
solid carbonic acid, and if cooled to - 55° it freezes to a colour-
less ice-like solid mass.
Its specific gravity (air=1) has been found to be 43737,
or 62·94 (H=1), thus closely corresponding to its theoretic
density, 63:45.
Hydriodic acid gas is easily decomposed by heat into iodine.
and hydrogen, as is seen by the violet colour which appears
when the gas is passed through a heated glass tube. A hot
2 Faraday, Phil. Trans. 1845, 1, 170.
1 Ber. 20, 3381.
HYDRIODIC ACID
209
net
12
363
T
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DES
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metallic wire plunged into the gas also causes an immediate
decomposition, violet fumes of iodine making their appearance.
109 The aqueous acid is obtained by passing the gas into water,
by which it is absorbed quickly and in large quantities, yield-
ing, when kept cold by ice, a solution which is twice as heavy as
water, having a specific gravity, according to De Luynes, of 1.99.
A simple mode of preparing a dilute aqueous solution of hydri-
odic acid consists in passing a current of sulphuretted hydrogen
gas through water in which finely-divided iodine is suspended,
the reaction which occurs being as follows-
H₂S+12=2HI+S
On standing, the clear liquid may be poured off from the
precipitated sulphur and boiled to expel any trace of sulphuretted
hydrogen. It is found that the strongest acid which can in this
way be prepared has a specific gravity of 1.56.
On distillation, aqueous hydriodic acid behaves like aqueous
hydrochloric and hydrobromic acids, both the strong and weak
aqueous acid yielding on distillation in an atmosphere of hy-
drogen (to prevent oxidation and liberation of iodine) an acid
of constant composition, boiling at 127° (under a pressure of
774 mm.), and containing 570 per cent. of hydriodic acid.
If dry hydrogen be led through aqueous acids of varying
strengths, each will attain the same constant composition at
the same temperature; thus from 15° to 19° the constant acid
contained 60-3 to 60.7 per cent. of HI. When the hydrogen
was passed through the liquid at 100°, the percentage of hy-.
driodic acid in the constant acid was 58.2,1 and hence it is
seen that no definite hydrate of the acid is obtained by boil-
ing, as was formerly supposed. Aqueous hydriodic acid also
rapidly undergoes oxidation with liberation of iodine when ex-
posed to the air, the colourless solution becoming brown owing
to the solubility of iodine in the acid. Gaseous hydrogen iodide
is also decomposed by dry oxygen when the mixed gases are
exposed to bright sunlight, and differs in this respect from
hydrogen chloride and bromide which are only decomposed by
Oxygen in presence of moisture.2
ILO The Iodides.-The metallic iodides possess great analogy
with the corresponding chlorides and bromides; they are all
1 Roscoe, Journ. Chem. Soc. 1861, 160.
2 Richardson, Journ. Chem. Soc. 1887, i. 805.
I
1
1
1
1
1
15
210
THE NON-METALLIC ELEMENTS
!
solid bodies, less fusible and volatile than the corresponding
chlorides and bromides. Silver iodide, AgI, mercurous iodide,
HgI, and mercuric iodide, HgI2, are insoluble in water, and lead
iodide, PbI,, sparingly soluble, whilst the other metallic iodides
dissolve readily in water. Most of the iodides are decomposed
on heating, either the metal or an oxide being formed and
iodine set free.
All the iodides, whether soluble or insoluble in water, are
decomposed by chlorine and nitrous acid, the iodine being liber-
ated. Some of the insoluble iodides possess a brilliant colour.
Thus, on adding a solution of corrosive sublimate (mercuric
chloride) to a soluble iodide, a salmon-coloured precipitate is
thrown down, which rapidly changes to a brilliant scarlet one of
mercuric iodide, HgI,, soluble in excess of either reagent; a
soluble lead salt, such as the nitrate or acetate, produces a bright
yellow precipitate of lead iodide, PbI,; silver nitrate gives a
light yellow precipitate of silver iodide, AgI, insoluble in nitric
acid and in ammonia. If a mixture of ferrous sulphate, FeSO4,
and copper sulphate, CuSO, be added to that of a soluble
iodide, a greenish white precipitate of cuprous iodide, CuI, is
formed. This reaction depends upon the fact that ferrous
sulphate is oxidized to ferric sulphate, Fe2(SO4)3, whilst cuprous
iodide is precipitated, thus:-
:-
2CuSO4 + 2FeSO4 + 2KI = 2CuI + K„SO4 + Fe2(SO4)3
This reaction serves as a means of roughly separating iodine
from a mixture containing chlorides and bromides.
The metallic iodides can be prepared by similar processes to
those which yield the chlorides and bromides.
(1) By the direct action of iodine on the metal, as in the cases
of the iodides of iron and mercury.
(2) By the action of iodine on certain of the metallic oxides,
hydroxides, or carbonates, as those of potassium, sodium, barium,
calcium, and silver.
(3) By the action of hydriodic acid on certain metals, such
as zinc, hydrogen being liberated.
(4) By the action of hydriodic acid on the metallic oxides,
hydroxides, or carbonates.
(5) By adding a soluble iodide, such as potassium iodide, to
a solution of the salt of the metal, when the metallic iodide is
thrown down in the form of a precipitate; this method, however,
can only be used when the iodide required is insoluble.
ESTIMATION OF IODINE
211
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de 5
Dr
at
ER&M a
Ke
orde
ar
402
III Detection and Estimation of Iodine.-For the detection of
iodine, the starch reaction, the violet-coloured vapours, and the
above-mentioned coloured precipitates are sufficient.
To estirnate iodine in the free state, a standard solution of
sulphurous acid is employed, and the point ascertained at which
sufficient of this solution has been added to reduce all the iodine
to hydriodic acid, thus:-
I½ + 2H2O + SO₂ = 2HI + H„SO.
The solution of sulphurous acid may be replaced by one of
sodium thiosulphate, a substance which reacts with free iodine
in the following manner :—
2Na¿S₂O3 + I2 = 2NaI + Na₂SO6
For the quantitative determination of iodine in a soluble iodide
and for the exact separation from chlorine or bromine, use may
be made of the fact that the palladium nitrate, Pd (NO3)2, pro-
duces with solutions of an iodide, an insoluble precipitate of
Pdl, which on ignition yields metallic palladium. Iodine when
in the form of an alkaline iodide can be weighed also as iodide
of silver, when neither chlorine nor bromine is present; 100
parts of silver iodide contain 54 128 parts of iodine. In the case
of the insoluble iodides, it is best either to transform them into
soluble iodide of sodium by fusing them with carbonate of soda,
or to digest them with zinc and dilute sulphuric acid, when
hydriodic acid is liberated, thus :-
—
2AgI + Zn + H„SO = 2HI + 2Ag + ZnSO4.
If it is required to determine chlorine, bromine, and iodine
when mixed in solution together the following method may be
-:
employed:
Field has shown that chloride of silver is completely de-
composed by digestion with bromide of potassium, the chlorine
and bromine changing places; and that both bromide and
chloride of silver are decomposed in like manner by iodide of
potassium. Hence, if a solution containing chlorine, bromine,
and iodine, be divided into three equal parts, each portion precipi-
tated by nitrate of silver, the first precipitate dried and weighed
1 Chem. Soc. Quart. Journ. 1858, 234.
212
THE NON-METALLIC ELEMENTS
the second digested with bromide of potassium, then dried and
weighed, and the third digested with iodide of potassium, then
dried and weighed, the relative quantities of the three elements
may be determined from the following equations:-
x+y+z=w
186.49
142 32
233.04
142·32x+
5x+y+z=w'
233.04
186-49Y+2=w"
where w, w' w", are the weights of the three precipitates, and
x, y, and z, the unknown quantities of chloride, bromide, and
iodide of silver respectively.
The mixture of the three salts of silver may also be treated
with a solution of potassium bichromate in sulphuric acid, which
converts the chloride and bromide into the soluble sulphate,
whilst the iodide is converted into the insoluble iodate. After
dilution and filtration the iodate may be reduced and the silver
in it determined, whilst the silver originally present as chloride
and bromide may also be determined in the filtrate.¹
In this case the equations became :-
:-
x + y + z = w
AgI
Ag
2=
X
Ag
AgCl
W
+ y
Ag
AgBr
= W2
where w is the weight of the mixed salts, w₁, the weight of
silver from silver iodate, and w₂ the weight of silver from the
chloride and bromide.
IODINE COMPOUNDS WITH OTHER
HALOGENS
112 Iodine burns with a pale flame in fluorine, yielding a
colourless fuming oil, which rapidly attacks glass, and is probably
an iodine fluoride. It is most likely identical with the compound
obtained by Gore by the action of iodine on silver fluoride.
(Moison.)
1 Macnair, Proc. Chem. Soc. 1893, 181.
IODINE TRICHLORIDE
213
sium the
e elery
bitates
omide, a
be the
acid,
e sulphas
ate.
d the s
as ch
weigh
er from th
HER
nellings
is probat
CoLip
er dans
Two compounds of iodine and chlorine are known :-(1) Iodine
monochloride, ICl. (2) Iodine trichloride, ICl. They are both
obtained by the direct union of chlorine and iodine, the higher
chloride being formed when the former element is in excess.
Iodine Monochloride, ICl, is prepared (1) by passing dry
chlorine gas over dry iodine until the latter is completely
liquefied; (2) according to Berzelius, by distilling 1 part of
iodine with 4 parts of potassium chlorate the chlorine evolved
by the reaction combining with a portion of the iodine;¹ (3)
by boiling iodine with strong aqua regia; after dilution with
water the liquid is shaken up with ether in which the chloride
of iodine dissolves and remains behind when the ether is
evaporated.²
-
The product thus obtained is a reddish brown oil, which, on
standing, solidifies, forming long well-defined crystals melting at
247; it boils at 101°3 and has a sp. gr. at that temperature of
288196 (Thorpe). It smells like a mixture of chlorine and
iodine, bleaches indigo solution, but does not colour starch-paste
blue. The following analyses show the composition of this
substance:-
.
Calculated.
Iodine
Chlorine 35.19
125.91 78.85
21.15
161.10 100.00
Liquid.
(Bunsen.)
77.85
22.15
100.00
-----
Found
Solid.
(Schützenberger.)
78.52
21.48
100.00
A second modification, known as ẞ iodine monochloride, is
obtained by heating the crude monochloride till it is free from
trichloride, and then cooling to 10°. It melts at 13.9°3
Iodine Trichloride, ICl,, is obtained (1) by acting on iodine,
gently heated, with a large excess of chlorine; (2) by treating
iodic acid, HIO, with hydrochloric acid; (3) by heating iodine
pentoxide, I,O, with pentachloride of phosphorus, PC15.
This compound forms long lemon-coloured crystals, and very
readily undergoes dissociation. When heated in the air to 25°
it decomposes, giving off chlorine gas, forming the monochloride;
but when heated in an atmosphere of chlorine it only de-
1 Thorpe and Perry, Journ. Chem. Soc. 1892, i. 925.
Bunsen, Annalen, 84, 1.
3 Tanatar, Journ. Russ. Chem. Soc., 25, 97.
214
THE NON-METALLIC ELEMENTS
composes at a much higher temperature, which rises as the
pressure of the chlorine is increased. Thus, under a pressure
of one atmosphere it decomposes at 67° into the monochloride
and free chlorine, and these again unite on cooling to form a
yellow sublimate of the trichloride.¹ The composition of the
compound is seen from the following analysis:-
Calculated.
Iodine.
Chlorine
·
•
·
125.91
105.57
231:48
54.39
45.61
100.00
Found.
54.34
45.66
100.00
Both the chloride and trichloride dissolve in water, ether, and
alcohol apparently without decomposition. When either of them
is acted upon by a small quantity of an alkali, an iodate and
chloride are formed and iodine is liberated.2
6KHO+5CII=5KCl+KIO3+21₂+3H₂O.
They are both very hygroscopic and give off irritating
vapours.
The formation of both of these compounds can be demon-
strated by inverting a small cylinder containing chlorine and
bringing its mouth in contact with that of another of the same
size filled with hydriodic acid gas. Iodine is first liberated,
which combines with the excess of chlorine, the yellow tri-
chloride being deposited on the sides of the upper jar, in which
chlorine is in large excess, whilst the brown monochloride,
mixed with iodine, is formed in the lower jar.
113 Iodine unites with bromine to form a solid, volatile,
crystalline compound which is probably the monobromide, and
also a dark liquid, possibly the tribromide. These bodies
possess properties similar to those of the chlorides of iodine.
OXYGEN. O=15.88.
114 Of the elements which occur on our planet, oxygen is
the most widely diffused, and is found in the largest quantity.
The old crystalline rocks, which constitute the chief mass of
the earth's crust, consist of silicates, or compounds of silicon
and various metals with oxygen. These rocks contain from
44 to 48 per cent. of oxygen. Water likewise is a compound
1 Brenken, Ber. 8, 487.
Philip, Ber. 3, 4.
2
PREPARATION OF OXYGEN
215
fa
Adt
her..
ate
ritat
dense
De S
TWEL
olat?
de s
backe
201
APKAN
――――
of oxygen and hydrogen, containing 88.81 per cent. of the
former element. Oxygen also exists in the free state in the
atmosphere, which contains about 21 per cent. of its volume of
this gas. Although the absolute amount of free oxygen con-
tained in the air is very great, yet the proportion which the free
oxygen bears to that in a state of combination is but very small.
It has already been mentioned, in the Historical Intro-
duction, that the air was believed to be a simple or elementary
substance until the investigations of Priestley, Rutherford, and
Scheele showed distinctly that it is a mixture of two different
gases, only one of which is capable of supporting combustion
and respiration. This constituent of the atmosphere is oxygen,
discovered on the 1st of August, 1774, by Priestley, who, by
heating "red precipitate" (mercuric oxide) by means of the
sun's rays, decomposed it into oxygen and metallic mercury.
The discovery of oxygen enabled Lavoisier to put forward the
true theory of combustion, and to the body capable of support-
ing this combustion was given the name "oxygène" (oğús
sour, and yevváw I produce), from the fact that the products of
combustion are frequently of an acid nature.
Preparation. (1) The simplest method of preparing
Oxygen is to heat mercuric oxide, HgO, in a small retort of hard
glass. The oxide decomposes at a red-heat into metallic mercury
oxygen; 100 parts by weight yield 74 parts by weight of
oxygen.
and
2 HgO=2Hg + O2.
The
apparatus in which this decomposition can be shown is seen
in Fig. 60. Owing to the comparatively high price of oxide
of mercury, this process is only used as a means of illustrating
the decomposition.
(2) The best and most usual mode of preparing oxygen con-
sists in heating potassium chlorate, commonly called chlorate of
potash, KCIO.
This salt loses the whole (3914 per cent.
of its weight) of its oxygen, leaving potassium chloride, thus :-
2KCIO₁ = 2KCI + 30.
The
preparation and collection of the gas, according to this
method, may be carried on in the apparatus shown in Fig. 61.
It
discovery,
appears
that Scheele had prepared oxygen prior to the date of Priestley's
but that these results were not published until after Priestley's
experiments had been made known. See Carl Wilhelm Scheele: Nachgelassene
Briefe und Aufzeichnungen, edited by A. E. Nordenskiöld (Stockholm, 1892).
216
THE NON-METALLIC ELEMENTS
The temperature has to be raised much above the melting point
of the salt (372°), before the evolution of the gas begins; and
after a certain time has elapsed, the fused mass becomes thick,

FIG. 60.
owing to the formation of potassium perchlorate, KC104, a part
of the evolved oxygen having united with the chlorate, whilst

FIG. 61.
potassium chloride, KCl, and oxygen are at the same time
formed, thus:-
10KCIO, = 6KCIO, + 4KCl + 30.
1 Frankland and Dingwall, Journ. Chem. Soc. 1887, 274; Teed, Journ. Chem.
Soc. 1887, 283.
PREPARATION OF OXYGEN
217
1
0300
When more strongly heated, the perchlorate also decomposes
into potassium chloride and oxygen.
The gas can be obtained at a lower temperature by employ-
ing a mixture of potassium and sodium chlorates (Shenstone).
(3) In order to obtain the evolution of oxygen at a lower
temperature, a small quantity of manganese dioxide is generally
mixed with the powdered chlorate; the oxygen then comes off at
about 350°, before the salt fuses, and thus the preparation of the
gas is greatly facilitated. The manganese dioxide is found,
mixed with potassium chloride, in the residue unaltered in
composition.
115 In order to prepare oxygen on a larger scale this mixture

b
FIG. 62.
thick
of potassium chlorate and manganese dioxide is heated in a
copper vessel a, Fig. 62, provided with a wide tube con-
nected with the wash-bottle b, containing caustic soda, for the
purpose of absorbing chlorine gas, which is always evolved
along with the oxygen.
It is difficult to give a satisfactory
explanation of this peculiar action of the manganese dioxide.
It may possibly be due to the fact that certain oxides, such as
this one, are capable of undergoing a higher degree of oxidation,
but that these higher oxides part very readily with a portion of
their
lower oxide. In this way such
oxygen, forming again the
oxides would perform the part of carriers of oxygen, first taking
it up and then setting it free. Indeed it has been rendered
218
THE NON-METALLIC ELEMENTS
probable that when manganese dioxide is used, potassium
permanganate is first formed according to the equation:
2KCIO 2MnO₂ = 2KMnO + Cl2 + O2.
4
3
This is borne out by the facts that traces of chlorine are in-
variably evolved and that when only a small quantity of
manganese dioxide is added, the pink colour of the permanga-
nate can be distinctly seen. The latter is then broken up by
the action of heat and the chlorine evolved by the further
decomposition of the chlorate, into potassium chloride, manga-
nese dioxide and oxygen, after which the reaction is repeated.¹
According to Brunck the gas prepared in this manner
invariably contains ozone.2
The decomposition of the chlorate is also aided by many
other oxides, the action of which is probably to be explained in
a similar manner to that of manganese dioxide. Finely divided
platinum (platinum black), kaolin, and other powdered sub-
stances also produce the same effect, but the above explanation
does not hold good in these cases.3
It not unfrequently happens that the commercial black oxide
of manganese may be accidentally mixed or adulterated with
carbon (pounded coal), and this impure material, when mixed
with chlorate of potash and heated, ignites, giving rise to even
fatal explosions. Hence care should be taken to try any new or
doubtful sample on a small scale beforehand by heating it with
chlorate of potash in a test-tube.
(4) Many other salts behave like potassium chlorate in yield-
ing oxygen on heating among these are the hypochlorites,
chlorites, perchlorates, bromates and perbromates, as well as
the iodates, periodates, nitrates, nitrites and permanganates;
but these compounds are not usually employed for this purpose.
(5) Several oxides, such as manganese dioxide, MnO2, lead
dioxide, PbO2, barium dioxide, BaO2, chromium trioxide, CrO,
lose a portion of their oxygen when strongly heated, and all
these may, therefore, be used for preparing oxygen. In order
to obtain oxygen by heating the first-named oxide, the sub-
stance is placed in a strong iron bottle which can be heated in
a furnace to bright redness (Fig. 63). The pure manganese
1 McLeod, Journ. Chem. Soc. 1889, 184.
2 Ber. 26, 1790.
3 Veley, Phil. Trans. 1888, i., 271; Fowler and Grant, Journ. Chem. Soc.
1890, 272.
PREPARATION OF OXYGEN
219
T
REERS
ALK
Ti
Be
Dithe
FOLLO
mHe
12
Tiend
A
1
ell as
Dates
176e
lead
CH
order
sub-
Dese
民
​dioxide loses one-third (12.4 per cent.) of its oxygen, being con-
verted into the brown oxide, Mn,04, thus:-
3MnO₁ = MnO4 + O2.
2
(6) By heating manganese dioxide in a glass flask with sul-
phuric acid, one-half of its oxygen is given off, and manganous
sulphate, MnSO4, is formed.
2MnO2 + 2H, SO, 2MnSO, + 2H2O + 02.
(7) Chromium trioxide can also be employed for the prepara-
tion of oxygen, but it is not necessary to obtain this substance
=

FIG. 63.
in the pure state, for if bichromate of potash, K,Cr,O,, be
heated with sulphuric acid, chromium trioxide is formed, thus:-
K₂Cr₂O,+2H2SO4 = 2KHSO4 + 2CrO3 + H₂O.
The chromium trioxide is then further decomposed by the
action of sulphuric acid with the formation of chromium sul-
phate,
a decomposition which is rendered visible by the change
of colour from the original red to a deep green, thus:-
4CrO3 + 6H2SO4 = 2Cr2(SO4)3 + 6H₂O + 302.
(8) Oxygen can be obtained by the decomposition of bleach-
ing powder (Mitscherlich, 1843; Fleitmann, 1865). For this
preparation a clear concentrated solution of bleaching powder
which contains calcium hypochlorite, CaCl,O,-is placed in
a flask, and a few drops of cobalt chloride solution added. An
220
THE NON-METALLIC ELEMENTS
oxide of cobalt, which probably has the formula¹ CoO2, is pre-
cipitated, and on heating the mixture to about 80° a rapid
effervescence of oxygen occurs. The cobalt oxide, which is
formed, is left unchanged after the operation, and may be
employed again; it probably acts, like the manganese dioxide,
by the formation of a higher oxide, which is again quickly
reduced, the oxygen being liberated as a gas. Instead of a
clear solution, a thick paste of bleaching powder may be used,
with the addition of a little cobalt salt and a small quantity of
paraffin oil, to prevent the frothing which usually occurs. The
best temperature for the evolution of gas is from 70-80°.
CaCl2O2 = CaCl2 + O2.
The same decomposition and the replacement of chlorine for
oxygen may be shown in a striking manner by passing chlorine
gas, generated in a flask from manganese dioxide and hydro-
chloric acid, into a second flask, which contains boiling milk of
lime, to which a little cobalt nitrate solution has been added.
Oxygen gas is then liberated in the second flask, and may be
collected as usual. The following equation explains the
replacement, and we see that two volumes of chlorine yield
their equivalent, or one volume of oxygen:-
2C1, + 2Ca (OH)₂ = 2CaCl2 + 2H2O + O₂
2
(9) Oxygen can also be prepared by the decomposition of
sulphuric acid. For this purpose a thin stream of sulphuric acid
flows into a platinum retort heated to redness; the acid splits
up into sulphur dioxide, water, and oxygen, yielding 15 68 per
cent. of its weight of the gas, or in practice 55 grams. of acid
yield 6 litres of gas, thus:-
2H₂SO₁ = 2SO₂ + 2H2O + O2.
4
The resulting sulphur dioxide and water can be absorbed, whilst
the oxygen can be collected in a gas-holder. An apparatus for
illustrating this decomposition is described under Sulphuric
Acid.
In order to prepare oxygen cheaply on the large scale several
1 Vortmann, quoted by McLeod, British Ass. 1892, 669.
PROPERTIES OF OXYGEN
221
‚ is pre
i rap
hich is
mari
icide
quickly
ad of s
e used
ne for
lorine
The
HUDAT
silk f
uded
Lar be
the
2
Field
on
ca
splits
S per
adid
for
eral
other processes have been suggested.
following is now widely used.
Amongst them the
(10) When baryta, BaO, is gently heated to dull redness in
the air, it takes up an additional atom of oxygen, forming the
dioxide, BaO, but at a bright-red heat this parts with the
additional atom of oxygen with the reproduction of baryta.
By thus alternately varying the temperature, first leading air
over the baryta contained in a porcelain tube, and then placing
the tube in connection with a gas-holder and raising the
temperature, and again repeating the process, a regular pro-
duction of gas can be obtained from a small quantity of
baryta. This simple method is now carried out on a large
scale according to a process patented by the Brin Oxygen
Company 2
Oxide of barium, prepared from the nitrate, is introduced in
pieces about the size of walnuts into steel or cast-iron retorts.
These retorts, placed in a vertical position in a gas furnace,
are heated to about 700°, and air, carefully purified from
moisture and carbonic acid then pumped through them under a
pressure of about 15 lbs. on the square inch. The baryta absorbs
the oxygen, becoming converted into peroxide, and as soon as this
peroxidation has been carried as far as is economical, the pump
is reversed and the pressure in the retorts thus reduced. When
the reduction of pressure has reached about 26-28 inches of
mercury below the normal, the peroxide begins to give up its
Oxygen, which passes through the pump and is delivered to a
gas-holder. A complete operation lasts about ten minutes, and
nearly 140 operations can be conducted per diem. The oxygen.
is then compressed at 120 atmospheres in wrought iron
cylinders.3
(11) Potassium manganate, K,MnO,, loses oxygen when heated
in a current of steam, forming caustic potash and lower oxides
of
manganese, which when again heated absorb oxygen, the
manganate being reproduced, so that the same portion may be
used over and over again (Tessié du Motay). A modified form
of this process in which sodium manganate is used, has
been applied by Fontana for the industrial preparation of
1
2
4
Oxygen.
It has also been proposed to utilise calcium plumbate
Boussingault, Ann, Chim. Phys. [3] 35.
Pat. 157, Oct. 5, 1885.
Journ. Soc. Chem. Ind. 1890, 246.
Journ. Soc. Chem. Ind. 1892, 312.
222
THE NON-METALLIC ELEMENTS
Ca,PbO, for this purpose. The material is heated at a low
temperature in the presence of carbonic acid, which causes
its decomposition into calcium carbonate, CaCO3, and lead
peroxide, PbO2. When the mixture of these is more strongly
heated oxygen is first given off and a lower oxide of lead
formed, after which the calcium carbonate loses its carbon
dioxide. The residual mass, consisting of a mixture of lime
and the lower oxides of lead is reconverted into calcium
plumbate by heating in a current of air.
116 Properties-Oxygen is a colourless, invisible, tasteless,
inodorous gas which is slightly heavier than atmospheric air.
According to Regnault's experiments 2 the density of the gas is
1·10562 (air=1), one litre at 0° and 760 mm. weighing 143011
grams, whilst according to Leduc 3 the density is 1-10503, and to
Rayleigh, 110535, one litre weighing 142961 grams.
1
The density of the gas compared with hydrogen has been
found by Rayleigh to be 15·882 (see p. 260); its molecular
weight is therefore about 31.76 and its formula O₂-
Oxygen was first liquefied by Cailletet and Pictet in December,
1877. It forms a light blue liquid, boiling at -181·4°, at which
temperature the density of the liquid is 1124. The critical
temperature of the gas is -118°, the corresponding pressure-
being 50 atmospheres.
As long ago as 1847 Faraday discovered that oxygen is less.
diamagnetic than air, and it was subsequently shown to be
actually paramagnetic. Liquid oxygen is strongly magnetic,
and when placed in a cup-shaped piece of rock salt between
the poles of a powerful electro-magnet suddenly leaps up to
the poles and remains there permanently attached until it
evaporates.
Liquid oxygen presents the same absorption spectrum as the
gas, characteristic bands being present in the orange, yellow,
green, and blue, but the bands are much more intense and well
marked than those of the gas. It also possesses a measurable
thermal absorption and presents a very high resistance to an
electric current. When cooled to -210° by its own rapid
evaporation, it is no longer capable of supporting combustion
or of combining with substances like phosphorus and sodium.
-
1 Kassner, Chem. Ind. 1890, 104, 120; Le Chatelier, Compt. Rend. 117, 109.
2 Regnault, Mem. Acad. Science, 21, 144; Crafts, Compt. Rend. 106, 1662.
3 Leduc, Compt. Rend. 113, 186.
Rayleigh, Proc. Roy. Soc. 53, 134.
5 Faraday, Phil. Mag. (3), 31, 401.
* Dewar, Proc. Roy. Soc. 50, 247.
PROPERTIES OF OXYGEN
223
It
24
1
t
1
Pictet has indeed found that chemical action in general entirely
ceases at temperatures approaching -150°. Thus sulphuric
acid and caustic potash when compressed together at this
temperature do not react, although the normal action takes
place at -90°. In the same manner sodium preserves its
metallic lustre in liquid alcohol of 84 per cent. at -78° and
does not commence to act upon it until the temperature reaches
-48°; and alcoholic litmus solution remains blue in contact
with solid sulphuric acid at all temperatures below -105°, at
which reddening suddenly takes place.
Oxygen dissolves appreciably in water; at 0° one volume of
water absorbs 0.04890 volume of oxygen, measured under
the normal temperature and pressure. When the tempera-
ture rises, the quantity of oxygen absorbed becomes less,
according to a complicated law, which is expressed by the
empirical formula.2
C = 0·04890 — 0·0013413t+ 0·0000283t2 – 0·00000029534t³.
Certain metals also absorb oxygen when in the molten state,
and give it off again on solidifying; thus, melted silver absorbs
about ten times its bulk of oxygen, and this is nearly all
emitted when the metal cools, giving rise to the peculiar
phenomenon of the "spitting" of silver.
much
As oxygen is the constituent of the air which supports com-
bustion, it naturally follows that bodies burn in oxygen with
greater brilliancy than they do in common air. A
glowing chip of wood, or the red-hot wick of a taper, ignites
slight detonation when plunged into oxygen gas, and
metals such as iron, which oxidize only slowly in the air,
burn brilliantly in oxygen. The following experiments serve to
illustrate this property of oxygen:-
with a
even
A bundle of thin iron wire, with the ends tipped with
lighted sulphur or a burning piece of twine, burns when
plunged into a jar of oxygen, forming the black oxide, Fe3O4
which falls down in glowing drops. A piece of watch spring
also burns easily with splendid scintillations if held in a flame
obtained by blowing a jet of oxygen into the flame of a spirit
lamp. An even more striking mode of showing the combustion
of iron is to place a heap of cast iron nails on a brick and
burn them by means of a blow-pipe fed with oxygen and coal-
1 Compt. Rend. 115, 814.
2 L. W. Winkler, Ber. 22, 1764; 24, 3607.
·
1
224
THE NON-METALLIC ELEMENTS
gas contained in separate gas-holders. Substances like sulphur
and phosphorus, which take fire readily in the air, burn with
much greater brilliancy in oxygen; combination takes place.
much more rapidly, and, therefore, the temperature reached is
much higher in oxygen than in the air, in which, moreover,
the inert nitrogen takes up a share of the heat. The best
method of exhibiting combustion in oxygen is to place the
substance to be burnt in a metal cup, riveted on to an
upright stem, carrying a round saucer containing water. As
soon as the body has been ignited, a large glass globe filled
with oxygen gas is placed over it, so that the cup occupies a

FIG. 64.
central position in the lower half of the globe, and then the
combustion can proceed with great rapidity without fear of
the globe being cracked by the heat evolved (Fig. 64). In
this way sulphur burns with a bright violet flame, with form-
ation of colourless sulphur dioxide gas, SO, whilst phosphorus,
thus burnt, emits a brilliant white light, which vies with
sunlight in intensity. In this case the white solid phosphorus
pentoxide, P,O,, is the product of the combustion.
117 An act of chemical union accompanied by the evolution
of light and heat, is termed a combustion, and hence oxygen is
commonly termed a supporter of combustion, whilst those bodies
which thus unite with oxygen are called combustible substances.
COMBUSTIONS IN OXYGEN AND IN HYDROGEN 225
Starfind
E
195
le
cet
To
1. 3
iples
I
form
horus
Wit
Des
A little consideration, however, shows that these terms are only
relative, and an experiment makes this plain. One of the stop-
pered bell-jars (Figs. 65 and 66) is filled with oxygen gas, the
other with hydrogen; two gas-holders, one containing hydrogen,
the other oxygen, are provided with flexible gas delivery-tubes at
the end of which is fixed a perforated caoutchouc stopper
carrying a metal tube with a nozzle. The hydrogen gas is
allowed to escape through the nozzle, then ignited, and the
flame of hydrogen plunged into the bell-jar filled with oxygen,
the caoutchouc stopper fitting tightly into the tubulus. A flame

0
H
FIG. 65.
of hydrogen burning in oxygen is then seen, the hydrogen being
the burning body and the oxygen the supporter of combustion.
A stream of oxygen gas is next allowed to issue from the nozzle
of the second gas-holder, the stopper of the bell-jar containing
hydrogen, is then removed, and the jet of oxygen is plunged
into the bell-jar, whilst the flame of a candle is brought at
the same instant to the tubulus. On pressing the caoutchouc
stopper into its place, a flame, not to be distinguished from
that burning in the other bell-jar, is seen, in which oxygen
is the burning body, and hydrogen is the supporter of com-
bustion.
16
226
THE NON-METALLIC ELEMENTS
Most bodies do not combine with oxygen rapidly enough at
the ordinary atmospheric temperatures to give rise to the
phenomena of combustion, but require to be heated before this
begins. Oxidation is, however, often slowly going on, as in
the case of the rusting of metals, the decay of wood and
organic bodies. Thus, we come to distinguish between quick
and slow oxidations in which the intensity of the heat and
light evolved is very different.
This slow oxidation frequently occurs in presence of certain
finely-divided metallic particles, probably owing to the con-

H
FIG. 66.
0
densation of the gases on the surface or in the pores of the
metal. Thus a small quantity of spongy platinum (obtained
by heating the double chloride of platinum and ammonium)
when held over a jet of coal gas or hydrogen, first becomes
red hot, owing to the combustion of the gas occurring on its
surface, and afterwards the temperature of the metal may rise
so high that the jet of gas is ignited.
In some cases the products of the slow oxidation of the sub-
stance are different from those formed by its rapid oxidation or
SLOW OXIDATION
227
e A
Are
U APAV
-Tig
ፍ
oda
combustion. When a coil of fine platinum wire is first heated in
a flame and then hung whilst warm over the surface of some
alcohol contained in a small beaker-glass, the coil soon begins
to glow, and remains red-hot until all the alcohol is consumed,
but no flame is seen. Alcohol has the formula C,H,O, and
when it burns with a flame its constituents unite with oxygen
to form water, H.O, and carbon dioxide, CO. When oxidised
at the lower temperature, a peculiar smelling body termed
aldehyde is formed, having the formula C₂H¸O; hence the
oxidation of the alcohol is partial or incomplete. Only two of
the bydrogen atoms of alcohol are then withdrawn, water being
formed, whilst the volatile aldehyde escapes, giving rise to a
peculiar choking smell.
The effect of mechanical division on the combustibility of
substances, especially of metals, is well known, and advantage
is taken of this in the preparation of the various pyrophori. If
tartrate of lead be gently heated in a glass tube, the lead is left
in a state of very fine mechanical division, and mixed with
carbon. After heating, the tube is hermetically sealed, and on
cooling it may be opened and the contents shaken out into the
air, when the finely divided particles will at once take fire. In
the same way, if the oxides of iron, cobalt, or nickel be reduced
by hydrogen at a moderate temperature, the metal is formed
in a pulverulent state, in which it takes fire spontaneously
on exposure to the air. The explanation of this is, that by
fine division, the ratio of the surface exposed, to the mass
to be heated becomes so great that the heat generated by
the oxidation of the surface is sufficient to bring the mass to
incandescence.
The spontaneous ignition of a mass of inflammable materials
like cotton or woollen rags, when mixed with a substance, such
as oil, capable of rapidly absorbing oxygen, and thereby
generating heat, is one of the most common sources of fire,
both in manufactories and on board ship.
Similar cases of
spontaneous combustion occur in hay-ricks, in which the hay
has been put up damp, for moisture greatly assists the process
of slow oxidation. Other examples of the same thing are seen
in the fires which break out in ships carrying coal, or in heaps
of coal or shale; these seem to be due to the oxidation of the
bituminous constituents of the coal, into carbon dioxide and
water by the oxygen of the air, which is absorbed by the
coal, especially when much broken up, and thus evolve heat
228
THE NON-METALLIC ELEMENTS
enough to set the mass on fire. All the supposed cases of
spontaneous combustion occurring in the human body have
been clearly proved to be mistakes or deceptions, as may
be seen by reading Chapter xxv. of Liebig's admirable Letters
on Chemistry, in which this matter is fully discussed.
118 Temperature of Ignition.—In order that a body may take
fire in air or in oxygen, a certain temperature must be reached:
this point is termed the temperature of ignition. The tempera-
ture at which inflammation occurs varies widely with different
substances; thus while the vapour of carbon bisulphide is ignited
by bringing in contact with it a glass rod heated only to 149°, a
jet of coal gas cannot be lighted with a piece of iron at a dull
red-heat and, again, certain substances, such as the liquid
phosphuretted hydrogen, or zinc ethyl, only require to be
exposed to the air at the ordinary temperature in order to
ignite, whilst nitrogen can only be made to unite with oxygen
by heating the mixture to the temperature of the electric
spark. The temperature at which slow oxidation commences
is of course lower than that of ignition; thus phosphorus begins
to enter into slow combustion in the air (exhibiting phospho-
rescence) below 10° C.; but we must heat it up to 60° C.,
before it begins to burn brightly, or to enter into quick com-
bustion.
The Davy Lamp.-A most striking example of the fact that a
certain temperature must be reached before a mixture of in-
flammable gas with air can take fire, is seen in the safety lamp
for coal mines, invented by Sir Humphry Davy.¹ The principle
upon which this depends is well illustrated by holding a piece
of wire gauze, containing about 700 meshes to the square inch,
over a jet of gas (Fig. 67). If the gas is lit, it is possible to
remove the gauze several inches above the jet, and yet the in-
flammable gas below does not take fire, the flame burning only
above the gauze. The metallic wires in this case conduct away
the heat so quickly that the temperature of the gas at the
lower side of the gauze cannot rise to the point of ignition. In
a similar way we may cool down a flame so much that it
goes out, by placing over it a small coil of cold copper wire
whereas it is impossible to extinguish the flame if the coil of
wire be previously heated. The "Davy lamp" consists of an
oil lamp (Figs. 68 and 69) the top of which is inclosed in a
covering of wire gauze, so that the products of combustion of
1 Phil. Trans. 1817, pp. 45-77.
TEMPERATURE OF IGNITION
229
the oil can escape, while no flame can pass to the outside of the
gauze. Hence no ignition is possible, even if the lamp is placed.
in the most inflammable mixture of fire-damp and air, although
the combustible gases may take fire and burn inside the gauze.
It is, however, necessary, to be careful that the flame thus
D+d
FIG. 68.
FIG. 67.
kindled inside the gauze does not heat it up to the point of
ignition of the inflammable gas, and especially to avoid placing
the lamp in draughts, which might blow the flame against a
point of the
gauze and thus heat it above the point of safety.


FIG. 69.
Indeed, it was pointed out by Davy himself that the lamp is
no longer safe if exposed to a draught of air, and several serious
accidents have occurred from the neglect of these precautions.
It has also been shown that the flame burning inside a wire
gauze may
be mechanically blown through the gauze by a cur-
230
THE NON-METALLIC ELEMENTS
rent or blast of air passing at the rate of eight feet per second,¹
and this has doubtless given rise to many serious accidents.
Several modifications of the original Davy lamp have been intro-
duced to lessen this danger, but the firing of shots in fiery pits
is, in any case, much to be condemned. It is almost unneces-
sary to say that the lamp ought not be opened whilst in use
in the pit.
119 Heat of Combination.-In every chemical reaction the
formation of new compounds is accompanied by a change in the
energy of the system. Thus when two grams of hydrogen at 0°
unite with 15.88 grams of oxygen, also at 0°, to form 17.88
grams of water at the same temperature, we not only have the
material conversion of the 33.3 litres of mixed gases into 17.88 cc.
of liquid water, but we find that sufficient heat is evolved to
raise the temperature of 67,940 grams of water (at 0° C) one
degree. The amount of heat necessary to raise the tempera-
ture of one gram of water (at 0° C.) through one degree is called
a calorie, and the heat evolved in chemical reactions is measured
in terms of this unit.
In order to measure the heat-change which accompanies the
chemical transformation of a mixture of hydrogen and oxygen
into water, oxygen is burnt in hydrogen contained in a platinum
globe immersed in water contained in a calorimeter. The latter
consists of a gilded brass cylindrical vessel, surrounded by two
concentric brass cylinders to prevent loss or gain of heat by
radiation. The air in the platinum vessel is displaced by a
current of hydrogen, and dry oxygen then introduced by means
of a tube and ignited by an electric spark, so that the jet of
oxygen continues to burn in the atmosphere of hydrogen
which is supplied through another tube. The excess of
hydrogen passes out of the globe by a third tube through
a weighed calcium chloride tube, which retains the water
vapour. The water of the calorimeter is stirred throughout
the experiment.
The temperature of the water is observed just before the
commencement of the combustion and a series of observations is
taken at the close of the experiment, the final temperature of the
water being calculated from these after allowing for loss of heat
by cooling. The amount of water formed is found by displacing
the hydrogen by air and weighing the platinum globe after the
¹ Galloway, Proc. Roy. Soc. 22, 441.
2 Thomsen, Thermochemische Untersuchungen 2, 45.
HEAT OF COMBINATION
231
PALE
TELE
de
Z
experiment, the moisture carried off by the escaping gases being
retained by calcium chloride and weighed. An actual experi-
ment gave the following results:---
Initial temperature 16.075°.
Final temperature (corr.) 19.357°.
Rise of temperature 3.282°.
Water value of calorimeter 2,460 grams.
Total weight of water formed 2·129 grams.
The water value of the calorimeter represents the amount of
water contained in it together with the amount which would
require as much heat to raise its temperature one degree as the
metal work of the calorimeter.
The heat developed, therefore, amounts to 2,460 × 3·282
8,074 cal.
To this must, however, be added the heat required
to raise the water produced by the combination to this final
temperature, and the latent heat which was absorbed by the
water passing off with the hydrogen as vapour, amounting in
all to 15 cal.
The total heat for 2129 grams of water is
therefore 8,089 units, and hence the heat of formation of
water is
8,089 x 17.88
2.129
-
67,934 cal.
=
The mean of a number of experiments gives 67,940 cal. for
the production of 17.88 grams of water. This may be ex-
pressed in an equation in the following manner, the chemical
formulae being understood to represent simply the quantities of
the reacting substances and not the molecular weights:-
2H + 0 = H₂0 + 67,940.
The product of this reaction possesses less energy than its
constituents, and the mixed gases are, therefore, said to possess
potential energy, which is converted on their combination into the
kinetic
of motion of the molecules of the heated
energy or energy
products. In order to reconvert the liquid water into the same
weight of the mixed gases this exact amount of energy must be
restored to it, and generally, the heat produced (or absorbed) in
the formation of a compound is exactly equal to that absorbed (or
evolved) by the decomposition of the substance into its original
constituents. Most substances are formed with evolution of
232
THE NON-METALLIC ELEMENTS
the compound so that it possesses more energy than its
constituents. This is the case, for example, with carbon bisul-
phide, the oxides of chlorine and nitrogen, ozone and many
other bodies. Such substances give out heat on decomposition,
and consequently can often be made to undergo explosive
decomposition. In these cases the heat evolved by the resolu-
tion of a small portion of the substance into its constituents
is sufficient to bring surrounding parts to the decomposition
point, and thus the whole mass rapidly breaks up. Chlorine
monoxide, for example, gives out 17,670 cal. on decomposition :
CI,O= 2 Cl + 0 + 17,670 cal.
and is an exceedingly unstable compound, exploding violently
when heated or even shaken.
120 It is frequently impossible to ascertain the heat of form-
ation of a compound directly, but an indirect determination is
rendered possible by the fact that the heat evolved in a chemical
reaction depends only on the initial and final states of the system
and is independent of the intermediate stages.¹
Thus if we wish to find the heat evolved in the production of
a dilute solution of ammonium chloride from the two gases,
ammonia and hydrogen chloride, we may either (1), allow
these two gases to combine directly and dissolve the product in
water; or (2), dissolve the two gases separately in water and
mix the solutions. In both cases we start with ammonia,
hydrochloric acid gas, and water, and end with a dilute solution
of ammonium chloride, and the amount of the heat-change is
found to be the same in both cases.
The determination of the heat of formation of hydriodic.acid
gas from its elements may serve as an instance of the applica-
tion of this indirect method.
When hydriodic acid, dissolved in water, is decomposed by
chlorine we have the reaction :-
HI Aq + Cl HCl Aq + I + 26,000 cal.
=
In this reaction the hydriodic acid has been decomposed and
the hydrogen and chlorine have combined to form hydrochloric
acid which has dissolved in the water. The second part of this
change gives rise to 39,000 cal.
H + Cl + Aq = HCl Aq + 39,000 cal.
¹ Hess. Pogg. Ann. 50, 385.
HEAT OF COMBINATION
233
From this it follows that the heat absorbed in the decomposi-
tion of the aqueous hydriodic acid into hydrogen and iodine is
39,000 - 26,000 = 13,000 cal. and therefore that the heat of
formation of the dilute acid is accompanied by an evolution of
13,000 cal.
-
H + I + Aq = HI Aq +13,000 cal.
It has further been found that the solution of hydriodic acid
gas in water gives rise to an evolution of 19,060 cal., so that the
actual formation of the gas from its elements is accompanied
by a heat-change of 13,000 19,060 6,060 cal. Gaseous
hydriodic acid is, therefore, produced from its elements with
absorption of this amount of heat.
This branch of chemical science, which is known as Thermo-
chemistry, has been studied by many chemists, among whom
may be mentioned Andrews, Favre and Silbermann, Julius
Thomsen and Berthelot, from whose researches the numbers.
given in the following table are taken.¹
HC1
HBr
HI
03
HO
H₂O₂
Cl₂O
LO
MOLECULAR HEAT OF FORMATION FROM THE ELEMENTS.
. 104213
•
. 144905
. 308626
217755
43770
314821
SO2
SO 3
NH,
3
N₂O
NO
NO,
N205
PH,
P₂O
PC13
•
·
-
-
29378
. 67940
-
21835 PCI,
8337 POCI,
6060
As O
As,O
AsHg
22927 BO
- 17670
-
•
=-
CH.
C₂H
44961
.70567 CO2
102526
- 17965
21438
2035
. 13000
4267
177062
74933
(a) charcoal.
11910 (b) gas carbon.
(c) diamond
(d) graphite
со
•
CS2
COCI,
CCI,
-
•
21637
- 2679
96253
95806
93190
92660
67490
- 2581
55183
20843
1 These numbers have been recalculated from the original results on the basis
of 0 = 15.88.
234
THE NON-METALLIC ELEMENTS
THE OXIDES.
121 All the elements with the single exception of fluorine are
found to unite with oxygen to form an important class of com-
pounds termed oxides, possessing very various properties acccord-
ing to the nature of the combining element and the quantity of
oxygen with which it unites. In many instances one element
is found to combine with oxygen in several proportions, giving
rise to distinct oxides. Oxides may be divided into three
classes, distinguished as Basic oxides, Peroxides, and Acid-
forming oxides.
(1.) The basic oxides, such as K,O, potassium oxide; Bao,
barium oxide; Fe,O,, ferric oxide, form in combination with
water a class of compounds termed hydroxides or hydrated oxides,
such as caustic potash, KOH; barium hydroxide or caustic
baryta, Ba(OH)2; ferric hydroxide, Fe(OH),; thus:
K₂O + H₂O = 2KOH.
BaO + H,O = Ba(OH),.
Fe₂03 + 3H₂0 = 2Fe(OH)3.
The characteristic property of these oxides as well as of the
corresponding hydroxides is their power of neutralizing acids
and forming compounds which are termed salts.
(2.) The peroxides contain more oxygen than the basic oxides.
A portion of it is loosely combined and is given off on heating;
thus 3MnO2 = MnO4 + O2; and although they can form hydrox-
ides, these peroxides have not generally the power of neutralizing
acids and forming stable salts. The following is a list of some
of the more important peroxides. Barium dioxide BaO,;
potassium tetroxide K₂O4; sodium peroxide Na₂O₂; manganese
dioxide MnO2; lead dioxide PbO2. The term peroxide is a
somewhat vague one and is applied to oxides possessing very
different properties.
(3.) The acid-forming oxides combine with water to form
hydrates, which are termed acids; thus-
Sulphur trioxide SO, yields Sulphuric acid H,SO.
Nitrogen pentoxide N₂O, yields Nitric acid HNO3.
Phosphorus pentoxide P₂O, yields Phosphoric acid H¸PO.
2
SO, + H₂O = H.SO.
3
2 5
N₂O + H₂O = 2HNO3.
P₂O₂ + 3H2O = 2H¸PO.
THE OXIDES
235
N
H
Jaki
1
F
sil
31
R
Acids possess a sour taste, turn blue litmus red, and neutralize
the basic oxides, which when they are soluble have the opposite
property, and turn red litmus blue.
Salts may be considered to be acids in which the hydrogen
is replaced by a metal. They are obtained by a variety of
reactions, of which the following are the most important.
(1) When certain metals are brought in contact with an acid;
thus-
:-
Zn + H2SO4 = ZnSO4 + H2.
(2) When a basic oxide, or a hydroxide acts upon an acid or
an acid-forming oxide, thus:-
=
PbO + H2SO4 PbSO4 + H₂O.
Ba(OH)2 + H₂SO₁ = BaSO4 + 2H₂O.
BaO + SO, = BaSO,
Ba(OH)2 + SO, = BaSO4 + H₂O.
3
(3) When a carbonate of a metal is acted upon by an acid:
CaCO3 + 2HNO, = Ca(NO3)2 + H2O + CO₂.
The division into these three classes of oxides cannot, however,
be strictly carried out. Thus, whilst the position of the extreme
members of each series, such as the strong bases or alkalis,
on the one hand, and the acids on the other, can be sharply
defined, it is often difficult to classify the middle terms such
alumina, Al,O,, manganese dioxide, MnO2, and tin oxide
SnO2, which act sometimes as weak bases and at other times as
weak acids.
as
OZONE. O₁ = 47·64.
Og
122 So long ago as 1785 Van Marum observed that oxygen
gas through which an electric spark had been passed possessed a
peculiar smell, and at once tarnished a bright surface of mer-
cury; but it was not until the year 1840 that the attention of
chemists was recalled to this fact by Schönbein.¹ This chemist
showed that the peculiar strongly-smelling substance, to which
he gave the name of ozone, from ow, I smell, is capable of
liberating iodine from potassium iodide, and of effecting many
oxidising actions. Schönbein, moreover, showed that
other
ozone is produced in other ways.
¹ Pogg. Ann. 4, 616.
236
THE NON-METALLIC ELEMENTS
(1.) It is evolved at the positive pole in the electrolysis of
acidulated water.
(2.) It is obtained by the slow oxidation of phosphorus in the
air.
(3.) It is formed by the discharge from an electrical machine
through air or through oxygen gas.
For many years much doubt existed respecting the exact
chemical nature of this oxidising principle. Williamson and
Baumert came independently to the conclusion that ozone is
an oxide of hydrogen having the formula H, O.; while Marignac
and De la Rive, as well as Frémy and Becquerel, found that ozone
is formed when electric sparks are passed through perfectly
3

CERCE
D
B
G
FIG. 70.
B
OE
dry oxygen gas. The explanation of these contradictory results
lies in the fact that it was found impossible to obtain ozone
except in very small quantities, and that an exact investi-
gation of its composition is rendered still more difficult by its
extremely energetic properties. Further researches, conducted
with the greatest care, have, however, shown that ozone is
nothing more than condensed oxygen, and the steps by
which this conclusion has been arrived at constitute an admir-
able example of the successful resolution, by the convergence
of many independent investigations, of an apparently insoluble
problem.
To Andrews belongs the credit of having first proved that
1 Phil. Trans. 1856, p. 13.
C
OZONE
237
ozone, from whatever source derived, is one and the same body,
having identical properties, and the same constitution, and also
that it is not a compound of two or more elements, but oxygen.
in an altered and allotropic condition.
If a series of electric discharges be sent through a tube con-
taining pure and dry oxygen, only a small portion of the gas is
converted into ozone; but if the ozone is absorbed as soon as it
is formed, by a solution of iodide of potassium, for example, the
whole of the oxygen can be gradually converted into ozone. In
order to obtain the maximum production of ozone, pure oxygen
gas is allowed to pass through an apparatus (Fig. 70), which con-
sists essentially of an iron tube (BB) turned very truly on its
outside, through which a current of cold water can be passed by
means of the tubes (cc). Outside this metal cylinder is one of
glass (AA) very slightly larger than the iron one. By means of
the tubes (DD) air or oxygen can be passed through the annular
space between the two cylinders. Part of the outer cylinder at
G is covered with tinfoil. The outer tinfoil coating and the inner
metal cylinder are connected with the poles of an induction coil
at E and F. By this means the oxygen is subjected to a series of
silent discharges, by which it is converted partially into ozone.
The action of this stream of ozonised oxygen upon a sheet of
paper covered with a solution of iodide of potassium and starch
is strikingly shown when the paper is held in front of the
current of issuing gas. The white surface assumes instantly
a deep blue colour.
123 That this ozonisation is accompanied by a change of bulk
was shown by Andrews and Tait.¹ These chemists filled a
glass tube (Fig. 71) with dry oxygen; one end was then sealed
off, whilst the other ended in a capillary tube, bent in form
syphon, and containing a liquid, such as strong sulphuric
acid, upon
which ozone does not act. On passing through the
gas a silent discharge, obtained by attaching one platinum wire
pole of a Ruhmkorff's coil, or to the conductor of a
to one
frictional electrical machine, a gradual diminution of volume
occurred, but this never reached more than 1th of the whole.
After the ozonized gas was heated to about 300° C. it was found
to have returned to its original bulk, and had lost all its active
properties.
This decomposition of ozone into oxygen can be readily shown
by
1 Phil. Trans. 1860, p. 113.
238
THE NON-METALLIC ELEMENTS
tube heated by the flame of a Bunsen-lamp. Every trace of
heightened oxidizing action will have disappeared and the blue
iodide of starch will not be formed; whilst on removing the
hot tube an immediate liberation of iodine is observed, if the
prepared paper is again brought into contact with the issuing
gas. In order to gain a knowledge of
the composition of ozone, Andrews
introduced into his ozone tube a sealed
glass bulb containing substances able
to destroy the ozone, such as iodide.
of potassium solution, or metallic mer-
cury. After transforming into ozone
as much as possible of the oxygen con-
tained in this tube, the bulb filled with
the iodide of potassium solution was
broken and the iodine liberated by
the ozone. On observing the column
of sulphuric acid in the syphon tube
it was found to have remained un-
altered after the ozone had reacted,
showing that the change had not been
attended with any alteration in volume,
whilst on afterwards heating up to
300° C. no further increase in the
volume occurred, proving that all the
ozone had been decomposed.
These facts are explained by the sup-
position that, in the formation of ozone,
three volumes of oxygen condense to
form two volumes of ozone
FIG. 71.
302 = 203
6 vols. 4 vols.
which, when heated, increase in bulk
again to form the original three volumes
of oxygen, whilst, when acted upon by potassium iodide, one-
third of the ozone is spent in liberating the iodine, and the
other two-thirds go to form ordinary oxygen thus:-
O3 + 2KI + H₂0 = 0₂ + I2 +2KOH.
2
This supposition has been proved to be correct by Soret, as follows:
Many essential oils, such as turpentine and oil of thyme, had
FORMULA OF OZONE
239
90
13
de
ART
T
܀
ܫܚܐ
been observed by Schönbein to possess the property of absorbing
ozone without decomposing it, and Soret¹ showed that the
diminution in volume which takes place on the absorption of
the ozone from a measured quantity of ozonised oxygen by
these oils is exactly twice as great as the increase of volume
observed when the ozone is decomposed by heating the gas.
A series of three experiments proved that for every 19.3 cc. of
ozone absorbed by the oil, 9-47 cc., instead of the exact number
965 cc., of common oxygen were formed on heating. Hence
ozone possesses the molecular formula Og, three volumes of
common oxygen having been condensed to two volumes by the
formation of ozone.
3
Soret obtained a confirmation of his results from a totally
different point of view. If the density of ozone is one-and-a-
half times as great as that of common oxygen, the rate of diffusion
(p. 65) will be inversely as the square roots of these numbers;
if, therefore, we know the rate at which ozone diffuses, compared
with the rate of diffusion of another gas whose density is also
known, we can draw conclusions respecting the density of ozone.
The gas chosen for experiment was chlorine, and it was found
by experiment that 227 volumes of chlorine diffused in the same
time as 271 volumes of ozone, or for one volume of ozone there
diffused 0-8376 volumes of chlorine; hence, according to the law
of inverse squares of the densities, the density of ozone is 248,
for
1:08376: √35 19: √24.8
whereas from the formula O, it should be the half of 3×15.88
3
that is 23-82.
Brodie 3 arrived, by a long series of most exact determinations,
at the same result, inasmuch as he obtained the ratio of 1 to 2 be-
tween the volume of the oxygen used in liberating iodine from
potassium iodide and that of the ozone absorbed by turpentine,
and also showed that all the oxidising effects of ozone upon the
most various substances can be explained upon this basis.
These experiments prove conclusively that dry oxygen is con-
verted by the action of the silent electric discharge into an allo-
tropic modification. But they do not decide the question whether
the strongly-smelling body obtained in the electrolysis of water
has an
analogous constitution, or whether it may not be an oxide
2
1 Ann. Chim. Phys. [4], 8, 113; Phil. Mag. [4] 31, 82, and 34, 26.
Ann. Chim. Phys. [4] 13, 257.
3 Phil. Trans. 1872, Part ii. 435.
240
THE NON-METALLIC ELEMENTS
of hydrogen. Andrews, however, proved that if such electrolytic
oxygen is perfectly dried, it does not lose its powerful smell, and
that if the dried gas be then passed through a hot glass tube, the
smell, as well as the oxidizing power, altogether disappeared with-
out the smallest trace of moisture being formed, and this must
have been deposited if the electrolytic oxygen had contained
an oxide of hydrogen.
124 Atmospheric Ozone.-The difficult question as to whether
ozone exists in the atmosphere can scarcely be regarded as settled
in the affirmative; it is, however, certain that hydrogen
peroxide (p. 311) is present, and it is very probable that it is
accompanied by ozone. The higher oxides of nitrogen, amongst
other substances, possess the same power as ozone of liberating
iodine from potassium iodide, and these oxides are certainly
formed in the atmosphere by electrical discharges, so that if the
ozone be measured, as is usually the case, by the amount of
iodine liberated, by observing the variation in tint of the so-
called ozone papers, we measure, along with the ozone, the higher
oxides of nitrogen.
The experiments of Andrews¹ have, however, decisively proved
that an oxidizing substance does occur in the atmosphere which
agrees in many of its properties with ozone. Thus when air at
the ordinary temperature was passed over ozone test-papers
contained in a glass tube, an indication of ozone was seen in two
or three minutes. When the air before passing over the test-
paper was heated to 260° C. not the slightest action occurred on
the test-paper, however long the current was allowed to pass.
Similar experiments made with an artificial atmosphere of ozone,
that is, with the air of a large chamber containing a little
electrolytic ozone, gave precisely the same results. On the
other hand, when air mixed with very small quantities of chlorine
or the higher oxides of nitrogen was drawn over the papers,
they were generally affected whether the air had been previously
heated or not. Houzeau has shown that a neutral solution of
iodide of potassium on exposure to air becomes alkaline with
the liberation of iodine, an effect which would not be produced
by the oxides of nitrogen, and which he believed to be due to
the presence of ozone in the air.
These effects may, however, possibly be due to hydrogen
peroxide. The only perfectly satisfactory distinction between
ozone and hydrogen peroxide is that the former produces a
1 Proc. Roy. Soc. 16, 63.
ATMOSPHERIC OZONE
241
deposit of peroxide of silver on a piece of silver foil, and this
has never been obtained with atmospheric air.¹
In spite of the uncertainty as to whether ozone really exists
in the air, many methods have been given for its determination.
Thus Zenger passed 100 litres of air through a dilute solution
of hydriodic acid and obtained iodine liberated, which corre-
sponded to 0.001 or 0·002 milligram of ozone; and it is doubt-
ful how far the iodine was really liberated by ozone. Certain
other observers 2 state that the proportion of ozone in the air
stands in a direct relation to the amount of atmospheric elec-
tricity present, whilst others again conclude from their ob-
servations that under the normal atmospheric conditions the
amount of ozone in the air is absolutely constant.
3
air
The usual method of estimating the amount of ozone present
in the air is a very rough one. It consists in exposing to the
papers which have been impregnated with a solution of starch
and iodide of potassium, for a given time (and best in the dark),
and noting the tint which they assume compared with certain
standard tints. The papers prepared according to the directions
of Dr. Moffat are those on which most reliance is placed. It
has indeed been proposed by Böttger to use papers impregnated
with thallious oxide as a test for ozone, as this substance is
not permanently changed in tint by the nitrogen oxides, but
this suggestion has not been generally adopted, and doubt has
been thrown by Lamy on the use of this reagent, as anything
more than a qualitative test of the presence of ozone.
It is scarcely necessary to remark that in thickly-inhabited
districts, especially in towns where much coal is burnt, ozone
is almost always absent, as it is reduced to ordinary oxygen
by the organic emanations as well as by the sulphurous acid
constantly present in such air.
Some observers state that in the air of the country, and
especially in sea air, the presence of ozone can almost always
be recognized, often indeed by its peculiar smell, this being said
by them to be the most reliable test for its presence. Respect-
ing the variations in the amount of atmospheric ozone in different
localities or in different seasons we possess at present no reliable
information.4
1
¹ Fremy, Compt. Rend. 61, 939; Schöne, Ber. 13, 1503.
* Neumann, Pogg. Ann. 102, 614, and Poëy, Compt. Rend. 65, 708.
3 Smyth, Proc. Meteoro. Soc. June 16, 1869.
4 Compare Ilosva, Bull. Soc. Chim. 3, 2,377.
I
1
1
17
242
THE NON-METALLIC ELEMENTS
A possible cause of the formation of ozone in the air has
been pointed out by Gorup v. Besanez,' inasmuch as he
has shown that an oxidizing substance is invariably formed
when water evaporates. This substance is however probably
hydrogen peroxide. The production of ozone by the slow
oxidation of phosphorus has already been mentioned. Several
other substances on oxidation also give rise to a formation of
ozone; thus turpentine and several other essential oils when
acted upon by atmospheric oxygen transform a portion of it
into ozone. This may be seen by shaking turpentine in a flask
containing air or oxygen, when the liquid will exhibit the
properties of ozone. Another method by which the active.
variety of oxygen may be obtained is by acting with strong
sulphuric acid upon dry barium dioxide, when oxygen is given
off, which is found to contain considerable quantities of ozone.
It is also stated that ozone is formed during combustion, and
can be recognized by its smell when a current of air is blown.
through the upper portion of a flame.
Properties.-Ozone prepared by any of the above methods.
is a gas possessing a peculiar odour, somewhat resembling
that of very diluted chlorine. It has a faint blue colour,
which is rendered more evident by compression. Ozone, when
dry, may be preserved in sealed glass tubes at the ordinary
atmospheric temperature for a very long time, but it changes
gradually into common oxygen. Not only is ozone destroyed
by heat, but also when agitated strongly with glass in fine
fragments (Andrews). It is one of the most powerful oxidiz-
ing agents known; it attacks and at once destroys organic
substances such as caoutchouc, paper, &c. One of the most
characteristic actions of ozone is its effect on mercury.
The
metal at once loses its mobility and adheres to the surface of the
glass in a thin mirror, and so delicate is this reaction, that a
single bubble of oxygen containing th of its bulk of ozone will
alter the physical characters of several pounds of mercury,
taking away its lustre and the convexity of its surface.
In many of its oxidising actions the volume of ozone does
not undergo any alteration, one molecule of ozone, O, yielding
one molecule of ordinary oxygen, O2, and one atom of oxygen
being employed for the oxidation. Ozone is converted into
ordinary oxygen by contact with certain metallic oxides,
such as oxide of silver and manganese dioxide or with
1 Annalen, 111, 232.
PROPERTIES OF OZONE
243
platinum black. These substances are not permanently altered
by the reaction, which is probably of somewhat the same nature
as that by which potassium chlorate is decomposed at low tem-
peratures in the presence of certain bodies (p. 217). Some non-
metals as well as most metals are at once oxidized in presence
of moist ozone phosphorus to phosphoric acid, sulphides to
sulphates, ferrocyanides to ferricyanides, whilst blood is com-
pletely decolorized, the albumen being entirely, and the other
organic matters being nearly all, destroyed. Ozone has not,
however, according to the experiments of Carius,¹ the power it
was formerly supposed to possess, of oxidizing nitrogen to nitric
acid in presence of water.
Ozone is somewhat soluble in water, imparting to water its
peculiar odour as well as its oxidizing powers. According to
Carius, 1,000 volumes of water dissolve 45 volumes of ozone,
and it is much more soluble in certain ethereal oils.
Ozone forms on condensation an indigo-coloured liquid
(Hautefeuille and Chappuis), which boils at -106° (Olszewski),
and like liquid oxygen is strongly magnetic. The compression
of the gas must be effected slowly, or the heat produced raises
its temperature to such a point that the gas is suddenly con-
verted into ordinary oxygen with explosion. A considerable
amount of heat is evolved in this change, ozone being formed
from oxygen with absorption of 29,378 cal. (Berthelot).
30₂ = 203 2 x 29378.
-
It is found that the change of one allotropic form of a substance
into another is always accompanied by either an evolution or an
absorption of heat.
Schönbein, and certain other chemists, believed that another
modification of oxygen besides ozone exists, to which they gave
the name of ant-ozone; the chief peculiarity of this body being
its power of combining with ozone to form ordinary oxygen.
Further experiments have, however, proved that ant-ozone is
nothing more than hydrogen dioxide.³
1 Liebig, Annalen, 179, 1.
2 Dewar, Proc. Roy. Soc. 50, 261.
3 See Brodie, Phil. Trans, 1862, 837.
"
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D
244
THE NON-METALLIC ELEMENTS
HYDROGEN AND OXYGEN
These elements form two compounds,
(1) HYDROGEN MONOXIDE or WATER, H₂O, and
(2) HYDROGEN DIOXIDE, H₂O.
WATER. H₂O = 17.88.
125 The question of the discovery of the composition of water,
a substance which up to nearly the end of the last century was
considered to be a simple body, has been fully discussed in the

FIG. 72.
m
historical introduction. We there learned that Cavendish first
ascertained that by the combustion of two volumes of hydro-
gen and one volume of oxygen, pure water and nothing else is
produced. Warped, however, as his mind was with the phlogistic
theory, he did not fully understand these results, and the true
explanation of the composition of water was first given by
Lavoisier in 1783, when the French chemist repeated and
confirmed the experiments of Cavendish. The apparatus, of
much historical interest, used by him for proving that hydrogen
gas is really contained in water, is seen in facsimile in Fig. 72.
WATER
245
The water contained in the vessel a was allowed to drop slowly
into the tube, e d, from which it flowed into the gunbarrel, df,
heated to redness in the furnace. Here part of the water is
decomposed, the oxygen entering into combination with the
metallic iron, whilst the hydrogen and some undecomposed
steam passed through the worm, s, where the steam was con-
densed and the hydrogen was collected and measured in the
glass bell-jar, m. The result of these experiments was found
to be that 13:13 parts by weight of hydrogen united to 86 87
parts by weight of oxygen, or 12 volumes of oxygen with 22.9
volumes of hydrogen.¹
Cavendish, by exploding air with hydrogen by means of the

FIG. 73.
electric spark had, on the other hand, come to the conclusion
that the relation by volume of the two gases combining to
form water was 1 of oxygen to 2 of hydrogen, and this was
confirmed in 1805 by the more exact experiments of Gay-
Lussac and Humboldt.2
The formation of water by the combustion of hydrogen in the
air can be readily observed by means of the arrangement shown in
Fig. 73. The hydrogen is dried by passing through the horizontal
tube filled with pieces of chloride of calcium, then ignited at the
end of the tube, and the flame allowed to burn under the bell-
jar. By degrees drops of water form, these collect on the sides
of the glass, and drop down into the small basin placed beneath.
1 Mémoire par MM. Meusnier et Lavoisier.
année 1781, p. 269, lu le 21 Avril, 1784.
Mém. de l'Acad. de Sciences,
2 Journ. de Phys. 60, 129.
I
1
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246
THE NON-METALLIC ELEMENTS
Another apparatus for exhibiting the same fact is seen in Fig.
74. It consists of a glass gasholder filled with hydrogen,
which is dried by passing through the chloride of calcium tube
(b), and then burns under the glass funnel (c). The water
formed collects in the tube (e), an aspirator (f) drawing the
steam formed by the combustion through the tube (d).
126 Eudiometric Synthesis of Water. The method which
Cavendish employed for the purpose of ascertaining the com-
position of water is still employed in principle, although the
modern processes are much superior in accuracy to the older
ones. Bunsen's modification of the method consists in bringing
known volumes of the constituent gases successively into a eudio-

FIG. 74.
C
meter and allowing these gases to combine under the influence
of the electric spark, carefully observing the consequent change
of volume. The eudiometer employed is a strong glass tube (c),
Fig. 75, one metre in length and 0.025 m. in breadth, closed
at the top and open at the bottom, having platinum wires
sealed through the glass near the closed end. The tube is accu-
rately divided into divisions of length by etching a millimetre
scale on the glass, and the capacity of each division of length on
the scale is ascertained by a process of calibration, consisting in
pouring successively exactly the same volume of mercury into
the tube, until the whole is filled with the metal, the height to
EUDIOMETRIC SYNTHESIS OF WATER
247
which each volume of mercury reaches being carefully read off
on the millimetre scale etched on the glass.
The eudiometer containing at the top one drop of water to
render the gases moist, is first completely filled with mercury and
inverted in the pneumatic trough (d) containing the same metal.

1:|:ཀུན དུ 1|:ཆེན ཆད པར ཤེས ཞིག ཏུང པོལ ས ན ར ནངEEEEFETA
FIG. 75.
Then a certain volume of perfectly pure oxygen gas, prepared
from pure potassium chlorate, is introduced, the volume is read
off, and the necessary reductions for temperature and pressure are
made. For this purpose a thermometer (b) is hung up near the
eudiometer, and the temperature as well as the level of the
meniscus of mercury in the tube read off by means of a tele-
248
THE NON-METALLIC ELEMENTS
scope placed in a horizontal position at such a distance that the
radiation from the observer does not produce any sensible effect
on the reading. The pressure to which the gas is subjected is
then ascertained by reading off the height of the barometer (a),
also placed near the eudiometer, and subtracting from this the
height of the column of mercury in the eudiometer above the
level of the mercury in the trough, this height being obtained by
reading the millimetre divisions at the upper and lower levels of
the mercury. The temperature of the mercurial columns in the
barometer and eudiometer must also be observed, so that cor-
rection may be made for the expansion of the mercurial column
the height of which must be reduced to that of a column at
0° C.
We have now, taking an actual example:-
(1) The observed volume of moist oxygen taken from the
reading of the upper level of mercury and from the calibration-
table of the eudiometer = 399.1.
(2) The temperature of the gas = 15° C.
=
(3) The height of the barometer (corrected to 0° C.) 765 mm.
(4) The height of the mercury column in the eudiometer
(corrected to 0° C.) = 500 mm.
From these data it is easy to obtain the volume of the gas at
the normal temperature (0°) and under some standard pressure
(either 1 m. or 760 mm. of mercury at 0°). The gas has,
however, been measured in the moist state; the water vapour
present exerts a certain pressure, and thus depresses the column
of mercury in the eudiometer and increases the apparent pres-
sure of the gas. In order, therefore, to obtain the actual
pressure of the dry gas it is necessary to subtract from the
height of the barometer, the column of mercury in the eudio-
meter and the vapour pressure of the water at the temperature
of the experiment, which may be found in a table of vapour
pressures (p. 277). This amounts at 15° to 127 mm. of mercury,
so that the true pressure of the gas considered dry is :
765 500 12.7 =252.3 mm.
---
Applying the laws of Boyle and Dalton it appears that 3991
vols. of gas at 15° and 2523 mm. pressure will, at 0° and a
pressure of 1 metre of mercury, occupy a volume of:
3991 × 273 × 2523
288 X 1000
= = 95'45.
EUDIOMETRIC SYNTHESIS OF WATER
249
The second part of the process consists in adding a volume.
of pure hydrogen, care being taken not to allow any bubbles.
of gas to remain attached to the sides of the tube. The
volume of hydrogen added must be such that the inflammable
mixture of two volumes of hydrogen and one volume of oxygen
shall make up not more than from 30 to 40 per cent. by volume
of the whole gas, otherwise the mercury is apt to be oxidized by
the high temperature of the explosion. Thus supposing we had
five volumes of oxygen, we must add ten volumes of hydrogen
65 × 15
to combine with this, and
= 28 volumes for the
purpose
35
of dilution.
As soon as the temperature equilibrium has been established,
the volume of the mixed gases contained in the eudiometer is
again read off with the same precautions, and the temperature
and pressure again ascertained as before. This having been
accomplished, the open end of the eudiometer is firmly pressed
down below the mercury in the trough upon a plate of caout-
chouc, previously moistened with corrosive sublimate solution,
and held firmly in this position by a stout clamp. By means
of an induction coil an electric spark is then passed from one
platinum-wire through the gas to the other wire; the mixed
gases are thereby ignited, and a flame is seen to pass down the
tube. On allowing the mercury from the trough again to enter
freely at the bottom of the tube a considerable diminution of
bulk is observed. The eudiometer is then allowed to remain
untouched until the temperature of the gas has again attained
that of the surrounding air, and the volume, pressure, tension,
and temperature are ascertained as before. The volume which
has disappeared does not, however, exactly correspond to the
true volume of gases which have united, inasmuch as the water
formed, occupies a certain although a very small, space. In
order to obtain the exact volume of the combined gases, the
volume of the explosive gases before the explosion must be
multiplied by the number 00005, which represents the frac-
tion of the total bulk of the component gases which is occu-
pied by the liquid water formed, and this volume must then
be added to the observed contraction. For other corrections
the article on this subject in Bunsen's Gasometry must be
consulted.
The following numbers illustrate the course of such an ex-
periment:-
250
THE NON-METALLIC ELEMENTS
Synthesis of water by volume.
Volume of oxygen taken.
Volume of oxygen and hydrogen.
Volume after the explosion
•
FIG. 76.
Reduced to 0° and 1 m. of mercury.
95.45
557.26
271.06
·
•
.
.
Hence 286 2 volumes disappeared, or 9545 volumes of oxygen
have combined with 19075 of hydrogen. Consequently 10000
volume of oxygen combines with 19963 volumes of hydrogen
to form water.

By careful repetition of the above experiments the com-
position of water by volume has been ascertained, within very
narrow limits, to be in the proportions of one of oxygen to two
of hydrogen.
Since the knowledge of the exact composition of water by
volume is of the greatest importance for the determination.
of the atomic weight of oxygen, many attempts have been
made to devise more accurate methods than that above de-
scribed, for ascertaining the exact ratio in which hydrogen and
oxygen combine by volume.
ELECTROLYSIS OF WATER
251
The most accurate of these is due to Morley,¹ who has re-
tained the principle of the eudiometric method, but greatly
improved its details. The greatest care was expended in purify-
ing the hydrogen gas employed, which was prepared by the
electrolysis of pure dilute sulphuric acid. Special precau-
tions were taken to free the gas from any traces of oxygen
by passing it over heated copper, after which it was care-
fully dried and used for the experiment. Portions of each
of the pure gases were then brought into the eudiometer
and exploded, the oxygen being in excess in some and the
hydrogen in other experiments. As soon as a large volume
of the gases had been brought into combination by successive
introductions followed by explosions, the residue remaining in
the eudiometer was analysed and the amount of residual hydro-
gen or oxygen, and impurity consisting of atmospheric nitrogen
from which it is impossible to free the hydrogen, ascertained.
This was then subtracted and the volumes of the two gases
thus determined. The average of twenty experiments con-
ducted in this manner gave the ratio of 1: 2·0002. It is clear
that if Gay-Lussac's law were strictly applicable, the volumes
must be in the simple ratio of 1: 2. The slight deviation from
this proportion, which seems to be well established, serves to
show that this law, like those of Boyle and Dalton, is only
approximately true, and that Avogadro's theory which is founded
upon it, is also only an approximate representation of the truth.
This deviation must of course vary with the pressure at which
the comparison is made, since the two gases deviate to different
extents from Boyle's law.
Electrolytic Analysis of Water.
127 A convenient form of voltameter for demonstrating the com-
position of water by volume is shown in Fig. 76. On passing a
current of electricity through the water, acidified with sulphuric
acid, which fills the U-shaped tube, bubbles of oxygen rise from
the surface of the platinum plate forming the positive pole,
whilst bubbles of hydrogen are disengaged from the negative
pole. The gases from each pole are collected separately, and
the volume which collects in the tube containing the negative
pole is seen to be a little more than double that which collects
from the positive pole. On trial the latter is found to be
¹ Chem. News, 63, 218.
252
THE NON-METALLIC ELEMENTS
oxygen, and the former hydrogen. In this experiment the
volume of the oxygen gas is found to be rather less than half
that of the hydrogen, because, in the first place, it is more
soluble in water than hydrogen, and, secondly, because a portion
of the oxygen is converted into ozone, which, being condensed
oxygen, occupies a less volume than oxygen in the ordinary
form. The fact that ozone is thus produced may be shown by
bringing some iodized starch paper in contact with the electro-
lytic gas, when iodine will be liberated, and the paper will

FIG. 77.
b
at once be turned blue. By raising the temperature of the
acidulated water to 100°, the solution of the gases is prevented,
and at the same time the formation of ozone is avoided, so
that the true volume relation is thus much more closely attained.
It must be remembered that the decomposition of the water
is not directly caused by the passage of the electric current.
The sulphuric acid which is present yields the ions H and SO,
(p. 116), the former of which give up their charges of electricity
at the negative pole and escape from the solution, the atoms
VOLUMETRIC COMPOSITION OF STEAM
253
combining to form molecules of hydrogen. The ions SO4, on
the other hand, give up their charges at the positive pole and at
once decompose, oxygen being liberated and a fresh quantity
of sulphuric acid formed :—
SO, H₂O H₂SO, + 0.
+
=
The atoms of oxygen then unite to form molecules which
escape from the solution, whilst the regenerated sulphuric acid
again undergoes a similar series of changes.
The apparatus, the construction of which is plainly shown in
Fig. 77, is used for collecting the mixed gases evolved by the
electrolysis of water. The mixed gases, thus prepared, combine
with explosive violence when a flame is brought in contact with
them, or when an electric spark is passed through the mixture.
In this act of combination, the whole of the hydrogen and the
whole of the oxygen unite to form water: in other words, sub-
ject to the correction above referred to respecting the volume of
water formed, the total volume of the detonating gas disappears.
That this is the case is seen from the following experiments
made by Bunsen, in which air was mixed with the electrolytic
gas, the mixture exploded, and then the volume of air deter-
mined. A second addition of the explosive gas was next made
and the volume of air again read off, and the operation repeated
a third time.¹
Original volume of air, in which detonating i
gas had been once exploded. .
After explosion, with 55:19 vols. detonating
gas
Ditto, measured again after 24 hours
After second explosion with 71:23 vols. deto-
nating gas.
to-}
112.68
112.68
112.57
112.66
128 Volumetric Composition of Steam.-Gay-Lussac not only
determined the composition of water by volume, but was the
first to ascertain that three volumes of the mixed gases com-
bine to form two volumes of gaseous steam; inasmuch as he
found the specific gravity of steam to be 06235, the number
deduced from the above composition being 0.6221.
This fact can be readily shown by exploding some of the
1 Gasometry, p. 65.
:
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254
THE NON-METALLIC ELEMENTS
electrolytic detonating gas evolved from the voltameter, Fig. 77
in the eudiometer E Fig. 78 which is so arranged that the
pressure on the gas can be altered at pleasure. Surrounding
C
1-1
F
FIG. 78.
сене
H
M
G
the eudiometer is a glass tube (T), and between the two tubes a
current of the vapour of amyl alcohol, which boils at 132°, can
be passed from the flask (F), and the vapour, after passing
COMPOSITION OF WATER
255
through the tube, condensed in the flask cooled in the trough of
water (H). When the temperature of the tube and of the gas
has risen to 132°, the volume of the gas is exactly read off on
the divided scale of the eudiometer, the height of the mercury
in the two limbs having been brought up to the same level by
means of the reservoir of mercury (M) attached to the iron foot
of the eudiometer by the caoutchouc tube (G). The pressure on
the gas is now reduced by lowering the level of the mercury
and, by means of the induction coil (C), a spark is passed. As
soon as combination has taken place, the level of mercury in the
two tubes is brought to the same height, and the volume of the
gaseous water is accurately read off, the temperature of the whole
being still kept up to 132° by the current of amyl alcohol vapour.
This volume is found to be almost exactly two-thirds of that of
the original mixed gases, and hence we conclude that 2 vols.
of hydrogen and 1 vol. of oxygen unite together to form 2 vols.
of steam or gaseous water.
The Composition of Water and the Atomic Weight of
Oxygen.
129 A knowledge of the composition of water by weight is of
the utmost importance, because the ratio in which hydrogen and
oxygen combine must be known before the exact atomic weight
of the latter (hydrogen being taken as the unit) can be deter-
mined. The earlier experimenters adopted a very simple method
for this purpose.
Many metallic oxides such as copper oxide, CuO, when heated
in a current of hydrogen lose their oxygen which combines with
the hydrogen to form water, the metal being produced. By
ascertaining the loss of weight which the oxide thus suffers, and
by weighing the water formed, we obtain all the data required
for determining the ratio by weight in which the two gases are
present in water, inasmuch as water contains no other constituent
besides oxygen and hydrogen.
This method of determining the synthesis of water by weight
was first proposed and carried out in 1820 by Berzelius and
Dulong, with the following results :-
1 Ann. Chim. Phys. 15, 386.
256
THE NON-METALLIC ELEMENTS
Synthesis of Water by Weight.
Loss of weight
Weight of
of copper oxide.
water obtained.
8:051
10.832
8.246
No.
1
2
3
•
•
•
•
No. 1.
88.942
Oxygen
Hydrogen 11.058
·
·
·
Hence we have the following numbers as representing the per-
centage composition of water according to these experiments:—
Percentage Composition of Water by Weight (Berzelius and
Dulong).
9.052
12.197
9.270
No. 2.
88.809
11.191
100.000
No. 3.
88.954
11.046
Mean.
88.90
11.10
100.000
100.000
100.00
It is thus seen that the separate experiments do not agree very
closely amongst themselves, and do not therefore yield us cer-
tain information as to the exact proportions by weight in which
the gases combine to form water.
In the year 1843, Dumas¹ undertook in conjunction with
Stas, a most careful repetition of these experiments; pointing
out the following probable sources of error in Berzelius's
experiments :-
(1) The weight of water formed ought either to be ascertained
in vacuo or reduced to a vacuum; this reduction would increase
the quantity of water by about 10 to 12 milligrams.
(2) The weight of oxygen ought also to be reduced to a vacuum.
(3) The hydrogen ought to be much more carefully dried than
was the case in the older experiments.
(4) Lastly, even supposing that the weights had thus been
adjusted, and if the hydrogen had been properly dried, Berze-
lius's determinations were made upon too small a scale to ensure
the necessary degree of accuracy.
A facsimile of the apparatus as used by Dumas is shown in
Fig. 79.
F is the vessel in which the hydrogen is evolved.
E is a funnel with stop-cock, containing sulphuric acid.
1 Ann. Chim. Phys. [3], 8, 189
COMPOSITION OF WATER
257

B1
A2
FIG. 79.
18
258
THE NON-METALLIC ELEMENTS
A is a cylinder filled with mercury under the surface of which
dips a safety tube.
The first U-tube contains pieces of glass moistened with
nitrate of lead.
The second U-tube contains glass moistened with silver
sulphate.
The third U-tube contains in the first limb pumice moist-
ened with potash, and in the second limb pieces of solid caustic
potash.
The fourth and fifth U-tubes contain fused solid caustic
potash.
The sixth and seventh U-tubes contain fragments of pumice
powdered over with phosphorus pentoxide, and are immersed in
a freezing mixture.
The eighth is a small weighed tube containing phosphorus
pentoxide.
B is a bulb blown on hard glass containing the dry oxide of
copper, furnished with a stop-cock (r) at its upper end, and
drawn out so as to pass into the narrow neck of the vessel B₁
at the lower end.
The bulb B can be heated by the Bunsen lamp placed on the
sliding holder of the retort-stand.
B, is the bulb in which the water, formed by the decom-
position, collects.
The U-tube placed next to the bulb B, contains pieces of
fused caustic potash.
The next U-tube contains phosphorus pentoxide and is sur-
rounded by a freezing mixture. Next to this is placed a small
weighed tube containing phosphorus pentoxide, whilst at the
end we find another tube like the last, but not weighed. A
cylinder A, filled with sulphuric acid, through which the excess
of hydrogen gas escapes, completes the arrangement.
With this apparatus Dumas made no less than 19 separate
experiments carried out with very great care. The hydrogen
evolved from zinc and dilute sulphuric acid, might contain oxides
of nitrogen, sulphur dioxide, arseniuretted hydrogen and sul-
phuretted hydrogen. These impurities are eliminated, and
the gas at the same time completely dried by passing over
the substances contained in the U-tubes; the nitrate of lead
absorbs the sulphuretted hydrogen; the sulphate of silver de-
composes any trace of arseniuretted hydrogen, and the rest of
COMPOSITION OF WATER
259
the tubes serve to arrest every trace of carbonic acid and mois-
ture, so that the gas passing through the stop-cock into the
bulb B consists of perfectly dry and pure hydrogen. In order to
render this certain, the small tube next to the bulb is weighed
before and after the experiment, and if its weight remain con-
stant we have proof that the gas has been properly dried. A
similar small weighed tube serves a like purpose at the other
end of the apparatus.
Great care must be taken that the oxide of copper contained
in the bulb B is perfectly dry, for this oxide being a hygroscopic
substance is liable to absorb water from the atmosphere. The
weight of the bulb containing the oxide is then accurately deter-
mined, and after all the air has been driven out of the U-tubes
by the dry hydrogen, the bulb is fixed in its place. The bulb
destined to receive the water is also carefully weighed before the
experiment, together with the 3 drying-tubes placed beyond it
for the purpose of absorbing every trace of aqueous vapour car-
ried over by the hydrogen. Then the oxide of copper is heated
to dull redness; the reduction commences, and the formation of
water continues for from 10 to 12 hours. After this, the bulb B
is allowed to cool in a current of hydrogen; the apparatus
is then taken to pieces, the bulb rendered vacuous and weighed,
whilst the hydrogen contained in the bulb and tubes serving to
collect the water is displaced by dry air before this portion of
the apparatus is weighed. It is clear the weight of hydrogen is
not directly determined by this method but that it is obtained
as the difference between the weight of water produced and that
of the oxygen consumed. As, however, the weight of the
hydrogen is only of that of the water formed, it is evident that
a percentage error of a given amount on the weight of water will
represent a much larger percentage error on the smaller weight
of hydrogen. The simplest way of reducing such errors is to
arrange the experiment so that a large quantity of water is
obtained, for the experimental errors remain, for the most part,
constant, and by increasing the quantity of substance experi-
mented upon, the percentage error is kept down. For this pur-
pose Dumas took such weights of copper oxide as would produce
in general about 50 grams of water, so that the experimental
error, on hydrogen taken as the unit, is reduced to 0.005 of its
weight. In the 19 experiments Dumas found that 840-161
grams of oxygen were consumed in the production of 945 439
1
260
THE NON-METALLIC ELEMENTS
grams of water; or the percentage composition of water by
weight is as follows:-
Percentage Composition of Water by Weight (Dumas).
88.864
Oxygen
Hydrogen
11.136
•
·
100.000
In other words two parts by weight of hydrogen combine with
15.9608 parts by weight of oxygen to form water.
This number is in almost exact accordance with the results
of the volumetric analysis of Bunsen, and the density deter-
minations of Regnault. According to the former, the ratio
of the volumes in which the two gases combine is exactly
1:2, whilst the latter found that oxygen is 15.96 times as
heavy as hydrogen. Dumas' results represented the most ac-
curate determinations made up to the year 1888, but since
that time a number of investigations on the subject has been
made with the utmost care and accuracy, the result of which
has been to show that the number 15-96 is undoubtedly too
high. The chief sources of error in the experiments of Dumas
are—(1) the presence of impurities in the hydrogen; (2) the
fact that heated copper takes up a certain amount of hydro-
gen, forming a hydride of this metal; (3) the action of the
hydrogen upon sulphuric acid, used by Dumas in some of his
experiments as a drying agent, by which sulphur dioxide is
liberated.
In the more recent determinations, the results of which are
given in the accompanying table, these errors have as far as
possible been avoided. Thus Cooke and Richards, Noyes and
Keiser, weighed the hydrogen employed, the last-named in the
form of palladium hydride, the others as the free gas, together with
the water produced by the reduction of copper oxide. Dittmar
and Henderson, and Leduc made use of Dumas' original method,
whilst Rayleigh weighed both the hydrogen and the oxygen,
but not the water. The atomic weight calculated from the
volumetric determination of Morley and the density as found by
Rayleigh leads to a number which is identical with the mean of
those directly obtained (if the result of Keiser's experiments,
which seems to be undoubtedly too high, be excluded). It may,
therefore, be concluded that two parts by weight of hydrogen
PROPERTIES OF ELECTROLYTIC GAS
261
combine with 15.88 of oxygen to form 17.88 parts of water, the
percentage composition of which is the following:-
Morley 8
Leduc 9
Hydrogen
Oxygen
Name.
Rayleigh 2
Leduc
100.000
The following table contains a summary of the results obtained
by the different investigators who have determined the relative
atomic weights, density and combining volumes of these two
elements:—
Dumas
Regnault
Regnault' corrected
2
Rayleigh
Cooke and Richards 3
Keiser.
Scott 5
Rayleigh 2
Noyes &
Dittmar and Henderson 7
•
Gay-Lussac and Humboldt 1805
1842
1845
Date.
Atomic
Weight.
1888-93
15.96
...
1888
1888 15.869
1888 15.949
1889 15.89
1890
15.896
15.866
1890
1891
15.879
1891
1892
1892
...
11.186
88.814
15.880
Density. Combining Volumes.
15.96
15.91
15.884
15.905
15.882
1:2
1 Regnault (corrected by Crafts), Compt. Rend. 106, 1662.
* Rayleigh, Proc. Roy. Soc. 43, 356; 45, 425; 50, 448.
3 Cooke and Richards, Amer. Chem. Journ. 10, 81, 191.
1: 1995-2.0025
1:2.00023
1:2·0037
Experiments with the Detonating Mixture of Oxygen and
Hydrogen.
130 In order to exhibit the explosive force of this detonating
gas a thin bulb (B), Fig. 80, of a capacity from 70 to 100 cubic
centimetres is blown on a glass tube. This is filled with the gas
evolved from the voltameter (A) as shown in the figure, and, when
* Keiser, Amer. Chem. Journ. 10, 249.
Scott, Proc. Roy. Soc. 42, 396; Brit. Assoc. Reports, 1888, 631.
6 Noyes, Amer. Chem. Journ. 12, 441.
7 Dittmar and Henderson, Proc. of Glasgow Phil. Soc. 1890–91; Chem. News
(1893), 54.
Morley, Amer. Journ. of Science [3], 41, 220; Chem News, 63, 218, &c.
* Leduc, Compt. Rend. 113, 186; 115, 41, 311; 116, 1248.
262
THE NON-METALLIC ELEMENTS
full, is placed over the perforated cork (c), through which two
insulated copper wires are inserted, these being connected at
the extremity by a fine platinum wire. The bulb is then sur-
rounded with a protecting cover of wire gauze (G), and a
current of electricity passed through the platinum wire, which
soon becomes heated to a temperature high enough to cause an
instantaneous combination of the oxygen and hydrogen to occur;
a sharp explosion is heard, and the bulb is shattered to fine.
dust.
The amount of the energy thus generated can be easily calcu-
lated when the quantity of heat developed by the combination is
known. Thus 1 grm. of hydrogen on burning to form water

B
FIG. 80.
evolves 33,970 thermal units, or heat sufficient to raise 33,970
grms. of water from 0° to 1°. But the mechanical equivalent of
heat is 423, that is, a weight of 423 grams falling through the
space of 1 metre is capable of evolving heat enough to raise 1
gram of water from 0° to 1°. Hence 1 gram of hydrogen on
burning to form water, sets free an amount of energy represented
by that required to raise a weight of 33,970 x 423 grams or
14,309 kilograms through the space of 1 metre.
The gases may, however, be made to combine not only
rapidly, as we have seen, but also slowly and quietly. High tem-
perature, the passage of the electric spark, and the presence of
platinum and other bodies effect the change in the first of these
ways. The smallest electric spark suffices to cause the combina-
tion of the largest masses of pure detonating gas, because the
PHENOMENA OF EXPLOSION
263
heat which is evolved by the union of those particles in whose
neighbourhood the spark passes is sufficient to cause the com-
bination of the adjacent particles, and so on. In every case a
certain minimum temperature, termed the temperature of igni-
tion, differing for each gas, must be reached in order that the
union shall take place, and the temperature may be so lowered
by mixing the detonating gas in certain proportions with inactive
gases that the explosive mixture cannot inflame.
Thus one
volume of detonating gas explodes when mixed with 2·82 vols.
of carbon dioxide, with 3:37 vols. of hydrogen, or with 9:35 vols.
of oxygen; but it does not explode when mixed with 2.89 vols.
of carbon dioxide, with 393 vols. of hydrogen, or with 1068
vols. of oxygen.¹
When the mixture of electrolytic gases is sealed up in a glass
bulb (which can be done without explosion occurring if the
side tubes are of capillary bore) and heated in the vapour of
boiling sulphur (448°) combination begins to take place slowly,
the whole of the mixed gases being at length converted into
water without any explosion occurring. If on the other hand
the bulb be immersed in the vapour of boiling zinc chloride
(606°) an explosion invariably takes place. This does not occur,
however, until the temperature reaches 650-730° if the gas
is passed through the bulb in a slow stream. The temperature
at which combination commences is much influenced by the
nature of the surface with which the gas is in contact. Thus
if the interior of the bulb be coated with silver, combination.
begins at as low a temperature as 155°.2
The following experiments indicate the slow combination
of oxygen and hydrogen. If a spiral of clean platinum wire is
held for a few seconds in the flame of a Bunsen burner, and
then the flame extinguished and the gas still allowed to stream.
out round the spiral, it will be seen that the spiral soon becomes
red hot, either continuing to glow as long as the supply of gas
is kept up, or rising to a temperature sufficient to ignite the
gas (Davy). A palladium wire acts in a similar way, but
wires of gold, silver, copper, iron and zinc produce no action of
this kind.
A perfectly clean surface of platinum plate also first effects a
1 Bunsen, Gasometry, p. 248.
V. Meyer and Krause, Annalen, 264, 85; V. Meyer and Askenasy, Annalen,
269, 49; V. Meyer and Freyer, Ber. 25, 622; Zeit. Phys. Chem. 11, 28; V.
Meyer, Ber. 24, 4233. See also Mitscherlich, Ber. 26, 160.
1
"
264
THE NON-METALLIC ELEMENTS
slow, but after a time even an explosive combination of the
detonating gas (Faraday). The finely-divided metal (spongy
platinum) which exposes a great surface to the action of the gas.
also induces, at the ordinary temperature, the combination of
hydrogen with air or oxygen; at first a slow combustion takes
place, but when the metal becomes red hot, a sudden explosion
occurs (Döbereiner).
Small traces of certain absorbable gases, such as ammonia,
destroy the inflaming power of the spongy platinum, but this
power is regained on ignition. The most probable explanation.
of this property of platinum is that this metal possesses the
power of condensing on to its surface a film of hydrogen and
oxygen, which gases, when brought under these circumstances
into intimate contact, are able to combine at the ordinary
atmospheric temperature, and by the heat which their com-
bination evolves, to excite the union of the remaining gaseous
mixture.
The following experiment strikingly shows that a mixture
of hydrogen and air becomes inflammable only when a definite
proportion between the two gases has been reached. Fig. 81
represents a suspended glass bell-jar closed at the top, and
covered at its mouth by a sheet of paper gummed on to the
glass.
A glass syphon passing through the paper cover is
fastened by copper wires to the bell-jar with the longer limb on
the outside. By means of a gas-generating apparatus the bell-
jar is filled with hydrogen by displacement, a rapid current of
the gas being made to pass in through the syphon, the air finding
its way out through the pores of the paper. When the bell-
jar is full of hydrogen, the vulcanized tube is removed from the
end of the long limb of the syphon, and the stream of hydrogen
gas which issues from the end (hydrogen being lighter than the
air, can be syphoned upwards) is then lighted and is seen to
burn with its usual quiet non-luminous flame. After a short
time, however, this flame may be seen to flicker, and is heard to
emit a musical note which begins by being shrill, but gradually
deepens to a bass sound, until, after a time, distinct and separate
impulses or beats are heard, and at last, when the requisite pro-
portion between the hydrogen and the air which enters through
the pores of the paper has been reached, the flame is seen to
pass down the syphon and enter the bell-jar, when the whole
mass ignites with a sudden and violent detonation.
131 The Phenomena of Explosion in Gases.-The phenomena
PHENOMENA OF EXPLOSION
265
which accompany the ignition of a detonating gas are of a very
interesting and important character. Bunsen, who was the first
to investigate this subject, directed his attention mainly to two
points, the rate of propagation of the explosion and the pressure
produced, both of which have been more fully studied by later
investigators.¹
Bunsen, in 1857, attempted to determine the rate at which
the explosion is propagated by igniting the gas as it issued from

FIG. 81.
the end of a narrow tube and measuring the rate at which the
detonating mixture had to be supplied to prevent the flame
passing back along the tube. In this way he found for hydrogen
and oxygen the rate of 34 metres per second.
1 Berthelot, "Sur la force des Matières Explosives (Paris)," Ann. Chim. Phys.
[5], 28, 289; Mallard and Le Chatelier, Compt. Rend. 1881, 145, "Combustions
des Mélanges Gazeux"; Dixon, Phil. Trans. 184, 97.
Phil. Mag. [4], 34, 493.
266
THE NON-METALLIC ELEMENTS
In 1881, however, it was found that when the explosive gas
is fired in a tube, the rate of explosion increases from its origin
until it reaches a certain maximum, after which it remains
constant whatever the length of the column of gas may be
(Berthelot, Mallard and Le Chatelier). The disturbance by which
the ignition is propagated throughout the gas is known as the
"explosion wave"; the maximum rate attained is exceedingly
high and has a perfectly definite and constant value for each
explosive mixture. The experiment of Bunsen, as will be seen,
referred to the initial period of the combination before the
explosion wave had attained its characteristic velocity.
In order to determine the rate, the detonating gas is brought
into a leaden tube, about 9mm. in diameter and 100 metres in
length, which is closed at either end by steel stop-cocks. Near
one end the gas can be fired by means of an electric spark, and
at about four feet from this point an insulated bridge of silver
foil is placed across the interior of the tube, a second similar
bridge being placed near the second. stop-cock. These bridges
convey electric currents, and are connected with a delicate
chronograph. The gas is fired by a spark, and the explosion
after attaining its maximum rate in the first four feet of the
tube breaks the first bridge, passes throughout the length of
the tube and finally breaks the second bridge, the time which
elapses between the two ruptures being recorded by the chrono-
graph. In the case of hydrogen and oxygen the enormous
velocity of 2,821 metres per second has been found, and the
rates in other gases are of the same order of magnitude.
The addition of an excess of one or other of the
gases or
of
inert gas is found to modify the rate of explosion by altering the
temperature which is attained and the density of the gases.
Thus, in a mixture of 8 vols. of hydrogen with 1 of oxygen the
rate is 3,532m. whilst in one containing 1 vol. of hydrogen with
3 vols. of oxygen it is 1,707m. per second. These velocities
correspond closely with those which would be attained by
sound in the gases concerned at the high temperature produced
by the combustion under the circumstances of the experiment
(Dixon). The ignition seems indeed to be propagated in
somewhat the same manner as a sound-wave. The numbers
calculated for hydrogen and oxygen on this supposition agree
well with the experimental results obtained with the diluted
gases. When the pure detonating gas is employed, however,
the calculated results are invariably higher than the experi-
an
OXYHYDROGEN FLAME
267
mental. This is probably due to the fact, that a certain fraction
of the gas escapes combustion in the explosion wave, the
temperature of which is probably above that at which the
dissociation of steam begins. The greater part of this uncom-
bined gas undergoes combustion as a secondary reaction after the
passage of the wave, but about 1 per cent. entirely escapes and
is found in the tube at the close of the experiment (Dixon).
According to Bunsen's experiments the pressure produced
when electrolytic gas is exploded is equal to 9.5 atmospheres,
and a similar result was subsequently obtained by Berthelot.
By comparing this result with the pressure as calculated from
the heat of combination, Bunsen concluded that only one-third
of the total volume of gas is burnt at the highest temperature
of the explosion. Later experiments made with more delicate
appliances have shown that the pressures produced in the
explosion wave, although of exceedingly short duration, are
considerably higher in value than those measured by Bunsen;
the pressure in the case of electrolytic gas for example probably
exceeds 20 atmospheres.
•
132 The Oxyhydrogen Flame.-By bringing a jet of oxygen
gas within a flame of hydrogen gas, burning from a platinum.
nozzle, a flame of the mixed gases is obtained which evolves
but very little light, although it possesses a very high tempera-
ture. A watch-spring held in the flame quickly burns with
bright scintillations. Platinum, one of the most infusible of
the metals, can be readily melted and even boiled, whilst silver
can thus be distilled without difficulty.
The arrangement of such an oxyhydrogen blowpipe is seen in
Fig. 82, the gases being collected separately in the two gas-
holders. The nozzle at s (Fig. 83) is screwed on to the tap of
the oxygen gasholder, the points a and b serving to keep the
oxygen tube in the centre, whilst the hydrogen enters the tube.
by the opening w, which is connected with the supply of this
gas by a caoutchouc tube. The hydrogen is first turned on and
ignited where it issues from the point of the nozzle; the
oxygen tap is then gently turned on so that the flame burns
quietly. No backward rush of gas or explosion can here occur,
for the gases only mix at the point where combustion takes
place.
If any solid, infusible and non-volatile substance, such as a
piece of quick-lime, be held in the flame, the temperature of the
surface of the solid is raised to a very high point and an intense
268
THE NON-METALLIC ELEMENTS
white light is emitted, which is frequently used, under the name
of the Drummond light, for illuminating purposes.
In certain metallurgical processes, especially in working the
platinum metals, this high temperature, of the oxyhydrogen

flame is turned to useful account. One of the forms of furnace
used for this purpose is shown in Fig. 84. It is built from a
block of very carefully burnt lime A, A, which has been cut in
half and then each piece hollowed out, so that when brought
Am
FIG. 82.
W
FIG. 83.
together they form a chamber into which the substance to
be melted is placed. The upper block is perforated, to
allow the nozzle (c, Q) of the blowpipe to fit in, and the
gases pass from the separate gasholders, into two concentric
OXIDATION AND REDUCTION
269
1
tubes C C and E' E', each provided with a stopcock (0 and H),
the hydrogen being delivered by the outer and the oxygen by
the inner tube. MM. Deville and Debray have in this way
melted 50 kilos. of platinum in one operation, and Messrs.
Johnson, Matthey, & Co. melted, by this process, a mass of pure
platinum weighing 100 kilos. which was shown at the Exhi-
bition of 1862. Since that time the same firm has melted no
less than 250 kilos. of an alloy of platinum and iridium for the
International Metrical Commission.
133 Alternate Oxidation and Reduction.-An interesting
experiment exhibiting the increase and loss of weight on the

D
C
P
EE
ELE
CQ
B
FIG. 84.
H
oxidation of metallic copper, and the subsequent reduction of
the copper oxide is carried out as follows: Copper oxide is
rubbed up into a stiff paste with gum-water, and the mass
rolled into the form of a cylinder about 1 cm. broad, and 3 cm.
long, which is then dried and ignited in the air. On heating
this in a current of hydrogen, the oxide is reduced to the metal,
and a cylinder of porous copper is obtained. A platinum wire
is then wound round it, and the cylinder held in the flame of a
Bunsen burner, so as to warm it, but not to bring it to a red
heat. On now plunging it into a jar of oxygen gas, it is seen to
glow from combination with oxygen, and this continues until
1 Ann. Chim. Phys. [3], 56, 385.
270
THE NON-METALLIC ELEMENTS
the whole is converted into oxide. When removed from the
oxygen, the colour of the metallic cylinder will be seen to be
changed from a bright red to the black colour of the oxide.
Next let this oxidised cylinder be suspended from one pan of a
balance, and a counterpoise placed in the other pan. The
cylinder is then removed from the balance, gently heated, and
whilst warm pushed up into a wide tube, placed mouth down-
wards, through which a current of hydrogen is passing; the
copper oxide is at once seen to glow, water being formed, which
condenses and drops down from the sides of the tube, whilst the
colour of the cylinder changes from black to the brilliant red of
the reduced metal. As soon as the reduction is complete the
cylinder is removed from the hydrogen and again hung on to the
pan of the balance, when a considerable loss of weight, due to
the loss of the oxygen, will be perceived.
PROPERTIES OF WATER.
134 Pure water is a clear, tasteless liquid, colourless when
seen in moderate quantity, but when viewed in bulk possessing
a bluish green colour, well seen in the water of certain springs,
especially those in Iceland, and in certain lakes, particularly
those of Switzerland, which are fed by glacier streams. This
blue colour is also observed if a bright white object be viewed
through a column of distilled water about six to eight metres in
length, contained in a tube with blackened sides and plate-glass
ends. Water is an almost incompressible fluid, one million
volumes becoming less by fifty volumes when the atmospheric
pressure is doubled; it is a bad conductor of heat, and probably
a non-conductor of electricity.
Expansion and Contraction of Water.-When heated from
0° to 4°, water is found to contract, thus forming a striking
exception to the general law, that bodies expand when heated
and contract on cooling; on cooling from 4 to 0° it expands
again. Above 4°, however, it follows the ordinary law, expand-
ing when heated, and contracting when cooled. This pecu-
liarity in the expansion and contraction of water may be
expressed by saying that the point of maximum density of water
is 4° C.; or according to the exact determinations of Joule,
3945; that is, a given bulk of water will at this temperature
PROPERTIES OF WATER
271
weigh more than at any other. Although the amount of con-
traction on heating from 0° to 4° is but small, yet it exerts a
most important influence upon the economy of nature. If it
were not for this apparently unimportant property, our climate
would be perfectly Arctic, and Europe would in all probability
be as uninhabitable as Melville Island. In order better to
understand what the state of things would be if water followed
the ordinary laws of expansion by heat, we may perform the
following experiment, first made by Dr. Hope. Take a jar con-
taining water at a temperature above 4°, place one thermometer
at the top and another at the bottom of the liquid. Now bring
the jar into a place where the temperature is below the freezing
point, and observe the temperature at the top and bottom of the
liquid as it cools. It will be seen that at first the upper ther-
mometer always indicates a higher temperature than the lower
one; after a short time both thermometers mark 4°; and, as the
water cools still further, it will be seen that the thermometer at
the top always indicates a lower temperature than that shown
by the one at the bottom: hence we conclude that water above
or below 4° is lighter than water at 4°. This cooling goes on
till the temperature of the top layer of water sinks to 0°, after
which a crust of ice is formed; and if the mass of the water be
sufficiently large, the temperature of the water at the bottom is
never reduced below 4°. In nature precisely the same phe-
nomenon occurs in the freezing of lakes and rivers;¹ the surface-
water is gradually cooled by cold winds, and thus becoming
heavier, sinks, whilst lighter and warmer water rises to supply
its place this goes on till the temperature of, the whole mass is
reduced to 4°, after which the surface-water never sinks, however
much it be cooled, as it is always lighter than the deeper water
at 4°. Hence ice is formed only at the top, the mass of water
retaining the temperature of 4°. Had water become heavier as it
cooled down to the freezing point, a continual circulation would be
kept up, until the mass was cooled throughout to 0°, when solidi-
fication of the whole would ensue. Thus our lakes and rivers
would be converted into solid masses of ice, which the summer's
warmth would be quite insufficient thoroughly to melt; and
hence the climate of our now temperate zone might approach
in severity that of the Arctic regions.
The following table gives the volume, and specific gravities
1 The point of maximum density of sea-water is considerably lower than that
of fresh, and is in fact below 0° C.
I
272
THE NON-METALLIC ELEMENTS
of water for temperatures varying from 0° to 100°, according to
the most accurate experiments.'
Tempe-
rature.
012348789
0°
5
6
10
11
12
13
14
15
16
17
18
Volume.
Specific Tempe-
Gravity. rature.
Volume.
Specific
Gravity.
0.99827
0.99806
1.000122 0.999878 19° 1:00153 0-99847
1.000067 0.999933 20 1·00173
1·000028 0.999972 21 1:00194
1:000007 0:999993 22 | 1.00216
1·000000 1·000000 23 1.00238
1·000008 0.999992 24 1·00262
0.99785
0-99762
0:99739
1.000031 0.999969
25
1·00287
0·99714
30
1·00425
0·99577
35
| 1·00586
0.99417
1·000067 0.999933
1·000118 0-999882
1·000181 0-999819 40
1:000261 0.999739 45
1·000350 0.999650 50
1:000456 0.999544
¦ 1·00770
1·00974
1:01197
0.99236
0.29035
0-98817
1.01436
0·98584
1.000570 0.999430 60
1.01694
0.98334
1:000703
0-999297
65
1:01967
0-98071
1·000847
0.999154
70 1.02261
0:97789
1.000997 0:999004
80 1:02891 0:37190
1001162 0.998839
90 1:03574 0.96549
1:001339
0-998663 100 1:04323 0.95856
135 Latent Heat of Water-In the passage from solid ice to
liquid water, we notice that a very remarkable absorption or
disappearance of heat occurs. This is rendered plain by the
following simple experiment:-Let us take a kilogram of water
at the temperature 0°, and another kilogram of water at 79°. If
we mix these, the temperature of the mixture will be the mean,
or 39°5; if, however, we take one kilogram of ice at 0° and mix
it with a kilogram of water at 79°, we shall find that the whole
of the ice is melted, but that the temperature of the resulting
2 kilograms of water is exactly 0°. In other words, the whole
of the heat lost by the hot water has just sufficed to melt
the ice, but has not raised the temperature of the water thus
produced. Hence we see that in passing from the solid to the
liquid state a given weight of water takes up or renders latent
just so much heat as would suffice to raise the temperature of
the same weight of water through 79° C.; the latent heat of water
1 Volkmann, Wied. Ann. 14, 260.
PROPERTIES OF WATER
273
-
is, therefore, said to be 79 thermal units—a thermal unit mean-
ing the amount of heat required to raise a unit weight of water
through 1 C. When water freezes, or becomes solid, this
amount of heat, which is necessary to keep the water in the
liquid form, and is, therefore, well termed the heat of liquidity,
is evolved, or rendered sensible. A similar disappearance of
heat on passing from the solid to the liquid state, and a similar
evolution of heat on passing from the liquid to the solid form,
occurs with all substances; the amount of heat thus evolved or
rendered latent varies, however, with the nature of the substance.
A simple means of showing that heat is evolved on solidification
consists in obtaining a saturated hot solution of acetate of soda,
and allowing it to cool. Whilst it remains undisturbed, it
retains the liquid form, but if agitated, it at once begins to
crystallize, and in a few moments becomes a solid mass.
If a
delicate thermometer be now plunged into the salt while solidi-
fying, a rapid rise of temperature will be noticed.
136 Freezing Point of Water and Melting Point of Ice.-
Although water usually freezes at 0° it was observed so long ago
as 1714 by Fahrenheit that under certain circumstances water
may remain liquid at temperatures much below this point.
Thus when brought under a diminished atmospheric pressure,
water may be cooled to 12° without freezing; or if water be
boiled in a glass flask, and the neck of the flask be plugged
whilst it is hot with cotton wool, the flask and its contents may
be cooled to 9° without the water freezing, but when the
cotton wool is taken out, particles of dust fall into the water,
and these bring about an immediate crystallisation, the tem-
perature of the mass quickly rising to 0°. Sorby¹ has shown
that when contained in thin capillary glass tubes, water may be
cooled to 15° without freezing, whilst Boussingault2 has
exposed water contained in a closed steel cylinder to a tem-
perature of 24° for several days in succession without its
freezing. The melting point of ice under the ordinary atmo-
spheric pressure is 0°, but this point is lowered by increase of
pressure; thus under a pressure of 81 atmospheres, ice melts at
-0°059 and under 16-8 atmospheres, at -0°129 or the melting
point is lowered by about 0°-00753 for every additional atmo-
sphere. This peculiarity of a lowering of the melting point
under pressure is common to all substances which, like water
-
――
-
1 Phil. Mag. [4], 18, 105.
2 Compt. Rend. 73, 77.
3 James Thomson, Edin. Roy. Soc. Trans. vol. 16, 575, 1849.
19
274
THE NON-METALLIC ELEMENTS
expand in passing from the liquid to the solid state, whilst
in the case of bodies which contract under like circumstances,
the melting point is raised by increase of pressure. Thus
Bunsen in the case of paraffin,¹ and Hopkins in the case of
sulphur, obtained the following results:-
""
Under a pressure of 1 atmosphere, paraffin melts at 46°3
85
48°.9
100
49°.9
39
""
1 atmosphere, sulphur melts at 107°0
519
135°.2
140° 5
792
29
39
""
99
99
"9
رد
د.
""
-
""
1 Ann. Chim. Phys. [3], 35, 383.
3 Phil. Mag. [4], 41, 165.
""
""
"2
From what has been stated we should expect that by increas
ing the pressure upon ice it could be melted, and Mousson has
shown that this is the case, for by exposing it to a pressure of
13,000 atmospheres he has converted ice into water at a tem-
perature of -18°. This lowering of the melting point of ice with
pressure explains the fact that when two pieces of ice are rubbed
together the pressure causes the ice to melt at the portions of
the surface in contact, the water thus formed running away, and
the temperature being lowered; then as soon as the excess of
pressure is taken away the two surfaces freeze together at a
temperature below 0°, one mass of solid ice being produced.
This phenomenon, termed regelation, was first observed by Fara-
day in 1850, and was afterwards applied by Tyndall to explain
glacier motion.
137 The crystalline form of ice is hexagonal, being that
of a rhombohedron. Snow crystals exhibit this hexagonal form
very clearly; they usually consist of crystals which have grown
on to another crystal in the direction of the three horizontal
axes, that so the snow crystal clearly exhibits these three
directions, as shown in Figs. 85, 86.
Ice is transparent, and when seen in small quantities it ap-
pears to be colourless, though large masses of ice, such as
icebergs or glaciers, possess a deep blue colour; like water
it is also a bad conductor of heat and a non-conductor of
electricity, and becomes electrical when rubbed.
Water on freezing increases nearly of its bulk, or, according
to the exact experiments of Bunsen,3 the specific gravity of ice
Pogg. Ann. 105, 161.
PROPERTIES OF WATER
275
at 0° is 0-91674, that of water at 0° being taken as the unit;
or one volume of water at 0° becomes 1:09082 volumes of ice at
the same temperature. This expansion plays an important
part in the disintegration and splitting of rocks during the
winter. Water penetrates into the cracks and crevices of the rocks
and on freezing widens these openings; this process being
repeated over and over again, the rock is ultimately split into
fragments. Hollow balls of thick cast-iron can thus easily be
split in two by filling them with water and closing by a
tightly fitting screw, and then exposing them to a temperature
below 0°.

**

·→
FIG. 85.
*
**
**
FIG. 86.
138 Latent Heat of Steam.-Under the normal barometric
pressure of 760mm water boils in a metal vessel at 100° C.
When liquid water is converted ir to gaseous steam, a large
quantity of heat becomes latent, the temperature of the steam
given off being the same as that of the boiling water, as, like all
other bodies, water requires more heat for its existence as a gas
than as a liquid. The amount of heat latent in steam is roughly
ascertained by the following experiment. Into 1 kilogram of
water at 0°, steam from boiling water, having the temperature
of 100°, is passed until the water boils: it is then found that
the whole weighs 1.187 kilos., or 0.187 kilo. of water in the form.
of steam at 100° has raised 1 kilo. of water from 0° to 100°; or 1
kilo. of steam at 100° would raise 5:36 kilos. of ice-cold water
276
THE NON-METALLIC ELEMENTS
through 100°, or 536 kilos. through 1º. Hence the latent heat of
steam is said to be 536 thermal units.
Whenever water evaporates or passes into the gaseous state,
heat is absorbed, and so much heat may be thus abstracted from
water that it may be made to freeze by its own evaporation.
A beautiful illustration of this is found in an instrument called
Wollaston's Cryophorus, Fig. 87; it consists of a bent tube,
having a bulb on each end, and containing water and vapour of
water, but no air. On placing all the water in one bulb, and
plunging the empty bulb into a freezing mixture, a condensation
of the vapour of water in this empty bulb occurs, and a corre-
sponding quantity of water evaporates from the other bulb to
supply the place of the condensed vapour; this condensation
and evaporation go on so rapidly that in a short time the water
cools down below 0°, and a solid mass of ice is left in the bulb.
By a very ingenious arrangement this plan of freezing water by
its own evaporation has been practically carried out on a large
A
FIG. 87.
B
scale by M. Carré, by means of which ice can be most easily
prepared. This arrangement consists simply of a powerful
air-pump (A, Fig. 88), and a reservoir (B), of a hygroscopic
substance, such as strong sulphuric acid. On placing a
bottle of water (c) in connection with this apparatus, and on
pumping for a few minutes, the water begins to boil rapidly,
and its temperature is so lowered by the evaporation that the
water freezes to a mass of ice.
139 Tension of Aqueous Vapour.-Water, and even ice, con-
stantly give off steam or aqueous vapour at all temperatures,
when exposed to the air. Thus we know that if a glass
of water be left in a room for some days, the whole of the
water will gradually evaporate. This power of water to rise in
vapour at all temperatures is due to the elastic force or tension
of aqueous vapour; it may be measured, when a small quantity
of water is placed above the mercury in a barometer, by the
extent to which the vapour thus given off is capable of
PROPERTIES OF WATER
277
depressing the mercurial column. If we gradually heat
the drops of water thus placed in the barometer, we shall
notice that the column of mercury gradually sinks; and when
the water is heated up to 100° C., the mercury in the barometer
tube is found to stand at the same level as that in the trough,
showing that the elastic force of the vapour at that tem-
perature is equal to the atmospheric pressure. Water is said to
boil in the air when the tension of its vapour is equal to the
superincumbent atmospheric pressure. On the tops of mountains,
where the atmospheric pressure is less than at the sea's level,
water boils at a temperature below 100°: thus at Quito, at a
height of 2914 metres above the sea's level, the mean height of

B
FIG. 88.
the barometer is 523 mm., and the boiling point of water is
90°1; that is, the tension of aqueous vapour at 90°1 is equal to
the pressure exerted by a column of mercury 523 mm. high.
Founded on this principle, an instrument has been constructed
for determining heights by noticing the temperatures at which
water boils. A simple experiment to illustrate this fact consists
in boiling water in a globular flask, into the neck of which a
stopcock is fitted: as soon as the air is expelled, the stopcock is
closed, and the flask removed from the source of heat; the
boiling then ceases; but on immersing the flask in cold water,
the ebullition recommences briskly, owing to the reduction of
the pressure consequent upon the condensation of the steam;
the tension of the vapour at the temperature of the water in
278
THE NON-METALLIC ELEMENTS
the flask being greater than the diminished pressure. All other
liquids follow a similar law respecting ebullition; but as the
tensions of their vapours are very different, their boiling points
vary considerably.
When steam is heated alone, it expands according to the
law previously given for permanent gases; but when water is
present, and the experiment is performed in a closed vessel,
the elastic force of the steam increases in a far more rapid
ratio than the increase of temperature. The following table.
gives the tension of aqueous vapour, as determined by experi-
ment, at different temperatures measured on the air ther-
mometer.
Temperature
Centigrade.
- 20°
- 10
0
+ 5
10
15
20
30
40
50
60
70
80
90
100
Table of the Tension of the Vapour of Water.
Tension in millime-
tres of mercury.
0.927
2.093
4.600
6.534
9.165
12.699
17.391
31.548
54.906
91.982
148.791
233.093
354.280
525 450
760·000
Temperature
Centigrade.
100°
111.7
- 120.6
127.8
133.9
144:0
159.2
170.8
180.3
188:4
1955
201·9
207.7
213.0
224.7
Tension in atmosphs.,
760
1 atmosphere
mm. of mercury.
1
1.5
2
2.5
84080242
10
12
14
16
18
25
140 Water as a Solvent.-Water is the most generally valuable
of known solvents. Not only do many solids, such as sugar and
salt, dissolve in water, but certain liquids, such as alcohol and
acetic acid, mix with it completely. Other liquids again, such
as ether, dissolve to a certain extent in water, although they do
not mix with it in all proportions. Gases also dissolve in water,
some, such as ammonia and hydrochloric acid, in very large
quantities, exceeding more than one hundred times the bulk
of the water; others, again, such as hydrogen and nitrogen,
WATER OF CRYSTALLIZATION
279
are but very slightly soluble, while carbon dioxide and some
other gases stand, as regards solubility, between these extremes.
Concerning the nature of solution, whether of solids, liquids,
or gases, we know at present but little. The phenomena of
solution differ, however, essentially from those of chemical
combination, inasmuch as in the former we have to do with
gradual increase up to a given limit, termed the point of
saturation, whereas in the latter we observe the occurrence of
constant proportions in which, and in no others, combination
occurs. Solution follows a law of continuity, chemical com-
bination one of sudden change or discontinuity.
The solubility of solids varies with the essential nature of the
solid, with that of the liquid, and with the temperature at
which they are brought together; the same may be said of the
solvent action of water upon liquids and upon gases, except that
the solubility of gases is also influenced by the pressure to which
the
gas and the water are subjected. The quantity of any solid,
liquid, or gas which dissolves in a solvent, such as water, must
be ascertained empirically in every case, as we are unacquainted
with any law according to which such solvent action takes place,
and we, therefore, cannot calculate the amount. The effect of
change of temperature on the solubility on a substance, whether
solid, liquid, or gaseous must likewise be determined by experi-
ment, but the effect of pressure upon the solubility of gases
is given by a simple law, known as the law of Dalton and
Henry (p. 284).
The solubility of most solids increases with the temperature,
a limit being reached at each temperature, beyond which no
further quantity of the solid dissolves. When the temperature
of such saturated solutions falls, or when the solvent is allowed to
evaporate, a portion of the dissolved substance is deposited from
solution usually in the form of a solid possessing some definite
geometrical form, and termed a crystal, whilst the substance is
said to crystallize.
Water of Crystallization.-Many salts owe their crystalline
character to the presence in a solid state of a certain definite
number of molecules of water. When this chemically combined
water is driven off by heat, the crystal falls to powder, and
hence it has been termed water of crystallization. Some salts
contain a large quantity of water definitely combined in this
form; thus the opaque white powder of anhydrous alum,
KAL(SO), unites with no less than twenty-four molecules of
280
THE NON-METALLIC ELEMENTS
water to form the well-known transparent octohedral crystals of
common alum, K¿Al„(SO) + 24H₂O; in like manner anhydrous
and powdery carbonate of soda, Na,CO,, when dissolved in
water deposits large monosymmetric crystals of common wash-
ing-soda, having the composition Na,CO, + 10 H₂O. The tem-
perature of the solution from which such crystals are deposited
materially affects the quantity of water with which the salt
combines; thus, in the case of carbonate of soda, whilst mono-
symmetric crystals of the ten-molecule hydrate are deposited at
the ordinary temperature, other crystals, having the composition
Na,CO + 8H₂O, or again others represented by the formula
Na,CO, + 5H,O, are deposited, when crystallization is allowed
to take place at higher temperatures.
The water in these crystals has a definite vapour tension and is,
therefore, given off when the temperature becomes so high that
the tension of the water of crystallization is greater than that in
the surrounding atmosphere. Different substances lose their
water in the air at very different temperatures, and even the
molecules of water combined with a single molecule of salt
behave differently in this respect. Thus potash alum loses ten
molecules of water at 100°, but it needs to be heated to 120° in
order to drive off a second ten molecules of water, and retains the
last four molecules until the temperature rises to 200°. Copper
sulphate in a similar manner loses four of its molecules of water
below 110°, whilst the fifth is only driven off at 200°. Sodium
carbonate, Na,CO, + 10H¸O, loses water on simple exposure to
the air, the tension of its combined water being greater than that
of the aqueous vapour in the atmosphere, and the salt becomes
covered with a white powder. Crystals which behave in this
manner are said to effloresce. Many salts which do not lose
water in the atmosphere do so when placed in dry air. Other
solid salts, such as calcium chloride, and potassium acetate,
combine with water with such avidity that when left exposed
to the air they begin to liquefy from absorption of the atmo-
spheric moisture; the salts are then said to deliquesce.
In the year 1840 Dalton observed that different salts, whose
water of crystallization has been driven off by heat, dissolve in
water without increasing the volume of the liquid, whereas if
the hydrated salt is dissolved, an increase of volume occurs which
is exactly that due to the water which is combined in the salt.
Playfair and Joule extended these observations, showing
1 Chem Soc. Mem. 2, 477; 3, 54, 199; Chem. Soc. Quart. Journ. 1, 121.
CRYOHYDRATES
281
I
for instance, that in the case of carbonate of soda, crystallizing
with ten molecules of water, and in that of the phosphates and
arsenates crystallizing with twelve molecules, the volume of the
whole molecule of hydrated salt is the same as that of its water
of crystallization would be if frozen to ice. The particles of
anhydrous salt would hence appear to occupy the spaces inter-
vening between those of the water without increasing its volume.
Thus the crystals of common washing soda have the following
composition:
Na,CO3
10 H₂O
2
•
Sodium Carbonate .
Hydrogen Sodium Phosphate
Normal Sodium Phosphate.
Hydrogen Sodium Arsenate.
Normal Sodium Arsenate
•
=
=
104.29
178.80
283.09
and 283-09 grams. of these crystals occupy exactly the space of
1788 grams. of ice.
The following table gives the specific gravities of the above
mentioned salts, first as observed by experiment, and secondly
as calculated upon the above hypothesis, and shows the close
agreement of the two sets of numbers.
-
Na, CO3 + 10 H₂O
Na, HPO + 12 H„O.
Nag PO + 12 H₂O
Na, H AsO + 12 H₂O
Na,AsO 12 HO
•
•
•
Specific Gravity.
Observed. Calculated.
1 454
1.463
1.527
1.622
1.736
1.834
1.525
1.622
1.736
1.804
·
·
•
•
•
"
In the case of certain other salts the volume of the crystal
was found to be equal to the sum of the volume of the water
when frozen and that of the anhydrous salt.
141 Cryohydrates.-It has been already mentioned that when
a dilute solution is cooled, ice separates out (p.114). If the ice be
removed and the cooling continued, a temperature is at length
reached at which the whole solution becomes solid. Thus
Guthrie, who has investigated this subject, found that when a
dilute solution of common salt is cooled down to -1°5, ice
begins to separate out, and this formation of ice continues until
the temperature sinks to 23°, at which the whole mass
becomes solid. A concentrated salt solution, on the other hand,
deposits at -7° crystals having the composition NaCl + 2H,O,
and the separation of this compound, in the form of iridescent
scales, also goes on until the liquid has cooled down to - 23°
when, as before, it freezes en masse.
1 Phil. Mag. [4], 49, 1, 206, 266.
282
THE NON-METALLIC ELEMENTS
The solid mass thus formed has been called a cryohydrate,
and resembles a chemical compound inasmuch as it has a
definite composition and a definite melting-point. It has,
however, been shown that these so-called cryohydrates are
simply mixtures of ice and a solid salt, and that their properties
are in all cases the mean of those of their constituents.¹
142 Freezing Mixtures.-The solution of a solid in water is
generally accompanied by a lowering of temperature, caused by
the conversion of sensible into latent heat by the liquefaction
of the solid. In the case, however, of many anhydrous salts,
solution is accompanied by a rise in temperature, which may
possibly be caused by the production of a definite chemical
compound between the solid and the solvent. By the solution
of many salts such a diminution of temperature is effected that
this process may be used for obtaining ice; thus when 500
grams. of potassium thiocyanate are dissolved in 400 grams.
of cold water, the temperature of the solution sinks to -20°.
When common salt is mixed with snow or pounded ice a con-
siderable reduction of the temperature of the mass occurs, the
two solid bodies becoming liquid and forming a concentrated
brine whose freezing-point lies at -23°. This solution contains
thirty-two parts by weight of salt to 100 parts of water, and in
order to bring about the greatest possible reduction in tempera-
ture the salt and snow must be mixed in the above proportions.
Equal weights of crystallized calcium chloride and snow when
mixed together give a freezing mixture whose temperature sinks
from 0° to
45°.
The temperatures produced in this way are those at which
the so-called cryohydrates are formed, since these are the lowest
temperatures at which the mass can remain liquid.
The Properties of Solutions.-Concerning the relation between
the dissolved substance and the solvent in a concentrated solu-
tion, but little is known. Many such solutions, when cooled,
deposit crystals which contain a portion of the solvent combined
with the substance which has been dissolved. It has, therefore,
been supposed that the solutions in these cases actually contain
these compounds, and this seems to be probable at all events at
low temperatures. On the other hand, it has been held that
no compounds are present in the solutions, but that the com-
bination takes place at the moment of separation of the solid
substance.
1 Offer, Wien. Akad. Ber. 81, ii. 1058.
PROPERTIES OF SOLUTIONS
283
The properties of dilute solutions can be to a large extent
explained, as already described (p. 111), on the supposition that
the dissolved substance is not combined with the solvent.
In the case of solutions of salts (electrolytes) in water a
special hypothesis, that of electrolytic dissociation, has been
introduced, according to which the salt is to a large extent split
up in the solution into its electrolytic ions. This view is
rendered probable not only by the facts already adduced (p. 116),
but by the circumstance that almost all the properties of such
solutions have been shown to be equal to the sum of the
properties of the ions. Each ion has a definite weight, volume,
colour, &c., and the effect produced by a salt is found to be
equal to the sum of the effects which would be independently
produced by the ions into which it is capable of being decom-
posed. This has already been explained for the osmotic
pressure, freezing-point and vapour pressure of solutions, and
also holds for the colour, specific gravity, refractive index, &c.
This theory is, therefore, in accordance with the physical
properties of such solutions, and it moreover throws great light
upon many chemical reactions. Chemical action occurring in
dilute solution, according to this view, takes place between the
ions of the substances concerned, so that the tests usually
applied for the metals and acids are in reality tests for the
corresponding ions. Thus, for example, the tests for ferrous
salts fail to detect iron in potassium ferrocyanide, K,FeC,No,
although they are given by ferrous sulphate, FeSO4. This is
due to the fact that in the latter case the ion of ferrous iron is
present in the solution, whilst in the former the ions are K and
the complex group FeCN
The fact that the same amount of heat is evolved by the
neutralisation of any of the strong acids by any of the strong
bases also receives a new and interesting interpretation in the
light of this theory. Taking the case of the action of hydro-
chloric acid on caustic soda, which is usually represented by the
equation,
NaHO + HCl = NaCl + H₂O,
we see that the action takes place between the ions in the
following way:
+
+
+
Na + HO+H+ Cl = Na + Cl + H₂O.
--
--
284
THE NON-METALLIC ELEMENTS
The result of the reaction is simply the formation of water,
since the Na and Cl ions remain dissociated. Exactly the same
reaction takes place if another acid and a different base be
employed:
+
+
+
K+ H+
K + HO + H + NO, = K + NO3 + H₂O.
---
The amount of heat evolved is, therefore, 'also the same.
143 Absorption of Gases by Water.-All gases are soluble to a
greater or less degree in water, the extent of this solubility
depending upon (1) the nature of the gas, (2) the temperature of
the gas and water, (3) the pressure under which the absorption
occurs. No simple law is known expressing the relation between
the amount of gas absorbed and the temperature. Usually the
solubility of a gas diminishes as the temperature increases, but
the rate of diminution varies with each gas, so that the amount
of gas dissolved in water at a given temperature can be ascer-
tained only by experiment. A simple relation has however
been found to exist between the quantity of gas absorbed under
varying conditions of pressure, the temperature remaining
constant.
In the year 1803 William Henry¹ proved that the amount of
gas absorbed by water varies directly as the pressure, or, in the
words of the discoverer of the law, "under equal circumstances
of temperature, water takes up in all cases the same volume of
condensed gas as of gas under ordinary pressure. But as the
spaces occupied by every gas are inversely as the compressing
force, it follows that water takes up of gas condensed by one,
two, or more additional atmospheres, a quantity which ordinarily
compressed, would be equal to twice, thrice, and so on, the
volume absorbed under the common pressure of the atmosphere."
Two years after Henry had enunciated this law, Dalton 2
extended the law to the case of mixed gases, proving that when
a mixture of two or more gases in given proportions is shaken
up
with water, the volume of the gas having a finite relation to
that of the liquid, the absorptiometric equilibrium occurs when
the pressure of each gas dissolved in the liquid is equal to that
of the portion of the gas which remains unabsorbed by the
liquid; the amount of each gas absorbed by water from such
a mixture being solely dependent on the pressure exerted by the
particular gas. This law, termed Dalton's law of partial pressures,
1 Phil. Trans. 1803, 29,
274.
2 Manc. Memoirs, 1805.
SOLUTION OF GASES IN WATER
285
may be illustrated by the following example: if two or more gases
which do not act chemically upon each other be mixed together,
and the mixture of gases brought into contact with water until
the absorptiometric equilibrium is established, the quantity of
each gas which dissolves is exactly what it would have been if
only the one gas had been present in the space. Thus, for
instance, the absorption co-efficient of oxygen at 0° is 0·04890,
that of nitrogen at the same temperature being 0.023481. Now
100 volumes of air contain on an average 7904 volumes of
nitrogen and 20·96 volumes of oxygen, hence the partial pressure
of the oxygen is 0-2096 of an atmosphere, whilst that of the
nitrogen is 0.7904, and as the solubility of each gas is propor-
tioned to its partial pressure,
0.2096 × 0.04890 = 0·010244
will be the proportion of oxygen dissolved, and
0.7904 × 0.023481 = 0.018559
will be the proportion of the nitrogen dissolved, or the per-
centage composition of the air dissolved in water will be:-
Nitrogen . .
Oxygen
Calculated.
64.4
35.6
100.0
•
•
•
Found.
64.9
35.1
100.0
Thus the relation between the dissolved gases as found by ex-
periment agrees closely with that calculated on the above
assumption, and the law of partial pressures is verified.
is
Every absorbed gas which follows the law of pressure will of
course be driven out of solution when the pressure on the gas
reduced to zero. This can be effected by removing the super-
incumbent pressure by means of an air-pump, by allowing
the liquid to come in contact with a very large volume of some
other indifferent gas, or, lastly, by boiling the liquid when
all the dissolved gas will be driven off with the issuing steam,
except where a chemical combination or attraction exists between
the
gas
and the water.
Some gases dissolve in water in very large quantities, whereas
others are only slightly soluble. Two distinct methods of
286
THE NON-METALLIC ELEMENTS
experimentation are needed for ascertaining the co-efficients of
solubility of these two classes. On the one hand, the amount
of the absorbed gas is determined chemically; on the other
the volume of gas absorbed by a known volume of water is
ascertained either by measuring the diminution in bulk of a
given volume of the gas when agitated with water, or by
saturating water with the gas and driving out the latter by
heat and then measuring it. The first of these methods was
that adopted by Bunsen to whom we are indebted for the first
exact and extended experimental investigation of this subject.
Among the gases whose solubility has been determined by
chemical methods are oxygen, sulphuretted hydrogen, sulphur
dioxide, ammonia, carbon dioxide, hydrochloric acid, and
chlorine. These gases, evolved in a state of purity, were
passed for a long time through a large volume of water, which
had been freed from air by continued boiling, and was kept at
a constant temperature during the experiment. After the gas
had passed so long through the water that the latter was com-
pletely saturated, the barometric pressure was read off, and a
known volume of the water withdrawn, special precautions to
avoid possible loss of the gas being observed. The gas con-
tained in this liquid was then quantitatively determined either
by means of volumetric analysis, or by the ordinary processes of
analytical chemistry. If the volume of the liquid does not
undergo any appreciable alteration in bulk owing to the ab-
sorption of the gas, we are easily able to calculate the co-efficients
of absorption from the data obtained by this process. If,
however, the volume of the saturated liquid is considerably
larger, as is usually the case, than that of the liquid before
saturation, either the specific gravity of the saturated liquid
must be ascertained, or only a small volume of water must be
saturated, and the absolute quantity of absorbed gas ascertained
by weighing before and after the experiment.
144 Bunsen's Absorptiometer, as shown in Fig. 892 consists
essentially of two parts: (1) a eudiometric tube, e, in which
a measured volume of the gas to be experimented upon is
brought in contact with a given volume of water; (2) an outer
vessel, consisting of a glass cylinder, fitting at the lower end
into a wooden stand, f, and having a water-tight lid at the
upper end. The eudiometer tube, which is divided and accu-
¹ Gasometry, p. 129, or Watts's Dictionary (1st Edition), article "Gases, Absorp-
tion of by Liquids."
2 Bunsen's Gasom. 43 and 44.
4.
SOLUTION OF GASES IN WATER
287
rately calibrated, is partially filled with the given gas in the
nsual way over a mercurial trough, and the volume of this gas

80
g
---d
go
hang pandit mai em con
FIG. 90.
FIG. 89.
read off with all due precautions; a measured volume of water
perfectly free from air is next admitted under the mercury into
288
THE NON-METALLIC ELEMENTS
the tube, and the open end of the tube then closed by screwing
it tightly against the caoutchouc plate of the small iron foot, a,
fixed on to its lower end, as shown in Fig. 90. The tube by
this means can be removed from the mercurial trough, without
any danger of losing gas or water, and placed in the glass cylin-
der, which contains mercury a in its lower part, and water
above, in which it can be safely shaken to ensure the establish-
ment of the proper absorptiometric equilibrium between gas and
water. The pressure in the tube can be readily adjusted from
time to time by unscrewing the open ends of the tube from the
caoutchouc plate, and thus placing the mercury inside in con-
nection with that outside the tube. The heights of the two
levels of mercury and the level of the water in the tube, as well
as the temperature (indicated by the thermometer ), can then
be read off through the glass cylinder, and thus all the data
are obtained for ascertaining exactly the volume of gas absorbed
by a given volume of water under given conditions of tempera-
ture and pressure.
Still more accurate results have been obtained by the use
of various modified forms of this apparatus, in which larger
volumes of water are employed.¹
145 The truth of the law of Dalton for pressures not greatly
higher than that of the atmosphere has been experimentally tested
by Bunsen, who showed that the results of an absorptiometric
analysis of a gaseous mixture-that is, of an experimental deter-
mination of its solubility in water, from which the composition of
the orginal gaseous mixture is calculated, on the supposition that
the law of partial pressures holds good-agrees exactly with a
direct eudiometric analysis of the same mixture. Thus it has
been shown by the same chemist that in mixtures of carbon
dioxide and carbon monoxide, of carbon monoxide and marsh
gas, of carbon dioxide and hydrogen, the component gases are
absorbed in quantities exactly corresponding to Dalton's law.
The limits of pressure beyond which gases do not follow the
law of pressures have not as yet been experimentally ascertained
in many cases; but, at any rate in the case of the more soluble
gases, the limits are reached within ranges of pressure varying
from 0 to 2 atmospheres. That under high pressures deviations
from the law must in many cases occur is clear, inasmuch as gases
do not conform to the law of Boyle under greatly increased
1 Winkler, L. W. Ber. 24, 89; Timofejew, Zeit. Phys. Chem. 6, 141.
2 Gasometry, p. 124.
NATURAL WATERS
289
pressure. So too, it is found that certain gases which follow
the law of absorption at one temperature do not conform at
another; thus, for instance, ammonia dissolves in water at 100°
under high pressures, in quantities exactly proportional to the
pressure, although at lower temperatures this is not the case.¹
Instances also occur in which certain gases, although agreeing
with the law of pressures when in the pure state, do not follow
it when mixed together with other gases. Thus in mixtures of
equal volumes of chlorine and hydrogen, and mixtures of vary-
ing proportions of chlorine and carbon dioxide, the carbon
dioxide and hydrogen do not dissolve in water in quantities
proportional to their partial pressures, although they both follow
the law when unmixed with other gases.2
Amongst the various applications of the laws of the absorption
of gases in water none is more interesting than the process pro-
posed by Mallet for solving the difficult problem of separating
the atmospheric oxygen from the nitrogen. We have already
seen that the percentage of oxygen contained in the air is 209,
whereas the mixture of oxygen and nitrogen dissolved in water
contains 351 per cent. of the former gas. If the gas thus dis-
solved be driven off by boiling, and then this again shaken up
with water, the dissolved gases will possess about the following
percentage composition:-
Nitrogen.
Oxygen
After the second absorption.
52.5
47.5
100.0
This again set free, and again shaken up with water, yields a
gaseous mixture, containing 75 per cent. of oxygen. Continuing
this process of alternately absorbing and liberating the mixture
of gases, the percentage of oxygen regularly rises, until after the
8th absorption the gas contains 97.3 per cent., or is nearly pure
oxygen gas.
NATURAL WATERS.
146 None of the various forms of water met with in nature
are free from certain impurities. These may be of two kinds;
(1) Mechanically suspended impurities; (2) Soluble impurities.
The first can be separated either by subsidence or by mechanical
1 Roscoe, Journ. Chem. Soc. 1856, 14.
2 Sims, Journ. Chem. Soc. 1862, 1.
1
!
20
290
THE NON-METALLIC ELEMENTS
filtration, the latter cannot be thus got rid of from the water, but
must be separated by distillation or by some chemical reaction.
Even rain or snow-water collected in clean vessels contains
in addition to the dissolved atmospheric gases traces of foreign
bodies which are contained in the air either as dust or vapour,
and no sooner does rain-water touch the earth than it at once
takes up into solution certain soluble constituents of the portion
of the earth's crust through which it percolates, thus gradually
becoming more and more impure until it again reaches the
ocean from which it had its origin.
147 Purification of Water.-The separation of suspended
matter is effected on the small scale for laboratory purposes by
filtration through porous paper placed in glass funnels, and on
the large scale by employing filtering beds of sand and gravel.
In order to separate suspended matter from water used for
drinking purposes it is usual to filter it through a layer of wood
charcoal, which not only holds back the solid matter but also
acts in other ways, as we shall hereafter learn, in improving the
character of the water.
The soluble constituents may be distinguished as (1) fixed,
and (2) volatile constituents, and water can be obtained free
from the first of these by the process of distillation, whilst the
latter may come over with the steam, and, therefore, require the
employment of other means. In order to obtain pure distilled
water, spring or rain-water is boiled in a vessel termed a still,
(B) Fig. 91 so arranged that the escaping steam is condensed
by passing through a cooled worm or tube made of block tin,
platinum, or silver, but not of glass, for if this substance be
used a trace of its more soluble constituents, the alkaline
silicates, is always dissolved. This process frees the water from
all non-volatile impurities, provided care has been taken to
prevent any mechanical spirting of the liquid, but substances
which are volatile will still be found in the distillate. Thus
ordinary distilled water invariably contains ammonia, as may
easily be proved by adding a few drops of Nessler's reagent.
This consists of an alkaline solution of mercuric iodide in
potassium iodide. If a few drops of this reagent be added to
about 100 cc. of ordinary distilled water contained in a cylin-
drical glass standing on a white plate, the water will be seen to
attain a distinct yellowish tint if small amounts of ammonia or
ammoniacal salts be present, whilst if larger quantities of
ammonia be present a brown precipitate will be formed.
GASES IN WATER
291
1
In order completely to free distilled water from volatile nitro-
genous organic bodies which it is likewise apt to contain, it is
necessary to re-distil it after solutions of potassium perman-
ganate and caustic potash have been added. These substances
oxidize the organic matter with formation of ammonia, and
after about one-twentieth of the water has come over, the
distillate is usually found to be free from ammonia, and to leave
no residue on evaporation. If ammonia can be still detected.
the water must again be distilled with the addition of a small
quantity of acid sulphate of potash which fixes the ammonia.¹
148 Gases Dissolved in Water.-All water contains in solution
the gases of the atmosphere, oxygen, nitrogen, and carbon

FIG. 91.
dioxide. In order to obtain water free from these dissolved
gases the water is well boiled and the glass vessel in which it is
boiled is then sealed hermetically. The arrangement Fig. 92
shows how this may be conveniently accomplished. After the
water has been quickly boiling for half an hour the caoutchouc
tube (a) from which the steam issues is closed by a clamp, the
lamp is removed, and the drawn out neck of the flask melted
off before the blow-pipe at (b).
Even when boiled for many hours a small residue of nitrogen
gas is left behind, and on condensing, the steam coming off
from such water leaves a minute bubble of nitrogen so that it
1 Stas, Recherches, p. 109.
292
THE NON-METALLIC ELEMENTS
appears impossible to obtain water quite free from nitrogen.
All water which is exposed to the air dissolves a certain quantity
of oxygen and nitrogen, a quantity which is determined by the
laws of gas absorption. It is indeed upon this dissolved oxygen
that the life of water-breathing animals depends. In every
pure water the proportion between the dissolved nitrogen and
oxygen is found to be constant, and it is represented by the
following numbers :-
Percentage Composition of Air Dissolved in Water.
Oxygen
Nitrogen.
FIG. 92.
•
35.1
64.9
100.0
1,000 cc. of pure water, such as rain-water, when saturated, dis-
solves 17.95 cc. of air. If the water is rendered impure by the
introduction of organic matter undergoing oxidation, the pro-
portion between the dissolved oxygen and nitrogen becomes
different owing to the oxygen having been partly or wholly used
for the oxidation of this material. This is clearly shown in the
1 Grove, Journ. Chem. Soc. 1863, 263.
GASES IN WATER
293
Total volume of
gas per litre.
following analyses, made by Miller, of the dissolved gases con-
tained in Thames water collected at various points above and
below London.
Carbon dioxide.
Oxygen
Nitrogen.
·
•
•
Kings-
ton.
CC.
52.7
303
Ham- Somerset Green- Wool-
mersmith House. wich. wich.
CC.
Thames Water taken at
-
7.4 4.1
150 15.1
cc.
CC.
62.9
45.2
1.5
0.25
16.2 15.4
CC.
71:25 63.05
55'6
48.3
0.25
14.5
Erith.
cc.
74.3
57
1.8
15.5
Ratio of oxy-
gen to nitrogen 1:2 1:37 1:105 1: 60 1:52 1: 8:1
This table shows that whereas the pure water at Kingston
contains the normal quantity of dissolved oxygen, the ratio of
oxygen to nitrogen decreases at a very rapid rate as the river
water becomes contaminated with London sewage, but that
this ratio again shows signs of a return to the normal at Erith.
Hence it is clear that an analysis of the gases dissolved in
water may prove of some help in ascertaining whether the water
is pure, or whether it has been contaminated with putrescent
organic matter. Indeed Miller concludes that whenever the
proportion between dissolved oxygen and nitrogen falls to less
than 1 to 2 the water is unfit for drinking purposes. It is,
however, found that oxygen is almost entirely absent from
certain deep spring waters of great purity, although they
contain their full complement of nitrogen.
In order to collect the gases dissolved in water, it is only
necessary to boil the water and to collect in a suitable measur-
ing apparatus the gases which thus become free. A simple form
of apparatus used for this purpose is shown in Fig. 93. It con-
sists of a globular flask, capable of holding from 500 to 1,000
cc. of water. This flask, connected with a bulb and long tube
by a strong piece of caoutchouc tubing, is filled with the water,
the tubing being closed by a screw-clamp. The bulb, also con-
taining water, is next heated so as to make the water boil
1
294
THE NON-METALLIC ELEMENTS
briskly, and thus the air contained in the bulb and tube is
driven out at the open end of the tube which dips under
mercury. As soon as all the air is driven out, the screw-clamp
is opened, and heat applied to the flask until the water boils,
which under the diminished pressure it will soon do. The dis-
solved gases then begin to come off, and are collected and
measured in the eudiometer filled with mercury, the operation
being continued for not less than an hour, until the last trace

FIG. 93.
of air has been expelled. A Geissler's mercury pump may also
be employed for this same purpose, and the apparatus thus
modified has been used for determining the amount of dissolved
oxygen in water.1
The amount of oxygen dissolved in water may also be deter-
mined chemically by several methods, the best of which depends
1 Roscoe and Lunt, Journ. Chem. Soc. 1889, i. 563.
2 Roscoe and Lunt, Journ. Chem. Soc. 1889, i. 565; Thresh, Journ. Chem.
Soc. 1890, i. 185.
RAIN-WATER
295
upon a measurement of the amount of sodium hyposulphite
which can be oxidised by a given volume of the water
(Schützenberger).
The several kinds of naturally occurring waters may be classed
as rain-water, spring-water, river-water, and sea-water.
149 Rain- Water.-Although this is the purest form of natural
water inasmuch as it has not come into contact with the solid
crust of the earth, it still contains certain impurities which are
washed out by it from the atmosphere. Thus rain-water
invariably contains ammoniacal salts, chloride of sodium, and
organic matter of various kinds in the state of minute suspended
particles which we see when a glass full of such water is held
up to the light. The amount of the constituents thus taken.
out of the air by the falling rain may serve as a means of
ascertaining the chemical climate of the locality, that is the
amount of those varying chemical constituents of the atmo-
sphere which are brought in by local causes. Thus, for instance,
the rain collected in towns where much coal is burnt is generally
found to have an acid reaction, owing to the presence of free
sulphuric acid derived from the oxidation of the sulphur con-
tained in the pyrites present in most coal. The amount of this
acid may reach, under certain circumstances, as much as 7
grains per gallon. In towns, the rain-water also contains a
larger proportion of ammoniacal salts and nitrates than that
falling in the country, whilst it is also found to hold in suspen-
sion or solution albuminous matter derived from decomposing
animal substances. An elaborate examination of the chemical
composition of a large number of samples of rain-water has
been made by Dr. Angus Smith, and in this work will be found,
not only a valuable series of original determinations of the
constituents of rain-water collected in various parts of the
country, but a statement of the results of the labours of
other chemists on the same subject.
According to the experiments of Lawes and Gilbert,²
the average amount of nitrogen contained in country rain-
water as ammonia, organic nitrogen, nitrous and nitric acids,
is about 07 parts in a million of rain-water, whilst the
rain of London (Hyde Park) contains 2:2 parts per million.
Boussingault, on the other hand, found in the rain of Paris
1 On Air and Rain; the Beginnings of a Chemical Climatology. Longmans,
Sixth Report of the River Commissioners, 1874.
1872.
296
THE NON-METALLIC ELEMENTS
4 parts of ammonia in one million, and of nitric acid 0.2 in a
million.
Spring-Water-The water flowing from springs, whether they
are surface- or deep-springs, is always more impure than rain-
water owing to the solution of certain portions of the earth's
crust, through which the water has percolated. The nature and
amount of the material taken up by the water must of course
change with the nature of the strata through which it passes,
and we accordingly find that the soluble constituents of spring-
water vary most widely, some spring-waters containing only a
trace of soluble ingredients, whilst others are highly charged with
mineral constituents. Those waters in which the soluble ingre-
dients are present only in such proportion as not sensibly to
affect the taste, are termed fresh waters; whereas, those in which
the saline or gaseous contents are present in quantity sufficient
to impart to the water a peculiar taste or medicinal qualities are
termed mineral waters. The salts which most commonly occur
in solution in spring-water are: (1) The carbonates of calcium,
magnesium, iron, and manganese, dissolved in an excess of car-
bonic acid. (2) The sulphates of calcium and magnesium. (3)
Alkaline carbonates, chlorides, sulphates, nitrates, or silicates.
The gaseous constituents consist of oxygen, nitrogen, and carbon
dioxide; the latter gas being present in varying amount though
always in much larger quantity than we find it in rain-water.
The nature and quantity of the inorganic, as well as of the
gaseous constituents of a fresh spring, or mineral-water must be
ascertained by a complete chemical analysis, frequently a long
and complicated operation.
150 Mineral-Waters and Thermal Springs.-Spring-waters
which issue from considerable depths, or which originate in
volcanic districts, are always hotter than the mean annual tem-
perature of the locality where they come to the surface. In
many of these springs the water issues together with a copious
discharge of undissolved gas, and in some cases, as in the cele-
brated Geysirs of Iceland, so carefully investigated by Bunsen,¹
steam accompanies the water or forces it out at certain intervals.
Several remarkable hot springs of this kind have been discovered
in New Zealand, but a still more extensive series occurs in the
district of the Yellowstone river in the United States. The
following is a list of the most important thermal springs, the
1 On the Pseudo-Volcanic Phenomena of Iceland, Cav. Soc. Memoirs, 1848,
p. 323.
MINERAL WATERS
297
Wildbad .
Aachen
temperature of all of which is much above that of the locality
where they occur.
Vichy
Bath
•
·
.
THERMAL SPRINGS.
Temp.
37°5
44 to 57°5
45°
47°
Temp.
.67°5
Baden-Baden
Wiesbaden
.70°
Karlsbad
. 75°
Trincheras (Venezuela) 97°
•
The chief
gases found free in these springs are carbonic acid.
and sulphuretted hydrogen.
According to the materials which the water contains in
solution these springs may be grouped as follows:-
(1) Carbonated Waters, which are cold, and are rich in
carbonic acid, and contain small quantities of alkaline carbon-
ates, chloride of sodium, and other salts. Amongst the best.
known of these are the waters of Seltzer, Apollinaris, and
Taunus.
(2) Alkaline Waters, containing a larger quantity of bicar-
bonate of soda, as well as common salt, and Glauber salt.
These are sometimes warm, such as the springs at Ems and
Vichy, but generally cold. They are often rich in carbonic
acid.
(3) Saline Waters, are those in which the alkaline bicarbonate
is replaced by other salts, thus Glauber-salt water, as Marienbad;
Magnesian water, such as Friedrichshall, Seidschutz and Epsom,
in which the sulphate and chloride of magnesium occur; Chaly-
beate waters in which ferrous carbonate is found dissolved in
carbonic acid, such as that of Pyrmont and Spa; Sulphuretted
water, containing sulphuretted hydrogen and the sulphides of
the alkaline metals, as the springs at Aachen and Harrogate.
Hot-springs also occur in which but very small traces of
soluble constituents are found, but which from their high
temperature are used for the purpose of medicinal bathing;
such springs are those of Pfäfers 44°, Gastein 35°, Bath 47°, and
Buxton 28°.
(4) Silicious Waters are those in which the saline contents
consist chiefly of alkaline silicates, such as the hot-spring waters
of Iceland.
The following analysis by Bunsen of the mineral waters
of Dürkheim and of Baden-Baden may serve as examples
298
THE NON-METALLIC ELEMENTS
of the complexity in chemical composition of certain mineral
waters:-
-
Analyses of 1,000 parts of the Mineral Waters in which the new
Alkali Metals Casium and Rubidium were discovered by
Bunsen.
Calcium bicarbonate
Magnesium bicarbonate.
Ferrous bicarbonate
Manganous bicarbonate
Calcium sulphate
Calcium chloride.
Magnesium chloride
Strontium chloride
Strontium sulphate
Barium sulphate
Sodium chloride
Potassium chloride
Potassium bromide
Lithium chloride
Rubidium chloride
Cesium chloride
Alumina.
Silica.
Free carbonic acid
.
·
.
•
Nitrogen
Sulphuretted hydrogen.
Combined nitric acid.
Phosphates.
Arsenic acid
•
•
.
.
Ammoniacal salts.
Oxide of copper
Organic matter
Total soluble constituents.
.
•
.
.
Dürkheim. Baden-Baden.
1.475
0.28350
0.01460
0.712
0.00848 . 0·010
traces
3:03100
0:39870
0.00818
0.01950
traces
12.71000
0.09660.
0 02220
traces
•
0.03910
0.00021
0.00017 .
0.00020 .
0.00040 .
1.64300 .
0.00460
traces
traces
·
·
18:28028 .
·
•
·
·
•
0.023
traces
. 20-834
1.518
•
•
•
·
·
•
·
traces
2.202
0.463
0.126
·
traces
0451
0.0013
traces
1.230
0.456
0.030
traces
traces
0.008
traces
traces
29.6393
151 Hard and Soft Water-Waters are familiarly distinguished
as hard and soft according as they contain large or small quantities.
of lime or magnesia salts in solution. These may exist either as
carbonates held in solution by carbonic acid, or as sulphates.
In both cases the water is hard, that is, it requires much soap to
HARDNESS OF WATER
299
be used in order to make a lather, because an insoluble compound
is formed by the union of the lime or magnesia with the fatty
acid of the soap which consists of sodium or potassium salts of
the acids of this series. But in the first instance, the hardness
is said to be temporary because it is removed either by the addi-
tion of milk of lime or by boiling the water, when the carbonic
acid holding the carbonate of lime in solution is either pre-
cipitated or driven off, whereas in the second instance, it cannot
be thus removed, and is therefore termed permanent hardness.
In order to ascertain the amount of this hardness a simple
method was proposed by the late Dr. Clark. It consists in
ascertaining how many measures of a standard soap solution
are needed by a gallon of water to form a lather. Thus this
soap test serves as a rough but convenient method of determin-
ing the amount of lime or magnesia salts which the water
contains.
The following is a description of a method employed for
determining the hardness of a water. 10 grams of good Castile
soap are dissolved in one litre of dilute alcohol containing about
35 per cent. of alcohol, and the strength is so proportioned that
1 cc. of this solution will precipitate exactly 1 mgrm. of calcium
carbonate when in solution. In order to standardise the soap-
solution 1 gram of calc-spar is dissolved in hydrochloric acid,
the solution evaporated to dryness in order to get rid of the excess
of hydrochloric acid, and the residue consisting of chloride of
calcium dissolved in one litre of distilled water. Of this solution.
12 cc. are brought into a small stoppered bottle, after being
diluted up to 70 cc. with distilled water. The soap-solution is
gradually added from a burette, until when vigorously shaken a
permanent lather is formed. If the solution has been made of
the right strength 13 cc. are needed for this purpose, inasmuch
as 70 cc. of distilled water will themselves require 1 cc of
soap-solution in order to make a permanent lather. For the
purpose of determining the hardness of a water, a measured
quantity of the water is taken, and the standard soap-solution
run in until a permanent lather is obtained. 70 cc. of water
are usually employed for this purpose, because every cc. of the
soap-solution will then correspond to one grain of calcium
carbonate in 70.000, or in a gallon of water. On the Continent
however, the hardness is usually calculated into parts per 100,000
of water. The hardness of a water is expressed in degrees, by
which is understood the number of parts of calcium carbonate
300
THE NON-METALLIC ELEMENTS
or of the corresponding magnesium, or other calcium salts, which
are contained in 70,000 or in 100,000 parts of the water. Thus
Thames water has a hardness of 15°0, or contains in solution
15 grains of carbonate of lime per gallon, whilst in the water of
Bala Lake, only 13 grains per gallon are present. The presence
of magnesium salts interferes to some extent with the accuracy
of the determination of hardness.
152 The Organic Constituents of Waters.-Spring-water not
only contains inorganic, but also soluble organic constituents, and
these likewise vary with the constituents of the strata through
which the water passes. This organic matter may be dis-
tinguished as (a) that which is derived from a vegetable, and
(b) that derived from an animal source. If the water has been
collected from moorland it will contain some soluble vegetable
matter, if it has come in contact with any decomposing animal
substances it will have taken up soluble animal matter. These
two forms of impurity are of a very different degree of importance
as regards the suitability of a water thus impregnated for drinking
purposes.
It is now generally admitted that a number of infectious
diseases, especially cholera and typhoid fever, are frequently
contracted and spread by means of drinking water containing
the bacteria of these diseases, derived from the excreta or other
discharges from persons suffering from them, or from the washing
of infected clothing. Hence it becomes very important to be
able to detect the presence of these pathogenic or disease pro-
ducing organisms. This is however a matter of considerable
difficulty, and hence the bacteriological examination of a water
is mainly valuable as a test of the efficacy of processes of purifi-
cation, since the conclusion may be safely drawn that any mode
of treating the water which proves fatal to the harmless bacteria,
which are generally present in considerable numbers, will also
prove fatal to the dangerous organisms.¹
The actual determination of the number of organisms of all
kinds present in a sample of water, without attempting to ascer-
tain their specific nature, is carried out by mixing a known volume.
of the water with sterilised nutrient gelatine and maintaining the
whole at a temperature of 20-23°, which is found to be favourable
to the development of these organisms, for several days. Under
these circumstances each organism produces a colony around
itself, and reproduction proceeds so rapidly that in a few days
1 Frankland, Journ. Soc. Chem. Ind. 1887, 6, 319.
ORGANIC CONSTITUENTS OF WATERS
301
a mass which is visible to the naked eye is produced. The
colonies are then counted and the number of bacteria which were
present in the volume thus ascertained. The result of a
determination of this character is not absolute because many
organisms do not develop under the conditions observed. In
1886 the water of the Lea unfiltered contained an average of
19781 organisms per cc., whilst after storage and filtration
the same water as supplied to the consumers contained only 253
or about 1.3 per cent. of this number. Filtration through sand,
charcoal, or spongy iron, has the effect of removing the whole of
these bacteria, but, unless the material of the filter is frequently
renewed, this high state of efficiency is not maintained, and the
filter may even serve as an incubating bed, so that the water
passing through is rendered worse by filtration.
Simply boiling the water for a few minutes does not entirely
destroy the bacteria contained in it but greatly diminishes their
number. Thus a sample of Parisian canal water which con-
tained 460,800 bacteria per cc. was found after boiling for ten
minutes only to contain a single living organism in every
2 cc.¹
The chemical examination of a water is a much more rapid
process, but yields even less direct information as to the whole-
someness of the sample than the bacteriological. It aims at
detecting and estimating in the water such substances as are
characteristic of the probable sources of pollution.
Nitrogen for example is one of the characteristic constituents
of animal matter, being present in considerable quantity in a
state of combination in every part of the flesh, nerves and tissues
of the body, whilst it is contained in plants in smaller quantity
and only in their fruit and seeds. Hence if water be impreg-
nated with animal matter this will be indicated by the presence
of nitrogen in solution, either in the form of albumin or
albuminous matter, if the animal matter be contained in the
water unchanged, or, if the animal matter has undergone oxida-
tion, in the form of ammonia, or nitrous or nitric acid. The
amount of the nitrogen which has been oxidized to ammonia.
can be easily determined by distilling the water with carbonate
of soda when the whole of the ammonia is obtained in the
distillate and estimated by Nessler's colorimetric test. The
Nessler's solution is prepared as follows:-35 grams of potas-
sium iodide, and 13 grams of mercuric chloride (corrosive subli-
1 Miguel quoted in article Water, Thorpe's Technical Dictionary.
302
THE NON-METALLIC ELEMENTS
mate) are dissolved in about 800 cc. of hot water and then a
saturated solution of mercuric chloride is gradually added until
the precipitate formed ceases to re-dissolve. 100 grams of
caustic potash are then dissolved in the liquid and the cold
solution is diluted to one litre, and is allowed to deposit any
undissolved matter. Half a litre of the water under examina-
tion must be distilled in a glass retort as shown in Fig. 94,
carbonate of soda having been previously added, and care
must be taken to free the apparatus from ammonia by a previ-
ous process of distillation. The distillate is collected successively
in volumes of 50 cc. and the ammount of ammonia in each of
these separate distillates determined. For this purpose the
distillate is collected in a high cylinder of white glass, 2 cc. of

FIG. 94.
If
Nessler's solution is added, and the mixture well stirred.
the smallest quantity of ammonia be present, a yellow colour is
noticed, and a corresponding degree of tint is obtained in a
second cylinder by gradually adding a standard solution of sal-
ammoniac to 50 cc. of water perfectly free from ammonia,
and containing some of the Nessler's reagent until the tint is
reached. This standard solution is prepared by dissolving 315
grams of ammonium chloride in one litre of water and diluting
10 cc. of this solution to one litre, so that each cc. will therefore
correspond to 0.01 mgrm. of ammonia.
The nitrates and nitrites present can be estimated in another
portion of the water by reducing these acids to ammonia by means
of the hydrogen evolved by aluminium in presence of pure
ORGANIC CONSTITUENTS OF WATERS
303
caustic alkalis. For other methods the treatises on water
analysis must be consulted.
In order to estimate the quantity of unaltered albuminous
matter which may possibly be contained in the water, two pro-
cesses have been proposed. The first of these, proposed by
Wanklyn and Chapman, depends upon the fact that these
albuminous bodies are either wholly or in part decomposed on
distillation with an alkaline solution of potassium permangan-
ate, the nitrogen being in this case again evolved as ammonia
which is determined as above. In the second process,
described by Frankland and Armstrong, the nitrogen gas con-
tained combined in albuminous matter in the water is liberated
as such by a combustion analysis performed on the dry residue
of the water, the volume of the free nitrogen being afterwards
carefully measured. In this latter process not only the organic
nitrogen but also the organic carbon, that is, the carbon de-
rived from animal and vegetable sources, can be quantitatively
determined.
If, now, by means of either of these processes, a water is
found to contain more than 0.15 parts of albuminoid nitrogen
in one million parts of water, it may be considered as unfit for
drinking purposes; many surface well-waters occur in large
towns in which the amount of albuminoid nitrogen reaches 0·3
to 08 parts per million, and such waters must be regarded as
little better than sewage, and, therefore, as absolutely poisonous.
But water in which no albuminous matter has been found may
also be largely impregnated with sewage or infiltrated animal
impurity, the greater part of which has undergone oxidation.
Thus when the amount of free ammonia exceeds 0:08 parts per
million, it almost invariably proceeds from the decomposition of
urea into carbonate of ammonia, and shows that the water consists
of diluted urine. In like manner, when the oxidation has pro-
ceeded further, the nitrogen will be found as nitrates and
nitrites, and should any considerable quantity of these sub-
stances be found in surface well- or river-water, the previous
admixture of animal impurity may be inferred. In some
instances, however, water from deep wells in the chalk has been
found to contain nitrates, which in such cases cannot as a rule
be supposed to indicate dangerous impurity.
153 The water analyst is also assisted in his attempts to
indicate the limits of wholesomeness in a water by the determina-
tion of the amount of chlorine present as chloride of sodium, &c.
which the water contains. Not that chlorides are in themselves
304
THE NON-METALLIC ELEMENTS
of importance, but because their presence serves as an indication
of sewage-contamination, for pure natural waters are almost free
from chloride of sodium, whilst urine and sewage are highly
charged with this substance So that if we meet with a water
almost free from chlorine, it cannot have come into contact with
sewage. Thus the water of Ullswater and that of the Thames
at Kew contain from 07 to 08 grains of chlorine in the gallon,
whilst many surface wells in large towns may be found which
contain from 10 to above 30 grains of chlorine per gallon.
Taken alone, the chlorine test cannot be relied upon, as many
pure well-waters occur, such as those in Cheshire, in the neigh-
bourhood of the salt beds, or near the sea, which contain com-
mon salt. If, however, this test be employed in conjunction
with those previously mentioned, the evidence for or against a
water is rendered much more cogent. As a rule it may be
said that waters containing more than two grains of chlorine per
gallon must be looked upon with suspicion, unless indeed some
good reason for the presence of common salt can be assigned.
The following analyses serve to show the difference between
a good potable water and one which is totally unfit for drinking
purposes. No 1, the water supplied by the Manchester Cor-
poration from the Derbyshire hills; No. 2 is a surface-well
water, at one time used for drinking purposes in a manufactur-
ing town, although little better than effluent sewage.¹
Total solids
Nitrogen as nitrites and
nitrates
Free ammonia.
Albuminoid ammonia
Chlorine..
Temporary hardness.
Permanent hardness.
Total hardness
No. 1. Good Water. No. 2. Bad Water.
Parts per Grains per
Million. Gallon.
63.0
0:25
0:03
0:07
11.4
4.4
530
0.017
7.8
0.002 4:32
0.005 0.9
69
Parts per Grains per
Million. Gallon.
0.8
0.1
2.4
2.5
37.1
0.546
0.303
0.063
4.8
7.2
144
21.6
1 For the special details of the processes of water analysis, the following works
or memoirs may be consulted:- Water Analysis, by Wanklyn and Chapman.
1889. Trübner, London. Frankland and Armstrong, Journ. Chem. Soc. 1868.
p. 77; Frankland, ibid. 109; also Journ. Chem. Soc. June, 1876. Percy Frank-
land, Agricultural Chemical Analysis, p. 257. Macmillan, 1883.
RIVER-WATERS
305
154 River- Waters.-The composition of river-water varies
considerably with the nature of the ground over which the
water runs; thus Thames water contains about 11 grains per
gallon of carbonate of lime; the Trent 21 grains of sulphate of
lime, or they are both hard waters, the first temporarily and the
second permanently hard. The waters of the Dee and the Don,
in Aberdeenshire, draining a granite district, are, on the other
hand, soft waters. The composition of these waters is shown in
the following table :-
TABLE GIVING THE COMPOSITION OF CERTAIN RIVER-WATERS.
Calcium carbonate
Calcium sulphate
Calcium nitrate.
Magnesium carbonate
Sodium chloride.
Silica
Ferric chloride and alu-
mina.
Calcium phosphate .
Organic matter
Hardness
•
21
•
•
Thames.
10.80
Grains per Gallon.
Trent. Dee.
0.32 0.85
21.55
0.12
3:00
0.17
1.25
1.80 17.63
0.56
0.72
1 P. 15.
-.
5.66 0.36
0.72
0.14
0.27
trace
2:36
20.21
14.0 26.5
0.50
0.06
trace trace
3.68 1.54
50.06 3.89
1.5
Don.
2.23
0.13
---
1:07
1.26
0.52
Unfortunately in England, as in other manufacturing and
densely populated countries, the running water seldom reaches
the sea in its natural or pure state, but is largely contaminated
with the sewage of towns, or the refuse from manufactures or
mines. So serious indeed is this state of things that steps have
been taken to prevent the further pollution of the rivers of the
country, and a Royal Commission has already reported and
several acts of Parliament have been passed with a view of
preventing the evil. The following analyses of the composition
of Lancashire rivers, taken from the First Report of the Com-
missioners¹ appointed in 1868, show clearly the pollution which
the originally pure waters of the Irwell and Mersey undergo on
flowing down to the sea.
0.27
trace
3:06
8.54
3:0
'.
·
+
306
THE NON-METALLIC ELEMENTS
COMPOSITION OF LANCASHIRE RIVERS.
Parts in 100,000.
Total soluble solids.
Organic carbon
Organic nitrogen
Ammonia
Nitrogen as nitrates and
nitrites
Total combined nitrogen
Chlorine
Hardness temporary
Total hardness
Organic.
Mineral.
Total
SUSPENDED MATTER-
IRWELL.
*1
*1. The Irwell near its source.
2. The Irwell below Manchester.
7.8
0.187
0.025
0.004
0.021
0.049
1.15
3.72
3.72
0
2
3.
4.
55.80
MERSEY.
3
7.62
1.173
0.222
0.332
0
0.740 0.002
2.71
2.71
5.42
0.707 0.021
1.648
0.023
9.63 0.94
15.04 4.61 10.18
15.04 4.61 10.18
OOO
0
0
39.50
0
1.231
0.601
0.622
From these numbers it is seen that the quantities of free
ammonia and nitric acid become increased 300 or 400-fold in
the river below Manchester, whilst the total combined nitrogen
is increased from 0.049 to 1.648.
0
1.113
155 Sea-Water.-The amount of solid matter contained in the
waters of the ocean is remarkably constant when collected far
from land. The mean quantity is about 35.976 grams in
1000 grams of sea-water; the average specific gravity of sea-
water is 102975 at 0°.
The Mersey, one of its sources.
The Mersey below Stockport.
SEA-WATER
307
COMPOSITION OF THE WATER OF THE IRISH SEA IN THE
SUMMER OF 1870.¹
One thousand grams of sea-water contain
1
.
Sodium chloride
Potassium chloride
Magnesium chloride.
Magnesium bromide .
Magnesium sulphate
Magnesium carbonate.
Magnesium nitrate
Calcium sulphate.
Calcium carbonate
Lithium chloride
Ammonium chloride
Ferrous carbonate
Silicic acid. .
Grams.
26.43918
0.74619
3.15083
0.07052
2 06608
traces
0.00207
1.33158
0.04754
traces
0.00044
0.00503
traces
33.85946
For the purpose of controlling the analysis, 1,000 grams of
water were evaporated to dryness, and the dry residue weighed.
Its weight was found to be 33.83855 grams.
The specific
gravity of the water at 0° C. was 102721, whilst that at 15° C.
was 1.02484.
Forchhammer found that 1,000 parts by weight of the water
of the mid-Atlantic Ocean contained 35.976 parts of dissolved
salts, whilst the mean of analyses of sea-water from different
localities gave 34-082 for the total salts in summer and 33-838
in winter. Dittmar,2 on the other hand, from 77 specimens
of sea-water collected on board the Challenger in various parts
of the world, concludes that the maximum quantity of salt
contained in the water of the Indian Ocean is 33-01, and in
that of the North Atlantic 37 37. In the neighbourhood of the
shore or in narrow straits the quantity of saline matter is often
much smaller.
According to Forchhammer the relation in which the several
Thorpe and Morton, Journ. Chem. Soc. 24, 506.
Challenger Reports, "On the Composition of Ocean Water." By Professor
Dittmar, F.R.S. London, 1884.
1
I
308
THE NON-METALLIC ELEMENTS
salts stand to one another is a constant one; and in this
conclusion Dittmar agrees. The former calculated the quantity
of lime, magnesia, potash, and sulphuric acid present with 100
parts of chlorine, including bromine. Dittmar estimated this
element and also soda and carbon dioxide, and then calculated
as Forchhammer, reckoning the bromine as its equivalent of
chlorine. Their results are:-
Cl
Br
SO3
CO₂
•
Forchhammer.
100.00
-
11.88
Sodium chloride
Magnesium chloride
Magnesium sulphate
Calcium sulphate
Potassium sulphate
Dittmar.
99.848
0.3
11.576
0.276
·
CaO.
MgO
K.0 .
Na,O
77.758
10.878
4.737
3.600
2.465
Forchhammer.
2.93
11.03
1.93
Dittmar calculated the following average composition of the
total saline constituents of sea-water, giving a somewhat different
arrangement to the acids and bases from that adopted by Thorpe
and Morton.
•
Magnesium bromide
Calcium carbonate.
.
BaO₂+ 2HCl = H₂O₂+BaCl₂
2
Dittmar.
3-026
11.221
2.405
74.462
"
0.217
0.345
All the elements are doubtless contained in sea-water.
In addition to those named above the following have been
detected:-
Iodine, fluorine, nitrogen, phosphorus, silicon, carbon, boron,
zinc, cobalt, nickel, copper, strontium, barium, manganese,
aluminium, iron, lithium, cæsium, rubidium (these three detected
spectroscopically), silver, lead, and lastly arsenic, making a total
of thirty elements.
1 Ann. Chem. Phys. 8, 306.
2 Duprey, Compt. Rend. 55, 736; Balard, Compt. Rend. 55, 758.
100 000
HYDROGEN DIOXIDE, OR HYDROGEN PEROXIDE. H₂O=33-76.
156 This body was discovered in 1818 by Thénard,' who
prepared it, by the action of dilute hydrochloric acid on barium
dioxide, thus:-
The compound is also easily formed by passing a current of
carbon dioxide through water, and gradually adding barium
dioxide in very small quantities,2 thus:-
BaO2 + CO2 + H₂O = H₂O₂ + BaCO3.
HYDROGEN PEROXIDE
309
Preparation.-Hydrogen dioxide is, however, most generally
obtained by decomposing pure barium dioxide with dilute
sulphuric acid; thus:-
BaO₂+H₂SO¸=H₂O₂+BaSO4.
The pure barium dioxide needed for these experiments is
prepared as follows:-commercial barium dioxide, very finely
powdered, is brought little by little into dilute hydrochloric acid,
until the acid is nearly neutralised. The cooled and filtered
solution is then treated with baryta-water, in order to precipitate
the ferric oxide, manganic oxide, alumina, and silica which are
always contained in the impure barium dioxide. As soon as a
white precipitate of the hydrated barium dioxide makes its
appearance, the solution is filtered, and to the filtrate concen-
trated baryta-water is added; a crystalline precipitate then falls
consisting of hydrated barium dioxide. This is well washed
and preserved, in the moist state, in stoppered bottles. In order
to prepare hydrogen dioxide by means of this substance, the
moist precipitate is gradually added to a cold mixture of not less
than five parts of water to one part of concentrated sulphuric
acid, until the mixture remains very slightly acid. The preci-
pitate of barium sulphate is allowed to settle and the liquid
filtered. The small trace of sulphuric acid which the filtrate
contains can be precipitated by careful addition of dilute baryta
solution. When the aqueous solution of the hydrogen dioxide
thus prepared is brought over sulphuric acid in vacuo, water
evaporates, and the solution of the dioxide becomes more con-
centrated. If, during the concentration, an evolution of oxygen
be noticed, a drop or two of sulphuric acid should be added, as
the dioxide is more stable in presence of free acid than when
perfectly pure (Thénard).
An aqueous solution of the peroxide can readily be prepared
for use in cases in which the presence of a soluble sodium
salt does not interfere, by dissolving sodium peroxide, Na,O,, a
compound now prepared on a large scale, in cold water and adding
a dilute acid.
2
2
Properties.-After the slow evaporation has been carried on
for some time a colourless transparent oily liquid is obtained,
having a specific gravity of 1452. This liquid evaporates
slowly in vacuo without the residue undergoing any change
1 Thomsen, Ber. 7,
74.
310
THE NON-METALLIC ELEMENTS
(Thénard). Hydrogen dioxide does not solidify at -30°; it
possesses no smell, has an astringent, bitter taste, and brought
on to the skin produces a white blister, which after a time
produces great irritation. It bleaches organic colouring matters
like chlorine, although acting more slowly, and is a very unstable
compound, easily decomposing into oxygen and water. One
volume of the liquid at 14° and under a pressure of 760mm.
yields, according to Thénard, 475 volumes of oxygen, whilst
the theoretical amount is 5018 volumes. The decomposition of
hydrogen dioxide takes place very slowly at a low temperature,
whilst at 20° the evolution of gas becomes plainly visible, and
if the concentrated solution is heated up to 100°, the separa-
tion of oxygen occurs so rapidly as sometimes to give rise to an
explosion. A dilute aqueous solution of the dioxide is, how-
ever, much more stable, and can even be concentrated up to a
certain point by ebullition, a portion of the dioxide passing
over undecomposed together with the vapour of water. This
explains why in the above decomposition the theoretical quantity
of oxygen is not obtained.
Hydrogen dioxide is easily soluble in ether, and if an aqueous
solution of the compound be shaken up with ether, the
dioxide dissolves in it. The ethereal is more stable than
the aqueous solution, and it can be distilled without decom-
position.
Hydrogen dioxide undergoes decomposition in presence of
a large number of different solid substances, and with the
greater rapidity the more finely divided these substances are.
Some of the phenomena which thus present themselves may,
to a certain extent, be accounted for, but for others we still
need an explanation. Thus, for example, the anhydrous com-
pound is decomposed with almost explosive violence into oxygen
and water, in presence of finely-divided silver, gold, platinum,
and other metals, the metals themselves, however, remaining
unaltered. The oxides of these metals also decompose hydrogen
dioxide easily, being reduced to the metallic state. The same
decomposition also occurs with a dilute aqueous solution of the
dioxide, and the reaction may be represented by the following
equation:-
Ag₂O+H₂O₂ = 2Ag+H₂O+O₂.
2 2
Here we have the remarkable phenomenon of a powerful
oxidizing agent exerting a reducing action upon metallic oxides,
HYDROGEN PEROXIDE
311
the metal being formed. The explanation of this fact is
however, not far to seek. The above-named metals possess
only a weak power of combination for oxygen, and their oxides.
accordingly decompose easily into their elements. When these
oxides are brought into contact with hydrogen dioxide, which
itself contains one atom of oxygen but feebly united, mutual
reduction takes place, the one atom of oxygen in the dioxide
combining with one atom of oxygen in the metallic oxide to
form a molecule of free oxygen. In the same way we explain
the fact that common oxygen is formed when ozonized oxygen
is brought into contact with aqueous hydrogen dioxide. Here,
too, both bodies contain a loosely combined atom of oxygen,
which unite together to form a molecule of free oxygen;
thus:-
----
H
80+0=0+20,
When baryta-water (barium monoxide) is mixed with hy-
drogen dioxide a precipitate of barium dioxide separates out;
thus:-
BaO+H,O,=BaO,+H,O.
If the hydrogen peroxide be left in contact with the barium
peroxide, oxygen is slowly evolved until the whole of the
hydrogen peroxide has been decomposed, the barium peroxide
being however left unaltered. According to Schöne this is due
to the formation of a compound, BaO2.H2O2. This is a white
substance, which decomposes in the presence of an excess of
hydrogen peroxide into barium peroxide, water and free
oxygen. Similar reactions occur with the alkaline hydroxides,
and this may explain the instability of hydrogen peroxide in
alkaline solution.
Hydrogen dioxide also transforms many other basic oxides.
especially in presence of an alkali, to peroxides. Thus manganous
salts become in this way converted into manganese dioxide. On
the other hand, these peroxides in presence of an acid are again
reduced by hydrogen dioxide to basic oxide. Thus, if hydrogen
dioxide be brought into contact with dilute sulphuric acid and
manganese dioxide, oxygen gas is given off, and manganous
sulphate is formed; thus:-
MnO₂+H₂O₂+H₂SO₁ = MnSO4+2H₂O+O₂.
2
1 Cf. Berthelot, Compt. Rend. 90, 572.
312
THE NON-METALLIC ELEMENTS
The decomposition here occurring is similar to that which takes
place in the reduction of oxide of silver, and this change is assisted
by the presence of the acid, which then combines with the basic
oxide to form a salt.
Detection and Estimation of Hydrogen Dioxide. In order
to detect the presence of hydrogen dioxide in solution, the
liquid is rendered acid with sulphuric acid, some ether and a
few drops of potassium chromate are added, and the solution
well shaken. If hydrogen dioxide be present, the solution
assumes a beautiful blue colour, and on allowing it to stand
the colour is taken up by the ether and a deep blue layer
separates out. This blue compound is probably perchromic acid,
and the reaction may, in a similar way, be employed for the
detection of chromium.¹
When hydrogen dioxide is added to a solution of iodide
of potassium and ferrous sulphate, iodine is set free, as may
easily be proved by the formation of the blue iodide of starch
(Schönbein). This reaction is so delicate that one part of the
dioxide in twenty-five million parts may thus be detected. Other
oxidizing agents have the power of liberating iodine from iodide
of potassium, but not in presence of ferrous sulphate.
For the purpose of determining the quantity of hydrogen
dioxide present in a solution, the liquid is acidified with sulphuric
acid, and then a standard solution of potassium permanganate
added, until the purple tint no longer disappears. The reaction
here occurring is thus represented :-
-
2KMnO4+3H₂SO¸+5H₂O₂ = K₂SO+2MnSO¸+8H₂O+50½
Hydrogen dioxide occurs in small quantities in the atmosphere
and has been found in rain and in snow to the amount of about
one mgrm. per litre; but the above method cannot be used for
estimating its quantity in this case, as other substances contained
in the air, such as the nitrites and organic matter, act upon the
permanganate solution In this case a colorimetric method
proposed by Schöne 2 may be used. It depends upon the fact that
a neutral solution of hydrogen dioxide gradually liberates iodine
from a neutral solution of potassium iodide; thus:—
H₂O₂+2KI = 2KOH+I2.
1 Moissan, Compt. Rend. 97, 96; Berthelot, Compt. Rend. 108, 24.
2 Ber. 7, 1695; Annalen, 195, 228.
OXIDES AND OXYACIDS OF CHLORINE
313
The liberation of iodine is accompanied by an evolution of oxy-
gen, and is very small in comparison to the amount of hydrogen
peroxide decomposed. Solutions containing small known but
varying quantities of hydrogen dioxide are first prepared, and to
each of these solutions potassium iodide and starch paste are
added, the degree of tint which the several solutions attain,
owing to the formation of the blue iodide of starch, is then com-
pared with that obtained in a similar way when rain-water, &c.,
is employed.
An aqueous solution of hydrogen dioxide is now largely used
for the bleaching of silk and wool. For this purpose a liquor is
prepared by dissolving sodium peroxide in water and acidulating
with sulphuric acid. The sodium peroxide is manufactured by
the combustion of sodium and is brought into the market under
the name of soda-bleach. Peroxide of hydrogen is also used for
the purpose of cleaning and bleaching old and stained engrav-
ings and oil paintings, and as an auricome for bleaching dark-
coloured hair.
OXYGEN AND CHLORINE.
OXIDES AND OXY-ACIDS OF Chlorine.
157 Although chlorine and oxygen do not combine directly,
two distinct compounds of these elements may be obtained by
indirect means.
A third oxide, described by Millon and others as
chlorine tri-oxide, has been proved to be a mixture of free chlorine
and chlorine peroxide.¹ We are acquainted with no less than
four compounds of chlorine with oxygen and hydrogen, which
are known as the oxy-acids of chlorine. Of these, the first
corresponds to the oxide of chlorine, that is, it is formed by the
action of water upon the latter. The following are the com-
pounds of chlorine, oxygen, and hydrogen as yet known:
Oxy-acids.
Hypochlorous acid, HCIO
Chlorous acid, HClO2
Oxides
Chlorine monoxide, Cl₂O
Chlorine peroxide, ClO2
Chloric acid, HClO3
Perchloric acid, HC104
¹ Garzarolli-Thurnlackh, Annalen, 209, 184.
314
THE NON-METALLIC ELEMENTS
Chlorine and oxygen can only be made to combine together
in the presence of a basic oxide; thus, if chlorine gas be led
over dry mercuric oxide, chlorine monoxide and mercuric oxy-
chloride are formed; thus:-
2HgO + 2Cl₂ = Hg₂OCl2 + Cl₂O.
The same reaction takes place in presence of water, and in
this case a colourless solution of the corresponding hypochlorous
acid is formed.
Cl₂O + H2O = 2C1OH.
When chlorine is passed into a cold dilute solution of an
alkali such as caustic potash, instead of the free hypochlorous
acid the corresponding salt, termed a hypochlorite, is formed;
thus:-
2KOH + Cl₂ = KOCI + KCl + H₂O.
If the solution of the alkali be concentrated, or if it be heated
whilst the gas is passed through it, a different reaction takes
place, in which a salt called a chlorate is formed; thus:—
6KOH + 3C1, = KClO3 + 5KC1 + 3H₂O.
2
From the potassium chlorate thus formed, chloric acid itself can
be obtained, and by reduction of this acid the oxide CIO, may
be prepared. Perchloric acid is prepared by the further oxidation
of chloric acid. The oxides corresponding to chlorous, chloric
and perchloric acids, viz., Cl₂O, Cl₂O, and Cl₂O,, have not yet
been prepared. The oxides and oxy-acids of chlorine are un-
stable compounds, as indeed might be expected, owing to the
feeble combining power which chlorine and oxygen exhibit to-
wards one another; in consequence of this they act as powerful
oxidizing substances, many of them being most dangerously
explosive bodies, which suddenly decompose into their con-
stituents on rise of temperature, or even on concussion. It is,
however, remarkable that perchloric acid, which contains the
most oxygen, is the one which is the most stable.
B 38
Be
***
CHLORINE MONOXIDE
315
CHLORINE MONOXIDE OR HYPOCHLOROUS ANHYDRIDE. Cl₂O.
158 So long ago as 1785 Berthollet noticed that chlorine could
be combined with an alkali and yet preserve the peculiar bleach-
ing power which had been previously discovered by Scheele, and
it is to Berthollet that we owe the practical application of this
important property. In his first experiments on this substance
he employed chlorine water, but afterwards he absorbed the gas
by a solution of caustic potash; and the liquor thus obtained,
called Eau de Javelles from the name of a bleach-works where
it was prepared, was employed for bleaching purposes on the large
scale. Berthollet described these experiments to James Watt,
who was at that time staying in Paris, and he brought the news
to Glasgow, where Tennant, in 1798, patented an improved
process for bleaching, in which lime was employed instead of the
potash, as being a much cheaper substance.¹
Up to the year 1809-10, when Gay-Lussac and Thénard
propounded the view that chlorine might be considered an
element, and when Davy proved that this supposition was correct,
the bleaching liquors were supposed to contain oxygenated muri-
ates of the base. Indeed their constitution remained doubtful
until the year 1834, when Balard 2 showed that the alkaline
bleaching compounds may be considered to be a mixture or
combination of a chloride and a hypochlorite. Eau de Javelles
therefore contains potassium chloride and hypochlorite, and
bleaching-powder solution the corresponding calcium salts;
solid bleaching-powder has, however, a different constitution
(Vol. II., Pt. I., p. 194).
of
Preparation.-Chlorine monoxide is obtained, as seen in
Fig. 95, by the action of dry chlorine gas upon cold dry oxide
mercury, which is contained in a tube (a b), which should be
cooled by means of ice or a stream of cold water.³ The crystal-
lized mercuric oxide can, however, not be used for this purpose,
as it is not acted on by dry chlorine, and hence the precipitated
oxide must be employed, it having been previously carefully
washed and dried at 300-400°. The reaction which takes place in
this case has already been described. Mercuric chloride, HgCl2,
is not formed in this reaction, but the oxychloride, HgO,HgCl.
1 Tennant's first patent was declared invalid three years after it had been
granted, as it was proved that bleachers in Lancashire and at Nottingham had
employed lime instead of potash before the year 1798.
2 Ann. Chim. Phys. 57, 225.
3 V. Meyer, Ber. 16, 2999; Ladenburg, Ber. 17, 157.
1
1
1
+
I
316
THE NON-METALLIC ELEMENTS
159 Properties.-Chlorine monoxide is a brownish yellow
coloured gas, which has a peculiar penetrating smell, somewhat
resembling, though distinct from, that of chlorine. Its density
is 435. By exposure to a low temperature the gas can be con-
densed, as in the tube (D), Fig. 95, to an orange-coloured liquid,
which boils at about + 5'. If an attempt is made to seal up
this liquid in the tube in which it has been prepared, or even if
the tube in which it is contained be scratched with a file, it de-
composes suddenly with a most violent explosion (Roscoe); and
when poured out from one vessel to another a similar explosion

B
C
FIG. 95.
takes place (Balard). It likewise explodes on heating, but not
so violently, two volumes decomposing into one volume of oxygen
and two of chlorine. According to Garzarolli-Thurnlackh and
Schacherl it does not, contrary to previous statements, undergo
decomposition in direct sunlight, and the liquid, if all organic
matter be carefully excluded, may be distilled without decom-
position. Most easily oxidizable substances and many finely
divided metals take fire in the gas and produce an explosion;
the gas is also decomposed in presence of hydrochloric acid into
free chlorine and water.
2HCl + Cl₂O 2Cl2 + H2O.
1 Thurnlackh and Schacherl, Annalen, 230, 273.
=
HYPOCHLOROUS ACID
317
Chlorine monoxide is readily soluble in water, the latter dis-
solving 200 vols. or 0.78 of its weight of the gas at 0°. The
solution has an orange-yellow colour.
__________
HYPOCHLOROUS ACID. HCIO.
160 The aqueous solution of chlorine monoxide must be con-
sidered as a solution of hypochlorous acid, a compound which
in the pure state is unknown.
Preparation. (1) The solution is best prepared by shaking
chlorine-water with precipitated mercuric oxide, when the oxide
quickly dissolves and the colour of the solution disappears,
thus:-
HgO+2Cl₂+H₂O= HgCl₂+2C1OH.
The liquid is now distilled in order to remove the mercuric
chloride, and a distillate is thus obtained, which, although it
contains only half as much chlorine as the original chlorine
water, possesses an equal bleaching power.¹
This fact is expressed in the following equations, which also
indicate that the bleaching effect produced by chlorine is in
reality due to a decomposition of water, the chlorine combining
with the hydrogen and liberating the oxygen. It is, therefore,
this latter element which is the true bleaching agent, inasmuch
as it oxidizes and destroys the colouring agent.
(a) Bleaching action of chlorine water,
2Cl₂+2H₂O=4IIC1+O₂.
(b) Bleaching action of the hypochlorous acid formed from
the chlorine water,
2C1OH = 2C1H+02.
(2) An aqueous solution of hypochlorous acid is also easily
obtained by adding to a solution of a bleaching compound
exactly the amount of a dilute mineral acid requisite to liberate
the hypochlorous acid (Gay-Lussac). For this purpose a dilute
nitric acid containing about 5 per cent. of the pure acid is
allowed to run slowly from a tap-burette into a filtered solution
of common bleaching-powder, whilst the liquid is kept well
stirred in order to prevent a local super-saturation, which would
1 Gay-Lussac, Ann. Chim. Phys. 43, 161.
318
THE NON-METALLIC ELEMENTS
cause a liberation of the hydrochloric acid of the chloride, and
thus again effect a decomposition of the hypochlorous acid into
chlorine and water. If this operation be conducted with care,
no chlorine is evolved, or at any rate only a trace if a slight
excess of nitric acid has been added, and the distillate is
perfectly colourless. Boric acid may also be employed with
advantage for liberating hypochlorous acid from its salts.¹
(3) Another method of obtaining the aqueous acid is to saturate
a solution of bleaching-powder with chlorine, then to drive off the
excess of chlorine by passing a current of air through the liquid,
and then to distil. The following equation represents the
reaction which here occurs :—
Ca(OCl)2+2Cl2 + 2H2O = CaCl₂+4CIOH.
In place of bleaching-powder baryta-water may be employed
when barium hypochlorite is at first formed, and this afterwards
decomposed as shown above.2
(4) Hypochlorous acid is so weak an acid that its salts are
decomposed by carbonic acid, so that if chlorine gas is led into
a solution of a carbonate, or passed through water containing
finely divided calcium carbonate in suspension (1 part to 40 of
water), no hypochlorite is formed, but only hypochlorous acid
(Williamson); thus:-
CaCO3+2C1, +H₂O = 2C1OH + CaCl2+CO2.
Other salts of the alkali-metals act in a similar way when a
stream of chlorine is passed through their aqueous solutions;
this is the case with sulphate and phosphate of sodium, in these
cases an acid salt is formed; thus:-
Na₂SO₁+H₂O+Cl₂ = NaCl + NaHSO₁+CIOH.
Concentrated aqueous solutions of hypochlorous acid have an
orange yellow or golden yellow colour, and an odour somewhat
resembling that of chloride of lime. Only dilute solutions of
hypochlorous acid can be distilled without decomposition; con-
centrated solutions are readily decomposed either on heating
or on exposure to sunlight, part splitting up into chlorine and
oxygen, whilst another part undergoes oxidation, yielding chloric
acid.
1 Lauch, Ber. 18, 2287.
2 Williamson, Chem. Soc. Mem. 2, 234.
"P
THE HYPOCHLORITES
319
The hypochlorites, like the acid, are unstable compounds,
which in the pure state are almost unknown. Of these the most
important is formed when bleaching-powder is dissolved in water
as calcium hypochlorite, Ca(OCI),, although it is probably not
present as such in the solid substance, and it is to the presence
of this compound that the bleaching properties of the solution
are due, inasmuch as when either hydrochloric or sulphuric acid
is added, a quantity of chlorine equal to that contained in the
compound is evolved. In the first case half the chlorine is derived
from the hypochlorite, the other half from the hydrochloric acid,
which first liberates hypochlorous acid, and then decomposes it
into chlorine and water; thus :—
manner:
(1) 2HCl+Ca(OCl), = 2HOC1+CaCl2.
(2) 2HC1+2HOCI = 2H₂O+2C12.
If sulphuric acid is used, the result is the same, as this acid
decomposes the calcium chloride; thus:-
CaCl₂+Ca(OCl)2+2H₂SO, = 2CaSO4+2H₂O+2C12.
When the solutions of hypochlorites are heated, they undergo
decomposition into chlorides and chlorates in the following
3KCIO=2KC1+KCIO„. .
Hence when chlorine is passed into a hot solution of an alkali
the product is a mixture of chloride and chlorate.
CHLOROUS ACID AND THE CHLORITES.
161 This acid, like chlorine trioxide, is not known in the
free state, but the chlorites can be prepared by adding potassium
hydroxide to an aqueous solution of chlorine peroxide, a mixture
of potassium chlorate, and potassium chlorite, KCIO,, being
obtained.1 The chlorites of the alkali metals are soluble in
water, and from their solutions the insoluble, or difficultly soluble
chlorites of silver, AgClO2, and of lead, Pb(ClO2)2, may be
prepared by double decomposition, as yellow crystalline powders.
All the chlorites are very easily decomposed. Thus if the lead-
¹ Garzarolli-Thurnlackh and v. Hayn, Annalen, 209, 203.
320
THE NON-METALLIC ELEMENTS
salt is heated for a short time to 100°, it decomposes with
detonation; and if it is rubbed in a mortar with sulphur or
certain metallic sulphides, ignition occurs. The soluble chlorites
possess a caustic taste, and bleach vegetable colouring matters,
even after addition of arsenious acid. This latter reaction
serves to distinguish them from the hypochlorites.
CHLORINE PEROXIDE. CIO.
162 This gas was first prepared and examined by Davy in
1815; it was obtained by him by the action of strong sulphuric
acid on potassium chlorate. In preparing this substance special
precautions must be taken, as it is a highly explosive and
dangerous body.
Preparation.--Pure powdered potassium chlorate is for this
purpose thrown little by little into concentrated sulphuric acid
contained in a small retort. After the salt has dissolved, the retort
is gently warmed in warm water. In this reaction chloric acid.
is in the first instance liberated, and then decomposes as follows
into perchloric acid, chlorine peroxide and water; thus:-
3HC10,= HC10+2C1O₂+H₂O.
Properties. The heavy dark yellow gas thus given off must
be collected by displacement, as it decomposes in contact with
mercury and is soluble in water; it possesses a peculiar smell,
resembling that of chlorine and burnt sugar. When exposed to
cold the gas condenses to a dark-red liquid, which boils at +9°,
and at 79° freezes to an orange-coloured crystalline mass. Its
vapour density corresponds to the formula CIO,, and the double
formula, ClO4, previously employed is therefore incorrect. The
gaseous, and especially the liquid and solid peroxide undergo
sudden decomposition, frequently exploding most violently,
hence their preparation requires extreme care. According to
Schacherl, liquid chlorine peroxide may be distilled without
decomposition if every trace of organic matter be excluded.³
In order to obtain an aqueous solution of the gas, a mixture of
potassium chlorate and oxalic acid may be heated in a water
bath to 70°, the mixture of chlorine peroxide and carbon dioxide
1 Pebal, Annalen, 177, 1.
* Pebal and Schacherl, Annalen, 213, 113.
3 Annalen, 206, 68.
1
CHLORIC ACID AND THE CHLORATES
321
then evolved being passed into water. The following equation
represents the action here occurring :—
2KCIO + 2H₂C₂O₁ = K₂C₂04+2H₂O+2CO₂+2C1O2.
Chlorine peroxide can be preserved without change in the dark;
it is, however, slowly decomposed into its elementary con-
stituents when exposed to light, and this decomposition takes
place quickly and with explosion when an electric spark is
passed through the gas, two volumes of the gas yielding one
volume of chlorine and two of oxygen.
When phosphorus, ether, sugar, or other easily combustible
substances are thrown into the gas they take fire spontaneously.
This oxidizing action of chlorine peroxide is well illustrated by
the following experiments. About equal parts of powdered
white sugar and chlorate of potash in powder are carefully
mixed together with a feather on a sheet of writing paper,
the mixture then brought on a plate or stone placed in a draught
chamber, and a single drop of strong sulphuric acid allowed
to fall upon the mixture, when a sudden ignition of the whole
mass occurs. This is caused by the liberation of chlorine
peroxide, which sets fire to a particle of sugar, and the ignition
thus commenced quickly spreads throughout the mass, and
the sugar is all burnt at the expense of the oxygen of the
chlorate. The combustion of phosphorus can be brought about
under water by a similar reaction: for this purpose some crystals
of chlorate of potassium and a few small lumps of yellow
phosphorus are thrown into a test glass half filled with water,
and a small quantity of strong sulphuric acid allowed to flow
through a tube funnel to the lower part of the glass where the
solids lie. As soon as the acid touches the chlorate, chlorine
peroxide is evolved, and this gas on coming in contact with
the phosphorus oxidizes it, and bright flashes of light are
given off.
Water at 4° dissolves about twenty times its volume of
chlorine peroxide gas, forming a bright yellow solution, whilst at
lower temperatures a crystalline hydrate is formed. If this
aqueous solution is saturated with an alkali, a mixture of chlorite
and chlorate is formed; thus:-
2KOH+2ClO,=KClO,+KClO,+H,O.
¹ Calvert and Davies, Journ. Chem. Soc. 2, 193.
22
322
THE NON-METALLIC ELEMENTS
When potassium chlorate is treated with hydrochloric acid a
yellow gas is evolved, first prepared by Davy, considered by him
to be a distinct oxide of chlorine, and termed Euchlorine. It
has, however, been shown by Pebal¹ that this body is a mixture
of free chlorine and chlorine peroxide in varying proportions.
This mixture possesses even more powerful oxidizing properties
than chlorine itself, and is therefore largely used as a disinfect-
ant, being not only very effective in removing by oxidation.
putrescent matter in the air, but being at the same time very
easily prepared.
CHLORIC ACID. HCIO.
163 Chloric acid is the most important member of the series
of chlorine oxy-acids. It was discovered by Berthollet in 1786,
and it is obtained when the lower acids or aqueous solutions of
the oxides of chlorine are exposed to light.
Preparation.-Chloric acid is best prepared by decomposing
barium chlorate with an equivalent quantity of pure dilute
sulphuric acid (Gay-Lussac, 1814); thus:-
Ba(ClO3)2+H2SO4 = BaSO¸+2HClO3.
The clear solution of chloric acid must be poured off from the
deposited precipitate of barium sulphate, and carefully evaporated
in vacuo over strong sulphuric acid. The residue thus prepared
contains forty per cent. of pure chloric acid corresponding to the
formula HCIO, +7H,O. When attempts are made to concen-
trate the acid beyond this point, the chloric acid undergoes
spontaneous decomposition with rapid evolution of chlorine and
oxygen gases, and formation of perchloric acid. Chloric acid
can also be prepared by decomposing potassium chlorate with
hydrofluosilicic acid, H,SiF, when insoluble potassium fluosi-
licate, K,SiF, is precipitated, and the chloric acid remains in
solution together with an excess of hydrofluosilicic acid. This
can be removed by the addition of a little silica and by subse-
quent evaporation when the fluorine passes away as gaseous
tetrafluoride of silicon, SiF4, and the pure chloric acid can be
poured off from the silica, which settles as a powder to the
bottom of the vessel.
164 Properties.—The acid obtained in this way in the greatest
state of concentration does not rapidly undergo change at the
1 Annalen, 177, 1.
CHLORIC ACID
323
ordinary temperature, but it forms perchloric acid on standing for
some time exposed to light. Organic bodies such as wood or paper
decompose the acid at once, and are usually so rapidly oxidized
as to take fire. Aqueous chloric acid is colourless, possesses
a powerful acid reaction and a pungent smell, and bleaches
vegetable colours quickly. It is a monobasic acid, that is, it
contains only one atom of hydrogen capable of replacement by
a metal, with the formation of salts.
The Chlorates. Of these salts potassium chlorate (or chlorate
of potash), KClO3, is the most important. It is easily formed by
passing chlorine in excess into a hot solution of caustic potash;
thus:-
3Cl₂+6KOH = 5KCl + KClO3+3H₂O.
The chlorate is much less soluble in water than the chloride
formed at the same time, so that by concentrating the solution
the chlorate is deposited in tabular crystals, which may be
purified from adhering chloride by a second crystallization.
Other chlorates can be prepared in a similar way; thus, for
instance, calcium chlorate is obtained by passing a current of
chlorine into hot milk of lime when the following reaction
occurs:-
6Cl₂+6 Ca(OH)2=Ca(ClO3)2+5CaCl₂+6H₂O.
All the chlorates are soluble in water, and many deliquesce
on exposure to the air. The potassium salt is one of the least
soluble of these salts, 100 parts by weight of water at 0° dis-
solving about 33 parts of this salt, whilst water at 15° dissolves
twice this amount. By the action of reducing agents such as
nascent hydrogen or sulphur dioxide, chlorates lose the whole of
their oxygen
and are converted into chlorides. A chlorate is
recognized by the following tests :-
(1) Its solution yields no precipitate with silver nitrate, but
on ignition the salt gives off oxygen gas, and a solution of
the residual salt (a chloride) gives a white precipitate on
addition of silver nitrate and nitric acid.
(2) To the solution of the chlorate a few drops of indigo
solution are added, the liquid acidulated with sulphuric acid,
and sulphurous acid (or sodium sulphite dissolved in water)
added drop by drop. If a chlorate be present the blue colour is
discharged, because the chloric acid is reduced to a lower
oxide.
324
THE NON-METALLIC ELEMENTS
(3) Dry chlorates treated with strong sulphuric acid yield a
yellow explosive gas (ClO2).
The composition of the chlorates has been very carefully
determined by Stas¹ and Marignac. The following numbers
give the percentage composition of silver chlorate according to
the analyses of Stas:-
-――――――――
Chlorine.
Oxygen
Silver.
18.5257
25.0795
56.3948
100.000
PERCHLORIC ACID. HCIO.
165 This acid was discovered by Stadion in 1816; it is formed
by the decomposition of chloric acid on exposure to heat or
light; thus:-
3HC103=HC10+Cl½ +20₂+H₂O.
4
It is best prepared from potassium perchlorate, which can be
obtained in any quantity from the chlorate. We have already
remarked under oxygen, that when potassium chlorate is
heated the fused mass slowly gives off oxygen, and a point is
reached at which the whole mass becomes nearly solid, owing
to the formation of perchlorate, together with potassium
chloride.
10KCIO, = 6KC10¸+4KCl+302.
The mass is then allowed to cool, powdered, and well washed
with water, to remove the greater part of the chloride formed.
In order to get rid of the unaltered chlorate, the crystalline
powder is gently heated with hydrochloric acid so long as
chlorine and chlorine peroxide gases are evolved: a subsequent
washing with water removes the remainder of the chloride, and
the pure, sparingly-soluble perchlorate is left.
Preparation. In order to prepare perchloric acid, the pure dry
potassium salt is distilled in a small retort with four times its
weight of concentrated (previously boiled) sulphuric acid. At a
temperature of 110° dense white fumes begin to be evolved, whilst
a colourless or slightly yellow liquid, consisting of pure perchloric
¹ Nouvelles Recherches Chimiques sur les Lois des Proportions, 208.
2 Bibl. Univ. 45, 347.
ANALYSIS OF PERCHLORIC ACID
325
acid,HCIO,,distils over (Roscoe).¹ If the distillation be continued,
this liquid gradually changes into a white crystalline mass,
having the composition HClO4+H2O. The formation of this
latter body can be readily explained; a portion of the pure per-
chloric acid splits up during the distillation into the lower oxides
of chlorine, oxygen and water, which latter combines with the
pure acid already formed. When the crystalline hydrate is again
heated it decomposes into the pure acid, which distils over, and
into an aqueous acid which boils at 203°, and therefore remains
behind in the retort. This reaction is employed in the prepara-
tion of the pure acid, HClO4, as that obtained by the first pre-
paration is generally rendered impure by sulphuric acid carried·
over mechanically.
An aqueous solution of the acid may be readily prepared
by the addition of hydrofluosilicic acid to a solution of the
potassium salt, and is sometimes used for the estimation of
potassium.2
Properties.-Pure perchloric acid is a crystalline substance
which melts at 15°,3 but is usually obtained as a volatile colour-
less or slightly yellow mobile liquid, having at 15°5 a specific
gravity of 1.782. It is strongly hygroscopic, quickly absorbing
moisture from the air, and emitting dense white fumes of
the hydrated acid. When poured or dropped into water it
dissolves, combining with the water so vigorously as to cause
a loud hissing sound and a considerable evolution of heat. A
few drops thrown upon paper and wood cause an instantaneous
and almost explosive inflammation of these bodies; and if the
same quantity be allowed to fall upon dry charcoal, the drops
decompose with an explosive violence which is almost equal
to that observed in the case of chloride of nitrogen. If the
pure acid, even in very small quantity, come in contact with
the skin it produces a serious wound, which does not heal for
months. Perchloric acid undergoes decomposition on distilla-
tion; the originally nearly colourless acid becomes gradually
darker, until it attains the tint of bromine, and at last suddenly
decomposes with a loud explosion. The composition of the
substance which is here formed is unknown. The pure acid also
undergoes spontaneous and explosive decomposition when pre-
served for some days even in the dark.
The methods employed in fixing the composition of this acid
2 Zeit. angew. Chem. 1893, 68.
1 Journ. Chem. Soc. 1863, 82.
3 Berthelot, Compt. Rend. 93, 240.
326
THE NON-METALLIC ELEMENTS
may here be referred to as illustrating the mode by which the
quantitative analysis of similar bodies is carried out.
A quantity of the pure acid (HClO) is sealed up in a
small glass bulb (Fig. 96), whose weight has been previously
ascertained, and the bulb and acid carefully weighed. The
sealed points of the tube are then broken, the acid diluted
with water, and the aqueous solution saturated with a slight
excess of solution of potassium carbonate. Acetic acid is
next added in slight excess, and the whole evaporated to
dryness on a water bath. The potassium acetate being
soluble, and the potassium perchlorate being insoluble in
· absolute alcohol, the whole of the latter salt formed by the
neutralisation of the acid is obtained in the pure state by
washing the dry mass with absolute alchohol and drying the
residue at 100°. Thus it was found that 07840 gram of
the acid thus treated yielded 10800 gram of potassium salt
corresponding to 07837 gram of pure perchloric acid, or the
FIG. 96.
acid under analysis contained 99·85 per cent. of HClO4, provided,
of course, that the salt obtained really had the composition
indicated by the formula, KClO. In order to obtain evidence
on this point, the quantities of oxygen, chlorine, and potassium
contained in the salt were determined as follows:-
(1) 0-9915 gram of the dry salt was mixed with pure dry
oxide of iron, and the mixture heated in a long tube of hard
glass, the weight of which, when thus filled, was determined.
The presence of the oxide of iron enabled the perchlorate to
yield up its oxygen at a lower temperature than it would
have done if heated alone. The loss of weight which the tube
experiences on heating represents the total weight of oxygen
contained in the salt; in this case it amounted to 04570 gram.
(2) The residue is next completely exhausted with warm
water, and the chlorine precipitated in the solution as silver
chloride; in the above analysis 0.7683 gram of pure silver
was needed for complete precipitation, and this corresponds to
40 252 gram of chlorine.
(3) 0:3165 gram of the salt was next carefully heated with
an excess of pure sulphuric acid, and the residue strongly
PERCHLORATES.
327
ignited. In this way the potassium perchlorate is converted
into sulphate, which was found to weigh 0·2010 gram.
From these numbers the percentage composition of the salt
can be easily obtained, and the results, as shown below, are
found to correspond, within the unavoidable errors of experiment,
with the numbers calculated from the formula :-
-
Analysis of Potassium Perchlorate.
Calculated.
Chlorine
Oxygen
Potassium.
.
Cl 35.19
04
63.52
K 38.84
137.55
Found.
25.58
25.46
46.18
46.09
28.24
28.58
100.00 100.13
166 Hydrates of Perchloric Acid.—The monohydrate, HClO+
H₂O, whose mode of formation has been mentioned, is obtained
in the pure state by the careful addition of water to the pure
acid, HCIO,, until the crystals make their appearance. This
substance, discovered by Serullas, was formerly supposed to be the
pure acid; it melts at 50° and solidifies at this temperature again
in colourless needle-shaped crystals, often several inches in length.
The liquid emits dense white fumes on exposure to the air, and
oxidizes paper, wood, and other organic bodies with rapidity.
As has been stated, the monohydrate decomposes at a higher
temperature into the pure acid, and a thick oily liquid, which
possesses a striking resemblance to sulphuric acid, boils at 203°,
and has a specific gravity of 182. This liquid contains 71-6
per cent. of HCIO,, and does not correspond to any definite
hydrate. An acid of the same composition, and possessing the
same constant boiling point, is obtained when a weaker acid is
distilled, the residue then becomes more and more concentrated,
until the above composition and boiling point is reached.
Aqueous perchloric acid, therefore, exhibits the same relations
in this respect as the other aqueous acids.
Perchlorates.-Perchloric acid is a powerful monobasic
acid, forming a series of salts, termed the perchlorates, which
are all soluble in water, and a few of which are deliquescent.
Potassium perchlorate, KCIO,, and rubidium perchlorate,
RbCIO,, are the least soluble of the salts, one part of the former
dissolving in 58, and the latter requiring 92 parts of water at
21° for solution. Both these salts are almost insoluble in
1
1
328
THE NON-METALLIC ELEMENTS
absolute alcohol, and they may be, therefore, employed for the
quantitative estimation of the metals.
The perchlorates are distinguished from the chlorates by the
following reactions:-
(1) They undergo decomposition at a higher temperature than
the chlorates.
(2) They are not acted upon by hydrochloric acid.
(3) They do not yield an explosive gas, CIO,, when heated
with strong sulphuric acid.
(4) They are not reduced to chlorides by sulphur dioxide.
167 Constitution of the Oxy-acids of Chlorine.-The con-
stitution of these acids has frequently been the subject of discus-
sion, and cannot as yet be regarded as definitely settled. On
the assumption that chlorine always acts as a monovalent, and
oxygen as a divalent element, the following formulæ are the
only possible ones for these acids:-
Hypochlorous acid, H-O-Cl.
Chlorous acid, H-0-0-CI.
Chloric acid, H-0-0-0-CI.
Perchloric acid, H-0-0-0-0-CI.
It has however been found, especially in the case of the carbon
compounds, that substances containing oxygen atoms united
together in this manner become more unstable as the number
of oxygen atoms increases, whilst with the oxy-acids of chlorine
the contrary is the case, hypochlorous acid being the most and
perchloric acid the least unstable.
Another theory is that chlorine behaves as a monad in hypo-
chlorous acid, a triad in chlorous acid, a pentad in chloric acid,
and a heptad in perchloric acid, the constitutional formulæ
being as follows:-
Cl-O-H
O-Cl-O-H
Cl-O-H
O
||
O-C1-0-H
OXY-ACIDS OF BROMINE.
329
This formula for perchloric acid readily explains the existence
of the hydrate of perchloric acid, HCIO,, H₂O, the constitution.
of which would then be represented by the formula-
HO
HO
O
11
Cl-O-H
A substance possessing this formula should from analogy be-
have as a polybasic acid (compare the phosphoric acids), but
hitherto no chemical or physical evidence of the polybasic
nature of perchloric acid has been obtained, all the salts cor-
responding to the formula, HClO4, and the same holds true
for chloric acid. On the whole, however, the balance of the
evidence is in favour of the second view, which is also in agree-
ment with the results obtained for periodic acid (p. 334).
Chlorine peroxide has been definitely proved to have the
molecular formula ClO2, and in this compound chlorine must be
regarded as a tetrad; this fact is also in favour of the view
that the valency of chlorine varies in its different compounds
with oxygen.
OXYGEN AND BROMINE.
OXY-ACIDS OF BROMINE.
168 No compound of bromine and oxygen has as yet been
obtained, but oxy-acids corresponding to those of chlorine are
known; viz:-
Hypobromous acid, HBrO.
Bromic acid, HBrO3.
HYPOBROMOUS ACID, HBrO.
This acid together with its salts, termed the hypobromites, are
formed, in a similar manner to hypochlorous acid, by the action
of bromine on certain metallic oxides (Balard). Thus if bromine
water be shaken up with mercuric oxide, and if the yellow liquid
1
1
1
I
I
I
1
•
330
THE NON-METALLIC ELEMENTS
thus formed be treated successively with bromine and the oxide,
a solution is obtained which contains in every 100 cc. 6-2 per
cent. of bromine combined as hypobromous acid, the reaction
being as follows:-
HgO+2Br₂+H₂O = 2HOBr+HgBr.
The greater part of the hypobromous acid contained in this
strong solution is decomposed on distillation into bromine and
oxygen. It can, however, be distilled in vacuo at a temperature
of 40° without undergoing this change.¹
Aqueous hypobromous acid is a light straw-yellow coloured
liquid, closely resembling in its properties hypochlorous acid,
acting as a powerful oxidizing agent and bleaching organic
colouring matters.
By the action of bromine on lime, a substance similar to
bleaching powder is formed and this salt is termed bromide
of lime.2
BROMIC ACID, HBrO3.
169 When bromine is dissolved in hot caustic potash or
soda, a colourless solution is produced which contains a mixture
of a bromide and a bromate; thus:-
3Br₂+6KHO=5KBr+KBr0¸+3H₂O.
The difficultly soluble potassium bromate may be easily separated
by crystallization from the very soluble bromide. Potassium
bromate is also formed when bromine vapour is passed into
a solution of potassium carbonate which has been saturated with
chlorine gas.
Preparation-Free bromic acid is formed when chlorine is
passed into bromine water; thus :—
Br₂+5Cl₂+6H₂O = 2HBrO₂+10HC1.
The acid is, however, best obtained by the decomposition of
the slightly soluble silver bromate. This salt is thrown down
on the addition of nitrate of silver to a solution of a soluble
bromate; the precipitate thus prepared is well washed with
¹ Dancer, Journ. Chem. Soc. 1862, 477.
2 Berzelius, Jahresb. 10, 130.
IODINE PENTOXIDE AND IODIC ACID
331
water and then treated with bromine; bromic acid remains in
solution and the insoluble silver bromide is thrown down; thus:—
5AgBrO3+3Br₂+3H₂O=5AgBr+6HBrO3.
Properties Obtained according to the foregoing methods,
bromic acid is a strongly acid liquid reddening and ultimately
bleaching litmus paper. On concentration at 100° the aqueous
acid decomposes into bromine and oxygen, and it is at once
decomposed by reducing agents such as sulphur dioxide and
sulphuretted hydrogen, as also by hydrobromic acid, the following
reactions taking place:-
2
(1) 2HBrO3+5SO₂+4H2O = Br₂+5H₂SO.
(2) 2HBrO3+5SH₂ = Br₂+6H₂O+5S.
(3) HBrO₂+5HBr=3Br₂+3H₂O.
Hydrochloric and hydriodic acids decompose bromic acid in
a similar manner with formation of the chloride or iodide of
bromine.
The bromates are as a rule difficultly soluble in water, and
decompose on heating into oxygen and a bromide, but unlike
the chlorates no perbromate is formed in the process.
PERBROMIC ACID, HBrO.
This substance is stated by Kämmerer¹ to be formed by the
action of bromine on dilute perchloric acid, the bromine
liberating chlorine. Other observers have, however, failed to
obtain the substance by this means, and the existence of the
acid and of its salts is, therefore, more than doubtful.2
OXYGEN AND IODINE.
OXIDE AND OXY-ACIDS OF IODINE.
170 Only one oxide of iodine is known with certainty. This
is the pentoxide I,O,, which unites with water to form iodic
acid, HIO. Besides these, hydrated periodic acid, HIO+2H₂O
is known and hypoiodous acid HIO appears to exist in solution.
1 J. Pr. Chem. 90, 190.
Muir, Journ. Chem. Soc. 1876, ii. 469; Wolfram, Annalen, 198, 95;
Mac Ivor, Chem. News, 33, 35; 55, 203.
I
I
1
1
332
THE NON-METALLIC ELEMENTS
HYPOIODOUS ACID, HIO.
When an alcoholic solution of iodine is treated with freshly
precipitated mercuric oxide, a yellow solution is formed which
does not turn starch blue at once, but only after a time; the
solution is then found to contains mercuric iodide and iodate.
It is probable that the solution contains hypoiodous acid, which
soon undergoes decomposition into free iodine and iodic acid.¹
An aqueous solution of iodine also yields with alkalis a solution
having a peculiar odour, which possesses bleaching properties,
and probably also contains a hypoiodite.2 Lunge and Schoch,
by the action of iodine on slaked lime and water at the ordinary
temperature, obtained a substance having a peculiar odour, and
resembling chloride of lime in its general properties. It has
probably the formula CaOI, or Ca(IO)2 + CaI¿.³
IODINE PENTOXIDE, IO, AND IODIC ACID, HIO.
171 This acid was discovered by Davy in the form of potas-
sium iodate, which he obtained by the action of iodine on
caustic potash; thus:—
312 + 6KOH = 5KI + KIO¸ + 3H₂O.
Preparation.-(1) Free iodic acid is best obtained by dissolv-
ing iodine in pure boiling concentrated nitric acid, which oxidizes
it as follows:-
31₂+10HNO3=6HIO¸+10NO+2H₂O.
For this purpose 1 part of iodine is heated in a retort with 10
parts of the acid until the whole of the iodine is dissolved, and
no further evolution of red fumes takes place. The solution is
then evaporated and the residue heated to 200° until every trace
of nitric acid is removed. The iodic acid thus loses water, and
a white powder of iodine pentoxide IO, is obtained. It has a
specific gravity of 4487, and when heated to 300° decomposes
into iodine and oxygen. This substance is very soluble in
water, dissolving with evolution of heat and from the thick
1 Köne, Pogg. Ann. 66, 302; Compt. Rend. 63, 968.
2 J. Pr. Chem. [1], 84, 385.
3 Ber. 15, 1883.
CONSTITUTION OF IODIC ACID
333
syrupy solution thus obtained rhombic crystals of iodic acid,
HIO,, are deposited.
(2) Iodic acid can also be obtained by the action of dilute sul-
phuric acid on barium iodate, which is prepared as follows: the
requisite quantity of iodine is dissolved in a hot concentrated
solution of potassium chlorate and a few drops of nitric acid
added; immediately a violent evolution of chlorine gas com-
mences, and, on cooling, the potassium iodate crystallizes out.
This salt is then dissolved in water and barium chloride added
to the solution, when barium iodate separates out as a white
powder. The potassium iodate may also be obtained by very
carefully heating a mixture of 2 mols. potassium chlorate
and 1 mol. iodine, a simple metathesis taking place.¹
2KClO3+I2=2KIO3+Cl₂.
(3) Iodic acid is likewise formed when chlorine is passed into
water in which iodine in powder is suspended; thus :—
I2+5Cl₂+6H₂O=2HIO₂+10HCI.
In order to separate the hydrochloric acid which is formed
at the same time, precipitated oxide of silver is added until
the acid is completely precipitated as the insoluble silver
chloride.
Properties.-Crystallized iodic acid has a specific gravity at 0°
of 4629; it is insoluble in alcohol, but easily soluble in water.
The concentrated aqueous solution boils at 104°, and first reddens,
and then bleaches litmus paper. Phosphorus, sulphur, and organic
bodies deflagrate when heated with iodic acid or with the
pentoxide. Sulphur dioxide or sulphuretted hydrogen as well
as hydriodic acid reduces iodic acid with separation of iodine;
thus:-
(1) 2HIO,+5SO₂+4H₂O=I₂+5H₂SO.
(2) 2HIO,+3H,S=I,+5S+6H₂O.
(3)
HIO₂+5HI=31₂+3H₂O.
3
The Iodates.-Iodic acid is a monobasic acid, and is distin-
guished from chloric acid and bromic acid by the fact that it
forms not only the normal salts, but salts which are termed
1 Thorpe and Perry, Journ. Chem. Soc. 1892, i. 925.
2 Ditte, Ann. Chem. Phys. [4], 21, 5.
334
THE NON-METALLIC ELEMENTS
acid- or hydrated-salts. Thus the following potassium salts are
known:-
Normal potassium iodate KIO.
Acid potassium iodate KIO, HIO,.
Di-acid potassium iodate KIO, 2HIO。.
The normal iodates are chiefly insoluble or difficultly soluble in
water; the more soluble are those of the alkali metals. On
heating they decompose either into oxygen and an iodide, or
oxygen and iodine are given off, leaving a residue of the metal
or its oxide; thus:-
2Ba(103)2 = 2BaO + 21+502.
2AgIOg = 2Ag + I2.
In order to detect iodic acid, the solution after acidifying with
hydrochloric acid, is mixed with a small quantity of starch paste
and then an alkaline sulphite or a solution of sulphurous acid
added drop by drop, thus liberating iodine which forms with the
starch the blue iodide.
Sodium iodate occurs in nature associated with sodium nitrate
in Chili saltpetre, and iodic acid is not unfrequently met with in
nitric acid prepared from this source.
The constitution of iodic acid is not known with certainty.
Unlike chloric acid it behaves as a polybasic acid, and the formula
H-0-0-0-I, is therefore even more improbable than in the
latter case. If, however, we assume that the iodine in this acid
is pentavalent and ascribe to it the constitutional formula
I-O-H, we are still unable to account for the existence of
the acid and diacid salts mentioned above, except by regarding
them as molecular compounds of the anhydrous salt and the
acid. Thomsen ¹ regards the molecular formula of the acid as
H₂I₂О, and ascribes to it a constitutional formula in which the
oxygen atoms are regarded as tetravalent, whilst another formula
has been suggested by Blomstrand; 2 the evidence is however
as yet insufficient to decide between the different suggestions.
2 J. Pr. Chem. [2], 40, 305.
6'
1 Ber. 7,
112.
PERIODIC ACID
335
PERIODIC ACID, HIO.
172 This substance was discovered by Magnus, and subse-
quently investigated by other chemists, especially by Ammer-
müller and Rammelsberg. Normal periodic acid, HIO,, is not
known. The hydrate H.IO, or HIO,+2H,O is formed either
by the action of iodine on aqueous perchloric acid, thus:-
6
HC10,+I+2H20=н¿I0¸+Cl,
or by the decomposition of silver periodate with bromine.
This hydrate is a colourless transparent crystalline deliquescent
solid which melts at 133° and at 140° is completely decomposed
into iodine pentoxide, water, and oxygen. The aqueous solution
has a strong acid reaction and it acts upon reducing agents in
a similar way to iodic acid.
Periodic acid forms a remarkable series of salts, the com-
position of which at first sight seems somewhat complex. Thus
we are acquainted with salts having the following general
formulae, M representing a monad metal:
4
MIO; MIO,; M.IO; MI,O,1; M.IO; M12I2013
11
6
7
7
6
If, however,
we regard iodine as a heptad in these compounds,
the hypothetical periodic anhydride, I,O,, would have the
following constitutional formula:
0 0
0=I-0-I=0
O
the above salts may then be looked upon as derived from acids
formed from this anhydride by union with varying numbers
of molecules of water, in the manner shown below:
I₂O;+H₂O=210¸.OH= 2HIO¸
I₂O₂+2H20=10,(OH),—O—IO,(OH),=H¸I₂O,
I₂O,+3H20=210,(OH),= 2HIO. 2H,O
I₂O₂+4H₂O=10(OH),—O—IO(OH), = H¸I₂011
I₂O₂+5H₂O=21O(OH)=2HIO, 4H₂O
1₂O₂+6H,O=I(OH)-O-I(OH)=H₁₂I₂013
L₂O₂+7H,0=21(OH),=2HIO,, 6H₂O.
¹ Pogg. Ann. 28, 514.
12 2
336
THE NON-METALLIC ELEMENTS
No salts are known corresponding to the acid I(OH),, but it will
be seen that the composition of the remaining six acids corre-
sponds exactly with one or other of the series of salts which has
already been prepared. (Compare the constitution of the
phosphoric acids.)
99
5'
The best known series of periodates are those derived from
the acids HIO, н¸¸О, н¸IO̟, and H.IO, which are termed
metaperiodates, diperiodates, mesoperiodates, and paraperiodates
respectively.
The periodates can be obtained in several ways; thus if
chlorine be allowed to act on a mixture of sodium iodate and
caustic soda, a mixture of two sodium paraperiodates (Na,H,
IO, and NaH₂10) and sodium chloride is formed. Another
method is to heat barium iodate, which is thus converted into
barium periodate, iodine and oxygen.
5Ba(IO3)2=Ba₂(106)2+412+802.
The barium paraperiodate may be heated to redness without
decomposition, whereas the other periodates are decomposed at
this temperature with evolution of oxygen.
The periodates are, as a rule, but slightly soluble in water;
their solutions give with silver nitrate and nitric acid a pre-
cipitate of silver periodate, a different silver salt being obtained
according to the proportion of nitric acid present.¹
SULPHUR. S
31.82.
173 SULPHUR has been known from the earliest times as it
occurs in the free or native state, in the neighbourhood of extinct
as well as of active volcanoes. It was formerly termed Brim-
stone or Brennestone, and was considered by the alchemists to
be the principle of combustibility, and believed by them to re-
present the alterability of metals by fire. The compounds of
this element occur in nature in much larger quantities, and are
much more widely distributed than free sulphur itself. The com-
pounds of sulphur with the metals, termed sulphides, and those
with the metal and oxygen termed sulphates are found in large
quantities in the mineral kingdom. The more important com-
pounds of sulphur occurring in nature are the following:-
1 Kimmins Journ. Chem. Soc. 1887, i. 356; 1889, i. 148.
=
OCCURRENCE OF SULPHUR
337
(1) Sulphides. Iron pyrites FeS2; copper pyrites CuFeS2;
galena PbS; cinnabar HgS; blende ZnS; grey antimony Sb,S;
realgar As,S,; orpiment As,S.
(2) Sulphates. Gypsum CaSO4 + 2H2O; gypsum anhydrite
CaSO4; heavy spar BaSO; kieserite MgSO4 + H2O; bitter spar
MgSO4 + 7H₂O; Glauber salt Na,SO, + 10H,O; green vitriol
FeSO, +7H₂O.
Volcanic gases almost always contain sulphur dioxide and
sulphuretted hydrogen, and when these two moist gases come
into contact they mutually decompose with the deposition of
sulphur, thus:-
SO₂+ 2H,S= 3S + 2H₂O,
2

FIG. 97.
B
It is very probable that native sulphur is, in some cases, formed
by the above reaction. The apparatus shown in Fig. 97 serves
to exhibit this change; the sulphuretted hydrogen gas evolved
in the bottle (C) is passed into the large flask (A) into which
is led at the same time sulphur dioxide from the small flask
(B). The walls of the large flask are soon seen to become
coated with a yellow deposit of sulphur.
Sulphur compounds are also found widely distributed in
the vegetable and animal world, in certain organic compounds
such as the volatile oils of mustard and of garlic, and in the
acids occurring in the bile. Sulphur is also found in small
quantities in hair, and wool, whilst it is contained to the
amount of about 1 per cent. in all the albuminous sub-
23
338
THE NON-METALLIC ELEMENTS
stances which form so important a constituent of the animal.
body.
174 Almost all the native sulphur of commerce comes from
Italy, where it is found in the Romagna and in other parts of
the country, but especially in very large quantities in the
volcanic districts of the island of Sicily, where it occurs in
widespread masses found chiefly on the south of the Madonia
range stretching over the whole of the provinces of Caltanissetta
and Girgenti, and over a portion of Catania. No fewer than
476 distinct sulphur workings exist in Sicily, from which the
annual production in the year 1886 amounted to 410,000 tons;
of this quantity only 30,226 tons were imported into the
United Kingdom, and since the above year the production
of the Sicilian mines has not been reduced, although the
number of the mines worked has decreased. The deposits of
Sicilian sulphur occur in the tertiary formation lying imbedded
in a matrix of marl, limestone, gypsum, and celestine. The
sulphur occurs partly in transparent yellow crystals termed
virgin sulphur and partly in opaque crystalline masses, to which
the name of volcanic sulphur is given. Both these varieties are
separated from the matrix by a simple process of fusion. The
method often described, in which the sulphur ore is repre-
sented as being placed in earthenware pots in a furnace, the
sulphur distilling out into other pots placed outside the
furnace, appears to be unknown in Sicily. In the Romagna
an apparatus made of cast-iron and provided with a receiver
of the same material is employed, but in Sicily a very simple
method of melting out the sulphur has long been, and still
continues to be, in vogue. This old process consists in placing
a heap of the ore in a round hole dug in the ground averag-
ing from 2 to 3 metres in diameter and about one-half metre
in depth. Fire is applied to the heap in the evening, and
in the morning a quantity of liquid sulphur is found to have
collected in the bottom of the hole; this is then ladled out,
the combustion being allowed to proceed further until the whole
mass is burnt out. By this process only about one-third of the
sulphur contained in the ore is obtained, whilst the remaining
two-thirds burns away evolving clouds of sulphurous acid.
This rough and wasteful process has been greatly improved
by increasing the quantity of ore burnt at a given time,
the excavation being made 10 metres in diameter, with
1 J. Soc. Chem. Ind. 1889, 313; 1899, 118.
EXTRACTION OF SULPHUR
339
a depth of 2 metres, and so arranged (on the side of a hill,
for instance) that an opening can be made from the lowest
portion of the hole so that the sulphur, as it melts, may flow
out. These holes are built up with masses of gypsum and the
inside covered with a coating of plaster of Paris (see Fig. 98).
The calcaroni, as these kilns are termed, are then filled with the
sulphur ore which is built up on the top into the form of a cone,
and air channels (b b b) are left in the mass by placing large
lumps of the ore together. The whole heap is then coated over
with powdered ore (c c), and this again covered with a layer of
burnt-out ore, after which the sulphur is lighted at the bottom.
By permitting the heat to penetrate very slowly into the mass, the

FIG. 98.
GUNN
sulphur is gradually melted, and running away by the opening (a)
at the bottom of the heap, is cast into moulds. By this process,
which takes several weeks to complete, the richest ores, con-
taining from 30 to 40 per cent., may be made to yield from 20
to 25 per cent. of sulphur, whilst common ores, containing from
20 to 25 per cent., yield from 10 to 15 per cent. of sulphur, the
remaining portion of the sulphur being used up for combustion.
Other methods of extraction by means of solvents, such
as bisulphide of carbon, or by the use of ordinary fuel instead
of sulphur itself, as a source of heat, have been proposed, but
from the nature of the country and its inhabitants these have
not yet proved successful in Sicily, and the raw sulphur still
remains the cheapest fuel for the purpose. Large deposits of
8
340
THE NON-METALLIC ELEMENTS
sulphur also exist in Iceland, Japan, and Mexico, some of which
are worked commercially.
175 Refining of Sulphur.-Commercial Sicilian sulphur con-
tains about 3 per cent. of earthy impurities which can be removed
by distillation, the arrangement being shown in Fig. 99. The
sulphur is melted in an iron pot (M) and runs from this by
means of a tube into the iron retort (G) where it is heated
to the boiling point; the vapour of the sulphur then passes into
the large chamber (A) which has a capacity of 200 cubic metres.

FIG. 99.
T
T
FIG. 100.
In this chamber the sulphur is condensed, to begin with, in the
form of a light yellow powder termed flowers of sulphur, just as
aqueous vapour falls as snow when the temperature suddenly
sinks below 0°. After a time the chamber becomes heated
above the melting point of sulphur, and then it collects as a
liquid which can be drawn off by means of the opening (0).
It is then cast in slightly conical wooden moulds, seen in Fig. 100,
and is known as roll sulphur, or brimstone. It is frequently
also allowed to cool in the chamber and then obtained in large
crystalline masses, known in the trade as block sulphur.
EXTRACTION OF SULPHUR
341
In France, Germany, and Sweden, sulphur is also obtained by
the distillation of iron pyrites, FeS2. This method, which was
described by Agricola in his work De Re Metallica, depends on
the following decomposition of the pyrites :-
――――――
3FeS=Fe,S,+S₂;
and the change which occurs is exactly similar to that by
means of which oxygen is obtained from manganese dioxide;
thus:-
3MnO₂ = MnO4+02.
This decomposition of the pyrites is sometimes carried on in
retorts, but more generally a kiln similar to a lime-kiln is
employed for the purpose, having a hole at the side into which.
a wooden trough is fastened. A small quantity of fuel is lighted
on the bars of the furnace, and then the kiln is gradually filled
with pyrites; a portion of the sulphur burns away whilst
another portion is volatilized; the burnt pyrites is from time
to time removed from below, and fresh material thrown on the
top so that the operation is carried on uninterruptedly. In this
way about half the sulphur which is contained in the pyrites
can be obtained, whilst only about one-third of the total sulphur
can be got by distilling in iron cylinders.
Sulphur is likewise obtained in this country, though in smaller
quantities, as a by-product in the manufacture of coal-gas.
The impure gas always contains sulphuretted hydrogen, which
can be removed by passing the gas over oxide of iron, when
iron sulphide is formed. This substance on exposure to air is
oxidised with separation of free sulphur, thus:-
Fe₂S₂+30+H₂O= Fe̱¸О¸‚H₂O+3S.
The mass can then be again employed for the purification of the
gas, and this alternate oxidization and sulphurization can be
repeated until a product is obtained containing 50-60 per cent.
of sulphur. The latter may be separated from the iron oxide
by distillation or by treatment with carbon bisulphide in a
suitable apparatus. In most cases, however, the spent oxide is
employed for the manufacture of sulphuric acid, and is then
burnt in kilns similar to those employed for burning pyrites.
Another and much more important source from which sulphur
is now obtained is the residue or waste in the soda manufac-
342
. THE NON-METALLIC ELEMENTS
ture; this consists of calcium sulphide mixed with chalk, lime
and alkaline sulphides. The sulphur which this material con-
tains was formerly altogether wasted; now, however, it is
economically regained in the alkali works on a very large scale.
For this purpose the waste, which consists mainly of calcium
sulphide CaS, is treated in the presence of water with car-
bonic acid gas, calcium carbonate being thus produced whilst
sulphuretted hydrogen is evolved:
CaS+H₂O+CO₂ = CaCO3 + H₂S.
The gas obtained is then burnt with an insufficient supply of
air in a specially devised kiln and the sulphur thus recovered
in the free and pure state.¹
176 Properties.-Sulphur exists in several allotropic modifica-
tions. Thus it can be obtained either crystalline or amorphous,
and at least two varieties of the amorphous form may be distin-
FIG. 101.
b
:
guished, one of which is soluble and the other insoluble in carbon
bisulphide. A form is also known which is soluble in water.
Rhombic or a-Sulphur occurs in nature in large yellow transparent
octahedra. Fig. 101 shows the form of the natural crystals of
sulphur (a being the simpler, and b the more complicated form)
which belong to the rhombic system, and have the following rela-
tion of the axes:-a: b c = 0·8106:1:1·898. In addition to this
primary form, no less than thirty different crystallographic modi-
fications are known to exist in the case of sulphur. Crystals of
rhombic sulphur have also been found in the sulphur chambers,
having been deposited by slow sublimation. The specific gravity
of this form of sulphur at 0°, is 2·05 - 207; it is insoluble in
water, very slightly soluble in alcohol, benzene, and ether, but
dissolves readily in carbon bisulphide, chloride of sulphur, petro-
leum, and turpentine. Artificial crystals of sulphur are best ob-
¹ Chance, Journ. Soc. Chem. Ind. (1888), 7, 162.
ALLOTROPIC FORMS OF SULPHUR
343
tained from solution in carbon bisulphide, which dissolves at
the ordinary temperature about one-third of its weight of
sulphur; the saturated solution on being allowed to evaporate
slowly deposits large transparent octahedral crystals. Very well
developed crystals can also be obtained by saturating pyridine
with sulphuretted hydrogen gas and allowing the solution to
stand exposed to the air for some time. The sulphuretted
hydrogen is partially oxidised by the air, and the sulphur which
is liberated crystallises out.¹ a-Sulphur melts at 114.5°
(Brodie), forming a clear yellow liquid, which has a
specific gravity of 1803, and when quickly cooled, solidifies
again at the same temperature; generally, however, it remains.
liquid at a temperature below its melting point, and then
solidifies at a slightly lower temperature, which varies with the
point to which the liquid has been heated.
177 The second or monosymmetric modification, known as B-
sulphur, is obtained when melted sulphur is allowed to cool at
FIG. 102.
the ordinary temperature until a solid crust is formed on the
surface. The crust is then broken through, and the portion of
sulphur still remaining liquid poured out; the sides of the
vessel will then be found to be covered with a mass of long,
very thin transparent crystals having the form of mono-
symmetric prisms (Fig. 102), the ratio of the axes of which are
expressed by the following numbers 2 a b c = 1·004 : 1 :
1004. B-Sulphur appears to be trimorphous, since no less than
three different types of crystals, all belonging to the mono-
symmetric system, but having different axial ratios and optical
properties, have been observed.3
Monosymmetric sulphur has a specific gravity of 196, its
melting-point is 120°, and like the rhombic modification, it is
readily soluble in carbon bisulphide.
178 According to the conditions under which the passage from
the fused to the solid state takes place, sulphur may separate
Mitscherlich, Pogg. Ann. 24, 264.
1 Ahrens, Ber. 23, 2708.
3 Muthmann, Zeit. Kryst. 17, 336.
344
THE NON-METALLIC ELEMENTS
either in the form of rhombic or of monosymmetric crystals.
The rhombic crystals are obtained by placing about 200 grams
of sulphur previously crystallized from solution in carbon bisul-
phide in a flask provided with a long neck, which is afterwards
bent backwards and forwards several times to prevent the entry
of floating dust. The sulphur is then melted by placing the
flask in an oil bath heated to 120°, and when the contents are
liquid the flask is immersed in a vessel filled with water at
95°. On standing for some time at a temperature of about
90°, crystals are seen to form, and when a sufficient quantity
have been deposited the flask is quickly inverted; the portion
of sulphur still liquid then flows into the neck and there at
once solidifies, leaving the transparent rhombic crystals in the
body of the flask.
When a large mass of molten sulphur is allowed to cool
slowly, rhombic crystals are also formed, and these cannot be
distinguished from the natural crystals. Thus Silvestri found
such rhombic crystals 5 to 6 centimetres in length in a mass
of sulphur which had been melted during a fire in a sulphur
mine. When a transparent rhombic crystal of sulphur is
heated for some time to some temperature above 976°, but
below its melting-point, it becomes opaque when touched
with a prismatic crystal, owing to its being converted into a
large number of monosymmetric crystals; whereas, on the other
hand, a transparent crystal of B-sulphur becomes opaque after
standing for twenty-four hours at the ordinary temperature,
having undergone a spontaneous change to the rhombic
modification, the crystal having been converted into a large
number of minute rhombic crystals. This conversion is
accelerated by vibration, as, for instance, when the crystals are
scratched, and also when they are exposed to sunlight; the
change is always accompanied by an evolution of heat, 2:25
cal. being liberated by the conversion of 31-82 parts of the
monosymmetric into the rhombic variety. A saturated
solution of sulphur in boiling carbon bisulphide deposits
rhombic crystals on cooling, whilst, on the other hand,
solutions in alcohol, ether and chloroform give rise to the
monosymmetric crystals. A saturated solution in boiling
benzene deposits the B-modification at temperatures between
75-80°, and the a-variety below 22°, whilst at intermediate
temperatures mixtures of the two are formed.
¹ Gernez, Compt. Rend. 98, 810 915, 100, 1343.
ALLOTROPIC FORMS OF SULPHUR
345
179 Amorphous sulphur is known in two forms, one of which
is soluble in carbon bisulphide and the other insoluble. The
soluble variety is produced by the decomposition of sulphuretted
hydrogen water in the air and by the action of acids upon the
polysulphides of the alkalis, etc.
Sulphur milk (lac sulphuris), a body known to Geber and now
used as a medicine, is sulphur in this form. It is deposited as a
fine white powder when two parts of flowers of sulphur are boiled
with thirteen parts of water and one part of lime slaked with
three parts of water, until the whole of the sulphur is dissolved.
The reddish-brown solution thus prepared contains calcium
pentasulphide, which is decomposed on the addition of hydro-
chloric acid with evolution of sulphuretted hydrogen and
deposition of milk of sulphur; thus :—
CaS+ 2HCl = CaCl₂+H₂S+4S.
The insoluble amorphous modification is known as y-sulphur,
and can be prepared in many different ways.
When melted sulphur is further heated, the pale yellow
liquid gradually changes to a dark red colour and becomes
more and more viscid, until at a temperature of from 220° to
250° it becomes almost black and so thick that it can only with
difficulty be poured out of the flask. Observed in thin films,
this change of colour from yellow to red is found to be as-
sociated with a distinct change in the absorption-spectrum,
inasmuch as the absorption in the red gradually disappears,
whilst that in the blue is gradually increased (Lockyer). If the
temperature be raised still higher, the liquid becomes less viscid,
although its dark colour remains, and on cooling down again.
the above-described appearances are repeated in inverse
order. If the viscid sulphur be rapidly cooled, or if the more
mobile liquid obtained at a higher temperature be poured in a
thin stream into cold water, the sulphur assumes the form of a
semi-solid transparent elastic mass, which can be drawn out
into long threads. This is known as plastic sulphur; its condi-
tion is an unstable one, and on standing it gradually becomes
opaque and brittle. The plastic variety of sulphur can be
obtained by the arrangement shown in Fig. 103. The sulphur
is first melted and then heated to its boiling-point in the retort.
The sulphur vapour condenses in the neck of the retort, and
the liquid sulphur runs in a thin stream into cold water.
I
1
I
1
I
346
THE NON-METALLIC ELEMENTS
If the brittle mass be treated with carbon bisulphide, a small
portion, less than one per cent., dissolves, the remainder being
left behind in the form of a dark brown powder, the colour of
which is due to small traces of fatty organic matter. In the
absence of this the colour of the residue is lemon yellow.
Together with the modification soluble in carbon bisulphide,
"flowers of sulphur" contains a light yellow insoluble modifica-
tion; and if a solution of sulphur in the above menstruum be
exposed to the sunlight, a portion of the sulphur separates out
in the insoluble form. The insoluble variety is also produced,
accompanied by the soluble form, by the decomposition of
chloride of sulphur with water, and in other similar reactions.
On preservation it gradually changes into rhombic sulphur,
FIG. 103.
and this transformation can also be produced by boiling it with
alcohol or by subjecting it to a pressure of 8,000 atmospheres.¹
The specific gravity of this variety, obtained from flowers of
sulphur, is 1.9556 at 0°.
Colloidal or 8-sulphur.-According to Debus an amorphous
variety of sulphur which is soluble in water is contained in
Wackenroder's solution (p. 420). It forms a yellow, semi-
liquid mass, and resembles colloidal silica in many of its pro-
perties. A variety which is also soluble in water is formed when
a saturated solution of sodium thiosulphate is decomposed by
two volumes of hydrochloric acid which has been saturated at
25° and allowed to cool to 10°.
1 Spring, Ber. 14, 2579. 2 Debus, Journ Chem. Soc. 1888, i. 282.
3 Engel, Compt. Rend. 112, 866.
19
28

VAPOUR DENSITY OF SULPHUR
347
Engel has also found that if the yellow solution be extracted
with chloroform before the deposition of this soluble sulphur and
the chloroform allowed to evaporate, rhombohedral crystals of
sulphur which belong to the hexagonal system are deposited.
These are denser than any other form of sulphur, having a
specific gravity of 2135, and on preservation become opaque,
the insoluble variety of sulphur being formed.
In addition to the modifications already mentioned, many
others have been described. All the numerous varieties may
however be referred to two fundamental forms, (1) the rhombic
form soluble in carbon bisulphide and (2) the insoluble form,
which is however less stable than the rhombic form, into which
it ultimately passes.
According to Berthelot,' the rhombic form, or some modifica-
tion referable to it, is always produced when the sulphur is the
electro-negative element of the molecule, as in sodium sulphide,
sulphuretted hydrogen, &c., whilst when it is the electro-
positive element, as in sulphur dioxide, sulphur chloride, &c.,
the amorphous form is produced on decomposition. In many
cases however one form is accompanied by more or less of the
other variety.
180 Sulphur boils, according to Regnault, under the normal
pressure at 448°4 or at 450° under a pressure of 779-9mm, form-
ing a red vapour. The density of the vapour was found by
Dumas at about 524° to be 6:56, corresponding to a molecular
weight of 189, whilst at 860°-1040° Deville and Troost found
it to be constant at a value of 223, corresponding to the
molecular weight of 64.2.
=
The conclusion was hence drawn that the molecules of sulphur
at temperatures near the boiling point consist of six atoms
(Mol. Wt. 1909), whilst at higher temperatures the molecules
are made up of only two atoms and have a weight of 636. The
experiments of Biltz5 have however shown that the vapour
density of sulphur varies gradually with the temperature, being
7.84 at 468°, 7·09 at 524°, and 4·73 at 606°, but finally reaches
the constant value 2:23. Since the vapour-density correspond-
ing to the formula S has not been found to be constant through
any definite range of temperature, the existence of molecules of
this formula cannot be considered as proved.
1 Compt. Rend. 44, 318, 378.
3 Ann. Chim. Phys. 50, 172.
5 Biltz, Ber. 21, 2013.
2 Relation des Erpériences, &c., tom. ii.
4 Ann. Chim. Phys. (3), 58, 257.
.
348
THE NON-METALLIC ELEMENTS
Determinations of the molecular weight of sulphur in solution
both by the freezing- and boiling-point methods seem to lead to
the formula S or Se¹
1
181 When sulphur burns in the air or in oxygen, a continuous
spectrum is observed, but if a small quantity of sulphur vapour
be brought into a hydrogen flame, a series of bright bands is
seen when the blue cone in the interior of the flame is examined
or when the sulphurized flame is brought on any cold surface.
This blue tint is almost always seen when a pure hydrogen
flame is brought for an instant against a piece of porcelain, the
blue colour being produced, according to Barrett, by the sulphur
contained in the dust in the air. The absorption spectrum of
sulphur has been obtained by Salet, and the emission spectra,
of which there are said to be two, a channelled-space and a line
spectrum, have been mapped by Plücker and Hittorf, and more
recently by Salet.5 According to Mr. Lockyer two other spectra
of sulphur occur, viz., a continuous absorption in the blue, and
a continuous absorption in the red. The change from the
channelled-space spectrum to that showing absorption in the
blue, is observed when the vapour-density changes, the first of
these spectra being seen when the vapour possesses a normal
density.
182 Detection and Determination of Sulphur.-The simplest
mode of detecting sulphur in a compound is to mix the
body with pure carbonate of soda, and fuse it before the
blowpipe on charcoal or, to avoid the introduction of sulphur
from the gas flame, to mix the substance with sodium car-
bonate and charcoal, and heat in a small closed crucible.
Sodium sulphide is thus formed, and this may then be re-
cognized by bringing the fused mass on to a silver coin and
adding water. The smallest quantity of sulphur can thus
be recognized by the formation of a brown stain of silver
sulphide. Sulphur is almost always quantitatively determined
as barium sulphate. If the body is a sulphide, as, for instance,
pyrites, it is finely powdered, and either fused with a mixture
of carbonate of soda and nitre, the fused mass dissolved in
water, and the filtrate, after acidifying by hydrochloric acid
precipitated with barium chloride, or the sulphide is oxidized
1 Paterno and Nasini, Ber. 21, 2153; Beckmann, Zeit. phys. Chem. 5, 76;
Hertz, Zeit. phys. Chem. 6, 358.
2 Phil. Mag. [4], 30, 321.
+ Phil. Trans. 1865, 1.
8 Compt. Rend. 74, 865.
6 Compt. Rend. 73 559, 561, 742, 744.
SULPHURETTED HYDROGEN
349
with a mixture of nitric and hydrochloric acids or fuming
nitric acid, the excess of acid removed by evaporation and
barium chloride added, whereby the insoluble barium sulphate
is formed, and this, after washing and drying, is ignited and
weighed. A further method is to fuse the mineral with 2 parts
of caustic soda and 4 parts of sodium peroxide, the melt being
then acidified and precipitated with barium chloride as above.¹
Atomic Weight of Sulphur.-This has been determined by
-Berzelius, Dumas, Stas, and other chemists with closely con-
cordant results. At a mean of five experiments Stas found that
100 parts of silver, when heated in sulphur vapour or in
sulphuretted hydrogen, yielded 114-854 parts of silver sulphide;
and as the mean of six others that 100 parts of silver sulphate,
Ag,SO,, when heated in a current of hydrogen, left a residue of
69-203 silver. Hence when O=15.88 and Ag=107-13, the
atomic weight of sulphur is 31.82.
SULPHUR AND HYDROGEN.
183 These elements unite to form at least two distinct com-
pounds, viz., hydrogen mono-sulphide or sulphuretted hydrogen,
HS, and hydrogen di-sulphide, H₂S2.
SULPHURETTED HYDROGEN, OR HYDROGEN MONO-SULPHIde.
H.S. 33·82.
The preparation of a solution of the polysulphides of calcium
by boiling lime with sulphur is described in the Papyrus of
Leyden, the oldest chemical manuscript known (p. 4), and
Zosimus frequently alludes to the unpleasant smell produced
from this liquid, which was known to him as the "divine water
(Greek Ostov, divine or sulphurous). Geber moreover described
the preparation of milk of sulphur, but we do not notice either
in his works or in those of the later alchemists that any further
mention is made of the fact that a fetid smell is given off in
the process. Not until we come to the writers of the sixteenth
and seventeenth centuries do we find any description given
of sulphuretted hydrogen, and then it is described under the
general name of sulphurous vapours. Scheele was the first to
1 Hempel, Zeit. Anorg. Chem. 3, 193.
350
THE NON-METALLIC ELEMENTS
investigate this compound with care. He found that it could
be formed by heating sulphur in inflammable air, and he
considered that it must be made up of sulphur, phlogiston
and heat.
Sulphuretted hydrogen is formed when hydrogen gas is passed
through boiling sulphur, when sulphur vapour is burnt in an
atmosphere of hydrogen, or when dry hydrogen gas is passed
over certain heated sulphides. Thus, if a little powdered
antimony trisulphide be placed in a bulb tube and heated
by a flame, and if a slow current of hydrogen be then passed
over the heated sulphide, the escaping gas when allowed
to bubble through a solution of lead acetate will produce a black
precipitate of lead sulphide, thus showing the formation of
sulphuretted hydrogen. The reaction is thus represented :—
Sb₂S3 + 3H2 = Sb₂ + 3H2S.
It is also produced in the putrefactive decomposition of
various organic bodies (such as albumin) which contain sulphur,
and it is to the presence of this substance that rotten eggs
owe their disagreeable odour. Sulphuretted hydrogen occurs,
as has been stated, in volcanic gases, whilst certain mineral
waters, such as those of Harrogate, contain dissolved sul-
phuretted hydrogen, which imparts to the waters their peculiar
medicinal properties as well as their offensive smell.
184 Preparation.—(1) Sulphuretted hydrogen is best prepared
by acting upon certain metallic sulphides with dilute acids; in
general, ferrous sulphide (sulphide of iron, obtained by melting
together iron filings and sulphur) is employed for this purpose.
Ferrous sulphide, FeS, dissolves readily in hydrochloric or in
dilute sulphuric acid, sulphuretted hydrogen gas being liber-
ated; thus:-
FeS + H2SO4 = H₂S + FeSO4
FeS2HCl = HS + FeCl2.
The apparatus shown in Fig. 104 may be used; the materials
are placed in the large bottle and the gas which, is evolved is
washed by passing through water contained in the smaller one.
When a regular evolution of gas for a long period is needed, the
Kipp's apparatus, Fig. 105, is employed. The two glass globes (a)
and (1) are connected by a narrow neck, whilst the tubulus of the
PREPARATION OF SULPHURETTED HYDROGEN 351
third and uppermost globe (c) passes air-tight through the neck of
(b). The sulphide of iron is placed in globe (b) and dilute sulphuric
FIG. 104.
acid poured through the tube-funnel until the lowest globe is
filled and a portion of the acid has flowed on to the sulphide
FIG. 105.
of iron. When it is desired to stop the current of gas, the stop-
cock at (e) is closed, and the acid is forced by the pressure
I

T
352
THE NON-METALLIC ELEMENTS
of the gas accumulating in the globe () up the tubulus into
the uppermost globe (c).
A still more convenient arrangement, especially for use in
laboratories, has been proposed by Ostwald¹ in which only
a small amount of acid is allowed to come in contact with
the sulphide of iron at once, and is thoroughly used up in
decomposing the sulphide, any excess of gas evolved being
stored in a specially arranged vessel.
(2) The gas thus obtained is, however, never pure, inasmuch as
the artificial ferrous sulphide always contains some particles of
metallic iron, and these coming into contact with the acid evolve
hydrogen gas. Hence in order to prepare pure sulphuretted
hydrogen, a naturally occurring pure sulphide, viz., antimony
trisulphide, is employed, and this substance, roughly powdered,
on being warmed with hydrochloric acid, evolves a regular
current of the pure gas; thus:-
Sb₂S₂+6HCl = 3H₂S + 2SbCl¸.
(3) A continuous current of sulphuretted hydrogen may like-
wise be obtained by heating a mixture of equal parts of sulphur
and paraffin (a mixture of solid hydrocarbons having the
general formula CnH2n+2). By regulating the temperature to
which the mixture is heated, the evolution of gas may be easily
controlled. The exact nature of the changes which here occur
remains as yet undetermined.2
---
(4) In order to obtain a perfectly pure gas the artificially pre-
pared sulphides of calcium or zinc may be decomposed by dilute
acids, or a solution of magnesium hydrosulphide, Mg(SH), may
be heated to 60°, at which temperature it is decomposed.
185 Properties.-Sulphuretted hydrogen obtained by any of
the above processes is a colourless, very inflammable gas, pos-
sessing a sweetish taste and a powerful and very unpleasant
smell resembling that of rotten eggs. The gas may be collected
over hot water in which it does not dissolve so readily as in
cold. When a light is brought to the open mouth of a jar filled
with the gas, it burns with a pale blue flame, the hydrogen
uniting with the oxygen of the air to form water, and the
sulphur partly burning to sulphur dioxide (which can be easily
recognized by its pungent smell) and partly being deposited as
a yellow incrustation on the inside of the jar. A mixture of
Galletly, Chem. News, 24, 162.
1 Zeit. Anal. Chem. 31, 100.
SULPHURETTED HYDROGEN
353
two volumes of sulphuretted hydrogen and three volumes
of oxygen explodes violently when an electric spark is passed
through it, complete combustion taking place.
Determination of Composition.-Sulphuretted hydrogen is
decomposed when heated by itself, this decomposition begin-
ning at as low temperature as 400°¹ and being complete at a
white heat. Upon this fact a method is based for the deter-
mination of the composition of sulphuretted hydrogen. This
may be readily accomplished by heating some metallic tin in a
given volume of the gas. The tin decomposes the gas, combin-
ing with the sulphur to form a solid sulphide, and setting free
the hydrogen, which is found to occupy the same volume as the
original gas.
The same result is obtained when a spiral of
platinum wire is heated to bright redness in the gas; sulphur
is deposited in the solid form and the hydrogen is left, whilst
no alteration occurs in the volume of the gas. Now, as the
specific gravity of the gas was found by the experiments of Gay-
Lussac and Thénard to be 1-1912, the molecular weight is 34-8,
or correcting this number by the more accurate results deduced
from analytical data, we have 33-82. Deducting from this the
weight of one molecule of hydrogen, we find the weight of the
sulphur contained in the molecule of sulphuretted hydrogen to
be 31-82, and hence the formula of the gas is SH, and the
theoretical density 1.175.
2
When inhaled, even when mixed with a considerable volume
of air, the gas acts as a powerful poison, producing insensibility
and asphyxia. From the experiments of Thénard it appears that
respiration in an atmosphere coutaining part of its volume
of this gas proves fatal to a dog, and smaller animals die when
only half the quantity is present. The best antidote to poison-
ing by sulphuretted hydrogen appears to be the inhalation of
very dilute chlorine gas as obtained by wetting a towel with
dilute acetic acid and sprinkling the inside with a few grains of
bleaching powder. The gas is dissolved to a considerable extent
by water, one volume of water absorbing, at 0°, 4-37 volumes, and
at 15°, 3.23 volumes of the gas. The general expression for the
solubility of sulphuretted hydrogen in one volume of water at
different temperatures between 2° and 43°3 is
c = 43706 0.083687 t + 0.0005213 t².
-
1 Myers, Annalen, 169, 124:
2 Langer and Meyer, Ber. 18,
135 c.
24
354
THE NON-METALLIC ELEMENTS
The solution reddens blue litmus paper (whence the name hydro-
sulphuric acid has sometimes been given to the substance), and
possesses the peculiar smell and taste of the gas. It however
soon becomes milky on exposure to air, owing to the hydrogen
combining with the oxygen of the air, whilst the sulphur separ-
ates out. At a temperature of -18° the gas combines with
water to form a hydrate which probably has the formula
HS+7H,0.2 Under a pressure of about seventeen atmo-
spheres, sulphuretted hydrogen gas condenses to a colourless
mobile liquid, which boils at 61°8 and has a tension of 146
atmospheres at 11°1 and at - 85° freezes to an ice-like solid.
Liquid sulphuretted hydrogen was first prepared in 1823 by
Faraday by means of the simple bent tube apparatus described
on page 165. In the closed limb of this tube Faraday brought
some strong sulphuric acid, and above it he placed some small
lumps of ferrous sulphide, taking care to separate them from
the acid by a piece of platinum foil placed in the tube. The
open end of the bent tube was then closed hermetically and
the sulphide shaken down into the acid. In making experi-
ments of this kind on the liquefiable gases, many precautions
must be taken if we would avoid serious accidents from ex-
plosions. The tube must be chosen of thick well-annealed
glass and the materials used must be pure; thus if metallic
iron be contained mixed with the sulphide, hydrogen gas will
be given off and the tube will probably burst; in sealing, the
sides of the glass tube must be allowed to fall together so as
to form a strong end, otherwise the tube will give way at the
weakest point. Prepared with care these "Faraday's tubes"
will withstand an internal pressure amounting to many tons
per square inch of surface, and the liquefied gases may be kept
in them with safety for years. Liquefied sulphuretted hydrogen
may also be prepared by passing the gas into a tube cooled to
about 70° in a bath of solid carbonic acid and ether.
-
-
Another mode of preparing liquid sulphuretted hydrogen is to
seal up a quantity of liquid hydrogen persulphide (p. 357) placed
in one limb of a Faraday's tube. This body spontaneously de-
composes into sulphuretted hydrogen and free sulphur, and by
degrees the tension of the gas inside the tube increases so much
that the gas becomes a liquid.
1 Wöhler, Annalen, 33, 125.
Forcrand, Compt. Rend. 106, 1357.
THE SULPHIDES
355
Both as a gas and in solution in water sulphuretted hydrogen
is converted into water and sulphur by nearly all oxidising
agents, and even by strong sulphuric acid, so that this acid
cannot be used for drying the gas.
H₂S + H₂SO₁ = S + 2H₂O + SO2.
If two cylinders, one filled with chlorine and the other with
sulphuretted hydrogen gas, are brought mouth to mouth, an
immediate formation of hydrochloric acid gas and deposition
of sulphur occurs. Fuming nitric acid dropped into a globe
filled with sulphuretted hydrogen gas causes decomposition with
explosive violence.
186 Both in the form of gas and as a solution in water, sul-
phuretted hydrogen is largely used in analytical operations as
the best means of separating the metals into various groups,
inasmuch as certain of these metals when in solution as salts,
such as copper sulphate, antimony trichloride, &c., are pre-
cipitated in combination with sulphur as insoluble sulphides
when a current of this gas is passed through an acid solution
of the salt or mixture of salts; thus:-
CuSO4 + H₂S = CuS + H₂SO.
2SbCl₂ + 3H¸S = Sb₂S¸ + 6HCl.
Certain other metallic salts are not thus precipitated because
the sulphides of this second group of metals are soluble in dilute
acids; thus sulphuretted hydrogen gas does not cause a pre-
cipitate in an acidified solution of ferrous sulphate, but if the
acid be neutralized by soda or ammonia, a black precipitate of
iron sulphide is at once thrown down.
FeSO4 + H2S+ 2NaHO = FeS + Na₂SO, + 2H₂O.
4
Again a third group of metals exists, the members of which are
under no circumstances precipitated by the gas, their sulphides
being soluble in both acid and alkaline solutions.
Many of the insoluble sulphides are distinguished by their
peculiar colour and appearance, so that sulphuretted hydrogen
is used as a qualitative test for the presence of certain metals
as well as a means of separating them into groups.
The reaction of sulphuretted hydrogen on several metallic salt
solutions may be exhibited by means of the apparatus seen in
356
THE NON-METALLIC ELEMENTS
Fig. 106. The gas evolved in the two-necked bottle, (A) passes
through the several cylinders, and precipitates the sulphides of
the metals whose salts have been placed in these cylinders;
thus, B may contain copper sulphate, C antimony chloride, D
a solution of zinc sulphate to which acid has been added, and E
an ammoniacal solution of the same salt.
Many of the sulphides can also be obtained by the direct
combination of the metal with sulphur. A class of bodies
termed hydrosulphides is also known in which only half of the
hydrogen of the sulphuretted hydrogen has been replaced.
Thus, in addition to potassium sulphide, KS, we have potassium

A
B
C
DE
FIG. 106.
hydrosulphide, KHS, and in addition to calcium sulphide CaS,
which is insoluble in water, we have the soluble calcium hydro-
sulphide, Ca(HS). Sulphuretted hydrogen therefore acts as a
weak dibasic acid (p. 123). The sulphides of the metals of the
alkalis and alkaline earths also combine with sulphur forming
unstable polysulphides such as KS, CaS.
Sulphuretted hydrogen immediately tarnishes silver with
formation of black silver sulphide; hence it is usual to gild
silver eggspoons to prevent them from becoming black by con-
tact with the sulphuretted hydrogen given off from the albumin
of the egg. Hence, too, silver coins become blackened when
carried in the pocket with common lucifer matches.
HYDROGEN PERSULPHIDE
357
HYDROGEN DISULPHIDE OR PERSULPHIDE. H₂S₂ = 65:64.
187 This substance was discovered by Scheele, and afterwards
more completely investigated by Berthollet. It was first
obtained in the form of a yellow transparent oily liquid by
pouring a concentrated aqueous solution of pentasulphide
of potassium, K,S,, into dilute hydrochloric acid, when the
liquid, on standing, deposits the substance in yellow drops.
Hence Berthollet believed that the substance possessed an
analogous composition to the body from which it was formed,
and gave it the formula HS. Thénard next examined the
compound, and came to the conclusion that, as in many of
its properties it resembled the then newly-discovered hydrogen
dioxide, the composition of the body would be, most probably,
represented by the formula HS; although it may be remarked,
that the analyses of Thénard¹ showed that it always contained
more sulphur than the above formula required. Hofmann has
observed, that when yellow ammonium persulphide, (NH4)2S3, is
mixed with strychnine, a crystalline compound is formed having
the composition C₂H₂N₂O₂+ H₂S, and this when treated with
hydrochloric acid, yields oily drops of the persulphide which,
therefore, probably has the formula H₂S. Ramsay ³ finds that the
compound obtained by Berthollet's process contains such varying
proportions of hydrogen and sulphur as are represented by the
formulæ HS, and HS10 Still more recently Schmidt has
shown that strychnine combines with sulphuretted hydrogen to
form a beautiful red crystalline body, but that it does so only in
presence of oxygen; thus:-
22 2
3
2C₂HËN₂O₂+ 6H₂S + 30 =
[22N2O2
21
(2C21H22NO2 + 3H2S½) + 3H₂O.
This compound yields hydrogen persulphide on treatment with
an acid, and the substance thus obtained appears to possess
identical properties with that prepared according to other methods.
The formula HS, has been obtained by Rebs for the persul-
phide obtained by decomposing the various polysulphides of the
alkali metals by acids. The formula of the compound has how-
1 Ann. Chim. Phys. 48, 79.
3 Journ. Chem. Soc. 1874, 857.
Annalen, 246, 356.
2 Ber. 1, 81.
4 Ber. 8, 1267.
358
THE NON-METALLIC ELEMENTS
ever been finally found to be that suggested by Thénard, for
Sabatier¹ has succeeded in distilling the substance under a pressure
of 40 mm its boiling point under these conditions being from
68-85°, and thus obtained it in colourless drops, the composition
of which was intermediate between those required by the
formulæ HS, and HS. Hence Thénard's view would be con-
firmed that this compound has a composition analogous to that
of hydrogen dioxide, but it seems that owing to the ease with
which it decomposes into sulphuretted hydrogen and sulphur,
which readily dissolves in the liquid, it usually is found to
contain an excess of the latter substance.
Hydrogen persulphide is usually prepared by boiling one part
by weight of slaked lime with sixteen parts of water and two
parts of flowers of sulphur, the clear cold solution being poured
into dilute hydrochloric acid. A heavy yellowish oil separates
out, sinking to the bottom of the vessel. It possesses a
characteristic pungent odour, gives a vapour which attacks the
eyes, and has a very acrid and unpleasant taste; its specific
gravity was found to be about 17. At the ordinary tempera-
ture it undergoes a slow decomposition yielding sulphur and
evolving sulphuretted hydrogen gas. Hydrogen persulphide
is however more stable in presence of an acid than in that of an
alkali; it is easily soluble in carbon bisulphide and benzene,
and scarcely soluble in alcohol. It bleaches organic colouring
matters, and, like hydrogen dioxide, reduces the oxides of gold
and silver so rapidly that it ignites (Odling). The persulphide
dissolves phosphorus and iodine, gradually changing these
bodies into phosphorus sulphide and hydriodic acid. On the
other hand, sulphur dioxide has no action on the persulphide,
which, in this respect, differs essentially from sulphuretted
hydrogen.
SULPHUR AND CHLORINE.
188 These elements combine together to form the following
compounds :-
Sulphur monochloride, S.Cl,, Sulphur dichloride, SC12, and
Sulphur tetrachloride, SCI.
1 Compt. Rend. 100, 1346 and 1500.
SULPHUR MONOCHLORIDE
359
B
SULPHUR MONOCHLORIDE. S₂Cl₂ = 134.02.
This compound, the most stable of the chlorides of sulphur, is
obtained as a dark yellow oily liquid by passing a current of
dry chlorine gas over heated flowers of sulphur.
Preparation. An apparatus arranged for this purpose is
shown in Fig. 107. The sulphur is placed in a retort and the
chloride which distils over is collected in the cooled receiver.
By rectification it can be obtained as a clear amber-coloured
liquid possessing an unpleasant penetrating odour, having a

C
FIG. 107.
E
specific gravity of 17055 (Kopp), and boiling at 138°. The
density of its vapour is 4.70, corresponding to a molecular
weight of 135-26, so that the compound contains two atoms of
sulphur in the molecule, and has the formula S,Cl₂, its exact
molecular weight being 134-02. When thrown into water
sulphur monochloride gradually decomposes with the formation.
of hydrochloric acid, thiosulphuric acid and sulphur; thus:-
2
2S,Cl₂+3H,O= 4HCl + S₂+ H₂SO.
A number of other products, among which pentathionic acid
may be named, are also formed by the further reaction of these
substances.
"
360
THE NON-METALLIC ELEMENTS
Metals decompose it on heating with liberation of sulphur
and formation of the chloride of the metal.
Sulphur dissolves in the monochloride so readily that the solu-
tion forms, at the ordinary temperature, a thick syrupy liquid
containing 66 per cent. of sulphur. This property has been largely
employed in the arts for the purpose of vulcanizing caoutchouc.
SULPHUR DICHLORIDE. SCl₂ = 102·2.
189 Chlorine gas is rapidly absorbed when passed into sulphur
monochloride at the ordinary temperature, and the liquid changes
colour until it finally assumes a dark reddish brown tint. When
this liquid is heated it begins to boil at 64°, but the thermometer
soon rises, as the compound undergoes decomposition into free
chlorine and the monochloride which remains behind. If, how-
ever, the monochloride be placed in a freezing mixture and then
saturated with dry chlorine, and the excess of chlorine be subse-
quently removed by a current of dry carbonic acid gas, a liquid
remains which analysis shows to be the dichloride.¹ The
molecular weight of the dichloride dissolved in acetic acid
determined by the freezing point method agrees with the
formula SC. In combination, the dichloride appears to be
much more stable. It forms definite compounds with arsenic
trichloride; thus:-
SCI,ASCI, (H. Rose),
and with ethylene and amylene; thus:-
C₂HSC12, CH10SCl₂ (F. Guthrie).
It is decomposed by water in a similar manner to the
monochloride.
SULPHUR TETRACHLORIDE. SCI₁ = 172:58.
190 The existence of this compound was for a long time a
matter of uncertainty, but Michaelis has shown that when the
dichloride is saturated with chlorine at -22° the tetrachloride
is formed. It is a light mobile yellowish-brown liquid, which
1 Hübner and Gueront, Zeitschrift für Chemie, 1870, 455; Thorpe and Dalziel,
Chem. News, 24, 159.
2 Costa, Gazz. Chim. 20, 367.
3 Annalen, 170, 1.
SULPHUR TETRACHLORIDE
361
.
at once begins to evolve chlorine when taken out of the
freezing mixture. Tetrachloride of sulphur forms crystallized
compounds with certain metallic chlorides, thus:-2AICI, SC1;
SnCl, 2SCI.
-
The decomposition of both of these bodies serves as an
excellent illustration of dissociation, which doubtless all com-
pounds undergo when their temperature is raised sufficiently
high, although we are as yet unable to obtain temperatures
elevated enough to decompose many of them. The follow-
ing table shows the composition of the liquid obtained by
saturating (1) the dichloride, and (2) the monochloride of
sulphur with chlorine at the given temperatures:—
-
-
11++
+
Dissociation of Sulphur Tetrachloride.
SC14.
100.0
Temp.
22°.
15
10
7
2
+ 0.7
6.2
·
65
85
90
100
110
•
120
130
·
·
•
Temp.
20°
30
50
Dissociation of Sulphur Dichloride.
SC12.
93.45.
·
41.95
27.62
21.97
11.93
8.87
2:43
·
-
•
87.22.
75.41.
66.78.
54.06.
26.48.
19.45.
12.35 .
5.44.
0·00.
SCI2.
0.00
58.05
72.38
78.03
88.07
91.13
97.57
SCI.
6.55
12.78
24.59
33.22
45.94
73.52
. 80.85
87.65
9456
. 100.00
From these tables it is seen that whilst a difference of 7°,
from - 22° to
15°, reduces the percentage in the case of
SCI,, from 100 to 41.95, a difference of 10° in the case of SCI,
reduces it only from 9345 to 87-22, and an elevation of 100°
does not completely dissociate the dichloride.
1
362
THE NON-METALLIC ELEMENTS
SULPHUR AND BROMINE.
SULPHUR MONOBROMIDE. S,Br₂ = 222·36.
191 Bromine and sulphur form compounds which are much
less stable than the corresponding chlorine compounds. The
monobromide is prepared by dissolving sulphur in a slight
excess of bromine, and volatilizing the excess by means of a
current of dry carbon dioxide.¹ It is a ruby red liquid, which
boils at about 200°, and which by repeated distillation can be
decomposed completely into sulphur and bromine.
Sulphur tetrabromide, SBr,, is formed, according to Michaelis,
by the action of phosphorus chlorobromide upon sulphur
dioxide; 2
SO₂+2PBr₂Cl₂ = 2POCI¸ + SBг.
3
It decomposes at once into bromine and the monobromide.
SULPHUR AND IODINE.
SULPHUR MONIODIDE. SI, 315·46.
192 When sulphur and iodine are heated together, even under
water, they combine to form a blackish grey crystalline solid,
resembling in its appearance the native sulphide of antimony,
and melting below 60°. This substance is sulphur moniodide,
SI2. The same body can be obtained, according to Guthrie,
in fine tabular crystals by acting upon ethyl iodide with sulphur
monochloride, when ethyl chloride and sulphur moniodide are
formed, thus:-
2C₂HI+S₂Cl₂
2C₂HCl + S₂I₂2.
1 M. M. Pattison Muir, Journ. Chem. Soc. 1875, 845.
2 Zeit. Chem. (2), 7, 185.
3 Journ. Chem. Soc. 1862, 57.
=
FLUORIDE OF SULPHUR
363
SULPHUR HEXIODIDE. SIG.
A compound of sulphur and iodine having the above formula
is stated to be deposited in crystals which are isomorphous with
iodine, when a solution of iodine and sulphur in carbon bi-
sulphide is evaporated (Landolt and vom Rath). This substance
however appears to be simply a mixture of the two elements,
(McLeod).¹
SULPHUR SUBIODIDE. SI₂ = 347.28.
This substance is formed by the action of H2S upon an
aqueous solution of the double compound of iodine trichloride
with potassium chloride.
It forms a red precipitate which loses iodine in the air, and
melts above 60°.
SULPHUR AND FLUORINE.
193 According to Davy and Dumas, a compound of sulphur
and fluorine is obtained by distilling lead fluoride with sulphur;
the composition of the body has, however, not yet been ascertained.
If sulphur is brought into contact with fused fluoride of silver,
silver sulphide is formed and a heavy colourless gas is given off,
which does not condense at 0°, fumes in contact with air, smells
like the chlorides of sulphur and sulphur dioxide, and etches
glass (Gore).
When sulphur is exposed to the action of gaseous fluorine,³
it becomes incandescent, and gives off a gas which has a smell
resembling that of chloride of sulphur. This gas is incom-
bustible, and does not take fire even when mixed with oxygen
or brought into contact with a flame. When the gas is heated
in a glass vessel the glass becomes etched, silicon fluoride being
formed.
1 Brit. Ass. Report, 1892, 690.
2 Ann. Chim. Phys. 31, 437.
3 Moissan, Ann. Chim. Phys. [6], 24, 239.
364
THE NON-METALLIC ELEMENTS
SULPHUR AND OXYGEN.
OXIDES AND OXYACIDS OF SULPHUR.
194 Sulphur forms with oxygen two compounds, which
belong to the class of acid-forming oxides, and which, therefore,
when brought into contact with water, both yield acids; thus:—
Sulphur dioxide, SO2, yields Sulphurous acid, H₂SO3.
Sulphur trioxide, SO3, yields Sulphuric acid, H₂SO4.
In addition to these we are acquainted with the oxides S₂O,
and S2O7, as well as with the following oxyacids of sulphur.
Persulphuric acid
Hyposulphurous acid
Thiosulphuric acid
Dithionic acid
Trithionic acid
Tetrathionic acid
Pentathionic acid.
Hexathionic acid.
·
•
HSO,.
HSO2.
H₂S₂O3.
H₂S₂O6
H₂SO
H₂SOε
H₂SO
H₂SO6
The names given to the six last acids are derived from cov
sulphur.
SULPHUR DIOXIDE. SO₂ = 63.58.
The ancients were aware that when sulphur is burnt pungent
acid smelling vapours are evolved. Homer mentions that the
fumes from burning sulphur were employed as a means of
fumigation, and Pliny states that they were employed for
purifying cloth. For a long time it was thought that sulphuric
acid was produced when sulphur was burnt, and it is to Stahl
that we are indebted for first showing that the fumes of
burning sulphur are altogether different from sulphuric acid,
standing in fact half way between sulphur and sulphuric acid,
and, therefore, termed, according to the views of the time, phlogis-
ticated vitriolic acid. Priestley in 1775 first prepared the pure
substance in the gaseous state, to which the name of sulphurous
acid was afterwards given.
SULPHUR DIOXIDE
365
Sulphur is an easily combustible body; according to Dalton
it ignites at a temperature of 260°, burning in the air with a pale
blue flame, and in oxygen with much greater brilliancy. In this ·
act of combustion sulphur dioxide is formed, and each atom of
sulphur evolves, according to the experiments of Favre and
Silbermann, 70,539 thermal units. The volume of the sulphur
dioxide formed is equal to that of the oxygen, as may be shown.
by the following experiment.
The apparatus employed, shown in Fig. 108, is similar in its
FIG. 108.
arrangement to the syphon eudiometer previously described
(p. 176), except that on one of the limbs a globe-shaped bulb has
beer blown, and this can be closed by a ground-glass stopper.
This stopper is hollow, and through it are cemented two stout
copper wires; one of these ends in a small platinum spoon,
whilst to the other a small piece of thin platinum wire is
attached, and this lies on the platinum spoon. A fragment of
sulphur is then placed over the thin wire in the spoon, and the
tube having been filled with oxygen gas, and the stopper placed
in position, the sulphur is ignited by heating the wire with a
current, care being taken to reduce the pressure on the gas by
366
THE NON-METALLIC ELEMENTS
allowing mercury to run out by the stop-tap, so as to avoid danger
of cracking the globe. The eudiometer is then allowed to cool
again, when it will be found that the level of the mercury rises
to the same point at which it stood before the experiment.
Hence one molecule of sulphur dioxide contains one molecule,
or 3176 parts by weight of oxygen and therefore the molecule
of the dioxide, which weighs 63:58, contains 31-82 parts, or one
atom of sulphur, and its molecular formula is SO
195 Preparation.-(1) Sulphur dioxide is formed not only
by the combustion of sulphur, but also by the action of certain
metals, such as copper, mercury, or silver, on concentrated
sulphuric acid; thus:-
Cu + 2H₂SO = CuSO4 + 2H2O + SO2.
Sulphur dioxide is easily prepared for laboratory use by
the above reaction. For this purpose a flask is half filled with
copper turnings or fine copper foil, and so much strong sulphuric
acid poured in that the copper is not quite covered. The mixture
is next heated until the evolution of gas commences; the lamp
must then be removed, as otherwise the reaction may easily
become too violent, and the liquid froth over.
(2) Pure sulphur dioxide is also produced when sulphur and
sulphuric acid are heated together; thus:-
S + 2H,SO₁ = 3SO, + 2H,O.
(3) It is also formed by the decomposition of a sulphite, such
as commercial sodium sulphite, which, when treated with warm
dilute sulphuric acid, easily evolves the gas; thus:-
A
Na,SO3 + HSO₁ = Na₂SO4 + H¸O + SO2.
4
A convenient method of employing this decomposition is to
allow concentrated sulphuric acid to drop into a saturated solu-
tion of sodium bisulphite, which is now an article of commerce.
(4) Sulphur dioxide is made on the large scale for the prepara-
tion of the sulphites, especially of sodium sulphite and calcium
sulphite, which are obtained by passing the gas either into a
solution of caustic soda or into milk of lime. For this purpose
charcoal is heated together with sulphuric acid, when carbon
dioxide is evolved, together with sulphur dioxide; but the
1
SULPHUR DIOXIDE
367
presence of the former compound for the purpose above men-
tioned is not detrimental; thus :-
:-
C + 2H,SO₁ = 2H2O + CO2 + 2SO2.
The sulphur dioxide evolved in the roasting of certain
metallic ores, which was formerly allowed to pass off into the
atmosphere, is now frequently utilized for the preparation of
the sulphites.
(5) Sulphur dioxide is used in enormous quantities for the
manufacture of sulphuric acid. For this purpose it is chiefly
obtained by roasting pyrites. When, however, perfectly pure
sulphuric acid is needed, the dioxide is prepared by burning
pure sulphur.
Temp.
0°
20°
40°
196, Properties.—Sulphur dioxide is a colourless gas, which
occurs in nature in certain volcanic emanations, as well as in solu-
tion in volcanic springs. It possesses the well-known suffocating
smell of burning sulphur. Its specific gravity is 2·2639 (Leduc),¹
and it can, therefore, be collected by downward displacement
like chlorine. If, however, the gas is required to be perfectly free
from air, it must be collected over mercury. Sulphur dioxide
does not support the combustion of carbon-containing material,
and a burning candle is extinguished when plunged into the
gas. Some of the metals however take fire when they are
heated in the gas; thus, potassium forms the thiosulphate and
sulphite, and tin and finely divided metallic iron are changed
partly into sulphide and partly into oxide; lead dioxide, PbO2,
ignites when plunged into the gas and loses its brown colour,
with formation of white lead sulphate, PbSO.
Sulphur dioxide is easily soluble in water, as is seen from the
following table : 2—
1 vol. of water
dissolves SO
79.789 vols. .
39.374
18.766
1 vol. of the solution
contains SO.
68.861 vols.
36.206
17.013
A solution of the gas saturated at 0° deposits a crystalline
hydrate which melts at a temperature of about 4° and which
probably possesses the formula H,SO, + 14H,O. The solution
of the dioxide has a strongly acid reaction, and therefore reddens
2 Bunsen und Schönfeld, Annalen, 95, 2.
1 Compt. Rend. 117, 219.
368
THE NON-METALLIC ELEMENTS
blue litmus paper, which the perfectly dry gas does not, as the
dioxide only forms sulphurous acid, H₂SO, by union with
water.
Sulphur dioxide condenses to a mobile liquid when exposed
to pressure or cold.
This liquid boils at -8°,¹ its vapour at 0°
having a tension of 116505 meters of mercury. The critical
temperature is +155°4 and the corresponding pressure 789
atmospheres. The condensation of this gas by pressure can
easily be shown. For this purpose an ordinary but strong glass
FIG. 109.
tube 20 mm. in diameter may be used; this is drawn out to a
point at one end, whilst into the other end fits a greased
caoutchouc plug, fastened on to an iron rod. The tube having
been filled with the dry gas by displacement, the plunger is
inserted, and the gas forcibly compressed; when the plunger
has been driven down so that the gas occupies about one-fifth
of its original bulk, drops of the liquid are seen to form and to
collect in the drawn-out point. At a temperature above its
boiling point, liquid sulphur dioxide evaporates quickly, absorb-
ing much heat, the temperature sinking to- 50° if a quick
1 Pierre, Compt. Rend. 70, 92.
M

LIQUID SULPHUR DIOXIDE
369
stream of air be driven through the liquid. If the liquid be
placed under the receiver of an air-pump and the air rapidly
withdrawn, evaporation takes place so quickly and so much heat
is absorbed that a portion of the liquid freezes, to a white snow-
like mass. The formation of the solid may also be observed in
the condensing tube when the plunger is quickly drawn out
again. According to Pierre its specific gravity at – 20·5° is
14911, and it dissolves iodine, sulphur, phosphorus, resins, and
many other substances which are insoluble in water.
In order to prepare liquid sulphur dioxide in larger quantity,
the apparatus Fig. 109 is used. The gas evolved by the action of
sulphuric acid on copper is purified by passing through the wash
bottle, and afterwards passes through the spiral glass tube,
surrounded by a freezing mixture of ice and salt. The liquid
which condenses and falls into the flask placed beneath may
be preserved by sealing the flask hermetically where the neck
I
FIG. 110.
has been drawn out. It may also be preserved in glass tubes
provided with well-closing glass taps, the construction of one
of which is seen in Fig. 110.
Sulphur dioxide, both in the gaseous state and in aqueous
solution, exerts a bleaching action on vegetable colouring
matters. This fact was known to Paracelsus, and it is still
made use of in the arts for bleaching silk, wool, and straw
materials, which are destroyed by chlorine. The decolorising
action of sulphur dioxide depends upon its oxidation in presence
of water with formation of sulphuric acid, the hydrogen which
is liberated uniting with the colouring matter to form a colour-
less body; thus:-
SO₂ + 2H2O = H₂SO4 + 2H.
Thus the bleaching action of this substance is a reducing one,
25
370
THE NON-METALLIC ELEMENTS
whilst that of chlorine is an oxidising one. The colouring matter
thus destroyed by bleaching with sulphur dioxide may often be
restored when the cloth is exposed to the air, as in the case of
linen marked with fruit stains, or when brought in contact with
an alkali, as when bleached flannel is first washed with soap.
The reducing action of sulphur dioxide is also made use of in the
paper manufacture, in order to get rid of the excess of chlorine
left in the pulp after bleaching, when the following decomposition
takes place :-
SO₂ + Cl2 + 2H₂O = H₂SO4 + 2HCl.
2
Sulphur dioxide is also a powerful antiseptic, and has been
successfully employed for preventing the putrefaction of meat,
as well as to stop fermentation. It is used in the sulphuring
of wine, and also as a disinfecting agent.¹
Sulphur dioxide has been shown by Tyndall to undergo a
remarkable decomposition when exposed to light. If a beam of
sunlight be passed through a long tube filled with the colour-
less gas a white cloud is seen to make its appearance, and this
consists of finely divided particles of sulphur and sulphur tri-
oxide, which are separated by the chemical action of the light.
The gas is also slowly decomposed, when a series of electric
sparks is passed through it, into sulphur and sulphur trioxide,
but this decomposition ceases when a certain quantity of the
latter compound is formed, and can only be fully carried out
when the trioxide is removed by allowing it to dissolve in strong
sulphuric acid.
197 In order to detect sulphur dioxide some paper steeped in
a solution of potassium iodate and starch is brought in the gas;
this will at once be turned blue by the formation of the iodide
of starch, if even only traces of the gas be present, iodine being
liberated, as is shown by the following reaction :-
―
2KIO, + 5SO₂ + 4H¸0 = I½ + 2KHSO + 3H„SO.
2
An excess of sulphurous acid however will bleach the blue paper
again with formation of hydriodic acid; thus:-
I½ + SO2 + 2H¸0 = 2HI + H¸SO4.
2
1 Liquid sulphur dioxide is now manufactured by Messrs. Boake & Co. of
Stratford, London, and stored in glass syphons or steel cylinders.
SULPHUROUS ACID
371
This last reaction serves as an excellent means of determining
the quantity of sulphur dioxide present in solution. For this
purpose a small quantity of starch paste is added to the solution,
and then a standard solution of iodine is added by means of a
burette to the solution until a permanent blue colour from the
formation of iodide of starch is observed. It is, however, to be
borne in mind that the above reaction does not take place unless
the solutions are sufficiently dilute, for in concentrated solution
sulphuric acid and hydriodic acid mutually decompose, forming
free iodine, sulphurous acid and water; thus:-
:-
2HI+ H₂SO, I₂+ H₂SO3 + HO.
2
=
Bunsen, who has investigated this subject thoroughly, finds
that aqueous sulphurous acid can only be completely oxidised to
sulphuric acid by means of iodine, when the proportion of sulphur
dioxide does not exceed 0:04 to 0.05 per cent. of the solution.
A standard solution of sulphurous acid may, of course, also be
used for the quantitative determination of iodine, and Bunsen
has made use of this reaction for the foundation of a general
volumetric method. The principle of this method depends on
the fact that a quantity of iodine, equivalent to that of the
substance under examination, is liberated, and the quantity of
this iodine is determined volumetrically by a dilute solution of
sulphurous acid.¹
SULPHUROUS ACID. H.SO.
198 This substance, like many other acids whose corresponding
anhydrides are gaseous, is only known in aqueous solution. This
solution smells and tastes like the gas and has a strongly acid
reaction. Exposed to the light it is decomposed with formation.
of pentathionic acid. When an aqueous solution saturated at 3°
is allowed to stand, crystals of a hydrate, H,SO, + 6H„O, are
obtained. Sulphurous acid differs from the acids which have
hitherto been described, inasmuch as it contains two atoms of
hydrogen, both of which may be replaced by metals. It is
therefore termed a dibasic acid; it forms two series of salts
termed sulphites, in one of which only half of the hydrogen is
1 Journ. Chem. Soc. 1856, 219.
2 Geuther, Annalen, 224, 219.
372
THE NON-METALLIC ELEMENTS
replaced by a metal, and which may therefore be considered as
being at once a salt and a monobasic acid, another in which the
whole of the hydrogen of the acid has been replaced by a metal.
The salts of the first series, are termed acid sulphites, and of the
latter normal sulphites.
The following serve as types of these different salts:-
Acid Sulphites, or Hydrogen Sulphites.
HNASO
HKSO
HAgSOg
Normal Sulphites.
Na,SO
KNASO
CaSO3.
The acid sulphites of potassium and sodium are obtained by
passing sulphur dioxide gas into caustic soda or caustic potash
as long as it is absorbed. If, then, exactly the same quantity of
alkali is added to this solution as was originally taken for the
preparation, the normal salts are obtained. All the sulphites of
the alkali metals are easily soluble in water, the normal sulphites
of the other metals being either difficultly soluble or insoluble
in water. They dissolve however in aqueous sulphurous acid
and exist in such a solution as acid salts, but on evaporation they
decompose with formation of the normal salt and sulphurous
acid. Salts of sulphurous acid are also known in which the
two hydrogen atoms of the acid have been replaced by differ-
ent metals. Thus when caustic soda is added to a solution of
acid potassium sulphite, or caustic potash to a solution of acid
sodium sulphite, a salt of the formula NaKSO3, sodium
potassium sulphite is formed. The salts produced by these
two methods are not however identical, but are found to
possess different properties. They crystallise, for example, with
different amounts of water of crystallisation, and yield different
products when acted upon by ammonium sulphide or
ethyl iodide. As these salts, although possessing the same
composition, have different chemical properties, the atoms in
their molecules must be arranged differently, in other words
they have a different chemical constitution. This difference is
expressed in the following formula, according to which one
atom of metal is combined directly with oxygen and the
other with sulphur.
Sulphurous Acid.
H.SO. OH.
Normal Sodium Sulphite.
Na. SO₂. ONa.
CONSTITUTION OF THE SULPHITES
373
Sodium Potassium Sulphite..
I.
Na. SO,. OK.
II.
K. SO₂. ONA.
Substances such as these, which possess the same molecular
weight and composition but differ in their chemical and
physical properties, are said to be isomeric. Comparatively
few instances of this kind occur among the inorganic compounds
but they are very frequent among the compounds of carbon.
Sulphurous acid also forms salts such as Na,S,O,, which are
known as metabisulphites, and other more complicated series,
for an account of which the original memoirs must be con-
sulted.¹
The normal sulphites have no odour, and those which are
soluble in water possess a sharp taste. They are readily detected
by the fact that when they are mixed with dilute sulphuric acid
they give off sulphur dioxide and also that their neutral solu-
tions give a precipitate with barium chloride which is soluble in
dilute hydrochloric acid, whereas if nitric acid be added to
this solution and the mixture warmed, a precipitate of barium
sulphate, formed by oxidation from the sulphite, is thrown down.
THIONYL CHLORIDE. SOCI,, = 118.08.
199 All oxy-acids and many other compounds contain the
group OH, which is a monad radical known by the name of
hydroxyl. It is thus termed because it is capable of taking the
place of monad elements such as chlorine and bromine, and in
its compounds may be replaced by other monad elements. If
the hydroxyl of an acid be replaced by chlorine an acid chloride
is obtained. The chloride of sulphurous acid, or thionyl chloride,
has been obtained by the action of phosphorus pentachloride on
sodium sulphite (Carius); thus:-
SO¸Ña¸ + 2PC1 = SOCI, + 2POCI, + 2NaCl.
Thionyl chloride is also obtained by passing sulphur dioxide over
pentachloride of phosphorus (Schiff); thus:-
:-
SO₂+PCI, SOCI₂+ POCI.
1 Schwicker, Ber. 22, 1728; Rörig, J. pr. Chem. 37, 250; Barth, Zeit. phys.
Chem. 9, 176; Divers, Journ. Chem. Soc. 1886, i. 533; Hartog, Compt. Rend.
109, 436.
"
1
374
THE NON-METALLIC ELEMENTS
It may likewise be prepared by the direct union of sulphur and
chlorine monoxide (Wurtz).
It is a colourless highly refractive pungent liquid which fumes
on exposure to air. It boils at 78° and has a specific gravity at
0° of 1675. Like all other acid chlorides, when brought in
contact with water it decomposes into its corresponding acid
and hydrochloric acid; thus:-
SOCI, + 2H2O = SO¸H₂ + 2HCI.
THIONYL BROMIDE, SOBг2,
206.42.
200 This substance cannot be prepared in the same way as
the chloride, but is formed by the action of bromine on
thionyl-aniline¹ and by that of thionyl chloride on potassium
bromide. It forms a brown liquid and boils at 136° but de-
composes largely on heating into sulphur dioxide and bromide
of sulphur.
=
SULPHUR TRIOXIDE. SO,, =79'46.
201 This body, which is called also sulphuric anhydride, and
formerly was termed anhydrous sulphuric acid, is formed when
a mixture of sulphur dioxide and oxygen is passed over heated
platinum-sponge. In place of pure platinum-sponge, platinised
asbestos may be employed; this is obtained by dipping some
ignited asbestos into a tolerably concentrated solution of platinum
chloride and then bringing it into a solution of sal-ammoniac.
The insoluble double chloride of platinum and ammonium
(NH),PtCl is deposited on the threads of the asbestos, and
when it has been dried and ignited this compound is converted
into finely divided platinum.
In order to show the oxidation of sulphur dioxide to the
trioxide the apparatus Fig. 111 may be employed. Sulphur
dioxide is evolved in the flask (@) and is mixed in the wash-
bottle, which contains strong sulphuric acid, with the oxygen from
a gas-holder coming in through the tube (b). The mixture next
passes through the cylinder () containing pumice-stone soaked
in strong sulphuric acid in order to remove every trace of
1 Michaelis, Ber. 24, 747.
2 Hartog and Sims, Chem. News, 67, 82.
#1
SULPHUR TRIOXIDE
375
♪
FIG. 111.
moisture, and then passes at (c) over the platinised asbestos.
As long as this is not heated no change is observed; so soon,
however, as it is gently ignited dense white fumes of the tri-
oxide are formed which condense in a receiver (d) cooled by a
freezing mixture in the form of long white needles. In order
to obtain these crystals, every portion of the apparatus must be
absolutely dry; if even a trace of moisture be present the needles
disappear at once, liquid sulphuric acid being formed. Wöhler
has shown that instead of platinum, certain metallic oxides, such
as copper oxide, ferric oxide, and chromic oxide, may be used.

A much more convenient process for preparing sulphur trioxide
than the above is by the distillation of fuming oil of vitriol.
This substance, sometimes called Nordhausen sulphuric acid,
consists of a solution of the trioxide in sulphuric acid, and is
obtained by the distillation of heated ferrous sulphate. The
writer known as Basil Valentine mentions that by this process
a "philosophical salt" can be obtained, but the preparation
of the "sal volatile olci vitrioli" from fuming acid was first
described by Bernhardt in the year 1775.
In order to prepare the trioxide, the fuming acid must
be gently heated in a retort, and the trioxide collected in a
I
I
376
THE NON-METALLIC ELEMENTS
well-cooled and perfectly dry receiver, where it is obtained in
the form of long transparent needles. Sulphur trioxide is also
obtained by heating concentrated sulphuric acid with an excess of
phosphorus pentoxide; thus:-
H₂SO+P₂O
SO3 + 2HPO3.
The same substance can be obtained in several other ways, for
instance by heating dry antimony sulphate. Sulphur trioxide
is now manufactured on the large scale by Messrs. Chapman
and Messel at Silvertown.¹
=
Properties. Sulphur trioxide forms transparent prisms which
melt at 14°8 and solidify at the same temperature. The melted
trioxide often remains for a considerable length of time in the
liquid state at a temperature below its ordinary point of solidi-
fication, but on agitation it at once solidifies, the temperature
rising to 148. The liquid trioxide has a specific gravity of 197
at 20°, and boils at 46°; its coefficient of expansion between
25° and 40° is 0-0027, a number which is much higher than that
ordinarily observed for liquid bodies and amounts to two-thirds
of the coefficient of the gases.
A second modification of the trioxide is said to be obtained
when the melted mass is allowed to stand at a temperature below
25°, being then transformed into a mass of silky needles which
do not melt below 50°. When these silky needles are melted
they undergo a change into the first modification. The exist-
ence of two modifications is denied by Weber,3 the difference
observed being due to impurities.
Sulphur trioxide absorbs moisture readily from the atmosphere
and evolves dense white fumes in the air. Thrown into water
it dissolves with a hissing sound, forming sulphuric acid and
evolving a large amount of heat. When brought into contact
with anhydrous baryta, BaO, a combination with formation of
barium sulphate, BaSO4, occurs with such energy that the mass
becomes red hot.
If the vapour of sulphur trioxide is led through a red-hot
porcelain tube it is decomposed into two volumes of sulphur
dioxide and one volume of oxygen. This fact indicates that the
formula of the substance is SO3, and this conclusion is borne out
by the vapour density which is 2.75.
1 Journ. Soc. Chem. Ind. 4, 520.
2 Schultz-Sellack, Ber. 3, 216.
3 Pogg. Ann. 159, 313.
SULPHURIC ACID
377
SULPHURIC ACID. H.SO.
202 Sulphuric acid is, without doubt, the most important and
useful acid known, as by its means nearly all the other acids are
prepared, whilst its manufacture constitutes one of the most
important branches of modern industry owing to the great
variety of purposes for which it is needed, as there is scarcely
an art or a trade in which in some form or other it is not
employed. It is manufactured on an enormous scale, no less
than 850,000 tons being at present annually produced in Great
Britain, and this production is undergoing constant increase.¹
It appears probable that Geber was acquainted with sulphuric,
or, as it was formerly called, vitriolic acid, in an impure state;
but the writer known as Basil Valentine was the first fully to
describe the preparation of this acid from green vitriol or
ferrous sulphate, and to explain that when sulphur is burnt
with saltpetre a peculiar acid is formed.
Originally, sulphuric acid was obtained exclusively by
heating green vitriol according to a decomposition which we
shall study hereafter. The present method of preparing the
acid is said to have been introduced into England from the
Continent by Cornelius Drebbel; but the first positive infor-
mation which we possess on the subject is that a patent for
the manufacture of sulphuric acid was granted to a quack doctor
of the name of Ward. For this manufacture he employed glass
globes of about 40 to 50 gallons in capacity; a small quantity
of water having been poured into the globe, a stoneware pot was
introduced, and on to this a red-hot iron ladle was placed. A
mixture of sulphur and saltpetre was then thrown in to this
ladle, and the vessel closed in order to prevent the escape of the
vapours which were evolved. These vapours were absorbed by
the water, and thus sulphuric acid was formed. This product, from
the mode of its manufacture, was termed oil of vitriol made by
the bell, as contradistinguished from that made from green vitriol,
and it cost from 1s. 6d. to 2s. 6d. per lb.
Dr. Roebuck of Birmingham was the first to suggest a great
improvement, in the use, instead of glass globes, of leaden
chambers, which could be constructed of any wished-for size.
For a complete account of the manufacture of sulphuric acid see Lunge's
excellent treatise on the subject. Gurney and Jackson, London, 1891.
* See Dossie's Elaboratory Laid Open, 1758, Intro. p. 44.
Ka
"
378
THE NON-METALLIC ELEMENTS
Such leaden chambers were first erected in Birmingham in 1746,
and in the year 1749 at Prestonpans in Scotland. The mode of
working this chamber was similar to that adopted with the glass
globes; the charge of sulphur and nitre was placed within the
chamber, ignited, and the door closed. After the lapse of a
certain time, when the greater portion of the gases had been
absorbed by the water in the chamber, the door was opened, the
remaining gases allowed to escape, and the chamber charged
again.
The leaden chambers first set up were only six feet square,
and for many years they did not exceed ten feet square, but in these
all the acid employed in the country was manufactured, whilst
much was exported to the Continent, where the chamber acid still
goes by the name of English sulphuric acid. The first vitriol
works in the neighbourhood of London were erected at Battersea
in the year 1772, by Messrs. Kingscote and Walker, and in 1783
a connection of the above firm established works at Eccles, near
Manchester. This manufactory, the first erected in Lancashire,
contained four chambers, each twelve feet square; and four others,
each of which was forty-five feet long and ten feet wide.
In the year 1788 a great stimulus was given to the manufacture
of sulphuric acid by Berthollet's application of chlorine, dis-
covered by Scheele in 1774, to the bleaching of cotton goods, and,
from that time to the present, the demand has gradually extended
until it has become enormous and almost unlimited in extent.
The next improvement in the manufacture consisted in making
the process continuous. The foundations of this mode of
manufacture appear to have been laid by Chaptal, and the
principle employed by him is that which is at the present day in
use. The improvements thus proposed were (1) the introduction
of steam into the chamber instead of water, (2) the continuous
combustion of the sulphur in a burner built outside the chamber,
(3) sending the nitrous fumes from the decomposition of nitre
placed in a separate vessel, along with the sulphur dioxide gas
and air into the chamber.
203 The theory of the formation of sulphuric acid in the
leaden chamber has been the subject of much discussion, and
even at the present day the views of chemists differ. The view
originally proposed by Berzelius may be simply expressed by
saying that although sulphur dioxide in presence of water or
1 Peligot, 1844, Ann. Chim. Phys. [3] 2, 263; R. Weber, 1866, Pogg. Ann.
127, 513; Raschig. Annalen, 241, 242; Lunge, loc. cit.
THEORY OF SULPHURIC ACID MANUFACTURE
379
3
steam is unable rapidly to absorb atmospheric oxygen, it is able
to take up oxygen from such oxides of nitrogen, as N₂O, or NO2.
If, therefore, these oxides are present in the chamber they give
up part of their oxygen to the sulphur dioxide, and are reduced
to nitric oxide, NO. This is, however, able to absorb free
oxygen, and is at once reconverted into N,O, or NO. This
continuous reaction may be represented as follows:-
(1) NO2 + SO2 + H2O = H₂SO4 + NO.
(2) NO + 0 = NO₂.
Another view, founded upon that of Davy, is due to Lunge.
According to him, nitrogen peroxide, NO2, is not formed in the
chambers, and is therefore not the essential agent which brings
about the oxidation of the sulphur dioxide, nor does reduction
to nitric oxide, NO, usually occur, the active substance being
nitrogen trioxide, N,O,, (p. 508). In the first instance, the
sulphur dioxide combines with nitrogen trioxide, oxygen and
water to form nitrosyl-sulphonic acid, SO,(OH)NO,, according
to the equation (1):—
(1) 2SO₂ + N₂O3 + O2 + H2O = 2SO,(OH)NO2.
(2) 280,(OH)NO2 + H₂O 2H₂SO4 + N₂O3.
=
This substance on meeting with an excess of water vapour is
decomposed into sulphuric acid which falls to the bottom of the
chamber, and nitrogen trioxide which is ready to react again
(equation 2). The existence of nitrosyl-sulphonic acid is well
known to the manufacturers of sulphuric acid, since it is formed
as a white crystalline substance when the supply of steam has
been insufficient, and is termed by them "chamber crystals."
It seems probable that in practice both these reactions occur,
the formation of sulphuric acid at the commencement of the
chambers being accompanied by the presence of nitric oxide, NO,
whilst in the subsequent stages the active substance is nitrogen
trioxide, N₂Og.¹
1
It is thus clear that according to either theory nitrous fumes
act as a carrier between the oxygen of the air and the sulphur
dioxide, so that, theoretically, a small quantity of these fumes will
suffice to cause the combination of an infinitely large quantity
of sulphur dioxide, oxygen, and water to form sulphuric acid.
1 A complete discussion of this somewhat intricate subject will be found in
vol. i. chap. x. of Lunge's Sulphuric Acid and Alkali, 1891.
380
THE NON-METALLIC ELEMENTS
Practically, however, this is not the case, because instead of
pure oxygen, air must be used, and four-fifths of this consists of
nitrogen, which so dilutes the other gases that in order to obtain
the necessary action a considerable quantity of these oxides of
nitrogen must be added. Besides this, nitrogen has to be
constantly removed from the chambers, and in its passage

FIG. 112.
carries much of the nitrous fume away with it, although most
of this can, as we shall see, be recovered and used over again.
The above reaction can be illustrated on the small scale by the
apparatus shown in Fig. 112, in which sulphur contained in the
bulb-tube is allowed to burn in a stream of air, supplied from
the double aspirator; the sulphur dioxide and air pass through
the wide glass tube into the large glass globe, but carry in on
their way the nitrous fumes generated in the small flask (a),
SULPHURIC ACID MANUFACTURE
381
from nitre and sulphuric acid. The flask () contains boiling
water, from which steam passes into the globe. The outlet tube
(e) of the globe communicates with a draught. By alternately
increasing and diminishing the supply of sulphur dioxide, the
disappearance and reappearance of the red nitrous fumes can be
readily shown. If the flask be kept dry whilst the two gases
are passed in, the white leaden-chamber crystals are seen to be
deposited on the glass. When aqueous vapour is admitted,
the crystals dissolve with formation of sulphuric acid and
ruddy fumes.

FIG. 113.
204 The leaden chambers for the manufacture of sulphuric
acid are now constructed of a much larger size than was formerly
the case, but vary considerably in different works; they are
frequently 30 meters in length, 6 to 7 meters in breadth, and
about 5 meters in height, and have therefore a capacity of from
900 to 1,000 cubic meters (about 38,000 cubic feet). The
chambers are made of sheet lead weighing 35 kilos per square
meter (or 7 lbs. to the square foot), and soldered together by
melting the edges of the two adjacent sheets by means of the
oxyhydrogen blow-pipe. The leaden chamber is supported by a
1
1
1
1
D
•
C

382
THE NON-METALLIC ELEMENTS
wooden framework to which the leaden sheets are attached by
strips of the same metal, and the wooden framework is generally
999
Z
B
K
m
m
103
No2
Ne 1
raised from the ground on pillars of brick or iron and the whole
erection protected from the weather, sometimes by a roof, but
F

SULPHURIC ACID MANUFACTURE
383
114.
Decim. 10
at any rate by boarding to keep off most of the rain. The space
below the chamber is used either for the sulphur burners or for
No 1
Nº 3
No 2
10
D
S
15 Meter.
the concentrating pans.
The general appearance or bird's-eye view of a sulphuric acid
384
THE NON-METALLIC ELEMENTS
chamber is shown in Fig. 113, whilst the arrangement and con-
struction of one of the most complete forms of sulphuric acid
plant now in use in this country is shown in Figs. 114, 115, and
116. Three chambers, termed respectively Nos. 1, 2, and 3
(Fig. 114), are placed side by side supported on iron pillars
ten feet high. Each chamber has the dimensions already given,
and each, therefore, has a capacity of 38,500 cubic feet. A
longitudinal section of the chamber (No. 2) in the direction (DC)
is shown in Fig. 115, and a sectional elevation in the direc-
tion (AB) is shown in Fig. 116. From this last figure it is
seen that the roof of the chamber is not horizontal but slightly
A
S
I-III-LE
n
DO
n
No 2
slanting so as to enable the rain to run off into gutters placed to
receive it.
205 Beginning at the first part of the process we find the
pyrites-kilns, or burners placed across the ends of the chamber
as seen in plan at A, Fig. 114, in longitudinal section and in
elevation at A, Fig. 116, and in cross section at A, Fig. 115.
The broken pyrites, FeS,, is filled, in moderately sized lumps,
into the burners, which have previously been heated to red-
ness, and when the burning is once started the fire is
kept up by placing a new charge on the top of that nearly
burnt out. The ordinary charge for each burner of pyrites,
containing about 48 per cent. of sulphur, is 6 to 8 cwt., which
SULPHURIC ACID MANUFACTURE
385
is burnt out in twenty-four hours, and the kilns are charged in
regular succession, so that a constant supply of gas is evolved
during the whole time, whilst the quantity of air which enters
the kiln is carefully regulated by a well-fitting door placed
below.
Decima 10
The hot sulphur dioxide, nitrogen, and oxygen gases, are
drawn from the pyrites burners, through the whole system of
tubes, towers, and chambers, by help of the powerful draught
from a large chimney which is placed in connection with
the apparatus. These gases first pass from each kiln into
a central flue, built in the middle of the kiln, and thence into
0
No 2
...
་
5
1
10
S
15 Meter.
an upright brick shaft through a horizontal earthenware flue, or
cast-iron pipe, into the lower part of the square denitrating tower
seen in section at G in Fig. 116. This tower, from a to b, is
about 45 feet, or 14 meters, in height; it is built up, from a to e,
to a height of 25 feet, or 8 meters, of lead lined with fire brick,
and of this about 15 feet, or 5 meters, from d to e, are filled up
with pieces of flint.
The object of this Glover's tower, or denitrating tower as it is
termed, is to impregnate the sulphur dioxide as it comes from
the burners with nitrous fumes derived from a later stage of the
operation. This is effected by allowing strong nitrated acid to
flow down the tower together with a stream of chamber-acid.
26
386
THE NON-METALLIC ELEMENTS
G
D
Strong sulphuric acid, as we shall see, has the power of absorb-
ing nitrous fumes, with formation of nitrosyl-sulphonic acid,
SO(OH)(NO); and these are given off again when the acid

TION
Op
M
No1
ZENTU
G
No3
1
comes into contact with the hot sulphur dioxide from the
kilns :-
280.(OH) (NO2)+SO₂+ 2H,O=3SO,(OH)2+2NO.
FIG. 116.
SULPHURIC ACID MANUFACTURE
387
______
Two reservoirs are placed at the top of the Glover's tower; one
containing the strong nitrated acid, the other containing the
chamber-acid. Both the strong nitrated and the chamber-acid
are allowed to flow down together over the column of flint stones
in given proportions, and when the mixture comes into contact
with the upward current of hot sulphur dioxide, the nitrous
fumes dissolved in the strong acid are given off and swept
away, together with the gases from the burners, direct into the
chambers. Although the nitrated acid loses its nitrous fumes
when diluted with water, it does not do so in presence of
chamber-acid of the ordinary strength, so that the denitrating
effect of the Glover's tower depends on the reducing action of
the sulphur dioxide, rather than on any dilution by the chamber-
acid. This addition is mainly made for the purpose of cheaply
concentrating the chamber-acid, for not only does the strong
acid lose its dissolved nitrous fumes, with the production of a
considerable amount of acid, but the weak chamber-acid coming
in contact with the hot dry gases which enter the tower at a
temperature of 340°, parts with a large quantity of its water,
which goes into the chamber as steam, whilst the concentrated
acid, falling to the bottom of the tower, flows into a reservoir, z,
Fig. 114, placed to receive it.
On issuing from the tower, the gas, having now been cooled
by contact with the stream of acid to a temperature of about 60°,
passes into the cast-iron pipe, x, Fig. 114 (4 feet 6 inches in
diameter), whence it is delivered at the further end of chamber
No. 1 at a height of 8 to 9 feet above the floor.
206 The supply of nitrous fumes, which is needed to act as
carrier of the atmospheric oxygen to the sulphur dioxide, is
furnished by the three nitre pots seen in plan and in section
in Fig. 117. The charges of 30 lbs., or 135 kilos, of nitrate
of soda, and 33 lbs., or 15 kilos, of sulphuric acid, of sp.
gr. 175, are run into the pots from the outside, and after the
lapse of two hours, when each charge is exhausted, the fused
bisulphate of soda (technically termed sale nixum) is run off into
a pan placed on a platform outside the oven, and a new charge
introduced. The decomposition of the nitre is accelerated by
heat from the pyrites burners, placed below the brick arch
which separates them from the pots, and the nitric fumes are
gathered into a cast-iron pipe, z, Fig. 117, which discharges its
contents into the long horizontal main carrying the products
from the pyrites burners into the chamber. Here also the
1
388
THE NON-METALLIC ELEMENTS
nitric acid vapour parts with some of its oxygen and is reduced
by the sulphur dioxide to the lower oxide which acts as a
carrier between the oxygen and sulphur dioxide.
The mixture of oxygen, nitrogen, sulphur dioxide, nitrous
fumes and vapour of water now meets with steam introduced
into the chamber by the tubes, s s, Fig. 115, and the reaction as
already described sets in. Having travelled through the length
of chamber No. 1, the gases pass by means of the connecting
shaft (v) shown in Fig. 116, into the second chamber, where
they likewise meet with steam jets, and having passed through
this chamber, and having deposited a further amount of liquid

B
Z
FIG. 117.
sulpnuric acid, which falls on the floor of the chamber, the gases
are drawn into the third or exhaust chamber by the flue (w)
shown in Fig. 116. Here, if the process is properly worked, all the
sulphur dioxide is converted into sulphuric acid, and red nitrous
fumes must always be visible. For the purpose of determining
the proper working of the process the percentage of sulphur
dioxide contained in the gases entering the first chamber, and
that of the oxygen in the gases leaving the third chamber, is
regularly ascertained in carefully managed works.
The nitrous fumes having been added in excess of the quantity
required to convert the SO, into H,SO, still remain in chamber
SULPHURIC ACID MANUFACTURE
389
No. 3, and, in order to absorb these, a Gay-Lussac tower (G",
Figs. 114 and 116) is employed, the capacity of which ought to
be at least one hundredth part of that of all the chambers.
This consists, like the Glover's tower, of a square tower 50 feet
in height, made of strong lead (7-8 lb.) and lined for 35 feet
with 2 inch thick glazed fire tiles, and filled with coke. The
exit gases
from chamber No. 3 are drawn in at the bottom of
this coke column, and escape to the chimney by the exit tube
(i, Fig. 116) at the top. In their passage they come in contact
with a finely divided shower of strong cold acid (sp. gr. 1·75)
obtained by concentrating the. chamber-acid. This strong sul-
phuric acid absorbs the excess of nitrous fumes which would
otherwise pass away up the chimney, and having thus become
saturated with nitrous fumes, runs away through the spout k
into reservoirs for the so-called nitrated acid, built under
chamber No. 3, the position of which (mm, Fig. 114) is shown
on the plan. From these reservoirs the nitrated acid is allowed
to run into one of the cast-iron air boilers (nnn) shown on the
plan, whence, by air pressure, it is forced up to the cistern on
the top of the Glover's tower for employment in the first part
of the process as already described.
207 The continuous process of acid making in the chambers.
is only carried on until the acid has attained a specific gravity of
1:53 to 162, or contains 62-70 per cent. of the pure acid, H₂SO4,
inasmuch as an acid stronger than this begins to absorb the
uitrous fumes. In order to obtain a stronger acid, either the
arrangement of the Glover's tower, as described, is employed
or, in works where the Glover is not used, the chamber-acid
is run into the leaden concentrating pans (00) placed under
chamber No. 2, shown in plan in Fig. 114, and in section in
Fig. 115. The flame and heated air from the fires (9) play
over the surface of the acid contained in these pans, the water
passes away in the form of steam, and the strong acid remains.
By this means the acid can be concentrated until it attains a
specific gravity of 172, or contains 79 per cent. of pure acid;
beyond this degree of concentration the hot acid begins rapidly
to attack the lead of the pans, and it therefore cannot be fur-
ther evaporated in them. It is then run off into the acid
cooler (p), a leaden trough surrounded by cold water, whence it
passes into the strong-acid cisterns (, Fig. 115). In this form
the acid is technically known as B. O. V., brown oil of vitriol,
as it is always slightly coloured from the presence of traces of
390
THE NON-METALLIC ELEMENTS

Centimeter, 100
T
090
FIG. 118.
T
B
1 Meter.
SULPHURIC ACID MANUFACTURE
391
organic matter, and it is in this condition that it is very largely
sold for a great variety of purposes.
208 In order to drive off the remaining portions of water, the
acid must be concentrated or rectified in platinum or glass vessels.
A common arrangement for concentrating in platinum stills is
shown in Fig. 118, as manufactured by Messrs. Johnson, Matthey,
and Co., of London. By means of this apparatus no less than
200 cwt. (10,000 kilos) of brown oil of vitriol can be daily
concentrated, yielding a product containing 98 per cent. of
real acid.
The retort or still (A, Fig. 118) consists of plates of platinum,
the joints of which are autogenously soldered. This rests on
the iron ring (c). The chamber-acid runs from the stopcock
through a platinum tube on to the heated thick bottom of the
still, where it is quickly concentrated, whilst the aqueous vapour
escapes by the head of the still (L). As soon as the level of the
concentrated acid reaches the top of the platinum funnel (D), it
begins to flow off by means of the tube (E) into the platinum
vessel (F), round which a current of cold water circulates.
Having been thus cooled, the acid passes into the stoneware jar
(H), surrounded by water, and thence, by means of the lead or
stoneware funnel (1) into the reservoir (K).
Messrs. Johnson, Matthey, and Co. have introduced an im-
proved form of platinum concentrating apparatus, by means of
which all evaporation in leaden pans is avoided, and thus the
operation not only considerably cheapened, but the acid ob-
tained in a purer condition. This new arrangement is repre-
sented in Fig. 119. A A are pans made of platinum plates,
which are corrugated at the bottom, and heated by a fire placed
below. In these the concentration proceeds until the acid
attains a strength of from 78 to 80 per cent. of HSO4. It then
runs into the retort (B), also having a corrugated surface, and
the perfectly concentrated acid which is thus obtained is cooled
by passing through the worm (D), made of platinum tube.
In many English works the sulphuric acid is rectified in glass
and not in platinum vessels. These glass vessels are large
retorts made of well-annealed and evenly-blown glass (Fig. 120
a), of such a size as to contain twenty gallons of the acid. Each
retort is placed on an iron sand-bath (b), round which the
flames from a fire are allowed to play, but so that the flame
does not touch the retort. A glass head (e) fits loosely into
the neck of the retort, and through this the aqueous vapour
392
THE NON-METALLIC ELEMENTS

B
D
FIG. 119.
SULPHURIC ACID MANUFACTURE
393
carrying with it a little acid fume, passes into a condensing
box. The plan of a rectifying house containing twenty-four
retorts is shown in Fig. 121. The acid having been concentrated
in the leaden pans (A A A), passes along the leaden tubes (BB B),
from which the retorts are filled by means of the upright leaden
tubes (d, Fig. 120), which can be bent so as to discharge the
acid into the neck of the retort. After the rectification is com-
plete the retorts are allowed to cool for twelve hours, and the
acid is then drawn out by means of leaden syphons into the
stoneware coolers (i, Fig. 120).
In order to effect a continuous rectification in glass vessels.
the following arrangement has been adopted in some works.
Three of the retorts are placed one above the other, as is shown
ان نا
FIG. 120.
in Fig. 122. As soon as the acid in retort (B) has attained a
specific gravity of 1·84, the retort is connected with a system of
syphon tubes (ƒƒƒ), and acid of the specific gravity of 1·74, and
having a temperature of 150°, is allowed to run into the upper-
most retort (D) by means of the stopcock. This acid gradually
passes through the three retorts, D, C, and B, and when it has
reached the last one it has attained a specific gravity of 184,
and is allowed to run off through a cooling chamber (h) into
the carboy. Another recent and efficient system of continuous
concentration in glass vessels has been patented by Webb,'
and improved by Levinstein,2 by means of which acid containing
about 96 per cent. of H,SO, (sp. gr. 1-84) can be obtained in a
Eng. patent, 2,343 (1891); 17,407 and 18,891.
2 Eng. patent, 19,213 (1892).
I
394
THE NON-METALLIC ELEMENTS
single operation from acid of sp. gr. 1625, containing about
71 per cent. of H₂SO..
209 According to theory, 100 parts of sulphur burnt should
yield 306-25 parts of pure sulphuric acid. In practice, however,
this theoretical yield is never attained, and for several reasons;
in the first place because a certain amount of loss must neces-
sarily take place in working with such enormous volumes of
gas, and in the second place inasmuch as an unavoidable loss
occurs in the processes of concentration; and thirdly, owing to

DALOU
B
OHIB
FIG. 121.
0000
B
the fact that an amount of sulphur varying from 2 to 5 per cent.
remains behind in the burnt ore, and this amount cannot be
accurately allowed for. As a general rule a yield of 270 to 280
parts of pure acid from 100 of sulphur is practically considered
about the proper production, so that about 5 per cent. of sulphur
is lost on the average, of which, however, only a portion passes
out in the gaseous form into the air. In cases where special
precautions are taken the yield sometimes reaches from 294 to
297, but when the manufacture is not carefully conducted much
more serious losses occur. Thus in his eighth annual report
SULPHURIC ACID MANUFACTURE
395
h
(1871) Dr. R. Angus Smith gives (p. 17) a table, showing the
total escape of sulphur acids (calculated as sulphuric acid) from
twenty-three chemical works. From this it appears that whilst
from some of the works no escape of these acids occurs, the average
loss of sulphuric acid in the twenty-three works in question is
7-606 per cent. on the total quantity obtainable from the sulphur
burnt, and that the loss in the case of four works actually rises
to more than 20 per cent., in one case amounting to an escape of
159 lbs. of sulphuric acid every hour. Facts like these, says the

B
FIG. 122.
C
D
A
inspector, dispose of the argument often used by the manu-
facturers, that they require the acid, and that it is to their
interest to keep it, and of course condense it to the best of their
power. Indeed, certain makers are fully aware that they are
allowing sulphuric acid to escape in large quantities, but their
reply is that it is cheaper to permit a large escape and work
rapidly rather than have large chambers and condense the whole
of their gases.
By the Alkali Act of 1881 the limit of 4 grains per cubic foot
has been set to the amount of sulphuric anhydride which may
"
396
THE NON-METALLIC ELEMENTS
be left in the gases escaping from the chambers. The average
actually attained is 1.284 grains per cubic foot.¹
The amount, again, of nitrate of soda or Chili-saltpetre used,
varies considerably even in the best works, according to the rate
at which the reaction is permitted to proceed, and the complete-
ness and rapidity with which the nitrous fumes can be recovered
in the Gay-Lussac tower and again brought into the chamber.
Manufacturers who employ Glover and Gay-Lussac towers use on
an average 35 to 65 parts of nitrate for every 100 of sulphur
burnt, whilst at works where these appliances are not in use the
quantity of nitre required may rise to from 12 to 13 parts. The
larger the quantity of nitrous fumes present in the chamber, the
quicker will be the formation of sulphuric acid, and the proportion
of fumes which pays best is a question for the manufacturer in each
instance to decide. A certain loss of oxides of nitrogen cannot,
of course, be avoided; the fumes are partly not completely con-
densed, and pass out by the chimney, and partly, in all prob-
ability, reduced by the sulphur dioxide to nitrous oxide or even
to nitrogen, which, as they cannot combine again with the
atmospheric oxygen, must escape into the air.
In order to convert 100 parts of sulphur into sulphuric acid,
about 210 parts of water in the form of steam are needed. This
steam is costly in its production, but an attempt made by
Sprengel to reduce this item of expenditure by employing a
jet of water in the form of spray or in a state of very minute
division has proved unsuccessful.
210 None of these processes yield, it must be remembered,
chemically pure acid, inasmuch as, in the first place, the water
cannot thus be completely removed, and secondly, because in-
purities, such as sulphate of lead, arising from the action of the
acid on the leaden concentrating pans, and arsenic derived from the
pyrites, are not got rid of by this process of simple concentration.
In order to prepare pure sulphuric acid, the commercial
product must be distilled in a glass retort until one-third has
passed over; then the receiver is changed and the acid dis-
tilled nearly to dryness. It not unfrequently happens that in
this process the acid bumps violently on ebullition, owing
to a small quantity of solid lead sulphate being deposited
on the bottom of the retort; the addition of small pieces of
platinum foil or wire stops this to a certain extent, but a better
preventive is either to heat the retort at the sides rather than
1 Report of Inspector of Alkali Works, 1892, p. 8.
PROPERTIES OF SULPHURIC ACID
397
at the bottom, or, when the ebullition becomes percussive, to
allow the liquid to cool, then to pour off the clear acid, leaving
the deposit behind, and to proceed with the distillation of the
clarified liquid. A slow current of air passed through the
boiling acid has also been found efficacious.
211 Properties.-The acid thus purified by distillation still con-
tains about 15 per cent. of water which cannot be removed by
this process. If, however, the distillate be cooled, the pure acid
containing 100 per cent. of H2SO4, separates out in the form of
crystals which melt at 105°. These crystals when once melted
generally remain liquid for a considerable time, even when
cooled below their freezing point, the liquid only solidifying
when it is agitated or when a small crystal of the acid is added,
the temperature then rising to 105°. The specific gravity of the
pure liquid acid is 1·837 at 15° compared with water at 4° (or, as
it is usually expressed, 15°/4°), whilst according to Lunge and
Naef, it is 18384. When the pure acid is heated, it begins to
fume at 30° inasmuch as it then partially decomposes into
water and sulphur trioxide. This dissociation increases with
increase of temperature until at 338°, the boiling-point of the
liquid (Marignac), a large quantity of trioxide is volatilized, so
that the residue contains from 984 to 988 per cent. of the
real acid, and then this liquid may be distilled without altera-
tion. The vapour of sulphuric acid when it is more strongly
heated completely decomposes into water and the trioxide.
According to Deville and Troost the vapour density at 440°
is 25, whilst for equal volumes of aqueous vapour and sulphur
trioxide the calculated vapour density is
= 24.36.
17.88 + 79:46
4
When heated still more strongly, the trioxide thus formed itself
splits up into oxygen and sulphur dioxide. This decomposition
may be readily shown by allowing sulphuric acid to drop slowly
into the platinum flask (a, Fig. 123), which is filled with pumice-
stone and heated strongly by the lamp; the mixture of gases
which escapes consists of one volume of oxygen to two volumes
of sulphur dioxide, which latter gas is absorbed by passing
through water containing caustic soda, the oxygen escaping in
the free state, whilst any undecomposed sulphuric acid is con-
densed in the U-tube and collects in the flask (d). It has been
¹ Marignac, Ann. Chim. Phys. [3], 192; Mendelejeff, Ber. 17, 2536.
2 Chem. Industrie, 1883, 37.
•
398
THE NON-METALLIC ELEMENTS
proposed to use this process for the preparation of oxygen on
the large scale, as the material is cheap and the sulphur
dioxide can again be used for the manufacture of sulphuric acid.
212 When sulphuric acid is mixed with water a considerable
evolution of heat takes place and a contraction ensues. The
amount of heat which is evolved by mixing sulphuric acid and
water has been exactly determined by Thomsen 1; his results
are given in the following table in which the columns marked I.

b
I.
X
1
2
3
5
9.1
19.1
a
FIG. 123.
give the number of molecules of water, and II. the heat evolved
in calories:-
I.
·
II.
H.SO +x H,O,.
6336
9355
11294
13020
14851
16117
From the above numbers it is seen that the addition of the first
molecule produces an amount of heat represented by 6,336
thermal units, or about one-third of the total, whilst the
addition of two molecules of water gives off about one-half
1 Therm. Unter. III. 34.
X
49.3
99.7
2004
401.7
804-4
1609-7
•
II.
H₂SO, x H₂O.
16572
.
. 16744
.
. 16950
. 17196
. 17522
17737
PROPERTIES OF SULPHURIC ACID
399
the total quantity. The heat evolved by a further addition of
water is less for each successive molecule of water, but it has
been found impossible to determine the point at which no further
evolution of heat is caused by further dilution.
4
1
Hydrates of Sulphuric Acid.—When a mixture of equal mole-
cules of acid and water is cooled down, the mixture solidifies to
a mass of prismatic crystals, which possess the composition
H₂SO, + H₂O, and melt, according to Pierre and Puchot, at 7°5.
Another hydrate of the formula H₂SO, + 4H¸O, has been
isolated by Pickering from a solution of sulphuric acid in
water. It forms large, well-defined crystals and melts at -25°.
This substance is characterised as a definite chemical compound
by the facts that it melts and freezes at a definite temperature,
and that its freezing point is lowered by the addition of either
of its components. The existence of many other hydrates of the
acid has been surmised from the properties of its solution in
water, but no others have as yet been isolated.
213 Sulphuric acid is largely used in the laboratory not only
for the preparation of most of the other acids, but also in con-
sequence of its powerful hygroscopic properties for the purpose
of drying gases. To effect this, the gas is best led through
tubes filled with fragments of pumice-stone which have been
boiled in strong sulphuric acid. The acid is also employed to
dry solid bodies, or to concentrate liquids, especially in cases
where the application of a high temperature is likely to pro-
duce a decomposition of the substance, the bodies to be dried
being placed over a vessel containing the acid in a closed space
or in a vacuum.
Sulphuric acid when concentrated does not act in the cold
upon many of the metals, although it does so in some cases when
heated. Thus copper, mercury, antimony, bismuth, tin, lead, and
silver are attacked by the hot acid, with evolution of sulphur
dioxide, but are not acted on by the cold dilute acid; thus:-
2Ag + 2H2SO4 = Ag₂SO4 + SO2 + 2H₂O.
Gold, platinum, iridium, and rhodium are unacted upon, even by
boiling sulphuric acid, and this acid is, therefore, employed in
the separation of silver and gold. The more easily oxidizable
metals, such as zinc, iron, cobalt, manganese, are dissolved by
the dilute acid with evolution of hydrogen and formation of a
sulphate, but also act upon the hot concentrated acid with
production of sulphur dioxide.
1 Journ. Chem. Soc. 1890, i., 1339.
400
THE NON-METALLIC ELEMENTS
Many organic bodies are decomposed by sulphuric acid, which
abstracts from them the elements of water. Thus, for instance,
oxalic acid, C₂H2O4, by heating with strong sulphuric acid is
decomposed into carbon dioxide, CO,, carbon monoxide, CO,
and water, H₂O; and alcohol, C,H,O, is transformed by means
of this acid into ethylene gas, C,H,, and water, H.O. Wood,
sugar, and other substances are blackened by sulphuric acid,
this body withdrawing from them the hydrogen and the oxygen
which they contain with production of water.
6
214 The Sulphates.-The salts of sulphuric acid are termed
sulphates, and as this acid is dibasic, like sulphurous acid, two
series of sulphates exist, viz., the normal salts, such as Na,SO
and CaSO,, and the acid salts such as NaHSO,.
Many sulphates occur native, existing as well-known and
important minerals; such are:-gypsum, CaSO4 + 2H₂O; heavy
spar, BaSO4; celestine, SrSO,; Glauber's salt, Na,SO + 10H¸0;
and Epsom salts, MgSO4 + 7H₂O.
Most of the sulphates are soluble in water, and crystallize
well, and these can be readily prepared by dissolving the metal
in dilute sulphuric acid, or the oxide or carbonate if the metal
does not readily dissolve. Some few sulphates, viz., calcium
sulphate and the sulphates of lead and strontium, are only
very slightly soluble, whilst barium sulphate is insoluble in
both water and dilute acids. This fact is made use of for the
detection of sulphuric acid. A soluble barium salt, usually
the chloride, is added to the solution supposed to contain a sul-
phate; if sulphuric acid be present, a heavy white precipitate
of barium sulphate, BaSO,, falls down, which is insoluble in
dilute hydrochloric acid. In order to detect free sulphuric acid,
together with sulphates, as for instance in vinegar, which is
sometimes adulterated with oil of vitriol, the liquid must be
evaporated on a water-bath with a small quantity of sugar. If
free sulphuric acid is present a black residue is obtained.
Free sulphuric acid is found in the water of certain volcanic
districts. It has already been mentioned that sulphur dioxide
occurs in volcanic gases, and these when dissolved in water
gradually absorb oxygen from the air and pass into sulphuric
acid. The Rio Vinagre in South America, which is fed from
volcanic springs and receives its name on the account of
the acid taste of the water, contains free sulphuric acid. A
singular occurrence of free sulphuric acid has been noticed in
the salivary glands of certain mollusca; thus, according to
SULPHURIC ACID
401
Degree
Twaddell
Bödeker and Troschel, those of the Dolium galca contain
about 2:47 per cent.
215 The following table by Lunge, Isler and Naef¹ exhibits the
percentage of real acid, H₂SO, contained in aqueous sulphuric
acid of varying specific gravities.
0
3
6
9
12
15
18
21
24
27
30
33
36
39
42
45
48
51
54
57
60
63
66
69
72
75
78
81
84
87
90
93
96
Specific
gravity
15°/4° in
vacuo
1.000
1.015
1:030
1.045
1.060
1.075
1 090
1.105
1.120
1.135
1.150
1.165
1.180
1.195
1.210
1.225
1.240
1.255
1.270
1.285
1:300
1315
1:330
1.345
1.360
1.375
1.390
1:405
1.420
1.435
1.450
1.465
1.480
Percentage
of H2SO4
0:09
2:30
4:49
6.67
8.77
10.90
12.99
15.03
17.01
18.96
20.91
22.83
24.76
26.68
28.58
30.48
32.28
34:00
35.71
37.45
39.19
40.93
42.66
44 28
45.88
47.47
49.06
50.63
52.15
53.59
55.03
56.43
57.83
Degree
Twaddell
99
102
105
108
111
114
117
120
123
126
129
132
135
138
141
144
147
150
153
156
159
162
165
166
167
168
Specific
gravity
15°/4° in
vacuo
1:495
1.510
1.525
1.540
1.555
1.570
1.585
1.600
1.615
1.630
1.645
1.660
1.675
1.690
1 Sulphuric Acid and Alkali, vol. i. p. 119 (1891).
Percentage
of H2SO4
59.22
60.65
62.06
63.43
64.67
65.90
67.13
68.51
69.89
71.16
72:40
73.64
74.97
76:30
77.60
78.92
80.24
81.56
1.705
1720
1.735
1.750
1.765
82.88
1.780
84.50
1.795
86:30
1.810
88.30
1.825
91.00
1.830
92.10
1.835
93.43
1.840
95.60
97.00
1.8410
1.8415 97.70
1.8410 98.20
1.8400 99.20
1.8390 99.70
1.8384
100.00
1
1
27
402
THE NON-METALLIC ELEMENTS
FUMING SULPHURIC ACID.
216 This substance, which is a solution of varying quantities
of sulphur trioxide in sulphuric acid, was known before the
sulphuric acid manufactured from sulphur, being termed Nord-
hausen sulphuric acid, from the fact that it was prepared at
Nordhausen in the Hartz, by heating roasted green vitriol.
Preparation. (1) When green vitriol or ferrous sulphate,
FeSO4+7H,O, is roasted in the air it loses water and becomes
oxidized to a basic ferric sulphate, Fe,S,O,, which is then

n
b
m
d
e
FIG. 124.
=
m
further heated in clay retorts, as shown in Fig. 124, when the
following decomposition takes place :-
FeS209
2SO3 + FeO3.
The sulphur trioxide thus formed, partly combines with the
water which is still present to form sulphuric acid, whilst
the other portion of the trioxide dissolves in the sulphuric
acid thus produced.
Fuming sulphuric acid is now almost entirely prepared in
Bohemia in the works of J. D. Starck. The solution of green
vitriol obtained by the oxidation of the pyrites is evaporated
down, and the residue ignited, care being taken that the "vitriol-
stone" thus obtained is as free as possible from ferrous sulphate,
inasmuch as if this body be present sulphur dioxide is formed
—
FUMING SULPHURIC ACID
403
in the subsequent distillation, and this carries away with it large
quantities of the easily volatile trioxide; thus:-
2FeSO4 = FeO3 + SO3 + SO2.
The more completely the vitriol-stone is oxidized, the larger
is the yield of fuming acid, which on an average amounts to
from 34 to 50 per cent. In some works the green vitriol is
allowed to crystallize out, and the oxidized mother liquors alone
used for the production of the fuming acid. Fuming sulphuric
acid was formerly chiefly used in the arts for dissolving indigo;
at present, however, it is largely employed in the preparation of
the soluble sulphonic acids of colouring.matters, and in the manu-
facture of artificial alizarin. For this purpose an acid is needed.
which contains more trioxide than the commercial substance, and
hence it is prepared specially by the manufacturers themselves
by heating the fuming acid in cast-iron retorts and absorbing
the trioxicle, which is given off, in another portion of the acid
in well-closed receivers.
(2) Sulphur trioxide is formed, as has been already stated
(p. 374), when a mixture of oxygen and sulphur dioxide is
passed over heated platinum sponge. It has been proposed to
employ this reaction for the preparation of common sulphuric
acid on the large scale, the sulphur dioxide being obtained by
the combustion of sulphur, or the roasting of pyrites, and this
together with air passed over heated platinized asbestos, the
fumes of the trioxide being collected in water. It was found
that this process cannot be practically carried out, inasmuch as
the platinum soon loses this peculiar property, probably owing
to the fact that dirt and particles of dust collect on the surface
of the metal. For the production however of a strongly fuming
acid, the following process, according to Winkler,' answers well.
Common sulphuric acid, as has been shown, decomposes on
heating, into aqueous vapour, sulphur dioxide, and oxygen. If
the mixture of gases be washed by sulphuric acid in order to
remove the water and particles of dust, and then the mixture
of dioxide and oxygen passed over heated platinized asbestos
the trioxide is formed and may be collected in sulphuric acid.
Properties.-Fuming sulphuric acid is a colourless, thick, oily
liquid when pure, but is generally coloured slightly brown from
the presence of organic matter. It has a specific gravity of from
186 to 1'89, and evolves on exposure to the air dense white
1 ¹ Dingl. Polyt. Journ. 218, 128.
404
THE NON-METALLIC ELEMENTS
fumes, inasmuch as the volatile trioxide escapes and combines
with the aqueous vapour of the air to form sulphuric acid.
When the fuming acid is cooled, white crystals of the com-
pound, H2SO4 + SO2, separate out. This substance melts, ac-
cording to Marignac, at 35°, fumes strongly in the air, and
decomposes easily on heating into its constituents. The name
disulphuric acid or pyrosulphuric acid, H₂SO,, has been given
to this substance as it forms a series of very stable salts; thus
sodium disulphate, Na,S,O,, is obtained by heating the acid
sodium sulphate, HNASO,, so long as water is given off;
thus:-
2 SO₂ {ONa
2
OH
=
ONa
SO2
O
SO, ONa
When still more strongly heated this salt decomposes into the
normal sulphate and sulphur trioxide.
2SO,
Sulphur trioxide and sulphuric acid also unite together to
form tetra-sulphuric acid, 3 SO, + H₂SO, H₂SO13, an oily
liquid, and another compound, having the composition
SO3+3H2SO4, a transparent crystalline mass melting at 26°.
+ H₂O.
CHLORIDES AND BROMIDES OF SULPHURIC
ACID.
JOH
OH
CHLOROSULPHONIC ACID, OR SULPHURYLHYDROXYCHLORIDE.
SO
S CI
OH
=
217 Williamson first obtained this substance by the direct
union of hydrochloric acid and sulphur trioxide.¹
It is, however, best prepared by the distillation of a mixture
of concentrated sulphuric acid and phosphorus oxychloride,
thus:-
= 115.65.
+ POCI₂ = 2 SO₂
JOH
7 Cl
+ HPO3 + HCl.
Hence it is seen that chlorosulphonic acid may be considered to
be sulphuric acid, in which the group hydroxyl, OH, is replaced
by chlorine. It is a colourless liquid, fuming strongly in the
1 Proc. Roy. Soc. 7, 11.
SULPHURYL CHLORIDE
405
.
air, having a specific gravity of 1766 at 18°, and boiling at
158°.
Its vapour decomposes on heating, and at 216° its
vapour density is found to be 32.8, or the dissociation is nearly
perfect.
This decomposition is probably represented by the equation : 1
2C1. SO,H SO₂+ Cl2 + H2O + SO3.
2
SO₂ { CI
3
When thrown into water it decomposes with explosive
violence, forming hydrochloric and sulphuric acids, and when
added to strong sulphuric acid, disulphuric acid and hydrochloric
acid are formed, thus :-
+
OH
=
2
(CI
Cl
=
OH SO₂
OH
SULPHURYL CHLORIDE. So, {CL
Cl.
SOH
O
SO, {OH
=
218 This body was first obtained, mixed with ethylene di-
chloride by Regnault in the year 1838 by the action of chlorine
upon a mixture of olefiant gas and sulphur dioxide.2 Sulphuryl
chloride is also formed when a solution of the two gases in
anhydrous acetic acid is allowed to stand. It is, however,
most readily obtained by heating chlorosulphonic acid in closed
tubes at a
temperature of 180° for 12 hours; 3 thus:-
250, { CI
SO2
So₂ {C+SO₂ {
CI
Cl
2
+ 2H₂O
=
+ HCL.
or by saturating camphor with sulphur dioxide and then passing
in chlorine, the camphor remaining unaltered.*
Sulphuryl chloride is a colourless liquid boiling at 70°, possess-
ing a strongly pungent odour, fuming strongly in the air and
having a specific gravity of 1·659 at 20°; it decomposes in presence
of a small quantity of water into chlorosulphonic acid and hydro-
chloric acid, and with an excess of water into sulphuric acid
and hydrochloric acid; thus:-
OH
OH
SO₂
OH.
OH.
=
SO2
Herrmann and Köchlin, Ber. 13, 604. 2 Ann. Chim. Phys. (2) 69, 170.
Behrend, Ber. 8,
4 Schulze, J. Pr. Chem. (2) 23, 351.
1004.
- 133.96.
+ 2HCl.
406
THE NON-METALLIC ELEMENTS
The vapour density of sulphuryl chloride is normal at 100°,
but falls to one half of this at 442, at which temperature it
is completely dissociated into sulphur dioxide and chlorine.¹
The affinity between the sulphur dioxide and chlorine is very
weak, and the latter is very readily withdrawn by any com-
pound having an attraction for chlorine; sulphuryl chloride is,
therefore, frequently used as a chlorinating agent in organic
chemistry 2
DISULPHURYL CHLORIDE. SO,Cl₂ = 213·42.
219 The chloride of disulphuric acid was first prepared by
Rose by the action of chloride of sulphur on sulphur trioxide,
thus:-
:-
3
S,Cl₂+ 5SOS₂O,Cl₂+5SO2.
The same compound has also been obtained by Michaelis by
heating sulphur trioxide with phosphorus oxychloride; thus:—
6SO3 + 2POCI¸=3S₂O¿Cl½ + P₂O5.
It is likewise formed when common salt is heated with
sulphur trioxide, and when this latter substance is brought in
contact with sulphuryl chloride; thus :—
Cl
SO3+SO₂ { CI
=
Cl.
0.
so₂ { cl.
1 Herrmann and Köchlin, Ber. 16, 602.
2 Armstrong, Proc. Chem. Soc., 1891, 60.
4 Zeitsch. Chem. (2) 7, 149.
SO2
It is colourless fuming liquid, boiling at 146°, and having a
specific gravity at 18° of 1819. Water decomposes it into
sulphuric acid and hydrochloric acid.
SULPHUR OXYTETRACHLORIDE. SO,Cl=252:04.
220 Millon first obtained this substance by the action of moist
chlorine upon sulphur or chloride of sulphur. It is best pre-
pared by cooling down a mixture of chloride of sulphur and
chlorosulphonic acid to -15°, and then saturating the liquid
with chlorine, when the following decomposition takes place :-
SO₂HCI+SC₁₁ = S₂O₂Cl + HCl.
3 Pogg. Ann. 44, 291.
SULPHUR SESQUIOXIDE
407
Sulphur oxytetrachloride forms a white crystalline mass, which
has a very pungent smell and attacks the mucous membrane
violently. It dissolves in water with a hissing noise, forming
hydrochloric, sulphuric, and sulphurous acids. Exposed to
moist air, it deliquesces with evolution of chlorine, hydrochloric
acid, and sulphur dioxide, leaving a residue of thionyl chloride
and disulphuryl chloride. When the compound is heated it
partially sublimes in fine white needles, whilst another portion.
decomposes into sulphur dioxide, chlorine, thionyl chloride, and
disulphuryl chloride. When kept in closed tubes this body
liquefies with the formation of thionyl and sulphuryl chlorides;
thus:-
S₂OCI = SOCI½ + SO₂CI¿.
An oxy-chloride, SOCI, is formed as a red liquid, boiling at
60°, by the action of chloride of sulphur on sulphuryl chloride.¹
Br.
SULPHURYL BROMIDE. SO,{
Br.
= 222.30.
221 Bromine combines with sulphur dioxide, forming a
volatile white solid crystalline mass, which when acted upon
with silver oxide forms sulphur trioxide; 2 thus:-
SO,Br₂+ Ag₂O = SO3 + 2AgBr.
Some doubt has been thrown on the existence of this com-
pound by later observers.³
SULPHUR SESQUIOXIDE. S₂O=111·28.
3
222 So long ago as the year 1804 Buchholz found that when
sulphur is heated with fuming sulphuric acid an intensely blue-
coloured solution is formed, and in the year 1812 F. C. Vogel
showed that this blue body is also produced by the action of
sulphur
on sulphur trioxide. In later years this subject has
frequently attracted the attention of chemists, but the nature of
the blue substance remained unexplained until R. Weber
showed that it consists of a new oxide of sulphur.
sulphur
In order to prepare this substance, carefully dried flowers of
are added, in small quantities, to recently prepared and
liquid sulphur trioxide, a fresh quantity of sulphur only being
added
when that already present has entered into combination.
10gier, Compt. Rend., 94, 446; Bull. Soc. Chem. 37, 293.
Odling, Journ. Chem. Soc. 1855, 2.
Michaelis, Lehrbuch, i., 733 (5th Edition).
4 Pogg. Ann. 156, 531.
4
C
408
THE NON-METALLIC ELEMENTS
In order to moderate the reaction, the test-tube in which the
solution is made must be placed in water at a temperature of
from 12° to 15°. The sulphur on falling into the trioxide dis-
solves in the form of blue drops which sink down to the bottom
of the test-tube and then solidify. As soon as a sufficient
quantity of this substance has been formed, the supernatant
sulphur trioxide is poured off and the residue removed from the
test-tube by very gently warming it.
Sulphur sesquioxide forms bluish-green crystalline crusts, in
colour closely resembling malachite. At the ordinary temperature
it slowly decomposes into sulphur dioxide and free sulphur, and
this decomposition takes place more readily when the substance
is warmed; thus:-
2S,O= 3SO₂+ S.
3
2
This compound dissolves in fuming sulphuric acid, giving rise to
a blue solution which on the addition of common sulphuric acid
gradually changes to a brown. Water decomposes the sesqui-
oxide with the separation of sulphur and the formation of
sulphuric acid, sulphurous acid, and thiosulphuric acid.
HYPOSULPHUROUS ACID. H.SO.
223 This compound was discovered by Schützenberger¹ who
gave it the name of hydrosulphurous acid and assigned to it the
formula H₂SO, its sodium salt being formulated as NaHSO,.
Bernthsen, however, subsequently showed that the simplest
formula of the sodium salt is NaSO2, the molecular formula
being probably double this, Na,SO, and the free acid H₂SO.
The zinc salt of the acid is obtained by the action of metallic
zinc upon an aqueous solution of sulphur dioxide contained in
a closed vessel; no evolution of hydrogen takes place, but the
zinc dissolves and a salt of hyposulphurous acid is formed,
the reaction being the following (Bernthsen):—
Zn + 2SO₂
ZnS₂O4.
=
The liquid thus obtained, which was looked upon by
Schützenberger as the free acid, possesses powerful reducing
properties. It bleaches organic colouring matters more quickly
than sulphurous acid and precipitates the metals silver and
2 Annalen, 208, 142; 211, 285.
1 Compt. Rend. 69, 169.
HYPOSULPHUROUS ACID
409
mercury from solutions of their soluble salts. None of the salts
have been prepared pure, but a solution of the sodium salt may
be obtained by the action of zinc on a solution of acid sodium
sulphite, the liquid, which is contained in a well closed bottle,
being kept well cooled by cold water. The following reaction
takes place (Bernthsen) :-
4NaHSO₂+ Zn =
3
-
ZnSO3 + Na2SO3 + H2O + Na2SO4.
A portion of the zinc sulphite is converted into a basic salt and
sulphur dioxide thus set free, which is then reduced in its turn
by the excess of zinc present. The greater portion of the
sodium sulphite crystallizes out along with the sulphite and
basic sulphite of zinc, but some still remains in solution. In
order to remove this, the mother liquor containing the hypo-
sulphite is poured off into a flask and three or four times its
bulk of strong alcohol added; the alcoholic solution, on standing
well corked, deposits a second crop of crystals of zinc-sodium
sulphite, and the supernatant liquid on again being poured off
into a well-stoppered flask, crystallizes into a mass of colourless
crystals of the hyposulphite which only needs to be pressed
between blotting paper or between a cloth and dried in a
vacuum. These crystals however only contain about 40 per cent.
of sodium hyposulphite, much of the sulphur being present as
sulphate, sulphite and thiosulphate. (Bernthsen.)
Sodium hyposulphite is also formed when a current of
electricity is passed through a solution of acid sodium sulphite,
the hydrogen, which is evolved at the negative pole, reducing
the salt. Sodium hyposulphite is employed by the dyer and
calico-printer for the reduction of indigo, as it possesses the
same reducing properties as the free acid. In the moist state
or in solution when exposed to the air it absorbs oxygen quickly
and changes at once into sodium metabisulphite.
Na₂S₂O₁ + 0 = Na,S.O
The aqueous solution decomposes, even when not exposed to
the air, with the formation of sodium thiosulphate. In order
to prepare hyposulphurous acid, a dilute solution of oxalic acid is
added to a solution of a hyposulphite; a yellow liquid is then
obtained which soon decomposes, thiosulphuric acid being
formed, and this being very unstable decomposes into sulphur
and sulphur dioxide.
N
H
+
1
1
1
1
1
410
THE NON-METALLIC ELEMENTS
The ease with which hyposulphurous acid undergoes decompo-
sition accounts for the fact that the existence of this compound
was so long overlooked. Berthollet showed so long ago as 1789
that iron dissolves in aqueous sulphurous acid without any
evolution of gas, and Fourcroy and Vauquelin found in 1798
that zinc and tin also act in a similar manner. That a lower
product of oxidation of sulphur is thus formed was even then
known, but up to the time of Schützenberger's discovery it was
supposed that the product of the reaction was thiosulphuric
acid.
224 According to Schützenberger's formula the acid may be
looked upon as derived from the unknown anhydride SO,
H₂SO, H₂O + SO;
=
whilst, if Bernthsen's view be correct, it must correspond to the
anhydride S,O3;
H₂S₂O
Bernthsen has succeeded in showing that for every two atoms
of sulphur in the form of hyposulphite, one atom of oxygen is
required to oxidise the substance to a sulphite, which may be
effected by means of an ammoniacal solution of copper sulphate,
and three atoms of oxygen to oxidize it to a sulphate, a reaction
which may be realised by the use of iodine solution. Both of
these results are in agreement only with the formula H₂SO, as
will be seen from a comparison of the following equations :-
=
H₂O + S₂O3.
2S02
2SO + 20 = 280,
(1) S₂O3 + 0 =
(2) S₂O + 30 = 2SO3
SO + 40 =2SO
SULPHUR HEPTOXIDE, SO, AND PERSULPHURIC ACID, H₂SO..
225 When a mixture of dry oxygen and sulphur dioxide is
subjected to the silent electrical discharge, combination takes
place and sulphur heptoxide is produced.¹
A substance of the same composition is formed at the positive
pole during the electrolysis of sulphuric acid of 40 per cent. and
1 Berthelot, Ann. Chim. Phys. (5) 14, 345, 363.
PERSULPHURIC ACID
411
also when hydrogen peroxide is mixed with concentrated
sulphuric acid.
The anhydrous substance is a viscid liquid which solidifies at
0° forming granules, needles or scales. It is readily volatile and
gradually decomposes on preservation, rapidly on heating, into
sulphur trioxide and oxygen.
―――――
When brought into water, it decomposes with evolution of
oxygen, according to the equation:-
4H2O + 2S2O, = 4H2SO4 + 02.
The corresponding acid has not been isolated, but its salts have
been prepared by Marshall¹ and also studied by Berthelot.2
The potassium salt, K₂SO̟¸, may be obtained by passing a
current of 3 to 3 ampères through a saturated solution of
potassium hydrogen sulphate. The solution is placed in a
platinum basin, cooled externally by a current of water, and
connected with the positive pole of the battery; the negative
pole consists of a platinum wire and is placed in a porous cell
filled with dilute sulphuric acid and suspended in the solution.
of potassium hydrogen sulphate. The potassium salt separates
out in the course of 24 to 48 hours as a mass of white crystals.
These may be recrystallised by dissolving in warm water and
allowing to cool, and then form large, tabular, apparently
asymmetric crystals. One hundred parts of water at 0° dissolve
1.77 parts of the salt, the solution being neutral to test paper.
On preservation decomposition commences after some time and
becomes rapid when the solution is heated, oxygen being evolved
and potassium sulphate formed. When the dry salt is heated
it loses oxygen and sulphur trioxide and is converted into the
sulphate:-
2K₂S,O=2K₂SO4+2SO3+02.
8
The molecular formula of the salt 3 is found, from a determina-
tion of the electrical conductivity of its dilute aqueous solution,
to be most probably K₂SO, and not KSO. The solution of
potassium persulphate has strongly oxidising properties; thus
ferrous sulphate is rapidly converted into the ferric salt, solutions
of silver, copper, manganese, cobalt and nickel salts, all yield
precipitates of the higher oxides on warming, hydrochloric
2 Ann. Chim. Phys. [6] 26, 526.
1 Journ. Chem. Soc. 1891, i. 771.
3 Löwenherz, Chem. Zeit., 16, 838.
1
412
THE NON-METALLIC ELEMENTS
acid evolves chlorine, potassium iodide is decomposed with
liberation of iodine, and alcohol is converted into aldehyde.
The ammonium salt, (NH)¿SO, is prepared in a similar
manner to the potassium salt; it forms lozenge-shaped, ap-
parently monosymmetric tablets, and is much more soluble than
the potassium salt, 100 parts of water at 0° dissolving 582
parts. The barium salt, BaS¸Î¸+4H¸0, is freely soluble in
water, whilst the lead salt, PbS,O,+3H,O, is deliquescent.
8
All the persulphates appear to be soluble in water, the
potassium salt being less soluble than any other which has yet
been examined.
2
THIOSULPHURIC ACID. H₂SO3.
226 This compound is better known under its old name of
"hyposulphurous acid," with which name however, we now
designate the body obtained by the reduction of sulphurous
acid. Thiosulphuric acid has not been prepared in the free
state, but it forms a series of stable salts which are known as
the thiosulphates (hyposulphites). When dilute sulphuric acid
is added to a solution of a thiosulphate, the solution remains, to
begin with, perfectly clear; but sulphur soon begins to separate
out as a white, very finely divided powder, and the solution
is found to contain sulphurous acid, sulphur dioxide being also
evolved; the thiosulphuric acid undergoes on liberation the
following decomposition:-
H₂S₂O3 = H₂O + SO₂ + S.
2
This decomposition is quantitative when the sulphur dioxide
is removed from the solution, but when it is retained, a limit
short of complete decomposition is attained, and other products,
among which is pentathionic acid, are formed.¹
Thiosulphuric acid is formed in small amount by the action
of sulphurous acid upon flowers of sulphur at the ordinary tem-
perature, more rapidly at 80-90°.2
The thiosulphates are formed in various ways: thus, for
instance, sodium thiosulphate (formerly called hyposulphite of
soda), which was first prepared by Chaussier in 1799, but after-
1 Colefax, Journ. Chem. Soc. 1892, i. 176.
2 Journ. Chem. Soc. 1892, i. 199.
1
THIOSULPHURIC ACID
413
wards more carefully examined by Vauquelin, is formed when
sulphur dioxide is passed into a solution of sodium sulphide;
thus:-
(a) SO2 + H2O + Na,S= Na,SO3 + SH2.
(b) SO, +2SH, 2H,0 + 3S.
=
(e) Na₂SO3 + S= Na₂S₂O3.
The same salt is also formed according to equation (c) when a
solution of sodium sulphite is boiled with flowers of sulphur
and this is the method by which the salt is usually prepared
(Vauquelin). Sodium thiosulphate is also formed when iodine
is added to a solution of sodium sulphite and sodium sulphide;
thus: 1-
Na,SO3 + NaS+I, Na,S,O,+2NaI.
Thiosulphates are also produced by the oxidation of sulphides,
either by the oxygen of the air or by means of suitable
oxidising agents.
2
These various methods of preparation point out that thiosul-
phuric acid is formed by the addition of sulphur to sulphurous
acid, just as sulphuric acid is formed by the addition of oxygen
to the same substance. Thiosulphuric acid may, therefore, be
regarded as sulphuric acid in which one atom of oxygen is
JOH.
replaced by sulphur, and its formula is accordingly SO SH.
The decompositions which its salts undergo bear out this inter-
pretation of its composition. Thus the decomposition of the
free acid into sulphur and sulphurous acid has already been
mentioned; and when a solution of sodium thiosulphate is
treated with sodium amalgam, sodium sulphite and sodium
sulphide are formed (Spring); thus:-
Na2S2O3 + 2Na Na₂SO3 + Na,S.
=
SO,
OAg
{
1 SAg
=
Again, when silver thiosulphate is warmed with water, black
sulphide of silver separates out, and the solution contains free
sulphuric acid; thus:—
+H,O= Ag₂S+SO₂ { OH
And, moreover, when a solution of sodium thiosulphate is treated
1 Spring, Ber. 7, 1157.
2 Schorlemmer, Journ. Chem. Soc. 1869, 256.
414
THE NON-METALLIC ELEMENTS
with cobalt chloride, black sulphide of cobalt is precipitated;
thus:-
SO2 +CoCl₂+H₂O = SO₂ {OH
(ONa
SNa
This formula is also borne out by the fact that the isomeric
sodium potassium sulphites (p. 372) are converted by the action
of ammonium polysulphides into isomeric thiosulphates, which
differ in solubility and specific gravity, and yield different
products when acted upon by ethyl iodide.¹
Sulphite.
OK
SO
SO,
Na
ONa
K
Nal
Na f
Thiosulphate.
OK
SO.
2
+CoS+2NaCl.
SO,
SNa
ONa
SK
The soluble thiosulphates generally crystallise well, and contain
water of crystallisation, the last molecule of which is very
difficult to remove by heat, being generally lost at such a high
temperature that the decomposition of the salt has already com-
menced. Hence it was formerly supposed that the thiosulphates
all contained hydrogen.
The thiosulphates exhibit a great tendency to form double
salts; those of the thiosulphates insoluble in water are found
to dissolve in an aqueous solution of sodium thiosulphate,
which also has the power of dissolving other insoluble salts,
such as silver chloride, silver bromide, silver iodide, lead iodide,
lead sulphate, calcium sulphate, &c; thus:-
Na
S₂O₂+ AgCl == SO3+ NaCl.
Ag f
3
Sodium silver thiosulphate forms distinct crystals, having the
composition AgNaS,O,+H,O, and these are distinguished
by possessing a sweet taste. The use of sodium thiosulphate
for fixing prints in photography, first suggested by Sir John
Herschel, probably depends on the formation of this salt. The
silver chloride with which the photographic paper is impregnated
when exposed to the light becomes blackened, the chloride
1 Schwicker, Ber. 22, 1733.
DITHIONIC ACID
415
undergoing a chemical change, after which it is insoluble in
sodium thiosulphate. In order, therefore, to fix such a photo-
graphic print, it is only necessary, after exposure to light, to soak
the paper in a bath of the thiosulphate; the unaltered chloride
of silver dissolves, and the picture, on washing, is found to be
permanent.
Several other complex thiosulphates of silver and sodium are
also known.
The thiosulphates are distinguished from the sulphites,
inasmuch as when dilute hydrochloric acid or sulphuric acid
is added to their solution not only is sulphur dioxide given
off as a gas, but free sulphur is deposited as a white powder.
On adding a thiosulphate solution to a silver or lead salt, a
white precipitate soluble in excess of the thiosulphate solution
is produced, whilst with a mercuric salt, a white precipitate of
the insoluble thiosulphate is first thrown down, but this quickly
becomes dark, and finally black, from its decomposition into
a metallic sulphide and sulphuric acid. Solutions of mercurous
salts give, with the thiosulphates, dense black precipitates, and
when a thiosulphate is boiled with an ammoniacal solution of
a ruthenium salt, the solution becomes of such an intensely
dark red colour that in the concentrated condition it appears
almost black. Ferric chloride added to a solution of a thio-
sulphate gives a dark violet coloration, which soon disappears,
sulphur being deposited.
DITHIONIC ACID. H2S2O6.
227 This acid, formerly called hyposulphuric acid, was dis-
covered by Welter and Gay-Lussac in 1819. The manganese salt
of the acid is prepared by passing sulphur dioxide into water
containing manganese dioxide in suspension; thus:—
2 SO₂+ MnO2 = MnS₂O6.
2
At the same time a portion of the manganese is converted into
manganese sulphate; thus:-
--
SO₂+MnO2
==
MnSO4
In order to obtain the dithionate free from sulphate, advantage
is taken of the solubility of barium dithionate in water; baryta
416
THE NON-METALLIC ELEMENTS
6
water, Ba (OH)2, is added until all the metal is precipitated as
manganese hydroxide, Mn (OH)2, and all the sulphuric acid is
thrown down as insoluble barium sulphate, BaSO4; on evapora-
ting and cooling, barium dithionate, BaSO̟+2H₂O, crystallises
out, and when this is decomposed by the requisite quantity of
dilute sulphuric acid, a solution of dithionic acid is obtained.
This solution may be concentrated in vacuo over sulphuric acid
until it has a specific gravity of 1347, but on attempting to
concentrate it further, the acid is resolved into sulphur dioxide.
and sulphuric acid; thus:-
H₂S₂O = SO₂+ H₂SO
In
Dithionic acid is also formed in small quantity by the oxidation
of sulphurous acid by means of potassium permanganate.¹
order to obtain the salts of this acid we may add the cor-
responding base to the acid solution, or they may be obtained
more simply by adding a soluble sulphate to barium dithionate.
Only the normal salts are known, most of which crystallise
well, and they contain, with the exception of the potassium
salt, water of crystallisation. Their aqueous solutions are not
oxidised in the cold either by atmospheric oxygen, by nitric
acid, or by potassium permanganate; though when they are
heated with these oxidising agents they are converted into
the sulphates. On heating they decompose partially at 100°,
and entirely at a higher temperature, into sulphur dioxide,
and a sulphate which remains behind; when sodium amalgam
is added to a solution of sodium dithionate, two molecules
of sodium sulphite are formed (R. Otto); thus :—
Na.SO.ONA.
SO, ONa
1
SOONa
+ 2Na
=
Na.SO2.ONA.
The dithionates are distinguished from the thiosulphates, inas-
much as they evolve sulphurous acid when heated with hydro-
chloric acid without separation of sulphur, whilst the solution
afterwards contains a sulphate.
TRITHIONIC ACID. H,SO
228 In 1842, Langlois obtained the potassium salt of this acid
1 Ann. Chim. Phys. (3) 55, 374.
TRITHIONIC ACID
417
by gently heating a solution of acid potassium sulphite with
sulphur; thus:-
S₂+ 6KHSO₂ = 2K2S3O6 + K2S2O3 + 3H₂O.
The same salt is also produced when a solution of potassium
thiosulphate is saturated with sulphur dioxide; thus :-
3SO₂+ 2K₂S₂O₂ = 2K₂S3O6 + S.
2K2S2O3
Further changes also occur, the trithionate combining with
the sulphur which is liberated to form tetra- and pentathionate
of potassium. (Debus.) The potassium salt is moreover formed
when potassium silver thiosulphate is heated with water;
thus:-
(OK
21 SAg
2S02
Ag₂S+ K2S3O6.
When iodine is added to a solution of sodium thiosulphate and
sodium sulphite, sodium trithionate is likewise formed (Spring);
thus:-
=
Na,SO+Na,SO3 + I2 = Na,SO + 2NaI;
6
whilst a solution of the trithionate treated with sodium
amalgam or caustic potash, decomposes again into sulphite and
thiosulphate.
In order to prepare the free trithionic acid, hydro-fluosilicic
acid is added to a solution of the potassium salt, when the
insoluble hydro-fluosilicate of potassium is precipitated. The
aqueous acid thus obtained has no smell, but has a strong acid
and bitter taste; it may be concentrated in a vacuum up to
a certain point, but it is an unstable compound, and at the
ordinary temperature easily decomposes even in dilute solution
into sulphur, sulphur dioxide, and sulphuric acid. The only one
of the trithionates which is well known is the potassium salt.
This on heating decomposes into sulphur, sulphur dioxide and
potassium sulphate and its solution undergoes the same decom-
position on standing :-
K₂SO = K2SO4 + SO2 + S.
In this case again secondary reactions occur, tetra- ana penta-
thionates being formed by the combination of the trithionate
with the sulphur. Its solution is not precipitated by barium
28
418
THE NON-METALLIC ELEMENTS
chloride in the cold, though on heating barium sulphate
separates out and sulphur dioxide is evolved, whilst silver
nitrate gives a yellow precipitate which very quickly becomes
black on standing.
Sulphuretted hydrogen has no action upon solutions of the
free acid but decomposes the potassium salt with formation of
sulphate and thiosulphate of potassium and deposition of
sulphur (Debus):
2K₂SO + 5H,S= K₂SO4 + K₂S¿О¸ + 5H₂O + 8S.
K2S2O3
3 6
The constitutional formula (p. 423) of potassium trithionate is
probably the following (Debus):
K.SO.O
KO.SO2.S.
TETRATHIONIC ACID. H.SO.
229 Fordos and Gélis in 1843 first prepared this acid and its
salts. They obtained the sodium salt by adding iodine to an
aqueous solution of sodium thiosulphate; thus:-
(ONa
SO,
2 SNa
SO,
SONa
SNa
+ I2
=
SO2
(ONa
is
SO₂ (O
SNa
2
}+2NaI.
Salts of this acid are also formed by the action of copper
sulphate upon barium thiosulphate and by the action of sul-
phuric acid upon a mixture of lead thiosulphate and peroxide.
In order to prepare the free acid, iodine in excess is added very
gradually to thiosulphate of barium suspended in a very small
quantity of water, the iodide of barium and excess of iodine
being removed by shaking up the semi-solid mass with strong
alcohol, leaving a white crystalline mass of barium tetrathionate,
BaSO. This may then be dissolved in a small quantity of
water and recrystallised, whilst from this pure salt the acid
may be prepared by adding exactly sufficient sulphuric acid
to decompose it completely.
Tetrathionic acid is a colourless, inodorous, very acid liquid,
which, when dilute, may be boiled without undergoing decom-
+3
TETRATHIONIC ACID
419
position, but in the concentrated state is easily decomposed into
sulphurous and sulphuric acids and sulphur. The tetrathionates
are all soluble in water, but their solutions cannot, as a rule, be
evaporated without decomposition into sulphur and a trithionate.
A solution of the potassium salt decomposes gradually on pre-
servation, sulphur dioxide being evolved but no sulphur
deposited. The decomposition proceeds in two stages, tri- and
pentathionate of potassium being first formed, and the tri-
thionate then decomposing in the usual way into potassium
sulphate, sulphur dioxide and sulphur which converts a portion
of the trithionate into pentathionate. (Debus.)
(1) 2K,S,O= K2S3O¸ + K₂S¿Ó¸·
6
(2) 3K₂S¸¸=2K,SO +2SO½ + K₂SO
6.
Dry potassium tetrathionate undergoes no alteration on pre-
servation. Sodium amalgam decomposes the compound into
two molecules of thiosulphate, and the same decomposition
occurs on the addition of potassium sulphide; thus :—
K₂SOK,S2K,S,O3 + S.
Potassium tetrathionate does not combine with ordinary sulphur
but combines with nascent sulphur to form a pentathionate.
Thus the free acid is decomposed by an excess of sulphuretted
hydrogen into water and sulphur :-
-
H₂SO6 + 5H₂S = 6H₂O+9S.
When however a slow current of sulphuretted hydrogen is
passed into a solution of the potassium salt, the sulphur
combines with a portion of the salt and forms potassium
pentathionate. Sulphur dioxide reacts with potassium tetra-
thionate to form a trithionate and a pentathionate, the latter
being probably produced by a secondary reaction from thio-
sulphuric acid or its anhydride:-
K₂SO® + SO₂ = K2S3O¸ † S₂O2.
2
PENTATHIONIC ACID. H₂SO
230 Wackenroder, in 1845, was the first to draw attention to
the existence of this acid in the liquid obtained by passing
1
420
THE NON-METALLIC ELEMENTS
sulphuretted hydrogen into a solution of sulphur dioxide; the
clear liquid obtained after the removal of the precipitated
sulphur was looked upon by him and his successors as a solu-
tion of the acid in water. The reaction was represented by
the equation :-
5H,S+5SO₂ = H₂S¿O+5S+4H₂O.
Later observers succeeded in preparing crystalline pentathion-
ates from this liquid and thus placed the existence of the acid
beyond doubt. The investigations of Debus¹ have shown
that the liquid in question, known as Wackenroder's solution,
has a very complex composition, which varies according to the
method of preparation. The liquid is best prepared by passing
sulphuretted hydrogen slowly for two hours into 480 cc. of
a saturated solution of sulphurous acid at a temperature near
0°, allowing the whole to stand for two days in a closed bottle
and repeating the operation until all the sulphurous acid has
disappeared. The liquid is then filtered from the sulphur
which has separated out and is thus obtained as a semi-trans-
parent milky fluid. The solution contains oily drops of sulphur
in suspension and a considerable amount of sulphur in a soluble
form (p. 346), which can be precipitated by the addition of
potassium nitrate or by concentrating the fluid on the water-
bath. The concentration of the clear liquid cannot be carried
further than a sp. gr. of 1.32 in this way without evolution of
sulphur dioxide and deposition of sulphur, but may be continued
over sulphuric acid in vacuo beyond this point.
An analysis of the solution thus obtained (sp. gr. 1:32) shows
that its composition is approximately that of pentathionic acid,
H₂SO6. When however the liquid is carefully treated with
about one third of an equivalent of caustic potash and then
allowed to evaporate spontaneously, a mixture of potassium
tetra- and pentathionates is obtained which can be separated
by recrystallisation from lukewarm water. The mother liquor
on evaporation yields a salt which contains more sulphur than
a pentathionate and is probably potassium hexathionate (p. 422).
In addition to these substances, Wackenroder's solution, pre-
pared in the manner described, contains sulphuric acid and
traces of trithionic acid, the amount of which becomes much
1 Journ. Chem. Soc. 1888, 278-357 (where references to the literature of the
subject will be found).
PENTATHIONIC ACID
421
greater when an insufficient quantity of sulphuretted hydrogen
is used.
The pentathionates are decomposed by caustic alkalis, sulphur
being deposited and a tetrathionate formed, and they are there-
fore best prepared from Wackenroder's solution by treating it
with the acetate of the metal and allowing the solution to evap-
orate spontaneously. The potassium salt, 2K,S,O,+3H,O, crys-
tallises in short rhombic prisms or six-sided plates. It forms a
clear, neutral solution in two parts of water and can be recrys-
tallised from water at 50° made acid with a little sulphuric acid.
The solution decomposes on standing, more rapidly on heating,
sulphur being deposited and a tetrathionate formed, but is more
stable in the presence of a little sulphuric acid;
H₂S₂O=H₂SO。 + S.
6
6
The salt itself decomposes on keeping if any moisture be present,
but if dried by washing the fine powder with dilute alcohol, and
then preserved over sulphuric acid, it remains unaltered for years.
On ignition it is converted into potassium sulphate, sulphur
dioxide and sulphur :-
K₂SO = K2SO4 + SO2 + 3S.
Potassium pentathionate is also formed when a slow current of
sulphuretted hydrogen is passed into a solution of the tetra-
thionate (p. 419). The barium salt, BaSO + 3H2O, and the
copper salt, CuS,O + 4H₂O, are also crystalline.
The pentathionates may be distinguished from the lower
members of the series by the facts that ammoniacal silver nitrate
solution produces a brown coloration which rapidly becomes
darker, a black precipitate being finally formed, and that caustic
potash produces an immediate precipitate of sulphur, tetrathionate
being also formed, whilst ammonia only gives this reaction on
standing. They are not affected by hydrochloric acid, ferric
chloride or barium chloride.
A solution of pure pentathionic acid, prepared by the decom-
position of the potassium salt, is converted by sulphuretted
hydrogen into water and sulphur :—
-
H₂SO + 5H₂S = 6H₂O+10S,
6
I
422
THE NON-METALLIC ELEMENTS
whilst the potassium salt yields a thiosulphate and a trithi-
onate:-
3K₂SO + 3H2S = K₂S₂O, + 2K₂S¸¸ + 3H2O + 10S.
3
Sulphur dioxide reacts both with the acid and its salts to form
trithionic acid and probably thiosulphuric anhydride :-
-
H₂SO。 + 2SO₂ = H2S3O6 + S2O2.
6
HEXATHIONIC ACID, H₂SO
231 This substance is only known in the form of its potassium
salt obtained from the mother liquors of potassium pentathionate.
This salt, which has not been obtained in a perfectly pure state,
forms a warty crust and yields an aqueous solution, which de-
composes even in the presence of acid. It reacts with caustic
potash and ammoniacal silver nitrate solution in the same way
as the pentathionates, but gives an immediate precipitate of
sulphur with ammonia.
FORMATION OF WACKENRODER'S SOLUTION.
It seems probable that in the preparation of Wackenroder's
solution, the direct product is tetrathionic acid :—
H₂S+3SO₂ = H₂SO。•
2
This substance is then acted upon by sulphurous acid with
formation of trithionic acid and thiosulphuric anhydride, SO,
(p. 419). Moreover the tetrathionic acid is converted by the
sulphuretted hydrogen into water and sulphur, and a number
of secondary reactions then go on, by means of which the thio-
sulphuric anhydride gives up some of its sulphur to the tetra-
thionic acid, forming pentathionic acid, whilst the trithionic
acid combines with nascent sulphur, forming tetra, penta-and
hexathionic acids.
When the action of the sulphuretted hydrogen is allowed to
continue until all action is at an end, water and sulphur are
found to be the final products:--
2H2S + SO₂ = 2H₂O + 3S.
SELENIUM
423
CONSTITUTION OF THE ACIDS OF WACKENRODER'S SOLUTION.
Potassium pentathionate is oxidised by bromine according to
the following equation :—
K₂S¿O + 4Br₂ + 6H₂O = 2KBr + 2S + 3H₂SO+6HBr.
2
Hence two of the sulphur atoms are differently combined in
the molecule from the other three; these two atoms are pre-
cipitated in the above reaction as free sulphur, whilst the other
three are converted into sulphuric acid and are therefore
probably already combined with oxygen in the molecule of pen-
tathionic acid and in those of tetra- and trithionic acid, which
stand in so close a relation to it. Trithionic acid moreover
strongly resembles sulphurous acid in its power of combining
with sulphur to form other acids and therefore probably contains
the group K.SO2, which is characteristic of sulphurous acid.
The following formulæ exhibit all these relations and are also in
accordance with the various decompositions of these substances;
they may therefore be assigned to the salts of these acids.
(Debus):-
Potassium Trithionate
·
Potassium Tetrathionate.
Potassium Pentathionate.
Potassium Hexathionate
Se
K. SO2. O
1
KO. SOą. S
KS. SO. O
=
KO. SO2. S
KS2. SO. O
KO. SO2. S
KS. SO2. O
KO. SO2. S
SELENIUM.
78'5.
232 We owe the discovery of this element to Berzelius.¹ He
first found it in the deposit from the sulphuric acid chambers at
Gripsholm in Sweden, in the year 1817, and it is to him that
we are indebted for the knowledge of its most important com-
¹ Schweigg Journ. 23, 309, 430; Pogg. Ann. 7, 242; 8, 423.
424
THE NON-METALLIC ELEMENTS
pounds. The name selenium is derived from Zeλývŋ, the moon,
on account of its analogy with the element tellurium (tellus, the
earth), discovered shortly before.
Although selenium is somewhat widely distributed, it occurs
only in small quantities. It is found together with sulphur in the
islands of Volcano and Hawai, and occurs, chiefly combined with
certain metals, at Clausthal and Zorge in the Harz, in La Plata,
the Argentine and Mexico, as the mineral clausthalite, PbSe;
a selenide of copper and lead, PbSe+Cu₂Se; lehrbachite,
PbSe+HgSe; selenide of silver, Ag,Se; selenide of copper,
Cu Se. We also find it as onofrite, HgSe+4HgS, in Mexico;
whilst eucairite, Cu,Se+Ag,Se, and crookesite (CuTlAg),Se,
occur at Skrikerum in Sweden. Selenium is also found in very
small quantity in many other minerals, especially in certain.
iron-pyrites and copper-pyrites, and where these are used
for the manufacture of sulphuric acid, a red deposit containing
selenium is found in the chambers.
Preparation. In order to prepare selenium from this deposit,
it is mixed with equal parts of sulphuric acid and water to a
thin paste and then boiled; nitric acid or potassium chlorate
being added until the red colour disappears. In this way a
solution of selenic acid, H₂SeO₁, is obtained, which is diluted
with water, filtered, and then heated with a quarter its volume
of fuming hydrochloric acid until three-quarters of the liquid
has evaporated. By this process chlorine is evolved and
selenious acid, H,SeO3, formed. The cold solution is then
poured off from the solid matter and saturated with sulphur
dioxide, when selenium separates out as a red powder. The
selenium thus obtained contains lead and other metals, from
which it may be separated either by distillation or by fusing it.
with a mixture of nitre and sodium carbonate, by which
means sodium selenate is formed, and this is then again treated
with hydrochloric acid and sulphur dioxide as above described
(Wöhler). Selenium may also be easily obtained from the
chamber deposit by heating it on a water-bath with a concen-
trated solution of cyanide of potassium until it assumes a pure
grey colour. On the addition of hydrochloric acid to the filtered
solution selenium is deposited in cherry-red flakes. This also
contains both copper and lead, and these impurities are removed
either by the process as described above or by evaporating the
selenium to dryness with nitric acid and reducing the aqueous
1 Handbook of Inorganic Analysis (1854), 154.
PROPERTIES OF SELENIUM
425
solution of selenium dioxide by means of sulphur dioxide
(Nilson).
Properties. Selenium, like sulphur, exists in several allotropic
modifications, one class being soluble, the other insoluble in
carbon bisulphide (Berzelius, Hittorf).
Soluble selenium is obtained as a finely divided brick-red
amorphous powder, when a cold solution of selenious acid is
precipitated by a current of sulphur dioxide; and as a black
crystalline powder when this gas is passed through a hot
solution. Other reducing agents, such as iron, zinc, stannous
chloride or phosphorous acid, also precipitate soluble selenium
from solutions of selenious acid. Selenium crystallizes from
solution in carbon bisulphide in small dark-red translucent
crystals, which are isomorphous with the monosymmetric form
of sulphur, and have a specific gravity of 45. Like sulphur,
selenium is polymorphous, the crystals which are deposited.
from a cold saturated solution differing in type from those just
described, although belonging to the same system. A third
variety, isomorphous with the crystals of tellurium and belong-
ing to the hexagonal system, is obtained at temperatures above
130°. When selenium is fused and allowed to cool quickly it
solidifies to a dark brownish-black glassy translucent amorphous
brittle mass, which is also soluble in carbon bisulphide, and has
a specific gravity of 43. Soluble selenium has no definite
melting point, softening gradually on heating.
The insoluble or metallic selenium is obtained by cooling
melted selenium quickly to 210° and then keeping the melted
mass at this temperature for some time. The selenium at length.
solidifies to a granular crystalline mass, the temperature rising
suddenly in the act of solidification to 217°. The solid mass
thus obtained has a specific gravity of 45, and is insoluble in
carbon bisulphide. This change from the soluble to the insoluble
condition also takes place, but more slowly, at lower tempera-
tures; thus, if a mass of soluble selenium be placed in an air-bath
at 100°, the change to the insoluble variety takes place gradually,
and the temperature rises up to 217°. If a concentrated solution
of potassium or sodium selenide be exposed to the air, black
selenium separates out in microscopic crystals which have a
specific gravity of 48, and are likewise insoluble in carbon
bisulphide. The insoluble or metallic selenium has a constant
melting point of 217°, and when quickly cooled is converted
1 Rammelsberg, Ber. 7, 669.
2 Muthmann, Zeit. Kryst, 17, 336.
426
THE NON-METALLIC ELEMENTS
into amorphous soluble selenium. The change of vitreous into
crystalline selenium is accompanied by an evolution of 279
units of heat. Both modifications of selenium are soluble in
chloride of selenium and separate out from this solution in the
form of metallic selenium.
In addition to these forms, a modification of selenium is also
known which is soluble in water. This colloidal selenium is
formed when a current of sulphur dioxide is passed into a
solution of selenious acid. It is a dark red powder which is
completely soluble in water, forming a red fluorescent solution,
but which gradually becomes insoluble on preservation. The
solution may be boiled without undergoing any change, but
the selenium is deposited on the addition of acids or salts.
The solution on spontaneous evaporation deposits the selenium
as a red transparent film.
2
According to the experiments of Carnelley and Williams,
selenium boils at 680°, and forms a dark-red vapour which con-
denses either in the form of scarlet flowers of selenium, or in
dark shining drops of the melted substance. As in the case
of sulphur, the vapour density of selenium diminishes very
rapidly with the temperature; thus at 860° the vapour density
is 1107, whilst at 1420° it has a density of 81-5, closely
corresponding to the normal vapour density of 78·5.
Metallic selenium conducts electricity, and exposure to light
increases its conducting power. The peculiar effect of light
is best exhibited on selenium which has been exposed for a
considerable time to a temperature of 210°, until it has attained a
granular crystalline condition. When selenium in this condition
is heated its electrical resistance is increased, whilst on exposing
it to the action of diffused daylight, the electrical resistance
instantly diminishes; this, however, is only a temporary change,
for on cutting off the light, the electrical resistance of the
selenium slowly increases, and after a short time reaches the
amount exhibited before the exposure. This remarkable pro-
perty of selenium may possibly be made use of for photometrical
purposes.
When selenium is heated in the air it burns with a bright
1 Schulze, Jr. Pr. Chem. [2], 32, 390.
2 Journ. Chem. Soc. 1879, i. 563.
3 Sale, Proc. Roy. Soc. 1873, 21, 283; W. G. Adams, Proc. Roy. Soc. 1875,
23, 535; W. Siemens, Berlin, Ber. Akad. 1875. S. Bidwell, Phil. Mag. (5),
20, 178.
SELENIURETTED HYDROGEN
427
blue flame, forming an oxide of selenium to which is due the
characteristic odour, resembling that of rotten horse-radish,
noticed when selenium burns. The emission spectrum of
selenium is seen when a small bead of the element is held in
a non-luminous gas flame; it is a channelled spectrum highly
characteristic and beautiful, consisting of a very large number
of bright bands, which in the green and blue are arranged at
regular intervals. According to Salet,2 selenium, like sulphur,
gives two emission spectra, one consisting of lines and the other
of bands. The absorption spectrum of selenium has been
examined by Gernez.3
The atomic weight of selenium has been determined by
Pettersson and Ekman, who determined the composition of the
oxide SeO2 and of silver selenite Ag₂SeO, and obtained the
number 785; their numbers agree with the older determina-
tions of Berzelius.5
SELENIUM AND HYDROGEN.
HYDROGEN SELENIDE OR SELENIURETTED HYDROGEN. H.Se.
233 This gas is formed when selenium vapour and hydrogen
are heated together, and the amount which is thus produced is
a function of the temperature. The quantity formed increases
when the temperature is raised from 250° to 520°, but above
this point the amount gradually diminishes. When selenium
is heated in a closed tube filled with hydrogen, it sublimes in
the cool part of the tube in the form of beautiful glittering
crystals, and these increase in number until the whole of the
selenium has volatilized. The formation of these crystals
depends upon the decomposition by heat of the seleniuretted
hydrogen which is formed; for when selenium is heated in a
tube filled with an indifferent gas, only red amorphous selenium
is found to sublime.
Seleniuretted hydrogen is easily obtained by the action of
dilute hydrochloric acid on potassium selenide, K,Se, or on iron
selenide, FeSe, which is prepared by adding selenium to heated
1 W. M. Watts, Index of Spectra, p. 56.
2 Compt. Rend. 73, 559 and 742.
* Ber. 9, 1290.
3 Compt. Rend. 74, 1190.
Pogg. Ann. 8, 21.
428
THE NON-METALLIC ELEMENTS
iron. It is also formed when organic materials such as colo-
phonium are heated with selenium. It is a colourless, inflam-
mable gas, possessing a smell which, to begin with, resembles
that of sulphuretted hydrogen, but afterwards is found to have
a much more persistent and intolerable odour, a small quantity
affecting the mucous membrane in a remarkable degree, attacking
the eyes, and producing inflammation and coughing which last
for days.
Berzelius describes the effects as follows:-"In order to become
acquainted with the smell of this gas I allowed a bubble not
larger than a pea to pass into my nostril; in consequence of its
action I so completely lost my sense of smell for several hours
that I could not distinguish the odour of strong ammonia, even
when held under my nose. My sense of smell returned after
the lapse of five or six hours, but severe irritation of the mucous
membrane set in and lasted for a fortnight."
In order to ascertain the composition of seleniuretted hydrogen
metallic tin is heated in a measured volume of the gas, when
tin selenide is formed, and a volume of hydrogen is liberated
equal to that of the original gas.
Hydrogen selenide is more soluble in water than the corre-
sponding sulphur compound, yielding a colourless solution which
reddens blue litmus paper, colours the skin a reddish-brown
tint, and possesses the foetid odour of the gas. Exposed to the
air, the aqueous solution absorbs oxygen with the separation of
red selenium. When added to solutions of salts of most of the
heavy metals, it produces precipitates of the insoluble selenides
in an analogous manner to sulphuretted hydrogen. Sulphur also
decomposes it, forming sulphuretted hydrogen and precipitating
selenium.
SELENIUM AND CHLORINE.
234 Selenium, like sulphur, forms several compounds with
chlorine.
SELENIUM MONOCHLORIDE. Se,Cl.
When a current of chlorine is passed over selenium, the latter
melts and is converted into a brown oily liquid, which is selenium
¹ Lehrbuch, 5 Aufl. 2, 213.
SELENIUM MONOBROMIDE
429
monochloride. It is more readily prepared by passing hydro-
chloric acid gas into a solution of selenium in fuming nitric
acid. Selenium monochloride is slowly decomposed by water
according to the equation :
2Se,Cl₂+ 3H2O = H₂SeO3 + 3Se + 4HCl.
-
SELENIUM TETRACHLORIDE.
SeCl.
This body is obtained by the further action of chlorine upon
the monochloride, as well as when selenium dioxide is heated
with phosphorus pentachloride (Michaelis); thus:-
3SeO2 + 3PC15 = 3SeCl + P₂O5 + POCI¸.
The tetrachloride is a light yellow solid body, which on heat-
ing volatilizes without previously melting, subliming in small
crystals. It also crystallizes from solution in phosphorus
oxychloride in the form of bright shining cubes. It dissolves
in water with formation of hydrochloric and selenious acids;
thus:-
SeCl, +3H,O= 4HCl + H₂SeO3.
When its vapour is heated its density diminishes, dis-
sociation taking place.2
2SeCl, Se,Cl₂+3C12.
=
SELENIUM AND BROMINE.
SELENIUM MONOBROMIDE. Se,Br.
235 Equal parts of bromine and selenium combine together
with evolution of heat to form a black semi-opaque liquid, having
a specific gravity at 15° of 36. It has a disagreeable smell
resembling that of chloride of sulphur, and colours the skin
a permanent red-brown tint. On heating it is decomposed,
and when brought into contact with water, selenium, selenious
acid and hydrobromic acid are formed; thus:-
2Se,Br₂+ 3H2O = 3Se + H₂SeO3 + 4HBr.
1 Divers and Shimose, Ber. 17, 866.
2 Chabrié, Bull. Soc. Chim. [3], 2, 803.
430
THE NON-METALLIC ELEMENTS
SELENIUM TETRABROMIDE. SeBг.
This compound is formed by the further action of bromine on
the monobromide. It is an orange yellow crystalline powder,
best obtained by adding bromine to a solution of selenium
monobromide in carbon bisulphide. It is very volatile, vapor-
ising between 75° and 80° with partial decomposition and
subliming in black six-sided scales. It possesses a disagree-
able smell similar to that of chloride of sulphur, decomposes
in contact with moist air into bromine and the monobromide,
and dissolves in an excess of water with formation of hydro-
bromic and selenious acids.
The chlorobromides of selenium, SeCl,Br and SeCIBr, are
orange yellow crystalline substances (Evans and Ramsay).
SELENIUM AND IODINE.
SELENIUM MONO-IODIDE. Se₂I2.
236 The compounds of selenium and iodine resemble those
of selenium with chlorine and bromine. The mono-iodide is ob-
tained when the two elements are brought together in the right
proportions (Schneider), and forms a black shining crystalline
mass, melting between 68° and 70° with the evolution of a small
quantity of iodine. When more strongly heated, it decomposes
into iodine which volatilizes, and selenium which remains
behind, and is decomposed by water in a similar way to the
corresponding bromide.
SELENIUM TETRA-IODIDE.
SeI.
It is a granular dark crystalline mass, which melts, at from 75°
to 80°, to a brownish-black liquid, translucent in thin films;
when more strongly heated it decomposes into its elements.
SELENIUM AND FLUORINE.
237 When the vapour of selenium is passed over melted
fluoride of lead, a fluoride of selenium sublimes in crystals.
SELENIOUS ACID
431
These are soluble in hydrofluoric acid, and are decomposed by
water (Knox). When selenium is exposed to gaseous fluorine it
fuses and finally takes fire, a white crystalline substance being
formed. This is soluble in hydrofluoric acid, but is decomposed
by water (Moissan).¹
SELENIUM AND OXYGEN.
OXIDES AND OXY-ACIDS OF SELENIUM.
238 Only one oxide of selenium, the dioxide, SeO2, is with cer-
tainty known to exist in the free state. A lower oxide is stated
by Berzelius to be formed when selenium burns in the air, and is
probably the cause of the peculiar smell then observed, which is
so penetrating that if 1 mgrm. of selenium be burnt in a room
the smell is perceptible in every part.
Selenium also forms two oxy-acids-selenious acid, HySeO
and selenic acid, H₂SeO.
SELENIUM DIOXIDE. SeO2.
239 When selenium is placed in a bent tube, as shown in Fig.
125 and strongly heated in a current of oxygen, contained in the
gasholder and dried by passing over pumice-stone saturated with
sulphuric acid, it takes fire and burns with a bright blue flame, a
white sublimate of solid selenium dioxide being deposited in
the cool part of the tube. Thus obtained it forms long four-
sided, white, needle-shaped crystals, which do not melt when
heated under the ordinary atmospheric pressure, but evaporate,
when heated to about 300°, yielding a greenish yellow-coloured
vapour possessing a powerful acid smell (Berzelius).
SELENIOUS ACID, H₂SeO3.
240 Selenious acid is formed when selenium is heated with
nitric acid, or when five parts of the dioxide are dissolved in one
part of hot water. On cooling, clear, long, colourless, prismatic,
nitre-shaped crystals of selenious acid separate out. These have a
strong acid taste, and, when heated, decompose into selenium.
1 Ann. Chim. Phys. [6] 24, 239.
432
THE NON-METALLIC ELEMENTS
dioxide and water. If sulphur dioxide be allowed to pass into
a hydrochloric acid solution of selenious acid, selenium is
deposited as a red powder. This decomposition takes place
but slowly in the cold and in absence of light; but when the
liquid is heated or in the sunlight, the change occurs quickly.
Organic substances also bring about this reduction, and the
colourless solution of the pure acid soon becomes tinged when
exposed to the air, owing to the presence of the dust in the
atmosphere. Selenious acid is distinguished from sulphurous
I

S
FIG. 125.
acid by this reaction, for sulphurous acid gradually absorbs
oxygen from the air.
Selenious acid is a dibasic acid, and forms not only acid
and normal salts, but also salts, containing acid selenites united
with selenious acid; thus, HKSeO3 + H₂SeO3.
The normal selenites of the alkali metals are easily soluble in
water; those of the alkaline earth metals and the heavy metals
are insoluble; whilst all the acid salts are soluble compounds.
When a selenite is heated on charcoal in the reducing flame, a
characteristic horse-radish-like smell is emitted, and when
SELENIC ACID
433
heated in a glass tube with sal-ammoniac, selenium sublimes.
The selenites are further distinguished by the fact that red
selenium is precipitated when sulphur dioxide is led into their
solution in water or in hydrochloric acid. The salts are very
poisonous.
SELENYL CHLORIDE, OR SELENIUM OXYCHLORIDE. SeOCl2.
241 This chloride of selenious acid is formed by the action of
selenium dioxide on selenium tetrachloride (Weber); thus:-
SeO +2SeCl₁ = 2SeOCl2.
and when the oxide is heated with sodium chloride: 2-
2SeO₂+ 2NaCl = SeOCl2 + Na2SeO3.
It is a yellow liquid which fumes on exposure to air, and
which, when cooled below 10°, deposits crystals having a
specific gravity of 2:44. It boils at 179°5 (Michaelis), and
decomposes with water in the same way as all acid chlorides.
When mixed with thionyl chloride, selenium tetrachloride and
sulphur dioxide are formed; thus:-
SeOCl2 + SOCI₂ = SeCl4 + SO2.
•
SELENYL BROMIDE. SEOBг.
Formed by the action of selenium tetrabromide on selenium
dioxide. The two substances are melted together, and the
compound, on cooling, crystallizes in long needles.
SELENIC ACID. H.SeO.
242 This acid, discovered in 1827 by Mitscherlich, is formed
by the action of chlorine on selenium or on selenious acid in
presence of water; thus:-
Se + 3Cl₂+4H₂O H₂SeO, + 6HC1.
By the same reaction selenites may be converted into selenates.
Bromine may for this purpose be employed instead of chlorine.
=
1 Pogg. Ann. 108, 615.
2 Cameron and Macallan, Chem. News, 59, 267.
29
434
THE NON-METALLIC ELEMENTS
Potassium selenate is also obtained when selenium is fused
with nitre.
In order to prepare selenic acid, a solution of sodium
selenite is treated with silver nitrate; and to the precipitate
obtained, suspended in water, bromine is added (J. Thomsen);
thus:-
Ag₂SeO3 + H2O + Br₂ = 2AgBr + H₂SeO.
The aqueous solution of selenic acid can be concentrated by
evaporation in the air, but it cannot be thus completely freed
from water, as at a temperature of about 280° it begins to de-
compose into oxygen, selenium dioxide and water. If heated
in vacuo to 180° and then cooled, the whole solidifies to a
crystalline mass, consisting of the pure acid.¹ The acid
crystallizes in long hexagonal prisms, similar to those formed
by sulphuric acid and melts at 58°. It combines eagerly with
water, forming a monohydrate which melts at 25° and, like
sulphuric acid, chars many organic substances. The specific
gravity of the solid acid is 26273 whilst that of the liquid at
15° is 23557.
The concentrated solution of the acid is a colourless, very
acid liquid, which is miscible with water in all proportions, heat
being thereby evolved. The heated aqueous acid dissolves gold
and copper with formation of selenious acid, whilst iron, zinc,
and other metals dissolve with evolution of hydrogen and pro-
duction of selenates. This acid is not reduced either by sulphur
dioxide or by sulphuretted hydrogen, and differs in this respect,
therefore, remarkably from selenious acid. When boiled with
hydrochloric acid it decomposes with the evolution of chlorine
and formation of selenious acid; thus:-
H₂SeO¸ + 2HCl = H₂SeO3 + Cl2 + H₂O.
This mixture dissolves gold and platinum, these metals com-
bining with the chlorine thus set at liberty.
The selenates exhibit the closest analogy with the sulphates
so far as regards amount of water of crystallization, crystalline
form, and solubility. Barium selenate, like the sulphate, is com-
pletely insoluble in water, and is employed for this reason in the
quantitative determination of selenic acid; it is, however,
distinguished from barium sulphate inasmuch as when boiled
with hydrochloric acid the insoluble selenate is decomposed into
1 Cameron and Macallan, Chem. News, 59, 219.
SELENOSULPHURIC ACID
435
•
the soluble selenite, whereas barium sulphate remains unchanged.
All the other selenates are also reduced to selenites by means
of hydrochloric acid, and this reaction serves as a means of
recognizing these compounds.
SELENIUM AND SULPHUR.
243 Several substances have been at various times described as
compounds of selenium and sulphur, but they have all proved
to be mixtures of the two elements.2
SELENOSULPHUR TRIOXIDE. SESO
It has long been known that when selenium is dissolved
in fuming sulphuric acid, a beautiful green colour is produced,
and it has been proved that this is due to the formation
of the above compound. This substance is best prepared by
dissolving selenium in freshly distilled well-cooled sulphur
trioxide. The compound then separates out in the form of
tarry drops which soon solidify to prismatic crystals, having a
dirty green colour, and yielding a yellow powder when
broken up. The compound dissolves in sulphuric acid with a
green colour, and is decomposed on addition of water with
separation of selenium and formation of sulphuric, sulphurous
and selenious acids. On heating, the body does not melt but
decomposes into selenium, selenium dioxide, and sulphur dioxide
(Weber).3
OH
SO₂ SeH.
SELENOSULPHURIC ACID. SO,
This compound, which corresponds to thiosulphuric acid, was
discovered by Cloez and like this latter acid is not known in
the free state. Its potassium salt is obtained when a solution of
sulphurous acid is mixed with one of potassium selenide, or,
together with the salts of selenotrithionic acid, when selenium
¹ Rathke, J. Pr. Chem. 108, 235; Ditte, Compt. Rend. 73, 625, 660;
Gerichten, Ber. 7, 26.
2 Divers, Chem. News, 51, 24; Bettendorf and von Rath, Pogg. Ann. 139,
Pogg. Ann. 156, 513.
Bull. Soc. Chim. 1861, 112.
329.
436
THE NON-METALLIC ELEMENTS
is dissolved in a solution of normal potassium sulphite.¹ The
selenosulphates are isomorphous with the thiosulphates, and are
decomposed by all acids, even by sulphurous acid, with the
separation of red selenium.
SELENOTRITHIONIC ACID. H,SeS₂O
The potassium salt of this acid is formed when a solution of
potassium selenosulphate is mixed with an excess of normal
potassium sulphate, and a concentrated solution of selenious.
acid, thus:-
SO, {OK
2
OK
+SO₂ { Se+SeO
2
K₂S₂SO+SeO
(OH
{
OH
SOK
(OK+H₂O
=
The potassium salt forms monosymmetric prisms and is stable
in the air. According to Schulze, the free acid is formed when
selenious acid is treated with an excess of sulphur dioxide.
The same chemist has found that when selenious acid is
kept in excess, an acid of the formula HSS,O, is obtained.
Sulphoselenoxytetrachloride, CISO, (OSeCl), is obtained by the
action of selenium tetrachloride upon chlorosulphonic acid as a
mass of white crystals; it melts at 165° and boils at 183°.
TELLURIUM. Te 124 (?).
244 Tellurium occurs in small quantities in the free state in
nature, and by early mineralogists was termed aurum paradoxum
or metallum problematum, in consequence of its metallic lustre.
In 1782 native tellurium was more carefully examined by Müller
von Reichenstein. He came to the conclusion that it contained
a peculiar metal, and at his suggestion, Klaproth in 1798
made an investigation of the tellurium ores confirming the views
of the former experimenter that it contained a new metal, to
which he gave the name of tellurium from tellus, the earth.
1 Schaffgotsch, Pogg. Ann. 90, 66; Rathke, J. Pr. Chem. 95, 1; Uelsmann,
Annalen 116, 122.
2 J. Pr. Chem. [2] 32, 405.
Crell. Ann. 1, 91.
3 Clausnizer, Ber. 11, 2007.
TELLURIUM
437
Berzelius in 1832, made a more exhaustive investigation of
tellurium; he likewise considered the substance itself to be a
metal, but its compounds were found to correspond so closely with
those of sulphur and selenium, that tellurium was placed in the
sulphur group.
Tellurium belongs to the rarer elements. It occurs in
Transylvania, Hungary, California, Virginia, Brazil, and Bolivia,
in small quantities in the native state, but it is generally
found in combination with metals as graphic tellurium, or
sylvanite (AgAu) Te,; black or leaf tellurium, or nagyagite
(AuPb), (TeSSb); silver telluride, Ag₂Te; tetradymite, or
bismuth telluride, Bi₂Te,, &c.
Preparation.—In order to prepare pure tellurium, tellurium-
bismuth containing about 60 per cent of bismuth, 36 per cent.
of tellurium, and about 4 per cent. of sulphur, is mixed with
an equal weight of pure carbonate of soda, and then the mixture
rubbed up with oil to a thick paste, and heated strongly in a
well-closed crucible. The mass is then lixiviated with water,
and the filtered solution, which contains sodium telluride and
sodium sulphide, is exposed to the air, when the tellurium grad-
ually separates out as a grey powder, which after washing and
drying may be purified by distillation in a current of hydrogen
(Berzelius).
In order to obtain the tellurium from graphic, or from black
tellurium, the ore is treated with hydrochloric acid in order
to free it from antimony, arsenic, and other bodies. The
residue is then boiled with aqua regia, and the filtrate eva-
porated to drive off the excess of nitric acid; ferrous sulphate
is then added, which precipitates the gold, and the tellurium
is thrown down in the filtrate by means of sulphur dioxide
(v. Schrötter).
The crude tellurium is best purified by treating it with aqua
regia, expelling the excess of nitric acid by means of hydro-
chloric acid and then diluting somewhat so that the tellurous
acid remains in solution whilst the chloride of lead is precipitated.
From the filtrate, the tellurium is precipitated by means of
sulphurous acid. The material thus obtained, which may
contain traces of selenium, lead and other metals, is then fused
in small portions with potassium cyanide, the mass extracted
with water in the absence of air, and the tellurium thrown
¹ Pogg. Ann. 28, 392; 32, 1 and 577.
1
D
438
THE NON-METALLIC ELEMENTS
down from the clear filtrate by a current of air. It is finally
melted and distilled in a current of hydrogen.¹
Properties. Tellurium is a silver-white body possessing
a metallic lustre, and crystallizing in rhombohedra. Tellurium
is a very brittle substance, and can therefore be easily powdered.
Its specific gravity is 6:24: it melts at 452° (Carnelley and
Williams), and boils at a still higher temperature, and may
accordingly be easily purified by distillation in a stream of
hydrogen gas.
Amorphous tellurium is obtained when a solution of tellurous
acid is precipitated by means of sulphurous acid. On heating
it is converted with evolution of heat into the crystalline
variety.2
Tellurium burns when heated in the air with a blue flame,
evolving white vapours of tellurium dioxide. It is insoluble in
water and carbon bisulphide, but dissolves in cold fuming sul-
phuric acid, imparting to the solution a deep-red colour, which
is probably due to the formation of a compound analogous to
sulphur sesquioxide, namely, STeO3, the tellurium being pre-
cipitated on the addition of water. On heating the sulphuric
acid solution, the tellurium is oxidized to tellurous acid, sulphur
dioxide being given off. In the same way it rapidly undergoes
oxidation in the presence of nitric acid.
Hydrochloric acid does not attack tellurium; caustic potash
dissolves it with formation of potassium telluride and tellurite.
When heated nearly to the melting-point of glass, tellurium
emits a golden-yellow vapour, which gives an absorption spec-
trum, consisting of fine lines stretching from the yellow to the
violet. The emission spectrum of tellurium has been mapped
by Salet and Ditte.5
4
According to Deville and Troost the vapour of tellurium.
possesses a specific gravity of 90 at 1390°, which number
corresponds to a molecular weight of about 259.6 The mole-
cule of tellurium therefore contains two atoms and has the
formula Te2.
245 Atomic Weight of Tellurium.-Considerable doubt exists
as to the value of the atomic weight of tellurium. The earlier
observers obtained the number 128, and this was confirmed in
1 Brauner, Monatsh. 10, 414.
2 Fabre and Berthelot, Compt. Rend. 104, 1405.
3 Gernez, Compt. Rend. 74, 1190.
5
Compt. Rend. 73, 262.
• Compt. Rend. 73, 559, 742.
Compt. Rend. 56, 871.
TELLURETTED HYDROGEN
439
1879 by Wills, who analysed the double bromide of potassium
and tellurium K,TeBr, and converted the element into tellurous
acid by means of nitric acid and of aqua regia. The close relation
of tellurium to sulphur and selenium however makes it highly
probable, from considerations connected with the arrangement
of the elements according to the periodic system (Vol. II.,
Part 2, p. 503), that the atomic weight of tellurium is less
than that of iodine (1259). Brauner 2 has therefore thoroughly
re-examined the question, but finds that all reliable methods
of determination (analysis of tellurous acid and of tellurium
tetrabromide) lead to about 126 5. He expresses the opinion
that the substance at present known as tellurium contains
some substance of higher atomic weight. In this he is con-
firmed by the fact that when the tellurium is previously sub-
limed in hydrogen its atomic weight is found to be considerably
lower (1265) than when it is sublimed in any indifferent gas
(1367). He supposes therefore that the impurity is partially
removed by the hydrogen.
TELLURIUM AND HYDROGEN.
TELLURIUM HYDRIDE, OR TELLURETTED HYDROGEN. H₂Te.
246 This compound, discovered by Davy in 1810, is a colour-
less gas possessing a foetid smell similar to that of sulphuretted
hydrogen. It is formed in small quantities when tellurium is
heated in hydrogen gas. If this is allowed to take place in a
sealed tube, the same phenomenon presents itself as is observed
with selenium, and the tellurium sublimes in long glittering
prisms.
In order to prepare telluretted hydrogen, tellurium is heated
with zinc, and the zinc telluride thus formed, decomposed by
hydrochloric acid; thus:—
ZnTe + 2HCl = ZnCl2 + H₂Te.
Tellurium hydride is easily combustible, burning with a blue
flame. It is soluble in water, and this solution absorbs oxygen
from the air, tellurium being deposited. Like sulphuretted
hydrogen, telluretted hydrogen precipitates many of the metals
from their solutions in the form of tellurides. The soluble
tellurides, such as those of the alkaline metals, form brownish.
2 Monatsh. 10, 411.
1 Journ. Chem. Soc. 1879, i. 704.
440
THE NON-METALLIC ELEMENTS
red solutions from which tellurium is deposited on exposure to
the air.
The density of tellurium hydride, like that of seleniuretted
hydrogen, has not been as yet directly determined, but Bineau
has shown that both gases, when they are heated with certain
metals, give up all their selenium or tellurium, leaving a
residue of hydrogen which occupies the same volume as the
original gas. Hence each molecule of these gases contains two
atoms of hydrogen combined, as analysis shows, with 785 and
about 124 parts of these elements. Consequently tellurium
hydride contains one part by weight of hydrogen and about
sixty-two parts by weight of tellurium.
TELLURIUM AND CHLORINE.
TELLURIUM DICHLORIDE.
TeCl.
247 This compound is formed together with the tetrachloride
when chlorine is passed over melted tellurium. It may be
separated from the less volatile tetrachloride by distillation;
and thus obtained, it forms an indistinctly crystalline, almost
black mass which gives a greenish yellow powder, melts at
175°, boils at 327°, and yields a deep red-coloured vapour,
having the density 6.89.2
Water decomposes the compound with separation of tellurium
and formation of tellurous acid; thus:-
2TeCl₂ + 3H₂O = Te + H¿TeO¸ + 4HCl.
TELLURIUM TETRACHLORIDE, TeCl,
Is formed by the further action of chlorine on the preceding
compound. It is a white crystalline body which melts at 224°,3
forming a yellow liquid, which when more strongly heated
becomes at last of a dark red colour and boils at 380°, without
decomposition. It is extremely hygroscopic and is decomposed
when thrown into cold water, an insoluble oxychloride being
1 Carnelley and Williams, Journ. Chem. Soc. 1879, i. 563; 1880, i. 125.
2 Michaelis, Ber. 20, 2488.
3 Carnelley and Williams, Journ. Chem. Soc. 1880, i. 125.
4 Michaelis, Ber. 20, 2491.
BROMIDES OF TELLURIUM
441
formed together with tellurous acid. Hot water, on the other
hand, gives rise only to the formation of the latter compound.
Like the corresponding tetrachlorides of sulphur and selenium,
it forms crystalline compounds with a large number of metallic
chlorides.
The vapour density of the tetrachloride is 9.2
(Michaelis).
TELLURIUM AND BROMINE.
TELLURIUM DIBROMIDE, TeBr
Is best obtained by heating the tetrabromide with tellurium.
On heating it volatilizes in the form of a violet vapour, which
condenses to black needles, melting at 280°, and again solidify-
ing to a crystalline mass; it boils at 339°.
Is formed
thus:-
TELLURIUM TETRABROMIDE, TeBr.
In order to prepare this compound, finely divided tellurium
is added to bromine which has been cooled down to 0°. It is
a dark yellow solid body which can be sublimed without de-
composition. It dissolves in a small quantity of water with a
yellow colour, but when added to a large quantity of water the
solution becomes colourless from the formation of hydrobromic
and tellurous acids. It boils at 420°. It forms double salts
with the bromides of the alkali metals.
TELLURIUM AND IODINE.
TELLURIUM DI-IODIDE. Tel.
Is obtained as a black crystalline mass by heating iodine
and tellurium together in the proper proportions.
TELLURIUM TETRA-IODIDE, Tel₁,
by the action of hydriodic acid on tellurous acid;
H₂TeO3 + 4HI= Tel +3H₂O.
442
THE NON-METALLIC ELEMENTS
It forms an iron-gray crystalline mass which melts when gently
heated, and when heated more strongly decomposes with separa-
tion of iodine. It is but slightly soluble in cold water, and is
decomposed by boiling water.
TELLURIUM AND FLUORINE.
TELLURIUM TETRAFLUORIDE. TeF4,
Is prepared by heating the dioxide with hydrofluoric acid in a
platinum retort. It distils over as a colourless, transparent, very
deliquescent mass.
When a solution of tellurous acid in hydrofluoric acid is con-
centrated, crystals of the formula TeF, + 4H₂O separate out.
Powdered tellurium becomes incandescent on exposure to
fluorine gas, the tetrafluoride being formed.¹
TELLURIUM AND OXYGEN.
OXIDES AND OXYACIDS OF TELLURIUM.
248 Three oxides of tellurium are known, TeO, TeO₂ and
TeO, as well as the acids, H,TeOg, and H,TeO, corresponding
to the last two of these.
3
TELLURIUM MONOXIDE, TeO.
249 This substance is left behind as a brownish black amor-
phous mass when tellurium sulphoxide, TeSO,, is heated to 230°
in vacuo, sulphur dioxide being evolved. On heating in the air or
on exposure to moist air it is gradually oxidised. Sulphuric acid
forms with it a red solution, from which a crystalline mass of
tellurium sulphate, Te(SO4)2, soon separates.
The monoxide
when heated in hydrochloric acid gas is converted into
tellurium dichloride.
TELLURIUM DIOXIDE, TEO₂,
250 Occurs in the impure state in nature as tellurite or tellu-
rium ochre, at Facebay in Transylvania. It is formed by the
1 Moissan, Ann. Chim. Phys. [6] 24, 239.
2 Divers and Shimose, Ber. 16, 1004; Journ. Chem. Soc. 1883, i. 319.
TELLUROUS ACID
443
combustion of tellurium in the air, and separates out in small
octahedra when tellurium is dissolved in warm nitric acid. It
is only very slightly soluble in water, and the solution does not
redden blue litmus paper. On heating it melts to a lemon-
yellow liquid which boils on further heating without decom-
position, whilst on cooling it solidifies to a white crystalline
mass. Although this is an acid-forming oxide, it also exhibits
basic properties, inasmuch as it combines with certain acids.
forming an unstable class of salts, which are decomposed by
The nitrate on ignition leaves a residue of TeO2.¹
water.
TELLUROUS ACID, H₂TeO,
251 Is obtained by pouring a solution of tellurium in dilute
nitric acid into water. It separates out in the form of a very
voluminous precipitate, which when placed over sulphuric acid
dries to a light white powder. It is but slightly soluble in
water, and the solution possesses a bitter taste. Tellurous acid
like sulphurous acid, is dibasic, and therefore forms two series of
salts: thus we have, normal potassium tellurite, K„TeO„, and acid
potassium tellurite, KHTeO,. Other more complicated series
of salts exist such as:—
KTеO, KTO, KTе013
The tellurites of the alkali metals are soluble in water, and are
formed by the solution of the acid in an alkali, or by fusing
the dioxide with an alkali. The tellurites of the alkaline earth.
metals are only slightly soluble, and those of the other metals
are insoluble in water, but soluble in hydrochloric acid.
Tellurous acid is converted into telluric acid by the action of
potassium
permanganate both in acid and alkaline solution.
The amount of tellurous acid in a solution may be estimated in
this manner?
TELLURIUM TRIOXIDE. TeOg.
252
This oxide is prepared by heating crystallized telluric acid
to nearly a
a red heat. If it is heated too strongly, a small quantity
of dioxide
is formed with evolution of oxygen, but this can be
separated by treatment with hydrochloric acid, in which it dis-
1 Klein, Ann. Chim. Phys. [6], 10, 1
108.
2 Brauner Monatsh. 12, 29.
444
THE NON-METALLIC ELEMENTS
solves whilst the trioxide is insoluble. Tellurium trioxide is an
orange yellow crystalline mass, which, when strongly heated
decomposes into oxygen and the dioxide.
TELLURIC ACID. HTеO.
253 When tellurium or the dioxide is fused with carbonate of
potash and saltpetre, potassium tellurate is formed; thus :—
Te₂ + K₂CO3 + 2KNO₂ = 2K,TеO₁ + N₂ + CO.
The same salt is produced when chlorine is passed through
an alkaline solution of a tellurite; thus:-
K₂TeO3 + 2KOH + Cl₂ = K¿TeО + 2KCl + H₂O.
On dissolving the fused mass in water, and adding a solution of
barium chloride, the insoluble barium tellurate is precipitated;
this is purified by washing with water, and afterwards decom-
posed by the exact amount of sulphuric acid necessary. The
clear acid solution, filtered from the sulphate of barium, gives on
evaporation crystals of the hydrated acid having the composition
H¸TeО + 2H,O. These are sparingly soluble in cold, but
readily soluble in hot water. When the crystals are heated to
160° they lose their water of crystallization, and the telluric acid
remains as a white powder nearly insoluble in cold, but readily
dissolving in hot water, forming a solution which deposits the
crystalline hydrate.
The Tellurates. Amongst the tellurates only those of the
alkali-metals are more or less readily soluble in water, those of the
remaining metals being either sparingly soluble or insoluble in
water, although generally dissolving readily in hydrochloric acid.
Certain of the tellurates are found to exist in two modifica-
tions, viz. :-
:-
(a) As colourless salts, soluble in water or in acids.
(b) As yellow salts, insoluble in water and in acids.
Besides these modifications we are acquainted not only with
normal and acid tellurates, but with several other series of
acid salts; thus we have:-
(1) Normal potassium tellurate, KTеÒ¸ + 5H¸O, obtained
upon evaporation of a solution, either in the form of crystalline
crusts, or as a gum-like residue, both being soluble in water.
THE TELLURATES
445
(2) K„Te0¸ + TeO3 + 4H„O, a salt sparingly soluble in cold,
but dissolving freely in hot water, and crystallizing to a woolly
mass.
(3) K₂Te0¸+3TeO¸+4H₂O (or 2KHTeO¸ + 2H„Te¤¸ + H₂O),
a salt sparingly soluble in water, obtained by adding nitric acid.
to salt No. 2.
The yellow modification of this salt is obtained when salt No. 2
is heated to redness. On adding water to the residue, normal
potassium tellurate dissolves out, and a tetratellurate remains
behind, being insoluble in water and in dilute acids.
Barium tellurate, BaTeO, + 3H,O, is a white powder, not pre-
cipitated in dilute solutions as it is not quite insoluble in water.
The di- and tetra-tellurate of barium, as well as calcium and
strontium tellurates, are similar white precipitates, whilst
magnesium tellurate is rather more soluble. All the other
tellurates are insoluble in water.
When a tellurate is heated to redness, oxygen is evolved and
a tellurite formed, and this reduction also occurs, with the
evolution
of chlorine, when a tellurate is heated with hydro-
chloric acid; thus:-
K¿TeО¸ + 2HCl = K„TeO3 + H₂O + Cl2.
254 Tellurium Oxychloride, TeOCl-This substance¹ is
obtained by heating the compound of tellurium dioxide with
hydrochloric acid, TeO, + 2HCl, above 90°,
TeO2, 2HCl=TeOCl2 + H₂O.
On further heating it decomposes into the tetrachloride and
the dioxide.
255 A
pared by
solutions
2TeOCl₂ = TeCl + TeO2.
4
in a
Tellurium Oxybromide, TeOBr.—This compound is obtained
similar manner to the oxychloride and is a faintly yellow
coloured mass. (Ditte).
TELLURIUM AND SULPHUR.
TELLURIUM DISULPHIDE, TES₂2.
disulphide, TeS,, and a trisulphide, TeS,, were pre-
Berzelius by the action of sulphuretted hydrogen upon
containing an alkali tellurite and telluric acid
1 Ditte, Compt. Rend. 83, 336, 446.
446
THE NON-METALLIC ELEMENTS
respectively. Both of these compounds yield up nearly all their
sulphur to carbon bisulphide and are therefore probably
mixtures.¹
Salts of the formula 3K,S, TeS, (potassium thio-tellurite) and
K,TeS (thio-tellurate) are however known.
TELLURIUM SULPHOXIDE. TeSO..
256 This compound is obtained by the direct union of tellurium
with sulphur trioxide. It is a red amorphous, transparent
solid, which melts at 30°. On heating for some time to 35°, it
becomes of a light reddish brown colour. Water decomposes it
with formation of tellurium and sulphuric acid, along with
other products. On heating to 230°, it loses sulphur dioxide
and forms tellurium monoxide.
NITROGEN. N
13'94.
257 Dr. Rutherford, Professor of Botany in the University of
Edinburgh, showed in the year 1772 that when animals breathe
in a closed volume of air, it not only becomes laden with impure
air from the respiration, but contains, in addition, a constituent
which is incapable of supporting combustion and respiration.
He prepared this constituent by treating air in which animals
had breathed with caustic potash, by means of which the fixed
air (carbonic acid) can be removed. The residual air was found
to extinguish a burning candle, and did not support the life of
animals which were brought into it.3
In the same year Priestley found that when carbon is burnt
in a closed bell-jar over water, one-fifth of the common air is
converted into fixed air which can be absorbed by milk of lime;
a residual (phlogisticated) air incapable of supporting either
combustion or respiration being left. Priestley, however, did not
consider that this air was a constituent of the atmosphere, and
it is to Scheele that we owe the first statement, contained in his
treatise on Air and Fire, that the "air must be composed of
two different kinds of elastic fluids." The constituent known
as mephitic or phlogisticated air was first considered to be a
=
1 Becker, Annalen, 180, 257.
2 Weber, J. Pr Chem. [2], 25, 218; Divers and Shimose, Ber. 16, 1009.
3 Rutherford, De aere Mephitico. Edinb. 1772.
1 PREPARATION OF NITROGEN
447
simple body by Lavoisier, who gave to this gas the name
azote (from a, privative, and (wn, life, by which it is still usually
designated in France). Chaptal first suggested the name
nitrogen, which it now generally bears (from virpov, saltpetre,
and yevvá I give rise to), because it is contained in saltpetre.
Nitrogen is found in the free state in the atmosphere, of
which it forms four-fifths by bulk, and occurs also in combi-
nation in many bodies such as ammonia, in the nitrates, and in
many organic substances which form an essential part of the
bodies of vegetables and animals.
258 Preparation. The simplest method for preparing nitrogen
is to remove the oxygen from the air. This can be done in a
variety of ways:-
-
(1) A small light porcelain basin is allowed to swim on the

FIG. 126.
water of
a
pneumatic trough, a small piece of phosphorus
brought into the basin and ignited, and the basin then covered
by a large tubulated bell-jar (see Fig. 126). The phosphorus
burns with the deposition of a white cloud of phosphorus
pentoxide,
one-fifth of
which, however, soon dissolves, whilst on cooling,
with water.
the contents of the bell-jar is found to be filled
The colourless residual gas is nitrogen; this may
be easily proved by first equalising the level of the water inside
and outside the bell-jar, after which, on opening the stopper
and plunging
seen to be instantly extinguished.
a burning taper into the bell-jar, the flame is
instantly extinguished. The nitrogen thus obtained
is never perfectly pure, as it always contains small quantities
of oxygen
which have not been removed by the combustion of
the phosphorus. In presence of aqueous vapour, phosphorus
448
THE NON-METALLIC ELEMENTS
slowly absorbs the oxygen of the air, at temperatures above
15, whilst the sulphides of the alkali metals as well as moist
´sulphide of iron (obtained by heating flowers of sulphur with
iron filings) act in a similar way.
(2) In order to obtain pure nitrogen, air contained in a gas-
holder is allowed to pass through tubes, T and T', Fig. 127,
containing caustic potash and sulphuric acid for the purpose of
purifying the air from carbon dioxide and drying it. The air
thus purified is passed over turnings of pure metallic copper
contained in a long glass tube (e f) which is heated to redness
in a charcoal furnace; copper oxide is thus formed, and pure
nitrogen passes over and is collected in the pneumatic trough.

T
FIG. 127.
(3) Copper also rapidly absorbs oxygen from the air at the
ordinary temperature in presence of a solution of ammonia; in
order to obtain nitrogen by this method a slow current of air is
passed through a tall cylinder containing copper turnings over
which a solution of ammonia is allowed to drop continuously.
The oxygen is absorbed and the resulting nitrogen, after
washing with water to remove traces of ammonia, is collected
in the usual manner. The last traces of oxygen are best
removed by a solution of chromous chloride, CrCl2, and this
method, when carried out with other precautions, yields abso-
lutely pure nitrogen.¹
(4) Pure nitrogen can also be prepared by heating a con-
1 Threlfall, Phil. Mag. [5], 35,,1.
PREPARATION OF NITROGEN
449
9
centrated solution of ammonium nitrite, the following reaction.
taking place:
NH,NO₂ = N₂+ 2H2O.
2
It is more convenient to employ a mixture of potassium nitrite
and ammonium chloride, which by double decomposition yield
potassium chloride and ammonium nitrite, the latter then
decomposing into nitrogen and water. As potassium nitrite
frequently contains free alkali it is advisable to add some
potassium bichromate in order to neutralise the latter; a
solution of one part of potassium nitrite, one part of ammonium

FIG. 128.
chloride, and one part of potassium bichromate in three parts.
of water gives good results.
(5) By action of chlorine upon ammonia, pure nitrogen is
also formed; thus:-
=
8NH3+3C12
N₂+6NH,CI.
The chlorine evolved in a large flask passes into a three-
necked Woulffe's bottle containing a strong aqueous solution of
ammonia. The nitrogen gas which is here liberated is collected
in the ordinary way over water, as shown in Fig. 128. Care
must, however, be taken in this preparation that the ammonia
30
1
450
THE NON-METALLIC ELEMENTS
is always present in excess, otherwise chloride of nitrogen may
be formed, and this is a highly dangerous body, which explodes
most violently (p. 477).
(6) On heating ammonium bichromate, nitrogen gas, chromium.
sesquioxide and water, are formed; thus:-
(NH4)2Cr2O7
Nitrogen may be obtained by this reaction at a cheaper rate
by heating a mixture of potassium bichromate and sal-ammoniac
instead of the ammonium bichromate, which is a somewhat
expensive salt; thus: -
K₂Cr₂O, +2NH4Cl = N₂+Cr₂O3 + 2KCl+4H₂O.
=
-
N₂+ Cr₂O3 + 4H₂O.
2
259 Properties-Pure nitrogen is a colourless, tasteless, inodor-
ous gas, which is distinguished by its inactive properties; hence it
is somewhat difficult to ascertain its presence in small quantities.
As has been said, it does not support combustion, nor does
it burn nor render lime water turbid. It combines directly
with but very few non-metals, although indirectly it can easily.
be made to form compounds with most of these elements,
and many of its compounds, such as nitric acid, ammonia,
chloride of nitrogen, &c., possess characteristic and remarkable
properties. It combines directly with a number of the metals,
such as calcium, barium, and magnesium, yielding compounds
termed nitrides. The specific gravity of nitrogen is 0.97209,
and one litre at 0°, and under the normal pressure, weighs
1·25749 grm. (Rayleigh).
Nitrogen was first liquefied by Cailletet by compressing it
to 200 atmospheres and allowing it suddenly to expand. It
has since been obtained in much larger quantities by
Wroblewski and Olszewski2 in the manner already described
(p. 84). Under a pressure of 33 atmospheres at -146° (its
critical temperature) it forms a colourless liquid, which boils
under atmospheric pressure at 194° and has
- 194 and has a sp. gr.
of
0885. If allowed to evaporate in a vacuum the temperature
is further lowered and at 214° the nitrogen solidifies, forming
a crystalline mass. If compressed nitrogen be cooled by means
of boiling oxygen and the pressure then somewhat diminished,
――――
1 Compt. Rend. 97, 1553; 98, 982; 102, 1010; Monatsh. 6, 204.
2
Compt. Rend. 99, 133; 100, 350; Ann. Chim. Phys. 31, 58.
PROPERTIES OF NITROGEN
451
the nitrogen separates out in crystalline flakes resembling
snow.
Nitrogen is but slightly soluble in water, one volume of water
absorbing only 002348 of the gas at 4°, and a smaller quantity
at higher temperatures.¹ The co-efficient of the solubility of
nitrogen (c) from 0-20° may be found by the following inter-
polation formula:-
c=0·023481 -0.0005799 +0.000008852.
The gas
is rather more soluble in alcohol than in water.
There are two characteristic spectra of nitrogen both obtained
by passing the spark from an induction coil through a Geiss'er's
tube containing a small quantity of highly rarefied nitrogen gas.
The nitrogen spectrum commonly obtained in this way is a
channelled one, exhibiting a large number of bright bands
especially numerous in the violet. If the spark is produced
by high tension, as when a Leyden jar is used, a spectrum of
numerous fine lines distributed throughout the length of the
spectrum will be obtained (Plucker).
Under ordinary conditions nitrogen is incombustible, but a
mixture of nitrogen and oxygen can be made to ignite under
certain circumstances. Thus if a powerful current of electricity
be passed through the primary of a large induction coil, an
arching flame is seen to issue from each secondary pole,
provided these are not too far apart, the two flames joining
in the centre; the flame is due to the combination of the
nitrogen and oxygen of the air to form oxides of nitrogen.
When the flame has once been formed the distance between
the secondary poles may be considerably increased without
extinguishing the flame, and the latter may then be blown out
and reignited by a taper. The flame does not extend to the
surrounding air because heat is absorbed in the combination,
and this can only therefore occur when energy is supplied to
the mixture from an external source.2
Assimilation of Free Nitrogen by Plants.-As already men-
tioned nitrogen forms an essential constituent of a very large
number of animal and vegetable substances and is necessary
for the maintenance of animal and vegetable life. It was how-
ever for a long time believed that members of the vegetable
1 Winkler, Ber. 24, 3605.
*Spottiswoode, Proc. Roy. Soc. 31, 173; Crookes, Chem. News, 1892, i. 302.
I
1
I
452
THE NON-METALLIC ELEMENTS
kingdom were unable to take up the free nitrogen of the air,
and that they were dependent for their supply of this element
on combined nitrogen contained in the atmosphere and in the
soil chiefly in the form of nitric acid and ammonia. About the
year 1886 it was shown that certain leguminous plants, such
as the white lupine, when grown in air free from ammonia and
other nitrogen compounds, contain more nitrogen than was
originally present in the seed and in the soil in which they were
sown, and they must, therefore, have obtained the excess from
the nitrogen of the air; since then it has been shown, certain
algæ, fungi and mosses behave in a similar manner, and it is
not improbable that further investigation will show that this
property is possessed by many other varieties. The assimilation
is brought about by the action of certain micro-organisms,
which absorb the free nitrogen from the air, forming compounds
of nitrogen which are then assimilated by the plant. Other
classes of micro-organisms also play a considerable part in
the assimilation of nitrogen by plants, as the latter are
incapable of directly assimilating ammonia, although they can
take up nitric acid; the micro-organisms in the soil bring
about the conversion of ammonia into nitric acid, one variety
of these converting the former into nitrous acid, and another
converting the nitrous into nitric acid.2
NITROGEN AND HYDROGEN.
AMMONIA. NH3 = 16·94.
260 Ammonia is found in the atmosphere in combination with
carbonic acid forming a small but essential constituent of the air.
It likewise occurs in combination with nitric and nitrous acids
in rain-water, and, especially as sal-ammoniac, NH,Cl, and as
sulphate of ammonia, (NH),SO,, deposited on the sides, the
craters, and in the crevices of the lava streams of active volcanoes,
as well as mixed with boric acid in the fumaroles of Tuscany.
Many samples of rock-salt also contain traces of ammoniacal
1 For further details see Lawes and Gilbert, Phil. Trans. 180 (B) 1; Journ.
Roy. Agric. Soc. 1891, 657; Proc. Roy. Soc. 46, 85; Schloesing, Compt. Rend.
113. 776; Marshall Ward, Nature, 49, 511.
2
Warington, Journ. Chem. Soc. 1891, i. 484.
AMMONIA
453
compounds, and all fertile soil contains this substance, which
is likewise found, although in small quantities, widely distributed
in rain and in running water, as well as in sea-water, in clays,
marls, and ochres. Ammoniacal salts are also found in the
juices of plants and in most animal fluids, especially in the
urine.
Ammonia was known to the early alchemists in the form of
the carbonate under the name of spiritus salis urinæ. In the
fifteenth century it was known that the same body may
be obtained by the action of an alkali upon sal-ammoniac;
and Glauber, in consequence, termed this body spiritus volatilis
salis armoniaci. Sal-ammoniac, which was known to Geber,
appears to have been brought in the seventh century from
Asia to Europe, and was known under the name of sal-armon-
iacum. It is possible that this sal-ammoniac was derived
from the volcanoes of Central Asia. Geber, however, describes
the artificial production of the salt by heating urine and common
salt together. In later times, sal-ammoniac was brought into
Europe from Egypt, where it was prepared from the soot
obtained by burning camel's dung. Its original name was
altered to sal-armoniacum, and then again changed to sal-
ammoniacum. This last name served originally among the
Alexandrian alchemists to describe the common salt (chloride
of sodium) and native sodium carbonate, which was found in
the Libyan desert in the neighbourhood of the ruins of the temple
of Jupiter Ammon. Boyle says in his "Memoirs for the Natural
History of Human Blood:"-" Though the sal-armoniac that is
made in the East may consist in great part of camel's urine, yet
that which is made in Europe, and commonly sold in our shops,
is made of man's urine." Later on, sal-ammoniac was obtained
by the dry distillation of animal refuse, such as hoofs, bones, and
horns; the carbonate of ammonia thus obtained being neutralized
with hydrochloric acid. From this mode of preparation ammonia
was formerly termed spirits of hartshorn.
Up to the time of Priestley, ammonia was known only in
the state of aqueous solution, termed spirits of hartshorn,
or spiritus volatilis salis ammoniaci. Stephen Hales, in 1727
observed that when sal-ammoniac is heated with lime in a
vessel closed by water, no air is given out, but, on the contrary,
water is drawn into the apparatus; Priestley, in 1774, repeated
this experiment, with the difference, however, that he used
1 Boyle, op. 4, 597, 1684.
T
1
454
THE NON-METALLIC ELEMENTS
mercury to close his apparatus. He thus discovered ammonia
gas, to which he gave the name of alkaline air. He also found
that, when electric sparks are allowed to pass through this
alkaline air, its volume undergoes a remarkable change, and
the residual air is found to be combustible. Berthollet, following
up this discovery in 1785, showed that the increase of volume
which ammonia gas thus undergoes is due to the fact that
it is decomposed by the electric spark into hydrogen and
nitrogen. This discovery was confirmed, and the composi-
tion of the gas more accurately determined by Austin (1788),
H. Davy (1800), and Henry (1809). It was shown by them
that, in the reaction above described, two volumes of ammonia.
are resolved into three volumes of hydrogen and one of
nitrogen.
It has been proved by Donkin that ammonia can be syntheti-
cally prepared by the direct combination of its elements,¹ the
silent electric discharge being, for this purpose, passed through
a mixture of nitrogen and hydrogen. If a mixture of 3 vols.
of hydrogen and 1 vol. of nitrogen is subjected to the action of
the electric discharge in a eudiometer, containing above the
mercury a little dilute sulphuric acid, the whole volume finally
disappears, the two gases slowly combining to form ammonia,
which dissolves in the acid as fast as it is formed. It is like-
wise obtained together with nitrous acid by passing nitrogen
over a mixture of platinum black and alkali.2
Ammonia is also formed:-
-
(1) By the putrefaction or decay of the nitrogenous con-
stituents of plants and animals.
(2) By the dry distillation of the same bodies; that is, by
heating these substances strongly out of contact with air.
(3) By the action of nascent hydrogen on the salts of nitric
or nitrous acid.
It is to the first of these processes that we owe the existence
of ammonia in the atmosphere, whilst the second serves for the
production of ammonia and its compounds, especially of sal-
ammoniac, on the large scale.
At the present day almost all the sal-ammoniac and other
ammonium salts are prepared from the ammoniacal liquor which
is obtained as a by-product in the manufacture of coal-gas.
Coal consists of the remains of an ancient vegetable world,
and contains about 2 per cent. of nitrogen, some of which, in
2 Loew, Ber. 23, 1443.
1 W. F. Donkin, Proc. Roy. Soc. 21, 281.
PREPARATION OF AMMONIA
T
455
the process of the dry distillation of the coal carried on in the
manufacture of coal-gas, is obtained in the form of ammonia
dissolved in the water and other products formed at the same
time.
In order to prepare sal-ammoniac from this liquor, which
contains very little if any free ammonia, but chiefly the sulphide,
carbonate, sulphite and thiosulphate of ammonia, it is boiled
with milk of lime to liberate the whole of the ammonia. This
ammonia distils over, and the distillate is neutralized with
hydrochloric acid, sal-ammoniac being formed as follows:-
NH,+HCl=NH,C.
The solution is then evaporated to dryness and the salt purified
by sublimation.
--
In most cases, however, the ammonia is passed into sulphuric
acid, thus forming ammonium sulphate, (NH4)2SO4, which is
much more readily purified, and from which most of the other
ammoniacal compounds are prepared.
Preparation. If any one of these ammoniacal salts be
heated with an alkali, such as potash or soda, or with an alkaline
earth, such as lime, the ammonia is set free as gas. In order to
prepare the gas it is only necessary, therefore, to heat together
sal-ammoniac and slaked lime; thus:-
2NH¸Cl + Ca(OH)₂ = 2NH¸ + CaCl₂ + 2H₂O.
In order to ensure the decomposition of all the sal-ammoniac, a
large excess of lime is usually employed. One part by weight
of powdered sal-ammoniac is for this purpose mixed with two
parts of caustic lime slaked to a fine dry powder; these are well
mixed together, and then introduced into a capacious flask, placed
on a piece of wire gauze and heated by a Bunsen-lamp. The
ammonia gas, which comes off when the mixture is gently heated,
is then dried by allowing it to pass through a cylinder filled with
small lumps of quick-lime, or by placing a layer of the latter
over the mixture of ammonium chloride and slaked lime in the
flask, and the gas thus dried may be collected either over
mercury, or, like hydrogen, by upward displacement in an
inverted dry cylinder as shown in Fig. 129, inasmuch as this
8:47
gas is
= 0·586 times as light as air, one litre weighing
14:391
at 0° and under 760 mm. pressure, 076193 grams.
I
456
THE NON-METALLIC ELEMENTS
261 Properties.-Pure ammonia is a colourless gas, possessing,
like its aqueous solution, a peculiar pungent alkaline odour and
caustic taste. In the solid state, however, it possesses but a
very faint smell. Ammonia gas turns red litmus-paper blue,
like the alkalis, neutralizes acids, and forms with them a series
of stable compounds, termed the ammonium salts. It is a
very stable compound, but at 1300° gradually decomposes into
its constituents.1
Ammonia is not combustible, and a flame is extinguished if

FIG. 129.
plunged into the gas. If, however, ammonia be mixed with
oxygen, the escaping gas may be ignited, and burns with a pale
yellow flame, with formation of water, nitrogen gas, and nitric
acid, HNO3. Another method of showing the combustibility
of ammonia is to put a jet of this gas into the air holes of an
ordinary Bunsen-burner, in which a flame of coal-gas is already
burning; the flame becomes at once coloured yellow, and in-
creases greatly in dimensions. A third experiment of this nature
is, to allow a stream of oxygen gas to bubble through a small
quantity of strong aqueous ammonia placed in a flask and
warmed as shown in Fig. 130; on bringing a light in contact
1 Crafts, Compt. Rend. 90, 309.
PROPERTIES OF AMMONIA
457
with the mixed gases issuing from the neck of the flask they
will be seen to burn with a large yellow flame.
Ammonia gas was first liquefied by Faraday, in 1823,
by heating a compound of silver chloride with ammonia,
placed in one limb of a strong hermetically sealed bent tube
whilst the other limb was placed in a freezing mixture. The
compound of silver chloride and ammonia is obtained by
saturating dry precipitated silver chloride with ammonia gas: it
has the formula AgCl(NH3), and fuses at 38°, whilst at about

FIG. 130.
115° it begins to part with its ammonia. The gas thus collects
in the tube until the pressure is reached under which it begins to
condense as a clear, highly refracting liquid. When the silver
chloride cools, the ammonia is again absorbed, the original com-
pound being re-formed. Liquid ammonia is also easily obtained
by leading the gas into a tube plunged in a freezing mixture
composed of crystallized calcium chloride and ice, and having a
temperature of -40° and forms a colourless highly refracting
liquid, boiling at -33°7 (Bunsen). When the temperature of
the liquid is lowered to below 75° in a bath of solid carbonic
-
458
THE NON-METALLIC ELEMENTS
acid and ether placed in vacuo, a mass of white translucent
crystals of solid ammonia is obtained (Faraday).
The co-efficient of expansion of liquid ammonia is 0.00204, and,
therefore, larger than that of most liquids having a higher boiling
point; its specific gravity compared with water at 0' is 0.6234.¹
The tension of liquid ammonia at 0° is 44 atmospheres, at
15°5, 69 atmospheres, and at 28°, 10 atmospheres.

B
P
a
C
m
m
FIG. 131.
f
C
10
The condensation of ammonia by pressure and the production
of cold by its evaporation can easily be shown by the following
experiment. The apparatus required for this purpose consists
essentially of two strong glass tubes (a and b, Fig. 131), which
are closed below and are connected together by the tubes (cc)
and (d d). The tube (dd) ends at () in a narrower tube
(mm), which is at this point melted into the tube (a). The tube
1 Joly, Annalen, 117, 181.
PROPERTIES OF AMMONIA
459
(a) is three-fourths filled with an alcoholic solution of ammonia
saturated at 8°, and then placed in the cylinder (A). The
syphon tube (g) and the tube (ff), which reach to the
bottom of the cylinder, are fixed in position through the cork.
In order now to perform the experiment, the cylinder (A) is
nearly filled with warm water; the glass stopcock (h) is
opened, and the tube (b) placed in ice-cold water. The water
contained in the flask is now quickly boiled, and thus the
water in (A) is rapidly heated to 100°, and the ammonia gas
driven out of solution until by its own pressure it liquefies in (b).
As soon as the condensation of liquid ammonia ceases, the
ebullition is stopped and a portion of the hot water is with-
drawn from the cylinder by means of the syphon (g), cold water is
allowed to enter the cylinder, and after a while this is replaced
FIG. 132.
by ice-cold water. The cylinder (B) is now removed, when the
liquefied ammonia begins to evaporate and is again absorbed by
the alcohol, though only slowly. But, on closing the stopcock (h),
the gas above the alcohol is quickly absorbed, and thus the
equilibrium is disturbed. The ammonia now passes rapidly
through the tube (mm), and is absorbed so quickly that the
liquid ammonia in (b) begins to boil, by which the temperature
is so much lowered, that if a test-tube containing water is placed
outside (b) it is soon filled with ice.
Ammonia may be used for the artificial production of ice.
For this purpose an apparatus (Fig. 132) has been invented by
M. Carré. It consists of two strong iron vessels connected by a
vent-pipe of the same metal. The cylinder (A) contains water
saturated with ammonia gas at 0°. When it is desired to procure
1
Ka Kang
p
460
THE NON-METALLIC ELEMENTS
ice, the vessel (A) containing the ammonia solution, which we may
term the retort, is gradually heated over a large gas-burner. The
ammonia gas is thus driven out of solution, and as soon
as the
pressure in the interior of the vessel exceeds that of seven
atmospheres, it condenses in the double-walled receiver (B).
When the greater portion of the gas has thus been driven out of
the water, the apparatus is reversed, the retort (A) being cooled
in a stream of cold water, whilst the liquid which it is desired
to freeze is placed in a cylinder which fits into the interior
portion of the hollow cylinder. A reabsorption of the
ammonia by the water now takes place, and a consequent
evaporation of the liquefied ammonia in the receiver. This
evaporation is accompanied by the absorption of heat which
becomes latent in the gas. Thus the receiver is soon cooled
down far below the freezing point, and the liquid contained
in the vessel (D) is frozen. For the production of larger
quantities of ice, a continuous ammonia freezing machine of more
complicated construction, but arranged on the same principle,
has been devised by M. Carré, in which 10 kilos. of ice can
be prepared by the combustion of 1 kilo. of coal.¹
262 Ammonia gas is very soluble in water; one gram. of water
absorbs at 0° and under normal pressure, 0.875 grms. or 1148 cc.
of the gas. The solubility at different temperatures and under
a pressure of 760 mm. of mercury is given in the following
table (Roscoe and Dittmar) :-
Temp. Gram. Temp. Gram.
0°
2°
0.875 16° 0:582
0.343 52°
32° 0.382 48°
0.833 18° 0.554 34° 0.362 50°
0.792 20° 0.526 36°
0.751 22° 0.499 38°
0:324 54°
0.713 24° 0.474 40°
0:307
10° 0.679 26°
0.449 42° 0.290
122 0.645 28° 0.426
56°
44° 0.275
14°
0.612 30° 0.403
46°
0.259
8°
Temp. Gram. Temp. Gram.
1
0.244
0.229
0.214
0.200
0.186
The same observers have found that the absorption of ammonia
1 Ice-making machines depending on the same principle are now in use in
which liquefied sulphur dioxide, or ether, are employed instead of aqueous
2 Journ. Chem. Soc. 1860, 128.
ammonia.
SOLUBILITY OF AMMONIA
461
in water does not follow the law of Dalton and Henry at the
ordinary atmospheric temperature, inasmuch as the quantity
absorbed does not vary directly as the pressure. Sims ¹ has
shown that at higher temperatures the deviations from the law
become less until at 100° the gas follows the law, the quantity

FIG. 133.
absorbed being directly proportional to the pressure, taken of
course under pressures higher than the ordinary atmospheric
pressure, inasmuch as boiling water does not dissolve any of the
gas under the pressure of one atmosphere.
1 Journ. Chem. Soc. 1862, 17.
462
THE NON-METALLIC ELEMENTS
In order to show the great solubility of ammonia gas in water
the same apparatus may be employed which was used for
exhibiting the solubility of hydrochloric acid in water (Fig.
133), the lower balloon being filled with water slightly coloured
with red litmus solution.
The liquor ammonia of the shops, a solution of the gas in
water, is prepared (as shown in Fig. 134) by passing the gas, which

M
FIG 134.
has been previously washed, into a flask containing water kept
cool by being placed in a large vessel of cold water, considerable
heat being evolved in the condensation of the gas.
The commercial liquor ammonia is now frequently prepared
directly from the ammoniacal liquor of the gas-works instead of
from sal-ammoniac. For this purpose the liquor is heated
together with milk of lime in an iron boiler having a capacity
COMPOSITION OF AMMONIA
463
of 1,000 gallons.
The gas which is evolved is first passed
through a long system of cooling tubes before it enters the
washing and condensing apparatus. The condenser consists of
a series of tubes filled with charcoal, by means of which any
remaining empyreumatic impurities, are removed. By this
method, if the system of tubes be sufficiently long, and by the
use of a sufficient number of wash-bottles, a perfectly pure
liquor ammoniæ can be obtained.
The fact that great heat is evolved in the production of the
saturated solution of ammonia is rendered evident by the
following experiment. If a rapid current of air be passed
through a cold concentrated solution of ammonia, the gas will
be driven out of solution, and an amount of heat will be absorbed
exactly equal to that which was given off when the solution of
the gas was made, in consequence of which the temperature
of the liquid will be seen to fall below -40°. A small quantity
of mercury may thus be frozen. Ammonia is very soluble
in alcohol, and this solution, which is frequently used in the
laboratory, is most conveniently prepared by gently warming a
concentrated aqueous solution of ammonia and passing the gas
thus evolved into alcohol. This method may also be employed
for preparing the gas in place of heating sal-ammoniac with
lime, or the concentrated solution may be allowed to drop from
a tap funnel on to lumps of caustic soda placed in a cylindrical
glass vessel, by which means a regular and continuous evolution
of the gas is obtained.
The following table gives the percentage of ammonia con-
tained in aqueous solutions of different specific gravity (Carius)
Specific Gravity.
0.8844
0.8864
0.8976
0.9106
0.9251
0-9414
0.9593
0.9790
Per Cent. NH3.
36.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
263 Composition of Ammonia.-In order to determine the
composition of ammonia the arrangement shown in Fig. 135 is
employed. The ammonia gas is placed in the closed limb of
I
464
THE NON-METALLIC ELEMENTS
the syphon eudiometer, after which the mercurial column in both
limbs is brought to the same height, the volume accurately
read off, and a series of electric sparks from the induction coil
allowed to pass through the gas until its volume undergoes no
further alteration. The tube and gas are next allowed to cool,
and the pressure in both limbs again adjusted. On the volume
being again measured it is seen to have doubled. Oxygen is
next added in such proportion that the mixture shall contain
no more than 35 per cent. of the explosive mixture of oxygen
and hydrogen (2 volumes of hydrogen to 1 of oxygen), and
an electric spark is passed through the mixture. From the
alteration of volume which takes place, the proportion of
hydrogen to nitrogen can readily be deduced, as is seen from
the following example :-
―
Volume of ammonia
nitrogen and hydrogen
After the addition of oxygen.
After the explosion .
""
20.0
40.0
157.5
112.5
Hence 45 volumes have disappeared, of which 30 consisted of
hydrogen; consequently two volumes of ammonia contain three
volumes of hydrogen and one volume of nitrogen.
That the relation between hydrogen and nitrogen in ammonia
gas is in the proportion of three volumes of the former to one
of the latter can be shown by the following experiment thus
clearly described by Prof. Hofmann :-¹
A glass tube for holding chlorine, having a small stoppered
portion separated from the rest of the tube by a glass stopcock
and used for receiving solution of ammonia, and admitting it,
drop by drop, to the chlorine, constitute the requisite apparatus
(Fig. 136). The glass tube is from 1 to 15 metre long, sealed
1 Introduction to Modern Chemistry.
COMPOSITION OF AMMONIA
465
at one end, open at the other, and marked off, by elastic
caoutchouc rings slipped over it and clipping it firmly, into
three equal portions.
The apparatus is thus employed. The long chlorine-tube having
been filled with lukewarm water and inverted over a pneumatic
trough, with its mouth immersed below the water-level,
is filled with chlorine in the usual way (see Fig.
137). When full, it is still allowed to stand for about
FIG. 136.
fifteen minutes over the chlorine delivery-tube, so that its
interior surface may be quite freed from the chlorine-
saturated water that would else remain adherent to it. The
stopcock is now closed and the chlorine thus shut in the
tube. This is then removed from the trough, and turned round
so that the stoppered end is uppermost. The small space
between the stopcock and the end is next two-thirds filled with
a strong solution of ammonia, and a single drop of the ammonia-
solution is suffered to fall into the chlorine tube, the stopcock
being opened for a moment for this purpose (Fig. 138). The
entrance of this drop into the atmosphere of chlorine is marked
by a small, lambent, yellowish-green flame at the point where
the drop enters the gas. Drop by drop, at intervals of a few
seconds, the ammonia solution is allowed to fall into the
chlorine-tube, the ammonia of each drop being converted, at the
instant of its contact with the chlorine, into hydrochloric acid
31
466
THE NON-METALLIC ELEMENTS
and nitrogen with a flash of light and the formation of a dense
white cloud. The addition of ammonia must be continued till
the whole of the chlorine present is supplied with hydrogen
at the expense of ammonia. To insure the ammoniacal solu-
tion being added in excess, a column of three or four centimetres
is abundantly sufficient. The result is that the hydrochloric
acid formed combines with the excess of ammonia to form a
compound, which makes its appearance as a white deposit lining
the interior of the chlorine tube. This deposit, being soluble,
is readily washed down and dissolved by agitating the liquor in

FIG. 137.
the tube, which now contains the whole of the nitrogen
separated.
We are now sure of two points, viz., that the whole of the
chlorine has been converted into hydrochloric acid at the
expense of the ammonia; and, that we possess within our tube
the whole of the nitrogen thus set free.
It becomes our next object to withdraw the excess of the
ammonia. For this purpose dilute sulphuric acid, which fixes
ammonia, is introduced by means of the portion of the tube
previously employed to admit ammonia.
COMPOSITION OF AMMONIA
467
The nitrogen, being thus freed from all intermixed gaseous
bodies, has only now to be brought to mean atmospheric
temperature and pressure in order that it may be ready for
measurement.
To equalize the pressure within and without the tube, the
bent syphon tube (Fig. 139) is employed. One end of this
communicates with the interior of the tube, while the other
plunges beneath the surface of water subject to atmospheric
in the tube
That this pressure exceeds that of the gas
pressure.
is at once seen by the flow of water through the syphon into

FIG. 138.
the tube. As the water-level in the tube rises, the nitrogen,
previously expanded, gradually approaches its normal volume,
which it exactly attains when the flow ceases, showing the
pressure within and without to be in equilibrium. Both tem-
perature and pressure being now at the mean, all the requisite
conditions are fulfilled for obtaining an exact knowledge of the
true volume of nitrogen; and this, on inspection, is found to
exactly fill one of the three divisions marked off at the outset on
our tube. Now, bearing in mind that we started with the three
divisions full of chlorine, and that we have saturated this
chlorine with hydrogen supplied by the ammonia; bearing in
1
1
1
B
1
468
THE NON-METALLIC ELEMENTS
mind, moreover, that hydrogen combines with chlorine, bulk for
bulk, it is evident that the one measure of nitrogen which
remains in the tube has resulted from the decomposition of a
quantity of ammonia containing three measures of hydrogen.
It is, therefore, clearly proved by this experiment that ammonia
FIG. 139.
is formed by the union of three volumes of hydrogen with one
volume of nitrogen (Hofmann).
Another method of demonstrating the same fact is by the
electrolysis of a strong solution of ammonia. For this purpose,
the solution, mixed with a little ammonium sulphate to increase
its conductivity, is introduced into a Hofmann apparatus
(Fig. 76, p. 250), and subjected to the action of a moderately
strong current of electricity; hydrogen is evolved at the neg-
COMBINATION OF AMMONIA WITH ACIDS
469
ative pole and nitrogen at the positive pole, the volume of the
former being three times as large as that of the latter.
264 Detection and Estimation of Ammonia.-The method
adopted for the detection and estimation of small traces of
ammonia with Nessler's reagent has already been described
under Natural Waters (see p. 300). If the quantity of ammonia
or of ammonium salt be larger, it may be detected by the
peculiar smell of the gas, by its alkaline reaction, and by the
formation of white fumes in presence of strong hydrochloric
acid. These fumes consist of ammonium chloride NH,Cl, and
are formed by the direct combination of the ammonia and
hydrogen chloride :
NH3 + HCl = NH₁Cl.
If, however, the gases are mixed together in a perfectly dry
condition, no reaction takes place, but on addition of a little
moisture combination immediately ensues. When an am-
moniacal salt is present, the ammonia must be liberated by
heating the solid salt, or its solution, with a caustic alkali. For
the estimation of ammonia in quantities larger than those for
which Nessler's method is applicable, it is usual to distil the
ammonia either into hydrochloric or sulphuric acid of known
strength, and then to ascertain, by volumetric analysis with a
standard solution of alkali, the amount of acid remaining free,
or into hydrochloric acid of unknown strength to which a
solution of chloroplatinic acid, H.PtCl, is added. On evaporat-
ing the resulting solution to dryness on a water bath, and
exhausting with alcohol, an insoluble yellow precipitate of am-
monium platinochloride, (NH),PtCl, is left, and this can either
be collected on a weighed filter, or it may be ignited and the
quantity of the metallic platinum remaining weighed, from
which the weight of ammonia is calculated.
In addition to its use in the laboratory and as a means of
obtaining artificial cold, ammonia is also largely employed for
the preparation of alum, carbonate of soda, aniline colours, and
in the manufacture of indigo.¹
Combination of Ammonia with Acids.-Mention has been
frequently made in the foregoing pages of the combination
which takes place between ammonia and acids. Thus ammonia
unites with hydrochloric acid to form the compound NH„HCl,
On the Manufacture of Liquor Ammoniæ see Lunge's work on Coal-Tar and
Ammonia, p. 667. London, 1887
CA LT-
I
470
THE NON-METALLIC ELEMENTS
or NHCl, and with hydrobromic acid to form NH¸HBr, or
NH,Br, whilst with sulphuric acid it yields 2NH„H₂SO or
(NH4)2SO4
In a similar manner ammonia combines with
almost all other acids yielding compounds containing the group
of atoms NH. Thus the compound NH4Cl may be regarded as
hydrochloric acid, HCl, in which the hydrogen atom has been
replaced by the group NH4, just as potassium chloride is hydro-
chloric acid in which the hydrogen atom is replaced by a
potassium atom. The two substances do in fact bear a strong
resemblance to each other both in their chemical and physical
properties, and all the other substances obtained by the com-
bination of ammonia with acids are similarly related to the salts
of potassium.
The groups of atoms NH, is therefore a never varying con-
stituent in a series of compounds, and behaves in these
compounds as though it were a simple substance: to such a
group the name of "compound radical" is given, and as in this
case the group in combination has the same effect on the
properties of the substance as a metal, it is termed ammonium,
the compounds with acids being known as the ammonium salts,
as they correspond to the salts of potassium, sodium, &c. They
will be described together with these in Vol. II., Part I.
HYDRAZINE OR DIAMIDE, N₂H.
=
265. This compound was first obtained by Curtius in 1887
by the action of hot dilute acids on triazo-acetic acid, a sub-
stance described in Vol. III., Part II. (2nd edition) p. 107, which
has the composition C,H,N(COOH), and contains the group
-N-N- three times.¹ It has since been obtained from other
organic compounds containing two nitrogen atoms combined
together; among these may be mentioned amidoguanidine,
which has the constitution NH,C(NH). NH. NH₂. As this
substance is obtained without difficulty from ammonium thio-
cyanate, it affords the best means for the preparation of hydra-
zine in quantity, and a description of the preparation may be
therefore shortly given here, although the intermediate com-
pounds will not be described till later.
Ammonium thiocyanate, NH,CNS, is heated for some time.
at a constant temperature of 170—180°, and the residue, which
consists chiefly of guanidine thiocyanate, treated first with
1 Curtius, Ber. 20, 1632.
———
HYDRAZINE
I
471
strong sulphuric acid, then with a little fuming sulphuric acid,
and the mixture after cooling mixed with nitric acid of
sp. gr. 15, and the whole poured into water. Crude nitro-guan-
idine, NH.C(NH).NH.NO.2, separates out, and is at once treated
with zinc dust and just sufficient dilute acetic acid for its
reduction. The solution obtained by the reduction of 208
grams of nitroguanidine is then evaporated to 1200 cc., mixed
with a solution of 260 grams of caustic soda in 500 cc. of
water, and boiled for 8-10 hours. The cooled liquid is poured
off from the sodium hydrogen carbonate which separates out,
and mixed with 260 cc. of concentrated sulphuric acid; the
greater part of the hydrazine separates out as the sulphate,
which is quite pure after a single recrystallization.¹
Free hydrazine has not up to the present been obtained in
a pure condition, owing to the great readiness with which it
combines with water to form a hydrate; hitherto it has not
been found possible to remove the water completely by the
action of even the strongest dehydrating agents. If a mixture
of the hydrate and anhydrous baryta is heated in a sealed tube
at 170°, and the latter opened after cooling, a gas is given off
which has an extremely penetrating odour, fumes in the air,
and probably consists of free hydrazine.
266. Hydrazine Hydrate, N,H,,H,O, is best obtained by dis-
tilling hydrazine sulphate with a solution of caustic potash in
a silver retort, connected without rubber or cork to a silver
condensing tube; the distillation is continued till the last drop
has passed over, and the distillate subjected to fractional dis-
tillation, by which the hydrazine hydrate is readily separated
from the excess of water.
Hydrazine hydrate forms a strongly refractive almost odour-
less caustic-tasting liquid, which fumes strongly in the air, but
may be kept unaltered in closed vessels; it boils without decom-
position at 118.5° under 739.5 mm. pressure, and solidifies in a
mixture of solid carbonic acid and ether, but melts again below.
-40°. It has a sp. gr. of 10305 at 21°, and a vapour density
at 100° under diminished pressure, agreeing with the formula
NH, H₂O, whilst at 170° it is completely split up into hydrazine
and water; if the temperature be further increased the vapour
density also increases, until at very high temperatures the latter
corresponds with a formula double that observed at 100°. The
cause of this abnormal behaviour has not yet been ascertained.
1 Thiele, Annalen, 270, 1.
2
8
·
I
I
I
1
I
1
W
472
THE NON-METALLIC ELEMENTS
In aqueous solution a determination of the molecular weight
gave numbers corresponding to the formula N,H,.2H₂O.
Hydrazine hydrate when hot attacks glass strongly, and also
quickly destroys cork and indiarubber; it is a very strong poison
for lower organisms. It is the strongest reducing agent known,
quickly precipitating all the more easily reducible metals from
their solutions even in the cold. It is a very strong base, and
like ammonia unites with acids to form well-defined salts, most
of which are readily soluble in water. Unlike ammonia, how-
ever, it forms more than one series of salts, giving with hydro-
chloric acid for example two salts, N,H,.HCl and N,H,.2HCl,
and with hydriodic acid three, NH,.HI, NH,.2HI, and
3NH2HI. These will be described, together with the
ammonium salts, in Vol. II., Part I.
AZOIMIDE OR HYDRAZOIC ACID, N¸H.
267. This interesting substance, was like the foregoing, dis-
covered by Curtius, who obtained it by the action of nitrites on
derivatives of hydrazine, the reaction being analogous to that of
nitrites on ammonia. As already mentioned, the interaction of
the latter substances leads to the formation of nitrogen, according
to the equation
NH,CI+ NaNO, N₂ + 2H2O + NaCl,
=
and the formation of azoimide is represented in a similar
manner by the equation
NH₂
N
+ NO.OH = ||
N
I
NH,
NH +2H₂O;
hydrazine itself may be used if only a dilute solution of
azoimide is required, but as a rule it is preferable to employ
one of its organic derivatives, in which one of the hydrogen
atoms is replaced by an organic radical. Curtius in his ex-
periments employed hippurylhydrazine, C,H,NO₂HN.NH₂,
which by the action of nitrous acid yields hippurylazoimide,
N
CH,NON ; the latter when treated with acids or alkalis is
N
8
1 Loew, Ber. 23, 3203.
2 Curtius and Jay, J. Pr. Chem. [2] 39, 33; Curtius and Schultz, J. Pr.
Chem. (2), 42, 521.
3 Ber. 26 1263.
AZOIMIDE
473
converted into azoimide and hippuric acid.¹ The former distils
over with the water on boiling the solution, which must first be
acidified if an alkali has been employed for the hydrolysis.
Numerous derivatives of azoimide in which the hydrogen
atom has been replaced by an organic radical had long been
known, the most readily prepared of which is the phenyl
N
derivative, C.H..NI (Vol. III., Part III., 2nd edition, p. 325),
N
this substance being known as triazobenzene or diazoben-
zeneimide. This compound itself cannot be converted into
azoimide by the action of acids or alkalis, but Noelting,
Grandmougin and Michel have found that if certain acid
radicals such as the group NO, be introduced into the phenyl
group, the resulting azoimide may be in many cases converted
into the mother substance by the action of alkalis. Tilden
and Millar have shown 3 that diazobenzeneimide is converted
N
[[, by the action of hot
N
into a nitro-derivative, CH,(NO)N
nitric acid of sp. gr. 14, and that this compound on boiling
with alcoholic potash and subsequent acidification yields.
azoimide, a dilute solution of which is obtained by distilling
the liquid.
Another convenient source of azoimide is the crude amido-
guanidine obtained in the preparation of hydrazine (p. 470);
this substance is converted by nitrous acid into diazoguani-
dine nitrate, NH.C(NH).NH.N:N.NO3, which on boiling with
alkalis yields azoimide and cyanamide; the former may be
isolated by acidification and distillation in the manner already
described.
A further very interesting synthesis of azoimide from purely
inorganic sources has also been described by W. Wislicenus,*
who obtained its sodium salt by the action of nitrous oxide on
sodamide :
NaNH,+N,O = NgNa + H,O.
3
1 Ber. 23, 3023.
3 Journ. Chem. Soc. 1893, i. 256.
The water formed acts on a further molecule of sodamide
yielding caustic soda and ammonia. The sodamide is obtained.
by passing ammonia over metallic sodium at a temperature of
150-250°, and as soon as all metallic sodium has disappeared,
2 Ber. 24, 2546; 25, 3328.
4 Ber. 25, 2084.
474
THE NON-METALLIC ELEMENTS
the stream of ammonia is replaced by one of nitrous oxide, and
continued till ammonia is no longer evolved; the product is
then dissolved in water and distilled with dilute sulphuric acid.
The dilute solution obtained by any of the above processes
may be concentrated by fractional distillation until the solution
contains 91 per cent. of azoimide, and the remainder of the
water is then removed by calcium chloride. Pure azoimide
forms a colourless mobile liquid which has a most pene-
trating, unbearable odour, boils without decomposition at 37°,
and dissolves readily in water and alcohol. When brought in
contact with a hot body it explodes with extreme violence,
giving a bright blue flash; the explosion also sometimes takes
place at the ordinary temperature, rendering the substance an
extremely dangerous one to work with. Thus on one occasion
005 gram was introduced into a barometric vacuum, and
exploded with such violence that the glass and mercury were
reduced to dust and spread over the whole of a large room, and
in another case 07 gram exploded when taken out of a freezing
mixture, breaking all the bottles in the neighbourhood, and
somewhat severely injuring one of the investigators.¹
The aqueous solution of azoimide behaves as a strong acid
and readily dissolves zinc, iron, magnesium, and aluminium.
with evolution of hydrogen and formation of salts of the
metals. It also gives salts with the metals of the alkalis and
alkaline earths and forms a silver salt, AgNg, and a mercurous
salt, HgNg, both of which are extremely explosive, but in other
respects closely resemble the corresponding chlorides. The
aqueous solution of the acid, as will be seen from the pro-
perties already given, behaves in a similar manner to aqueous
hydrochloric acid. Hence the group Ng must be itself
electronegative, and is analogous in its chemical properties
with the halogens.
The
The salts formed by azoimide with the metals of the alkalis
of the alkaline earths are not nearly so explosive as those
formed with the heavy metals, and azoimide itself when in
dilute aqueous solution may with reasonable care be handled
without danger.
Other compounds of nitrogen and hydrogen have been
prepared by Curtius; these consist of the salts formed by
ammonia and hydrazine with azoimide, and will be described
with the other ammonium and hydrazine salts.
1 Curtius and Radenhausen, J. Pr. Chem. [2], 43, 207.
HYDROXYLAMINE
475
HYDROXYLAMINE, NH₂O.
268. This compound was discovered in 1865 by Lossen,¹
but until 1891 was only known in the form of salts or in
aqueous solution. Lossen obtained it by the action of nascent
hydrogen on nitric oxide, the reaction taking place as follows:
2NO+6H = 2NH₂O.
Since that time it has been obtained in other ways, such for
example as the reduction of nitric acid by metals under suitable
conditions, as a product of decomposition of the fulminates 3
(Vol. III, part I., p. 524), and by the action of sulphuretted
hydrogen on silver nitrite.¹
By the interaction of nitrites and sulphites under suitable
conditions, salts of hydroxylaminedisulphonic acid (p. 520) are
formed, and these on heating with water are converted into
hydroxylamine sulphate. To prepare it by this, the most con-
venient method, a concentrated aqueous solution of sodium hy-
drogen sulphite (2 mols.) is added to a similar solution of sodium
nitrite (1 mol.), the temperature not being allowed to rise much
above 0°; a sufficient quantity of potassium chloride is then
added to convert the whole of the disulphonic acid into the
less soluble potassium salt, which separates out in compact
crystals. The latter are then heated in neutral aqueous solution
at 100° for a long time, or for a short time at 130°, and the
resulting hydroxylamine sulphate separated from the sparingly
soluble alkaline sulphates by fractional crystallisation.5
•
For a long time free hydroxylamine was only known in solution,
but in 1891 the anhydrous compound was prepared almost
simultaneously by Lobry de Bruyn, and Crismer. The former
prepared it by dissolving hydroxylamine hydrochloride in
absolute methyl alcohol, adding a solution of sodium methylate,
NaOCH,, in the same solvent, separating the sodium chloride
formed, and distilling off the greater portion of the methyl
alcohol under 100 mm. pressure; the residue is then distilled
in small portions under 40 mm. pressure with the addition
of a little vaseline to prevent frothing. As soon as solid
2 Journ, Chem. Soc. 1883, i. 443.
1 Annalen Suppl, 6, 240.
3 J. Pr. Chem. [2], 25, 233; Ber. 19,
Journ. Chem. Soc. 1887, i. 48.
Rec. Trav. Chim. 10, 100; 11, 18.
993.
5
Raschig, Annalen, 241, 161
7 Bull. Soc. Chim. [3], 6, 793.
476
THE NON-METALLIC ELEMENTS
hydroxylamine passes over, the receiver is changed and cooled
to 0°, care being taken that the hydroxylamine vapour is
not exposed to air at 60-70° for any length of time, as violent
explosions then take place. Crismer prepared the base by
heating a double compound which it forms with zinc chloride,
ZnCl2, 2NH₂OH.
Hydroxylamine forms white inodorous scales or hard needles,
has a sp. gr. of about 13, melts at 33 05°, and boils at 58° under
22 mm. pressure, the density of its vapour agreeing with the
formula NH,O. When heated to 90-100° it decomposes, and
detonates at a higher temperature. It inflames in a current of
chlorine gas and combines violently with bromine and iodine, but
without evolution of light. In the pure state it is stable, but in
presence of alkali it gradually decomposes, the alkali dissolved
from the less resistant forms of glass being sufficient to bring
about this change. Strong oxidizing agents such as potassium
permanganate or bichromate decompose it with production of
flame or explosion, and sodium likewise attacks it with produc-
tion of flame. On exposure to the air it liquefies owing to the
absorption of moisture, and then undergoes oxidation with form-
ation of a solid substance containing nitrous acid and ammonia ;
it dissolves readily in water and to a less extent in ethyl and
methyl alcohol and in boiling ether, and separates from the last-
named solution in acicular crystals on cooling. The solutions
have a strongly alkaline reaction, and cause the separation of
the more easily reduced metals from their salts. From a
solution of copper sulphate it precipitates red cuprous oxide,
this reaction being sufficiently delicate to recognize one part of
hydroxylamine in 100,000 parts of water. If sodium nitroprusside
be added to a neutralized solution of hydroxylamine, and then
a little caustic soda, the whole assumes on boiling a beautiful
magenta red colour.¹ When hydroxylamine or its salts are treated
with nitrous acid decomposition takes place rapidly, nitrous oxide
being evolved. The reaction proceeds in two stages, hyponitrous
acid being first formed according to the equation:
HO.NH, + ON.OH = HO.N:N.OH + H₂O.
2
This substance then splits up into its anhydride, nitrous oxide,
and water.2
The salts of hydroxylamine will be more fully described after
1 Angeli Gazzetta, 23, ii, 102,
Wislicenus, Ber. 26, 771; Thum, Monats. 14, 294.
NITROGEN CHLORIDE
477
the ammonium salts in Vol. II., Part I. They all decompose
on heating with effervescence, the nitrate yielding nitric oxide.
and water.
NH₂O, HNO3 = 2NO + 2H₂O.
The behaviour of hydroxylamine towards organic substances
shows that it must contain the hydroxyl group OH, and it is there-
fore ammonia in which one atom of hydrogen is replaced by that
OH
group, its constitutional formula being NH.
H
1
COMPOUNDS OF NITROGEN WITH THE
ELEMENTS OF THE CHLORINE GROUP.
NITROGEN CHLORIDE, NC.
269. This dangerous body was discovered by Dulong¹ in 1811,
who, notwithstanding the fact that he lost one eye and three
fingers in the preparation of this body, yet continued its in-
vestigation. A similar accident happened in 1813 to Faraday
and Davy, who had, however, been made aware of the ex-
plosive properties of this substance. "Knowing that the liquid
would go off on the slightest provocation, the experimenters
wore masks of glass, but this did not save them from injury. In
one case Faraday was holding a small tube containing a few
grains of it between his finger and thumb, and brought a piece
of warm cement near it, when he was suddenly stunned, and on
returning to consciousness found himself standing with his hand
in the same position, but torn by the shattered tube, and the
glass of his mask even cut by the projected fragments. Nor was
it easy to say when the compound could be relied on, for it.
seemed very capricious; for instance, one day it rose quickly in
vapour in a tube exhausted by the air-pump, but on the next
day, when subjected to the same treatment, it exploded with a
fearful noise and injuring Sir H. Davy."
The compound is formed by passing chlorine into a warm
solution of sal-ammoniac, or when a solution of hypochlorous
acid is brought into contact with ammonia (Balard). If a
galvanic current be passed through a concentrated solution of
sal-ammoniac, chloride of nitrogen is formed at the positive
pole (Böttger, Kolbe). The liquid obtained by the action of
2 Gladstone, Life of Faraday, p. 10.
Schweigy Journ. 8, 302.
ܕ
478
THE NON-METALLIC ELEMENTS
chlorine on a solution of ammonium chloride has been carefully
examined by Gattermann, who finds that it is not a homogeneous
compound, but is a mixture of more or less completely chlorinated
ammonias. If however it be mixed with a little water and
chlorine passed over it for half an hour, it is converted into the
completely chlorinated compound NC. The analysis of the
liquid was carried out by placing a weighed quantity in water,
and carefully adding ammonia solution which slowly converts it
into nitrogen and ammonium chloride, the chlorine in the
solution being then estimated as silver chloride.
1
Chloride of nitrogen is a thin yellowish oil, which evaporates
quickly on exposure to the air, has a sp. gr. of about 16,
and possesses a peculiar smell, the vapour attacking the eyes
and mucous membrane violently. Gattermann has shown that
FIG. 140.
it does not readily explode spontaneously; and may even be
subjected to operations such as washing without much risk of
explosion provided direct sunlight is excluded.
When heated by itself to 95°, or if it is brought in contact
with certain bodies, such as phosphorus or turpentine, it explodes
with great violence, giving out light, and pulverizing any glass
or porcelain vessels in which it may be contained. In cold
water it undergoes spontaneous decomposition with the evolution.
of chlorine, nitrogen, hydrochloric acid, and nitrous acid.
The following method is employed for preparing this dangerous
¹ Ber. 21, 751.
T
a
T
D
St
37

IS
NITROGEN CHLORIDE
479
substance in small quantities, and for showing its explosive
properties without risk. A flask of about two litres capacity,
having a long neck, is filled with chlorine, and placed mouth
downwards in a large glass basin filled with warm saturated
solution of sal-ammoniac as shown in Fig. 140. Below the neck
of the flask is placed a small thick leaden saucer, in which the
chloride of nitrogen is collected. The solution of sal-ammoniac
absorbs the chlorine, and, as soon as the flask is three parts filled
by the liquid, oily drops are seen to collect on the surface inside
the flask. These gradually increase, and at last drop one by one
down into the leaden saucer. When a few drops have collected,
the leaden saucer may be carefully removed by a pair of clean
tongs, another being placed in its stead. A small quantity of the
chloride of nitrogen may be exploded by touching it with a
feather moistened with turpentine attached to the end of a long
rod. Another drop of the oil may be absorbed by filtering
paper, and when this is held in a flame a loud explosion like-
wise ensues.

B
FIG. 141.
The force of the explosion may be rendered still more evident
by placing the flask in a strongly constructed box with glass
sides, and when the drops of chloride of nitrogen collect on the
surface of the solution, passing in some turpentine from a
stoppered funnel; the latter is kept outside the box and is con-
nected with it by means of rubber and glass tube the end of
which dips under the neck of the flask. When the turpentine
reaches the surface of the solution in the flask a bright flash is
seen, and a violent thunderlike explosion occurs, completely
shattering the flask.¹
The formation and properties of chloride of nitrogen may
be also exhibited in the following way. A solution of sal-
ammoniac, saturated at a temperature of 28°, is brought into a
glass basin (A, Fig. 141), and a cylinder (B), the lower end of
1 V. Meyer, Ber. 21, 26.
480
THE NON-METALLIC ELEMENTS
which is closed by a piece of bladder, is also filled with the
solution and placed upright in the basin. A layer of oil of
turpentine is then poured on to the top of the liquid in the
cylinder, and a platinum plate (a) in contact with the positive
pole of a battery of six cells placed in the cylinder, whilst the
negative pole (b) is placed under the bladder in the basin. Yellow
oily drops soon begin to form on the surface of the positive pole,
and these gradually become detached from the pole, and rise in
the liquid until they come in contact with the layer of turpentine,
when they explode.
BROMIDE OF NITROGEN.
When bromide of potassium is added to chloride of nitrogen
under water, potassium chloride and bromide of nitrogen are
formed. The latter is a dark red very volatile oil, possessing
a powerful smell, and is as explosive as chloride of nitrogen
(Millon).
IODIDE OF NITROGEN.
270 When iodine is brought into contact with aqueous or
alcoholic ammonia, a black powder is formed which, when dried,
decomposes spontaneously with a very violent detonation either
when touched, or when slightly heated, violet vapours of iodine
being emitted. This compound gradually decomposes under cold
water, pure iodine being left behind. Warm water facilitates
this decomposition, and the body explodes violently when
thrown into boiling water.
The composition of this black powder appear to vary accord-
ing to the process employed in it preparation. Serullas,¹
who first prepared this body, precipitated an alcoholic solu-
tion of iodine with aqueous ammonia; the compound thus
obtained possesses, according to Gladstone, the composition
NHI,; but according to Stahlschmidt, its composition is
represented by the formula NI, a body having the former
composition being obtained when alcoholic solutions of iodine
and ammonia are mixed. Bunsen, on the other hand, found
that under these circumstances, and when absolute alcohol is
used, a precipitate having the composition NIH, or NH¸‚NI¸
is formed, and that on precipitating an aqueous solution of
2 Journ. Chem. Soc. 1852, 34.
3-
1 Ann. Chim. Phys. 42, 200.
3 Pogg. Ann. 119, 421.
OXIDES OF NITROGEN
481
chloride of iodine with ammonia a black powder is obtained,
which consists of NH4NI.¹ These observations render
it probable that under different circumstances at least two
distinct iodides of nitrogen are formed, one being derived
from ammonia by the replacement of the whole, and the other
by the replacement of a portion of the hydrogen by iodine,
thus:-
(a) 4 NH¸+31, = NI¸+3NH¸I.
2
(¹) 3NH3 + 21₂ = NHI, + 2NH¸I.
Iodide of nitrogen would thus appear to be capable of combining
with ammonia, giving rise to the compounds described by
Bunsen.
Szuhay has recently shown that the compound obtained by
adding an excess of aqueous ammonia to a strong solution of
iodine in concentrated aqueous potassium iodide has the
composition NHI,, its formation being represented by equa-
tion (b) above. When suspended in water and mixed with an
ammoniacal solution of silver nitrate it yields a black substance
having the composition NAgI,, which is likewise explosive,
and is decomposed by boiling water and dilute acids. If it be
treated with a solution of a soluble cyanide the silver may be
replaced by other metals, but the compounds thus formed
have not been isolated; they regenerate the compound NAgI
on addition of a silver salt. The compound NHI, therefore
possesses acid properties.
NITROGEN AND OXYGEN.
OXIDES AND OXY-ACIDS OF NITROGEN.
271 As already mentioned nitrogen burns in oxygen when
both gases are heated to a sufficiently high temperature (p. 451),
and direct combination also takes place more slowly when a
series of electric sparks is passed through fairly dry air, whilst
if much moisture is present nitric acid is produced. When a
mixture of nitrogen and oxygen is passed over platinum black at
250°, oxides of nitrogen are also formed, and they are further
produced when carbon is burnt in highly compressed air. We
1 Annalen, 84 1
3 Loew, Ber. 23, 1443.
2 Br. 26, 1933.
4 Hempel, Ber. 23, 1457.
32
482
THE NON-METALLIC ELEMENTS
are acquainted with five oxides of nitrogen,¹ and three oxygen
acids corresponding to the oxides numbered 1, 3, and 5.
Acids.
Oxides.
1. Nitrous Oxide, or
Nitrogen Monoxide NO Hyponitrous Acid.
2. Nitric Oxide, or
Nitrogen Dioxide
NO
NO
3. Nitrogen Trioxide No Nitrous Acid
O
NO
4. Nitrogen Peroxide, or
Nitrogen Tetroxide
NO2
5. Nitrogen Pentoxide }
NO,
NO, S
O Nitric Acid.
*
HO.N
HO.N
NO}
H
NO.)
H
0
0
NITRIC ACID. HNO3.
272 In Geber's tract, De Inventione Veritatis, we find the
following description of a mode of preparing nitric acid or aqua-
fortis:-" Sume libram unam de vitrioli de cypro. et libram salis
petræ, et unam quartam aluminis Jameni, extrahe aquam (the
acid) cum rubidine alembici." That is, by strongly heating a
mixture of saltpetre, alum, and sulphate of copper, the nitric
acid distils over, owing to the decomposition of the saltpetre by
the sulphuric acid of the other salts. Nitric acid was commonly
prepared and used as a valuable reagent by the alchemists,
especially as a means of separating gold and silver. The
method of preparation which we now use from nitre and oil of
vitriol appears to have been first employed by Glauber, for long
afterwards the acid thus obtained was called Spiritus nitri
fumans Glauberi.
The first theory respecting the composition of nitric acid
was proposed by Mayow in 1669. He believed that the acid
contained two components, one derived from the air and
having a fiery nature, and the other derived from the earth.
More than a century later, in 1776, Lavoisier showed that
one constituent of nitric acid is oxygen, but he was unable to
satisfy himself as to the nature of the other components, and
it was reserved for Cavendish 3 to prove the exact composition
1 According to Berthelot and Hautefeuille and Chappuis, a sixth oxide, pos-
sessing the composition NO, exists. It is formed by the action of an electric
discharge on a mixture of nitrogen peroxide and oxygen. (See Ann. Chem. Phys.
[5], 22, 432; Compt. Rend. 92, 80, 134, 94, 1111, 1306.)
2 Mayow, De sal-nitro et spiritu nitri aereo.
3 Phil. Trans. 1784, p. 119. Do. 1785, p. 372.
NITRIC ACID
483
and mode of formation of this acid or its salts by the direct
combination of oxygen and nitrogen gases in presence of water
or alkaline solutions. Priestley had already observed that when
a series of electric sparks was made to pass through common
air included between short columns of a solution of litmus, the
solution acquired a red colour and the air was diminished in
volume. Cavendish repeated the experiment, using lime-water
and soap-lees (caustic potash) in place of the litmus, and he
concluded that the lime-water and soap-lees became saturated
with some acid formed during the operation. He proved that this
was nitric acid by passing the electric discharge through a mix-
ture of pure dephlogisticated air (oxygen) and pure phlogisticated
air (nitrogen) over soap-lees (caustic potash), when nitre (potas-
sium nitrate) was formed. Cavendish clearly expresses his
FIG. 142.
views in the following words:-" We may safely conclude that
in the present experiments the phlogisticated air was enabled,
by means of the electric spark, to unite to, or form a chemical
combination with, the dephlogisticated air, and was thereby
reduced to nitrous acid, which united to the soap-lees, and
formed a solution of nitre; for in these experiments those two
airs actually disappeared, and nitrous acid was actually formed
in their room." The apparatus which he employed, represented
in Fig. 142, consisted of a bent syphon-tube containing, in the
bent portion, the mixture of gases, and mercury in the limbs,
the open ends dipping under mercury in the glasses.
Nitric acid is also formed when various bodies are burnt in a
mixture of oxygen and nitrogen. Thus, if three to five volumes
of the detonating mixture of oxygen and hydrogen be mixed
with one volume of air in a eudiometer over mercury, and
1 Phil. Trans. 1785, p. 379.
1

484
THE NON-METALLIC ELEMENTS
an electric spark passed through the mixture, instantaneous
combination takes place and nitric acid is formed, the surface
of the mercury becoming covered with crystals of mercurous
nitrate.¹
In order to exhibit the direct combination of oxygen and
nitrogen, it is only necessary to allow the sparks from an in-
duction-coil to pass between two platinum wires placed in the
interior of a large glass globe containing dry air, as shown in
Fig. 143. Red fumes of nitrogen peroxide are rapidly formed,
and their presence may be distinctly recognized by plunging
a piece of iodized starch paper into the globe, when the blue
iodide of starch will at once be produced. On pouring a few

0000
eeeeeeee
FIG. 143.
5000022
felledellee
drops of water into the globe and shaking it up, the red fumes
are absorbed and nitrous and nitric acids formed, as may be
shown by the acid reaction of the liquid.
In a similar manner, if a flame of hydrogen be allowed to
burn in a large flask into which oxygen is led, but not in such
quantity as to displace the whole of the air, nitric acid is
formed in large quantity. The same acid is also produced when
ammonia burns in oxygen. The formation of nitric acid by
the discharge of electricity through the air accounts for the pres-
ence of this acid in the atmosphere.
Another and more productive source of the nitrates has yet
to be described. When nitrogenous organic matter is exposed
1 Bunsen, Gasometry, p. 58.
2 Kolbe, Annalen, 119, 176; Hofmann, Ber. 3, 663.
PREPARATION OF NITRIC ACID
485
to the air the nitrogen assumes the form of ammonia; but when
alkalis, such as potash, soda, or lime, are present, a further
slow oxidation of the nitrogen takes place, and nitrates of
these metals are formed. Hence these nitrates are widely
diffused in all surface soils, especially in hot countries such
as India, where oxidation takes place quickly. Soil in the
neighbourhood of the Indian villages, which contains con-
siderable amounts of potash, thus becomes rich in nitre
or potassium nitrate, KNO, originating from the decomposition
of the urea of the urine. It is from this source that the
largest quantity of nitre imported into this country is obtained.
Another nitrate, lime-saltpetre or calcium nitrate, Ca(NO3), is
often found as an efflorescence on the walls of buildings, such as
stables, or cellars of inhabited houses, and this source was made
use of during the French revolution for the manufacture of
nitre. Chili saltpetre, or sodium nitrate, NaNO3, occurs in
large deposits in the province of Tarapaca, a rainless district
on the Peruvian coast lying near the 20th degree of south
latitude. The nitrate of soda is found mixed with chloride
of sodium and other salts, probably pointing to the fact
that the locality has been covered by the sea, and that the
nitrate is a product of the decomposition of sea-plants and
animals.
Preparation.—In order to prepare pure nitric acid, saltpetre
which has been previously well dried is placed in a tubulated
retort together with an equal weight of concentrated sulphuric
acid. On heating the mixture, the volatile nitric acid passes
over, and is collected in a well-cooled receiver, see Fig. 144,
hydrogen potassium sulphate (commonly called bisulphate of
potash) remaining behind in the retort; thus:-
KNO₂+H₂SO₁ = HNO3 + KHSO.
4
The distillate thus obtained has a yellowish colour, caused by
the presence of nitrogen peroxide, and it is not free from water,
as the concentrated acid, when heated, decomposes partially into
peroxide, oxygen and water. In order to purify the acid thus
obtained it must be again distilled with its own volume of con-
centrated sulphuric acid, and the distillate freed from traces of
the peroxide by gently warming the acid, and leading a current
of dry air through it until it is cold. Thus prepared it
contains from 995 to 998 per cent. of the anhydrous acid,
HNOg
486
THE NON-METALLIC ELEMENTS
273 Properties.-Nitric acid is a colourless liquid fuming
strongly in the air. It possesses a peculiar though not very
powerful smell, and absorbs moisture from the air with the
greatest avidity. Nitric acid is an extremely corrosive sub-
stance, which, when brought in contact with the skin, produces
painful wounds, being used in surgery as a powerful
cautery. The dilute acid acts less energetically, and colours
the skin, nails, wool, silk, and other organic bodies of a bright
yellow tint.
When pure concentrated nitric acid is heated, it begins to
boil at 86°, and becomes of a dark-yellow colour owing to
the decomposition of a portion of the acid into nitrogen

FIG. 144.
peroxide, oxygen, and water. As soon as about three-fourths
of the acid has distilled over, the residue becomes colour-
less, and then contains only 95.8 per cent. of acid.¹ If the
distillation is pushed further, the boiling point continually rises,
a strong acid distils over, and the residue becomes constantly
weaker until it contains 68 per cent. of acid, when the liquid is
found to boil unaltered at 120° 5 under the normal atmospheric
pressure, yielding an acid of the above constant composition,
and with a specific gravity of 1414 at 15°5. This constant
acid is always obtained, whether a stronger or a weaker acid be
subjected to distillation. If this acid of constant composition
be distilled under an increased, or under a diminished pres-
1 Roscoe, Journ. Chem. Soc. 1861, i. 147.
PROPERTIES OF NITRIC ACID
487
sure, the composition of the residual acid again undergoes a
change, until for each pressure a constant boiling point is
reached. Thus, under the pressure of 122m. of mercury, an
acid containing 68.6 per cent. of HNO, distils without alteration,
whilst under a pressure of 0·070m. an acid distils over at a tem-
perature of from 65° to 70°, having a constant composition of 66-7
per cent. When a current of dry air is passed through aqueous
nitric acid, either a stronger or a weaker acid is volatilized,
according to the concentration or the temperature of the acid,
until at length a residue is obtained which volatilizes unchanged.
Thus, when the experiment is made at 100°, the residual
acid contains 66.2 per cent.; when at 60°, 64 5 per cent.;
and at 15° the residual acid contains 640 per cent. of HNO3.
From this it will be seen that nitric acid behaves in a
similar way in this respect to hydrochloric and the other
aqueous acids.
When the concentrated acid is mixed with water an in-
crease of temperature and a contraction of bulk is observed.
This attains its maximum when one molecule of the acid is
mixed with three molecules of water (Kolbe). The following
table gives the specific gravities of aqueous acids at 0° and
15°:-¹
Per cent.
HNO3.
100.0.
900..
80.0..
700..
60.0 ..
50.0.
40.0.
30.0.
20.0.
15.0.
10.0.
5.0.
·
•
•
•
·
·
Sp. gr.
at 0°.
1.559.
1.522.
1.484.
1.444.
. 1.393.
. 1·334.
.
•
•
·
•
1.267. .
1.200.
. 1.132.
. 1·099 ..
1.070.
. 1.031.
•
•
.
1.530
. 1·495
1.460
1.423
1.374
1:317
1.251
1.185
. 1.120
. 1.089
. 1.060
. 1.029
•
Sp. gr.
at 15.
•
It has been already remarked that concentrated nitric acid
begins to decompose, at a temperature of 86°, into water, oxygen,
and nitrogen peroxide. If the acid be more strongly heated in
closed glass tubes this change takes place so rapidly that at
260° the whole of the nitric acid is thus decomposed (Carius).
¹ Kolbe, Ann. Chim. Phys. [4], 10, 140.
488
THE NON-METALLIC ELEMENTS
In order to exhibit this decomposition by means of heat, the
same apparatus serves which was employed for the decomposi-
tion of sulphuric acid, Fig. 145. Strong nitric acid is allowed
to fall on the hot pumice-stone contained in the platinum
flask. Immediately red vapours are emitted, and these are
condensed in a U-tube placed in a freezing mixture to a
brown liquid, NO2, whilst the cylinder placed over the
pneumatic trough becomes filled with a colourless gas which
can be easily shown to be oxygen (Hofmann).

b
a
C
FIG. 145.
Pickering has succeeded in isolating two hydrates of nitric
acid, having the composition HNO, H2O and HNO3, 3H,O
respectively. The former separates in large fairly transparent
crystals, and the latter in smaller, more opaque, gritty
crystals.¹
274 Commercial Manufacture.-In order to prepare nitric acid
on the commercial scale, sodium nitrate is substituted for nitre, as
it is much cheaper, and the salt is decomposed with sulphuric
acid as before. The proportions of these two substances em-
ployed are not the same in all works. If one molecule of
1 Journ, Chem. Soc. 1893 i. 441.
MANUFACTURE OF NITRIC ACID
489

2
35
↓
FIG. 146.
490
THE NON-METALLIC ELEMENTS
sulphuric acid and two of sodium nitrate be taken, the follow-
ing are the reactions. In the first place we have:-
H₂SO4 + NaNO3 = NaHSO4 + HNO3.
When the heat is raised, the acid sodium sulphate acts upon a
second molecule of sodium nitrate; thus:-
NaHSO4 + NaNO, = Na,SO4 + HNO3.
In this case, however, a part of the acid is decomposed owing
to the high temperature, and nitrogen peroxide is evolved in the
form of red fumes which dissolve in the concentrated acid, giving
it the red appearance usually noticed in the strong commercial
product. When a large excess of sulphuric acid is employed,
a certain quantity of acid sodium sulphate is formed, which
lowers the melting point of the residual mass so that it can
be withdrawn from the retorts in the fused state, whereas in
the other case the residue can only be removed in the solid
state after the cylinder has been cooled. The ordinary com-
mercial acid has a specific gravity of from 1:38 to 141, and is
usually prepared by means of chamber (sulphuric) acid; but if
a more concentrated acid is required a stronger sulphuric acid
must be employed. The strongest nitric acid occurring in com-
merce has a specific gravity of 1:53, and this is obtained by
distilling well-dried Chili saltpetre with sulphuric acid having a
specific gravity of 185. The retorts in which nitric acid is
usually prepared on the large scale in England consist of cast-
iron cylinders built in a furnace in such a way that they may
be heated as uniformly as possible, as shown in Fig. 146. Some
manufacturers cover the upper half of the cylinder with fire-
bricks in order to protect the iron from the action of the nitric
acid vapours. This, however, is unnecessary, if the retorts are
so thoroughly heated that no nitric acid condenses on the surface
of the iron. The ends of the cylinders which are not exposed
to the action of the flame are closed by plates of Yorkshire
flag cemented on to the iron with a mixture of iron filings,
sulphur, sal-ammoniac, and vinegar. In the upper part of one of
these flags is a hole, through which the sulphuric acid is intro-
duced after the charge of Chili saltpetre. The hole is then closed
by a clay plug. A similar hole in the other flag is furnished
with a bent earthenware tube (c) passing into a series of large
Woulffe's bottles (bb), one placed behind the other, containing
MANUFACTURE OF NITRIC ACID
491
small quantities of water in which the nitric acid condenses,
and from which the acid is withdrawn by leaden syphons.
The last of these Woulffe's bottles is placed in connection with a
tower filled with coke, down which a current of water runs.
Any uncondensed nitrogen peroxide passes up this tower, and,
coming in contact with the water and the oxygen of the air, is

FIG. 147.
oxidized to nitric acid. When the operation is complete, one of
the flags is removed and the residual fused sulphate of soda
scraped out.
A usual charge for one retort is 305 kilos of Chili saltpetre
and 240 kilos of strong sulphuric acid. The mixture is
heated uniformly for about eighteen hours, and the quantity
of water placed in the Woulffe's bottles is such that a yield of
363 kilos of nitric acid, of specific gravity 1:35, is obtained,
492
THE NON-METALLIC ELEMENTS
whilst 295 kilos of fused sodium sulphate remain behind in the
cylinder.
In a German factory, where the strongest nitric acid is made,
a cast-iron vessel is employed for its generation, the construction
of which is seen in Fig. 147. The charge, in this case, consists of
700 kilos of sulphuric acid, of specific gravity 184, and 600 kilos
of Chili saltpetre. The retort is placed in connection with two
series of receivers, twenty-five in number. According to this
process 100 parts of Chili saltpetre, containing 96 parts of pure
nitrate, yield 68 parts of nitric acid, of specific gravity 15, and
17 parts of weaker acid. This corresponds to about 96 per cent.
of the theoretical yield.
A new arrangement has lately been devised, whereby a more
rapid generation and condensation is possible than by the older
processes. According to this plan, proposed by Guttmann, a charge
of 610 kilos can be worked off in 10-11 hours with a yield of
strong acid only 3-7 per cent. below the theoretical amount,
whilst if the fumes are condensed by water and the weak acid
is used, there is absolutely no loss in the process.
The charge of sodium nitrate and sulphuric acid is brought into
an iron retort similar to that shown in Fig.147, but which allows
of the spent sodium bisulphate being drawn off at the bottom;
in order to reduce the quantity of nitrogen peroxide in the
condensed acid to a minimum, a current of air is injected into
the gaseous products, the mixed gases then passing into a very
carefully made system of earthenware pipes arranged in a some-
what similar manner to the well-known atmospheric condensers
of the gas-works, which are cooled by water. Each set of upright
pipes is connected with a main, into which the acid falls, and
flows away into a receiver placed below. The uncondensed
gases pass away into a tower built of earthenware, and filled
with perforated plates of special construction, down which water
is continuously allowed to trickle. The arrangement of the
perforated plates is the invention of Prof. Lunge.
The wash tower is of great value even if the weak acid be
thrown away, inasmuch as without it the draught becomes too
great and serious escape of nitrous gas takes place. This process
is particularly useful where a very strong acid is needed, as for
example in the preparation of nitro-glycerine and gun-cotton.¹
Commercial red nitric acid always contains chlorine, and
sometimes iodine in the form of iodic acid, originating from the
1 Journ. Soc. Chem. Ind. 1893, 203.
1
NITRIC ACID
493
Chili saltpetre. In addition it also contains nitrogen peroxide,
iron oxide, sulphuric acid, and sodium sulphate, which have
been mechanically carried over. In order to purify the acid, it
must be distilled in glass retorts, chlorine and nitrogen peroxide
coming over in the first portion. As soon as the acid distillate
is free from chlorine the receiver is changed, and the liquid may
be distilled until only a small residue is left, containing the
whole of the iodic acid, sulphuric acid, and sodium sulphate.
Concentrated, as well as dilute nitric acid is largely used in the
arts and manufactures. Large quantities are employed in the
manufacture of the various coal-tar colours, of nitro-glycerine,
of gun-cotton, of sulphuric acid, and of nitrate of silver, which
is largely used for photographic purposes. The acid is
also used in large quantities for the preparation of certain
nitrates, especially lead nitrate, iron nitrate, and aluminium
nitrate, all of which are employed in the processes of dyeing
and calico-printing, whilst the nitrates of barium and strontium
are used for pyrotechnic purposes. In the laboratory it is an
indispensable reagent, and is used in the preparation of a
large number of inorganic and organic substances.
Nitric acid is a monobasic acid forming a series of salts
which are termed the nitrates. These are almost all easily soluble
in water, and as a rule crystallize well. They may be obtained
by neutralizing the acid with an oxide or a carbonate, and
are almost all formed by dissolving the metal in nitric acid. In
this case the metal is oxidized at the expense of a portion of the
acid which, according to the concentration or the temperature, is
reduced to NO2, NO, NO, NO, and even to nitrogen and
ammonia.
Several other bodies, such as sulphur, phosphorus, carbon,
and many organic substances, are easily oxidized, especially
by the concentrated acid. In order to exhibit this action,
some nitric acid may be poured upon granulated tin, which is
then oxidized with the evolution of dense red fumes, whilst
a white powder of tin oxide is deposited. Turpentine when
poured into the concentrated acid is likewise oxidized with
almost explosive violence, light and heat being evolved. In
like manner ignition may take place when straw or sawdust
becomes impregnated with the strong acid.
Other organic bodies treated with nitric acid undergo no
apparent alteration. Thus, for instance, with cotton-wool no
ignition or evolution of red fumes occurs. If, however, the
"
|
!
י
1
I
I
T
494
THE NON-METALLIC ELEMENTS
cotton-wool after having been thus soaked in strong nitric
acid is washed and dried, it is found to possess very different
properties from ordinary cotton, although in appearance it can
hardly be distinguished from it. Cotton-wool, or cellulose, has
the formula C12H20O10, whilst after treatment with nitric acid
it consists of nitro-cellulose, gun-cotton or collodion-cotton.
This consists of cellulose in which some of the hydroxyl groups
varying in number from four to six according to the strength of
nitric acid employed, are replaced by NO, the changes which
have occurred being represented by the equation:-
C12H20010+4HNO3=C12H1606(NO3),+4H¸O,
or C12H20010+ 6HNO3=C₁₂H₁₁O4(NO3)6+6H₂O.
12 14
Nitric acid acts in a similar way on many other organic bodies.
275 Action of Nitric Acid on Metals.-As already mentioned,
nitric acid dissolves a large number of metals with formation of
nitrates. Hydrogen is not evolved at the same time as is the
case with sulphuric and hydrochloric acids, but in its place lower
oxides of nitrogen and even nitrogen itself and ammonia are
formed. The explanation usually given of this change is that
hydrogen is first evolved, but that it at once acts on the excess of
nitric acid present, forming water and the lower oxides of
nitrogen. Thus, for example, the formation of nitric oxide by
the action of copper on nitric acid is supposed to take place
in the two following stages:-
Cu+2HNO3 = Cu(NO3)2+2H,
6H+2HNO3=2NO+4H₂O.
According to Veley, however, this explanation is not correct,
inasmuch as pure copper, mercury and bismuth do not dissolve
in pure dilute nitric acid, but dissolve readily when nitrous acid
is present, the change at any moment being directly proportional
to the mass of nitrous acid in the solution, and the more rapid
the greater the proportion of the former to the latter. He there-
fore believes that the reaction is started either by traces of nitrous
acid already present, or by impurities in the metal inducing a local
galvanic current; the first product of the reduction of the nitric
acid is nitrous acid, and the production of lower oxides of nitro-
gen he regards as due to the subsequent changes occurring be-
tween nitrous acid and cuprous and cupric nitrate or nitrite in
DETECTION OF NITRIC ACID
495
presence of an excess of nitric acid, the nitrous acid being
decomposed as fast as it is formed.¹
Detection and Estimation.-The presence of nitric acid or
of its salts is easily ascertained. Thus for instance, if we heat a
few drops of not too dilute nitric acid with copper turnings,
brownish red vapours of nitrogen peroxide are emitted. The
nitrates give the same reaction if they, or their concentrated
aqueous solutions, are treated with sulphuric acid and copper.
In order to detect nitric acid, or a nitrate in a very dilute
solution, a cold solution of ferrous sulphate is added to the
liquid, and about an equal volume of strong sulphuric acid is
then allowed to flow slowly down to the bottom of the inclined
test-tube, care being taken that the two liquids do not mix.
If nitric acid be present it is liberated by the action of the
sulphuric acid, and at once oxidises the ferrous sulphate where
it comes in contact with it, nitric oxide being simultaneously
given off. The latter at once unites with the excess of ferrous
sulphate forming a dark brown compound, the solution of which
is seen as a dark-coloured ring at the point where the dense
sulphuric acid joins the lighter aqueous solution.
Another very delicate test for the presence of nitric acid is
aniline. In order to apply this test, ten drops of aniline are
brought into 50 cc. of a dilute sulphuric acid containing 15
per cent. of pure acid, and 0.5 cc. of this solution is poured on
to a watch-glass together with 1 cc. of concentrated sulphuric
acid. If a glass rod moistened with the solution under exami-
nation be now brought in contact with the edge of the
liquid in the watch-glass, a red streak will be produced if a
nitrate be present, and the colour will increase in intensity until
the whole liquid becomes red. If a larger quantity of nitric acid
be present, the whole mass will assume more or less of a brown
tint (C. D. Braun).
The organic base called brucine, bearing a close resemblance
to strychnine, serves as a test for nitric acid even more delicate
than aniline. If to half a drop of a solution of one part of nitric
acid to 100,000 parts of water, one or two drops of a solution
of brucine be added, and then a few drops of concentrated
sulphuric acid, a distinct pink coloration will be observed if
the solution be viewed against a white ground.2
In order quantitatively to determine the amount of nitric acid
¹ Proc. Roy. Soc. 46, 216; 52, 27; Phil. Trans. 1891 (a), 312; Journ. Soc.
Chem. Ind. 1891, 204.
2 Reichardt, Jahresbericht, 1871, 893.
496
THE NON-METALLIC ELEMENTS
contained in potassium- or sodium-saltpetre, the well-dried sub-
stance is heated to dull redness for half an hour with freshly-
ignited and finely-powdered quartz or silica, SiO2. The nitrates
are thus completely decomposed, whilst any sulphates or
chlorides which may be present undergo no change. The
decomposition which here takes place may be represented as
follows:-
2KNO3
=
K₂O + 2NO₂+ 0.
Oxygen and nitrogen peroxide are evolved, whilst the potash
combines with the silica to form silicate of potash. From the
loss of weight thus ensuing the amount of nitre present can
easily be calculated.
Another good method, which is particularly useful in the
determination of the nitrates contained in drinking water,
depends upon the fact that a thin zinc plate, which has been
covered with a deposit of spongy metallic copper by dipping it
into a solution of copper sulphate, on being heated with water
containing nitrates reduces them to ammonia, zinc hydroxide
and free hydrogen being at the same time formed (Gladstone
and Tribe); thus:-
KNO₂ + 8H = NH¸ + KOH + 2H₂O.
The ammonia thus obtained is distilled over into an excess of
hydrochloric acid and determined in the usual way, or if present
in only small quantities by nesslerisation (see p. 300).
A further method, proposed by Lunge, consists in mixing the
nitrate with sulphuric acid and shaking in a graduated tube
with mercury; the whole of the nitrogen is liberated in the
form of nitric oxide, the volume of which is measured. The
reaction which takes place is represented by the following
equation:-
3Hg + 3H¿SO + 2HNO₂ = 3HgSO4 + 4H₂O + 2NO.
A special apparatus known as the nitrometer has been
designed by Lunge for carrying out this reaction which is
especially useful in the determination of the nitrogen contents
of organic nitrates such as the nitro-celluloses.
¹ Ber. 11, 434.
1
1
NITROGEN PENTOXIDE
497
AQUA REGIA.
276 This name is given to a mixture of nitric and hydro-
chloric acids which is frequently employed for dissolving the
noble metals, such as gold and platinum, as well as many
metallic ores and other bodies. A method of preparing this
substance was described by Geber in his work De Inventione
Veritatis, by dissolving sal-ammoniac in nitric acid; and he
states that the liquid thus obtained has the power of dissolving
gold and sulphur. The name, aqua regia, is first found in the
writings of Basil Valentine. He, like Geber, prepared it by
dissolving four ounces of sal-ammoniac in 1 lb. of aqua-fortis;
he also states that strong aqua regia can be obtained by mixing
hydrochloric and nitric acids. The solvent power of aqua regia
depends upon the fact that, on heating, this mixture of acids.
evolves chlorine; thus:-
HNO3 + 3HC1 = 2H2O + NOCI + Cl₂_
The compound nitrosyl chloride, NOCI, which is liberated at the
same time is described on page 510.
NITROGEN PENTOXIDE, NO=107.28.
277 This substance, which is commonly called nitric anhydride,
was discovered in 1849 by Deville,¹ who obtained it by leading
perfectly dry chlorine gas over dry silver nitrate contained in a
U-tube placed in a water-bath. The reaction begins at 95°,
and when cooled to 60°, the decomposition of the nitrate goes
on regularly. The pentoxide is collected in a bulb tube
surrounded by a freezing mixture. The following equation
represents the reaction which takes place :-
――――――
4AgNO3 + 2C1, = 4AgCl + 2N₂O, + O2.
In the preparation of the substance all joints of cork and caout-
chouc must be avoided, and the parts of the glass apparatus
must be connected either by fusion, or by placing the end of
one tube inside the other and closing the space between the
tubes with asbestos, the pores of which are filled up with melted
paraffin.
Nitrogen pentoxide can be also prepared still more simply
1 Ann. Chim. Phys [3] 28, 241.
33
498
THE NON-METALLIC ELEMENTS
from pure perfectly anhydrous nitric acid by withdrawal from
this substance of the elements of water. For this purpose
nitric acid is distilled two or three times with concentrated
sulphuric acid to remove all water, and is then intro-
duced into a retort to the neck of which a long glass tube
is fused to serve as a condenser. Phosphorus pentoxide is next
gradually added until the mixture has a syrupy consistency,
the retort being cooled with ice-water during the addition; if the
nitric acid has been properly dehydrated no hissing noise is
heard on addition of the anhydride. The reaction which here
takes place may be thus represented :-
2HNO3 + P₂O₂ = N₂O5 + 2HPO3.
2
By gently heating the retort, a deep orange-coloured distillate
is obtained, which on standing separates out into two layers.
The upper, or lighter layer is then poured into a thin stoppered
tube and cooled down by plunging the tube into ice-cold water.
Crystals of the pentoxide soon separate out, and these may be
purified by pouring off the orange-coloured liquid from which
they are deposited, melting the crystals at a moderate heat,
again allowing them to deposit, and finally pouring off the
mother liquor.
-:
Properties.-Nitrogen pentoxide is a white colourless solid,
crystallizing in bright rhombic crystals, or in six-sided prisms
derived from these. When heated to 15°-20° the crystals
become of a yellowish colour, and melt at about 30° to a dark
yellow liquid, which decomposes between 45° and 50° with the
evolution of dense brown fumes. When suddenly heated, the
pentoxide decomposes with explosive violence into nitrogen
peroxide and oxygen, and this sudden decomposition occurs
sometimes even at ordinary temperatures, if the crystals have
been kept for some time. The lower the temperature is kept
the longer does the substance remain unaltered, and below 30°
it may be sublimed in a closed vessel, depositing in crystals in
the cool part of the tube. In dry air the pentoxide volatilizes
very quickly, whilst in moist air it deliquesces with formation
of nitric acid. Thrown into water, it dissolves with evolution
of heat, forming nitric acid; thus:-
0
NO.
=
}
NO2
0+
H
NO; } 0 + H}
The pentoxide possesses very powerful oxidizing properties.
}
0.
NITROUS OXIDE
499
Thus, if brought in contact with sulphur it forms white vapours,
which condense to a white sublimate of nitrosulphonic anhy-
dride, SO(NO2)2 Phosphorus and potassium burn with
brilliancy in the slightly warmed anhydride. Charcoal does
not decompose even the boiling anhydride, but when ignited
and brought into the vapour it burns with a brilliant light.
When brought in contact with nitric acid, the anhydride com-
bines to form the compound N2O5+2HNO3. This substance,
which is a liquid at the ordinary temperature, possesses at 18°
a specific gravity of 1642, and solidifies at 5° to a crystalline
mass. It forms the heavy layer obtained in the preparation of
the pentoxide, and decomposes with explosion when heated.
Its formation is perfectly analogous to that of disulphuric acid,
and the constitution is probably represented by the formula—
NO₂-O-NO-OH.
CO
NO₂-O-NO-OH.
NITROGEN MONOXIDE OR NITROUS OXIDE, NO=43·76.
278 This gas is formed by the action of easily oxidizable sub-
stances, such as potassium sulphide, moist iron filings, the sul-
phites, and other bodies upon nitric oxide, and according to these
methods it was first prepared by Priestley in the year 1772. It
is moreover formed when zinc and other metals are dissolved in
very dilute nitric acid, and by the action of sulphur dioxide
on nitric oxide in presence of moisture ¹
Preparation. In order to prepare the gas we do not, how-
ever, usually employ any of these methods, but we have re-
course to the decomposition which ammonium nitrate undergoes
on heating. This salt splits up into water and nitrous oxide
gas; thus:-
NH¸NO₂ = N₂O + 2H₂O.
3
It is best, before the experiment, to melt the nitrate, in order to
free it from moisture; and the powdered dry substance is then
introduced into a flask furnished with a cork and delivery tube.
The flask must be heated gently until a regular evolution of
gas begins, and then the flame moderated, as sometimes, if the
¹ Lunge, Ber. 14, 2196.
500
THE NON-METALLIC ELEMENTS
heat applied be too great, the decomposition takes place so
violently, with evolution at the same time of nitric oxide, that an
explosion may occur. In order to free the gas from traces of
nitric oxide it can be shaken up with a solution of ferrous
sulphate, which combines with the latter gas; whilst in order to
remove traces of chlorine derived from the chloride of ammo-
nium, which the commercial nitrate often contains, it must be
allowed to stand over a solution of caustic potash or soda.
These precautions are especially needed when the gas is used
for inhaling. A regular stream of the gas may also be obtained
by heating sodium nitrate with a slight excess of ammonium
sulphate at 240°.¹

FIG. 148.
As nitrous oxide is somewhat soluble in cold, but not nearly
so soluble in hot water, it is best to fill the pneumatic trough
with warm water before collecting the gas. The arrangement
used for this purpose is seen in Fig. 148, and requires no further
explanation.
279 Properties.-Under ordinary circumstances nitrogen mon-
oxide is a colourless gas possessing a pleasant smell and sweet,
agreeable taste, and having a specific gravity of 1.52 (Colin). Its
solubility in water between 0° and 25° is represented by the
formula
c = 130521 0.045620t+ 0.000684312;
or its coefficients of absorption are as follows-2
1 W. Smith, Journ. Soc. Chem. Ind., 11, 867, 12, 10.
2 Carius, Annalen, 94, 140.
—
NITROUS OXIDE
501
5°
0°
15°
20°
25°
10°
13052 10954 0·9196 0.7778 0.6700 0.5962
It is still more soluble in alcohol, one volume of this liquid
absorbing, according to the experiments of Carius, a quantity
of the gas found by the formula.
c = 4·17805 - 0.0698160 + 0·000609012.
Nitrous oxide was first liquefied by Faraday in 1823 by heating
nitrate of ammonium in a bent tube (Fig. 149). The liquid is
now prepared on the manufacturing scale by compressing the
gas into strong cylinders; it is a colourless very mobile liquid,
which has a specific gravity of 0.9369¹ at 0° and boils at 87°9
under 767-3 mm. pressure. For experimental purposes the liquid
may be transferred to open vessels by placing the cylinder with
the valve downwards, and carefully opening the latter, the liquid
rushing out in a fine stream through the nozzle. For, although
FIG. 149.
the liquid boils at a temperature more than 80 degrees below
the freezing-point of water, it may be kept for more than half
an hour in tubes or other open vessels.
Like other condensed gases, liquid nitrous oxide has a very
high coefficient of expansion; one volume of the liquid at 0°
becoming 11202 volumes at 20°, whereas one volume of the
gas at 0° becomes only 10732 volumes when raised to 20°. A
drop of the liquid brought on to the skin produces a blister, and
when water is thrown into the liquid it at once freezes to ice,
at the same time producing a dangerously explosive evolution
of gas. Phosphorus, potassium, and charcoal do not undergo
any change when thrown into liquid nitrous oxide, but if a
piece of burning charcoal be thrown on the liquid, it swims
on the surface and continues to burn with great brilliancy.
On pouring a little mercury into a tube containing the liquid
nitrous oxide, the metal solidifies, whilst at the same moment a
piece of ignited charcoal may be seen to be brilliantly burning
on the surface of the liquid.
1 Andreef, Annalen, 110, 11.
502
THE NON-METALLIC ELEMENTS
When liquid nitrous oxide is poured into carbon bisulphide
the two liquids mix, and if the mixture be brought under the
receiver of an air-pump the temperature sinks to 140°.
tube filled with liquid nitrous oxide be dipped into a bath of
solid carbon dioxide and ether, and if this mixture be allowed
to evaporate in vacuo, the liquid nitrous oxide freezes to
colourless crystals, whose tension is less than one atmosphere.
Poured into an open vessel, liquid nitrous oxide cools down by
evaporation to a temperature of - 100°, and if the alcoholic
thermometer be taken out of the liquid, a portion of the ad-
hering substance solidifies and the temperature sinks to 115°.
Solid nitrous oxide in the form of snow has also been prepared
by Wills, who obtained it by a modification of Thilorier's method
of obtaining solid carbon dioxide.
Gaseous nitrous oxide, like oxygen, supports combustion
vigorously. A red-hot splinter of wood rekindles when brought
into the gas, a watch spring burns with bright scintillations, and
a bright flame of sulphur continues to burn with a brighter flame.
If, however, the sulphur be only just kindled, the flame is ex-
tinguished on bringing it into the gas, as then the temperature
of the flame is not sufficiently high to decompose the gas into
its constituents. All combustions in this gas are simply
combustions in oxygen, the burning body not uniting with the
nitrous oxide, but with its oxygen, the nitrogen being liberated.
Potassium and sodium also burn brightly in the gas when
slightly heated, with formation of the peroxides of these metals,
and these again when more strongly ignited in the gas yield the
nitrates of the metals.
-
The very remarkable effects on the organism produced by
the inhalation of nitrous oxide, first observed by Davy, have
been further investigated by Hermann.2 The first effects
noticed are singing in the ears, then insensibility, and, if the
inhalation be continued, death through suffocation. In the
case of small animals, such as birds, fatal effects are observed
in 30 seconds, and in rabbits after the expiration of a few
minutes. If, however, air be again allowed to enter the lungs.
as soon as insensibility has set in, the effects quickly pass away
and no serious results follow. When a mixture of four volumes
of this gas and one volume of oxygen is breathed for from
one-and-a-half to two minutes, a curious kind of nervous ex-
citement or transient intoxication is produced, without loss of
Jahresbericht, 1865, 662.
1 Journ. Chem. Soc. 1874, p. 21.
NITROUS OXIDE
consciousness, and this soon passes off without leaving any evil
consequences. Hence this substance received the name of
laughing-gas. Nitrous oxide is now largely employed as an
anæsthetic agent instead of chloroform in cases of slight surgical
operations, especially in dentistry, where only a short period of
unconsciousness is needed. Care must, however, be taken that
for these purposes the gas is free from chlorine and nitric oxide
503
The composition of nitrous oxide may be ascertained in
various ways; thus, a given volume of the gas is brought into a
bent glass tube over mercury in the upper part of which (Fig.
150) a small piece of sodium is placed. The lower and open
end of the tube is then closed under the mercury by the finger,
and the part of the tube containing the sodium heated with
a lamp. After the combustion, the tube is allowed to cool, and
the volume of the residua as measured. This is found to be
the same as the original volume taken, and to consist entirely
FIG. 150.
of nitrogen. Now, as 22-3 litres of nitrous. oxide are found by
experiment to weigh 43·76, and 22-3 litres of nitrogen are known.
to weigh 27.88, it is clear that the difference, or 15 88, is due
to the oxygen. Hence we see that the molecule of nitrous oxide
consists of two atoms of nitrogen combined with one of oxygen.
The same result is attained when a spiral of steel wire is
placed in a given volume of the gas and heated to redness by a
galvanic current. The iron then burns in the gas, and a
volume of nitrogen remains equal to that of the nitrous oxide
employed.
By means of eudiometric analysis we may likewise determine
the composition of the gas, and for this purpose the nitrous
oxide must be mixed with hydrogen and the mixture exploded
by an electric spark, when water is formed and nitrogen gas is
left behind; thus:-
N₂O + H₂ = N₂ + H₂O.
2
·
1
504
THE NON-METALLIC ELEMENTS
HYPONITROUS ACID, H₂N₂O2.
280 The salts of this acid were first obtained by Divers by the
reduction of an aqueous solution of potassium nitrate with
sodium amalgam; the first product of the reaction is potassium
nitrite, which on further reduction yields, in addition to some
hydroxylamine, potassium hyponitrite, which was found to have
the empirical composition KNO.¹ It is also obtained by the
electrolysis of a solution of potassium nitrite, and by the
action of nitric oxide on alkaline ferrous or stannous hydroxides,³
and is always isolated by the addition of silver nitrate which
yields silver hyponitrite as a yellow precipitate. The best
method of preparation of hyponitrites is however by the
action of alkalis on the salts of hydroxylaminesulphonic acid,
HO.NH.SOH (p. 321). It has already heen stated that
dilute acids convert this substance into hydroxylamine sulphate,
but with alkalis no trace of hydroxylamine is formed, the
sole products being a sulphite, a hyponitrite, and nitrous
oxide, the latter being a decomposition product of the hyponi-
trites.¹
2HO.NH.SO,K+ 4KOH= K₂N₂O2 + 2K₂SO3 + 2H₂O.
The formation of hyponitrites from derivatives of hydroxylamine
shows that in these salts the oxygen atom must be between the
nitrogen atom and that of the metal N.O.K; nitrogen is,
however, never known to act as a monad element, and it is,
therefore, probable that the hyponitrites have the double
formula
N.O.K
N.O.K
This is further confirmed by the fact that the vapour density
of ethyl hyponitrite corresponds to the formula (CH),N¸0..5
Hyponitrous acid is further obtained by adding a solution of
sodium nitrite to one of hydroxylamine sulphate and rapidly
heating to 60°, the following reaction taking place :—
HO.NH, + ON.OH = HO.N: N.OH + H₂O.
1 Divers, Proc. Roy. Soc. 19, 425.
2 Zorn, Ber. 12, 1509.
3 Divers and Haga, Journ. Chem. Soc. 1885, i. 561; Duustan and Dymond,
Journ. Chem. Soc. 1887, i. 646.
Divers and Haga, Journ. Chem. Soc. 1889, i. 760.
5 Zorn, Ber. 11, 1630.
NITRIC OXIDE
505
The acid quickly splits up into nitrous oxide and water, but if
silver nitrate solution be at once added a considerable quantity
of silver hyponitrite is thrown down.¹
Free hyponitrous acid has not been prepared, as when it is
liberated from its salts by addition of acids it very rapidly splits
up into its anhydride (nitrous oxide) and water. Even the
salts split up readily in a similar manner yielding nitrous oxide
and alkali. According to Berthelot, the reason that nitrous
oxide does not combine with bases to form hyponitrites is that
the difference between the heat of formation of hyponitrous
acid in solution (-574 cal.) and that of nitrous oxide.
(-206 cal.) is greater than the heat of neutralization of the
acid.
An aqueous solution of hyponitrous acid may be obtained by
the action of the calculated amount of dilute hydrochloric acid
on the silver salt; it is colourless and strongly acid, and is
stable towards acids and alkalis even on boiling. On titration
with potash in presence of either phenolphthalein or litmus it
remains acid until the acid salt is formed and then becomes
alkaline, but it does not expel carbon dioxide from the alkaline
carbonates. In acid solution it is converted by potassium per-
manganate quantitatively into nitric acid, but in alkaline solu-
tion yields nitrous acid. It neither decolorises iodine nor
liberates the latter from solutions of potassium iodide.²
With lead acetate solution the salts give a white precipitate
which becomes dense and yellow after a time, and with silver
nitrate a yellow precipitate, which dissolves in acids, but is
reprecipitated by ammonia. The acid solution does not colour
ferrous sulphate, but on addition of strong sulphuric acid the
black coloration characteristic of nitric oxide is observed.
-
NITROGEN DIOXIDE, OR NITRIC OXIDE, NO = 29.82.
281 This gas was first observed by Van Helmont, who
included it under the term gas sylvestre. It was afterwards
more fully investigated by Priestley, who named it nitrous air
(see "Historical Introduction").
Preparation. The gas is formed when nitric acid acts on certain
metals such as copper, silver, mercury, zinc, &c., as also upon
1 Wislicenus, Ber. 26, 772; Thum, Monatsh. 14, 294.
2 Thum, Monatsh 14, 294.
I
·
I
506
THE NON-METALLIC ELEMENTS
phosphorus and some other easily oxidizable substances. It is
usually prepared by dissolving copper foil or copper turnings in
nitric acid of specific gravity 12, washing the gas by passing it
through water and caustic soda, and collecting it over cold
water in the pneumatic trough. The reaction occurring in this
case is expressed as follows:--
3Cu + 8HNO3
2NO + 3Cu(NO3)2 + 4H₂O.
The gas thus obtained is, however, not pure, as it invariably
contains free nitrogen and nitrous oxide, the quantity of the
latter gas increasing with the amount of copper nitrate which
is formed.¹ In order to obtain a much purer gas the nitric oxide
must be passed into a cold concentrated solution of ferrous sulphate
or chloride, with which it forms a singular compound, to be
described hereafter, and which dissolves in water with formation
of a deep blackish-brown solution. On heating this solution,
nitric oxide is given off, but this gas still contains one-500th of
its volume which is not absorbable by ferrous salts.2 The almost
pure gas can also be obtained by heating ferrous sulphate with
nitric acid, or by heating a mixture of ferrous sulphate and sodium
nitrate with dilute sulphuric acid; or by allowing a concentrated
solution of sodium nitrite to drop into a hydrochloric acid
solution of ferrous sulphate or chloride. The pure gas may be
prepared by acting on mercury with a mixture of sulphuric and
nitric acids, a reaction which has been made use of by Lunge
in his nitrometer for the estimation of the oxides of nitrogen
in sulphuric acid.
=
Properties.-Nitric oxide is a colourless gas having a
specific gravity of 1.039 (Bérard). It was first liquefied by
Cailletet in November, 1877, by exposing the gas to a pressure of
104 atmospheres at a temperature of -11°. It forms a colourless
liquid which boils under 712 atmospheres pressure at — 93°5
(critical temperature), and at 153°6 at atmospheric pressure.5
When mixed with an excess of oxygen it yields almost entirely
nitrogen peroxide, but, according to Lunge, when nitric oxide
is in excess, nitrogen trioxide is also formed; with an excess
of oxygen and moisture it is chiefly converted into nitric acid,
whilst with even an excess of oxygen in presence of concentrated
1 Ackworth, Journ. Chem. Soc. 1875, 828.
2 Leduc, Compt. Rend., 116, 323.
4 Emich, Monatsh. 13, 73.
3 Thiele, Annalen, 253, 246.
5 Olszewski, Compt. Rend. 100, 940.
NITRIC OXIDE
507
sulphuric acid it yields solely nitrosylsulphuric acid (p. 516),
and no nitrogen peroxide or nitric acid.¹ On exposure to air,
this gas at once combines with the atmospheric oxygen, with
evolution of heat and formation of red fumes of higher oxides.
No combination occurs when the perfectly dry gas is mixed
with dry oxygen.2
Nitric oxide is extremely stable when heated, not being com-
pletely decomposed until the temperature of melted platinum is
reached. Its formation from its elements is attended by absorption
of heat (p. 232), and when exposed to the shock from an ex-
plosion of mercuric fulminate, it is resolved into its elements.
This property is also shown by other substances which are
formed with absorption of heat such as carbon bisulphide and
acetylene.
If a spiral of iron wire be heated to redness in this gas by
means of a galvanic current, the iron burns brilliantly so long as
any nitric oxide remains undecomposed, and after the combustion
the residual gas is found to consist of nitrogen exactly equal in
volume to one-half of the gas employed.
When a stream of the gas is passed over heated potassium this
metal takes fire and burns brilliantly; whereas metallic sodium,
even when heated with a spirit-lamp, remains unaltered in the
gas. Phosphorus also burns with a dazzling brilliancy in nitric
oxide, but only when it is brought into the gas already brightly
burning, The flame of feebly burning phosphorus, as well as that
of sulphur and of a candle, are on the other hand extinguished
on plunging them into nitric oxide, because the temperature of
these flames is not sufficiently high to decompose this gas into
its elementary constituents. If a few drops of carbon bisul-
phide be poured into a long glass cylinder filled with nitric
oxide vapour, and the cylinder well shaken so that the vapour
of the bisulphide is well mixed with the gas, the mixture
burns with a splendid blue and intensely luminous flame, which
is characterised by its richness in the violet or chemically active
rays. So intense is this light for the violet and ultra-violet
rays, that a lamp in which the two gases are burnt has been
constructed for the use of photographers.5
The name nitrogen dioxide has been given to this gas because
for the same quantity of nitrogen it contains twice as much
1 Ber. 18,
1384.
3 Emich, Monatsh. 13, 78.
Sell, Ber. 7, 1522.
2 Baker, Proc. Chem. Soc. 1893, 129.
Berthelot, Compt. Rend. 93, 613.
508
THE NON-METALLIC ELEMENTS
oxygen as nitrous oxide. or nitrogen monoxide. Its density,
however, which remains constant down to - 70°,¹ is about 14, and
the gas has therefore the formula NO, and consequently possesses
a simpler constitution than nitrous oxide. The chemical and
physical properties of nitric oxide bear out this view. Thus, it
does not condense under circumstances which effect the lique-
faction of nitrous oxide; it is also much more stable than this
latter gas, so that it follows a law which we find to hold good
with regard to analogous gaseous bodies, viz. that those possess-
ing the simpler constitution are much less easily condensible,
and much less easily decomposable, than those of more compli-
cated constitution.
NITROGEN TRIOXIDE OR NITROUS ANHYDRIDE, N₂Og. = 75·52.
282 When starch, sugar, arsenious anhydride and other easily
oxidisable bodies are heated with nitric acid, red fumes are
given off, which consist of a varying mixture of nitric oxide,
nitrogen trioxide and nitrogen peroxide, the relative propor-
tions varying with the strength of the acid employed. The
largest proportion of trioxide is obtained by acting on arsenious
anhydride with nitric acid of specific gravity 1:35, or on starch
with an acid of specific gravity 1:33.¹ A red gas is thus formed,
which on passing through a freezing mixture condenses to a
deep blue mobile liquid, and does not solidify at – 90°.
-
2
During the past few years doubts have arisen as to whether
nitrogen trioxide exists in the gaseous condition. Witt,
Ramsay, and others 3 maintain that the red fumes having the
composition NO3 consist in reality of a mixture of equal
volumes of NO and NO₂ (or N2O4, see p. 513). Ramsay and
Cundall have shown that when nitrogen peroxide and nitric
oxide are mixed over mercury no contraction of volume takes
place, as would be the case if any nitrogen trioxide were formed,
and they conclude from the density of the gas itself that it does
not contain any N,O. Lunge on the other hand has urged
that whereas nitric oxide when mixed with an excess of oxygen
yields nitrogen peroxide almost entirely, the red fumes having
the composition NO, even when mixed with a large excess of
3
1 Daccomo and V. Meyer, Annalen, 240, 326.
2 Lunge, Ber. 11, 1641.
3 Witt, Ber. 11, 756; 12, 2188; Ramsay and Cundall, Journ. Chem. Soc.
1885, i. 187, 672; Geuther, Annalen, 245, 96.
NITROUS ACID
509
oxygen at temperatures varying from 4° to 150° are only con-
verted to a comparatively small extent into the peroxide; hence
he concludes that the dissociation of the trioxide into dioxide
and peroxide is only a partial one. He further makes the
objection to Ramsay and Cundall's experiment that nitrogen
peroxide is not unacted upon by mercury, and that no valid
conclusions can be drawn from experiments made with gases
confined by that liquid.¹
Ramsay concludes that even liquid N₂O, dissociates partially
at as low a temperature as - 90°, and can only exist dissolved in
an excess of nitrogen peroxide.2
NO} 0+
NITROUS ACID, HNO2.
283 Nitrogen trioxide dissolves in ice-cold water, giving rise
to a beautiful blue liquid, which contains nitrous, acid, as shown
in the following equation :—
H}0=
0:
Н
0}
NO } O + NO} 0.
H
Nitrous acid is not known in the pure state, it being a very
unstable substance, which even in aqueous solution rapidly
undergoes decomposition when warmed, giving rise to nitric
acid and nitric oxide gas; thus:-
3HNO₂ = HNO3 + 2NO + H₂O.
This reaction is, however, according to Veley, a reversible one,
nitric oxide yielding with nitric acid and water a small quantity
of nitrous acid.3 The salts of this acid, or the nitrites, are, on
the contrary, very stable bodies. They are not only formed by
the action of the acid upon oxides, but also by the reduction of
nitrates and by the oxidation of ammonia. Thus, for instance,
potassium nitrite, KNO,, is formed either by fusing saltpetre,
or, more easily, by heating this salt with lead or copper;
thus:-
2KNO, = 2KNO2 + O2.
It is also formed to some extent by the action of sunlight on a
sterilized solution of potassium nitrate. Nitrites also occur in
¹ Lunge, Ber. 11, 1229, 1641; 12, 357; 15, 495; 18, 1376.
2 Journ. Chem. Soc. 1890, i. 597.
4 Laurent, Bull. acad. Belg. [3], 21, 337.
3 Proc. Roy. Soc. 52, 23.
510
THE NON-METALLIC ELEMENTS
nature. Thus the atmosphere contains small quantities of
ammonium nitrite, and traces of nitrites have been detected in
the juices of certain plants (Schönbein). All the normal nitrites
are soluble in water, and most of them soluble in alcohol. The
silver salt is the nitrite which is most difficultly soluble in cold.
water, crystallizing out in long glittering needle-shaped crystals
when the hot aqueous solution is cooled. The nitrites deflagrate
when thrown on to glowing carbon, as do the nitrates. They
can, however, be distinguished from the latter salts by the
action of dilute acids, which produce an evolution of red fumes
from the nitrites but not from the nitrates.
In a similar way aqueous solutions of the neutral nitrites
become of a light brown colour when mixed with a solution of
ferrous sulphate, and this colour deepens to a dark brown on the
addition of acetic acid. In order to detect the presence of a
nitrite in dilute solution, iodide of potassium, starch paste, and
dilute nitric acid are added. The latter acid sets free the nitrous
acid, and this instantly decomposes the iodide with liberation of
iodine. As, however, other oxidising agents act in a similar
way, a small quantity of potassium permanganate solution is
added to another portion of the liquid; if a nitrite be really
present, the colour of the permanganate solution will be at
once destroyed.
In the case of the presence of nitrites in very small quantities,
as in certain waters, Fresenius recommends the distillation of
the water previously acidified with acetic acid, the first few drops
of the distillate being allowed to fall into a solution of iodide
of potassium and starch, to which a small quantity of sulphuric
acid has been added. Another very delicate reagent employed
for the detection of nitrites in water is meta-diamido-benzene,
which gives in sulphuric acid solution a brown coloration with
nitrites (Griess).
NITROSYL CHLORIDE, NOCI. = 65.01.
284 This chloride of nitrous acid is formed by the direct
union of nitric oxide and chlorine, as well as by the action of
phosphorus pentachloride upon potassium nitrite; thus:-
PC15+ NOOK = NOCI + KCl + POCI„.
-
It is likewise formed together with free chlorine when a mixture
1
NITROSYL BROMIDE
511
SO4
of hydrochloric and nitric acids, the so-called aqua regia, is
slowly heated; thus:-
HNO3 + 3HCl = NOCI + Cl2 + 2H₂O.
In order to obtain the chloride in the pure state, a mixture of one
volume of nitric acid of specific gravity 142 and four volumes
of hydrochloric acid of specific gravity 116 is gently warmed,
the gases which are evolved being first dried by passing through
a chloride of calcium tube, and then led into strong sulphuric
acid. The chlorine and hydrochloric acid gases thus escape,
whilst nitrosyl sulphate, SO,H(NO), a body to be described
later on, is formed. As soon as the sulphuric acid is saturated,
the liquid is heated with an excess of perfectly dry sodium
chloride, when nitrosyl chloride is evolved;¹ thus :-
――――――
{NO + NaCl = SO,
Nitrosyl chloride is an orange-yellow gas, the colour of which
is quite different from that of chlorine. It liquefies readily
when passed through a tube surrounded by a freezing mixture,
forming a deep orange limpid liquid which boils about -8°.
This substance combines with many metallic chlorides, forming
peculiar compounds, whilst, brought into contact with basic
oxides, it is decomposed with formation of a nitrite and
chloride; thus:-
Na
S
+NOCI
H
NOC1+2KOH= KNO₂+ KCl + H₂O.
2
NITROSYL BROMIDE, NOBr. = 109.18.
285 In order to prepare this compound, nitric oxide is led
into bromine at a temperature of -7° to -15° as long as it is
absorbed. In this way a blackish-brown liquid is obtained,
which begins to decompose at the temperature of -2°, nitric
oxide being evolved. If the temperature is allowed to rise
to +20°, a dark-brownish red liquid remains behind, which
has the composition NOBr,; and this is also formed when
bromine is saturated with nitric oxide at the ordinary
atmospheric temperature. Nitrosyl tribromide, NOBr, is
volatilized when quickly heated, almost without decomposition,
but if it is slowly distilled it decomposes into its constituents
(Landolt).
Tilden, Journ. Chem. Soc. 1860, 630.
512
THE NON-METALLIC ELEMENTS
NITROGEN TETROXIDE, OR NITROGEN PEROXIDE, NO2, OR N₂O,
286 The red fumes which are formed when nitric oxide comes
into contact with oxygen or air, consist chiefly of nitrogen.
peroxide. If one volume of dry oxygen be mixed with two
volumes of dry nitric oxide and the red fumes produced led
into a tube surrounded by a freezing mixture, the peroxide
condenses in the tube either as a liquid or in the form of
crystals.
Nitrogen peroxide is also formed by the decomposition
which many nitrates undergo when heated, and is usually
prepared by strongly heating lead nitrate in a retort of hard

FIG. 151.
glass, as shown in Fig. 151, when the following decomposition
occurs:-
2Pb(NO)2=2PbO + 4NO₂+ 0.
2
This mode of preparation is, however, not very convenient,
and a considerable loss of material occurs, as the oxygen which
is evolved carries away some quantity of the peroxide even
when the tube into which the fumes are led is plunged into
a freezing mixture.
The following method is free from the above objections;
arsenious oxide (white arsenic) in the form of small lumps is
placed in a flask and covered with ordinary nitric acid, or
NITROGEN PEROXIDE
513
according to Cundall,¹ with a mixture of nitric acid of specific
gravity 1.5 and half its weight of sulphuric acid; the red fumes,
which are given off in quantity on gently heating, are led into
a receiver surrounded by a freezing mixture, where a mixture
of trioxide and tetroxide of nitrogen collects. The mixture is
freed from the trioxide by the addition of strong nitric acid
and a large quantity of phosphorus pentoxide, the tetroxide is
then poured off from the syrupy layer, and distilled.
287 Properties.-Nitrogen tetroxide is a liquid at the ordinary
atmospheric temperatures; at -10°1 it solidifies to a mass of
colourless crystals. Slightly above this temperature the liquid
compound is also colourless, but when warmed above this, it
first becomes of a pale greenish yellow, then at +10° it attains
a decided yellow colour, whilst at 15° it becomes orange-coloured,
and at higher temperatures it assumes a still darker tint. The
absorption spectrum of gaseous nitrogen tetroxide is a cha-
racteristic band spectrum which has been mapped by Brewster
and Gladstone.
Temperature.
97°5
24°5
11°3
4°.2
Liquid nitrogen tetroxide has a specific gravity of 1·49 at 0°
and boils at 22°. forming a reddish brown vapour, possessing a
very strong and unpleasant smell. When the temperature of the
gas is raised, the colour becomes darker and darker, until at
last it appears almost black and opaque. This is well shown
by sealing some of the gaseous tetroxide in two wide glass
tubes, and heating one for some little time in the flame of a
lamp whilst the other remains at the ordinary temperature.
These remarkable changes in appearance cannot be recognized
by any equally striking changes in the absorption spectrum of
the gas, although it is probable that the peroxide exists in two
distinct forms, as indeed has been shown by the variations
which its density exhibits. Thus, at low temperatures the
density corresponds to the formula N,O,, and at higher ones
to NO2. The density of the vapour at different temperatures
was found by Playfair and Wanklyn 2 to be as follows:-
•
1 Journ. Chem. Soc. 1891, i. 1076.
Density.
Air=1.
1.783
2:520
2.645
2.588
Corresponding mole-
cular weight.
51.5
74.8
2 Journ. Chem. Soc. 1863, 156.
34
514
THE NON-METALLIC ELEMENTS
The density required for the compound NO, is 1585, that
for the compound N,O,, the double of this, or 3:17. It will
be seen that the numbers obtained all lie between these two,
the density at the highest temperature not lying far from
the lower number, whilst those found for the lower tempera-
tures correspond more nearly to the density of the substance.
NO. From these facts we draw the conclusion that at low
temperatures the molecule of the compound is represented by
the formula N,O,, and the density 46, but that as the tempera-
ture rises a gradual change in the density of the gas takes
place, one molecule of N,O, splitting up, or becoming dissociated,
into two of NO2, possessing a density of 23. From Deville and
Troost's ¹ experiments the following percentage composition of
the gas at various temperatures has been calculated:-
at 26° 7
60° 2
100°1
..
د.
""
29
135°
140°
•
NO.
20:00
50.04
79.23
98.96
100.00
•
N2O4
80.00
49.96
20.77
1.04
0:00
We thus see that at 140° the black vapour consists entirely
of the simpler molecule NO.
Liquid nitrogen peroxide when diluted with chloroform under-
goes a similar dissociation, the amount of dissociation increasing
with dilution and rise of temperature 2
Nitrogen tetroxide is decomposed by cold water with pro-
duction of nitric and nitrous acids; thus:-
2NO2 + H2O = HNO3+HNO2.
This decomposition, however, only occurs at low temperatures
and with small quantities of water. When nitrogen peroxide
is added to an excess of water at the ordinary temperature, the
nitrous acid is at once decomposed into nitric oxide and nitric
acid; thus:-
3NO₂+H₂O=2HNO3 + NO.
If oxygen be present at the same time, this of course com-
bines with the nitric oxide, forming nitrogen peroxide, which is
again decomposed by water, and in this way the peroxide may
1 Jahresb. 1867, p. 177.
2 Cundall, Journ. Chem. Soc. 1891, i. 1076.
NITROGEN PEROXIDE
515
be completely transformed into nitric acid. In order to exhibit
this decomposition, as well as to show the formation of the
peroxide and nitric acid, the following apparatus may be used
(Fig. 152) The upper vessel, containing a little water, is filled
with nitric oxide, and is connected with the lower vessel by the.
tube (4), which is drawn out to a point. This lower vessel
contains water, coloured with blue litmus. If oxygen be now

B
FIG. 152.
led slowly into the upper vessel by means of the tube (B), red
fumes are formed, which are absorbed by the water. A vacuum
is thus produced, and the coloured water rises in the form of a
fountain into the upper vessel, and becomes coloured red. If
the nitric oxide be perfectly pure, and if care be taken that the
oxygen is allowed to enter but slowly towards the end of the
operation, the whole of the upper vessel may be filled with
water.
516
THE NON-METALLIC ELEMENTS
COMPOUNDS OF NITROGEN WITH SULPHUR
AND SELENIUM.
NITROGEN SULPHIDE, NS2.
288 This body is obtained, together with other compounds, by
the action of dry ammonia upon chloride of sulphur, or upon
thionyl chloride. It is a yellow powder, crystallizing from
solution in bisulphide of carbon in yellowish-red rhombic
prisms. When heated to 120° it becomes darker coloured, and
emits vapours which attack the mucous membrane violently.
Heated to about 135°, it sublimes in the form of fine yellowish-
red crystals, and at 158° begins to melt with evolution of gas.
At 160° it decomposes rapidly with the evolution of light and
heat, and on percussion detonates very violently.
When sulphur dichloride is added to a solution of nitrogen.
sulphide in bisulphide of carbon, several different compounds
are produced, according to the quantity of the chloride of
sulphur which is present. If this last body be present in excess,
a yellow crystalline precipitate is formed, having the com-
position (NS)SCI,, which, on heating, sublimes in needles. If
after this compound has separated out, more nitrogen sulphide
solution be added, the yellow powder is changed into a red
substance, (NS),SC, and this again, on addition of more
nitrogen sulphide, or on heating, yields a compound (NS) SCI,
which forms a beautiful yellow powder, unalterable in contact
with the air (Fordos and Gélis).
Nitrogen Selenide, N,Se,.-This compound is formed by the
action of ammonia on selenium tetrachloride. It is an orange-
yellow mass, which, on heating to 200° as well as by slight
pressure, detonates strongly.
NITROSULPHONIC ACID, SO2 OH.
S O.NO
289 This compound, which is frequently termed nitrosyl-
sulphuric acid, is commonly known as the crystals of the leaden
chambers or nitrosyl sulphate, produced during the process of
the manufacture of sulphuric acid whenever the supply of
steam is insufficient to produce sulphuric acid. Nitrosulphonic
NITROSULPHONIC CHLORIDE
517
acid is, however, best prepared by acting upon sulphur dioxide
with concentrated nitric acid; thus:-
SOH
SO₂+ NO₂OH =SO NO
2
2.
For this purpose, dry sulphur dioxide is led into cold fuming
nitric acid until the mass becomes syrupy. The semi-solid
mass is then drained on a dry porous plate over sulphuric acid.
Another mode of preparing the substance is to pass the
vapour of nitrosyl chloride into sulphuric acid:-
Он
NOCI + SO₂ {OH
N₂O₂+2SO₂ OH
SOH
=
SO.NO
SO,
21 OH
It is likewise formed when nitrogen trioxide is brought
together with sulphuric acid.
+
H
SO2
}
O.NO
HS
{0
{
•
OH + HCl.
Nitrogen peroxide also yields the same product with sulphuric
acid together with nitric acid.
= 280₂ {NO?
OH
The substance obtained by these reactions crystallizes in
four-sided rhombic prisms, or sometimes in tabular or nodular
crystalline masses, which begin to melt at 30°, with evolution
of vapour.
0 =
The crystals dissolve in small quantities of cold water without
any evolution of gas, forming a blue liquid, which contains
sulphuric and nitrous acids :
2
+ H₂O.
(OH
SO₂OH + HO.NO.
Nitrosulphonic acid dissolves in concentrated sulphuric acid
without decomposition, and this solution can be distilled.
NITROSULPHONIC CHLORIDE, SO, {CNO
2 Cl.
290 This chloride is formed by the direct union of sulphur
trioxide and nitrosyl chloride, and by the action of thionyl
chloride on silver nitrate. It forms a white crystalline mass,
which melts on heating, with separation of nitrosyl chloride. It
1 Thorpe, Journ. Chem. Soc. 1882, i. 297.
518
THE NON-METALLIC ELEMENTS
is decomposed in contact with water into sulphuric, hydrochloric,
and nitrous acids (Weber).
NITROSULPHONIC ANHYDRIDE,
291 When nitrosulphonic acid is heated, it decomposes into
water and this anhydride, but the latter substance cannot be
obtained in a pure state in this way, as sulphuric acid is formed
by the further decomposition of nitrosulphonic acid, and these
bodies cannot be separated, since they both boil nearly at the
same temperature (Michaelis). It is, however, easy to obtain
this anhydride by passing dry nitric oxide into sulphur trioxide,
so long as it is absorbed, and warming the solution until the
boiling point of the liquid is nearly reached; thus:-
3S0,0 + 2NO =
SO₂
O.NO.
O
SO, O.NO.
SO₂ONO
2
0
SO, ONO.
The same compound is formed when sulphur dioxide acts
upon nitrogen peroxide, as well as when the next compound to
be described is heated. Nitrosulphonic anhydride crystallizes
in hard colourless quadratic prisms, melts at 217° to form a
yellow liquid, becomes darker on further heating, and distils
over unchanged at about 360°. The compound dissolves readily
in strong sulphuric acid, forming nitrosulphonic acid.
+ SO2.
OXYNITROSULPHONIC ANHYDRIDE,
NITROXYPYROSULPHURIC ACID,
SO, {ONO
2
O
SO, ONO
2
292 When sulphur trioxide and nitrogen peroxide are brought
together in the cold, the above compound separates out as a
white crystalline mass, which on heating gives off oxygen, and
forms the anhydride last described (Weber).
SOO.NO.
0
SO, ONO,.
When sulphur trioxide and nitric acid are mixed together
in the cold, a thick oily liquid is formed, from which the above
¹ Ber. 7, 1075.
NITRILOSULPHONIC ACID
519
compound crystallizes out under certain conditions of concen-
tration. It is soluble without decomposition in warm dilute
nitric acid, and on cooling the liquid, crystals of this substance
separate out, containing one molecule of water of crystallization.
SULPHONIC ACIDS OF AMMONIA AND HYDROXYLAMINE.
293 In the year 1845 a number of crystalline salts derived from
acids containing both nitrogen and sulphur were obtained by
Fremy by the interaction of nitrites and sulphites. He only
determined their empirical formulæ, but they were further in-
vestigated by Claus, who showed that they are sulphonic acids,
that is, compounds containing the group SO,OH. He, however,
wrongly regarded a number of them as sulphonic acids of the
hypothetical substance NH,, whereas the later investigations of
Berglund and especially of Raschig have shown that all these
compounds are sulphonic acid derivatives of ammonia, hydroxyl-
amine and the hypothetical dihydroxylamine, NH(OH)2.
Potassium nitrilosulphonate, N(SO,K); + 2H₂O.-This com-
pound, which was termed by Claus potassium trisulphammonate,
is obtained by adding an excess of a concentrated solution of
potassium sulphite to a similar solution of potassium nitrite,
and separates out as a crystalline precipitate which may be
recrystallized from alkaline solution. The reaction which takes
place may be represented as follows:-
KNO, +3K₂SO,¸ + 2H2O = N(SO¸K); + 4KOH.
The reaction is remarkable inasmuch as one of the most
powerful alkalis is produced by the interaction of two neutral
salts.
----
Potassium nitrilosulphonate crystallizes with 2 mols. H₂O,
and forms thin silky rhombic needles, which are decomposed
in the following manner by boiling water:-
N(SO,K) + 2H2O = NH₂SO₂H + K₂SO̟ + KHSO̟.
3
4
Hence only two-thirds of the sulphur is precipitated as barium
sulphate on adding barium chloride to the hot solution, amido-
sulphonic acid, which will be subsequently described, not being
affected by barium chloride.
1 Annalen, 56, 315.
2 Annalen, 152, 336, 351; 158, 52, 194.
3 Lunds Universitas Årskrift, 12 and 13.
4 Annalen, 241, 161.
520
THE NON-METALLIC ELEMENTS
Potassium imidodisulphonate, NH(SO,K)-When the nitrilo-
sulphonate is boiled for only a short time with water, the
reaction does not proceed so far as shown in the above equation,
the chief product being potassium imidodisulphonate:—
N(SO₂K); + H₂O = NH(SO₂K), + KHSO.
This is more readily obtained by moistening the nitrilo-
sulphonate with dilute sulphuric acid, allowing to stand for a
day, and recrystallizing from dilute ammonia. It forms mono-
symmetric crystals, and is decomposed by boiling water into
amidosulphonic acid and potassium sulphate.
A large number of other imidodisulphonates have been pre-
pared by Divers and Haga;¹ the most interesting of these are
ammonium imidodisulphonate, NH(SO,NH)2, and basic am-
monium imidodisulphonate, NH.N(SO¸NH), which were
first prepared by Rose in 1834 by the action of ammonia on
sulphur trioxide and termed parasulphatammon and sulphatam-
mon respectively.
Free imidodisulphonic acid has only been obtained in aqueous
solution.
Amidosulphonic acid, NH.SO₂H.—This acid is, as already men-
tioned, the final product of the action of boiling water on the
salts just described. It is a very stable compound sparingly
soluble in water, and forms colourless rhombic prisms. Its
potassium salt, NH,.SO,K, crystallizes in very similar forms.
Hydroxylaminedisulphonic acid, HO.N.(SO,H)2.-This acid is
not known in the free state, but its potassium salt is formed by
the action of potassium hydrogen sulphite on potassium nitrite
in aqueous solution at 0°. The reaction proceeds more readily
with the sodium salts, which are mixed in the proportions
required by the equation :—
NaNO2+2NaHSO, HO.N(SO,Na)2 + NaOH.
The sodium salt is however readily soluble in water, and it
is therefore best converted into the potassium salt by adding
an equivalent quantity of a concentrated potassium chloride so-
lution. Potassium hydroxylaminedisulphonate, HO.N(SO,K),
2H₂O, separates out on standing in compact crystals, which may
be readily separated from the small quantity of slender needles
of potassium nitrilosulphonate simultaneously produced. It
1 Journ Chem. Soc. 1892, i. 943.
=
DIHYDROXYLAMINESULPHONIC ACID
521
forms monosymmetric crystals, which are very slightly soluble
in water; when boiled with the latter or with acids it is first
converted into hydroxylaminesulphonic acid and then into
hydroxylamine, which combines with the sulphuric acid simul-
taneously produced to form the sulphate. This method is now
employed for the preparation of hydroxylamine (p. 475).
Hydroxylaminesulphonic acid, HO.NH.SO,H, like amido-
sulphonic acid, is stable in the free state and forms a syrupy
liquid. The potassium salt, HO.NH.SOK, is best obtained by
the partial hydrolysis of the disulphonate with hot water, and
may be purified by quick recrystallization from that liquid.
When treated with alkalis it yields salts of hyponitrous acid
(p. 504).
Nitrosohydroxylaminesulphonic acid, or Dinitrososulphonic acid,
HSNO-The alkali salts of this acid, which are colourless
crystalline substances, are formed when nitric oxide is passed
into an alkaline solution of a sulphite. The free acid is
unknown, as all acids, even carbonic acid, decompose the salts
into sulphates and nitrous oxide. When reduced with sodium
amalgam the salts are converted into a hyponitrite and sulphite,
from which it appears that they contain the group NO, com-
bined with both potassium and the sulphonic acid group.
These reactions are satisfactorily explained by the constitutional
formula suggested by Raschig, viz. ON.N(OK)SOK, according
to which the potassium salt is a basic salt of hydroxylamine-
sulphonic acid, in which one of the hydrogen atoms has been
replaced by the group NO.
Dihydroxylaminesulphonic acid, (HO), N. SO,H, is unknown, but
its basic potassium salt is sometimes formed in the preparation
of hydroxylaminedisulphonates from potassium nitrite and
hydrogen sulphite. It is a crystalline unstable substance which
is decomposed by acids with evolution of nitrous oxide. It is
probably identical with Fremy's potassium sulphazinite.
Fremy also described a salt under the name potassium
sulphazinate, which according to Raschig is identical with
another derivative of dihydroxylamine, obtained by gradually
adding a solution of potassium hydrogen sulphite with con-
stant shaking to a solution of potassium nitrite, till the
solution forms a magma of crystals. It has the composition.
KHN,O,(SO,K),, decomposes on heating with evolution of red
fumes, gives off nitrous oxide on addition of acids, and decom-
poses in aqueous solution into hydroxylaminedisulphonic acid and
522
THE NON-METALLIC ELEMENTS
potassium nitrite. It is probably formed from two potassium
salts of dihydroxylaminesulphonic acid with elimination of
water in the following manner :—
SO,K.NOH+HO,
OK, HO
N.SO,K=SO₂K.N.O.N.SO,K+H₂O
1
OK OH.
In addition to the foregoing Raschig¹ has investigated a
series of salts known as the sulphazotinates, which appear to be
derived from an acid having the constitution.
(SO,H)2: N
N: (SO,H)2.
NATURE OF THE REACTION BETWEEN NITRITES AND
SULPHITES.
294 Nitrous acid is usually supposed to have the constitutional
formula O: N.OH, but it is not unlikely that in aqueous solution
it may further combine with another molecule of water, having
then the constitution N(OH). The reactions which occur with
sulphites may then be readily explained as consisting in the
substitution step by step of the group SOH for the hydroxyl
group with elimination of water and the successive formation of
dihydroxylaminesulphonic acid, hydroxylaminedisulphonic acid,
and nitrilotrisulphonic acids, or rather their potassium salts, as
shown in the following equations:-
N(OH)3+H.SO¸K=(HO),N.SO₂K+H₂O.
(HO),N.SO,K+HSO,K=HO.N(SO,K),+H,O.
HO.N(SO,K)2 + HSO₂K = N(SO„K)3 + H¿O.
AMIDE AND IMIDE OF SULPHURIC ACID.
295 When ammonia is allowed to act on a solution of sul-
phuryl chloride in chloroform, it yields a mixture of the amide
and imide of sulphuric acid, SO,(NH2), and SO₂: NH, together
with another substance which is probably imidosulphurylamide
NH(SO,NH,)..
SO„Cl₂+4NH¸=2NH¸Cl+SO₂(NH2)2
SO¿Cl₂+3NH¸ = 2NH¸Cl+SO₂NH.
1 Annalen, 241, 232.
3
THE ATMOSPHERE
523
The three compounds are separated from one another and
from the ammonium chloride simultaneously formed by taking
advantage of the different properties of their silver salts, which
after separation are converted into the free amides by the action
of hydrochloric acid.
Sulphamide, SO(NH2)2, forms large colourless crystals, which
are extremely soluble in water, and on heating soften at 75°
and melt at 81°. It is converted by alkalis into salts of
amidosulphonic acid (sulphaminic acid), NH.SO,OH (p. 520),
and yields a silver compound, SO2(NHAg), which is a white
amorphous powder.
Sulphimide, SO,NH, is formed together with sulphamide in
the manner stated above, and is also obtained when the latter is
heated above its melting point. It has not been obtained pure;
its aqueous solution is fairly stable at the ordinary tempera-
ture, but on boiling is converted into ammonium hydrogen
sulphate. Its silver compound, SO,NAg, crystallizes from hot
water in long needles, which require 500-600 parts of cold-
water for their solution.¹
THE ATMOSPHERE.
296 The term air was used by the older chemists in the
general sense in which we now employ the word gas, to signify
the various kinds of aeriform bodies with which chemistry has
made us familiar. At the present day, however, we confine the
signification of the word air to the ocean of aeriform fluid or
the atmosphere (aτμós, vapour, and opaîpa, a sphere) at the
bottom of which we live and move.
Of the existence of an invisible gaseous envelope lying
above the solid mass of the earth's crust, we become aware
by the resistance offered to our bodies when we pass rapidly
from place to place, as well as by the effects produced by the
motion of the particles of the atmosphere which we term wind.
The most convincing proof of the existence of the air is how-
ever given by showing that air has weight. This can readily
be done by hanging a large thick-walled glass globe, closed
with a cork through which a tube passes, furnished with a
stopcock, on to one end of the beam of a balance, and placing
¹ Traube, Ber. 26, 607.
524
THE NON-METALLIC ELEMENTS
FIG. 153.
weights in the opposite pan until the arrange-
ment is in equilibrium. On exhausting the
globe by placing it in communication with an
air-pump, and again weighing the globe par-
tially freed from air, the weight will be seen to
be considerably less than that which it possessed
before the evacuation.
A knowledge of the composition of the at-
mosphere forms the beginning of the present
epoch of chemical science, experiment having
shown, in opposition to the older views, that
the air is not a simple body, but consists mainly
of two different kinds of air or gases, oxygen
and nitrogen.
Although some of the ancients, especially
Vitruvius, appear to have held the view that
the air possesses weight, yet it is to Torricelli
that we owe the first distinct proof that this is
the case. In the year 1640 a Florentine pump-
maker observed that his lift-pumps would not
raise water to a height greater than thirty-two
feet, and consulted his great townsman Galileo
as to the cause of this phenomenon. Galileo
does not appear to have given the correct solu-
tion, as he compared the water column to an
iron rod hung up by one end, which, when long
enough, will at last break with it own weight.
Torricelli, however, in 1643, made an experi-
ment which gave the true explanation of the
pump-maker's difficulty. Filling with mercury
a glass tube three feet in length, and closed
at one end, but open at the other, he closed
the open end with his finger and inverted the
tube in a basin filled with mercury. The mer-
cury then sank in the tube to a certain level,
whilst above this level there was an empty
space, which is still called the Torricellian
vacuum. Above the mercury in the basin
was water, and Torricelli then raised the tube
so that the open end came into the water.
The mercury then flowed out and the water
rushed up, completely filling the tube. Fig. 153 represents
THE ATMOSPHERE
525
the actual tubes employed by Torricelli, photographed from
the original instruments placed in the Science Loan Exhibition
at South Kensington. The rise of mercury or water in a vacuous
tube is caused by the pressure of the atmosphere; the water
is, however, 135 times lighter than the mercury, and hence the
column of the former liquid which is supported by the atmo-
spheric pressure is 135 times as high as that of the latter
liquid. Thus was the barometer discovered, though this name
was first made use of by Boyle.¹
Hearing of Torricelli's discovery Blaise Pascal resolved to put
this theory to a further test. If, argued he, the suspension of the
mercury in the barometric tube is due to the pressure or weight
of the air, the mercurial column must sink when the barometer
is taken to the top of a mountain, owing to the pressure on the
mercury being lessened. Unable to try this experiment himself
Pascal instructed his brother-in-law, Périer, to ascertain whether
this is so or not, and on September 19, 1648, Périer took a
barometer to the summit of the Puy-de-Dôme, and showed
that the mercury sank as he ascended, proving conclusively the
correctness of Torricelli's explanation.
A simple arrangement enables us to reproduce this experiment
in the lecture-room; a barometer tube filled with mercury is
inverted over mercury contained in a trough, when the mercury.
will be seen to sink to a certain level, the space above this being
vacuous. A tubulated receiver furnished with a tight-fitting
caoutchouc stopper is then brought over the tube, and the air
pumped out from the interior of the receiver. As the pressure
of the air is by degrees removed, the level of mercury in the
tube will gradually become lower, until at last it will nearly,
but not quite, reach the level of the mercury in the trough. On
opening the stopcock the air will rush in, and the level of the
mercury in the tube will rise until it has attained its former
elevation.
297 Following then the laws
of gravitation, the air forms
part of the earth's body, and accompanies the solid and liquid
portions in their axial and orbital motions. The absolute height
to which the atmosphere extends above the earth's surface has
not been ascertained with accuracy. As its density is not
1 See New Experiments on Cold, published 1664-5, Boyle's Works (Edn. 1772),
vol. ii. p. 487. "The barometer, if to avoid circumlocutions I may so call
the whole instrument, wherein a mercurial cylinder of 29 or 30 inches is kept
suspended, after the manner of the Torricellian experiment."
526
THE NON-METALLIC ELEMENTS
uniform, but diminishes as the distance from the earth's surface
increases, the exact point at which the atmosphere terminates
is difficult to determine. The height is certainly not uniform,
inasmuch as owing to the variation of gravitation at the poles
and at the equator, and owing also to the different velocities of
rotation as well as to changes of temperature, a column of polar
air is considerably shorter than a column of equatorial air.
The atmospheric pressure at the sea's surface would naturally
be constant if it were not that owing to the variations in the
solar radiation, the temperature, and, therefore, the pressure of
the air, undergoes frequent alterations. These irregular varia-
tions necessitate our reading off the height of the barometer
whenever volumes of gas have to be measured. There is no
doubt that the atmosphere has a definite limit, and, from
observations of the time during which the twilight extends to
the zenith, it appears that the atmosphere reaches in a state of
sensible density to a height of from forty to forty-five miles
above the earth's surface. The relation between this height
and the diameter of the earth may be illustrated by the state-
ment that if a globe of one foot in diameter represents the
earth, a film of air of an inch in diameter will represent the
atmosphere.
12
If the air were an incompressible fluid, instead of being an
elastic one, and if it had throughout the density which it
possesses at the sea's level, the height of the atmosphere
would be
10513 × 0·760 = 8360 meters, or 5·204 English miles.
As, however, the air is elastic, it diminishes in density as the
distance from the earth's level increases; thus at a height of
5528 metres, the air expands to twice its volume; whilst at a
height of twice 5528 metres the density of the air is only of
that which it possesses at the sea's level. At greater elevations
the volume increases in the following ratios:-
Geographical Miles.
0.000
0.587
1.174
1.761
2.348
2.935
3.522
Volume.
1
2
4
8
16
32
64
THE ATMOSPHERE
527
According to the corrected determinations of Regnault (p. 105),
one litre of dry pure air at 0°, and under the pressure of 760 mm.
at the latitude of Paris weighs 1.29349 gram, whilst according to
Lasch¹ the weight at Berlin is 1.293635 gram, or almost exactly
s of the weight of water. It must be remembered that
since the composition of air varies within certain narrow
limits, the weight of a given volume of it must also vary
slightly.
The weight of the air at the level of the sea in our latitude is
equal to that of a column of mercury at 0° of a height of
760 millimetres, and this is taken as the normal barometric
pressure. Hence, as one cc. of mercury weighs 13:596 grams,
the pressure exerted by the air on one square centimetre of
surface at the sea's level will be 13:596 x 76 = 10333 grams
(or nearly 15 lbs. on every square inch).
In common with all bodies at the earth's surface, the
human frame has to support this weight, but, under ordinary
circumstances the pressure is exerted in all directions, and it is
not felt. If, however, the pressure in one direction be removed,
as when the hand is placed over the open end of a cylinder from
which the air is being pumped out, the weight of the air is at
once perceived. As the air follows Boyle's law, its density
being directly proportional to the pressure to which it is sub-
jected, it follows that when the height above the sea level
increases by equal intervals, the density of the air decreases in
a geometric ratio. The difference in height of two stations in
the same vertical line is therefore in the ratio of the difference
between the logarithms of the barometric readings at the two
stations, and, if the temperature of the two stations be the same,
we need only multiply the difference of the logarithms of the
two readings (reduced to 0° for the expansion of mercury) by
the number 18363 to obtain the elevation in meters.
The average or mean annual temperature of the air, like its
density, is not the same throughout the mass. It diminishes
as the elevation above the earth's surface increases, so that at
a certain height, differing for different latitudes, a line is reached
at which the mean temperature of the air does not rise above
the freezing point. This is called the line of perpetual snow. At
latitude 75° it reaches the sea level, in the latitude of 60' it
is found at a height of 3,818 feet, whilst under the equator the
snow-line exists at a height of 15,207 feet above the sea.
1 Pogg. Ann. Ergänzungsbd. 3, 321.
528
THE NON-METALLIC ELEMENTS
The height of the snow-line is also affected by local causes
and is found to vary considerably even in the same latitude.
Owing to the unequal heating effect produced by the sun on
different portions of the earth's surface, great variations are
observed in the temperature of the atmosphere in different
places, and these give rise to those motions of the atmosphere
which are termed winds. Winds may either be caused by
local alterations of temperature confined to narrow limits, as
with the land and sea-breezes of our coasts, or they may be
produced by a general unequal diffusion of heat over the sur-
face of the globe, as with the so-called trade-winds, which are
caused by the temperature of the air in the equatorial zones
being higher than that of the air in the polar regions.
298 Air was first liquefied by Cailletet, and liquid air has
since been more closely investigated by Wroblewski2 and Dewar.³
It has not a constant boiling point, as the nitrogen boils off more
rapidly than the oxygen, the ebullition proceeding somewhat
roughly. Dewar has also succeeded in obtaining air in the solid
state, but it is as yet doubtful whether this is solid throughout,
or consists of a mixture of solid nitrogen and liquid oxygen,
the latter gas not having yet been solidified when in the pure
condition.
THE COMPOSITION OF THE ATMOSPHERE.
299 Atmospheric air, in addition to oxygen and nitrogen,
contains as normal constituents, aqueous vapour, carbon dioxide,
ammonia, and perhaps ozone. Other gases and vapours do
indeed occur in different places, under a variety of circum-
stances, and in varying quantities. Furthermore certain sub-
stances, such as common salt, ammonium nitrate, and some
other salts, occur as finely-divided solid particles, together with
other minute floating particles of animal, vegetable, and mineral
⚫ origin.
The discovery of the composition of the atmosphere has
been described in the Historical Introduction. We saw there
that we owe to Cavendish the first exact determination of the
relation existing between the two important constituents
oxygen and nitrogen. "During the last half of the year
1 Compt. Rend 85, 1270.
2 Monatsh. 6, 204; Compt. Rend. 102, 1010.
3 Proc. Royal Institution.
4 Phil. Trans. 1783, p. 106. "An Account of a new Eudiometer."
COMPOSITION OF THE ATMOSPHERE
529
.
1781," he writes, "I tried the air of near sixty different days
in order to find whether it was more phlogisticated at one
time than another, but found no difference that I could be sure
of, though the wind and weather on these days was very
various, some of them being very fair and clear, others very
wet, and others very foggy."
"" 1 This result was founded on a
long series of experiments, for seven or eight analyses of air
collected on the same day were made by different processes, so
that altogether Cavendish cannot have made fewer than 400
determinations of the composition of atmospheric air. Ex-
periments were likewise made to see whether London air
differed from that of the country, and slight differences were
sometimes found in favour of London air, in Marlborough
Street, sometimes in favour of country air, in Kensington. On
taking a mean of the numbers no difference whatever was
perceptible, the result of all his experiments being that 100
volumes of air contain 20-83 parts by volume of dephlogisticated
air or oxygen.
The constant results thus obtained by Cavendish led several
chemists, such as Prout, Döbereiner, and Thomson, to maintain
that the air is a chemical compound of one volume of oxygen
with four volumes of nitrogen. Against this assumption John
Dalton protested, insisting that the air is merely a mechanical
mixture of constant composition, and contending that, because
nitrogen is lighter than oxygen, the relative amounts of the two
gases must vary at different heights above the earth's surface,
the oxygen diminishing and the nitrogen increasing as we
ascend. This view was, however, shown to be erroneous by
Gay-Lussac and Thénard, who collected air in a balloon at an
elevation of 7,000 metres, and found it to contain exactly the
same proportional quantity of oxygen as that collected at the
same time in Paris and analysed in the same way. Their
results have since been corroborated by the more exact in-
vestigations of Brunner, who analysed the air collected at the
top and at the bottom of the Faulhorn and found in each case
exactly the same proportion between the oxygen and the
nitrogen.
The very important question whether the composition of the
air undergoes variation, under varying conditions of time and
place and what is the percentage of oxygen it contains was
1 Phil. Trans. 1783, p. 126.
2 Manchester Memoirs, 2nd series, vol i. p. 244.
35
530
THE NON-METALLIC ELEMENTS
further investigated by many chemists. Gay-Lussac and
Humboldt in Paris found that the air contained from 20.9 to
21.1 per cent. of oxygen; Davy in London obtained from 20.8
to 211 per cent.; Thomson in Glasgow 211, and Kuppfer in
Kasan 211 per cent. of oxygen. It was, however, necessary
that more accurate methods of analysis should be employed.
300 Two processes are now used, viz. (1) measurement of
the volumes of the component gases, (2) determination of their
weight. The first of these processes, or the eudiometric method,
has been practised by Regnault, Bunsen, Lewy, and Angus
Smith, whilst the latter method has chiefly been used by
Dumas and Boussingault in their celebrated research carried
out in the year 18411 In their analyses of air by the latter

FIG. 154.
method, the two French chemists employed an apparatus
shown in Fig. 154. The large balloon (v) was rendered as
perfectly vacuous as possible, and brought in connection with
the vacuous tube (a b), containing metallic copper reduced
by means of hydrogen, and placed in a furnace in which it
could be heated to redness by means of charcoal or gas. At
the other end this tube was connected with the tubes c and
B, and with the bulbs A; these last contained caustic potash, and
the others pumice-stone moistened with strong sulphuric acid
for the purpose of taking up all moisture from the air, as well
as of abstracting from it the whole of its ammonia and carbon
dioxide. As soon as the tube a b had been heated to dull red-
1 Ann. Chim. Phys. [3], 3, 257.
COMPOSITION OF THE ATMOSPHERE
531
ness the stopcocks r were opened so as to let the air pass slowly
into the apparatus. Entering the tube a b, and coming in
contact with the glowing copper, the whole of the oxygen was
absorbed, only the nitrogen going over into the vacuous globe.
When the experiment was complete, the stopcocks were closed,
the tube and balloon detached from the apparatus and each
accurately weighed. Both tube and balloon were then again
rendered vacuous, and weighed a third time. The tube contain-
ing the copper oxide, which had been weighed empty to begin
with, is thus weighed full of nitrogen, after which the nitrogen
is withdrawn by the air-pump and its weight again determined.
The following details of an actual experiment may illustrate the
method:-
---
Vacuous tube containing copper before the
experiment.
Tube filled with nitrogen and copper after the
experiment.
Vacuous tube after experiment
Balloon containing nitrogen at 19° and under
pressure 762.7 mm.
:}
Balloon, vacuous, at 19°4 and 7627 mm.
grams.
Or the percentage composition is :—
93
Hence the weight of oxygen is found to be 3680 grams, whilst
the weight of nitrogen in the balloon was 12:304 grams, and the
weight of nitrogen in the tube 0069, giving a total of 12:373
1841.
April 27.
28.
29.
""
Oxygen.
Nitrogen
•
•
Dumas and Boussingault used two balloons, with which they
obtained the following results :-
Mean.
22.92
77.08
100.00
Grams.
647.666
651-415
651.346
1403.838
1391.554
balloon.
22.92
23.03
23.03
22.993
Percentage by weight of oxygen.
With small
With large
balloon.
22.92
23.09
23.04
23.016
532
THE NON-METALLIC ELEMENTS
The mean of these determinations is 23-005 parts by weight of
oxygen and 76.995 of nitrogen.
These experiments were subsequently repeated by other
chemists, with the following results:-
Lewy, in Copenhagen, 1841
Stas, in Brussels, 1842 .
Marignac, in Geneva, 1842
According to these experiments, therefore, the air contains
about 23 parts by weight of oxygen and 77 parts by weight of
nitrogen.
If the composition of air by weight be calculated from its
volumetric composition by the aid of the known specific
gravities of nitrogen and oxygen, the numbers obtained are:-
Oxygen.
Nitrogen
Mean Percentage by
weight of oxygen.
22.998
23.100
22.990
23.2
76.8
100.00
and these numbers have also been obtained experimentally by
a gravimetric method by Leduc.¹
301 This method, although capable of giving exact results,
requires large apparatus and good air-pumps and balances. It can
only be carried on in a laboratory, and it necessitates the employ-
ment of large volumes of air. The eudiometric, or volumetric
method is less difficult and tedious, so that the determinations
may be repeated by thousands and require a much smaller volume
of air. This method is liable to so small an experimental
error that, with an observer well skilled in its use, it never
reaches the Too part, and may sometimes not exceed the
100000 part of the whole.
The process depends on the well-known fact that when
oxygen and hydrogen gases are mixed and fired by an electric
spark, they unite to form water in the proportion of one
volume of the former to two volumes of the latter. The volume
of the liquid water formed (p. 249) is so small in proportion
to that of the constituent gases that, except in cases of very
exact estimations, it may be altogether neglected. Hence if
¹ Compt. Rend. 113, 129.
COMPOSITION OF THE ATMOSPHERE
533
we bring together a given volume of air and more hydrogen
than is needed to combine with the oxygen in it, and if an
electric spark be passed through the mixture, one-third of the
observed contraction will be due to the oxygen. In order to
obtain by this method exact results a very carefully calibrated
eudiometer is employed 1 meter long and about 0025 wide,
and the observations are conducted in a space within which
the changes of temperature are as small and as gradual as
possible.
The air for these determinations is collected either in small
flasks of about half a liter in capacity, the necks of which have
been previously elongated before the blowpipe, or in long tubes,
the ends of which have been drawn out. Inside the flask or
tube a small piece of fused chloride of calcium is placed for
the purpose of absorbing the ammonia, and a similar piece of
fused caustic potash to absorb the carbonic acid, and both
substances are allowed to crystallize on the sides of the glass
by the addition of a drop of water. It is quite necessary to
remove the carbonic acid of the air previous to analysis. Even
if the quantity present were only 0.05 per cent. of the total
volume, it would produce an appreciable error in the oxygen
determination, carbonic acid when exploded with an excess of
hydrogen in presence of the detonating mixture of oxygen
and hydrogen being decomposed into an equal volume of
carbonic oxide, while an equal volume of hydrogen disappears,
so that the volume of combined gas would be 0.05 per cent.
too large.
In carrying out this method with exactitude the same mani-
pulatory precautions have to be attended to which have already
been described in connection with the determination of the
composition of water (p. 246).
The following numbers show the approximation obtained.
by Bunsen in two analyses of the same sample of air collected
in Marburg on 9th January, 1846:-
Volume.
•
Pressure
at 0°.
Temp.
C.
Vol. at 0°
and 1 m.
pressure.
428.93
. 841.8
0.3
Air employed
0.5101
After addition of hydrogen 1051.7 0.7137 0.3
749.77.
After the explosion
878.8 0.5460 0.3 480.09
1 For details on these points Bunsen's Gasometry may be consulted.
2 Bunsen's Gasometry, 71.
534
THE NON-METALLIC ELEMENTS
1
Composition of Air in 100 parts by volume.
. 79.030
. 20.970
100.000
Air Employed
Air employed.
. 859.3
After addition of hydrogen 1051.9
After the explosion . . 870·3
After addition
of Hydrogen.
Nitrogen
Oxygen
After the Ex-
plosion.
39
Composition of Air in 100 parts by volume.
. 79.037
. 20.963
100.000
Nitrogen.
Oxygen
6h O'
7
8 0
Bunsen, who has brought all the processes of gas analysis to
a marvellous degree of perfection, points out¹ that in normal
determinations of the composition of the air still greater
precision may be attained by repeating, several times, and at
regular intervals, the observation of the height of the mercury
in the eudiometer. From the agreement between the reduced
volumes which are read off, the point in the series of observa-
tions is found at which the temperature has been most constant.
As an example of such an accurate analysis Bunsen gives the
following:-
11
•
•
12 0
000
1
3 0
0
5 0
•
·
Vol.
Hence Nitrogen
Oxygen
0 5045
754.9
755.0 0.5046
755.2 0.5047
904.0 0.6520
904-6
904.9
Press.
•
·
•
0.6521
06518
0.5225
0.6
0.6
0.7079
0.5317 0.6
732.3
0.4781
16.1
732.5 0.4777
16.1
732.7 0.4777 16.1
Temp.
1 Loc. cit. 77.
15.4
15:4
15.5
15.8
16.0
16 0
. 20.964
100.000
Vol. at 0° C. and 1 meter
pressure.
448.00
743:01
461.72
360.52
360 63 360-62
360.70
79.036 volumes.
557.20
}
557 24 557-20
557-17
330.64
330-45 330.54
330-54
39
"
COMPOSITION OF THE ATMOSPHERE
535
As the result of twenty-eight analyses Bunsen found the
average percentage of oxygen to be 20-924 volumes, whilst the
lowest percentage found was 20-840.
Regnault, using a different volumetric method, has
analysed a very large number of samples of air collected in a
uniform manner in various quarters of the globe, according to
instructions which he had given. The error of two analyses
made on the same sample rarely reached 0.02 per cent. In
more than 100 samples of air collected in or near Paris, Reg-
nault found a maximum amount of 20.999, a minimum of
20-913, and a mean of 20.96 percentage of oxygen. The differ-
ence of 0.086 per cent. is too large, according to Regnault, to be
due to errors of experiment, and it must, therefore, be ascribed
to variations in the composition of the air occurring from day
to day.
Air collected from other localities gave Regnault the following
results-
9 samples from Lyons gave .
30 samples from Berlin
10
Madrid
23
17
5
2
2
2
99
"3
""
""
"9
"2
:)
""
""
""
""
""
""
در
•
·
Geneva and Chamounix
Toulon Roads and Mediter-
ranean
Atlantic Ocean
Ecuador
Summit of Pichincha
Antarctic Seas . .
·
•
•
•
Percentage of oxygen.
20.918 to 20 966
20.908 20.998
20.916 20.982
20.909 20.993
""
33
"9
}
. 20.918 20.965
. 20.96
. 20.949 20.988
20.86
20.94
20.912 20.982
99
""
""
""
The conclusion which Regnault draws from these determi-
nations, and which all subsequent observers have confirmed,2
is, that the atmosphere shows perceptible, though very small,
alterations in the amount of oxygen at different times and
in different localities. This variation ranges from 209 to 210
per cent, but from special unknown causes the amount of oxygen
seems sometimes to sink in tropical countries as low as 204
per cent., as was seen in the Bay of Bengal on March 8, 1849.
302 Dr. R. Angus Smith has extended our knowledge
of the variations which the percentage of oxygen undergoes
in the air of towns and that of closed inhabited spaces.
He
1 Ann. Chem. Phys. [3], 36, 385.
Morley, Amer. Journ. of Science, 22, 429; Chem. News, 45, 283; Hempel,
Ber. 18, 267, 1800; 20, 1864.
3 On Air and Rain, Longmans, 1872.
536
THE NON-METALLIC ELEMENTS
finds that the percentage of oxygen in air from the sea shore,
and from Scotch moors and mountains, is as high as 20-999.
In the free air of towns, and especially during foggy weather, it
may sink to 20-82. In inhabited rooms and crowded theatres,
the percentage of oxygen may sink sometimes to 20-28 (Lewy),
whilst according to the very numerous (339) analyses of Angus
Smith,¹ the percentage of oxygen in mines does not average
more than 20-26. The exact composition of the air at high
elevations has been investigated by Frankland,2 who has shown
that, as far as nitrogen and oxygen are concerned, the composi-
tion of the air up to an elevation of 14,000 feet is constant,
and that the variations exhibited in air collected at the above
height fall within the limits noticed by former experimenters.
303 That the air is a mechanical mixture and not a chemical
combination of oxygen and nitrogen is seen from the following
facts:-
(1) The quantities of nitrogen and oxygen in the air do not
present any simple relation to the atomic weights of these
elements, and, indeed, the proportions in which they are mixed
are variable.
(2) On mixing oxygen and nitrogen gases mechanically in
the proportion in which they occur in air, no contraction or
evolution of heat is observed, and the mixture behaves in every
way like air.
(3) When air is dissolved in water, the proportion between
the oxygen and nitrogen in the dissolved air is quite different
from that of the undissolved air, the difference being in strict
accordance with the laws of gas-absorption on the assump-
tion that the air is a mixture. When water is saturated with
air at any temperature below 30°, the following is the proportion
of oxygen and nitrogen contained in the dissolved and the
original air :-
Oxygen
Nitrogen
.
Air dissolved
in water.
. 35.1.
64.9.
•
•
.
100.0
•
·
Air undissolved
in water.
. 20.96
79.04
100.00
1 On Air and Rain, p. 106.
2 Journ. Chem. Soc. 13, 22.
·
If the air were a chemical combination of oxygen and
nitrogen, such a separation by solution would be impossible.
COMPOSITION OF THE ATMOSPHERE
537
(4) When liquefied air is allowed to boil, the nitrogen passes
off much more rapidly than the oxygen, which could not take
place if the two gases were chemically combined.
In order to show the composition of the air, the apparatus
Fig. 155 is often used. This consists of a calibrated and divided
glass tube filled to a given point with air over mercury. Into
this is introduced a small piece of phosphorus supported upon
a copper wire. Gradually all the oxygen is absorbed and the
mercury rises in the tube. After a while the volume of residual
nitrogen is read off, and, corrections having been made for tem-

FIG. 155.
FIG. 156.
perature and pressure, it is found that 100 volumes of the air
contain about 21 volumes of oxygen.
Another less exact but more rapid method of exhibiting the
same fact is carried out by help of the arrangement shown in
Fig. 156. In the beaker glass (c) is placed the iron stand (d)
carrying the iron cup (e), containing a small piece of phosphorus.
Over this stand is placed the tubulated cylinder (a). The upper
part of this cylinder is graduated into five equal divisions,
and water is poured into the beaker glass until the level reaches
the first division. The phosphorus in the cup is ignited by
dropping down on it a chain which has been heated in a
538
THE NON-METALLIC ELEMENTS
flame. The phosphorus then burns, the fumes of phosphorus
pentoxide are absorbed by the water, and, when the gas has
cooled, and the pressure been equalized by bringing the level of
the water outside up to that inside the cylinder, four-fifths of
the original volume of the air remain unabsorbed.
SUBSTANCES PRESENT IN SMALLER QUANTITY IN THE
ATMOSPHERE.
The carbonic acid, aqueous vapour, organic matter, and the other
constituents of the atmosphere vary in amount in different places
and at different times much more than the oxygen and nitrogen.
304 Atmospheric Carbonic Acid.—Reference has already been
made to the part played by atmospheric carbon dioxide (carbonic
acid). The whole vegetable world depends for its existence on
the presence of this gas, which serves, in the sunlight, as the
chief food of plants.
The normal amount of carbonic acid existing in the air was
formerly supposed to be 4 vols. in 10,000, but the investigations
of the last 25 years have shown that the methods formerly in
use gave too high results and that the amount is nearly 3 vols.
in 10,000.¹ This number is the average of a large number of
analyses of country air, but it appears that the amount is slightly
greater in the night than in the day, probably owing to the in-
fluence of vegetation; over the sea the amount is also 3 vols. in
10,000, but no difference is observed between the day and night
values. In dull cloudy weather the amount is somewhat larger
than in bright fine weather, and variations are also observed
according to the direction of the wind, which when it has come
over a large extent of land causes an increase in the quantity
of carbonic acid.
In large towns where much coal is used the amount of car-
bonic acid may rise as high at 60 and even 70 vols. per 10,000,
and at high elevations the proportion of carbonic acid is
generally, but not invariably greater than at lower levels. In
closed inhabited spaces the volume of carbonic acid proceeding
from respiration and from the combustion of illuminating
materials is usually much higher than in the open air,³ and the
proportion is much higher in mines than in the air above ground.
1 For a full discussion of the results of the various investigators see Blochmann,
Annalen, 237, 39.
2 Thorpe, Journ. Chem. Soc. 1867, 189.
3 Roscoe, Journ. Chem. Soc. 1867, 189.
ATMOSPHERIC CARBONIC ACID
539
Air containing more than 70 vols. of carbonic acid per
10,000 is as a rule harmful for respiration; this is, however,
probably not due to the carbonic acid itself, but to the other
organic impurities which are formed along with it especially
during respiration, for the deleterious properties of such air
remain after the removal of the former gas.
As the quantity of carbonic acid serves as the readiest and
most reliable test of the healthiness or otherwise of an atmo-
sphere, it becomes a matter of importance to ascertain its
amount with accuracy.
The methods in use for this purpose are
(1) the gravimetric and (2) the volumetric method. In the first

FE
រៀល ឬ
D
C B
FIG. 157.
U
1
of these the carbonic acid is absorbed from a known volume.
of air, freed from ammonia and aqueous vapour, drawn through
weighed tubes containing caustic potash. This necessitates the
passage of not less than forty litres of air drawn by means of
the aspirator (v, Fig. 157), filled with water, through the tubes, in
order that a sufficient weight of carbonic acid for an exact
weighing may be obtained. The tubes A and B are filled with
pumice-stone moistened with sulphuric acid. The air is thus
dried and freed from ammonia. C and D contain moist but solid
caustic potash for the absorption of the carbon-dioxide. E and
F are also drying-tubes, prepared like the tubes A and B, and
1 Saussure, Pogg. Ann. 19, 391.
540
THE NON-METALLIC ELEMENTS
serving to hold back any moisture which the dry gas might have
taken up from the tubes C and D. The following example of a
determination made by this plan illustrates the process :-
Gravimetric Determination of Carbonic Acid in London Air,
Feb. 27, 1857.
Volume of aspirated air at 8° and under
772.5 mm. of mercury
}
Weight of absorbed carbonic acid.
•
=43.2 litres.
=0.0308 grm.
Hence 10,000 volumes of air contain 37 volumes of carbonic
acid.
305 A much more convenient process is the volumetric one
proposed by Pettenkofer. For this a volume of about 10 litres
of air only is needed, a glass cylinder closed by a caoutchouc cap
being employed, and no balance being required. The method
depends upon the fact that a solution of hydrate of baryta of
known strength when shaken up with a closed volume of air
containing carbonic acid, abstracts the whole of that carbonic
acid from the air with formation of insoluble carbonate of
barium; thus:
Ba(OH)2 + CO₂ = BaCO3 + H₂O.
The quantity of baryta in excess which remains in solution, after
shaking up with the air, is ascertained by adding to an aliquot
portion of the milky fluid a standard solution of oxalic acid or
sulphuric acid until the alkaline character of the baryta water
disappears. When this point is reached, the whole of the
residual soluble baryta has been neutralised by the oxalic acid.
The baryta and oxalic acid solutions are made of such a strength
that equal volumes exactly neutralise each other, and so that
1 cc. of baryta solution will precipitate exactly 1 mgrm. of car-
bonic acid. The following example will serve to explain this
process:-
Volumetric Determination of Carbonic Acid in Manchester Air,
Nov. 10, 1873.
Volume of air employed, 10-80 litres at 7° and under a pressure
of 765 mm. of mercury.
50 cc. of standard baryta solution (1 cc.1 mgm. CO) was
1 Journ. Chem. Soc. 1858, 292.
ATMOSPHERIC MOISTURE
541
shaken up with this air. Of this, after the experiment, 25 cc.
needed 22 cc. of standard oxalic acid (1 cc. = 1 ingm. CO₂) for
complete neutralization. Hence 6 cc. of baryta solution were
neutralized by the carbonic acid in 108 litres of air; or 10,000
volumes of air contain 2·85 volumes of carbonic acid.
To obtain very accurate results by Pettenkofer's method,
which is in any case more reliable than the gravimetric method,
a number of precautions must be taken. For these reference
may be made to Blochmann's paper.¹
306 The moisture contained in the air is liable to much more
extensive changes than even the carbonic acid. Amongst the
circumstances which affect the atmospheric moisture, distance
from masses of water, and the configuration of the land, seem
the most important. A given volume of air cannot take up
more than a certain quantity of aqueous vapour at a given
temperature, and then the air is said to be saturated with
moisture. The weight in grams of water capable of being taken
up by 1 cubic meter of air, at different temperatures, is given in
the following table :—
Co. Grams.
2.284
-10°
0°
4.871
5° 6.795
10° 9-362
C°.
Grams.
15° 12.746
20° 17.157
25° 22.843
30° 30-095
C. Grams.
35°
39.252
40°
50-700
100° 588.73
804-75 × 273 × 9·163
283 X 760
This quantity is wholly dependent on the temperature of the air,
being represented by the tension of the vapour of water at that
temperature. Thus the weight of aqueous vapour which can be
taken up by 1 cubic metre of air at 10°, at which temperature
the tension of its vapour is 9.163 mm., is obtained as follows:-
1 cubic metre of aqueous vapour at 0° and 760 mm. weighs
804-75 grams. Hence one cubic metre of air needs for satura-
tion the following quantity of aqueous vapour :—
= 9.362 grm.
When the temperature of saturated air is lowered, the aqueous
vapour is precipitated in the form of rain, snow, or hail, accord-
ing to the temperature of the air during, or after, the deposition.
If one cubic mile of air saturated with water at 35° be cooled to
1 Loc. cit.
542
THE NON-METALLIC ELEMENTS
0°, it will deposit upwards of 140,000 tons of water as rain, for
one cubic metre of air at 35° is saturated when it contains 39.25
grams of aqueous vapour, whereas as 0° it can hold only 487
grams in the state of vapour. It seldom happens that the air is
completely saturated with moisture, and as seldom that the
amount of moisture sinks below of the saturating quantity.
Even over the sea the air is never completely saturated with
moisture. Large amounts of watery vapour are, however, driven
by the winds into the interior of the continents, and more in
summer than in winter, when the land is colder. than the sea,
and when, therefore, the aqueous vapour is more easily con-
densed, and does not so readily penetrate for great distances.
The following mean tensions of aqueous vapour in the air at
different places exhibit this fact clearly:-
London
Utrecht
Halle
Berlin.
Warsaw
•
St. Petersburg
Kasan
Barnaul
Urtschusk
.
Mean Tension of Aqueous Vapour in mm.
Mean yearly. January.
July.
5.5
4.8
4.5
4.3
3.4
2.7
1.5
14
0.4
8.6
7.6
7.5
7.3
6.9
5.7
5:0
4.8
4:0
•
.
·
•
12.2
10.2
11.6
11.1
11.7
10.5
9.8
11.3
11.3
In order to determine the amount of moisture in the air, we
may employ either a chemical or a physical method. According
to the first, a known volume of air is drawn through weighed
tubes containing hygroscopic substances, the increase in weight of
these tubes giving the weight of the aqueous vapour. Thus
43.2 litres of London air at 8° and 7725 mm., when passed
through drying tubes, deposited 0·241 grm. of water.
In the second method Hygrometers are employed; of these
Regnault's dew-point hygrometer is the best.¹ For the physical
determinations of atmospheric moisture, works on Hygrometry
must be consulted.
307 Ammonia is another important constituent of the air,
originating in the decomposition of nitrogenous organic matter.
The relative proportion in which this substance is contained in
1 Ann. Chem. Phys. [3] 15, 129.
ATMOSPHERIC AMMONIA
543
I
the atmosphere is extremely small, and probably very varying,
inasmuch as it is not present as free ammonia, but combined
with atmospheric carbon dioxide and other acids, and these
ammoniacal salts are washed down by the rain or absorbed
by the earth. This constituent plays an important part in vege-
tation, for it is from it that unmanured crops derive the chief
portion of the nitrogen which they require for the formation of
seed and other portions of their structure.
According to the experiments of Goppelsröder,¹ the rain water
collected at Basel contained the following amounts of ammonia in
parts per million :-
1871.
January.
February
March
April.
May.
June.
·
Minimum.
4.6
3.2
3.8
3.2
3.2
3.2
.
·
·
•
.
.
Small quantities of nitrous and nitric acids are also found in
the atmosphere, in combination with ammonia; their formation
is probably due partly at any rate to the passage of electric
discharges through the air, as the quantity of these compounds
found in rain water falling during thunderstorms is greater than
that occurring in ordinary rain. Ozone also is supposed to occur
frequently in small quantity, but as already stated, this cannot
yet be regarded as definitely proved (p. 240).
The following observations have been made by Bechi 2 on the
amount of ammonia and nitric acid contained in the rain water
falling in Florence and at Vallambrosa in the Apennines 957
metres above the sea-level for one square hectometer of surface.
Florence.
Vallambrosa.
Maximum.
7.8
6.5
. 18.2
6.8
14.8
9.1
Rain in cubic metres
Ammonia in grams 13236 10572 12917
Nitric acid in grams.
1870. 1871. 1872.
9284 10789 12909
1 J. Pr. Chem. [2] 4, 139 and 383.
3 Journ. Chem. Soc. 1892, ii. 234.
20278
10433
15728 9153 13057 . 11726
·
•
Tuxen has also published tables showing the amount of nitric
acid and ammonia in the rain falling in Denmark during the
years 1880 to 1885.3
2 Ber. 8, 1203.
544
THE NON-METALLIC ELEMENTS
308 Atmospheric Organic Matter.-The atmosphere also, of
course, contains gases arising from the putrefactive decomposition
of organic substances. These gases do not remain in the air
any length of time, but undergo pretty rapid oxidation. The
particles of dust which we see dancing in the air as motes in the
sunbeam are partly organic and partly inorganic. Bechi found
that a thousand litres of rain water which fell in November
1870 in a garden in Florence contained 4123 grams of total
solid residue, of which one-half consisted of organic bodies and
ammoniacal salts and one quarter of gypsum and common salt.
Amongst the organic substances the germs of plants and animals
always occur, as has been proved by the classical labours of
Pasteur. These bodies are the propagators of fermentation and
putrefaction, and air which has been freed from these particles
either by filtration through asbestos or cotton-wool, or by igni-
tion (Pasteur), or by subsidence (Tyndall), may be left in contact
for any length of time with liquids such as urine, milk, or the
juice of meat, without these organic liquids undergoing the
slightest change. Air which has thus been filtered is termed by
Tyndall optically pure. When a ray of light is allowed to
pass through air thus freed from solid particles no reflection is
noticed, and the space appears perfectly empty, the motes which
in ordinary air reflect the light being absent. (See also p. 546.)
The organic nitrogen contained in the air, probably chiefly
contained in such germinal bodies, has been quantitatively
determined by Angus Smith in the form of ammonia.
obtained the following results :-
Innellan (Frith of Clyde)
London
Glasgow.
Manchester.
Near a midden
·
He
1 kilogram of air contains of organic
nitrogen calculated as ammonia
Grams.
. 0·11
0.12
0.24
0.20
0.31
The volatile organic products arising from putrefaction, which
are always present in the air, appear to exist in larger quantities
in marshy districts than elsewhere, and in all probability they are
the cause of the unhealthiness of such situations. The unpleasant.
odour invariably noticed on entering from the fresh air into a
¹ Ann Chim. Phys. [3] 64, 5.
2 Air and Rain, 438.
VENTILATION
545
closed inhabited space is also due to the presence of the same
organic putrescent bodies, whilst the oppressive feelings which
frequently accompany a continued habitation of such spaces do
not proceed from a diminished supply of oxygen, or an increase
in the atmospheric carbonic acid, but are to be ascribed to the
influence of these organic emanations.
309 Hence the subject of ventilation is one of the greatest
consequence to well-being as well as to comfort, and it is neces-
sary to provide for a continual renewal of the deteriorated air.
Fortunately this renewal takes place to a considerable extent
in a room, even when doors and windows are shut, by what may
be called the natural means of ventilation, by the chimney, by
cracks and crevices in doors and windows, and especially through
the walls. Almost instinctively man appears to have chosen

B
b
FIG. 158.
€
porous building materials, thus permitting by gaseous diffusion
an exchange of fresh for deteriorated air. The well-known
unhealthiness of new and damp houses, as well as of those
built of iron, is to a great extent to be attributed to the fact
that the walls do not permit a free diffusion to go on.
The fact that gases readily pass through an ordinary dry
brick- or sandstone-wall, is clearly shown by the following
experiment proposed by Pettenkofer. (A) Fig. 158 is a piece of
wall built of ordinary brick or sandstone 82 centimetres in
height, 40 cm. broad, and 13 cm. thick. On each side of
the wall two rectangular plates of iron (C) are fixed, and
the whole of the outside of the wall is then covered over with
a coating of tar, and thus made air-tight. A tube (cc)
is soldered into a hole in the centre of each iron plate. If
36
546
THE NON-METALLIC ELEMENTS
a candle-flame be held in front of the opening of the tube
at one side of the wall, and a puff of air be blown from the
lungs through the open end of the tube at the other side
of the wall, the candle will at once be blown out; whilst if
the one tube be connected by a caoutchouc tube to a gas jet,
and the coal gas be allowed to pass through the tube, a flame
of gas can, in a few seconds, be lighted at the open end of the
opposite tube. If the bricks or stones of the experimental
wall be well wetted, it will be found very difficult to blow out
the candle as described.
BACTERIOLOGY OF AIR.
310 The air always contains, as has been stated, a certain
number of micro-organisms. These belong to the groups of
moulds, yeasts, and bacteria. One litre of air contains on an
average from four to five microbes. A pure unfiltered river
water, on the other hand, contains from 6,000 to 20,000 in one
cc., and a fresh undisturbed soil about 100,000 microbes in
one cc. The atmosphere is therefore relatively poor in micro-
organisms. Their great storehouse in Nature is the soil, and those
present in the air are almost entirely derived from this source.
The majority of the air microbes consist of the spores of
moulds and yeasts. The greater number of the bacteria found
belong to the group of the micrococci, and many of these are
characterised by the production of pigment when grown on
culture media. Sarcina forms are constantly met with. The
bacteria, whether bacilli or cocci, are almost entirely saprophytic
organisms ie. organisms which are not disease-producing. It is
only occasionally that pathogenic or disease producing forms are
found, as for example the pus cocci. The spores of moulds
are so light that the isolated cells can float freely in the
atmosphere. The conditions are otherwise with regard to
bacteria. These are not found isolated in the air, but aggre-
gated in small groups and adhering to particles of dust. Dust
is the vehicle by which they are transmitted to the air, and the
bacteria therefore belong to the more ponderable elements of
the air-dust. Thus for instance, if the dust in a room be stirred
up, large numbers of bacteria will be found in the air; but after
the dust has once again settled to the ground, the bacteria.
1 Flügge, Grundriss der Hygiene. Leipzig. 1889.
BACTERIOLOGY OF THE AIR
547
disappear in great part, leaving the lighter free spores of the
moulds in the air.
Bacteria do not pass into the air from a moist surface or from
the surface of water. Indeed the air of sewers is found to
contain fewer organisms than the outside air, the damp walls
and the sewage retaining the organisms which are not carried
about by wind. It is only when the surface containing them
becomes dry that the wind is able to carry up dust and with
it bacteria. The factors favouring the distribution of bacteria in
the air are dryness of the soil and wind currents; the factors
hindering their presence are moistness of the soil and a still
atmosphere. The number of microbes present in the air will,
accordingly, vary with the atmospheric conditions. The air
contains few microbes after a prolonged fall of rain, and many
after prolonged heat and dry weather. The spores of moulds,
however, form an exception to this rule, as they are most
abundant in the air during damp weather, moisture favouring
their production. The less dust there is in the air, the less
the number of bacteria, and vice versa. The air over the open
sea and on high mountains contains few or no bacteria; the air
of a town more than the air of the country. Drying being
essential to the passage of microbes into the air, the forms most
largely present will be those least affected by dryness, e.g. moulds
and their spores. In the open air from 10 to 20 times as many
moulds are found as bacteria. On the other hand drying is
fatal to a large number of bacteria, and especially to the
pathogenic organisms, e.g. Koch's comma bacillus, glanders
bacillus, &c. It would therefore seem that the dangers of air
infection have been over-estimated. The air of enclosed
spaces and dwelling-houses is richer in microbes than the open
air.
It thus appears that the most dangerous factor, hygienically,
is not the air itself nor the gases and sewage emanations that
may be present in it, but the dust to which bacteria cling.
The dust of dwellings may, therefore, become an important
factor in carrying infectious microbes, more especially by
fragments of clothing, linen, &c., from sick people and their
attendants. Cornet found that the dust of dwellings contains
the tubercle bacillus, and that such dust can produce an infec-
tion with tubercle.2 The dust in the air of dwellings and
¹ Petri, Zeitschrift für Hygiene, 1887.
2 Zeitschrift für Hygiene, Bd. v. 1888.
548
THE NON-METALLIC ELEMENTS
hospitals is infectious for animals, and may produce tubercle,
malignant œdema, tetanus, or septic peritonitis.
311 The great majority of air microbes are however sapro-
phytic, and amongst them always occur the organisms to which
fermentation and putrefaction are invariably due (Pasteur,
Tyndall.) A knowledge of their nature and action is important
for industries in which fermentation is an essential feature, as
the presence of strange forms may be injurious to the processes
involved. The number of these special forms present in the air
also varies according to the locality, weather, and season of the
year. There are fewer in the open air than in dwellings, in
cold weather than in hot weather, and in winter than in summer.
Pasteur, and later Hansen, found a relatively small number of
saccharomyces (yeast-plant) in the open air. The germs of the
alcoholic fermentation are however already present on the
surface of fruits, &c., and do not require to be introduced
from the air.¹
The methods of examining the air for microbes fall into two
groups
I. The methods of Miquel and their modifications. The air
is filtered through glass-wool or soluble filters, and the filters
are then distributed in flasks of nutrient bouillon.
:-
II. The methods in which Koch's nutrient gelatin is used
directly or indirectly. The air may be aspirated over nutrient
gelatin in Hesse's tubes, or through soluble or insoluble filters
(sugar, sand, &c.). In the latter case the filters and the microbes
are distributed on gelatin culture plates. One of the best me-
thods is that devised by Petri, in which the air is aspirated
through previously sterilised sand filters.
By means of the above methods a qualitative and quantita-
tive examination of the air microbes can be carried out. For
other points concerning the hygienic relations of the air,
Dr. Renk's book should be consulted ("Die Luft," Handbuch
Hansen's important investigations, and the zymotechnical methods employed
by him are fully described in Jörgensen's book, Die Mikroorganismen der
Gährungsindustrie (Berlin, 1893). The book also contains references to all the
most important papers. The micro-organisms of the air have been specially
studied by Miquel, and a full account of his results is contained in the issues of
the Annuaires de Montsouris (1879, 1884, 1886, &c.).
2 Welz, "Bacteriologische Unterschung der Luft," Zeitschrift für Hygiene,
Bd. xi. 1891. Also Uffelman, "Luft Untersuchungen," Archiv für Hygiene,
Bd. viii.
3 Mittheil. d. k. Gesundheitsamt. 1884.
4 Zeitschrift für Hygiene, Bd. iii. 1887.
PHOSPHORUS
549
d. Hygiene, Leipzig, 1886), and also the monograph on “Air"
in A Treatise on Hygiene and Public Health, vol. i. (London:
J. & A. Churchill, 1893).
PHOSPHORUS, P = 30.8.
312 A considerable amount of uncertainty surrounds the dis-
covery of phosphorus, inasmuch as several chemists have claimed
the first preparation of this body, while each has contradicted the
other in a variety of ways. It seems, however, tolerably certain
that phosphorus was first prepared (1674) by the alchemist Brand,
of Hamburg, who obtained it from urine in a process which had
been previously made use of for the purpose of preparing a
liquid supposed to have the power of turning silver into gold.
By a secret process, Brand succeeded in preparing phosphorus
from this liquid, and he is said to have sold the secret of the
manufacture to Krafft, from whom it appears that Kunkel
learnt what he knew, and published in the year 1678 a pam-
phlet on this remarkable product.¹
In these early days phosphorus was a very costly body, being
valued as one of the most remarkable and interesting of
chemical substances. Krafft exhibited it as one of the wonders
of nature to various crowned heads, amongst others, in the year
1677, to King Charles II. of England. Robert Boyle became
acquainted with its existence without, as he tells us, having
been informed by Krafft of the mode of preparation except
so far as that it was obtained from an animal source, and
he succeeded in the year 1680 in the preparation of phosphorus,
as Kunkel and Brand had done before him, by strongly heating
a mixture of evaporated syrupy urine and white sand in an
earthenware retort. The difficulty of thus preparing phosphorus
was considerable, and so many chemists failed in the attempt
that the price, as late as the year 1730, was extremely high,
ranging from ten to sixteen ducats the ounce. Gahn, in 1769,
discovered the existence of calcium phosphate in bones, but it
was not until this fact was published by Scheele in 1771 that
1 "Oeffentliche Zuschrift vom Phosphor Mirabile und dessen leuchtenden
Wunderpilulen.”
-
2 Boyle, Phil. Trans., 1693 — “A Paper of the Hon. Robert Boyle, deposited
with the Secretary of the Royal Society on the 14th of October, 1680, and
opened since his death."
550
THE NON-METALLIC ELEMENTS
phosphorus was obtained from bone-ash, which has from that
time invariably served for its preparation.
The name phosphorus (pos, light, and, pépw I bear) was
originally used to designate any substance which was capable
of becoming luminous in the dark. The first chemical substance
in which this property was noticed was termed Bonnonian
phosphorus (see barium sulphide). In order to distinguish
true phosphorus from this body, the name of phosphorus
mirabilis, or phosphorus igneus, was given to it. In the
eighteenth century it was usually termed Brand's, Kunkel's,
or Boyle's phosphorus, or sometimes English phosphorus,
because it was then prepared in London by Hankwitz in
quantity according to Boyle's receipt.
Up to the time of Lavoisier, phosphorus was considered to be
a compound of phlogiston with a peculiar acid; but in 1772
the great French chemist showed that the acid body formed by
the combustion of phosphorus weighed more than the phosphorus
itself, the augmentation in weight being due to a combination
with a constituent of the air. In a memoir communicated to the
Academy in 1780, he represented phosphoric acid as a compound
of phosphorus and oxygen, and investigated its salts.
313 Phosphorus, being a very easily oxidizable body, does not
of course, exist in the free state in nature. It is, however, very
widely distributed, especially in combination with oxygen and
calcium as calcium phosphate. The most important minerals
containing phosphorus are, estramadurite or phosphorite
Ca,(PO4)2; sombrerite, an impure calcium phosphate; apatite
3Ca3(PO4)2 + CaCl2; wavellite 4A1(PO) + 2A1(OH)3 + 9H¸O ;
vivianite Fe(PO4)2 + 8H₂O. Calcium phosphate also forms
the chief constituent of coprolites, and occurs in small quantity
throughout the granitic and volcanic rocks, whence it passes
into the sedimentary strata, and thus finds its way into the soil.
The original observation of Gahn, that phosphorus forms an
essential constituent of the animal body, might, it may be
thought, have led to the conclusion that this element is
very widely distributed. It was, however, reserved for a later
time to show that almost all substances found on the earth's
crust contain phosphorus, that it is always present in sea-water,
and in all river as well as in almost every spring-water.
All fruitful soils contain phosphorus, and no plants will grow on
a soil destitute of it, as it is required to build up certain
1 Opuscules Physiques et Chimiques, 1774.
PREPARATION OF PHOSPHORUS
551
essential parts of the vegetable structure, especially the fruit
and seeds. From the plant the phosphorus passes into the
animal body, where it is found in the juices of the tissues, but
especially in the bones of vertebrate animals, the ashes of which
consist almost entirely of calcium phosphate. Phosphorus is
likewise connected with the higher vital functions of animals,
the substance of the brain, as well as nervous matter in general,
always containing it. When the animal tissues, whether
muscular or nervous, are worn out, they are replaced by fresh
material, and the phosphorus in them is excreted in the urine
chiefly as sodium ammonium phosphate or microcosmic salt
Na(NH)HPO + 4H₂O. Phosphorus is likewise found in small
quantity in meteoric stones, a fact which indicates its wide
cosmical distribution.
314 Preparation.—The preparation of phosphorus from bones
was first described by Scheele in 1775. He warmed bone-ash for
many days with dilute nitric acid, precipitated the lime with sul-
phuric acid, evaporated the liquid from which the gypsum had
separated out to a thick syrup, and distilled the residue with
charcoal. This process was afterwards simplified by Nicolas
and Pelletier,2 inasmuch as they treated the bone-ash at once
with sulphuric acid. The yield of phosphorus by this process
was, however, but small, until Foureroy and Vauquelin ³ deter-
mined the exact proportion of sulphuric acid required for the
complete decomposition of the bone-ash, thus preparing the way
for the economical production of the element.
3
In order to obtain calcium phosphate from bones, the bones
were formerly burnt in ovens. At present, however, the
organic material contained in them is made use of in several
ways. Thus the bones are either boiled with water, or treated
with superheated steam, to extract the gelatine which they
contain, or they are distilled in iron retorts to obtain the
ammonia and other volatile matters which they yield. In the
latter case bone-black or animal charcoal, which consists of
a mixture of charcoal and calcium phosphate, is left behind.
This bone-black is largely used by sugar refiners for clarifying
the syrup, and it is only when it has become useless for this
purpose that it is completely burnt in an open fire, and obtained
in the form of bone-ash. To prepare phosphorus from bone-ash
it is treated with sufficient dilute sulphuric acid to convert the
2 Journ. Phys. 11, 28.
1 Gazette Salutaire de Bouillon, 1775.
3 Journ. Pharm. 1, 9.
552
THE NON-METALLIC ELEMENTS
whole of the calcium present into sulphate,¹ which remains as
an insoluble precipitate, whilst the liberated phosphoric acid
dissolves in the water. The sulphate is separated by filtering
through large filter beds, washed with water till nearly free
from phosphoric acid, and the filtrate concentrated to a syrup.
The liquor is then mixed with about a quarter of its weight of
coke or charcoal, and carefully dried in cast-iron pots or a muffle

b
FIG. 159.
furnace, the orthophosphoric acid, H,PO, being thus converted
into metaphosphoric acid, HPO,. The dried mixture is then
heated to bright redness in earthenware retorts of the shape
shown in Fig. 159, when the following reaction takes place :-
4HPO, +12C = 2H, + 12CO + 4P.
The mixture of phosphorus vapour, hydrogen and carbonic
1 Readman, Journ. Soc. Chem. Ind. 9.
PREPARATION OF PHOSPHORUS
553
oxide passes through the bent earthenware pipe (a), which dips
under the surface of water contained in the vessel (b), whereby
the phosphorus vapour is condensed.
The
carbon
A large proportion of the phosphorus made in England is pre-
pared from sombrerite, an impure calcium phosphate found on
the Island of Sombrero in the West Indies. It appears that
almost the whole of the phosphorus made in the world is
manufactured in two works-namely, that of Messrs. Albright
and Wilson, at Oldbury, near Birmingham, and that of
MM. Cognet et Fils, in Lyons. The manufacture of phos-
phorus is somewhat dangerous, on account of the
inflammability of the product, and it is also difficult, inasmuch
as the distillation requires forty-eight hours for its completion,
and necessitates, during the whole of this time, constant
watching.
easy
out on
charcoal
Many other processes have been proposed for obtain-
ing phosphorus, but as a rule these have not been very
successful. Recently, however, a process, known as the Read-
man, Parker and Robinson system, has been successfully carried
the large scale, in which the phosphate is directly
converted into phosphorus without previous treatment with
sulphuric acid. In this process, the phosphate is mixed with
and suitable fluxes, and after heating to as high a
temperature as possible, is introduced into an electrical furnace,
consisting of an iron tank lined with refractory material,
and containing large carbon electrodes in the sides. Through
the latter a powerful electrical current is passed, and at the
high temperature thus attained phosphorus vapour mixed with
other gases distils over, and is condensed in the usual manner;
the residue forms a liquid slag which may be drawn off at
intervals, and fresh raw material introduced into the furnace,
thus rendering the process continuous.2
100
the
crude phosphorus always contains small particles of
mechanically carried over. To get rid of this and other
impurities, the phosphorus is either melted under water, and
presseci through chamois leather, or, more frequently, the crude
material is mixed with sulphuric acid and bichromate
of potash, three-and-half parts of each being used for every
parts of phosphorus. This oxidizing mixture acts upon
1
Hofmann, Report on the Vienna Exhibition.
Patents No. 14,962 and 17,719 (1888); Thorpe's Dict. 3, 192.
2
554
THE NON-METALLIC ELEMENTS
liquid, whilst the pure phosphorus remains clear and colourless
at the bottom. It was formerly cast into sticks by the work-
men sucking the melted phosphorus up with the mouth into
glass tubes. Instead of this dangerous operation an apparatus
is now employed by which the phosphorus is cast in brass or
copper tubes by a continuous process, proposed by Seubert.¹
The apparatus consists of a copper vessel, in which the phos-
phorus is melted under water; from this the molten phosphorus
is allowed to flow into a tube consisting of glass or copper.
One-half of this tube is surrounded by hot and the other by cold
water, the phosphorus being thus obtained in the form of solid
sticks, and cut under water into pieces of a convenient length.
The quantity of phosphorus manufactured in the year 1874
amounted to 250 tons, and is at the present time (1893) prob-
ably not less than 1,200 tons, the greater portion of which is
used for the manufacture of lucifer matches, a certain amount.
being employed as a vermin poison, and a small quantity being
used in chemical laboratories.
The distillation of phosphorus can easily be shown in
the lecture-room, by placing some pieces of dry phosphorus
in a small tubulated retort attached to a tubulated receiver
containing water, which communicates with the air by means
of a tube one metre in length dipping under mercury. A
current of carbon dioxide gas is passed through the tubulus of
the retort so as to drive out the air. As soon as all the air has
been removed the phosphorus can be heated and is seen to boil,
the colourless vapour condensing in transparent yellow drops on
the neck of the retort and in the receiver. The barometer-tube
prevents the entrance of atmospheric oxygen.
315 Properties. Like sulphur and other elements, phosphorus
exists in different allotropic modifications. Common or octo-
hedral phosphorus is, when freshly prepared and kept in the
dark, a slight yellow or almost colourless body, which, when
slowly solidified, is perfectly transparent, but when quickly cooled
is translucent, and of a wax-like character. At low temperatures
phosphorus is brittle, but at 15° it becomes soft like wax, so that
it may be easily cut with a knife. The mass of the substance has,
however, a crystalline structure. This may be seen by leaving the
solid for some time in contact with dilute nitric acid, when the
surface becomes distinctly crystalline. According to v. Schrötter
its specific gravity at 10° is 183, and it melts at 443, forming
1 Annalen, 49, 346.
PROPERTIES OF PHOSPHORUS
555
a colourless or slightly yellow strongly refractive liquid, having a
specific gravity of 1764. Melted phosphorus, under certain cir-
cumstances, remains liquid for a long time at temperatures much
below its melting point. This is especially the case when it is
allowed to cool slowly under a layer of an alkaline liquid, or
when the solution in carbon bisulphide is slowly evaporated
under water. Neither solid nor melted phosphorus conducts
electricity (Faraday). When heated in an atmosphere free from
oxygen to a temperature of 290°, phosphorus boils, yielding
a colourless vapour which, according to the experiments of
Mitscherlich, has a specific gravity of 458 at 515°, and of 4·50
at 1040° according to those of Deville and Troost (Air=10).
Hence the molecular weight of phosphorus between these
temperatures is 123 84, or the molecule consists of four atoms.
At a temperature of 1500-1700°, the vapour-density decreases,
the numbers obtained falling between the values required for
the formulæ P, and P2, so that at this temperature the molecules
P are partially dissociated into lighter molecules.¹ The
determination of the molecular weight of phosphorus in solution
has led to different results with different investigators; thus
Beckmann 2 from the boiling point of solutions of phosphorus
in carbon bisulphide deduced the molecular formula P₁, which
was also obtained by Hertz 3 from the freezing point of its
benzene solutions. On the other hand Paterno and Nasini 4
obtained by the latter method numbers which point to the
existence of molecules P, and P, in the solution.
4
4
4
2
Phosphorus also evaporates at temperatures below its boiling
point. If a small piece of phosphorus be placed in the Torri-
cellian vacuum, it gradually sublimes and is deposited again in
the form of bright colourless crystals. Large crystals of
phosphorus are obtained by placing phosphorus in a flask
filled with carbon dioxide, then hermetically sealing it and
allowing the bottom of the flask to be heated on a water-bath
for some days to 40°; or in another way by keeping phosphorus
in vacuous tubes in the dark for some time, when it sublimes
and crystallizes on the side of the tube in colourless transparent
brightly shining crystals (Hermann and Maskelyne).
Phosphorus is nearly insoluble in water and slightly soluble in
1 Biltz and V. Meyer, Ber. 22, 726; V. Meyer and Mensching, Annalen.
240, 317.
2 Zeit. Phys. Chem. 5, 76.
3 Zeit Phys. Chem. 6, 358.
4 Ber. 21, 2155.
556
THE NON-METALLIC ELEMENTS
ether, oil of turpentine, and the essential oils. It is readily soluble
in chloride of sulphur, phosphorus trichloride, sulphide of phos-
phorus, and carbon bisulphide, of which one part by weight will
dissolve from seventeen to eighteen parts of phosphorus. From
solution in carbon bisulphide, phosphorus can easily be obtained
in the crystalline state, usually in the form of rhombic dodeca-
hedra. The same crystals are obtained, according to Mitscherlich,
by heating under water a mixture of one part of sulphur with
two parts of phosphorus. In order to obtain phosphorus in the
state of fine powder, the melted substance is well shaken with
cold water containing a little urea, the small drops thus formed
congealing into solid particles.
Phosphorus is an extremely inflammable substance, and
is always kept under water. In presence of air and light it
becomes covered under water with a white crust, which gradually
falls off, whilst the phosphorus becomes darker coloured. The
crust is common phosphorus, which falls off from an unequal
oxidation of the mass, and on melting it under water it
assumes the ordinary appearance of the element.
Phosphorus appears luminous in the dark, when in contact
with moist air, and it evolves fumes possessing a strong garlic-
like smell. These fumes are poisonous, producing phosphorus-
necrosis, a disease in which the bones of the jaw are destroyed, and
one by which scrofulous subjects are the most easily affected. The
luminosity of phosphorus in the air depends upon its slow oxida-
tion, with formation of phosphorous acid. In this act of combina-
tion so much heat is evolved, that if a large piece of phosphorus be
allowed to lie exposed to the air it at last melts and then takes
fire. The luminosity and oxidation of phosphorus are best seen
by pouring a few drops of the solution of this body in carbon
bisulphide on to a piece of filter paper and allowing the solution
to evaporate. In the dark the paper soon begins to exhibit a
bright phosphorescence, and after a short time the phosphorus
takes fire and burns. It was formerly believed that phosphorus
becomes luminous in gases upon which it can exert no chemical
action, such as hydrogen or nitrogen. This is, however, not so,
the luminosity which has been observed in these cases being due
to the presence of traces of oxygen. From these facts it would
naturally be inferred that phosphorus must be more luminous in
pure oxygen than in air. Singularly enough, this is not the
case. At temperatures below 20° phosphorus is not luminous.
1 Quart. Journ. Science, 1829, ii. 83.
PROPERTIES OF PHOSPHORUS
557
in pure oxygen, indeed it may be preserved for many weeks.
in this gas without undergoing the slightest oxidation.¹ If,
however, the gas be diluted by admixture with another
indifferent gas, or if it be rarefied, the phosphorescence is at once
observed (Graham). The phenomenon can be very beautifully
FIG. 160.
shown by placing a stick of phosphorus in a long tube (a, Fig.
160), closed at one end and open at the other, and partly filled
with mercury, into which some pure oxygen is brought. The
open end of the tube is connected by a caoutchouc tube with
the vessel (b) containing mercury, so that, by raising or lower-
1 W. Müller, Ber. 3, 84.
558
THE NON-METALLIC ELEMENTS
ing the vessel the pressure on the gas can be regulated. If the
pressure be so arranged that it does not amount to more
than one-fifth of an atmosphere, the phosphorus will be seen
to be brightly luminous in the dark. If the pressure be
then gradually increased, the light will become less and
less distinct, until, when the level of the mercury is the same
in both vessels, the luminosity has entirely ceased. The phos-
phorescence can, however, at once be brought back again by
lessening the pressure. The luminosity of phosphorus is also
stopped when certain gases, such as sulphuretted hydrogen, or
the vapours of certain compounds, such as ether or turpentine,
are present even in minute quantities.
When phosphorus is heated slightly above its melting-point
in moist air or oxygen it takes fire and burns with a brightly
luminous flame and with evolution of dense white fumes of phos-
phorus pentoxide PO. The greatest precautions are necessary
in working with phosphorus on account of its highly inflammable
character. It must always be cut, as well as kept, under water,
and must not be rubbed either in contact with the skin, or when
it is being dried with blotting paper, as burning phosphorus
produces deep wounds, which heal only with great difficulty.
Phosphorus does not, however, combine with oxygen in absence
of moisture, and may be distilled in the perfectly dry gas without
undergoing any change. ¹
1
Phosphorus combines directly with the elements of the chlorine
and sulphur groups of elements, but not directly with hydrogen.
If it is heated with aqueous vapour to a temperature of 250°,
the water is decomposed with formation of phosphorous acid
and phosphuretted hydrogen. It also combines with most of
the metals at a high temperature, and on account of its easy
oxidizibility, it acts as a powerful reducing agent, precipitating
certain metals, such as gold, silver, and copper, when it is
brought into solutions of their salts. This reaction may also
be made use of for the purpose of detecting free phosphorus.
Thus, if the material under examination be boiled with water,
and the escaping vapour allowed to come in contact with a
piece of paper which has been wetted with a solution of
nitrate of silver, any phosphorus present will cause a black
stain of metallic silver to appear on the paper. No reduction,
however, takes place of a lead salt under similar circumstances,
whilst if the black stain be due to the presence of sulphuretted
1 H. B. Baker, Journ. Chem. Soc. 1885, i. 349.
•
DETECTION OF PHOSPHORUS
559
hydrogen, the lead as well as the silver paper will become
stained.
A
316 Detection of Phosphorus.-The water in which phosphorus
has been kept is also luminous in the dark, as phosphorus is
soluble, though only very slightly so, in water.
When phos-

b
FIG. 161.
B
phorus is boiled with water, it is partially volatilized, issuing in
the state of vapour together with the steam. This property of
phosphorus is made use of for its detection in cases of phos-
phorus poisoning. The apparatus which is used for this
purpose is shown in Fig. 161. The contents of the stomach
supposed to contain the poison are diluted with water and
560
THE NON-METALLIC ELEMENTS
placed in the flask A. This flask is connected by the tube b,
with a condensing tube ccc, surrounded by cold water. As
soon as the liquid contained in the flask is heated to boiling,
some of the phosphorus, if present, is volatilized together with the
steam, and if the whole of the apparatus be placed in the dark,
a distinct luminosity, usually in the form of a ring, is observed
at the point where the steam is condensed. If the quantity of
phosphorus is not too small, some of it is found in the receiver in
the form of small solid globules (Mitscherlich).
317 Action of Phosphorus as a Poison.-Ordinary phosphorus
is a powerfully poisonous substance capable of inducing death
in a few hours, or, when given in small doses, of producing
a remarkable train of poisonous symptoms lasting for many
days, or even for weeks. Red phosphorus appears, on the
other hand, to be without action on the animal economy,
when introduced into the stomach.
Although cases occur in which the administration of phos-
phorus is followed by death in a few hours, more commonly
some days elapse between the date of administration and
death.
In the more common' cases of phosphorus poisoning, some
time after the poison has been taken there supervenes pain in
the stomach, with vomiting of garlic-smelling substances, and not
unfrequently diarrhoea; all these symptoms of gastro-intestinal
irritation may be, and often are, absent. Whether they are
present or absent the patient soon becomes very weak, a febrile
condition ensues, and the skin assumes a jaundiced hue.
Hæmorrhages may occur, and, towards the end, convulsions or
coma usually make their appearance.
The appearances observed in the bodies of animals and men
poisoned with phosphorus are very interesting, and indicate that
this substance produces a powerful effect upon the nutrition of
the body. Minute extravasations of blood are frequently seen
in the lining membrane of the stomach and intestines, and
not unfrequently small ulcers occur in those organs. The
common and remarkable appearances are fatty degeneration
of the liver, kidneys, heart and voluntary muscles. It is to
be remarked that the same changes are observed, although in
a less marked degree, after chronic poisoning by arsenic, an-
timony, and vanadium. These fatty degenerations probably
indicate that phosphorus and the allied poisons exert an in-
fluence whereby the oxidation changes, which have their seat
RED PHOSPHORUS
561
in the animal tissues, are more or less slowed or arrested
(Gamgee).
Death has in man followed the administration of doses of
phosphorus not exceeding a decigram.
318 Red Phosphorus.-This peculiar modification of phos-
phorus, frequently termed amorphous phosphorus, was dis-
covered by v. Schrötter in 1845.¹ Other chemists had indeed
previously noticed the existence of this substance, but its nature
had been misunderstood. It is obtained by the action of light
FIG. 162.
and heat on ordinary phosphorus. This change occurs with
tolerable rapidity when the yellow phosphorus is heated from
240° to 250°. At a higher temperature red phosphorus begins
to undergo the opposite change, yellow phosphorus being formed.
Hence the passage from one allotropic modification to another
is more readily shown in the case of phosphorus than in that
of any other element. For this purpose all that is needed is a
glass tube containing three bulbs (Fig. 162), and having the
open end bent at right angles and dipping under mercury. In
¹ Pogg. Ann. 81, 276.
37
562
THE NON-METALLIC ELEMENTS
the last of these three bulbs is placed a piece of phosphorus.
The phosphorus is then heated, the whole of the oxygen con-
tained in the apparatus being very soon absorbed, and the
remainder of the phosphorus is distilled from the last into the
middle bulb. By gently heating this, it is transformed into the
red modification, which by a further application of heat, is re-
converted into the ordinary modification, which distils into the
third bulb. This experiment should be made on a leaden table
or on a surface covered with a coating of sand, in case the bulbs
should burst.
Red phosphorus is also formed when ordinary phosphorus is
heated in closed vessels to 300°, or about 10° above the
boiling point. In this case the change takes place in a few
minutes. The conversion of ordinary into red phosphorus
can also be brought about by certain chemical actions. E.
Kopp¹ found, in 1845, that by the action of iodine on phos-
phorus a red body is formed, which on heating yields the
ordinary modification of phosphorus, and B. C. Brodie 2 has
shown that only a trace of iodine is needed to bring about the
change from the yellow to the red modification, and that when
common phosphorus is heated with a trace of iodine to 200°,
a very violent reaction takes place and the red modification is
formed.
4
Red phosphorus is usually obtained as a compact solid sub-
stance, which has a dark reddish brown colour, and generally pos-
sesses a metallic iron-grey lustre. It has a specific gravity of
2106, exhibits a conchoidal fracture, and yields a reddish brown
coloured powder closely resembling finely divided oxide of iron;
its hardness lies between that of calcspar and fluorspar. It was
formerly supposed to be amorphous, but Pedler and Retgers
have shown that it is at any rate partially crystalline. Accord-
ing to Muthmann red phosphorus is a mixture of an orange
red amorphous powder with small crysta having a violet tint.
It is a tasteless, odourless substance, insoluble in all those
solvents which dissolve common phosphorus, and when intro-
duced into the system in the ordinary manner is not poisonous,
the whole being excreted unchanged; if, however, it is injected
into the blood the usual symptoms of phosphorus poisoning
occur.6
1
Compt. Rend. 18, 871.
3 Journ. Chem. Soc. 1890, i. 599.
5 Zeit. Anorg. Chem. 4, 303.
2 Journ. Chem. Soc. 1853, 289.
4 Zeit. Anorg. Chem. 3, 399.
6 Neumann, Ber. 21, 748c.
RED PHOSPHORUS
563
It is usually stated that red phosphorus, when perfectly free
from the ordinary modification, is unalterable in the air, but
Pedler¹ has shown that at any rate in hot moist climates, this
is not the case, but that it slowly undergoes oxidation, with
formation of phosphorous and phosphoric acids. When heated
by itself in absence of air, it does not undergo any alteration
until a temperature of 350° is reached, when it is slowly con-
verted into ordinary phosphorus, the change taking place more
quickly at a higher temperature. When heated in the air it
takes fire at about 260°. Ordinary phosphorus takes fire spon-
taneously when brought into chlorine gas, but the red phos-
phorus requires heating before ignition takes place. Similar
differences between the two modifications present themselves
in a large number of other chemical reactions, and the red
modification conducts electricity, although but feebly, whilst
the yellow does not do so at all.
The mode of manufacturing red phosphorus is simple.
Ordinary phosphorus is placed in an iron vessel and heated to a
temperature of 240°. This vessel is closed by means of a cover,
through which passes a long narrow pipe open at both ends, so
that the air has limited access to the phosphorus contained in
the vessel. Thus all danger of explosion is avoided, and the
air in the narrow tube undergoing but little change, only a
little of the phosphorus takes fire as soon as the oxygen
has been withdrawn from it by the combustion of the first
portion of the phosphorus. The red phosphorus thus prepared
is ground under water and freed from common phosphorus
by boiling with a solution of caustic soda, washing and drying.
The commercial red phosphorus, when in large compact
masses, almost always contains a small quantity of enclosed
yellow phosphorus, and not unfrequently takes fire when
it is rubbed or broken. In consequence it is usually packed
in vessels containing water, whilst the ground substance, as
above described, may be sent in the dry state in tin boxes.
It frequently also contains traces of graphite, originating
from the iron pots in which it is heated.
319 Metallic or Rhombohedral Phosphorus.-When phosphorus
is heated in sealed tubes in contact with metallic lead for ten
hours at a temperature approaching a red heat, a third modi-
fication of phosphorus is formed. On cooling, the whole mass
of lead is found to be permeated with small crystals, which
1 Loc. cit.
564
THE NON-METALLIC ELEMENTS
have been formed by the phosphorus dissolving in the
melted lead at a high temperature and crystallizing out on
cooling. In order to separate these crystals from the metallic
lead, the mass is placed in dilute nitric acid, when the lead is
dissolved. The crystals of phosphorus are still further purified
by subsequent boiling in strong hydrochloric acid. Metallic
phosphorus is a brightly lustrous dark crystalline mass, which
in thin plates possesses a red colour and consists of micro-
scopic rhombohedra. Its specific gravity at 15°5 is 2·34; it
is stated to conduct electricity better than the red variety,
and requires to be heated to a temperature of 358° before it
is converted into ordinary phosphorus. This variety is also
formed when red phosphorus is heated under pressure to a
temperature of 580° (Troost and Hautefeuille).
The recent investigations of the properties of red phosphorus
have shown that its properties agree more nearly with those
of metallic phosphorus than was formerly supposed, and it
appears probable that the latter is simply a better crystallized
variety of the red modification (Pedler, Retgers).
According to Thénard a fourth modification of phosphorus
exists. This substance has a black colour, and is obtained when
melted phosphorus is quickly cooled. Recent observations have,
however, shown that this black phosphorus is only formed when
foreign bodies, especially mercury or other metals, are present,
these bodies uniting with phosphorus to form a black metallic
phosphide. A further modification has been described by
Vernon, but its existence is as yet doubtful.
When hydrogen is passed over phosphorus or when phos-
phates, hypophosphites, or phosphites, or the corresponding acids,
are brought into a vessel in which hydrogen is being evolved,
the hydrogen is seen to burn with an emerald green flame; and
if the quantity of phosphorus be not too small, a white porce-
lain plate held in the flame is stained with a red deposit. This
reaction does not occur in the presence of alcohol, ether, or
animal matter. The spectrum of the phosphorised hydrogen
flame exhibits three bright green lines, of which one is almost
coincident with one of the lines of the barium spectrum, the
third being not quite so bright, and lying between the two
bright ones and the sodium line."
·
1 Hittorf, Pogg. Ann. 126, 193.
3 Dusart, Compt. Rend. 43, 1126.
5 Christofle and Beilstein, Ann. Chem. Phys. [4] 3, 280.
2 Proc. Chem. Soc. 1891, 3.
4 Blondlot, Compt. Rend. 52, 1197.
LUCIFER MATCHES
565
Phosphorus is frequently used in the laboratory. It is largely
employed in the manufacture of the iodides and bromides of methyl
and ethyl, bodies much used in the preparation of certain aniline
colours. The main purpose for which phosphorus is employed in
the arts is, however, the manufacture of lucifer matches, for which
purpose more than 1,000 tons are employed every year.
320 Lucifer Matches.-The application of this substance to
the artificial production of heat and light is only of recent date.
The oldest mode of artificially obtaining fire is that, still made
use of by certain rude tribes, of rubbing together a piece of
hard wood and a piece of soft wood, turning the former quickly
on the latter until it takes fire. At a later time it was found
that, when a piece of iron pyrites was struck with a mass of
iron, sparks flew off, by means of which, dry inflammable
materials, such as tinder, might be ignited. In place of iron
pyrites flint was next used, and the iron replaced by a rough
piece of steel. The tinder employed was made of charred linen,
and the glowing tinder was made use of to ignite a match, con-
sisting of a splint of wood, the ends of which were coated with sul-
phur. Up to the year 1829 this was the usual method employed
for obtaining a light. The first lucifer matches consisted of pieces
of wood the ends of which had been dipped into sulphur, and
which were coated in addition with a mixture of sugar and
chlorate of potash. In order to bring about the ignition
of these matches, they were dipped into a bottle containing
asbestos moistened with fuming sulphuric acid. Friction
matches were invented in the year 1832, the material composing
the inflammable mixture consisting of two parts of sulphide of
antimony and one part of chlorate of potash, mixed together to
a paste with gum and water. The matches, which had been
previously coated with sulphur, were then dipped into this
mixture and dried. In order to ignite them, these matches
were drawn through two layers of sand-paper, held between the
thumb and first finger. The antimony sulphide was soon
replaced by phosphorus, and the first matches which were made
in this way were sold in boxes containing from 50 to 60 for
twopence. Chlorate of potash is now supplanted by nitre,
especially in the case of Continental makers, inasmuch as the
latter substance is less liable to give rise to an explosive ignition.
A further improvement consisted in the replacement of sulphur,
which produces a disagreeable smell, by wax or paraffin.
The discovery of red phosphorus naturally led to the idea
566
THE NON-METALLIC ELEMENTS
of the employment of this substance in the manufacture
of lucifer matches, and this improvement was especially valu-
able, as, in spite of all care, the phosphorus disease made
its appearance in match manufactories, where ordinary phos-
phorus was employed. The substitution of the red phos-
phorus for the white modification rendered its recurrence
impossible.
Many difficulties had to be overcome in the employment of
this new substance, and it was only after some time that the
following mixture applied to the head of the matches was found
to serve the required purpose:
Potassium Chlorate
Potassium Bichromate
Red Lead
Sulphide of Antimony
•
2. Liquid
3. Solid
·
This mixture contains no phosphorus, and, as a rule, it will
only ignite on a surface strewn with a mixture of amorphous
phosphorus and sulphide of antimony. If, however, these so-
called safety matches be quickly rubbed over a non-conducting
surface such as that of glass or a smooth sheet of paper they
can be made to take fire.
""
.
PHOSPHORUS AND HYDROGEN.
321 Three compounds of phosphorus with hydrogen are
known-
""
1. Gaseous Hydrogen Phosphide, PH¸.
P₂H.
PH₂.
1 Crell. Ann. 1, 450.
3 Crell. Ann. 1796, ii. 148.
.
32
12
32
24
""
""
GASEOUS HYDROGEN PHOSPHIDE OR PHOSPHINE, PH₁ = 33·8.
2
3
By heating together phosphorus and caustic potash Gengembre
in 1783¹ obtained a gas which was spontaneously inflammable.
Some years later Davy and Pelletier prepared a very similar
gas by heating phosphorous acid. This gas differed, however, from
the former, inasmuch as, although very easily inflammable, it did
2 Phil. Trans. 1809, i. 67.
PHOSPHINE
567
not take fire spontaneously on coming in contact with the air
Both compounds were at that time recognized to be compounds
of hydrogen and phosphorus. The true explanation of the
difference between these two gases was given by Paul Thénard.¹
He showed that the spontaneous inflammation of the one gas
was due to the presence in it of small traces of the vapour of a
liquid hydride of phosphorus.
Preparation.—(1) In order to prepare spontaneously inflam-
mable phosphuretted hydrogen, as the impure gas has been called,
phosphorus is heated with milk of lime or with a solution of
caustic potash. The spontaneously inflammable gas is evolved.
and calcium or potassium hypophosphite left behind; thus:—
3KOH + 4P + 3H₂O=3KH₂PO₂ + PH3.
(2) The same mixture of gases is also readily formed when
phosphide of calcium is thrown into water. Each bubble of the
gas ignites, on coming to the surface of the water, with a sharp
explosion, burning with a bright white flame, and a ring-like cloud
of phosphorus pentoxide is formed, which on ascending shows the
remarkable vortex motions. In order to exhibit this phenomenon,
a small flask (a Fig. 163) is three-quarters filled with strong
potash solution, a few pieces of phosphorus are thrown in, and
the whole is gently warmed. As soon as small flames are seen
at the mouth of the flask, a gas delivery-tube (c) is fixed in with
cork, the lower end dipping under water.
When the spontaneously inflammable phosphuretted hydrogen
is exposed to the light, or when it is passed through a freezing
mixture, or left in contact with carbon or potassium, it loses its
power of spontaneous inflammability, inasmuch as the liquid
hydride contained in it is either decomposed or condensed.
(3) The non-spontaneously inflammable phosphuretted hydro-
gen is obtained by warming phosphorus with an alcoholic.
solution of potash, or by decomposing phosphide of calcium by
means of hydrochloric acid.
(4) Phosphine may also be prepared by the action of dilute.
acids on the phosphides of zinc, iron, tin, or magnesium.2
The phosphine prepared by any of these methods is, however,
not pure, but contains more or less hydrogen mixed with it
In order to obtain pure phosphuretted hydrogen we make use
of its property of combining with hydriodic acid to form the
1 Ann. Chim. Phys. [3] 14, 5. Lüpke, Journ. Chem. Soc. 1891, ii. 397.
568
THE NON-METALLIC ELEMENTS
crystalline compound termed phosphonium iodide, PHI. This
substance, when thrown into water, decomposes into its con-
stituents, namely, phosphine, PH, and hydriodic acid, HI. The
solid iodide, the preparation of which will be hereafter described,
is employed as follows for the preparation of the pure gas.¹
Some pieces of the iodide of the size of peas, mixed with broken
glass, are brought into a small flask. The flask is closed by a
cork having two holes bored through it, in one of which is
placed a stoppered funnel-tube, and in another a gas-delivery
tube. The funnel is filled with concentrated solution of potash,

a
FIG. 163.
and this is allowed to run into the flask slowly, when the
following decomposition occurs :-
PH,I + KOH = PH¸ + KI + H₂O.
According to Messinger and Engels 2 it is preferable to mix the
phosphonium iodide with ether and gradually add water, a
regular stream of phosphine being thus obtained. The gas
prepared in this manner is not spontaneously inflammable, at any
rate at the beginning of the operation, although if the evolution
2 Ber. 21, 326.
1 Hofmann, Ber. 4, 200.
PHOSPHINE
569
be carried on for a considerable length of time the spontaneously
inflammable gas is formed (Rammelsberg).
-
322 Properties.-Phosphine is a colourless gas, smelling like
rotten fish; it liquefies at -85°, solidifying at — 133°5 (Olszewski).
The pure gas takes fire only above a temperature of 100°, and is
so inflammable that the heat evolved by the friction of the
stopper on opening the bottle containing the gas is sometimes.
sufficient to produce its inflammation. It may be mixed with
oxygen without undergoing any alteration, but if this mixture
be suddenly exposed to diminished pressure an explosion occurs.
This remarkable phenomenon reminds one of the non-luminosity
of phosphorus in pure oxygen and its luminosity in diluted
oxygen at the same temperature.
Phosphine also takes fire when a few drops of dilute nitric
acid are brought in contact with it, or when it is mixed with
the vapours evolved from chlorine- or bromine-water. If the
gas free from air be led through common nitric acid containing
nitrous fumes, it becomes spontaneously inflammable, and it
explodes in chlorine gas with great violence and with the evo-
lution of a bright greenish-white light. Phosphine is somewhat
soluble in water, and imparts to it a peculiar and disagreeable
taste; the solution decomposes in the light with the evolution
of hydrogen and the separation of red phosphorus. When
a series of electric sparks is passed through the gas it also
decomposes into phosphorus and hydrogen, the volume of the
latter bearing to that of the original gas the proportion of three
to two. In order to show this a eudiometer similar to the one
already described (Fig. 135) may be employed, but instead
of platinum wires pieces of gas-coke are melted through the
glass, inasmuch as platinum and phosphorus in contact unite,
forming a silver white compound which is brittle and easily
fusible (Hofmann).
Phosphine combines, like ammonia, with certain metallic
chlorides; thus, for instance, with aluminium chloride AlCl3,
tin chloride SnCl, titanium chloride TiCl,, and antimony
chloride SbC15.
Phosphine is a very poisonous gas, producing, when present
in small proportions in respired air, in turn dyspnoea and death.
It possesses the power of combining with the respiratory oxygen
linked to hæmoglobin; in this way its toxic action has been
explained, although in reality it is almost entirely due to more
complex operations.
570
THE NON-METALLIC ELEMENTS
323 Phosphonium Compounds.-Phosphine possesses feebly basic
properties, and combines with hydrobromic acid and hydriodic
acid to form salts, in a similar manner to ammonia. These
salts contain the compound radical PH,, which is usually
termed phosphonium, just as the compound radical NH (p. 470)
is termed ammonium.
4
Phosphonium Bromide, PH,Br, crystallizes in colourless cubes,
which boil at 30°. The vapour possesses a specific gravity of
1906, so that we conclude that it is a mixture of phosphine
and hydrobromic acid.
Phosphonium Iodide, PH,I.-This beautiful compound, which
crystallizes in large transparent glittering quadratic prisms, can
easily be obtained by placing in a retort of a litre capacity (see
Fig. 164) 400 grams of common phosphorus, allowing an equal
FIG. 164.
weight of dry carbon bisulphide to run in, and gradually adding
680 grams of pure iodine, care being taken to keep the retort
well cooled. The carbon bisulphide is next completely removed
by distillation in a water bath, and the retort connected with
a long wide tube placed in a slightly slanting position, and
furnished at its lower end with a tubulated receiver. This,
again, is connected by a series of bulb tubes with two absorp-
tion-vessels, the first of which contains a dilute solution of
hydriodic acid, and the second water. The object of this
arrangement is to absorb the hydriodic acid formed during the
reaction, and at the same time to prevent the liquid from entering
the wide tube into which the iodide of phosphonium is sub-
limed. The apparatus is then filled with pure carbon dioxide,
a slow current of the gas being passed through during the
1

LIQUID HYDROGEN PHOSPHIDE
571
operation. The experiment being thus far arranged, 240 grams
of water are allowed to drop slowly by means of a stoppered
tube-funnel into the retort, which is slightly warmed. The
heat evolved by the action then taking place is sufficient to
sublime the greater part of the iodide of phosphonium into the
wide tube. Towards the end of the operation, which usually
requires about eight hours for its completion, the retort is heated
somewhat more strongly. When no further increase in the
amount of sublimate takes place, the apparatus is dismounted,
the end of the long tube closed with corks, and the thick crust
of phosphonium iodide loosened by means of a stout iron
wire, and preserved in stoppered bottles.¹ The formation of
phosphonium iodide is represented by the following equation:-
51+9P+15H₂O
5PHI+ 4H¸Р.
=
An excess of phosphorus is, in practice, employed because a part
of this substance is converted, during the reaction, into the
red modification. The formation of the hydriodic acid which
escapes is due to the decomposition of the iodide of phos-
phonium in the presence of warm water. Phosphonium
iodide boils at about 80°, but it easily vaporizes at a lower
temperature. It is used in the laboratory as a powerful reducing
agent, as well as for the preparation of many organic phos-
phorus compounds.
LIQUID HYDROGEN PHOSPHIDE, P₂H=65·60.
324 This substance, discovered by Thénard in the year 1845,
is obtained by the action of water on calcium phosphide. The
latter is prepared by the action of phosphorus on lime at a red
heat and is best converted into liquid hydrogen phosphide by
carefully adding it in small quantities at a time to water heated
to 60°, the air in the apparatus having previously been displaced
by hydrogen. The gas evolved is passed through a small con-
denser placed in ice water, in which a portion of the liquid
phosphide condenses, the remainder passing on with the gaseous
hydride simultaneously formed. The liquid phosphide is colour-
less, has a sp. gr. of about 101, and boils at 57-58° under a
pressure of 735 mm., and leaves no residue if not too strongly
heated. It has the empirical formula PH,, but its molecular
1 Hofmann, Ber. 6, 286.
572
THE NON-METALLIC ELEMENTS
formula is in all probability PH, corresponding to that of
hydrazine, NH₁; no determination of its vapour density could,
however, be made as it so readily undergoes decomposition.¹
Liquid hydrogen phosphide is spontaneously inflammable,
taking fire at once on exposure to the air and burning with a
bright phosphorus-like flame. On exposure to light, or when
heated above its boiling point it decomposes into phosphine
and solid hydrogen phosphide, according to the following equa-
tion :
—
5P₂H¸ = 2P₂H + 6PH3.
The same reaction takes place in contact with hydrochloric acid
and hydriodic acid, 1 cc. of hydrochloric acid being sufficient,
according to Thénard, to decompose an indefinite quantity of the
phosphide.
T
FIG. 165.
In order to exhibit the properties of the liquid phosphuretted
hydrogen, Hofmann employs a U-tube made of strong glass,
3 to 4 mm. in diameter, each of the limbs of which is furnished
with a glass stopcock (Fig. 165). This tube is placed in a freez-
ing mixture of pounded ice and salt and connected with a flask,
into which from 30 to 50 grams of freshly prepared calcium
phosphide are gradually thrown. This being decomposed by
water, the phosphuretted hydrogen evolved passes through the
U-tube, in which the liquid hydride condenses, whilst the
spontaneously inflammable gas escapes. As soon as all the
calcium phosphide has been decomposed, a current of dry
carbon dioxide is led through the apparatus; the flame of
the issuing gas is then changed to a faintly luminous cone.
If, however, the carbon dioxide be replaced by a current of
hydrogen, the luminous flame is again seen.
1 Gattermann and Haussknecht, Ber. 23, 1174.
FLUORIDES OF PHOSPHORUS
573
SOLID HYDROGEN PHOSPHIDE, P„H.
325 This compound is formed in the manner already de-
scribed, and may be readily obtained by passing the uncondensed
vapours obtained in preparing the foregoing compound into a
large flask containing concentrated hydrochloric acid; it forms
a yellow powder, having the empirical formula P₂H, but its
molecular formula is as yet unknown. On heating this in a stream
of carbon dioxide to 70° it decomposes into phosphorus and
hydrogen. It does not take fire in the air until it attains a
temperature of 160°.
PHOSPHORUS AND FLUORINE.
PHOSPHORUS TRIFLUORIDE, PF3.
326 This compound is obtained by the action of copper
phosphide on lead fluoride, or by allowing arsenic trifluoride
to drop into phosphorus trichloride, moisture being excluded.
It is a colourless gas which does not fume in the air, and
condenses to a colourless liquid at 10° under a pressure
of 40 atmospheres. It is only slowly absorbed by water, but
forms an explosive mixture with oxygen, and unites with
ammonia and bromine. The dry gas is dissociated by the
passage of electric sparks into fluorine and phosphorus penta-
fluoride.
PHOSPHORUS PENTAFLUORIDE, PF5,
Was discovered by Thorpe,' who prepared it by the action
of arsenic trifluoride on phosphorus pentachloride.
5AsF3 + 3PC15 = 5AsCl₂ + 3PF5.
It is also formed by heating phosphorus trifluorodibromide
(p. 579) which splits up at 15° into the pentabromide and
pentafluoride. It is a colourless gas and decomposes in contact
with water into phosphoric and hydrofluoric acids; it possesses
a strongly irritating smell, and attacks the mucous membrane.
1 Proc. Roy. Soc. 25, 122.
2 Moissan, Compt. Rend. 99, 655, 570; 100, 272, 403.
574
THE NON-METALLIC ELEMENTS
1
It partially condenses to a liquid at 16° under a pressure of 46
atmospheres, and solidifies at a very low temperature. Accord-
ing to Thorpe the gas is unaffected by passing a series of electric
sparks through it whether alone or mixed with hydrogen or
oxygen, whilst Moissan ¹ found that with sparks of high tension
a partial dissociation into fluorine and the trifluoride occurs.
On the other hand when phosphorus trifluoride comes in
contact with free fluorine it takes fire and burns with a yellow
flame forming the pentafluoride. With dry ammonia it yields
a white solid compound having the composition 2PF, 3NH„.
The vapour density of the compound is 63 (H=1) and the
molecular formula is therefore PF5. The existence of this
gaseous pentafluoride taken in conjunction with its stability even
at high temperatures is of great theoretical interest, inasmuch
as it shows that phosphorus can form pentavalent derivatives.
capable of existing in a state of vapour.
PHOSPHORUS AND CHLORINE.
Ordinary phosphorus takes fire in dry chlorine gas, and burns
with a pale-greenish flame, with formation of phosphorus tri-
chloride, PC13, or, with an excess of chlorine, phosphorus penta-
chloride, PCl5.
PHOSPHORUS TRICHLORIDE, PCI,
= 136.37.
327 This compound was discovered by Gay-Lussac and
Thénard in 1808. It is best prepared by placing red phos-
phorus in a retort (D, Fig. 166), and heating it whilst a
stream of chlorine gas evolved in the flask (A), and dried by
passing through the tube (c), is led over it. The distillate is
purified from any pentachloride which is formed by allowing it
to remain in contact with ordinary phosphorus for some time
and then rectifying.
The trichloride is a mobile colourless liquid, which has a very
pungent smell, boils at 76°, and does not solidify at 115°.
The specific gravity of the liquid at 0' is 161294.3 When
1 Bull. Soc. Chim. [3] 5, 880.
2 Dumas, Ann. Chim. Phys. [3], 55, 172.
3 Thorpe, Proc. Roy. Soc. 24, 295.
PHOSPHORUS PENTACHLORIDE
575
A
exposed to the air it evolves white fumes, absorbing the atmo-
spheric moisture, and decomposing into hydrochloric and phos-
phorous acids; thus:-
PCl₂+ 3H20=3HCl + P(OH)3.
Sulphur trioxide acts violently on this compound, with
formation of phosphorus oxychloride, and sulphur dioxide.
Heated with concentrated sulphuric acid, chlorosulphonic acid

A
B
C
AND
=
FIG. 166.
D
E
and phosphorus pentoxide are produced according to the
equation:-
2PC1, +3SO,(OH)2
HO
Cl
2SO₂+ 5HCl + CI SO₂+ P₂O5.
It also unites with ammonia forming additive compounds.¹
PHOSPHORUS PENTACHLORIDE, PC1, = 206-75.
328 This compound was discovered by Sir Humphry Davy
in the year 1810, though it was first analysed by Dulong in
1816. It is easily formed by the union of phosphorus
trichloride with chlorine. To prepare it, a current of dry
chlorine is led through a wide tube on to the surface of the
liquid trichloride contained in a flask surrounded by cold water.
1 Besson, Compt. Rend. 111, 972.
576
THE NON-METALLIC ELEMENTS
As the absorption of the chlorine is accompanied by the evolu-
tion of much heat, it is necessary to take care, in the beginning
at least, that the liquid is well cooled. The reaction is finished
as soon as the product assumes the condition of a perfectly dry
mass.
Phosphorus pentachloride is a white or yellowish-white
lustrous crystalline powder, possessing a very sharp unpleasant
smell, and violently attacking the eyes and the mucous mem-
brane. On heating, it is found to sublime below 100°, but it
cannot be fused under the ordinary pressure of the atmosphere.
When, however, it is heated under increased pressure, it melts
at 148°, solidifying on cooling in transparent prisms. When
heated still more strongly it boils, emitting a colourless vapour,
which becomes coloured on further heating, the coloration
increasing with the temperature. This is due to the fact that
the vapour gradually undergoes dissociation into equal molecules
of free chlorine and phosphorus trichloride. That this is the
case is fully proved by Dumas' determination of the density of
this mixture at different temperatures :—
Temperature
Density
·
•
•
182° 200° 250° 300° 336°
73.3 70.0 57.6 52.4 525
These numbers clearly exhibit the gradual dissociation of the
vapour, the density undergoing a continuous diminution until the
temperature of 300° has been reached. Above that point it
remains constant. The vapour at these temperatures consists of
a mixture of an equal number of molecules of the trichloride and
chlorine, possessing the density: 136:37 + 2 × 35·19
= 51.69
4
That the vapour thus obtained contains free chlorine was
proved by Wanklyn and Robinson in the following way. The
vapour of the pentachloride was allowed to diffuse into an
atmosphere of carbon dioxide. Chlorine gas being lighter than
the vapour of the trichloride, must diffuse more quickly than
the latter. Accordingly, if, after the experiment, the vessels
containing the pentachloride were found to contain the trichlo-
ride, whilst the atmosphere of carbon dioxide was admixed with
free chlorine, the fact of dissociation would be proved. This
was the result. The dissociation of the pentachloride may be
prevented, or at any rate much diminished, by allowing it to
volatilize in a space saturated with the vapour of the trichloride.
1 Proc. Roy. Soc. 12, 507.
PHOSPHORUS PENTACHLORIDE
577
Wurtz¹ obtained, under these circumstances, a vapour possessing
a density close upon 1034 (the normal density of PC15) at
temperatures varying from 160° to 175°.
In perfectly dry air pentachloride of phosphorus undergoes no
alteration; on exposure, however, to moist air, it decomposes
with formation of phosphorus oxychloride; thus:—
PC1, + H₂O POCI₁+ 2HCl.
This compound as well as the pentachloride, dissolves in water
with evolution of heat and formation of phosphoric and hydro-
chloric acids; the two reactions being :-
POCI₂+3H₂O=PO(OH)3+3HCI,
PC15+4H₂O = PO(OH)3 + 5HCI.
SO₂
2
=
One of the most important properties of phosphorus penta-
chloride is its action on the acid-forming oxides and on the acids.
These it converts into the acid chlorides. Many examples of
this kind of decomposition have already been given. For in-
stance, it forms, with sulphur trioxide, the chloride of sulphuric
acid or sulphuryl chloride; thus:-
SO₂+ PC15=SO¿CI½ + POCI¸.
3
In this reaction two atoms of chlorine are replaced by one
atom of oxygen. When, on the other hand, the pentachloride is
allowed to act upon a hydroxy-acid, chlorine replaces the radical
hydroxyl, OH; thus:—
(OH
OH
+ PCI, = SO₂ { OH + POCI¸ + HCl.
In a similar way the pentachloride acts upon organic acids
and other compounds containing hydroxyl; and in consequence
of this property it is much used in the preparation of organic
chlorides.
Phosphorus pentachloride forms a crystalline compound with
iodine monochloride; thus :-PC1, + ICl, as also with different
metallic chlorides; thus:-PC1, + FeCl3, &c.
At the ordinary temperature ammonia acts upon phosphorus
pentachloride with formation of chloramido-derivatives (p. 610);
if, however, ammonia be passed into a cooled solution of the
1 Compt. Rend. 76, 601.
88
578
THE NON-METALLIC ELEMENTS
pentachloride in carbon tetrachloride, the two combine forming
the additive compound PC, 8NH,, which separates out as a
white powder and is stable in the air.¹
PHOSPHORUS TRIFLUORODICHLORIDE, PF,Cl₂,
Is formed by the direct combination of chlorine and phos-
phorus trifluoride, and is a colourless uninflammable gas which is
instantaneously absorbed and decomposed by water and alkalis.
It has a vapour density of 5·4 (air = 1) corresponding to the above
formula. It condenses to a liquid at -8° under atmospheric
pressure, and is decomposed at 250° or by the passage of electric
sparks into phosphorus pentafluoride and pentachloride. When
treated with a small quantity of water it yields phosphorus
oxyfluoride POF, and with sulphur forms the corresponding
thiofluoride PSF.2
PHOSPHORUS AND BROMINE.
PHOSPHORUS TRIBROMIDE, PBr
268.88.
329 Bromine and phosphorus act so violently upon each other
that small pieces of phosphorus thrown upon bromine may
cause a dangerous explosion. In order to prepare the tribro-
mide, dry carbon dioxide is allowed to pass through bromine, a
small portion of the vapour of which is carried over and is then
brought into contact with dry phosphorus (Lieben). `Another
method of preparation consists in dissolving both of these
elements separately in dry carbon bisulphide, and then gradually
pouring the bromine solution into that containing the phos-
phorus and distilling, when the carbon bisulphide boiling at 43°
comes off, leaving behind the phosphorus tribromide which boils
at 175° (Kekulé).
=
The simplest process for preparing the tribromide is to
place red phosphorus in a flask closed by a doubly bored
cork, one opening of which is connected with an inverted con-
denser, and round which cold water is allowed to flow, whilst
through the second opening a funnel with glass stopcock is
placed, by means of which bromine is allowed to fall slowly on
the phosphorus. The first drops combine with evolution of
2 Poulenc, Compt. Rend. 113, 75.
1 Besson, Compt. Rend. 111, 972.
PHOSPHORUS PENTABROMIDE
579
light and heat, but this rapid combination soon ceases, and the
bromine can be allowed to drop in without causing any violent
action. The product is then separated by distillation from the
excess of phosphorus which must be present (Schorlemmer).
The tribromide is a colourless mobile liquid possessing at 0° a
specific gravity of 2925. It has a strong unpleasant pungent
odour, and is decomposed in presence of water into phosphorous
acid and hydrobromic acid.
PHOSPHORUS PENTABROMIDE, PBr
= 427.6.
This body is obtained when bromine is added to the cold
tribromide. It is a lemon-yellow crystalline body which on
heating melts, forming a red liquid which decomposes at 100°
into the tribromide and bromine (Gladstone). When a current of
carbon dioxide is led through the melted compound, the bromine
is carried over and the tribromide remains behind. The penta-
bromide possesses an extremely pungent odour, and forms,
when brought into contact with a small quantity of water,
phosphorus oxybromide and hydrobromic acid.
PHOSPHORUS TRIFLUORODIBROMIDE, PF,Br₂ = 246.2
Is obtained by the union of phosphorus trifluoride and bromine,
and forms an amber yellow fuming liquid, which solidifies at
– 20°, forming pale yellow crystals (Moissan).
PHOSPHORUS CHLOROBROMIDE, PC1,Br₂
= 305.1.
330 If bromine and phosphorus trichloride are brought
together in molecular proportions, heat is evolved and the liquid
separates into two layers. The upper layer consists of a solution
of bromine in phosphorus trichloride, whilst the lower consists of
a solution of trichloride in bromine. If the mixture be cooled
down to a temperature of -20° the whole solidifies to a yellowish-
red crystalline mass which again splits up at ordinary tempera-
tures into two distinct layers (Wichelhaus). If this mixture,
placed in a sealed tube, be exposed for some weeks to about
15°, a crystalline compound is formed which is more stable, in-
asmuch as it does not decompose into its constituents until a
temperature of 35° is reached.
Phosphorus chlorobromide combines with bromine, and forms.
580
THE NON-METALLIC ELEMENTS
2
the compound PCI,Br₂+ Br, which solidifies in large crystals, red
by transmitted, but blue by reflected, light. A second compound
PCl, Br, + 2Br, is also formed at the same time, crystallizing in
needle-shaped crystals of a greenish lustre, which soon turn.
brown. These compounds correspond to those formed by the
union of phosphorus trichloride with iodine monochloride and
with the metallic chlorides (Michaelis).
PHOSPHORUS AND IODINE.
PHOSPHORUS DI-IODIDE, PI
331 This compound, which has an analogous composition to
liquid phosphuretted hydrogen, is obtained by dissolving one
part of phosphorus in carbon bisulphide and then adding
gradually 8.2 parts of iodine. On gently warming the solution
so as to distil off the carbon bisulphide, the di-iodide remains
behind as a yellow crystalline mass. When the bisulphide is
cooled down to 0°, the same compound separates out in long
orange-red crystals (Corenwinder). The crystals melt at 110°
and are decomposed by water with formation of red phosphorus,
phosphorous acid, and hydriodic acid; thus:-
= 565.24.
3P₂I + 12H₂O = 2P + 4P(OH)3 + 12ḤI.
Its vapour density corresponds to the formula PI.¹
1
PHOSPHORUS TRI-IODIDE, PI = 408.53.
This compound is obtained in a similar way to the foregoing,
but using 1 times as much iodine. By gently heating the
solution, the greater portion of the bisulphide of carbon is got
rid of, and the residue is cooled down by a mixture of salt and
ice. Red six-sided crystals separate out which melt at 55° and
which on gently cooling may be obtained of large size. On the
addition of water to this compound hydriodic and phosphorous
acids are formed; thus:-
PI,+3H,O= P(OH)3 + 3HI.
Phosphorus pentiodide PI, has been described by Hampton,
but its existence cannot as yet be regarded as definitely proved.
2 Chem. News, 42, 180.
1 Troost, Compt. Rend. 95, 293.
PHOSPHORUS SUBOXIDE
581
OXIDES AND OXYACIDS OF PHOSPHORUS.
Oxygen forms with phosphorus four compounds, and with
hydrogen and oxygen a large number of acids:-
Oxides.
Phosphorus suboxide, PO.
Phosphorous oxide, POË
Phosphorus tetroxide, P₂O.
Phosphorus pentoxide, P₂O.
5
Acids.
Hypophosphorous acid, PH(OH)2.
Phosphorous acid, P(OH)3.
Hypophosphoric acid, P₂O₂(OH),.
Phosphoric acid, PO(OH)3.
Pyrophosphoric acid, P₂OŽ(OH).
Metaphosphoric acid, POOH.
PHOSPHORUS SUBOXIDE, PO.
332 This oxide was first obtained by Leverrier by allowing
a solution of phosphorus in phosphorus trichloride to oxidise
in the air,¹ and is also obtained by the action of zinc on phos-
phorus oxychloride, and together with the other oxides when
phosphorus is burned in an insufficient supply of air. It is a
red apparently amorphous compound and closely resembles red
phosphorus in appearance.
HYPOPHOSPHOROUS ACID, H,PO2
333 The salts of this acid were discovered by Dulong in 1816.
They are formed when the phosphides of the metals of the
alkaline earths are decomposed by water, or when phosphorus is
boiled with an alkali or an alkaline earth. For the purpose of
preparing the acid, baryta is best employed, as it is from the
barium salt that not only the other salts but also the free acid.
is easily prepared. The formation of barium hypophosphite
is shown in the following equation:-
3Ba(OH)2 + 8P + 6H₂O = 3Ba(PH₂O2)2 + 2PH¸.
At the same time a small quantity of barium phosphate is
formed, but this can readily be separated from the hypophos-
phite by filtration. To the clear solution, the requisite quantity
of dilute sulphuric acid is added, and the filtered solution
2 Reinitzer and Goldschmidt, Ber. 13, 847.
1 Annalen, 27, 167.
3 Thorpe and Tutton, Journ. Chem. Soc. 1890, i. 549; 1891, i. 1019.
582
THE NON-METALLIC ELEMENTS
evaporated to a syrupy consistency. On cooling the solution,
the hypophosphorous acid is obtained in the form of a thick very
acid liquid. Hypophosphorous acid can also be obtained in the
form of a white crystalline mass melting at 17°4 as follows.¹
The tolerably concentrated solution is gently evaporated in a
platinum dish at a temperature below its boiling point and then
gradually heated from 110° up to 130°, at which temperature
it is allowed to remain for ten minutes. The solution thus
obtained is cooled, poured into a stoppered bottle, and this placed
in a freezing mixture at a few degrees below 0°. The acid
is then found either to crystallize spontaneously or to do so on
being touched with a glass rod.
Hypophosphorous acid when strongly heated decomposes into
phosphuretted hydrogen and phosphoric acid; thus:—
2H,PO₂ = PH¸ + H3PO4·
3
Its aqueous solution precipitates gold and silver from solutions
of their salts, phosphoric acid being formed; thus :—
4AgNO3 + 2H2O + H„PO₂ = 4Ag + 4HNO3 + H₂PO.
2
When a solution of this acid is added to mercuric chloride
solution, either calomel (mercurous chloride) or metallic mercury
is precipitated according to the proportions in which the acid is
present.
Hypophosphorous acid is also oxidized by chlorine and other
oxidizing agents to phosphoric acid, and when exposed to the
air it takes up oxygen with the formation of phosphorous acid.
Nascent hydrogen reduces it to phosphuretted hydrogen.
Although hypophosphorous acid contains the group hydroxyl
twice, only one atom of hydrogen can be replaced by metals,
This is easily explained when we remember that this acid may
OH
be considered as dihydroxyl-phosphine P OH, or as a weak
H
basic body which has become a weak acid by the addition of
two atoms of oxygen.
The Hypophosphites.-Most of the salts of hypophosphorous
acid are soluble in water, some being also soluble in alcohol,
and crystallizable. In the dry state they do not undergo altera-
tion in the air and may be boiled in water, free from absorbed
oxygen, without decomposition. Like the free acid, all the
1 Thomsen, Ber. 7, 994, 996.
PHOSPHOROUS OXIDE
583
hypophosphites possess strong reducing properties, giving with
solutions of gold, silver, and mercury, the same reactions as
the acid itself.
PHOSPHOROUS OXIDE OR PHOSPHOROUS ANHYDRIDE, PO.
334 This oxide was first obtained by Sage in 1777, and
was also observed by Cabell, but was first thoroughly examined
by Thorpe and Tutton. In order to prepare it the apparatus
shown in Fig. 167 is employed. Pieces of phosphorus about
an inch in length are placed in the wide combustion tube
(a), which is open at one end to admit air and bent into the
shape shown to prevent escape of melted phosphorus. The
la
b
Ilm
FIG. 167.
other end of the tube is fitted into a brass tube enclosed
in a second wider brass tube (b), water being introduced into
the space between the tubes by means of d. A loose plug of
glass wool is placed at the further end of the brass tube be-
fore the connection with the U-shaped condenser (c), surrounded
by ice and salt, which is in turn connected with the wash bottle
(f) containing sulphuric acid. The outlet of ƒ is in connection with
a water pump, by means of which a slow current of air is drawn
through the apparatus. The phosphorus is ignited by carefully
heating, and the temperature of the water in the jacket allowed
to reach 50° and maintained at that temperature till nearly the
end of the experiment, when it is allowed to rise to 60°. The
red oxide formed condenses in the portion of the tube nearest
the phosphorus, and the passage of the pentoxide is prevented
by the plug of glass wool, whilst the phosphorous oxide passes
1 Loc. cit.
584
THE NON-METALLIC ELEMENTS
through to the condenser. The reaction is stopped as soon as
four-fifths of the phosphorus has been burnt, and the U tube
detached and warmed, when the phosphorous oxide melts and
falls into the small bottle below.
Phosphorous oxide is thus obtained as a wax-like mass, but
may also be condensed in the form of feathery crystals, and
when the melted substance is allowed to cool it separates out
in thin prisms capped by pyramids, probably belonging to the
monosymmetric system. It has an unpleasant garlic like odour,
melts at 22:5° and resolidifies at 21° but frequently exhibits the
property of superfusion; the liquid has a specific gravity of
1·9358 at 248°/4°. It boils at 1731 (corr.) in an atmosphere of
nitrogen, its vapour density being 774; its molecular formula,
like that of the corresponding oxide of arsenic, is therefore PОë
When quite pure it is unaltered in the light, but it usually assumes
a dark red colour owing to the separation of red phosphorus.
Phosphorous oxide is only attacked very slowly by cold water,
with the gradual formation of phosphorous acid; with hot water
however a violent reaction takes place, spontaneously inflam-
mable hydrogen phosphide being evolved, and phosphoric acid
and either red phosphorus or the suboxide remaining behind.
On exposure to the air or oxygen it spontaneously oxidises to
phosphorus pentoxide, and when heated to 50-60° ignites and
burns with great brilliancy. It is acted upon with great
violence by alcohol, ignition taking place, but dissolves without
alteration in ether, carbon bisulphide, benzene, and chloroform.
When heated in a sealed tube to 210° it becomes turbid and at
440° is completely converted into red phosphorus and phos-
phorus tetroxide. When thrown into chlorine it burns with a
green flame, but by the slow action of the gas it yields a mixture
of phosphoryl and metaphosphoryl chlorides. With sulphur it
yields phosphorus sulphoxide, and with ammonia the diamide of
phosphorous acid HOʻP(NH2)2.
PHOSPHOROUS ACID, H₂PO.
In
335 This acid is formed together with hypophosphorous and
phosphoric acids when phosphorus is oxidised in moist air.
order to prepare pure phosphorous acid the reaction employed by
Davy in 1812 is employed. This consists in the decomposition of
the trichloride by water; thus:—
PCl₂+ 3H2O = P(OH); + 3HCl.
PHOSPHOROUS ACID
585
For this purpose it is not necessary to prepare the pure
trichloride separately, for, if chlorine be led through melted
phosphorus under water, the trichloride is first formed, and, on
coming in contact with the water, this is decomposed as shown
in the above equation. Care must be taken to stop passing the
chlorine in before all the phosphorus has disappeared, as the
chlorine would otherwise oxidize the phosphorous to phosphoric
acid. It is difficult to prevent this altogether even if phosphorus
is present in excess.
By evaporating the solution until the residue attains a tem-
perature of 180° a thick syrupy substance is obtained which is
transformed more or less rapidly on cooling into a crystalline
mass melting at 70°1.1 Phosphorous acid has an acid, garlic-
like taste, absorbs moisture rapidly from the air and deliquesces.
When strongly heated, phosphorous acid decomposes into
phosphuretted hydrogen and phosphoric acid; thus:-
4H,PO
3H3PO4 + PH₂.
And when treated with phosphorus pentachloride, phosphorus
trichloride is formed :—
-
P(OH)3 + 3PC15 = PC13 + 3POCl3 + 3HCl.
Hence the trichloride is the chloride of phosphorous acid.2
The aqueous solution of the acid also slowly absorbs oxygen
from the air, and in presence of nascent hydrogen it is reduced
to phosphuretted hydrogen. It acts as
It acts as a strong reducing
agent, precipitating gold, silver, and mercury from their solutions
like hypophosphorous acid.
The Phosphites. Although a weak acid, phosphorous acid is
tribasic, but under ordinary circumstances, only two atoms of its
hydrogen can be replaced by metals. A normal tribasic salt,
P(ONa), is known, but this has not yet been obtained in the
anhydrous state. On the other hand, ethers of phosphorous
acid are known in which the three atoms of hydrogen are replaced
by a radical, such as ethyl, CH, and the resulting tri-ethyl
phosphite, P(OC₂H;), is a body which can be distilled without
decomposition.
The phosphites which are soluble in water possess an acid and
2 Geuther, J. Pr. Chem. [2] 8, 359.
1 J. Thomsen, Ber. 7, 996.
3 Zimmermann Annalen, 175, 21.
586
THE NON-METALLIC ELEMENTS
garlic-like taste. They act upon the salts of the noble metals
like the hypophosphites, from which, however, they are distin-
guished by giving a precipitate with baryta- or lime-water.
PHOSPHORUS TETROXIDE, PO4
336 This compound is prepared by heating the mixture
of oxides obtained by the combustion of phosphorus in an
insufficient supply of air to 290° in a sealed tube. Phosphorus
tetroxide then volatilises and condenses in the cool portions of
the tube, in the form of clear transparent crystals, belonging to
the rhombic system. It may also be prepared from phos-
phorous oxide by heating it to 440° when it decomposes into red
phosphorus and phosphorus tetroxide.
The tetroxide is a deliquescent substance and dissolves in
water with evolution of heat yielding a strongly acid solution
which is unaltered by boiling, reduces mercuric to mercurous
chloride, and gives with silver nitrate a white precipitate rapidly
changing to black. It only decolourises permanganate solution
slowly, and on evaporation in vacuo yields a thick colourless
syrup containing phosphorous and phosphoric acids. It is not the
anhydride of hypophosphoric acid, as the latter reduces per-
manganate quickly, and also forms a sparingly soluble sodium salt,
which cannot be obtained from the solution of the tetroxide.¹
HYPOPHOSPHORIC ACID, H,P₂O6
337 It was formerly supposed that the acid obtained by the
slow oxidation of phosphorus in moist air, prepared by exposing
thin strips of phosphorus to a limited supply of moist air as
shown in Figs. 168 and 169, is phosphorous acid. Salzer has,
however, shown that in addition to phosphoric and phosphorous
acids this liquid contains hypophosphoric acid. On neutralising
with caustic soda, a slightly soluble salt, sodium hypophosphate,
H,Na,PO, separates out. A solution of this salt yields with
lead acetate an insoluble salt, Pb,P,O,, from which on decom-
position with sulphuretted hydrogen, the acid can be prepared,
as an odourless acid liquid. The aqueous solution decomposes
1 Thorpe and Tutton, Journ. Chem. Soc. 1886, i. 833.
2 Annalen, 187, 222; 194, 28; 211, 1.
PHOSPHORUS PENTOXIDE
587
on concentration. Hypophosphoric acid is tetrabasic, and yields
two classes of salts. The sodium salts crystallize well:
2 6
Na P₂O+ 10H₂O Normal sodium hypophosphate.
H₂Nа¿P₂O+9H₂O Hydrogen sodium hypophosphate.

FIG. 168.
FIG. 169.
PHOSPHORUS PENTOXIDE, PO5
338 The thick white clouds which are formed when phosphorus
burns brightly in the air consist of this oxide. If a small piece of
phosphorus is burnt on a dry plate covered with a bell-jar, these
fumes condense partly on the sides of the glass and partly on the
plate, in the form of a white flocculent powder, whilst the por-
tion near the burning phosphorus forms a glassy mass. This
white powder is amorphous, and may be sublimed in a test-
tube heated over a gas lamp. The pure powder is perfectly
colourless and odourless. If it should possess any garlic-like
smell it contains phosphorous oxide, and if it has a yellowish or
reddish colour it is mixed with red phosphorus or phosphorus
suboxide. It possesses no action upon dry blue litmus paper,
and is excessively hygroscopic, deliquescing very quickly when
exposed to the air, with formation of metaphosphoric acid :-
-
P₂O+H₂O=2HPO3.
When thrown into water it dissolves with a hissing noise
and with the evolution of a large amount of heat. On heating
with carbon, or other reducing agents, phosphorus is formed :-
P,0+5C=2P+5CO.
Phosphorus pentoxide is often used in the laboratory as a
desiccating agent, especially for the purpose of removing the last
588
THE NON-METALLIC ELEMENTS
traces of moisture from gases or from liquids. Owing to its
power of combining with water it is able to withdraw the
elements of water from many compounds containing oxygen
and hydrogen. Thus it is used for the preparation of nitrogen
pentoxide and other compounds.
In order to prepare larger quantities of the pentoxide the ar-
rangement shown in Fig. 170 is employed. (A) is a large dry
glass balloon, provided with three necks (a, d, and g); the neck (y)
is connected with a powerful water-aspirator by means of which

A
a
FIG. 170.
B
air, dried by passing through the drying tube (f), is drawn into
the balloon; g communicates with a large wide-necked bottle
(B) into which a portion of the light powder is driven by the
current of air; through the neck (a) passes a straight glass tube,
closed with a cork at the top but open at the bottom, reaching
nearly to the centre of the balloon and having a small copper
crucible (c) fixed to its lower end. A piece of phosphorus is
dropped down the straight tube into the crucible and ignited by a
hot wire, a good current of air being kept up until the operation is
complete; a second piece of phosphorus is then dropped down
PHOSPHORUS PENTOXIDE
589
into the crucible (c), and when this is burnt a third piece is
introduced, and so on, until a sufficient quantity of the pentoxide
has been obtained.
A more practically useful arrangement for preparing the pen-
toxide in quantity consists of a cylinder (a, Fig. 171), open at
both ends, made of common sheet-iron, fourteen inches high and
twelve inches in diameter, having a cover provided with a bent
chimney (b), one inch in diameter, closed by a cork. The cylinder
is supported by a wooden tripod, and rests in a sheet-iron

b
C
FIG. 171.
funnel (h), fitting into the neck of a wide-mouthed bottle (g). A
copper spoon (d), fixed to an iron rod, serves to receive the
phosphorus which is from time to time renewed by drawing
back the spoon to the opening (e) and dropping in a fresh
piece. In order to renew the supply of air, which during dry
weather does not require desiccation, the board (i) is occasionally
removed, and air allowed to enter between the funnel and the
cylinder. On account of its being so exceedingly hygroscopic,
1 V. Grabowski, Annalen, 136, 119.
590
THE NON-METALLIC ELEMENTS
the pentoxide thus obtained must be preserved in well-stoppered
bottles, or, better, in hermetically-sealed flasks.
In order to free phosphorus pentoxide from all traces of lower
oxides, it must be distilled over platinum sponge in a current of
oxygen,¹
PHOSPHORIC ACID.
339 The history of this acid possesses a peculiar interest for
the scientific chemist. In the year 1746, Marggraf observed
that, when fusible salt of urine, NH,NaHPO4, is mixed with a
solution of nitrate of silver, a yellow-coloured silver salt is
precipitated. It was afterwards noticed that other salts of
phosphoric acid, as, for instance, the ordinary phosphate of soda,
gave the same reaction; but Clark, in the year 1828, pointed
out that when the salt is heated and then dissolved in water,
the solution so obtained gives with nitrate of silver a white
precipitate. He, therefore, distinguished the acid contained in
the heated salt from that contained in the common phosphate,
and termed the former pyrophosphoric acid. In 1829 Gay-
Lussac proved that the acid thus prepared can be converted
into other salts without losing its peculiar properties; and
Berzelius and Engelhardt had previously found that freshly-
prepared solution of well-ignited phosphoric acid is able to
coagulate clear solutions of albumen, whilst this is not the case
when the solution of the acid has been standing for any con-
siderable length of time. These and similar observations led to
the conclusion that phosphoric acid, with which the pentoxide
was then classed, can exist in several isomeric conditions.
The classical researches of Thomas Graham 2 first threw a
clear light on this subject. He showed, in the first place, that,
in addition to ordinary phosphoric acid and pyrophosphoric
acid, a third modification exists, to which he gave the name of
metaphosphoric acid, and that this is the substance which has
the power of coagulating albumen. He also ascertained that
common phosphates, when they are saturated with a base,
contain three times as much of that base, in proportion to the
same weight of phosphoric acid as the metaphosphates, whilst
the pyrophosphates contain twice as much as the metaphosphates.
Graham likewise proved that, when the acids are liberated from
1 Threlfall, Phil. Mag. 1893; Shenstone and Beck, Journ. Chem. Soc. 1893,
i. 473.
2 Phil. Trans. 1833, ii. 253.
PHOSPHORIC ACID
591
these different salts, they may be regarded as containing different
quantities of water. Hence the composition of the three acids
is as follows:-
Old notation.
Common, or Orthophosphoric acid P.0, +3H,0
P₂O5 + 2H₂O
Pyrophosphoric acid .
Metaphosphoric acid
P₂O₂+ H₂O
2
HO
HO
H
Orthophosphoric Acid.
•
Hypophosphorous Acid.
HO
P-O-OH
The exact constitution of these acids has been the subject
of a considerable amount of discussion. The supporters of the
theory of the constant valency of the elements regarded these
three acids as well as hypophosphorous and phosphorous acids
as derivatives of phosphine PH, and ascribed to them the
following constitutional formulæ :
HO
HO-P = 0
HO
P-OH
·
.
Phosphorous Acid.
HO
P-OH
HO
Pyrophosphoric Acid.
OH-P-O-OH
b
OH-P-O-OH
New notation.
H,PO
HP₂O
HPO3.
Pyrophosphoric Acid.
HO
P = 0
HO
O
HO
P = 0
HO
Metaphosphoric Acid.
HO-P
O
Other chemists however believed that in the phosphoric acids
phosphorus behaves as a pentad, and represented them by the
following formulæ :-
Orthophosphoric Acid.
Metaphosphoric Acid.
0 = P = 0
OH
Since it has been shown that phosphorus forms a stable pen-
tafluoride, the vapour density of which remains constant through
a considerable range of temperature and corresponds to the
molecular formula PF, the pentad nature of phosphorus in
many of its derivatives has been generally admitted, and the
592
THE NON-METALLIC ELEMENTS
second series of formulæ for the phosphoric acids is now almost
universally accepted.
Poisonous Action of the Acids of Phosphorus.-When ad-
ministered in an uncombined condition, the various oxides of
phosphorus produce apparently the same symptoms which
follow the administration of other mineral acids. Sufficient
data do not exist as to the specific physiological action of all
these compounds.
In the case of ortho- meta- and pyrophosphoric acids it would
appear that the first, when in combination with inactive bases,
acts as a perfectly inert body. The second possesses some
activity as a poison, while the pyro-salts when introduced
directly into the blood are found to be very powerful poisons
(Gamgee).
ORTHOPHOSPHORIC ACID, H,PO₁.
340 In order to prepare this acid, red phosphorus is heated in
a retort with common concentrated nitric acid. The phosphorus
is oxidized at the expense of the nitric acid, and red fumes are
slowly evolved. When the phosphorus has dissolved, the residue
is evaporated in a porcelain dish, and the concentrated solution
repeatedly treated with nitric acid, in order to oxidize com-
pletely any phosphorous acid which may have been formed. As
soon as the further addition of nitric acid is unaccompanied by
the evolution of red fumes, the operation is concluded, and the
residue only requires to be evaporated again, in order to get
rid of the excess of nitric acid (v. Schrötter). Common phos-
phorus was formerly employed instead of red phosphorus for
this purpose; but this undergoes oxidation much more slowly
than the red variety, inasmuch as it melts forming round
globules, which are only slowly attacked by the nitric acid. In
addition to this, when common phosphorus is employed, weak
nitric acid can alone be used, as the strong acid is apt to
produce an explosion when brought in contact with it.
The residue obtained in the preparation of hydriodic acid
by means of iodine and phosphorus consists of a mixture
of phosphoric and phosphorous acids, containing a small quan-
tity of hydriodic acid. In order to prepare pure orthophos phoric
acid from this residue, it may be heated with a little fuming
nitric acid, and filtered, in order to separate it from the solid
iodine which is liberated. More nitric acid is then added, in
PHOSPHORIC ACID
593
order to oxidize phosphorous acid, and the liquid is evaporated
to a syrupy consistency.
Orthophosphoric acid is prepared on the large scale from
bone-ash, which consists chiefly of tri-calcium phosphate,
together with a small quantity of magnesium phosphate and
calcium carbonate. According to Liebig, equal weights of
sulphuric acid and bone-ash are taken; the sulphuric acid
is diluted with ten times its weight of water, and is allowed
to remain in contact with the bone ash for some time. The
acid solution is then filtered through linen, and the filtrate
evaporated to a small bulk. On the addition of strong sul-
phuric acid, the calcium, still present in solution, is precipitated
as gypsum. The clear solution is then poured off, evaporated to
dryness, and freed from an excess of sulphuric acid by ignition.
The residue is free from lime and sulphuric acid, but it contains
small quantities of magnesia, which can only with difficulty be
removed.
In order to prepare a pure acid from bone-ash, the powdered
ash is dissolved. in the smallest possible quantity of nitric
acid, and to the clear liquid a solution of acetate of lead
is added. A precipitate of lead phosphate falls down, which
must be warmed for some time with the liquid, in order to free
the precipitate from any calcium phosphate which is thrown
down with the lead salt. It is well washed with boiling water,
and decomposed by sulphuretted hydrogen. According to
Berzelius's method, the lead salt is decomposed by dilute
sulphuric acid, and the excess of acid removed by ignition of
the evaporated filtrate. The residue is then dissolved in water
and freed from traces of lead by sulphuretted hydrogen.
Another method which may be employed is to dissolve the
bone-ash in its own weight of hydrochloric acid of specific
gravity 118, diluted with four times its weight of water. To
this solution one-and-a-half parts of dry sodium sulphate is
added, whereby a precipitate of gypsum is produced; this is
then filtered off, and the boiling solution neutralized with
sodium carbonate; the solution is again filtered, to separate
any calcium carbonate which may fall down, and the whole
precipitated with barium chloride. The precipitate thus formed
consists of a mixture of barium sulphate and barium phosphate
and this is decomposed by one part of sulphuric acid, having a
specific gravity of 1.71 (Neustadt).
A further method is to treat calcium phosphate with hydro-
39
594
THE NON-METALLIC ELEMENTS
fluoric acid in a leaden dish; the excess of acid is then evapor-
ated off and the solution filtered from separated calcium fluoride
and evaporated.¹
Commercial phosphoric acid frequently contains arsenic acid,
derived from the sulphuric acid or hydrochloric acid employed
in its manufacture. In order to free it from arsenic, it must be
dissolved in water, sulphur dioxide led through the warm
solution, in order to reduce the arsenic acid to arsenious acid,
then the solution boiled to remove the excess of sulphur dioxide,
and sulphuretted hydrogen passed through, by which means the
whole of the arsenic is precipitated as the insoluble trisulphide.
341 Phosphoric acid is extremely soluble in water; it has a
pleasant purely acid taste, and is perfectly free from smell.
When the aqueous acid is evaporated down until the residue
possesses the composition, H,PO,, it presents the appearance of
a thick syrup, from which, on standing, a crystalline mass
is deposited. If a crystal of this acid be dropped into a freshly-
prepared solution of the requisite strength, crystals begin to
form at once, and soon spread throughout the mass. These
crystals belong to the rhombic system, forming six-sided prisms
terminated by six-sided pyramids, which melt at 38.6°. The
crystallized acid may be heated to 160° without undergoing any
alteration, but above this temperature it loses water, and at 213°
is completely converted into pyrophosphoric acid, H,P,O,. This
substance, in its turn, loses water when heated to redness, with
formation of metaphosphoric acid, HPO.
The table on the following page gives the variation of the
specific gravity, with the percentage composition of aqueous
solutions of ortho-phosphoric acid :—³
342 The Orthophosphates.-Orthophosphoric acid, being tri-
basic, forms three classes of salts according as one, two, or three
atoms of hydrogen are replaced by their equivalent of metal.
Thus we know three orthophosphates of sodium :-
Trisodium or normal sodium phosphate, Na¸PO,+2H,0.
Hydrogen disodium phosphate, HNаPO̟¸+12H₂O.
Dihydrogen sodium phosphate, H.NaPO,+H₂O.
Of the normal salts those of the alkalis with the exception of
the lithium salt are easily soluble in water and their solutions
2 J. Thomsen, Ber. 7, 997.
1 Nicolas, Compt. Rend. 111, 975.
3 John Watts, Chem. News, 12, 160.
I
THE ORTHOPHOSPHATES
595
Specific
Gravity.
have a strong alkaline reaction. The normal orthophosphates
which are insoluble in water are easily soluble in dilute acids
by which means they are converted into the soluble hydrogen
Per Cent. Specific
P205
Gravity.
1.508
49.60
1.492
48.41
1.476
47.10
1.464
45.63
1.453
45.38
1.442
44.13
1-434
43.95
1.426
43.28
1.418
42.61
1:401
41.60
1:392 40.86
1.384 40.12
1.376
39.66
1.369 39.21
1.356
38.00
1:347
37.37
1.339 36.74
1.328
1.315
1:302
1.293
1.285
1.276
1.268
1.257
1.247
1.236
1.226
1.211
1.197
1.185
1.173
1.162
1.153
Per Cent.
P₂05.
36.15
34.82
33.49
32.71
31.94
31.03
30.13
29.16
28.24
27:30
26.36
24.79
23.23
22:07
20.91
19.73
18.81
Specific
Gravity,
1.144
1.136
1.124
1.113
1.109
1.095
1.081
1:073
1.066
1.056
1.047
1.031
1.022
1.014
1.006
Per Cent.
P.OS.
17.89
16.95
15.64
14.33
13.25
12.18
10.44
9:53
8.62
7:39
6.17
4.15
3:03
1.91
0.79
orthophosphates. These latter salts are readily obtained from
the normal compounds; even carbon dioxide brings about the
change; thus, if this gas be led into a solution of trisodium
phosphate the following reaction takes place :-
―――――
Na3PO + CO2 + H₂O = HNа,PO4 + HNaCO3.
The hydrogen disodium phosphate which is here formed.
together with hydrogen sodium carbonate is the common
phosphate of soda of the shops. This salt, although according
to its constitution it must be considered as an acid salt,
inasmuch as it still contains hydrogen replaceable by a
metal, has a slightly alkaline reaction, and, like other soluble
hydrogen orthophosphates, is easily obtained by adding a solution
of soda to phosphoric acid until the liquid has a weak alkaline
reaction. The hydrogen disodium orthophosphate is converted
on heating, with loss of water, into the pyrophosphate.
The dihydrogen orthophosphates of the alkalis are soluble in
1
596
THE NON-METALLIC ELEMENTS
water, and possess a slight acid reaction. Dihydrogen potassium
phosphate, H,KPO, forms large well defined crystals, and this
salt may be heated to a temperature of 400° without losing
water. At a higher temperature, however, one molecule of
water is driven off and potassium metaphosphate, KPO,
formed.
is
The orthophosphates can readily be recognized by the follow-
ing reactions. The normal as well as the acid salts give with
nitrate of silver a yellow precipitate of silver orthophosphate,
Ag,PO; thus:-
-----
NaPO4 + 3AgNO, = 3NaNO3 + Ag¿PO.
HNa,PO + 3AgNO₁ = 2NaNO3 + HNO¸ + Ag¿PO
H,NaPO4 + 3AgNO, NaNO3 + 2HNO3 + Ag₂PO-
=
In the case of common phosphate of soda and nitrate of silver,
we have the singular fact of two neutral or slightly alkaline
solutions, when mixed, yielding a strongly acid liquid.
When to a solution of an ortho-salt a mixture of sal-ammoniac,
ammonia, and magnesium sulphate solutions is added, a crystal-
line precipitate of ammonium magnesium phosphate,
(NH₁)MgPO4 + 6H₂O,
in a
am-
is thrown down. In order to detect orthophosphoric acid
substance insoluble in water, the body may be dissolved in
nitric acid and an excess of a solution of molybdate of
monia in nitric acid added to the liquid. If phosphoric acid be
present, this solution on slightly warming, or on standing in the
cold, yields a dense yellow precipitate. The composition of
this precipitate is approximately represented by the following
formula:-
14M0O3 + (NH₁)PO + 4H₂O.
4
METAPHOSPHORIC ACID, HPO3.
343 This modification of phosphoric acid was discovered by
Graham in 1833. It is obtained when a solution of phosphoric
acid is heated until the residue does not give off any
water. The acid thus prepared solidifies on cooling to
pasty mass which on exposure to the air readily absorbs
moisture and deliquesces. The glacial phosphoric acid of the
more
soft
a
1
METAPHOSPHORIC ACID
597
shops is metaphosphoric acid which usually contains soda as
impurity.¹ Metaphosphoric acid is also formed when crystalline
phosphorous acid is heated in a sealed tube with bromine
(Gustavson); thus :-
H,PO3 + Br₂ = HPO3 + 2HBr.
When phosphorus pentoxide is allowed to deliquesce in moist
air or when it is dissolved in cold water metaphosphoric acid is
formed. This aqueous solution on standing at the ordinary
temperature of the air, gradually undergoes change with forma-
tion of common phosphoric acid and this conversion takes place
quickly without the formation of the intermediate pyrophos-
phoric acid if the liquid be boiled (Graham). Metaphosphoric
acid is volatile at a bright red heat and when heated with
sulphates expels sulphuric acid from them, for although sul-
phuric acid is a stronger acid than phosphoric acid, the former
is more easily volatile than the latter. An aqueous solution of
metaphosphoric acid is also obtained by passing a current of
sulphuretted hydrogen gas through a liquid containing lead
metaphosphate in suspension.
The solution of metaphosphoric acid is distinguished from
those of the other two modifications inasmuch as it produces
a white precipitate with solutions of calcium chloride, barium
chloride and albumen,
344 The Metaphosphates.-The salts of metaphosphoric acid
are obtained by neutralizing the aqueous solution of the acid by
a base or by heating a dihydrogen orthophosphate; thus:-
KH¿PO₁ = KPO, + H₂O.
3
No less than five distinct modifications of the metaphosphates
are known to exist.2
(1) Monometaphosphates. Of this class only those of the
alkali metals are known, such as KPO,. The monometaphos-
phates are remarkable as being insoluble in water. The potas--
sium salt is formed, as above shown, when dihydrogen
potassium phosphate is heated. The monometaphosphates are
distinguished from the other modifications inasmuch as they do
not form any double salts.
(2) Dimetaphosphates.-These salts are formed when aqueous
phosphoric acid is heated to a temperature of 350° (Fleitmann)
¹ Brescius, Zeit. Analyt. Chem. 6, 187.
2 Maddrell, Mem. Chem. Soc. 3, 373.
598
THE NON-METALLIC ELEMENT
a
or 316° (Maddrell) with the oxides of zinc, manganese, or copper.
If the copper salt be then decomposed by potassium sulphide
soluble potassium dimetaphosphate, KPO, is obtained and if
sodium sulphide be employed a soluble sodium dimetaphosphate
is in like manner produced. In addition to the dimetaphosphates
containing only one metal, double salts such as CuK₂(P¸O、;)½
can be prepared. Only the dimetaphosphates of the alkali
metals are soluble in water and are crystallizable; the other
are insoluble or only very slightly soluble (Fleitmann).
(3) Trimetaphosphates.-The sodium salt, Na,PO,, is obtained
together with the monometaphosphate when microcosmic salt,
(NH)HNaPO4, is gently heated until the fused mass becomes
crystalline (Lindbom). By double decomposition other trimeta
phosphates can be obtained from this salt. These are
all
soluble in water, including the silver salt, and they form double
salts such as NaBaPO,. The silver salt may be obtained in the
form of large transparent monosymmetric crystals by allowing a
mixture of the sodium salt and nitrate of silver solution to stand
for some days. In the same way the crystalline lead salt may
be prepared by the substitution of nitrate (but not of acetate)
of lead for the silver salt.
(4) Tetrametaphosphates.-The lead salt, Pb,P,O,,, is formed
by treating oxide of lead with an excess of phosphoric acid and
heating up to a temperature of 300°. If this is then decom.
posed by sodium sulphide the sodium salt is obtained as
tetrametaphosphate. This, however, is not a crystalline salt
but forms with a small quantity of water a viscid elastic mass
and on the addition of a larger quantity of water a gum-
solution which will not pass through a filter. The tetrameta
phosphates of the alkalis produce viscid precipitates with the
soluble salts of the alkaline earths. If sodium dimetaphosphate
is fused with copper dimetaphosphate and the mixture allowed
gradually to cool a double compound having the composition
Cu Na,P,O12 is formed (Fleitmann and Henneberg).
-like
4
a
is
(5) Hexametaphosphates.-The sodium salt, Na, P¸O₁S
obtained when fused sodium metaphosphate is allowed
cool slowly. It is a crystalline mass, deliquesces on exposure
to the air, and produces with barium chloride a flocculent
precipitate, and with the salts of the heavy metals gelatinous
precipitates.
It also forms characteristic double salts in which fine equiva
lents of the monad metal are replaced by the corresponding
to
PYROPHOSPHORIC ACID
599
amount of a dyad metal, the calcium salt, for example, having
the composition Ca”¿Na₂(PO3)12 or (NaPO¸)¿(Ca″P₂O6)5°
The metaphosphates which are soluble in water have a neutral
or slightly acid reaction. When their solutions are boiled they
are converted into orthophosphates. All the metaphosphates
undergo this change on boiling with nitric acid or when they
are fused with an alkali.
The different varieties of metaphosphates are derived from
acids, all of which possess the same composition, but differ, as
the double salts show, from one another in molecular weight.
Compounds of this description are termed polymeric bodies. If
we assume the constitution formula now usually accepted for
metaphosphoric acid, namely HO—P_O
1!
the constitution of these hypothetical acids may be represented
graphically as follows:-
O OH
0=P-OH
O=P-OH
Dimetaphosphoric Acid.
and
P
O P P=0
HO O OH.
Trimetaphosphoric Acid.
=
PYROPHOSPHORIC ACID, HP,O,.
345 Common phosphate of soda, or hydrogen disodium phos-
phate, HNa,PO,, when heated to a temperature of 240° loses
water, and is converted into sodium pyrophosphate (Graham):
2HNA,PO, H₂O+Na,P₂O,.
2
The salt thus obtained dissolves in water, but does not again
form an orthophosphate, and is distinguished from the original salt
inasmuch as its solutions yield on the addition of silver nitrate, a
white, and not a yellow, precipitate. This fact was first observed
by Clark, of Aberdeen, in the year 1828.¹ It is, however, to
Graham that we are indebted for a knowledge of the fact that
1 Edinburgh Journal of Science, 7, 298.
600
THE NON-METALLIC ELEMENTS
the heated sodium salt as well as the silver salt obtained from
it is derived from an acid having the composition H,PO,, and
that this can be obtained from orthophosphoric acid by heating
it for a considerable length of time to a temperature of 215°.
Pyrophosphoric acid is also formed when equal molecules of
ortho- and metaphosphoric acids are heated together at
100°.1
OH
|| OH
HO-P-O+HO-P
!
=
HO
HO
P-O-P
188
!!
OH
OH
Pyrophosphoric acid forms either a soft glassy mass (Graham)
or an opaque indistinctly crystalline mass (Peligot).
An aqueous solution of pyrophosphoric acid is obtained by
precipitating the sodium salt with a solution of acetate of lead
and decomposing the well-washed lead pyrophosphate with sul-
phuretted hydrogen. The acid solution undergoes no change
at the ordinary temperature, even after standing for some
time, but when heated it is converted into orthophosphoric
acid.
Pyrophosphoric acid may be distinguished from the ortho-
modification inasmuch as its solution produces a white granular
precipitate with silver nitrate, and from the meta-variety inas-
much as it does not produce a precipitate either with a solution
of chloride of barium or with one of albumen.
346 Pyrophosphates.-These salts are prepared from the mono-
hydrogen orthophosphates by heat, or by neutralizing a freshly
prepared solution of the acid by means of a base. Both normal
pyrophosphates, such as NaPO7, and acid or hydrogen pyro-
phosphates, such as H,Na,P₂O, exist; the first have an alkaline
reaction, the second a slightly acid one. The pyrophosphates of
the alkali metals are soluble in water, those of the other metals
insoluble, but many of them dissolve in an excess of sodium
pyrophosphate. The pyrophosphates in solution remain un-
altered in the cold and even on heating do not change, but
when boiled with an acid are decomposed, the orthophosphates
being formed. The same change takes place on fusion with
an alkali.
Tetraphosphates.-A sodium salt having the composition
NaPO13, was obtained by Fleitmann and Henneberg by fusing
1 Geuther, J. Pr. Chem. [2], 8, 359.
ESTIMATION OF PHOSPHORIC ACID
601
sodium hexametaphosphate with pyrophosphate or orthophos-
phate, in quantities represented by the following equations:-
6
2 7
Na PO18 + 3NaP₂O, = 3Nа¸P4013; or
Na PO18+2Na¸PO₁ = 2Nа¸PO,
он
13.
4
The salt thus obtained can be crystallized from solution in warm
water and gives with silver nitrate a white precipitate, AgeP013
which does not dissolve in an excess of the sodium salt. The
constitution of the pyrophosphates, or the diphosphates as they
may be called, and of the tetraphosphates may be exhibited as
follows:-
Diphosphoric Acid.
Tetraphosphoric Acid.
OH
T
PO-OH
PO-OH
PO-OH
PO-OH
|
OH
PO-OH
PO-OH
1
OH
347 Quantitative Determination of Phosphoric Acid.-Phos-
phoric acid is best determined in the soluble phosphates by
adding to the solution a mixture of sal-ammoniac, ammonia and
magnesium sulphate solutions, when a precipitate of ammonium
magnesium phosphate occurs, NH¸MgPО¸ + 6H,O. After this
has stood for some time, the solution is filtered off, and the
precipitate washed with dilute ammonia, dried, converted by
ignition into magnesium pyrophosphate, Mg,P,O,, and weighed.
The insoluble phosphates must be converted into soluble salts
before the determination of the phosphoric acid. If they are
soluble in nitric acid, the method proposed by Sonnenschein
may be adopted; namely, to precipitate the nitric acid solution
of the phosphate by an excess of ammonium molybdate dis-
solved in nitric acid. On standing for some time at a moderate
temperature, a yellow precipitate, containing all the phosphoric
acid, is formed; the excess of molybdenum salt is removed by
washing with water, the precipitate dissolved in ammonia, and
the phosphoric acid precipitated by magnesium sulphate as
described above.
602
THE NON-METALLIC ELEMENTS
If the phosphates will dissolve in acetic acid the phosphoric
acid may be precipitated with uranium acetate as uranium
phosphate. This method may also be employed for the volu-
metric determination of phosphoric acid, the point of complete
precipitation of the phosphoric acid being ascertained by the
addition of a drop of the solution to a drop of a solution of ferro-
cyanide of potassium with which the slightest excess of uranium
acetate produces a brown colour.
Metaphosphates and pyrophosphates must first be converted
into orthophosphates before precipitation. In like manner
phosphorus itself as well as the hypophosphites and phosphites
may also be quantitatively determined in the form of phosphoric
acid by previously oxidizing them with nitric acid.
HALOGEN DERIVATIVES OF PHOSPHORIC ACID.
348 The hydroxyl groups contained in the three modifications of
phosphoric acid may be replaced, as is the case with other
hydroxyacids, by chlorine or bromine thus giving rise to the oxy-
chlorides and oxybromides of phosphorus, as they are commonly
termed.
PHOSPHORUS OXYFLUORIDE, POF₁ = 1034,
was first obtained by Moissan 1 by exploding a mixture of
phosphorus trifluoride and oxygen and is also prepared by
heating a mixture of two parts of cryolite and three parts of
phosphorus pentoxide in a brass tube,2 or by dropping phosphorus
oxychloride on to dried zinc fluoride, or by the action of
anhydrous hydrogen fluoride on phosphorus pentoxide.* It is
a gas which fumes in the air and condenses to a liquid at-50°
or at 16° under a pressure of 15 atmospheres; it solidifies at a
very low temperature.
PHOSPHORUS OXYCHLORIDE OR PHOSPHORYL CHLORIDE,
POCI¸. = 152·25.
349 This compound was discovered in the year 1847 by
Wurtz, who obtained it by decomposing the pentachloride
with the requisite quantity of water:
PC15+ H2O = POCI¸ + 2HCl.
1 Compt. Rend. 102, 1245.
2 Thorpe and Hambly, Journ. Chem. Soc. 1889, i. 759.
3 Bull. Soc. Chim. (3), 4, 260.
5 Ann. Chim. Phys. [3] 20, 472.
4 Bull. Soc. Chim. (3), 5, 458.
PHOSPHORUS OXYCHLORIDE
603
The best method of preparing this substance is the one pro-
posed by Gerhardt; namely, by heating dried oxalic acid with
phosphorus pentachloride, when the following reaction takes
place:-
PC15 + H₂C₂O₁ = POC] + 2HCl + CO₂ + CO.
2
Instead of oxalic acid, boric acid may be employed, thus:-
3PC15 + 2B(OH), = 3POCI¸ + B2O3 + 6HCl.
Pure phosphorus oxychloride may also be easily obtained by
heating the pentachloride and the pentoxide together in sealed
tubes in the proportion of 3 molecules of the former to one
molecule of the latter: 2
3PC1, + P₂O5 = 5POCI¸.
The same compound is also obtained by distilling phosphorus
pentoxide with common salt:
3
2P,O,+3NaCl = POCI, +3NaPO3.
5
Phosphorus oxychloride is also formed as a product of decom-
position in the preparation of many acid chlorides. Several ex-
amples of this mode of formation have already been mentioned
and many more will have to be described.
Phosphorus oxychloride is a colourless mobile liquid boiling
at 107°2 and having a specific gravity of 1.7118 at 0° (Thorpe).
When strongly cooled it solidifies in the form of tabular or
needle-shaped crystals which melt at -1°5 (Geuther and
Michaelis). It fumes strongly in the air and has a very pene-
trating and acrid smell resembling that of phosphorus trichloride.
According to Cahours the specific gravity of the vapour is 5:334.
The oxychloride when thrown into water sinks, and slowly
dissolves with the formation of phosphoric and hydrochloric
acids:-
:-
POCI, + 3H2O = PO(OH)3 + 3HCI.
When brought in contact with many metallic chlorides it forms
crystalline double compounds.
1 Ann. Chim. Phys. [3] 44, 102.
2 Gerhardt and Chiozza, Annalen, 87, 290.
Kolbe and Lautemann, Annalen, 113, 240.
604
THE NON-METALLIC ELEMENTS
PYROPHOSPHORYL CHLORIDE, POC.
350 When the four hydroxyl-groups in pyrophosphoric acid,
HP₂O, are replaced by chlorine, pyrophosphoryl chloride is
formed. It was first prepared by Geuther and Michaelis,¹ by
the action of nitrogen peroxide on phosphorus trichloride. The
reaction which takes place in this case is a somewhat compli-
cated one, nitrogen, nitrosyl chloride, phosphoryl chloride, and
phosphorus pentoxide being formed.
Pyrophosphoryl chloride is a colourless strongly fuming liquid
boiling between 210° and 215° and then undergoing partial
decomposition into pentoxide and common oxychloride :
3P₂OC, P₂O+4POCI.
5
At 7° the specific gravity of the liquid is 178, it decomposes
violently in contact with water without sinking in it, and in
this decomposition orthophosphoric acid and not pyrophosphoric
acid is formed. When it is treated with pentachloride of
phosphorus, phosphoryl chloride is produced:
P₂O₂C₁₁ + PC15 = 3POCI¸.
=
By the action of phosphorus pentoxide on phosphorus oxy-
chloride, Gustavson 2 obtained a syrupy mass which he regarded
as metaphosphoryl chloride PO,Cl. Hamblys has however
shown that this is a mixture of at least two compounds, one of
which is pyrophosphoryl chloride; the other constituent has a
constant composition but cannot from the analysis have a simpler
formula than PO₁Cl, and is probably itself a mixture.
15
PHOSPHORUS OXYBROMIDE, OR PHOSPHORYL BROMIDE.
POBr, 28476.
=
351 This body is formed by the action of a small quantity of
water upon the pentabromide, but it is best prepared by distil-
ling pentabromide of phosphorus with oxalic acid (Baudrimont).
It forms a mass of flat tabular crystals which have a specific
gravity of 2-822 (Ritter), melt at 46° and boil at 195°. Water
decomposes the oxybromide into phosphoric and hydrobromic
acids.
¹ Ber. 4, 766.
2 Ber. 4, 853.
3 Journ. Chem. Soc. 1891, i. 202.
SULPHIDES OF PHOSPHORUS
605
PHOSPHORYL BROMOCHLORIDE. POBrCl. = 196 42.
When phosphorus trichloride is allowed to fall drop by
drop into absolute alcohol, C,H,(OH), the first action that takes
place is the formation of the compound (OC₂H5)PCI. This body
is decomposed by the addition of bromine into ethyl bromide,
CH,Br, and phosphorus oxybromochloride,. POBrCl₂. This
latter compound is a highly refractive liquid, boiling at 136° and
having at 0° a specific gravity of 2049 (Menschutkin). When
the liquid is cooled it solidifies in the form of tabular crystals
melting at 11°, and probably isomorphous with the crystals of
phosphoryl chloride and phosphoryl bromide (Geuther and
Michaelis).
1
PHOSPHORUS AND SULPHUR.
352 The compounds formed by these two elements were
formerly divided into two classes according as they contained a
greater number of atoms of phosphorus or sulphur. Two com-
pounds of the former class have been described, namely, sulphur
tetraphosphide SP, and sulphur diphosphide SP,, but later
researches have shown that these substances are in reality
simply solutions of sulphur in phosphorus, for no heat is
evolved in their formation from the elements, and a current
of an indifferent gas removes the whole of the phosphorus at
temperatures below its boiling point.
The sulphides of phosphorus are solid bodies formed when
phosphorus and sulphur are gently heated together, the reaction
being accompanied by the evolution of heat. If common
phosphorus is employed for this purpose violent explosions
may occur, and hence it is advisable to
employ red
phosphorus. Even in the latter case, when finely divided
sulphur, such as flowers of sulphur, is employed, the reaction is
often very violent, and for this reason the sulphides of phos-
phorus are best prepared by mixing the necessary quantity of
red phosphorus with small lumps of roll sulphur. The
mixture is effected in a flask, the cork loosely placed in, and the
flask then heated on a sand bath by means of a Bunsen flame,
1 Schultze, J. Pr. Chem. [2] 22, 113; Ber. 16, 2066: Isambert, Compt.
Rend. 96, 1499, 1628, 1721.
606
THE NON-METALLIC ELEMENTS
until the reaction begins, when the flame is removed. After
the flask has cooled it may be broken to obtain the solid mass
which is then preserved in dry well-closed bottles (Kekulé).
TETRAPHOSPHORUS TRISULPHIDE, PS3.
This compound is obtained as a yellow mass, crystallizing
from solution in carbon bisulphide or phosphorus trichloride in
the form of rhombic prisms. It melts at 166° (Ramme), forming
a reddish liquid, which boils about 380° (Isambert). When
heated in a current of carbon dioxide it sublimes at 260° and
condenses to form crystals, which appear to belong to the regular
system. The specific gravity of its vapour is 7.90.
It is very
easily inflammable and is slowly decomposed in contact with
boiling water with formation of sulphuretted hydrogen, phos-
phuretted hydrogen, and phosphorous acid :—
P₁S + 9H₂O3SH₂ + 3P(OH), + PH3.
=
TETRAPHOSPHORUS HEXASULPHIDE, PS,
Is obtained according to Isambert¹ by heating the calculated
quantities of the elements in an atmosphere of carbon dioxide.
It crystallizes in thin needles, and boils at 490°, the vapour
density corresponding to the above formula.
TRIPHOSPHORUS HEXASULPHIDE, P.Se
This is obtained in crystalline needles, melting from 296-
298°, by heating yellow phosphorus and sulphur with carbon
bisulphide for some hours to 210°. The above molecular
formula was established by a vapour density determination
(Ramme).2
PHOSPHORUS PENTASULPHIDE, P₂S5.
353 In order to prepare this compound in the pure state, the
sulphide prepared by Kekule's method is distilled in a current of
carbon dioxide. A pale yellow crystalline mass is thus obtained,
and frequently distinct crystals.3 When a mixture of yellow
2 Ber. 12, 940, 1350.
1 Compt. Rend. 102, 1386.
3 Carl Meyer and V. Meyer, Ber. 12, 610.
THIOPHOSPHORIC ACID
607
phosphorus and sulphur in the proper proportions is heated with
carbon bisulphide to 210°, fine, pale, yellow crystals separate
out (Ramme). Phosphorus pentasulphide melts at 274—276°,
boils at 518° (Goldschmidt), and yields a brown vapour having a
specific gravity of 7·67 (C. and V. Meyer). Water decomposes
this substance as follows:-
P₂S + 8H₂O = 2PO(OH)3 + 5H₂S.
Phosphorus pentasulphide is often used in making organic
preparations for the purpose of replacing oxygen in organic
compounds by sulphur. Thus, for instance, if common alcohol,
CH,OH, be heated with this body, thioalcohol or mercaptan,
C₂H,SH, is formed.
PHOSPHORUS SULPHOXIDE, POS
Is obtained by carefully heating phosphorous oxide with the
calculated quantity of sulphur to 150-170°, the product when
heated in vacuo subliming in colourless strongly refractive crystals.
It melts at about 102°, and boils at 295°, yielding a vapour
whose density corresponds to the formula POS. It rapidly
deliquesces in the air and is quickly dissolved by water, form-
ing sulphuretted hydrogen and metaphosphoric acid, which is
eventually converted into orthophosphoric acid :—
POS₁ + 6H₂O = 4HPO¸ + 4H₂S.
6 4
MONOTHIOPHOSPHORIC ACID, H₂PSO..
354 This substance may be regarded as phosphoric acid in
which one atom of sulphur replaces one atom of oxygen. It is
however, not known in the free state. The sodium salt, Na, PSO,,
is produced when the chloride (described below) is heated with
caustic soda. The salt forms distinct crystals, which have an
alkaline reaction, and are decomposed by the weakest acids, the
thiophosphoric acid which is thus liberated being at once
decomposed into sulphuretted hydrogen and phosphoric acid ;
thus:-
PS(OH)2 + H2O = PO(OH); + SH2.
Dithiophosphoric acid, H,PS,O,, and trithiophosphoric acid,
HPS,O, are also known in their salts.¹
1 Kubierschky, J. Pr. Chem. [2], 31, 93.
608
THE NON-METALLIC ELEMENTS
THIOPHOSPHORYL FLUORIDE, PSF.
355 This compound is obtained by the action of thiophos-
phoryl chloride on arsenic fluoride, but is best prepared by
heating a mixture of carefully dried lead fluoride and phos-
phorus pentasulphide to 170-250°, the following reaction taking
place :-
-
P₂S + 3PbF=3PbS + 2PSF.
At the ordinary temperature it is a transparent colourless
gas, which condenses to a liquid under a pressure of 10-11
atmospheres. It does not attack dry glass, but is spontaneously
inflammable in air or oxygen, and is decomposed by heat or the
electric spark with separation of sulphur and phosphorus and
formation of phosphorus fluorides. The products obtained by
the action of oxygen consist of phosphorus pentafluoride, phos-
phorus pentoxide and sulphur dioxide, and with water, of
sulphuretted hydrogen, phosphoric and hydrofluoric acids.¹
THIOPHOSPHORYL CHLORIDE, PSCI.
This compound, which corresponds to oxychloride of phos-
phorus is best obtained by acting upon pentachloride of phos-
phorus with phosphorus pentasulphide (Weber):
3PC15 + P₂S5=5PSC13.
It is a colourless, mobile, very refractive liquid, which fumes
strongly in the air, possesses a powerful pungent odour, boils at
125°, and has a specific gravity at 0° of 1·16816 (Thorpe). The
specific gravity of the vapour according to Cahours is 5878 at
298°. It is decomposed by water into hydrochloric acid and
thiophosphoric acid, which is then further decomposed as
described above.
THIOPHOSPHORYL BROMIDE, PSBг.
According to Baudrimont this body is obtained by distilling
the tribromide of phosphorus with flowers of sulphur. It is
however best prepared by dissolving equal parts of sulphur and
1 Thorpe and Rodger, Journ. Chem. Soc. 1889, i. 306.
PHOSPHOROUS DIAMIDE
609
phosphorus in carbon bisulphide, and adding gradually to the
well-cooled liquid eight parts of bromine. On distillation the
carbon bisulphide first comes over and then the thio-bromide.
This is purified by shaking it up several times with cold water;
a hydrate is formed in this way having the composition
PSBr,+H,O, which on being warmed to 35° separates into
its constituents. The anhydrous compound is obtained by the
evaporation of the solution in carbon bisulphide as a yellow
liquid, which, when touched by a solid body, at once solidifies
to a crystalline mass. This compound can also be obtained
in the crystalline state from solution in phosphorus tri-
bromide, when it separates out in regular octahedra, having
a yellow colour and melting at 38°. When heated with water
it is slowly decomposed, and on distillation is resolved partly
into sulphur and into a compound, PSBr,PBrg, which boils at
205°1
PYROPHOSPHORYL THIOBROMIDE, P₂SBг.
356 This compound, the sulphur analogue of pyrophosphoryl
chloride, is obtained by pouring carbon bisulphide over phos-
phorus trisulphide and adding to it, drop by drop, a solution
of bromine in carbon bisulphide. It forms a light yellow
oily liquid which fumes in the air, has an aromatic and
pungent smell, and on heating decomposes into the ortho-
compound and phosphorus pentasulphide; thus:-
3P,S,Br₁ = 4PSBг3 + P₂S
PHOSPHORUS AND NITROGEN.
357 No compound of phosphorus and nitrogen is known, but
derivatives of phosphorous and phosphoric acids containing this
element have been prepared.
Phosphorous diamide HO.P(NH2), is obtained by the action of
ammonia on a solution of phosphorous oxide in ether or benzene.
It is a white powder which dissolves in water instantly, with
such violence that it becomes incandescent. When treated with
moderately dilute hydrochloric acid, a violent reaction occurs
with liberation of non-spontaneously inflammable phosphine,
2 Michaelis, loc. cit.
1 Michaelis, Annalen, 104, 9.
40
610
THE NON-METALLIC ELEMENTS
separation of free phosphorus, and formation of a solution of
ammonium chloride, phosphorous and phosphoric acids.¹
Amidophosphoric acid or Phosphamidic acid, NH₂PO(OH),
The potassium salt of this acid is obtained by the action of
potash on the corresponding phenyl salt. The free acid, which
is prepared by decomposing the silver salt with sulphuretted
hydrogen and adding alcohol to the filtered solution, crystallises
in colourless microscopic crystals, which have a sweet taste, and
are not hydrolysed by caustic alkalis. It forms both normal
and acid salts.2
Diamidophosphoric acid, (NH),PO.OH, is obtained in a
similar manner to the foregoing compound. It is a crystalline
substance and is converted by nitrous acid, first into monamido-
phosphoric acid and then into orthophosphoric acid. In addi-
tion to the normal silver salt, (NH),PO.OAg, it forms a
remarkable compound of the formula (NHAg),P(OAg), derived
from the unknown acid (NH2), P(OH)3.
358 When dry ammonia is passed over phosphorus penta-
chloride as long as it is absorbed, a white mass is obtained
consisting probably of a mixture of the compound PCl¸(NH2)2
and NH Cl. No means of separating these are known, but if
the product be treated with water ammonium chloride dissolves,
and the chloramido-compound is decomposed with loss of hydro-
chloric acid, and is converted into phosphamide PO(NH)NH.
This remains behind as a white insoluble powder, which is
slowly converted by the boiling liquid into acid ammonium
phosphate:-
PO(NH)NH,+ 3H,O = PO(OH)(ONH,),.
6
Phospham, PH,N (?). If the product of the reaction of
ammonia and phosphorus pentachloride is heated in absence of
air until no further fumes of ammonium chloride are evolved, a
light white powder having the above composition remains behind
to which the name phospham has been given. It is insoluble
in water and does not melt at a red heat, but oxidises on heating
in the air with evolution of white fumes. If it be moistened
with water and then heated, metaphosphoric acid and ammonia
are formed:—
P₂H₂N + 9H₂O = 3P0¸H + 6NH¸.
6
1 Thorpe and Tutton, Journ. Chem. Soc. 1891, i., 1027.
3 Stokes, Ber. 27,
2 Stokes, Amer. Chem. Journ. 15, 198.
4 Liebig and Wöhler, Annalen, 11, 146; Hofmann, Ber. 17, 1909.
565.
IMIDODIPHOSPHORIC ACIDS
611
When fused with potash it decomposes with evolution of light
and heat :-
PH₂N + 9KOH + 3H20=3PO(OK)3 + 6NH¸.
It's exact molecular weight has not yet been ascertained, but from
analogy with the corresponding phosphorus chloronitride it has
probably the molecular formula given above.
Phosphoryl nitride, PON, is obtained by heating phosphamide
to redness in absence of air, or by subjecting the product of the
reaction of ammonia and phosphorus oxychloride at 0° to the
same treatment. It forms a white amorphous powder, which
melts at a red heat, and resolidifies to a glassy mass. It is not
acted upon by nitric acid, but when fused with caustic alkalis is
converted into ammonia and potassium orthophosphate.
PON + 3KOH= PO(OK), + NH3.
IMIDODIPHOSPHORIC ACIDS.
359 The action of ammonia on phosphorus oxychloride was
investigated many years ago by Gladstone ¹ and by Schiff. The
former succeeded in obtaining from the product a number of
acids, which he regarded as pyrophosphaminic acids, or acids
derived from pyrophosphoric acid by the replacement of hydroxyl
by the group NH. The later investigations of Mente³ have
however shown, that these acids are in reality formed from
diphosphoric acid by the replacement of one or more oxygen
atoms by the imido group NH and they may therefore be
termed imidodiphosphoric acids.
The constitution of these acids is probably as follows:-
Imidodiphosphoric acid, HO.PO
Diimidodiphosphoric acid, HO.PO
NH
O
NH,
NH
PO.OH,
PO.OH,
NH
NH
Diimidodiphosphaminic acid, HO.PO
PO.NH
1 Annalen, 76, 74; Journ. Chem. Soc. 1850, 121; 1851, 135, 353; 1864,
215; 1866, 290; 1868, 64, 261; 1869, 15.
2 Annalen, 101, 299; 102, 113; 103, 169.
3 Annalen, 248, 232.
612
THE NON-METALLIC ELEMENTS
In addition to these Mente has also prepared nitrilotrimeta-
OH
PO
phosphoric acid, which has the constitution N
-PO:
O
OH
Phosphoryl triamide, PO(NH), has been described by Schiff,
but later investigators have failed to obtain it, his compound being
probably impure diimidodiphosphaminic acid.
PO
Phosphorus chloronitride, PNC. This substance was first
obtained by Liebig and Wöhler among the products of the
reaction of ammonia and phosphorus pentachloride, and has
been more closely investigated by Gladstone¹ and Wichelhaus.2
It is best prepared by subliming a mixture of one part of
phosphorus pentachloride and two parts of dried ammonium
chloride, purifying the sublimate by washing with water and
distilling in a current of steam. It crystallises in thin trans-
parent six-sided rhombic tablets, melts at 114° and boils at 250°;
its vapour density is 12:05, corresponding to the formula given
above. It is insoluble in water, but is slowly decomposed by it
with formation of diimidodiphosphoric acid.
The substance PNCI, obtained by Besson by heating the
additive compound PCl5, 8NH3, is probably identical with this
substance. Besson has obtained phosphorus bromonitride in a
similar manner; it forms refractive apparently rhombohedral
crystals, melts at 188-190°, and begins to sublime at 150° in
vacuo.3
1 Journ. Chem. Soc. 1869, 15.
3 Compt. Rend. 114, 1264, 1479.
ARSENIC. As = 74.4.
360 The yellow and red sulphides of arsenic, now termed
orpiment and realgar, were known to the ancients, although they
did not distinguish between them. Aristotle gave to them the
name of oavdapáxn, which was also applied to cinnabar and red
lead, and Theophrastus mentioned them under the name of
ἀρσενικόν.
White arsenic or arsenious oxide, As,O,, is first distinctly
2 Ber. 3, 163.
ARSENIC
613
mentioned in the writings of the Greek alchemist Olympiodorus,
who describes it under the name of white alum, and gives a
recipe for its preparation from the sulphide by roasting in the
air. The later alchemists were all acquainted with these sub-
stances. Thus, for instance, in the works attributed to Basil
Valentine they are described as follows: "In its colour the
arsenicum is white, yellow, and red; it is sublimed by itself
without any addition, and also with addition according to
manifold methods."
The alchemists made use of arsenic especially for the purpose
of colouring copper white (see p. 7). The change thus brought
about was believed to be the beginning of a transmutation
although Albertus Magnus was aware that on strongly heating
the alloy, the arsenic is volatilized. He describes this fact in his
work "De rebus metallicis" as follows: "Arsenicum aeri con-
junctum penetrat in ipsum, et convertit in candorem; si tamen
diu stet in igne, aes exspirabit arsenicum, et tunc redit pristinus
color cupri, sicut de facile probatur in alchymicis."
Free arsenic was known to the Greek alchemists, who obtained
it as a sublimate capable of turning copper white, and hence
looked upon it as a kind of mercury.2
That a metal-like substance is contained in white arsenic was
probably known to Geber, but Albertus Magnus was the first to
state this distinctly: "Arsenicum fit metallinum fundendo
cum duabus partibus saponis et una arsenici." Metallic arsenic
was considered by the later alchemists and chemists to be a
bastard or semi-metal, and was frequently termed arsenicum rex.
It was, however, Brandt who in the year 1773 first showed that
white arsenic is a calx of this substance. After the overthrow
of the phlogistic theory the views concerning the composition of
white arsenic were those which are now held, namely, that it is
an oxide of the elementary substance.
Arsenic occurs in the free state in nature, usually in mam-
millated or kidney-shaped masses, which readily split up into
laminæ. Occasionally, however, native arsenic is met with in
distinct crystals. It occurs in large quantity at Andreasberg in
the Harz, in Joachimsthal in Bohemia, at Freiberg in Saxony,
at Zimeoff in Siberia, in Borneo, and at Newhaven in the United
States.
Arsenic occurs much more commonly in a state of combination
in many ores and minerals, of which the following are the most
1 Berthelot, Introduction à l'Élude, &c., p. 68.
2 Ibid. p. 99.
614
THE NON-METALLIC ELEMENTS
important: arsenical iron, FeAs,; tin white cobalt (CoNiFe)As₂ ;
arsenical nickel, NiAs; arsenical pyrites or mispickel, Fe„S¿As;
realgar, As,S2, and orpiment, As₂S. Less frequently we find
white arsenic or arsenious oxide as arsenite, AsО¸, and several
salts of arsenic acid, such as
Pharmacolite (HCaAsO4)2 + 5H₂O,
Cobalt bloom Co₂(AsO4)2 + 8H₂O,
Mimetesite 2Pb(AsO4)2 + Pb₂(PO)CI.
Small quantities of arsenic also occur in many other minerals.
Thus, for instance, it is contained in almost all specimens of iron
pyrites so that it is often found in the sulphuric acid which is
manufactured from pyrites, and in the various preparations for
which this acid serves. Especially remarkable is the occurrence
of arsenic in almost all mineral waters, in which of course it is
only contained in traces (Will, Fresenius). Similarly it has been
detected in sea-water.
361 Preparation.-The arsenic occurring in commerce is either
the natural product, which is never quite pure but contains iron
and other metals mixed with it, or, more generally, that obtained
by heating arsenical pyrites in earthenware tubes in a furnace.
These tubes are 1 metre in length and about 32 cm. wide. In
the open end of this earthenware tube another is placed, made
of sheet iron, about 20 cm. long, so that half of the iron tube
is inside the earthenware one. The arsenic sublimes into the
iron tube, from which it is obtained by unrolling the sheet iron.
Prepared by this process, arsenic forms a compact, brittle,
crystalline mass having a strong metallic lustre. The decom-
position which takes place is represented by the equation :-
Fe,SAs = 2FeS + As.
In Silesia it is prepared from arsenious oxide, which is heated
with charcoal in an earthen crucible covered with a conical iron
cap, into which the arsenic sublimes.
In order to purify the commercial arsenic it is sublimed with
the addition of a small quantity of powdered charcoal. On the
small scale, arsenic may be purified by introducing the mixture
into a glass flask which is then placed in a large crucible, sur-
rounded by sand and heated to redness. As soon as the
sublimation begins, a loosely-fitting stopper of chalk is placed in
the neck of the flask, a second crucible is placed over the first,
and the whole heated until the arsenic is sublimed into the
PROPERTIES OF ARSENIC
615
upper portion of the flask. In this way rhombohedral crystals
of arsenic are obtained, which have a bright metallic lustre and
are isomorphous with tellurium and antimony (Mitscherlich).
Properties.-Arsenic has a steel-grey colour. Its specific
gravity at 14° is 5727, its specific heat 0083 (Wüllner and
Bettendorf), and it is a good conductor of electricity. If pure
arsenic is quickly sublimed in a stream of hydrogen gas it is
deposited in the neighbourhood of the heated portion of the
tube in crystals, but, at a little distance, as a black glittering
mass, and, still further on, as a yellow powder. The last
two modifications of arsenic are both amorphous; the first
has a specific gravity of 4.713, the last of 3.70.¹ When heated
to 360° they are both transformed into the crystalline variety.
According to Berthelot and Engel amorphous and crystalline
arsenic evolve the same amount of heat on oxidation.2
It was formerly supposed that arsenic could not be melted,
for when heated under ordinary circumstances to about 450° it
passes at once from the solid to the gaseous state. Landolt³
has, however, shown that under an increased pressure it melts
at 500°. On cooling, the fused mass forms a dense crystalline
solid which has, at 19°, a specific gravity of 5.709. Its melting
point lies between those of antimony and silver (Mallet).
The vapour of arsenic is of a lemon yellow colour and smells
disagreeably of garlic. It is, however, still uncertain whether
this smell is due to the element itself, or to a low oxide which
has not yet been isolated. The specific gravity of the vapour at
860° was found by Deville and Troost to be 10-2, whilst Meyer
and Biltz obtained the values 5'45 at 1714°, and 5:37 at 1736°.
The molecule of arsenic at these high temperatures therefore
consists of two atoms, whilst at 860° the vapour density points
to the presence of four atoms in the molecule.
Arsenic oxidizes somewhat rapidly in moist air at the ordinary
temperature, becoming covered with a blackish-grey 'coating.
Heated in oxygen it burns with a bright white flame, forming
arsenious oxide, which is also produced when arsenic is heated in
the air; at the same time the alliaceous smell of its vapour is
perceived. In the act of combination with oxygen, 1 gram of
arsenic evolves, according to Thomsen, 1031 thermal units,
whilst by its combination with chlorine, in which gas finely
powdered arsenic takes fire spontaneously, it evolves 953 thermal
1 Geuther, Annalen, 240, 208.
3 Jahrbuch f. Min. 1859, 733.
2 Compt. Rend. 110, 498.
4 Ber. 22, 725.
616
THE NON-METALLIC ELEMENTS
units. It is easily oxidized by nitric acid, and also by con-
centrated sulphuric acid with evolution of sulphur dioxide. It
combines with various non-metals, and with most of the metals.
It stands in such close proximity to the latter class of elements,
especially in its physical properties, such as lustre, specific
gravity, &c., that some chemists have placed it amongst them.
Metallic arsenic is chiefly used for the purpose of hardening lead
in the manufacture of shot.
6
=
The atomic weight of arsenic was first determined by Berzelius,¹
who heated 2.203 grm. of As40 with sulphur, and obtained
1069 of SO,, hence As 7444. By the analysis of the tri-
chloride Dumas 2 obtained As = 7439, whilst Kessler by oxi-
dizing As,O to As,O, found As=747. The most accurate
value of the atomic weight is probably 744.
6
ARSENIC AND HYDROGEN.
These two elements unite to form a gaseous compound, AsH,,
and a solid compound, As,H.
HYDROGEN ARSENIDE, OR ARSINE, ASH,
77.4.
362 The existence of this gas was first noticed by Scheele in
1775 on treating a solution of arsenic acid with zinc. He found
that the gas thus evolved deposited white arsenic on burning, and
explained this as being due to the fact that the inflammable air
had dissolved some arsenic. Proust showed in 1799 that the
same gas is given off when arsenious acid and dilute sulphuric
acid are brought together in the presence of zinc, and also when
sulphuric acid is allowed to act on arsenical metals. The gas
which is thus given off is a mixture of hydrogen and arsine.
In order to prepare hydrogen arsenide in the pure state,
zinc arsenide, As,Zn,, must be decomposed by dilute sulphuric
acid, thus:-
-
As₂Zn + 3H2SO4 = 2AsH¸ + 3ZnSO4.
Zinc arsenide is obtained by heating zinc and arsenic together
in a closed crucible, when heat enough is evolved to melt the
The greatest care must be taken in the preparation of
this gas, as it is extremely poisonous, a quantity no larger than
mass.
1 Pogg. Ann. 8, 1. 2 Annalen, 113, 29. 3 Pogg. Ann. 95, 204; 113, 134.
+ Soubeiran, Ann. Chim. Phys. [2], 23, 307; 43, 407.
ARSINE
617
one bubble having been known to produce fatal effects. Gehlen
lost his life in this way in the year 1815.
Arseniuretted hydrogen, as the gas was formerly called, is
also formed by the electrolysis of arsenious and arsenic acid
(Bloxam) and in small quantity when arsenic is boiled with
water.¹
363 Properties.-Arseniuretted hydrogen has a very peculiar
and disagreeable smell, and, according to Dumas, possesses a
specific gravity of 2.695. It liquefies at -40° (Stromeyer), but
it does not solidify when cooled to a temperature of —110°.
Arsine is formed from its elements with absorption of heat
(p. 232), and like many other compounds of the same class can be
made to undergo explosive decomposition into its elements when
exposed to the shock produced by the detonation of fulminate of
mercury, although it does not explode when heated. It burns.
with a pale bluish flame, emitting dense clouds of arsenious oxide.
If a cold piece of white porcelain be held in the flame, metallic
arsenic is deposited as a brown or black shining mirror, and
when the gas is passed through a glass tube which is heated in
one or in several places by means of a gas flame, the arsenic is
deposited near the heated portions of the tube in the form of a
bright shining mirror, the hydrogen being liberated. This
decomposition takes place at 230°.3
Whenever hydrogen is liberated by means of an acid from any
liquid containing arsenic in solution, traces of arseniuretted
hydrogen are evolved. This may be easily detected either by
the smell or by the above-mentioned reactions, which are of
such delicacy that 0.01 mgrm. (7 of a grain) can with
certainty be recognized (Otto).
Hydrogen is evolved during the growth of moulds and of cer-
tain fungi, and it is possible that if arsenic compounds are present
where such growths are going on, arseniuretted hydrogen may be
evolved. This may, perhaps, explain the evil effects noticed
when arsenical wall papers are employed. At the same time
it must be remembered that in these cases arsenic doubtless
finds its way into the system in the form of dust, which in such
rooms invariably contains it.
When arseniuretted hydrogen is led over heated oxide of
copper, water and copper arsenide are formed. In this way
2 Berthelot, Compt. Rend. 93, 613.
¹ Cross and Higgin, Ber. 16, 1198.
3 Brunn, Ber. 22, 3205. * Selmi, Ber. 7, 1642; Giglioli, Ber. 14, 2295.
5 Fleck, Dingl. Polyt. Journ. 207, 146.
618
THE NON-METALLIC ELEMENTS
arseniuretted hydrogen ean easily be determined quantitatively.
When metals such as tin, potassium, or sodium are heated in
the gas the arsenides of the metals are formed, free hydrogen,
which occupies 1 times the volume of the original arseniu-
retted hydrogen, being generated. When potassium and sodium
are heated in the gas, the compounds AsK, and AsNa, are
formed. When decomposed by dilute acids these substances
yield arseniuretted hydrogen which is purer than that obtained
from zinc arsenide (Janowsky).
Arseniuretted hydrogen when passed into a dilute solution
of a gold or silver salt precipitates the metal, the arsenic
entering into solution in the form of arsenious acid:-
2ASH, +12AgNO3 + 6H₂O = 2H¸AsO3+12HNO3 + 12Ag.
3
A very small quantity of arseniuretted hydrogen can, in this
way, be detected by the precipitation of finely divided silver
'from the clear solution, the liquid becoming acid at the same
time. When on the other hand, arsine is passed into a strong
solution of silver nitrate, a yellow coloration is produced which
is due to the formation of a double salt, As,Ag,3AgNO3:-
AsH + 6AgNO3 = AsAg,3AgNO3 + 3HNO¸.
3
―――
On the addition of water this substance is decomposed with
formation of arsenious acid and nitric acid, metallic silver being
precipitated.¹
One volume of water absorbs about five volumes of the gas,
and the solution on exposure to the air deposits arsenic.
Chlorine decomposes arseniuretted hydrogen with great violence,
bromine and iodine also act upon it less energetically. Sulphu-
retted hydrogen does not act upon the pure gas, but in the
presence of oxygen or at a temperature above 230° the
yellow sulphide of arsenic is formed.2
SOLID HYDROGEN ARSENIDE. As₂H2.
364 This substance is formed as a brown silky mass when
sodium arsenide, AsNag, is decomposed by water. The brown
powder produced by leaving the gaseous compound in contact
with moist air, or by decomposing one part of arsenic and five
of zinc by hydrochloric acid, which was formerly supposed to
1 Poleck and Thümmel, Ber. 16, 2438.
2 Brunn, Ber. 22, 3202.
3 Janowsky, Ber. 6, 220.
ARSENIC TRICHLORIDE
619
be this substance, has been shown to be nothing but the element
in a finely divided state.
According to Ogier¹ a solid hydride of the formula As¿H is
formed by the action of the electric current on arsine.
ARSENIC AND FLUORINE.
ARSENIC TRIFLUORIDE, ASF,
= 131.5.
2
365 This compound was obtained by Unverdorben by distil-
ling a mixture of four parts of arsenious oxide and five parts of
fluor-spar with ten parts of sulphuric acid. It is a transparent
colourless liquid boiling at 63° and having a specific gravity of
2-73; it fumes strongly in the air, has a pungent and powerful
odour, and when brought in contact with the skin produces
serious wounds which only heal after a long time (Dumas.) It
attacks glass, and is decomposed by water into arsenious and
hydrofluoric acids. It is soluble in ammonia, and forms with
this gas a crystalline compound.
Arsenic Pentafluoride is not known in the free state. Marignac
obtained a double compound AsF+KF in colourless crystals
by dissolving potassium arsenate in hydrofluoric acid.
ARSENIC AND CHLORINE.
ARSENIC TRICHLORIDE. AsCl
180.
366 This is the only known compound of arsenic and chlorine.
Glauber first prepared this substance; his work "Furni novi
philosophici," published in the year 1648, contains the follow-
ing recipe:Ex arsenico et auripigmento to distil a butter or
thick oil. As has been described under antimony, so likewise
from arsenicum or auripigmentum with salt and vitriol can a
thick oil be distilled."
=
Preparation. In order to prepare arsenic trichloride accord-
ing to this plan, 40 parts of arsenious oxide must be heated
with 100 parts of sulphuric acid to the boiling point of water
in an apparatus which is connected with a well-cooled receiver.
Small pieces of fused chloride of sodium are then carefully
1 Compt. Rend. 89, 1068.
2 Pogg. Ann. 7, 316; see also Moissan, Compt. Rend. 99, 874.
620
THE NON-METALLIC ELEMENTS
thrown in.
follows:-
The decomposition which takes place is as
12HCl + As,O=4AsCl₂ + 6H₂O.
The water, which is formed at the same time, remains behind
in combination with the sulphuric acid, whilst the trichloride
distils over.
The same compound is easily obtained by passing dry chlorine
over heated arsenic which burns to chloride of arsenic. In
order to purify it from excess of chlorine it must be rectified
over some more arsenic.
Properties.—Arsenic trichloride is a colourless oily liquid and
has a specific gravity of 2.205 (0°/4°). It solidifies at -18° in
pearly needles¹ and boils at 130°-2 (Thorpe), evolving a colour-
less vapour, which has a specific gravity of 63 (Dumas). It is
an extremely powerful poison, and evaporates in the air with
the emission of dense white fumes. When arseniuretted hydro-
gen is led into the liquid, arsenic separates out (Janowsky);
thus:-
AsCl,+ASH, As₂+3HCl.
When brought in contact with a small quantity of water.
star-shaped crystalline needles of arsenic oxychloride, As(OH),Cl
separate out (Wallace). In contact with a large quantity of
water, it decomposes into arsenious oxide and hydrochloric acid,
and when the solution is distilled, arsenic trichloride comes
over together with the vapour of water; this explains the fact
that the hydrochloric acid prepared from arsenical sulphuric
acid invariably contains arsenic. Arsenic trichloride absorbs
dry ammonia, forming a solid compound having the composition
AsCl₂+3NH3, which dissolves in alcohol, being deposited from
this solution in white crystals.2
=
ARSENIC AND BROMINE.
ARSENIC TRIBROMIDE, AsBr₂ = 312·4.
367 In order to prepare this compound, powdered arsenic is
added to a solution of one part of bromine in two parts of carbon
bisulphide until the solution becomes colourless. Then bromine
and arsenic are added alternately until the colour of the first
¹ Besson, Compt. Rend. 109, 940. 2 Wallace, Phil. Mag. [4], 16, 358.
ARSENIC TRI-IODIDE
621
disappears; the clear liquid is poured off, and the bisulphide of
carbon allowed to evaporate spontaneously.
Arsenic tribromide forms colourless deliquescent crystals
which possess a strong arsenical odour (Nicklès), and melt at
about 20°. It has a specific gravity of 366, and boils at 220°.
By the action of water, it is decomposed in a similar way to the
chloride.
ARSENIC AND IODINE.
ARSENIC DI-IODIDE, ASI₂
326.2.
368 When arsenic tri-iodide is heated with arsenic together
with a little carbon bisulphide in a sealed tube, arsenic di-iodide
is formed, and the same compound is produced when arsenic is
heated with iodine in the proper proportions at 230° in a sealed
tube. It crystallises from carbon bisulphide in cherry-red,
brittle prisms, and easily oxidises in the air. When heated
with water or alkalis it turns black, arsenic and arsenic tri-iodide
being formed.¹
=
ARSENIC TRI-IODIDE, ASI, = 452·1.
When arsenic and iodine are brought together they com-
bine with considerable evolution of heat. For the purpose of
preparing the tri-iodide a method is adopted similar to that em-
ployed for the preparation of the tribromide. It is obtained in
the form of bright red hexagonal tables, which have a specific
gravity of 4:39. It may also be prepared by passing hydriodic
acid into arsenic trichloride, when hydrochloric acid is evolved,
and the iodide separates out in the form of crystals, or by adding
a hot solution of arsenious oxide in hydrochloric acid to a con-
centrated solution of potassium iodide.²
1 Bamberger and Philipp, Ber. 14, 2643.
4 Sloan, Chem. News, 46, 194.
When it is heated in the air or in oxygen it burns with forma-
tion of arsenious oxide. When ammonia is passed through its
solution in ether or benzene, a compound of the formula
2ASI,+9NH, is formed. On heating with alcohol to 150°, ethyl
iodide, C,H,I, is formed. It is used in medicine as a remedy
for certain skin diseases.
3
3
Arsenic Penta-iodide, AsI,, has been prepared by heating the
2 Ibid.
3 Ibid.
622
THE NON-METALLIC ELEMENTS
tri-iodide with iodine at 150°. It is a brown crystalline mass
which melts at 70°, has a sp. gr. of 393 and is soluble in water
and alcohol. The solutions deposit the tri-iodide on standing.
OXIDES AND OXYACIDS OF ARSENIC.
369 Arsenic unites with oxygen in two proportions producing
two acid-forming oxides, the composition of which corresponds
with the oxides of phosphorus: viz.—
Arsenious Oxide, As,Og
Arsenic Pentoxide, As,O.
ARSENIOUS OXIDE, OR ARSENIC TRIOXIDE, AS406
= 361.1.
Arsenious oxide has long been known under the names of
white arsenic and arsenious acid. In the writings of Basil
Valentine the name Hüttenrauch, or furnace-smoke, is given
to this substance because it is obtained by roasting arsenical
pyrites, and is emitted during the process in the form of a
white smoke which condenses to a white powder.
Arsenious oxide is prepared on the large scale in many
metallurgical processes by the roasting of arsenical ores. The
vapours of the oxide which are given off are condensed in long
passages or chambers called poison chambers (Giftkanäle), or
in towers termed poison-towers (Giftthürme) in the form of
crude flowers of arsenic or poison-flour (Giftmehl). For the pre-
paration of white arsenic, arsenical pyrites are usually employed.
It is obtained as a by-product in the roasting of cobalt ores,
which are employed in the manufacture of smalt. The crude
sublimate obtained in the Freiberg works contains about 75
per cent. of arsenious oxide. Open roasters are now supplanting
the muffle furnaces in which the operations were formerly con-
ducted. The hearth of such a furnace is about four metres in
length and about 28 metres in breadth, and in it 900 kilos. of
the ore can be roasted at once; four charges are made during
the day, and the white powder which comes off collects in long
underground passages of some 200 metres in length. Large
quantities of white arsenic are manufactured in the Harz, and
also in Devonshire and Cornwall. At the Great Devon Consols
and other mines in England 400 tons are manufactured monthly
by roasting tin-ore containing arsenical pyrites, and also the grey
ARSENIOUS OXIDE
623
flue deposits of the tin mines. In the year 1872 in England no
less than 5,171 tons of white arsenic were made.¹
Of late years Oxland's self-acting calciner has been much
used for the manufacture of arsenious oxide from the Cornish
and Devonshire ores. This furnace consists of an iron tube,
from three to six feet in diameter, and thirty feet long, set at an
inclination of from half to one inch per foot, varying according
to the nature of the ore. This tube is heated by a fire placed
at its lower end, whilst at its upper it is placed in connection
with the flues in which the white arsenic is deposited. The
tube is made to revolve by suitable machinery at the rate of
about one revolution in four minutes, and the crushed ore is
admitted in a regular stream through a feed-pipe at the back
end of the tube. Great economy of fuel is effected by this
furnace; indeed, if properly worked with a good ore, the heat of
combustion of the sulphur and arsenic is itself sufficient to carry
on the process. One such cylinder turns out upwards of twenty-
five tons of ore per diem, and the calcined product contains less
than 0.5 per cent. of arsenic.
A part of the arsenious oxide comes into the market in the
form of a white crystalline powder, the rest in the form of
arsenic glass, or amorphous arsenic obtained from the powder by
resublimation. For this purpose the arsenic powder which is
not white enough to be sent into the market is in the Ger-
man manufactories placed in iron pots heated by a furnace and
cylinders placed over them, in which the arsenious oxide con-
denses in the vitreous form. The pots hold 4 cwt. of the
crude arsenious oxide, and this is sublimed in from ten to
twelve hours. In order to produce a pure product two sublima-
tions are necessary. In this operation care must be taken
that a reduction to the state of metallic arsenic does not occur,
as this not only colours the arsenic-glass. of a dark tint, but
is apt to form a fusible alloy with the iron of the pots, and
thus to destroy them; if this occurs the oxide then falls
into the furnace and escapes into the air, and this is a continual
source of danger to the workmen employed in the operation.
In England a common reverberatory furnace is used for the
resublimation of the crude white arsenic, but, to prevent dis-
coloration by smoke, either coke or anthracite is used (Phillips).
Properties.-Arsenious oxide possesses no smell, but has a weak
metallic sweetish taste; it forms a colourless and odourless vapour
1 Phillips' Metallurgy, p. 463.
624
THE NON-METALLIC ELEMENTS
which has a density of 1977 at temperatures varying from 570°
(Mitscherlich) to 1560° (V. and C. Meyer).¹ Hence its molecular
formula is As Og. The vitreous modification of arsenious oxide is
translucent or transparent, and perfectly amorphous. Its specific
gravity is 3.738. On heating, it melts without volatilization at
a temperature of about 200°. When kept for any length of time
it becomes opaque, being changed into a porcelain-like mass.
This change is due to the passage from the amorphous or
vitreous to the crystalline condition; the change commences at
the outside of the mass, and gradually penetrates into the interior.
Arsenious oxide is slightly soluble in water. It is deposited
on cooling from a hot saturated solution in transparent regular
octahedra. It is very much more soluble in hydrochloric acid
than in water, and it may be easily obtained from the solution
in the form of large crystals. It also occurs in this form as
arsenic-bloom, being found together with native arsenic, having
been formed by the oxidation of this substance. The naturally
occurring crystals sometimes assume the form of octahedra and
sometimes of tetrahedra. The specific gravity of octahedral
arsenious oxide is 3689, and in its passage into the amorphous
modification an amount of heat is evolved represented by 5,330
thermal units (Deville and Troost). When the crystals are
deposited from hydrochloric acid solution, a bright and con-
tinuous luminosity is observed in the dark; this, however, does
not occur on crystallizing a second time. Water and alcohol
dissolve different quantities of the amorphous and of the crystal-
line varieties of the oxide. Thus eighty parts of cold water
dissolve nine parts of the crystalline modification, whilst one
part of the amorphous variety dissolves in twenty-five parts of
water (Bussy). One part of the crystallized oxide requires 400
parts of absolute alcohol for solution, whilst the amorphous
variety dissolves in ninety-four parts of alcohol (Giradin). A
constant solubility of each variety at different temperatures
cannot, however, easily be obtained, as the two modifications
pass readily from one into the other. When the crystallized
oxide is heated it evaporates at 125-150° without melting,
but, under an increased pressure, it melts and passes into the
amorphous modification.
Arsenious oxide also occurs in a third form in which it is
found crystallized in rhombic prisms, first observed by Wöhler
in a deposit from a cobalt roasting-furnace. Claudet found
¹ Ber. 12, 1116.
ARSENIOUS ACID
625
this same modification in a mineral occurring at San Domingo
in Portugal, and hence this substance has received the name of
Claudetite. According to Groth, the relations of the axes of this
form are 0.3758:1:03500. This rhombic form of the oxide
occurs when a boiling solution of potash is saturated with the
amorphous oxide, and then the solution allowed to cool
(Pasteur). Debray has observed that both crystalline forms may
be obtained by heating the oxide in a closed glass tube, half of
which is heated to a temperature of 400°. On cooling, the lower
part of the tube is found to contain glassy arsenious oxide, the
middle part rhombic crystals, and the upper part regular
octahedral crystals.
Arsenious oxide unites with many substances to form double
compounds. Thus, when dissolved in sulphuric anhydride
mixed with different amounts of water, compounds of one
molecule of the oxide, As40, with 16, 8, 4, and 2 molecules
of sulphuric anhydride are formed, whilst when heated with
sulphuric anhydride compounds with 12 and 6 molecules are
obtained.2 These compounds are very unstable and are
decomposed by water.
Many other double compounds with other substances have
also been described.
Arsenious oxide serves for the preparation of a large number
of other arsenic compounds, especially of the acids of arsenic
and their salts. It is also employed in the manufacture of
arsenical-pigments and is largely used in the manufacture of
glass.
ARSENIOUS ACID, AS (OH)3.
370 An aqueous solution of arsenious oxide has an acid
reaction, and contains tribasic arsenious acid. This has, however,
not been prepared in the pure state, although a large number
of well-defined salts is known. The salts, which are very
stable, are termed the arsenites. Of these there are several
series known. The most important are: the ortho-arsenites,
such as silver ortho-arsenite, Ag,ASO, and calcium ortho-arsenite,
Ca,(ASO). Then we are acquainted with metarsenites such
as potassium metarsenite, KASO,; and besides these, other salts
are known such as Ca,As,O,, and some having a still more
complicated constitution.
1 Adie, Journ. Chem. Soc. 1889, i. 157.
2 Weber, Ber. 19, 3185.
41
626
THE NON-METALLIC ELEMENTS
The arsenites of the alkali metals are soluble in water; those
of the other metals insoluble, but easily soluble in acids. A
neutral solution of an arsenite produces with ferric chloride a
reddish-brown precipitate, with silver nitrate a yellow precipitate,
and with copper sulphate a grass-green precipitate. The last is
soluble in caustic soda, and when the solution is boiled cuprous
oxide, CuO, is precipitated. Oxidizing agents convert arsenious
into arsenic acid. Thus, for instance, all the elements of the
chlorine group effect the change;
As(OH)3 + I2 + H₂O = AsO(OH)3 + 2HI.
Hence, arsenious acid is often employed for the volumetric
determination of chlorine, bromine, and iodine, and of sub-
stances which are capable of liberating these elements from
their compounds.
Arsenious oxide and the soluble arsenites act as very powerful
poisons, a dose of 0.06 gram (one grain) being very dangerous,
but from 0.125 to 0 25 gram (two to four grains) almost always
producing fatal effects unless speedily ejected from the system
by vomiting, or at once rendered harmless by its precipitation
as an insoluble compound. In small doses, however, arsenious
oxide and the arsenites of the alkali metals are largely used
in medicine, especially in skin diseases, nervous complaints, and in
intermittent fevers. Fowler's solution or the liquor arsenicalis
of the pharmacopoeia is made by dissolving eighty grains of
arsenious acid (arsenious oxide) with the same weight of
carbonate of potash, in one pint of water. The solution then
contains four grains of arsenious oxide in one fluid ounce.
It is a very singular fact that persons can accustom themselves
to sustain the action of quantities of arsenic which if taken without
preparation would certainly have proved fatal. Well-authenticated
cases of such arsenic eating occur especially in Styria.¹ In
one case, a woodcutter was seen by a medical man to eat a piece
of pure arsenious oxide weighing 45 grains, and the next day he
crushed and swallowed another piece weighing 5 5 grains, living on
the following day in his usual state of health. The reasons which
the arsenic-eaters give for the practice, which is usually carried
on in secret, is that it enables them to carry heavy weights with
ease to great elevations. The workmen in the arsenic works.
Roscoe, "On the alleged Practice of Arsenic Eating in Styria "—Memoirs
of the Lit. and Phil. Soc. of Manchester. 1860.
ARSENIC ACID
627
also appear to possess the power of withstanding doses of arsenic
which, given to ordinary persons, would produce fatal effects.
As an antidote against arsenic poisoning, sulphuretted
hydrogen was formerly employed, but this substance does
not act satisfactorily. Bunsen and Berthold proposed the
best antidote for poisoning by arsenic, namely, freshly pre-
cipitated hydrated oxide of iron. This converts the soluble
arsenious acid or even an alkaline arsenite into basic ferric
arsenite insoluble in water and in the liquids of the stomach.
The hydrated oxide should be freshly prepared, as when kept
it becomes crystalline, and loses its power.
ARSENIC PENTOXIDE, ASO = 228.2.
371 Arsenic is distinguished from phosphorus, which it other-
wise closely resembles, inasmuch as when burnt in the air or in
oxygen it oxidizes to arsenious oxide which does not directly
combine with more oxygen. If, however, the arsenious oxide be
treated with an oxidizing agent in presence of water, arsenic
acid, AsO(OH)3, is formed, and this, when heated to a tempera-
ture slightly below a red-heat, gives off water, and arsenic pent-
oxide remains behind as a white porous mass :—
2AsO(OH), = As₂О5 + 3H₂O.
If the substance be heated still more strongly, it melts and
decomposes into arsenious oxide and oxygen.
The specific gravity of arsenic pentoxide is 3734 (Karsten);
it dissolves slowly, but to a considerable extent, in water,
and deliquesces in moist air with formation of arsenic acid. It
is easily reduced to free arsenic when heated in presence of
charcoal, potassium cyanide, or other reducing agents.
ARSENIC ACID, ASO(OH)3.
Arsenic acid was first prepared by Scheele, in the year
1775, by dissolving arsenious oxide in aqua regia, and also by
acting with chlorine upon this oxide in the presence of water,
thus:-
:-
As40+10H,0 + 4Cl₂ = 4AsO(OH), + 8HC1.
6
1 Das Eisenoxydhydrat, ein Gegengift der arsenigen Säure. Göttingen. 1834.
628
THE NON-METALLIC ELEMENTS
Arsenic acid is also formed very readily when the lower oxide.
is warmed with nitric acid, and this is the process which is em-
ployed for the manufacture of the substance on the large.
scale. Nitrous fumes escape, and pass, together with air, up a
tower filled with coke where they meet a current of water;
they are then oxidized to nitric acid which is condensed.
The arsenic acid as it appears in commerce is a thick very
acid liquid, having a specific gravity of 20. It deposits, when
cooled, transparent crystals having the formula 2ASO(OH)¸+H₂O.
These crystals melt at 100°, and give off water, the anhy-
drous ortho-arsenic acid, H,ASO, remaining behind as a
crystalline powder. Arsenic acid possesses a very acid and
unpleasantly metallic taste, and acts as a poison, though not so
powerful a one as the trioxide.¹ The concentrated solution of
the acid acts on the skin as a strong cautery. Heated to a
temperature of 180°, it loses water, and hard glittering crystals
of pyro-arsenic acid, H,As,O,, separate out. This on heating
to 200° again loses water, a white crystalline mass of metarsenic
acid, HASO, being left. The pyro- and meta-acids both dis-
solve in water with evolution of heat, and are transformed at
once into the ortho-acid. This property serves to distinguish
these two varieties from the corresponding forms of phosphoric
acid, both of which can be obtained in solution.
Crystals of the formula As₂O, + 4H₂O, melting at 35—36°,
may be obtained from a syrup of the same composition, and
these when heated in a sealed tube give crystals of As,О5 +
3H,0.2
The specific gravity of aqueous solutions of arsenic acid of
known strength is according to the experiments of Schiff, as
follows:-
-
Specific
Gravity.
1.7346
1.3973
1.2350
Percentage of
H&ASO
67.4
45.0
30.0
Specific
Gravity.
1.1606
1·1052
1.0495
Percentage of
H₂ASO4.
22.5
15.0
7.5
Arsenic acid is completely reduced to arsenious acid by warm-
ing with an aqueous solution of sulphurous acid.
The Arsenates.-The salts of arsenic acid, or the arsenates, are
isomorphous with the corresponding phosphates. Indeed, it was
by the comparison of these two series of salts that Mitscherlich
3 Annalen, 113, 183.
1 Wöhler and Frerichs, Annalen, 65, 335.
2 Joly, Compt. Rend. 101, 1262.
SULPHIDES OF ARSENIC
629
in the year 1819 was led to the discovery of the law of isomor-
phism. In their reactions, as well as in their general properties,
both classes of salts exhibit great analogy. Thus the soluble
arsenates give with ammonia, ammonium chloride and a mag-
nesium salt, a crystalline precipitate of MgNH,AsО + 6H₂O,
corresponding exactly to the phosphorus compound, and when
gently warmed with a solution of ammonium molybdate in
nitric acid, the arsenates give a yellow precipitate similar to
that obtained with a phosphate. They may, however, be dis-
tinguished from the latter class of salts, inasmuch as a neutral
solution produces with silver nitrate a dark reddish-brown
precipitate, and when acetate or nitrate of lead is added to a
neutral solution, a white precipitate of lead arsenate Pb(AsO4)2
is obtained, and this when heated on charcoal before the blow-
pipe is reduced to arsenic, which emits its peculiar garlic-like
smell.
Being tribasic, arsenic acid forms three series of salts. Of
the normal salts only those of the alkali-metals are soluble in
water. Two series of acid salts or hydrogen-salts also exist, such
as Na,HASO, and NaH,ASO. The former of these salts when
carefully heated yields sodium pyro-arsenate, Na As₂O7, and
the latter sodium metarsenate, NaASO. These salts can, how-
ever, only exist in the solid state, as when they are dissolved
in water they are at once transformed into the ortho-compound.
Arsenic acid is used largely in commerce in the manufacture
of magenta; and its salts, especially sodium arsenate, are
employed to a great extent in the processes of calico-printing.
ARSENIC AND SULPHUR.
372 Arsenic forms three definite compounds with sulphur,
viz.:-
Arsenic disulphide or realgar, As₂S₂ ;
Arsenic trisulphide or orpiment, As₂S;
Arsenic pentasulphide, As,S.
Of these, the last two correspond to the oxides, and act as
acid-forming sulphides, giving rise to well-defined series of
salts termed the thio-arsenites and the thio-arsenates.
630
THE NON-METALLIC ELEMENTS
ARSENIC DISULPHIDE, ASS.
This compound occurs native as realgar, crystallizing in ob-
lique prisms belonging to the monosymmetric system. These
possess an orange-yellow colour and resinous lustre, and are
more or less translucent; the streak varies from orange yellow to
red. The specific gravity is 34 to 36, and hardness 15 to 2.0.
It occurs together with silver and lead ores at Andreasberg in
the Harz and other localities, and imbedded in dolomite on
the St. Gothard, and has been found in minute crystals in
Vesuvian lavas. Strabo mentions the occurrence of "Sandaraca "
in a mine in Paphlagonia.
When it is heated to 150° with a solution of sodium bicar-
bonate it dissolves in the liquid and afterwards separates out in
the crystalline form (Sénarmont).
The red arsenic glass or ruby sulphur which occurs in
commerce is an artificial disulphide of arsenic prepared in
various arsenic works. In Freiberg, arsenical pyrites and
common pyrites are used for this purpose, mixed in such pro-
portions that the mixture contains about 15 per cent. of arsenic
and 27 per cent. of sulphur. Such a mixture is then sublimed
in a furnace in which are placed twelve iron tubes. Each tube
holds about thirty kilograms of the ore, and the charge is re-
newed every twelve hours. In order to give the product the
right degree of colour it is again melted with sulphur.
Ruby sulphur is a red glassy mass, translucent at the edges.
It does not possess a constant composition; the material
manufactured at Freiberg contains generally 75 per cent. of
arsenic and 25 per cent. of sulphur, and that made at
Reichenstein in Silesia is a mixture of ninety-five parts of disul-
phide with five parts of sulphur. This body was formerly much
used as a pigment, and is still employed in the manufacture of the
so-called Indian- or white-fire, which is a mixture of two parts of
the disulphide with twenty-four parts of nitre, and burns with
a splendid white light when ignited. The disulphide is also
employed in tanning, being mixed with lime and employed for
removing the hair from the skins.
ARSENIC TRISULPHIDE, ASS,.
373 This substance occurs in nature and is known under the
name of orpiment (auri pigmentum) or the yellow sulphide of
ORPIMENT
631
arsenic, and was known to Pliny as arsenicum. It crystallizes
in translucent lemon-coloured prisms belonging to the mono-
symmetric system, and has a specific gravity of 3:46.
When sulphuretted hydrogen is passed through an aqueous
solution of arsenious oxide the liquid becomes of a yellow colour,
but no precipitate is formed, the liquid containing arsenic
trisulphide in the colloidal form.¹ If a small quantity of
hydrochloric acid be present, a beautiful yellow precipitate of
arsenic trisulphide is at once thrown down. On heating this sub-
stance it melts to a yellowish red liquid which volatilizes without
decomposition at a temperature of about 700°. Heated in the
air it takes fire and burns with a pale blue coloured flame to
arsenious oxide and sulphur dioxide. It is soluble in solutions
of the alkalis' and their carbonates, a metarsenite and a meta-
thioarsenite being formed :-
2As¿S + 4KOH= KAsO₂ + 3KAsS½ + 2H₂O.
When hydrochloric acid is added to the solution thus obtained
the whole of the arsenic is again precipitated as trisulphide :-
8
—
KAsO₂ + 3KAsS₂ + 4HCl = 2As¿S¸ + 4KCl + 2H₂O.
The sulphide of arsenic occurring in commerce is prepared by
subliming a mixture of seven parts of pulverized arsenious oxide
with one part of sulphur, and it is really a mixture of arsenious
oxide with more or less sulphide of arsenic. The material
thus prepared, which is very poisonous, from the excess of
arsenious oxide which it contains, was formerly much used as a
pigment under the name of King's yellow, but it is now almost
entirely superseded by the comparatively innocuous chrome
yellow. The yellow sulphide of arsenic is also used in the arts
and manufactures, for instance in the printing of indigo colours;
and a mixture of orpiment, water, and slaked lime, is used in
the East under the name of Rusma as a depilatory, its action
depending upon the formation of a hydrosulphide of calcium.
The thio-arsenites.-These salts, which are frequently termed
the sulpho-arsenites, stand in the same relation to the trisulphide
as the arsenites to arsenious oxide. They are formed by the com-
bination of the trisulphide with the sulphide of a metal, and
they may be arranged, like the arsenites, in different groups.
¹ Schulze, J. Prakt. Chem. (2), 25, 431; see also Picton, Journ. Chem. Soc.
1892, i. 127, 140.
II.1
632
THE NON-METALLIC ELEMENTS
Acids decompose them with precipitation of the trisulphide.
The thio-arsenites of the alkali metals are. soluble in water
yielding yellow solutions; those of the other metals exist in
the form of coloured precipitates.
ARSENIC PENTASULPHIDE, As„S5.
374 When sulphuretted hydrogen is passed through a solution
of arsenic acid at some temperature between 4° and 80°, the pre-
cipitate consists of arsenic pentasulphide mixed with arsenic
trisulphide and sulphur, a partial reduction of the arsenic acid
having taken place. In the presence of hydrochloric acid, how-
ever, and when the gas is passed rapidly into the warm solution,
the precipitate consists entirely of the pentasulphide.¹ The
pentasulphide can also be obtained by fusing the trisulphide in
the proper proportions with sulphur. It forms a yellow fusible
mass, which can be sublimed without decomposition in absence
of air. It is more readily obtained by acidulating a dilute
solution of sodium thio-arsenate
thio-arsenate with hydrochloric acid
(Fuchs):-
2Na,ASS, +6HCl = 6NaCl + 3H₂S + As₂S
The thio-arsenates or sulpharsenates are formed by dissolving the
pentasulphide in a solution of the sulphide of an alkali-metal
or by treating the trisulphide with an alkaline polysulphide :-
As,S+ K₂Sg2KASSg.
They may be likewise obtained by the action of sulphuretted
hydrogen on a solution of an arsenate :-
――――――
K¸AsО¸ + 4H„S = K¸AsS, + 4H₂O.
4
As will be seen by the above formula, both ortho- and meta-
thio-arsenates exist, and we are likewise acquainted with pyro-
thio-arsenates such as K.AsS. The thio-arsenates of the
alkali metals are soluble in water, yielding yellow solutions,
whereas the corresponding salts of the other metals form
insoluble precipitates.
ARSENIC AND SELENIUM.
375 Certain compounds of arsenic and selenium containing
sulphur at the same time have been prepared by von Gerichten.
1 Brauner and Tomitscheck, Ber. 21, 221c.
2 Ber. 7, 29.
DETECTION OF ARSENIC
633
These substances were obtained by fusing the components in the
proper proportions. Thus arsenic seleno-sulphide AsSeS, is a
red translucent mass which dissolves in the state of powder in
ammonium hydrosulphide, yielding a brownish-red coloured
solution. Arsenic thio-selenide AsSSe, is an opaque crystalline
mass which can be distilled without decomposition, and dissolves
in ammonium hydrosulphide, forming a deep yellow coloured
solution.
ARSENIC AND PHOSPHORUS.
ARSENIC PHOSPHIDE, ASP.
376 When dry arseniuretted hydrogen is led into phosphorus
trichloride, the above compound is precipitated in the form of a
brownish-red powder. It is slightly soluble in bisulphide of
carbon, and is oxidized with ignition when acted upon by nitric
acid. Heated in absence of air, it decomposes into its con-
stituents, and when heated with an aqueous solution of an
alkali it gives off arseniuretted hydrogen and phosphuretted
hydrogen, leaving behind a residue of an arsenite and a
phosphite.¹
THE DETECTION OF ARSENIC IN CASES OF POISONING.
377 With a few exceptions, all the compounds of arsenic are
to some extent poisonous. Arsenious oxide is a particularly
powerful poison, and as this substance is largely used in the
arts and manufactures, and likewise employed on a large scale as
a rat- and vermin-poison, it is not difficult, in spite of legislative
enactments respecting the sale of poisons (Sale of Poisons Bill),
to obtain this body in quantity, and hence cases of accidental
poisoning with this substance are not uncommon, and, in addi-
tion, it is too frequently made use of for the express purpose of
destroying human life.
In medico-legal investigations, in cases of poisoning by
arsenic, whether it be in the contents of the stomach, in
vomited matter, or in the several portions of the body itself, it
is not sufficient for the toxicologist to ascertain with certainty
the presence of arsenic. He is bound also to determine, with as
great a degree of accuracy as is attainable, the absolute amount
1 Janowsky, Ber. 6, 216.
634
THE NON-METALLIC ELEMENTS
of this poison found in the body, in order that he may be able
to give a distinct opinion as to whether the quantity is sufficient
to produce fatal effects. Traces of arsenic are, as we have seen,
widely distributed in nature.
It is liable to occur, although in
small quantities only, in various pharmaceutical and chemical
preparations. Hence it is the first duty of the chemist who
investigates such matters, to ascertain that the whole of the
reagents employed as well as the apparatus used in his experi-
ments are altogether free from arsenic. This is especially
necessary, as hydrochloric and sulphuric acids, which are in-
variably employed for the investigation, are very apt to con-
tain traces of arsenic; and inasmuch as arsenic is sometimes.
present in the glaze of certain kinds of porcelain. The opera-
tion which is conducted for the purpose of satisfying the ex-
perimenter as to the freedom of his apparatus and chemicals
from arsenic is termed a blind experiment, and must be carried
on with the same quantities of the same materials, and with
the same kind of apparatus, side by side with the real ex-
periment in which the substance supposed to contain the
poison is examined.
In cases of poisoning with white arsenic, the quantity of the
poison employed is almost always more than is necessary to
produce death, and, as arsenious oxide is very difficultly soluble
in water, white particles may frequently be found either adher-
ing to the coatings of the stomach and intestines, or found in
the vomit. These white particles must be carefully looked for
by help of a lens, amongst the folds and in the inflamed portions
of the stomach and intestines as well as in the contents of the
stomach itself, and picked out with pincettes, washed with cold
water, dried, and then brought into a tube made of hard glass
drawn out to a point of the form and size shown in Fig. 172.
FIG. 172.
In the tube, above these dried white particles, a small splinter
of ignited wood-charcoal is placed, and then this charcoal
heated in the flame. As soon as this is red-hot, the tube,
which is at first held in a horizontal direction, is gradually
slanted, keeping the charcoal still heated in the flame, and
DETECTION OF ARSENIC
635
gradually brought into a nearly vertical position, so that at
last the point of the tube becomes red-hot. The vapour of the
trioxide then passes over the red-hot carbon and is reduced to
the metal which is deposited in the form of a bright metallic
mirror, shown in Fig. 173, on the part of the tube above the
FIG. 173.
carbon. When cooled, the charcoal is shaken out, and the
metallic arsenic is heated by itself so as to drive it up into the
wider portion of the tube. If it be not present in large quantity,
it is wholly oxidized in the act of volatilization to the oxide,
which forms a white sublimate in the upper part of the tube,
consisting, as may be seen with the lens, of small glittering
octahedra, shown in Fig. 174. This is then dissolved in a small
FIG. 174.
quantity of boiling water, and when the solution is cold, a solu-
tion of silver nitrate is added, and then very dilute ammonia
drop by drop until the liquid is neutral. A yellow precipitate of
silver arsenite is then formed :-
:-
As O+ 12AgNO3 + 12NH,HO
6H₂O.
4AgAsO +12NH¸NO₂+
The arsenious oxide may also be dissolved in warm hydro-
chloric acid and then sulphuretted hydrogen passed through the
solution, when the yellow sulphide of arsenic is precipitated.
If no particles of undissolved arsenious oxide can be found, the
organic matter under investigation, which if it consists of the
solid portion of the body must be cut up into small pieces, is
brought into a retort provided with a well-cooled receiver and
containing fused common salt or pure rock salt. This mixture
is then distilled with pure sulphuric acid, whereby arsenic
trichloride is formed, and this substance volatilizes with the
vapours of water and hydrochloric acid. In order that this
operation should be successful, it is necessary that a smaller
=
·
636
THE NON-METALLIC ELEMENTS
quantity of sulphuric acid should be added than that needed
to decompose the whole of the salt. The cooled distillate
is then treated with sulphuretted hydrogen, and the precipi-
tate, consisting of impure sulphide of arsenic, is treated as
hereafter described.
The above method can only be employed when the arsenic is
in the state of arsenious oxide, or as an arsenite. In order to
ascertain with certainty whether arsenical compounds in general
are present, the following method must be adopted. The organic
matter contained in the substance must first be removed as
completely as possible. For this purpose the best process to
employ is that of Fresenius and Babo. The contents of the
stomach, or solid portions of the stomach or other organs,
obtained in as fine a state of division as possible by trituration
in a mortar or by other means of mechanical division, are
brought into a large porcelain dish and diluted with a quantity
of water sufficient to make it into a thin paste. A volume of
pure hydrochloric acid of specific gravity 112 is then added
equal to that of the solid substance taken, and the mixture
warmed on a water-bath, whilst every five to ten minutes from
one to two grams of pure potassium chlorate is added, the water,
as it evaporates, being from time to time renewed. This opera-
tion is continued until the whole assumes the form of a thin
homogeneous yellow liquid. A few more grams of potassium
chlorate are then added and the mass is heated until the smell
of chlorine has completely disappeared. The solution is next
filtered, and into the clear solution, heated to about 70°, a
current of pure sulphuretted hydrogen is passed until it smells.
strongly of the gas. The saturated solution is then loosely
covered, and allowed to stand in a warm place for twenty-four
hours. If after this time the smell of sulphuretted hydrogen
has disappeared, it must be treated a second time with the
gas. As soon as the liquid, after standing, smells distinctly of
sulphuretted hydrogen, the precipitate of impure arsenic tri-
sulphide is collected on a filter, and the filtrate again treated
with sulphuretted hydrogen in order to ensure the complete
precipitation of the arsenic. Great care must of course be taken
that the sulphuretted hydrogen is prepared from pure materials
free from arsenic.
The precipitate, obtained according to one or other of these
methods, invariably contains organic matter, and that obtained
according to the last process may contain, in addition, the sul-
DETECTION OF ARSENIC
637
phides of other poisonous metals such as antimony, tin, lead,
mercury, &c. The precipitate is now washed, first with water
containing sulphuretted hydrogen, and afterwards with pure
water, and then treated with dilute ammonia, which dissolves
the sulphide of arsenic, the organic matter, and traces of any
sulphide of antimony which may be present. The filtrate
is evaporated to dryness, and the residue placed in a small
porcelain crucible, repeatedly moistened with pure concentrated
nitric acid, and the solution evaporated to dryness. The pale yel-
low residue is then neutralized with a few drops of pure caustic
soda, and the liquid obtained evaporated to dryness. The residue
is next mixed with the requisite quantity of a finely powdered
mixture of one part of sodium carbonate and two parts of sodium
nitrate, and the mixture gently heated in a porcelain crucible
until the mass fuses. The fused mass is treated with water, the
soluble portion filtered off, and the residue washed with a mixture
of equal parts of water and alcohol, when any antimony present
remains behind as an insoluble sodium antimoniate. The mixed
aqueous and alcoholic filtrates containing the arsenic are then
evaporated, having been acidified with pure hydrochloric acid
for the purpose of removing all traces of nitric and nitrous acids.
The residue is boiled with water, diluted, and the solution then
heated to 70°, and treated with sulphuretted hydrogen in the
manner before described. The precipitate thus formed, after
having been washed, is dissolved in dilute ammonia, the solution
evaporated, and the residual sulphide of arsenic weighed. A very
necessary point to attend to in these operations is that all the
chemicals employed shall be free from chlorine, as otherwise some
arsenic trichloride may be formed and volatilized (Wöhler).
The next operation is to convert the sulphide into metallic
arsenic, in order to be positively certain that this substance is
present. A portion of the weighed precipitate is heated
with pure nitric acid, the solution obtained evaporated on the
water-bath, and the dry mass dissolved in water. Metallic
arsenic is best obtained from this solution, which of course
contains arsenic acid, by employing the well-known reaction of
Marsh. Two forms of the apparatus employed for this purpose
are shown in Figs. 175 and 177. The first of these is a simple
form usually employed in most laboratories, the second is
one recommended by Otto. The first form requires no ex-
planation, the second consists of a gas-generating flask (A)
1 New Edin. Phil. Journ. 1836, p. 229.
638
THE NON-METALLIC ELEMENTS
of 100-200 cc. capacity, provided with a tube-funnel (b)
and a drying-tube (a), containing in the nearer part some
plugs of cotton wool for the purpose of retaining any liquid
which may be mechanically carried over, and in the further
O
FIG. 176.
FIG. 175.
portion some pieces of solid caustic potash for the purpose of
drying the gas and withdrawing from it any traces of acid which
may be carried over. The end of this tube is connected with
a reduction-tube (d), consisting of hard glass, of the diameter
and thickness shown in Fig. 176, which may be
drawn out at various points as shown in Fig. 177.
The flask contains pure granulated zinc, together
with some water. A cold mixture of one part of
sulphuric acid and three parts of water is now
gradually added by the funnel tube when the hydrogen is
quickly evolved. As soon as the whole of the air has been
driven out of the apparatus, the portion of the reduction tube
nearest the flask is strongly heated by means of the flame for
fifteen minutes, whilst the gas which escapes may be allowed
to pass, by means of the tube bent at right angles, through a
dilute solution of nitrate of silver. If, after the expiration of
this time, the tube does not exhibit any dark mirror-like
deposit, and if the silver solution remains clear, the materials
are free from arsenic. The solution under examination is then
gradually added to the contents of the flask by means of the
tube-funnel, and the reduction-tube is heated again, a little in
DETECTION OF ARSENIC
639
A
advance of the first drawn-out portion, so that the arsenic is
deposited in the narrow portion of the tube. As soon as a
sufficient quantity has been formed a second portion of the
tube is heated in the same way. In spite of the ignition of
the tube, a certain quantity of undecomposed gas often escapes,
the flame of the gas burning at the end assumes a pale
lavender tint, and pieces of white porcelain when placed in the
flame become covered with a dark coating of metallic arsenic.
This stain is at once dissolved by sodium hypochlorite solution,
the soluble sodium arsenate being formed by oxidation.

FIG. 177.
The gas escaping during the operation may preferably be passed
through a dilute solution of silver nitrate, when the whole of
the arseniuretted hydrogen is decomposed, silver being precipi-
tated, and arsenious acid remaining in solution. In order
that this reaction should be carried on satisfactorily it is
necessary that the hydrogen gas be evolved at a low tempera-
ture, for when the liquid becomes hot, sulphuretted hydrogen
is formed, and this precipitates the arsenic as the insoluble
sulphide of arsenic, thus withdrawing it from the reducing
action of the hydrogen. In addition to this neither nitrates,
nitrites, chlorides, nor free chlorine must be present.¹
1 For further particulars respecting the detection of arsenic, Fresenius's Quan-
titative Analysis may be consulted.
41.
640
THE NON-METALLIC ELEMENTS
If care be taken to dilute the arsenic solution sufficiently and
to have a slow evolution of hydrogen, the whole of the arsenic
may be obtained in the form of a mirror and its amount
estimated by a comparison of the mirror with a set of standard
mirrors prepared from known amounts of arsenic.¹
It has already been remarked that arsenious oxide is em-
ployed largely for the manufacture of Scheele's green and
emerald green, and that arsenic acid is employed on a very
large scale in the manufacture of the aniline colours. The
employment of arsenical wall-papers is much to be deprecated;
still more is the employment of the insoluble arsenical green
for colouring light cotton fabrics, such as gauze, muslin, or
calico, to be condemned. The colour is merely pasted on with
size, and rubs off with the slightest friction.
For the purpose of detecting arsenic in wall-papers or cotton
fabrics a very convenient test is that of Reinsch. The colour
is dissolved by hydrochloric acid, into this arsenical solution a
small piece of bright copper foil or wire is brought, and this
soon becomes dark coloured from the deposition on its surface
of metallic arsenic. In order still further to prove the presence
of arsenic the metallic copper is well washed with water, dried,
and then strongly heated in a dry test tube. A portion of the
arsenic is thus oxidized and volatilized, and the arsenious oxide
deposited as a white sublimate on the cold portions of the tube.
This is then fully examined by the methods already described.
This process, however, can only be employed when the quantity
of arsenic present is considerable, inasmuch as the greater part
of the arsenic remains behind in combination with the copper.
BORON. B = 10.74.
378 Boron does not occur in nature in the free state, but is
found combined with hydrogen and oxygen, forming boric or
boracic acid, B(OH), and its salts. Of these latter the most
important are tincal or native borax, Na,B,O,+10H,O, boracite,
2MgBO15+MgCl2, and borocalcite, CaB,O,+4H₂O.
The name borax is found in the writings of Geber and other
alchemists. It is, however, doubtful whether they understood
¹ Selmi, Gazzetta, 10, 435; Sanger, Proc. Amer. Academy of Arts and Sciences,
26, 24.
2 Schw. Journ. 53, 377.
PREPARATION OF BORON
641
by the word the substance which we now denote by it. Nothing
satisfactory was known concerning the chemical nature of this
salt for a long time. Homberg¹ first prepared boric acid from
borax in the year 1702, and he termed it sal sedativum, for he
was unacquainted with the composition of the acid. It was not
till 1747-8 that Baron showed in two memoirs read before the
French Academy that borax was a compound of sal sedativum
and soda. After the establishment of the Lavoisierian system,
the name boracic acid was given to sal sedativum, and it was
then assumed that this acid contained an unknown element, for
the isolation of which we are indebted to Gay-Lussac and
Thénard,² as well as to Sir Humphry Davy,³ who about the year
1808 obtained elementary boron.
Boron occurs in two allotropic modifications: amorphous and
crystallized.
Amorphous Boron.-This modification was obtained by Gay-
Lussac by heating boron trioxide, B,O, (obtained by the ignition.
of boric acid), with potassium in an iron tube. It may also be
obtained by mixing ten parts of coarsely-powdered boron tri-
oxide with six parts of sodium, bringing the mixture into a
crucible already heated to redness, and covering it with a layer
of powdered chloride of sodium previously well dried. As soon
as the reaction, which is very violent, has subsided, the mass is
stirred with an iron rod until all the sodium has been oxidized,
and then carefully poured into water acidified with hydrochloric
acid. The soluble salts dissolve in the water, whilst the boron
remains behind as an insoluble brown powder. This is then
collected on a filter, and it must be very carefully dried, as it is
easily oxidized and may take fire.
The boron obtained in this way is impure, and a similar
product is got when boron trioxide or borax is heated with
magnesium powder. In this case a boride of magnesium is
probably formed of the formula Mg,B,, which is decomposed by
acids. The product left after this treatment, however, still
contains about 5 per cent. of magnesium and about 1 per cent.
of hydrogen, which is also found in boron prepared by means
of sodium, and is probably present in the form of a solid
hydride.
In order to prepare pure boron, 70 gr. of magnesium powder
2 Recherches, i. 269.
1 Crell. Chem. Archiv. 2, 265.
* Decomposition of boracic acid, Phil. Trans. 1809, i
75.
4 Gattermann, Ber. 22, 195; Winkler, Ber. 23, 772; Lorenz, Annalen, 247,
226.
42
642
THE NON-METALLIC ELEMENTS
is heated with 210 gr. of boron trioxide. The product, which
consists of boron, accompanied by magnesium boride and
borate, is treated with dilute acid which dissolves the borate
and the greater part of the boride. In order to remove
the last portions of the boride the residue is then fused with
borax and again treated with hydrochloric acid, which leaves
the boron containing only traces of silicon, iron, and magnesium.
The presence of nitride of boron in the final product can only
be avoided by carrying out the heating in an atmosphere of
hydrogen or by placing the mixture in a crucible lined with
titanic acid.
Amorphous boron is a chestnut-brown powder of specific
gravity 2:45. It is infusible even at the temperature of the
electric arc, but partially volatilizes, and burns when heated to
700° in the air. It is a more powerful reducing agent than
carbon or silicon, inasmuch as it is oxidized by carbonic
oxide and by silica. It combines with bromine at 700° but
not with iodine, and forms compounds with many metals,
among which are silver and platinum, on heating. It is
acted on by the oxy-acids, is oxidized by water vapour, and
combines with nitrogen at a high temperature.¹ It is a non-
conductor of electricity, and when freshly prepared and not
strongly ignited is slightly soluble in water, imparting to it a
yellow colour and being precipitated unchanged from its aqueous
solution on the addition of acids or salts.
379 Crystallized or Adamantine Boron.-This substance was
first obtained in the year 1856 by Wöhler and Deville. It can
be prepared by several processes. Thus, if amorphous boron be
pressed down tightly in a crucible, a hole bored in the centre of
the pressed mass, and a rod of aluminium dropped into the hole,
and the crucible then heated to whiteness, the boron dissolves
in the molten aluminium and separates out in the crystalline
form when the metal cools. The aluminium is then dissolved
in caustic soda, and thus the insoluble boron is left in large
transparent yellow or brownish-yellow crystals. The same
modification may be obtained in smaller crystals, which are
often joined together in the form of long prismatic needles, by
melting together boron trioxide and aluminium. In order to
prevent the action of the oxygen of the air upon the fused mass,
the crucible in which the operation is conducted must be placed
inside a larger one and the space between them filled up with
2 Annalen, 101, 113.
1 Moissan, Compt. Rend. 114, 392, 617.
CRYSTALLINE BORON
643
powdered charcoal. In this process, however, the boron takes
up carbon to the amount of from 2 to 4 per cent. This carbon
must be in the form of diamond carbon, inasmuch as the boron
crystals containing this impurity are transparent, and more
transparent the larger the percentage of carbon. In addition to
carbon the boron thus prepared is found to contain a certain
quantity of iron and silicon from the crucible used. These im-
purities can be removed by treatment with hydrochloric acid,
and afterwards with a mixture of nitric and hydrofluoric acids.
According to the experiments of Hampe,¹ the crystals of
adamantine boron contain aluminium as well as carbon, and
possess a constant composition which is represented by the
formula BCAl¸.
In the preparation of crystalline boron the occurrence of cer-
tain graphite-like lamina has been
observed. These were at one time
supposed to be a third modification
of boron until Wöhler and Deville 2
showed that they consist of a com-
pound of boron and aluminium,
having the formula AlB¿.
In addition to these substances, large black plates of an
aluminium boride, AlB12, and smaller black crystals of carbon
boride, CB, have been observed.3
FIGS. 178 AND 179.
According to W. H. Miller, boron crystallizes in monosym-
metric pyramids or prisms, shown in Figs. 178 and 179, which
have a lustre and hardness exceeded only by that of the diamond,
as they scratch both ruby and corundum. The specific gravity
of this form of boron is 268. When heated in the air or in
oxygen it ignites at the same temperature as the diamond does,
and then does not oxidize throughout the mass, but becomes
covered with a coating of the melted trioxide (Wöhler). Con-
centrated nitric acid exerts no action upon it, and even aqua
regia attacks it but slowly. Boiling caustic soda solution like-
wise does not act upon it, but if it is fused with the solid alkali
it dissolves slowly with formation of sodium borate and with
evolution of hydrogen.
380 The Atomic Weight of Boron.-The composition of borax,
Na₂BO,+10H₂O, has been made the basis of the determina-
tion of the atomic weight of boron by many chemists, although,
1 Annalen, 183,
2 Ibid., 141, 268.
75.
3 Joly, Compt. Rend. 97, 456.
644
THE NON-METALLIC ELEMENTS
of
owing to the volatility of boric acid and the presence of water
crystallization in the salt, it is difficult to attain great accuracy
in its analysis. By determining the water of crystallization,
Berzelius 1 obtained the number 10.98, Laurent 2 10-84, and
Ramsay and Aston 3 10.85. The last-named chemists moreover
obtained the number 10'89 by converting anhydrous borax into
sodium chloride by distillation with methyl alcohol and hydro-
chloric acid, whilst Rimbach, by the titration of borax with
hydrochloric acid in presence of methyl orange as an indicator,
found 10.86. Deville analysed the chloride and bromide of
boron, obtaining the numbers 10.73 and 1088, and Abrahall
by the analysis of the bromide found 10.74.
6
BORON AND HYDROGEN.
BORON HYDRIDE, BH, = 13·74.
mass
381 Davy showed that when amorphous boron is heated with
potassium, and the product treated with water, a gas is evolved
possessing a peculiar odour, and supposed by him to be a
hydride of boron. On dissolving in hydrochloric acid the
obtained by heating iron filings and boron trioxide to whiteness,
hydrogen is evolved, and this on ignition burns with a green
flame. Gmelin at first supposed that this gas contained boron,
but afterwards believed that its properties were due to other
impurities. Deville and Wöhler doubted the existence of this
hydride, and hence it was generally assumed that boron is the
only non-metallic element which does not combine with hydro-
gen, until Francis Jones' succeeded in preparing the compound.
He heated boron trioxide with magnesium dust in the following
proportions:-
----
6Mg+B₂O₂=3MgO+B,Mgg.
The grey friable mass thus obtained contains, in addition to
magnesia and magnesium boride, boron nitride, free
and magnesium. On the addition of hydrochloric acid,
hydride is evolved, but it is mixed with so much hydroge
2
1 Pogg. Ann. 8, 19.
2 Compt. Rend. 29, 7, and Journ. Prakt. Chem. 47, 415.
8 Journ. Chem. Soc. 1893, 207.
Ann. Chim. Phys. [3], 55, 181.
7 Ibid. 1879, i. 41.
boron,
boron
I that,
Ber. 26, 164.
6 Journ. Chem. Soc. 1892,
i. 650.
BORON HYDRIDE
645
so far, its preparation in the pure state has not been possible.
The gas is colourless, possesses an extremely unpleasant and
very characteristic odour, and when inhaled even in small
quantities produces sickness and headache. It burns with a
bright green flame, which deposits a brown spot of boron on
porcelain. It dissolves slightly in water, and its solution does
not undergo change on standing. At a red heat it decomposes
into its constituent elements, and on passing it into a solution
of silver nitrate a black precipitate is thrown down, although
only in small quantities, this being probably owing to the forma-
tion of free nitric acid. This precipitate contains boron and
silver, and is decomposed by hot water with evolution of boron
hydride. When this gas is passed into ammonia the peculiar
smell of boron hydride disappears, but the gas has a fœtid odour,
and burns with a yellow-green flame. The characteristic smell
of the gas is again observed when the ammoniacal solution
is acidified. An accurate analysis of the gas was not found to
be possible, but the results obtained by combustion with copper
oxide agreed as nearly as could be expected with the formula
BH₂.¹
BORON AND FLUORINE.
BORON TRIFLUORIDE, BF, = 67·4.
382 This gas was discovered in the year 1808 by Gay-Lussac
and Thénard. They obtained it by heating a mixture of one
part of boron trioxide with two parts of fluor-spar to whiteness
in a gun-barrel. It may be more easily prepared by heating
the above mixture with twelve parts of strong sulphuric acid in
a glass flask (J. Davy):—
B₂O3+3CaF₂+3H₂SO₁ = 2BF¸+3CaSO4+3H,O.
2
Another method is to heat a mixture of potassium fluoborate
and the trioxide with sulphuric acid (Schiff) :-
2
6KFBF3+B₂O₂+6H₂SO4=8BF¸+6KHSO₁+3H₂O.
The colourless gas evolved by any of these processes must be
collected over mercury or by displacement, as it is dissolved
when brought into contact with water. Both amorphous and
crystalline boron become ignited in fluorine gas, the trifluoride.
1 Jones and Taylor, Journ. Chem. Soc. 1881, i. 213.
646
THE NON-METALLIC ELEMENTS
being formed. It fumes strongly in the air, does not attack
glass, possesses an intensely pungent odour, and has a specific
gravity of 2:37 (J. Davy). It acts upon certain organic bodies
by withdrawing from them the elements of water, and carbonizes
them like sulphuric acid. Potassium and sodium burn very
brilliantly when heated in the gas.
When equal volumes of ammonia and boron trifluoride are
brought together, a compound BF,NH, is produced. This is a
white opaque solid which can be sublimed without alteration.
If one volume of the fluoride be brought in contact with two or
three volumes of ammonia, the gases condense and form colour-
less liquids having the composition BF,(NH), and BF¸(NH¸)¸.
These substances lose ammonia on heating, and are converted
into the solid compound (J. Davy).
Hydrated Boron Trifluoride.-No less than 700 volumes of
boron trifluoride are absorbed by one volume of water. Great
heat is evolved during the act of absorption, and the solution
when saturated is an oily fuming liquid, having a specific
gravity of 1.77, and acting as a powerful cautery like strong
sulphuric acid. When the saturated solution is heated, about
one-fifth of the absorbed gas is evolved, and a liquid which
possesses approximately the composition represented by the
formula BF₂+H₂O remains behind. This boils between 165°
and 200°, and undergoes partial decomposition, boric acid
being formed. The specific gravity of the vapour shows
that this substance is wholly dissociated in the gaseous
condition.2
FLUOBORIC ACID, HBF
383 When fluoride of boron is brought in contact with water,
boric and fluoboric acids remain in solution; thus:-
8BF+6H₂O=6HBF¸+2B(OH)3.
This acid is also formed when aqueous hydrofluoric acid is
saturated with boric acid (Berzelius). When the solution is
allowed to evaporate, the foregoing hydrate is formed, and this
may be considered to consist of a solution of boric acid in
fluoboric acid.3
Fluoboric is a monobasic acid, and when brought into contact
with bases, a series of salts termed the fluoborates is obtained
1 Moissan, Ann. Chim. Phys. [6], 24, 244.
2 Basarow, Ber. 7, 824 and 1121.
3 Basarow, loc. cit.
BORON TRICHLORIDE
647
(Berzelius). The same salts are formed by bringing together
the acid fluoride of an alkali with boric acid. In this case
the peculiar phenomenon presents itself of solutions which, to
begin with, have a neutral reaction, becoming alkaline when
they are mixed together. This is explained by the following
equation :-
B(OH),+2NaHF,= NaBF,+NaOH+2H₂O.
Most of the fluoborates are soluble in water, and crystalline.
When these salts are heated trifluoride of boron is given off,
and a fluoride remains behind.
BORON AND CHLORINE.
BORON TRICHLORIDE, BCI, = 116.3.
384 Amorphous boron takes fire spontaneously when brought
into chlorine gas with formation of boron trichloride,¹ and this
same body is formed, also with the evolution of light and heat,
when hydrochloric acid gas is passed over amorphous boron
(Wöhler and Deville). In order to prepare boron trichloride a
current of dry chlorine gas is passed over a strongly heated
mixture of boron trioxide and charcoal:-2
B₂O3 + 3C + 3Cl₂ = 2BC1¸ + 3CO.
For the purpose of preparing boron trichloride by this method
the arrangement represented in Fig. 180 is employed. It consists
of a porcelain tube, a b, placed in a furnace, and containing an
intimate mixture of fused boric acid and charcoal. The vapour
of the volatile trichloride is admitted into a Y-shaped tube, e, the
lower limb of which is placed in a freezing mixture; this con-
denses the chloride, and the excess of chlorine passes away by
the other limb. The temperature of the furnace must be high,
as the reduction of the oxide in presence of chlorine only takes
place at a bright red heat. In order to free the product from the
excess of chlorine, the liquid is shaken with mercury, and the
pure trichloride distilled off.
Boron trichloride is also obtained when the finely powdered
1 Berzelius, Pogg. Ann. 2, 147.
2 Dumas, Ann. Chim. Phys. [2], 31, 436, and 33, 376.
!
648
THE NON-METALLIC ELEMENTS
trioxide is mixed with double its weight of phosphorus penta-
chloride, and heated in a sealed tube for three days at a tempera-
ture of 150°. The tube is well cooled, opened, first warmed in a
water bath, and afterwards heated to redness, in order to drive
off the last portions of the chloride which remain in combination
with boron trioxide (Gustavson).
The chloride can also be readily obtained by passing chlorine
over the crude boron obtained by heating borax with half its
weight of magnesium powder.¹
Boron trichloride is a colourless liquid boiling at 18.2°
FIG. 180.
b
(Regnault). At 17° it has a specific gravity of 1:35. On heat-
ing in closed tubes it expands very rapidly, and yields a colourless
vapour, which has a specific gravity of 4-065. Boron trichloride
fumes strongly in the air, and is decomposed in contact with
water into hydrochloric and boric acids. When brought together
with small quantities of cold water it forms a solid hydrate, and
this when ignited in a current of hydrogen decomposes into
hydrochloric acid and amorphous boron. Boron trichloride can
be distilled over sodium without undergoing decomposition, and
zinc dust does not act upon it at temperatures below 200°.
1 Gattermann, Ber. 22, 195.
1

BORON IODIDE
649
When the chloride and trioxide are brought together in the pro-
portion of two molecules of the former to one of the latter, and
the mixture heated in a closed tube to 150°, a white gelatinous
mass is produced, from which half of the chloride is driven off at
100°, whilst the other half is not volatilized below a red heat.
would thus appear that an oxychloride is formed (Gustavson);
thus:-
:-
It
3BOCI = B.0, + BCl3.
When boron trichloride is heated with sulphuric anhydride
in a closed tube to 120°, sulphuryl chloride is formed
(Gustavson) :-
3SO3 + 2BC1, = 3SO₂Cl2 + B₂O3.
Ammonio-Chloride of Boron, 2BC1, + 3NH3.-This substance
is obtained as a white crystalline body, when ammonia gas is
led into very well-cooled boron trichloride. The compound
can be sublimed without decomposition, but in contact with
water it splits up into sal-ammoniac and ammonium borate
(Berzelius).
BORON AND BROMINE.
= 248.7.
BORON TRIBROMIDE, BBr
385 This compound can be obtained by the direct union of the
two elements at a red heat, but it is best prepared by passing
the vapour of bromine over a mixture of charcoal and boron
trioxide and rectifying the product over mercury. It is a
colourless, strongly fuming liquid, having a specific gravity of
2.69 and boiling at 90°5 (Wöhler and Deville). Its vapour
is colourless, and has the normal specific gravity of 8·78. It
behaves towards water and ammonia exactly in the same way
as the chloride.
BORON AND IODINE.
BORON TRIIODIDE, BI
= 388.47.
386 Pure boron does not combine directly with iodine, but the
iodide may be prepared by passing boron trichloride and
hydrogen iodide through a hot porcelain tube, or by passing
dry hydrogen iodide over impure amorphous boron heated to
the softening point of potash glass. It forms white crystalline
1
650
THE NON-METALLIC ELEMENTS
plates, melts at 43°, boils at 210°, is very hygroscopic, and is
decomposed by water in a similar manner to the chloride. It
dissolves in carbon bisulphide, benzene, &c., and has a density
of 3.3 at 50°. It very readily parts with its iodine in chemical
reactions, and has therefore been used in the preparation of
iodine derivatives of other elements.¹
OXIDES AND OXY-ACIDS OF BORON.
BORON TRIOXIDE, B₂O, AND BORIC ACID, HBO,.
387 Boron trioxide, the only known oxide of the element,
is obtained when boron burns in the air, or in oxygen. It is,
however, best prepared by heating boric acid to redness; thus:—
2B(OH)3 = B₂O3 + 3H₂O.
The fused mass thus obtained solidifies to a brittle glassy
solid, having a specific gravity at 4° of 183. It is a very
hygroscopic substance, uniting easily with water to form boric
acid. It is not volatile at a red heat, but volatilizes when
heated to whiteness. In consequence of its non-volatility,
boron trioxide decomposes, at a red heat, all salts whose acids
or corresponding oxides are volatile at a lower tempera-
ture, and thus carbonates, nitrates, sulphates, and other salts
are converted into borates. Most metallic oxides dissolve in
fused boron trioxide at a red heat, many of them imparting to
the mass characteristic colours; hence this substance is much
used in blowpipe analysis.
Orthoboric Acid, B(OH)3.-This compound is formed by the
union of the oxide with water. It was first prepared by the
decomposition of borax by means of a mineral acid, and known
under the name of Homberg's sal sedativum.
In the year
1774 Höfer, a Florentine apothecary, observed the occurrence
of this compound in the water of the lagoons of Monte Rotondo
in Tuscany, and in 1815 a manufactory was erected on the
spot for the purpose of obtaining boric acid from the water.
The undertaking did not flourish until the year 1828, owing
to the cost of fuel needed for the evaporation of the water
containing the acid in solution. In that year Larderel gave a
new impetus to the manufacture by using the natural heat of
1 Moissan, Compt. Rend. 112, 717; 114, 617.
BORIC ACID
651
the volcanic jets of steam, termed suffioni, to evaporate the water
charged with the acid.
Almost the whole of the boric acid brought into the European
market is derived from these Tuscan lagoons. Large volumes
of steam issue from volcanic vents near Monte Rotondo, Lago
Zolforeo, Sasso, and Larderello, and this steam is condensed in
the lagoons. The vapours themselves, the temperature of which
varies between 90° and 120°, contain only traces of boric acid,
but when this steam is allowed to pass into the lagoons, the water
soon becomes charged with the substance, and on evaporation
yields crystals of the acid. During the last fifty years many
borings have been made through the eocene strata, and thus
artificial suffioni have been formed.
In order to obtain the boric acid, the suffioni are surrounded
by basins, built of bricks or of glazed masonry, large enough
to contain two or three of the vents. Several of these basins
are usually built on the side of a hill, as shown in Fig. 181, and
the water of a spring or lagoon is allowed to run into the upper-
most one. The steam and gases are then permitted to pass
through this water for twenty-four hours, after which it is
conducted by a wooden pipe to the second basin, and so on
until the liquor has passed through from six to eight basins
and cannot take up any more boric acid. It then contains
about 2 per cent. of this substance. After settling, the clear
liquid is run in a thin stream on to a large sheet of corrugated
lead, 125 meters in length and 2 meters in breadth, placed in a
slightly inclined position and kept hot by the vapours from the
suffioni, which are allowed to pass underneath. In this way
20,000 liters of water can be evaporated every twenty-four
hours. The liquid running off the end of the plate is then
further evaporated in leaden pans until the boric acid begins
to crystallize out.
Hot water flows out from some of the artificial suffioni,
and this sometimes contains as much as 04 per cent. of
boric acid, and may be directly brought on to the pan. The
water of the Lago Zolforeo formerly contained only 0.05 per
cent. of boric acid, but this percentage has been considerably
raised by cutting off all ingress of fresh water, and dam-
ming off that part of the lake to which the suffioni have
access. The temperature in this portion of the lake was thus
raised to 65°, and the water contained from 02 to 03 per
cent. of the acid, whilst in the other portion the water had a
652
THE NON-METALLIC ELEMENTS
D
FIG. 181.
temperature of 26°
and only contained
0.08 per cent. of
boric acid. These
lagoons produce no
less than from 1,200
to1,500 kilos of boric
acid daily. In order
to purify the com-
mercial acid, which
contains about 25

per cent. of foreign
matter, it is re-
crystallized from
hot water and then
dried in chambers
heated by the suf-
fioni.
It is still a matter
of doubt in what
form the boric acid,
thus obtained, oc-
curs in the earth.
The occurrence of
ammoniacal salts
and sulphide of am-
monium, together
with the boric acid,
is very remarkable.
The most probable
hypothesis appears
to be that of Wöhler
and Deville,¹ ac-
cording to which
the acid is derived
from the decompo-
sition of a nitride
of
boron, BN.
Boron is one of the
elements which can
combine directly
1 Annalen, 74, 72, and 105, 71.
BORIC ACID
653
with nitrogen, and the compound thus formed is decomposed
by steam into boric acid and ammonia.¹
This theory is rendered the more probable by the observation.
made by Warrington, that the boric acid and sal-ammoniac found
in the crater of the Island of Volcano contain traces of boron
nitride. It is, however, possible that boric acid may be derived
from a sulphide of boron, which is decomposed by water into
sulphuretted hydrogen and boric acid (Sartorius von Walters-
hausen).
Boric acid is also manufactured from certain minerals, such as
borocalcite, which occurs in considerable quantities in the nitre
beds of Peru and Chili. It is likewise prepared from the natural
borax or tincal, which was first obtained from
the basins of dried-up lagoons in Central Asia,
and has lately been found in the borax lake
in California in such quantities that the amount
there obtained is sufficient to supply the whole
demand of the United States. For the purpose
of preparing boric acid from these sources, the
minerals are dissolved in hot hydrochloric acid, the boric acid,
which separates out on cooling, being recrystallized from hot
FIG. 182.
water.
Boric acid crystallizes from aqueous solution in shining six-
sided laminæ unctuous to the touch, and belonging to the asym-
metric system, having the form shown in Fig. 182. It has a
specific gravity of 14347 at 15° (Stolba), and is much more
readily soluble in hot than in cold water, as is shown by the
following table (Brandes and Firnhaber) :-
""
One part of boric acid requires for solution
""
""
39
""
""
""
""
""
29
29
""
22
در
23
99
در
ور
""
99
""
Parts of water. At
25.66 19°
14.88
25°
12.66
10.16
6.12
4.73
3.55
2.97 100°
37°.5
50°
62°5
75°
87°.5
Boric acid is a weak acid, and its cold saturated solution
colours blue litmus tincture of a wine-red colour like car-
bonic acid, but is without action on methyl-orange. When
1 Compare Popp, "Ueber die Bildungsweise der Borsäure in den Fumerolen
Toscanas." Annalen, Suppl. 8, 5.
2 Chemical Gazette, 1855, 419.
I
654
THE NON-METALLIC ELEMENTS
the solution is boiled, the acid volatilizes with the aqueous
vapour, and this property explains the presence of the acid in
the suffioni. Boric acid is also easily soluble in alcohol, and
when this solution is inflamed it burns with a characteristic
green-edged flame. The same green tint is seen when a small
bead of the molten acid on the end of a very fine platinum
wire is brought into the fusion-zone of a non-luminous flame.
The spectrum of this green flame consists of several bright
bands the brightest of these (a) is situated in the yellowish
green, and two others (B and y), equally characteristic, occur in
the green.¹
The action of boric acid upon the colouring matter of
turmeric is highly characteristic. If a piece of paper
coloured with turmeric be moistened with a solution of the
acid it turns brown, and this coloration increases when the
paper is dried. Alkalis give a similar coloration to turmeric
paper, but the colour thus produced disappears on the addition
of an acid, whilst the brown tint imparted to turmeric paper by
boric acid remains unaltered in presence of free hydrochloric
acid and is converted into blue or green by dilute alkalis.
Metaboric Acid, BO(OH), is produced when boric acid is
heated to 100°. It forms a white powder which at the above
temperature undergoes a slow but complete volatilization.2
Pyroboric Acid, BО(OH)2.—This substance is a brittle glass-
like mass obtained when boric acid is heated for a long time
to 140°.
Boric acid dissolves readily in fuming sulphuric acid, and
from-this solution tabular crystals separate out having the
composition-
2SO OB+SO.
SH
4
These crystals when heated evolve sulphur trioxide.
When boric acid is evaporated with an excess of concentrated
phosphoric acid, and the dry residue treated with water, in order
to separate the phosphoric acid, a white amorphous mass is left
which possesses the composition BPO,. This substance is in-
fusible, it is not attacked by strong acids, but dissolves in
aqueous potash. The existence of these compounds points to
the conclusion that boron trioxide possesses feebly basic pro-
perties, resembling in this respect alumina, Al2O. This oxide
¹ Lecoq de Boisbaudran, Spectres Lumineux, 193.
2 Schaffgotsch, Pogg. Ann. 107, 427.
THE BORATES
655
also sometimes acts as a weak acid, and sometimes as a weak
base forming a corresponding phosphate, AIPO.
388 The Borates.-Boric acid, like phosphoric acid, forms
many series of salts, several of which are derived from the above-
named modifications of the acid. Ortho-boric acid is tribasic;
but its salts are very unstable, and the only well-defined ortho-
'borate which is known is Mg(BO3)2. The tribasic character of
boric acid is however clearly shown by its volatile ethereal
salts, compounds in which the hydrogen of the boric acid is
replaced by the organic radical, C₂H. Thus ethyl orthoborate,
B(OC₂H), is a colourless liquid, which volatilizes without
decomposition, and has a vapour density corresponding to
the above formula.
The metaborates are much more stable compounds. Thus we
are acquainted with the following:-
Potassium metaborate
Sodium metaborate
Magnesium metaborate
Calcium metaborate.
Larderellite.
Lagonite
Boracite
•
•
•
·
The pyroborates are also stable compounds, and to this class
belong the following:-
Borax, or sodium pyroborate
Na2B4O7.
Borocalcite, or calcium pyroborate. CaB,O,.
Boronatrocalcite
•
•
Na₂BO,+2CaB,O,.
In addition to these, other salts are known which have a more
complicated constitution, corresponding in this respect to cer-
tain classes of phosphates. The following are examples of
such borates :-
KBO2.
NaBO2.
Mg(BO2)2.
Ca(BO2)2.
•
(NH₁),B₂O13-
Fe,BO12
2MgBO+MgCl2.
The crystalline borates almost always contain water of crystalli-
zation, and, with the exception of those of the alkalis, are either
insoluble or only slightly soluble in water. They are all easily
decomposed by acids, and therefore, when they are warmed with
656
THE NON-METALLIC ELEMENTS
sulphuric acid and alcohol, and the mixture is ignited, the
characteristic green flame of boric acid is observed. The same
green coloration is observed when a trace of a borate is brought
on to a platinum wire with a small quantity of acid potassium
sulphate, and this held in the non-luminous flame.
BORON AND SULPHUR.
BORON TRISULPHIDE, BS.
389 Berzelius first obtained this compound by heating boron
in the vapour of sulphur. It can be most readily prepared
by the action of the vapour of carbon bisulphide upon an
intimate mixture of lamp-black and boron trioxide (Wöhler and
Deville) :-
――――――――――――
3CS₂+ 3C + 2B₁O¸ = 2B₂S¸ +6CO.
3
It may also be obtained by heating boron to bright redness in
a current of sulphuretted hydrogen, or by heating boron iodide
with sulphur above 440°.
Boron trisulphide occurs generally as a white, glassy, fusible
solid, but it is sometimes obtained in the form of silky needles.
It melts on heating, softening at 310°, has a density of 1·55, and
can be distilled in a current of sulphuretted hydrogen. It is
at once decomposed in contact with, water with formation of
boric acid and sulphuretted hydrogen :-
BS + 6H₂O = 2B(OH)3 + 3H₂S.
It possesses a pungent smell and attacks the eyes.
BORON PENTASULPHIDE, B₂S.
This compound is formed as a white crystalline powder by
treating a solution of sulphur in carbon bisulphide at 60° with
boron iodide. It melts at 390°, has a density of 185, and
is decomposed by water into sulphuretted hydrogen, sulphur,
and boric acid.2
1 Sabatier, Compt. Rend. 112, 862; Moissan, Compt. Rend. 115, 203.
2 Moissan, Compt. Rend. 115, 271
BORON NITRIDE
657
BORON AND NITROGEN.
BORON NITRIDE, BN.
390 Amorphous boron combines directly with nitrogen at a
white heat to form the above compound, which is also produced
when the compound of chloride of boron and ammonia is
passed together with ammonia gas through a red-hot tube
(Martius). Boron nitride was first obtained by Balmain, in
the year 1842, by heating boron trioxide with the cyanide of
potassium or of mercury:-
B₂O3 + Hg(CN), = 2BN + CO + CO₂ + Hg.
2
The best mode of preparing the substance is by heating an
intimate mixture of one part of anhydrous borax with two parts
of dry sal-ammoniac to redness in a platinum crucible (Wöhler).
The mass is washed first with water containing hydrochloric acid,
and afterwards with pure water, and lastly, treated with hydro-
chloric acid in order to remove completely the boric acid which
is mixed with the nitride. The reaction which takes place is
represented by the equation :-
Na,BO, +4NH CI = 4BN + 2NaCl + 2HCl + 7H₂O.
The nitride thus obtained is a white, light, perfectly amorphous
powder resembling finely-divided tale. When it is heated in
the flame it phosphoresces with a bright greenish-white light.
Heated in a current of steam it yields ammonia and boric
acid:-
:-
BN + 3H₂O = H,BO3 + NH3.
Hydrofluoric acid dissolves the nitride slowly with formation of
ammonium fluoboride; thus:-
BN + 4HF
=
NH,BF.
BORON AND PHOSPHORUS.
391 Boron does not combine directly with phosphorus, but a
phosphide can be prepared by the use of boron iodide.¹ When
this substance and yellow phosphorus are dissolved in carbon bi-
sulphide, a red insoluble powder is formed, which can be sublimed
1 Moissan, Compt. Rend. 113, 624, 726.
43
658
THE NON-METALLIC ELEMENTS
in vacuo at 200° in red crystals. This compound has the formula
PBI,, is very hygroscopic, and is decomposed by water. When it
is heated at 160° in hydrogen, it loses one atom of iodine, form-
ing a volatile crystalline substance of the formula PBI, which
on further heating in hydrogen is converted into boron phos-
phide, PB. This compound is a colourless insoluble powder,
which burns brilliantly in the air at 200°, and on strong ignition
in a current of hydrogen is converted into a lower phosphide of
the formula PB5. Phosphide of boron is also formed when the
white solid compound of boron bromide with phosphine, BBr
PH,, is heated at 300°, hydrobromic acid being evolved.¹
CARBON. C=1191.
392 Carbon occurs in the free state in nature in two distinct
allotropic modifications as diamond and graphite. It forms,
moreover, an invariable constituent of all organized bodies, and
when any such substance is heated in absence of air, a
portion of the carbon remains behind in the form of amorphous
carbon or charcoal.
Diamond.-On account of its brilliant lustre and remarkable
hardness the diamond has been valued for ages as a precious
stone. Manilius appears to be the first to mention it in his
Astronomia: "Adamas punctum lapidis, pretiosior auro." Up
to the year 1777 the diamond was believed to be a species of
rock-crystal, but Bergman in that year proved, by means of
blowpipe experiments, that the diamond contained no silica,
and came to the conclusion that it was composed of a peculiar
earth to which he gave the name of terra nobilis. But as soon
as the fact of its combustibility had been definitely ascertained
it was classed amongst the fossil resins.
This combustibility of the diamond appears to have been
observed at an early period, although the fact does not seem to
have attracted the general attention of the older chemists, as
statements of a contrary character are recorded by them.
Thus, for instance, Kunkel states that his father, at the com-
mand of Duke Frederick of Holstein, heated diamonds in his
gold-melting furnace, for nearly thirty weeks, without their
undergoing any change. It is to Newton, however, that we
owe the first argument which went to prove that the diamond
¹ Besson, Compt. Rend. 113, 78.
DIAMOND
659
was capable of undergoing combustion on account of its high
refractive power, a property characteristic of the class of oily
bodies. In the second book of his Opticks, Newton says upon
the subject," Again the refraction of camphire, oyl-olive, lint-
seed oyl, spirit of turpentine and amber, which are fat sulphu-
reous unctuous bodies, and a diamond, which probably is an
unctuous substance coagulated, have their refractive powers in
proportion to one another as their densities without any con-
siderable variation." The conclusion to which Newton was
led by theoretical considerations was experimentally proved to
be correct in the year 1694-5 by Averami and Targioni, members
of the Academia del Cimento, who, at the request of the Grand
Duke Cosmo III., of Tuscany, placed a diamond in the focus
of a large burning-glass and observed that it entirely' disap-
peared. Francis I., who is said to have received from an
alchemist an anonymous receipt for melting diamonds, exposed,
in the year 1751, diamonds and rubies of the value of 6,000
gulden for twenty-four hours to the action of a powerful fire;
the rubies were found unaltered, but the diamonds had altogether
disappeared. The volatilization of the diamond by means of heat
was from this time forward made the subject of numerous expe-
riments. Thus, Darcet observed in 1766 that diamonds dis-
appear when they are heated in a cupel-furnace, even in closed
crucibles, but, continuing his experiments at the request of the
Paris Academy, he, together with Rouelle, found that when heated
in perfectly hermetically-sealed vessels, the diamond did not
disappear. Macquer, in the year 1771, was the first to observe
that when the diamond undergoes volatilization it appears to
be surrounded by a flame. In conjunction with Cadet and
Lavoisier, he afterwards found that a true combustion takes
place. In continuation of these experiments Lavoisier, to-
gether with Macquer, Cadet, Brisson, and Baumé,¹ placed a
diamond in a glass vessel containing air collected over mercury,
and on igniting the diamond by means of a burning-glass, they
found that carbonic acid gas was produced.
Charcoal and diamond were now placed together under the
head of carbon, and their chemical identity was fully proved.
Smithson Tennant, in 1796, corroborated these results by show-
ing that equal weights of these two substances yielded equal
weights of carbon dioxide on burning, whilst Mackenzie in 1800
added to this the proof that the same weight of graphite also
1 Lavoisier, Euvres, tome ii. 38, 64.
660
THE NON-METALLIC ELEMENTS
gives the same weight of carbon dioxide. Allen and Pepys, in
1807, came to the same conclusion, and Davy, from experiments
made upon diamonds with the same lens which the Florentine
Academicians had used in 1694, showed, in 1814,¹ that no trace
of water is formed in the combustion of diamonds, thus proving.
that this substance contains no hydrogen, but consists of chemi-
cally pure carbon. Davy likewise reduced the carbonate of lime
obtained from the air in which a diamond had burnt, by means
of potassium, in this way preparing a black powder which, like
ordinary carbon, took fire when thrown into a flame.
393 The diamond came to Europe from the East. The mines
in Purteal, which in former days were famous as those of
Golconda, and where the Koh-i-noor was found, are at present
almost entirely exhausted. The diamond fields of Minas Geraes
in Brazil which have been worked since the year 1727 are prob-
ably the richest in the world, and yield yearly about 2,000
kilogr. of stones. Of late years the diamond fields in the Cape
have become celebrated, and they now supply nearly the whole
demand of the world. During the year 1888 the mines of
British South Africa produced diamonds weighing in all
3,567,744 carats, the value of which amounted to £3,608,212.2
Diamonds are also found in Borneo, in the Ural, in New
South Wales, in Bahia, in California, in Georgia, as well as in
other localities.
The substance termed carbonado is a porous and massive form
of impure diamond, and occurs in black or brownish fragments,
which sometimes weigh as much as 1 kilogr. When examined
with a lens it exhibits cavities filled with small octohedra.
The diamond always occurs in alluvial deposits in the neigh-
bourhood of a certain kind of micaceous rock which was
first observed in the Brazils, and has been termed itacolumite.
This rock is distinguished by the fact that in thin plates or bars
it is very flexible. It was for a long time doubtful whether the
diamonds occur in situ in the rock; small diamonds have,
however, been found embedded in the matrix, and Jeremejew 3
has observed the existence of microscopic diamonds in a talcose
schist occurring in the Southern Ural. This rock contains a
hydrated silicate termed xanthophyllite occurring in yellow
tabular crystals, in the inside of which the small crystals of
1 "Some Experiments on the Combustion of the Diamond and other Carbona-
ceous Substances," Phil. Trans. 1814, p. 557. Read June 23, 1814.
2 Thorpe's Dict. Article "Diamond."
3 Ber. 4, 903.
ARTIFICIAL DIAMOND
661
diamond are embedded in a direction parallel to the cleavage
of the xanthophyllite.
"The "blue earth" in which the diamonds occur at the Cape
contains, in addition to the occasional diamonds large enough to
be picked out, about 0·5-0·1 grams of crystallized carbon per
cubic metre, and this can be isolated from the whole mass by
first of all boiling with sulphuric acid, washing with water, and
treating with aqua regia, after which the mass is again washed
with water and then treated successively with hydrofluoric acid,
sulphuric acid, and water twelve or fourteen times to remove
the whole of the mineral matter, the residue, which amounts
to about 0.094 mgr. from 2 kilogr. of earth, then consisting
almost entirely of graphite, carbonado, and microscopic trans-
parent diamonds, the graphite being present in greater propor-
tion than the diamond.¹
All diamonds when burnt leave a residue consisting of a small
quantity of incombustible ash, those which are colourless leaving
the least. The ash amounts to from 0·05-02 per cent. in the
case of diamond, and to as much as 45 per cent. in the case of
carbonado. It has a reddish colour, and always contains iron
and silica, which are usually accompanied by lime and
magnesia.2
3
Diamond has also been found, both in the form of colourless
crystals and of carbonado, accompanied by graphite, in a meteo-
rite found in Canon Diablo, and this occurrence is of the
highest importance, as showing that the diamond has been
probably formed by crystallization from a mass of iron heated to
a high temperature.
This probability has been converted into a certainty by
Moissan, who has succeeded in preparing both diamond and
carbonado artificially. He has found that when carbon is
allowed to crystallize from solution in molten iron or silver under
a high pressure diamond is produced, whilst under ordinary
conditions the carbon separates out as graphite.
In order to obtain artificial diamond, pure sugar charcoal is
strongly compressed in a cylinder of soft iron, which is then
closed by a plug of the same metal. This is placed in a crucible
containing about 200 gram. of molten iron, melted by means of
1 Couttolenc; Moissan, Compt. Rend. 116, 292.
2 Roscoe, Proc. Manchester Lit. and Phil. Soc.; Moissan, Compt. Rend. 116,
458.
3 Friedel, Compt. Rend. 116, 290; Moissan, Compt. Rend. 116, 288.
?
662
THE NON-METALLIC ELEMENTS
an electric furnace, and the crucible at once withdrawn from the
furnace and cooled as rapidly as possible. Water does not cool
the mass quickly enough, owing to the formation of a badly
conducting layer of steam, and it is, therefore, better to cool
the crucible by immersion in molten lead. Iron, like water,
expands when it solidifies, and the solid crust first formed on
the outside of the mass therefore exerts an enormous pressure
on the interior portion during the crystallization of the latter.
The mass is then treated with hydrochloric acid, which dis-
solves the iron, and the residue is then subjected to a treat-
ment resembling that employed in isolating the crystallized
carbon from the blue earth. The residue consists of graphite,
a maroon-coloured variety of carbon, carbonado, and trans-
parent colourless diamond. The densest part of this is isolated
by further treatment with hydrofluoric and sulphuric acids,


FIG. 183.
Ju
followed by the action of fuming nitric acid and potassium
chlorate, to remove the graphite, after which the remaining
fragments are separated according to their density by placing
them in liquids of different densities. In this way small
fragments having the crystalline form, density, and hardness of
diamonds are obtained, and these are found on combustion to
yield the expected amount of carbonic acid. Enlarged sketches
of some of these artificial diamonds are shown in Fig. 183,
which exhibits their crystalline structure and octahedral shape.
The size and transparency of the diamonds produced depends
largely upon the rapidity of the cooling, diamonds of 05 mm.
diameter and of the brilliant limpidity of the natural diamond
having been obtained by the use of molten lead, whilst in the
fusions which were cooled by water the diamonds were much
smaller and less limpid.
PROPERTIES OF DIAMOND
663
d
One of the crystals thus produced showed the interesting
property, which has been observed in certain Cape diamonds, of
splitting into fragments when preserved, owing probably to a
state of strain produced by the rapid cooling.¹
The diamond crystallizes in hemihedral forms belonging to the
regular system, the crystals being usually octohedral in type,
although the simple form rarely occurs alone. Combinations of
two tetrahedra or two rhombic dodecahedra, the hexakistetra-
hedron (a), and the hexakisoctahedron, a forty-eight sided
figure (b and c), or combinations and twins (d) of this form as
well as combinations of the hexakistetrahedron with two tetra-
b
FIG. 184.
hedra (e), are those which usually occur (Fig. 184). The faces
of the crystals are not unfrequently curved, and the form
of the crystal distorted, whilst twin crystals are also found.
All diamonds cleave easily in directions parallel to the faces
of the regular tetrahedron, showing this to be the primary
form. The fracture is conchoidal. The crystals are usually
colourless and transparent, though sometimes they are green,
brown, and yellow. Blue and black crystals rarely occur.
The specific gravity of diamond varies from 35 to 36, that of
1 Moissan, Compt. Rend. 116, 218; 118, 320; see also Friedel, Compt. Rend.
116, 224.
1
664
THE NON-METALLIC ELEMENTS
the purest specimens being, according to Baumhauer, 3.518 at
4°, whilst carbonado is found to vary in density from 3 to 3.5.
The diamond possesses a peculiar and characteristic lustre, and
refracts light very powerfully, its index of refraction being 2.439.
It is also the hardest of known substances. The lustre (termed
adamantine) of the natural faces of the diamond is greatly in-
creased by cutting and polishing and by giving it numerous
facets which render it capable of reflecting and dispersing light
in all directions and thus add greatly to its lustre, which
depends on the fact that it reflects all light which falls on
its posterior surface at an angle of incidence greater than
24° 13′. Diamonds are cut or polished by pressing the surface
of the gem against a revolving metal wheel covered with a
mixture of diamond dust and oil, no other substance except
boride, and perhaps silicide of carbon being hard enough to
abrade the diamond.
Most diamonds when examined under the microscope exhibit
cloud-like darker portions. Dark spots are also frequently seen
in them, which Brewster considers to be cavities, but Sorby
has shown that they consist of small crystals of much lower
refractive power than the diamond itself.
Crystals have been found within which impressions of other
diamonds are seen. Kenngott observed a yellow octohedron
which was enclosed in a colourless diamond, whilst Goeppert
has noticed in certain diamonds the occurrence of a cell-like
structure resembling that obtained when a jelly undergoes
solidification.
The specific heat of diamond at ordinary temperatures is
0·1469 (Regnault), whilst at 985° it is 0459 (Weber).
394 The diamond burns with tolerable ease when heated in
the air or in oxygen. Thus if a diamond is placed on a piece of
platinum foil it may be ignited by the flame of the mouth blow-
pipe. To demonstrate the combustibility of the diamond in
oxygen and the production of carbonic acid, the apparatus
shown in Fig. 185 may be employed. Two thick copper wires
(c) pass through the caoutchouc cork fitting into the cylinder
containing oxygen. These are connected together by a spiral
of thin platinum wire (bb) wrapped round the copper. Into
this spiral a splinter or small diamond (a) is placed, and on
allowing a current from 6 to 8 Grove's cells to pass through the
platinum it is heated to whiteness. The diamond then takes
fire, and on breaking the circuit it is seen to burn brilliantly
PROPERTIES OF DIAMOND
665
until it is completely consumed. A small quantity of clear
lime-water may be poured into the cylinder before the experi-
ment; the liquid remains clear until after the diamond is
burnt.
The exact identity of the gas produced by burning the
diamond in oxygen with carbon dioxide has been proved by
combining it with caustic soda. The resulting substance was
found to be in every respect identical with ordinary sodium
carbonate,¹ The spectrum given by the gas is also identical
with that given by carbon dioxide prepared in the usual manner.2
The temperature of ignition of the diamond varies from
760°-875°,3

FIG. 185.
When heated in hydrogen, diamond undergoes no change
even when heated to whiteness, and a crystal which was heated
embedded in charcoal powder to the melting point of cast
iron remained unchanged, while a cut brilliant, on the other
hand, when thus heated became black, owing to a thin
coating of graphite being formed on its surface. 4 When
the diamond is heated between the carbon poles of a powerful
electric battery it swells up and becomes converted into a
black mass of graphite.
1 Krause, Ber. 23, 2409.
2 Roscoe and Schuster, Proc. Manchester Lit. and Phil. Soc. 7, 80.
3 Moissan, Compt. Rend. 116, 460.
4 G. Rose, Berlin Acad. Ber. 1872, p. 516.
666
THE NON-METALLIC ELEMENTS
The diamond is by no means easily attacked by most
chemical reagents. It is unaffected by chlorine or hydrochloric
acid at 1200°, and is not acted on when heated with potassium
chlorate or iodic acid. It is moreover not attacked by boiling
sulphuric acid, hydrofluoric acid or a mixture of nitric acid and
potassium chlorate, and these reagents are therefore used, as
already described, for separating the diamond from the other
forms of carbon and all mineral substances. When fused with
carbonate of sodium or potassium the diamond gradually
disappears, being converted into pure carbon monoxide. The
diamond is also attacked by sulphur at 1000°.¹
The diamond is the most valuable of precious stones, those
which are colourless and have the purest water being espe-
cially prized. The most beautiful of this kind is the Pitt or
Regent diamond, which weighs 136 25 carats or 431 grains,
and is worth £125,000. More rarely some of the transparent
but coloured stones are highly prized. Thus the celebrated
blue Hope diamond, weighing only 4 carats, but of peculiar
beauty and brilliancy, is valued at £25,000.
Among other celebrated diamonds that in the possession of
the Nizam of Hyderabad must be mentioned. It was found
about half a century ago, having been used by a child as a toy.
During the Indian mutiny a portion of this diamond was broken
off, and the remainder, of which there is a model in the
collection of the British Museum, weighs about 277 carats
(Maskelyne). The largest diamond in Europe is set on one
end of the Russian sceptre; it has a yellow colour, and weighs
1944 carats. The yellow Tuscan diamond of the Emperor of
Austria weighs 139 carats. The Koh-i-noor, one of the British
crown diamonds, originally came from India, and when brought
to this country in its rough state, weighed 186 carats. Owing to
the imperfect character of its original cutting it had to be recut,
and was thus reduced to 106 carats. The largest diamond ever
found in the Brazils, termed the Star of the South, originally
weighed 254 carats, but was reduced by cutting to 127 carats.
A Cape diamond of 288 carats has been found, but has since
been cut, and in consequence has lost a considerable amount
of its weight. The diamonds in the possession of the Shah of
1 Moissan, Compt. Rend. 116, 460.
The word "carat" (Arabic, "qîrat ") is derived from Kepáriov, St. John's
bread, or karob, the seeds of this plant having been formerly used as weights.
1 diamond carat = 3.17 grains or 0.2054 grm.
GRAPHITE
667
Persia were undoubtedly derived from the plunder of Delhi by
Nadir Shah. Their weight is unknown. According to Tavernier,
the Great Mogul possessed a diamond, which before cutting
weighed 900 carats, whilst afterwards it only weighed 279-6
carats.
Rough or small diamonds, which cannot be used as brilliants,
are termed "Boart" and are employed for a number of other
purposes. Their powder is largely used for the purpose of
cutting diamonds and other precious stones, whilst the splinters
are used for the purpose of writing upon glass, although they
will not cut that substance. For that purpose we require a
naturally-curved edge of the crystal; the curved edge pro-
ducing a deep slit determining the fracture of the glass with
certainty, whilst the straight edges merely scratch the surface.
Diamonds are also largely used in the construction of rock-boring
tools.
395 Graphite was also known to the ancients, but up to the
time of Scheele no distinction was made between it and the
closely analogous substance, sulphide of molybdenum, MoS2, and,
at that period, both these metal-like minerals which leave a mark
on paper, were termed indiscriminately plumbago or molybdæna.
Graphite appears to have been first distinguished by Conrad
Gessner in his work De rerum fossilium figuris, in 1565. A
picture of a black-lead pencil occurs there, and underneath
is written "Stylus inferius depictus ad scribendum factus est,
plumbi cujusdam (factitii puto, quod aliquos stimmi Anglicum
vocare audio) genere, in mucronem derasi, in manubrium ligneum
inserti." 1
For many years graphite was supposed to contain lead, whence
the name plumbago, or black-lead. The former name seems to
be derived from the Italian grafio piombino, which also, like the
other name graphite, from ypápw, I write, indicates its use.
In the year 1779 Scheele showed that molybdenum-glance
is totally different from graphite, and that this latter body
when treated with nitric acid is converted into carbonic acid, so
that it must be looked upon as a kind of mineral carbon.
It
Graphite occurs in nature tolerably widely distributed.
is usually found in lumps or nodules in granite, gneiss, and
other crystalline rocks. The best graphite for the purpose of
1 "The pencil represented below, is made, for writing, of a certain kind of lead
(which I am told is an artificial substance termed by some, English antimony,)
sharpened to a point and inserted in a wooden handle."
668
THE NON-METALLIC ELEMENTS
making black-lead pencils, was that formerly found exclusively
at Borrowdale in Cumberland, in green slate. These mines are
however, now almost exhausted, not having been worked for the
last forty years. In the sixteenth and seventeenth centuries
they were so productive as to yield an annual revenue of
£40,000, although they were only worked a few weeks in the
year for fear of exhausting the mine. Graphite is also found at
Passau in Germany, in Bohemia and in Styria. It likewise
occurs in many places in the United States, the deposits at
Sturbridge, Mass., and at several localities in New York being
large enough to yield a considerable supply. By far the largest
mine in the United States is the "Eureka Black-Lead Mine "at
Sonora in California. The graphite here forms a layer of some
twenty to thirty feet in thickness. It is so pure that it may be
obtained in large blocks. In the year 1868 not less than one
million of kilos. were raised each month.¹ Large quantities of
graphite are also found in Ceylon. In Southern Siberia this
substance occurs in considerable quantities in the Batougal
mountains, and is largely exported to Europe.
Graphite commonly occurs in compact foliated or granular
masses, but occasionally in small six-sided tables, which accord-
ing to Kenngott belong to the hexagonal, but according to
Nordenskjöld to the monosymmetric system. It has a steel-gray
colour, an unctuous touch, and it is so soft that it gives a black
streak on paper. Its specific gravity varies from 2015 to 2.583,
and this considerable variation is due to the fact that almost
all natural graphite contains more or less impurity which,
when the graphite is burnt, remains behind as ash and consists
of alumina, silica, and ferric oxide, with small traces of lime
and magnesia. It usually contains 0.5 to 13 per cent. of
hydrogen, a fact which seems to point to its organic origin.
Graphite is a good conductor of heat and electricity, whilst the
diamond is a non-conductor. The intumescent graphite obtained
by Moissan commences to burn in oxygen at 575°, but the
temperature of ignition varies for the different specimens of
natural graphite. The specific heat at ordinary temperatures
is 0.202 (Regnault), whilst at 978° it is 0:467 (Weber).
The artificial production of graphite by melting cast iron
containing a large proportion of carbon, and allowing it to cool
1 Chem. News, 1868, 299.
2 Mène, Compt. Rend. 64, 1019 and 1867.
3 Regnault, Ann. Chim. Phys. [2], 1, 202.
GRAPHITE
669
slowly, was first observed by Scheele in 1778. Cast iron
contains a certain amount of carbon combined with iron; in the
molten condition, however, it can dissolve a much larger
quantity of carbon, up to as much as 4 per cent. of its weight.
This excess crystallizes out as graphite when the metal cools.
The coarsely crystalline gray pig-iron owes its peculiar pro-
perties as well as its appearance to the presence of graphite, and
when this form of iron is dissolved in acid, scales of graphite
remain as an insoluble residue.
Graphite also occurs in many meteoric masses, as, for
instance, in the meteorite which fell in 1861 at Cranbourne
near Melbourne, and this meteoric graphite is, according to
Berthelot, identical in properties with iron-graphite. We may
thus conclude that the meteoric mass in which it has been found
has been exposed to a very high temperature.
According to Wagner, the black deposit which is formed.
by the spontaneous decomposition of hydrocyanic acid, CNH,
contains graphite, as may be seen by washing the deposit
with strong nitric acid, when the insoluble scales of graphite
are left behind.¹ Another remarkable mode of production
of graphite, most likely from cyanogen compounds, was first
observed by Pauli 2 in the manufacture of caustic soda from the
black-ash liquors. These liquors are evaporated to a certain
degree of consistency, and Chili saltpetre is added to oxidise
the sulphur and cyanogen compounds present. Torrents of
ammonia are thus evolved, and a black scum of graphite is
observed to rise to the surface.
When the vapour of chloride of carbon is led over melted cast
iron, ferric chloride is evolved, and carbon dissolves in the iron
until it becomes saturated, after which hexagonal plates of
graphite separate out.3
The carbon which occurs in crystalline boron remains as
amorphous carbon when the diamond boron is heated to red-
ness. When it is heated to whiteness it takes the form of
graphite. Both diamond and amorphous carbon are converted into
graphite at the temperature of the electric arc, and when the
arc is maintained between two poles of amorphous carbon the
negative pole is found to be largely converted into graphite.
Many specimens of graphite possess the remarkable property
1 Wagner's Jahresb. 1869, p. 230.
2 Phil. Mag. [4], 21, 541.
3 H. Deville, Ann. Chim. Phys. [3], 49, 72.
I
I
1
¦
1
I
·
670
THE NON-METALLIC ELEMENTS
of swelling up and leaving a very voluminous residue of finely-
divided graphite, in the same manner as mercuric sulphocyanide
(Pharaoh's serpents) when moistened with strong nitric acid and
then strongly heated. This property, however, is not common
to all specimens of graphite, and it has even been proposed
to divide the known graphites into two classes, those which
give this nitric acid reaction being recognized as graphite,
whilst those which do not, such as the graphite of the electric
arc and that obtained from cast iron, are distinguished as
graphitite.¹
The intumescent variety may be prepared by fusing platinum
in a carbon crucible in an electric furnace. When the ingot is
treated with aqua regia a residue of graphite of this kind is
left. It is also found in the interior of an ingot of cast iron
which has been cooled by water, whilst the external layers
only contain the non-intumescing variety. When intumescence
takes place oxides of nitrogen and a little carbon dioxide are
given off and a residue of pure graphite left, which is scarcely
altered by further treatment with nitric acid. The intume-
scence is, therefore, probably due to the sudden evolution of
gas produced by the action of traces of nitric acid remaining
in the graphite upon a little amorphous carbon entangled
among the plates of graphite.2
Graphite in its chemical relations occupies a position totally
distinct from that of all other forms of carbon. When graphite
is repeatedly treated with fuming nitric acid and potassium
chlorate it is converted into a compound which contains
both hydrogen and oxygen, and is known as graphitic acid
or graphitic oxide (p. 729), whereas under the same treat-
ment diamond is unaltered and amorphous carbon completely
dissolved. This reaction, as has been pointed out, provides a
means of distinguishing graphite from the other allotropic
forms of carbon, and is the basis of Berthelot's method for
estimating the proportions of the three varieties in a mixture.
When the electric arc is maintained between carbon poles, it
is observed that, however powerful the current, a maximum
degree of brightness is attained. This seems to indicate that
the carbon of the pole volatilises at the temperature corre-
sponding to this brightness. By heating a portion of the pole
to this maximum temperature and then shaking it off into a
1 Luzi, Ber. 24, 4085; 25, 216, 1378; 26, 890.
2 Moissan, Compt. Rend. 116, 608.
GRAPHITE
671
calorimeter it has been found that the temperature of volatili-
sation of carbon is about 3,500°, and this is, therefore, the
maximum temperature which can be attained in an electric
furnace in which carbon poles are employed.¹
396 As already mentioned, many specimens of graphite in-
tumesce when moistened with nitric acid and strongly heated. It
is also found that when finely-powdered graphite is heated with a
mixture of one part of nitric and four parts of strong sulphuric
acid, or when a mixture of fourteen parts of graphite and one
part of potassium chlorate is warmed with twenty-eight parts of
strong sulphuric acid, the graphite assumes a purple tint, but
on subsequent washing returns to its original colour. It
contains now oxygen, hydrogen, and sulphuric acid, and when
it is heated to redness swells up with a copious evolution of
gas, and then falls to an extremely finely-divided powder of
pure graphite, which has a specific gravity of 2.25.2 This
process is employed for the purpose of purifying natural gra-
phite. With this object it is first ground, and the powder well
washed in long troughs in order to remove as much as possible
of the earthy matrix with which it is mixed. The graphite thus
obtained is pure enough for many uses; but if it is required in
the pure state, the powder must be treated with potassium
chlorate and sulphuric acid as above described. The fine powder
is then thrown upon water, on the surface of which it swims,
whilst the earthy matters sink to the bottom. The foliated
graphite answers best for this purpose, amorphous graphite
being more difficult to purify. This variety may, however, also
be rendered pure if a small quantity of fluoride of sodium be
added to the mixture as soon as the evolution of chlorine
and its oxides has ceased, the object of this addition being to
remove the silica as silicon tetrafluoride.
Graphite is employed for a great variety of purposes. The
Cumberland black-lead pencils, the first of their kind, were
originally manufactured by cutting slips of graphite out of the
solid block. Experiments were afterwards made for the purpose
of making use of the graphite powder, which was fused with
sulphur or antimony. The pencils thus prepared were, however,
hard and gritty. A remarkable improvement in the manufac-
ture, used up to the present day, was introduced by Comte. The
powdered graphite is mixed with carefully-washed clay, and the
1 Violle, Compt. Rend. 115, 1273.
2 Brodie, Ann. Chim. Phys. [3], 45, 351.
-4
1
1
1
672
THE NON-METALLIC ELEMENTS
mixture placed in short iron cylinders having an opening at the
bottom. The semi-solid mass is then pressed through the hole
and assumes the form of a fine thread, which can be cut up into
the required lengths and used for pencil-making.
Another important application of graphite is for the prepara-
tion of the black-lead crucibles largely used in metallurigical
operations, especially in the manufacture of cast steel. The
Patent Plumbago Crucible Company at Battersea employ Ceylon
graphite for the manufacture of their crucibles, whilst the graphite
employed by Krupp of Essen in the manufacture of the cruci-
bles in which his celebrated cast steel is melted is obtained
from Bohemia. The finely-ground graphite is well mixed with
Stourbridge fire-clay and water, so as to obtain a homogeneous
mass; the water is then pressed out and the mass formed into
blocks, which have to lie for many weeks. The plasticity of the
mass is thus much increased. The crucible is next moulded by
hand on a potter's wheel, or sometimes formed in a mould;
it is then slowly dried, and afterwards ignited in a pottery
furnace, being placed in saggers in order to prevent the com-
bustion of the graphite. These crucibles are good conductors
of heat, they do not crack readily on change of temperature.
and they likewise possess a clean surface, so that the metal can
be poured out completely.
Finely-divided graphite purified according to Brodie's process
is largely used for polishing gunpowder, especially the large-
grain, blasting, and heavy ordnance powder. This coating
of black lead gives a varnish to the corn and prevents it
from absorbing moisture. The explosive force of the powder is,
however, somewhat diminished by this coating of graphite, for
Abel has shown that the explosive force of unpolished powder
being 107 6, the same powder polished with common graphite
has an explosive force of 89.9, and when varnished with Brodie's
graphite of 99.7.
The particles of the finely-divided and purified graphite
adhere together when they are brought into close contact, as by
great pressure, and the mass thus obtained may be used for
pencil-making and for other purposes. The pressed graphite is
found to conduct electricity very much more readily than the
ordinary graphite, and better than gas-coke. Thus, according to
Matthiesen, its conducting power is eighteen times greater than
that of natural graphite, and twenty-nine times as great as that
of the dense coke used for the Bunsen's batteries. From this
AMORPHOUS CARBON
673
property, graphite powder is used largely in electrotyping, the
moulds upon which it is desired to deposit the metal being
covered with a fine coating of powdered graphite. This acts as
a conductor of electricity, and a uniform coating of the metal is
deposited upon it. Graphite is used not only for the purpose of
preventing the rusting of iron objects, but in some cases also
instead of oil for lessening the friction in running machinery.
397 Amorphous Carbon.-In early ages the attention of
chemists was attracted to charcoal from the fact that it is a body
which cannot be acted upon by any solvent. The supporters of
the phlogistic theory made this substance a special study, because
they believed that it contained more phlogiston than any
other known body. This modification of carbon is produced
when substances which contain that element are strongly
heated in the absence of air. The amorphous carbon thus
obtained has received various names which indicate its origin
or mode of production, but the different varieties are identical
in chemical properties. The chief forms which are thus dis-
tinguished are (1) lamp-black, (2) gas-carbon, (3) charcoal, (4)
animal charcoal and (5) coke. The last three of these usually
contain mineral matter which was originally present in the
wood, bone or coal from which they are derived. The sub-
stances known by these names also generally contain small
amounts of both hydrogen and oxygen.
(1) Lamp-black. The luminosity of flame is probably due to
the presence in it of intensely heated particles of carbon, which
are deposited as soot on any cold surface held in the flame.
When the burning substance is rich in carbon, the flame smokes
even without its being cooled, and it does so the more strongly
the smaller the supply of air. This fact is made use of for the
purpose of preparing finely-divided amorphous carbon or lamp-
black.
In the manufacture of lamp-black, tar, resin, turpentine or
petroleum is burnt in a supply of air insufficient to burn it
completely, the smoky products of this imperfect combustion
being allowed to pass into large chambers hung with coarse
cloths, on which the lamp-black is deposited. The finest kind
of lamp-black is obtained by suspending metallic plates over
oil-lamps, or revolving over them metallic cylinders, on which
the soot is deposited. It is purified by heating it in closed
vessels, and is used for preparing Indian ink and in calico-
printing for producing gray shades; while common lamp-black
44
674
THE NON-METALLIC ELEMENTS
is employed as a black paint and for manufacturing printer's
ink. Soot or lamp-black is, however, not pure carbon, but
contains about 80 per cent. of that element along with oily and
fatty matters and a small amount of mineral matter, for it
always contains appreciable quantities of hydrocarbons arising
from the incomplete combustion of the tar. In order to
remove these impurities it is not sufficient to ignite the lamp-
black strongly. It must be heated to redness in a current of
chlorine for a considerable length of time, the hydrogen then
combining with the chlorine, and the carbon remaining unacted
upon.
(2) Gas carbon, which, next to lamp-black, is the purest form
of amorphous carbon, is formed in the preparation of coal-gas,
and probably owes its origin to the decomposition of the gaseous
compounds of hydrogen and carbon, which are evolved, by
the intensely heated walls of the retort. It is found as a
deposit in the upper portion of the retort in the form of an
iron-gray mass, so hard that it strikes fire like a flint. The
portions of this carbon deposited on the sides of the retort
contain no hydrogen, and have a specific gravity of 2.356,
whereas those lying further from the surface of the retort
contain some hydrogen. Gas carbon is also obtained by passing
olefiant gas, C₂H (which is one of the chief constituents
of coal-gas), through a red hot porcelain tube. This form
of carbon conducts heat and electricity well, and is used for
the preparation of the carbon cylinders or plates employed
in Bunsen's battery, and the carbon-poles for the electric
light.
(3) Charcoal. This substance is obtained in the pure state
by heating pure white sugar in a platinum basin. The carbon
thus obtained is purified by ignition in a current of pure
chlorine. It is tasteless and possesses no smell. It is a good
conductor of electricity, and has a specific gravity of 1:57,
Like the other modifications of carbon it is infusible, and is in-
soluble in every solvent. Pure sugar-charcoal is used as a
reducing agent, especially in the preparation of volatile metallic
chlorides, and it is peculiarly valuable inasmuch as its freedom
from silica prevents the formation of volatile tetrachloride of
silicon.
Amorphous carbon is converted by many oxidising agents
into complex soluble compounds, among which the most
important are oxalic acid, C₂H,O,, and mellitic acid, C₁₂HO12
6
CHARCOAL
675
(Vol. III. Part v. p. 374). Both of these acids are formed by the
action of alkaline potassium permanganate solution on amor-
phous carbon, whilst mellitic acid is also formed by the oxida-
tion of wood charcoal with concentrated sulphuric acid,2 carbon
dioxide, and sulphur dioxide being evolved at the same time
(p. 367), and when a current of electricity is passed through a
solution of caustic potash between carbon poles. The aluminium
salt of mellitic acid, known as honey-stone, occurs in seams of
brown coal and is possibly formed by oxidation from that
material.
The specific heat of amorphous carbon (wood charcoal) is
0.241.
On the large scale, wood charcoal is prepared in the same way
from wood as coke is from coal. Charcoal-burning is a very
old process, and the simple methods which were originally
adopted are carried on up to the present day. The method
consists in allowing heaps of wood covered with earth or sods to
burn slowly with an insufficient supply of air. It is usual to
build up large conical heaps with billets of wood placed verti-
cally and covered over with turf or moistened soil, apertures being
left at the bottom for the ingress of air, and a space in the middle
serving as a flue to carry off the gases (Fig. 186). The pile
is lighted at the bottom, and the combustion proceeds gradually
to the top. Much care is needed in regulating the supply of
air, and the consequent rate of the combustion. One hundred
parts of wood thus treated yield, on an average, sixty-one to
sixty-five parts by measure, or twenty-five parts by weight,
of charcoal. In Austria and Sweden charcoal is made from
long logs of fir-wood, which are placed horizontally in a rect-
angular pile (Fig. 187).
In countries like our own where wood is scarce, charcoal is
obtained from small wood or sawdust by a more modern process.
It consists in the carbonisation of the wood in cast-iron retorts,
and in this process not only is charcoal obtained, but the volatile
products, especially wood spirit, and pyroligneous acid, as well
as tar, are collected.
Good charcoal possesses a pure black colour and a bright
glittering fracture. When struck with a hard object it emits a
sonorous tone, and it burns without smoke or flame. The
specific gravity of charcoal, when the pores are filled with
1 Schulze, Ber. 4, 802, 806.
3 Bartoli and Papasogli, Gazetta, 13, 37.
2 Verneuil, Compt. Rend. 118, 195.
676
THE NON-METALLIC ELEMENTS
air, varies between 0.106 (ash charcoal) and 0.203 (birch char-
coal). Such charcoal will swim on water, but it sinks when the
pores which were filled with air become filled with liquid.
Charcoal rapidly absorbs gases and vapours (Saussure), and

2
FIG. 186.
possesses the remarkable property of precipitating certain sub-
stances from solution, and absorbing them in its pores (Lowitz,
1790). Animal charcoal possesses this absorptive power in a
much higher degree than common charcoal.

FIG. 187.
The properties and chemical composition of charcoal vary
much according to the temperature to which the wood is
heated. According to Percy, wood becomes perceptibly brown
at 220°, whilst at 280° it becomes after a time a deep brown-
1 Metallurgy, Fuel, p. 107.
་
CHARCOAL
677
black, and at 310° it is resolved into an easily pulverisable black
mass. Charcoal made at 300° is brown, soft, and friable, taking
fire easily when heated to 380°; whilst that prepared at a high
temperature is a hard, brittle substance, which does not take
fire till it is heated to about 700°. The proportion of carbon
contained in charcoal prepared at different temperatures varies
considerably. That prepared at the lowest point contains much
more of the volatile constituents of the original wood than the
charcoal made at a red-heat, though, as a matter of course, the
yield of charcoal is greater at the low temperature. According
to Violette¹ 100 parts of buckthorn wood yield the following
amounts of charcoal :-
99
At 250° 50 parts of charcoal containing 65 per cent. carbon
300° 33
73
400° 20
80
1500° 15
96
در
""
Carbon.
Hydrogen.
Oxygen with
some nitro-
gen
Ash
•
.
""
39
99
•
Carbon
Hydrogen
Oxygen and nitrogen
Ash.
Water
99
•
""
•
""
Beech
charcoal.
85.89
2:41
1.45
3:02
7.23
.
Thus charcoal, like lamp-black, as ordinarily prepared, never
entirely consists of pure carbon (Davy). The following table gives
the composition of charcoal obtained at different temperatures.
270° 350° 4320 1023° 1100° 1250° 1300° 1500°
70-45 76-64 81 64 81-97 83-29 88-14
4.64 4.14 1.96 2:30 1.70 1:41
24.06 18.61 15-24 14 13 13 79
0.85 0.61 1.16 1.60 1.22
The following results were obtained by Faisst.2
Ann. Chim. Phys. [3], 32, 305.
""
""
100.00
""
""
100.00 100.00 100.00 100.00 100 00 10000 10000 10000 100.00
"3
""
2.88
3:44
2:46
6:04
""
13
""
90 81 94.57 96.51
1.58 0.74 0.62
9.25 6.46 3.84 0.93
1.20 1.15 0.66 1.94
Over
15000
Hard charcoal, Light charcoal,
made in iron
from wood-
cylinders.
85.18
gas-works.
87.43
2.26
0.54
1.56
8.21
100.00
100.00
2 Wagner, Jahresb. 1855, p. 457.
678
THE NON-METALLIC ELEMENTS
(4) Animal Charcoal, which is obtained by the carbonisation.
of animal materials, differs from that prepared from vegetable
sources, inasmuch as it contains considerable quantities of nitro-
gen. Bone-black is obtained by charring bones in iron cylinders
or retorts, and is formed as a by-product in the manufacture of
animal, or Dippel's oil (Oleum animale Dippelii). Dry bones
contain about 70 per cent. of inorganic material, consisting
chiefly of calcium phosphate. This inorganic matter remains
behind with the charcoal when the bones are charred, and thus
the charcoal is deposited in an extremely finely-divided con-
dition. It is probably for this reason that bone-black possesses
a greater absorptive and decolourising power than wood charcoal.
The presence of the phosphate of lime, however, prevents the
application of this decolourising power of the charcoal to cases of
acid liquids which dissolve the phosphate. In such cases it is
necessary to make use of blood-charcoal. This is obtained by
evaporating four parts of fresh blood with one part of carbonate
of potash, and heating the residue in a cylinder. The charred
residue is then boiled out with water and hydrochloric acid, and
again heated to redness in a closed vessel. The addition of potash
serves the purpose of making the charcoal as porous as possible.
The following is an analysis of a good sample of dried animal
charcoal.¹
Carbon.
Calcium and magnesium phosphates, calcium fluoride,
&c.
Calcium carbonate
Other mineral matter
•
·
10.51
80.21
8:30
0.98
100.00
398 The power possessed by wood charcoal and, in a much
higher degree, by animal charcoal, of withdrawing many sub-
stances from their solution, seems to be almost entirely of a
physical nature. Thus, if a solution of iodine in iodide of potas-
sium be shaken up with finely-divided charcoal, the iodine is
completely withdrawn from solution. In the same way certain
metallic salts, especially basic salts, are decomposed when their
solutions are filtered through charcoal. This power of withdrawing
bodies from solution is exerted most effectually upon colouring
1 ¹ Thorpe's Dictionary of Applied Chemistry, Vol. I. 171.
ANIMAL CHARCOAL
679
matters and astringent principles. Thus, if a quantity of
red wine (claret or port), or a solution of indigo in sulphuric
acid, be shaken up and gently warmed with a quantity of
freshly-ignited bone charcoal and the mixture filtered, the
liquid which comes through is colourless. In the same way, if
alcohol containing fusel oil be shaken up with animal charcoal,
the characteristic smell of the fusel oil disappears. Wood char-
coal is largely used for the latter purpose, whereas animal
charcoal is employed for decolourising the juice of raw sugar.
Animal charcoal is employed in a similar way in the purifica-
tion of many organic compounds, although it not unfrequently
happens that the compounds themselves are attracted by the
porous charcoal. Occasionally this property is made use of
for the separation of compounds which thus adhere to the
charcoal, as, for instance, the alkaloids, from other bodies, these
compounds being afterwards dissolved from the charcoal by
the addition of hot alcohol. Like many other porous bodies
both of these forms of charcoal possess the property of largely
absorbing gaseous bodies. This power depends upon the fact
that all gases condense in greater or less degree on to the
surface of solid bodies with which they come in contact, and as
charcoal is very porous, or possesses a very large surface to a
given mass, its absorbent power is proportionately great. Char-
coal, when exposed to the air, condenses large quantities of the
latter upon its surface. This may be easily shown by attach-
ing a piece of metal to it and sinking the mass in a cylinder
filled with water. If the cylinder be now placed under the
receiver of an air-pump and the air exhausted, a rapid stream
of bubbles will be seen to rise in brisk effervescence from the
charcoal. The remarkable absorptive power of charcoal for
certain gases is also well illustrated by inserting a stick of
recently calcined charcoal into a tube filled over mercury with
dry ammonia gas. The gas is so quickly absorbed by the
charcoal that the tube soon becomes filled with mercury.
Another mode of showing a similar absorption of sulphuretted
hydrogen gas is to plunge a small crucible filled with freshly-
ignited and nearly cold powdered charcoal into a jar of sul-
phuretted hydrogen. This gas is then absorbed by the charcoal
in such quantity that if it be removed when saturated and
plunged into a jar of oxygen the charcoal will burst into vivid
combustion, owing to combination occurring between the ab-
sorbed sulphuretted hydrogen and oxygen gases.
680
THE NON-METALLIC ELEMENTS
The absorptive power of wood charcoal for gases was
first investigated by Saussure. In his experiments he made
use of beech-wood charcoal which had been recently heated
to redness and then cooled under mercury in order to remove
the air from its pores. The following numbers were obtained
by him:-
:-
1 volume of charcoal absorbs at 12° and under 724 mm. the
following (Saussure):
Ammonia
Hydrochloric acid
Sulphur dioxide .
Sulphuretted hydrogen
Nitrogen monoxide.
Carbon dioxide
Ammonia
Cyanogen
Nitrous oxide.
Methyl chloride .
Methyl ether.
Ethene.
Nitric oxide
.
•
•
•
•
Absorption of gases by charcoal (Hunter).
Vols.
171.7
107.5
•
Vols.
90 Ethylene
85
65
Oxygen.
55 Nitrogen
40 Hydrogen
35
Hunter, who also made experiments on the same subject,
found that one volume of charcoal absorbed the following
quantities of gas at the temperature 0° and under a pressure of
760 mm.
Carbon monoxide
.
86.3
764
76.2
74.7
70.5
Phosphine
Carbon dioxide
Carbon monoxide.
Oxygen
Nitrogen.
Hydrogen
.
Vols.
35
•
9.42
9.25
6:50
1.25
Vols.
69.1
67.7
21.2
17.9
15.2
4.4
The differences observed between these two series of experi-
ments with the same gases are to be explained by the fact that
the charcoals employed were not of equal porosity. Both series,
however, show that the more readily the gas is condensible the
more it is absorbed by charcoal, which seems to show that the
gases condensed by charcoal undergo at any rate a partial lique-
faction. This view is rendered possible by the observation of
Melsen, that when dry hydrogen is brought into contact with
charcoal saturated with chlorine a considerable quantity of
1 Phil. Mag. [4], 25, 364; 29, 116.
2 Compt. Rend., 76, 81, 92.
ABSORPTIVE POWER OF CHARCOAL
681
hydrochloric acid is formed, even when the experiment is
carried on in complete darkness; and that when charcoal
saturated with chlorine is brought into a Faraday's tube and
the other limb placed in a freezing mixture, liquid chlorine
is obtained. In a similar way ammonia, cyanogen, sulphur
dioxide, sulphuretted hydrogen, and hydrobromic acid have been.
liquefied.
Wood charcoal, like bone charcoal, has the power of absorbing
the unpleasant effluvia evolved in the processes of decay and
putrefaction as well as the moisture from the air. Stenhouse,¹
who has investigated this subject, has shown that charcoal not
only absorbs these gases and effluvia, but has the power, espe-
cially in contact with air, of oxidising and destroying them,
inasmuch as when absorbed by charcoal these substances are
brought into such close contact with the atmospheric oxygen,
which is also absorbed by the charcoal, that a rapid oxidation
is set up, and the odoriferous products of decomposition are
instantly resolved into carbon dioxide and water, and other
simple compounds. This property is retained by the charcoal
for a long time, and when it has been lost it can be renewed by
ignition. Hence charcoal filters are largely used for preventing
the foul sewer gases from polluting the air of the streets and
houses, and charcoal respirators and ventilators have been pro-
posed by Stenhouse as protections against the ingress of dele-
terious gases into the lungs. For the same reason, trays filled
with heated wood charcoal, placed in the wards of hospitals or
other infected apartments, have proved very effective in absorb-
ing noxious emanations. Charcoal filters are also largely em-
ployed for filtering water for drinking purposes, as in its
passage through the charcoal the water is decidedly improved
in quality, not only organic and soluble colouring matters being
removed as well as all suspended matter, but the water under-
going aeration. Such filters do not, however, necessarily free the
water from bacterial life; but may, on the other hand, increase
its amount unless the filter is frequently sterilized. (See ante,
p. 301.)
(5) Coke. This substance remains behind when bituminous
coal is heated to redness in absence of air. Coke is obtained.
as a by-product in the manufacture of coal-gas, but it is also
specially manufactured in coke ovens, and sometimes by burn-
1 On Charcoal as a Disinfectant; Proc. Roy. Inst. 2, 53; also Pharm. Journ.
16, 363.
682
THE NON-METALLIC ELEMENTS
ing coal in heaps and stopping the combustion at a certain
stage by quenching with water. When prepared by heating in
covered ovens or kilns, the coke is harder, more lustrous, and
less combustible than that obtained by burning in heaps. It is
termed hard-coke or engine-coke, and is largely used for iron
smelting, whereas the other variety is termed soft-coke or
blacksmith's coke, and is the more combustible. Coke takes fire
at a much higher temperature than common coal, and, when
burning, gives rise to a very high temperature, but without the
elimination of smoke, as it consists almost entirely of carbon.
Coke not only contains the inorganic material, or ash, present in
the coal from which it is manufactured, but in addition small
quantities of hydrogen, oxygen, and nitrogen, as the following
analyses show :-
COKE ANALYSES.
Carbon
Hydrogen
Nitrogen and oxygen
Ash
First Sample. Second Sample.
91.30
0.33
2.17
6.20
91.59
0:47
2.05
5.89
The amount of coke produced in Great Britain during 1885
was 11,077,375 tons.¹
COAL.
399 When vegetable matter decays in absence of air and under
water, or in the earth, it undergoes a change similar to that
which occurs when it is heated. Water, carbon dioxide, and
marsh gas are given off, and the residual material becomes
richer in carbon. The fact of the occurrence of marsh gas and
carbon dioxide as products of decomposition is rendered evident
by their presence in a highly compressed condition in the coal
measures at the present day, from which the, former is evolved
as fire-damp in enormous quantities which often produce fatal
accidents. It is in this way that the coals of various kinds,
lignites and peats, have been formed. Their composition com-
pared with that of cellulose is shown in the following table, the
1 Thorpe's Dictionary of Applied Chemistry, vol. ii. 162.
COAL
683
Cellulose
Irish peat.
Lignite from Cologne
Earthy coal from Dax
Cannel coal from Wigan
Newcastle Hartley
Welsh anthracite
amount of mineral matter which is left behind as ash on com-
bustion having been subtracted :-
Carbon.
50.00
60.02
66.96
74.20
85.81
88.42
94.05
Oxygen
Hydrogen. and
Nitrogen.
6:00 44.00
5.88
34.10
5.25
27.76
5.89
19.90
5.85
8.34
5.61
5.97
3.38
2.57
The above table shows that coal is a less pure form of carbon
than wood charcoal. It consists of the more or less altered
remains of a vegetable world which once flourished at various
points on the earth's surface. The plants of the coal formation.
consist of Calamites, the representatives of the living Equisetums,
of Lepidodendra, and Sigillariæ, which were the principal forest-
trees, and which, though of gigantic dimensions, were true crypto-
gams, represented by the living Lycopods and Selaginellæ, and
an important group of Conifers and Cycads, as yet not well
understood, and chiefly known through their seeds, which are
numerous and varied. Besides these arborescent types, ferns
formed an abundant undergrowth, many of these also having
been representatives of the living tree-ferns.
In the passage into coal the original woody fibre has not only
undergone a loss of hydrogen and oxygen but it has at the same
time become bituminised, so that for the most part all vegetable
structure has disappeared, and the coal possesses a fatty lustre
and coarse slaty fracture. There are many different kinds of
coal containing more or less of the hydrogen, oxygen, and
nitrogen of the original woody fibre. Cannel coal and boghead
coal contain the most hydrogen, and anthracite contains the
least, whilst the various kinds of bituminous coals lie between
these extremes.
Anthracite Coal of all coals contains the largest percentage of
carbon, and has therefore, undergone the most complete change
from woody fibre. It is found in the oldest deposits of the car-
boniferous series, especially in Wales, Pennsylvania, and Rhode
684
THE NON-METALLIC ELEMENTS
Island, and in smaller quantities in France, Saxony, and South-
ern Russia. It has a conchoidal fracture, possesses a bright lustre,
often sub-metallic, an iron-black colour, and is frequently iri-
descent. It burns with a smokeless flame. Anthracite gradu-
ally passes into bituminous coal, becoming less hard and
containing more volatile matter. Its specific gravity varies
from 1.26 to 1.8.
The Bituminous Coals consist of a large number of varieties
differing considerably from one another in their chemical com-
position, as also in their products of decomposition by heat.
They have the common property of burning with a smoky
flame when placed in the fire, and yielding on distillation
volatile hydrocarbons, tar or bitumen, whence their name is
derived.
The most important kinds of bituminous coals are (1) caking
coal, which softens and becomes pasty or semi-solid in the fire,
and yields, when completely decomposed, a greyish-black cellu-
lar mass of coke; (2) non-caking coal, agreeing with the last-
named variety in all its external characters and even in its
chemical composition, but burning freely without softening, and
without any appearance of incipient fusion; the residue which
it yields is not a proper coke, being either in powder or in the
form of the original coal.
Cannel Coal, sometimes called parrot coal. This is a variety
of coal differing from the preceding in texture, and yielding
usually more volatile matters and being therefore specially
employed for the purpose of gas-making. Cannel coal is more
compact than bituminous, possesses little or no lustre, does not
show any banded structure, breaks with a conchoidal fracture
and smooth surface; and has a dull black or grayish-black
colour. Its name is derived from the fact that small fragments,
when lighted, will burn with flame, and hence it was termed
candle- or cannel-coal.
400 The tables on pages 685 to 687, give examples of the
composition of the different forms of coal.
Connected with the true coals is a peculiar form, termed
Boghead coal or Torbane Hill mineral, which was first found
near Bathgate in Linlithgowshire, and has since been observed
in other places, especially in New South Wales. This deposit
occurs in the carboniferous formation, but it is not pro-
perly speaking a coal, but belongs to the class of bituminous
shales. Its specific gravity is lower than that of true coal, and
COMPOSITION OF COALS
685
South Staffordshire
""
""
Locality.
shire
Scotland
""
St. Helens, Lanca-
در
""
""
"3
Dowlais, S. Wales
·
در
""
Blanzy, France
Pas de Calais
Hungary
Aix-la-Chapelle
""
Sp. Gravity.
1.278
1.279
1.362
1-423
1.343
Carbon.
78.57
70-41
71.13
NON-CAKING BITUMINOUS COALS.
75.81
76.08
80.63
80.93
89.33
87.62
82.60
76.48
82.68
73.38
91.45
Hydrogen.
5.29
4.69
5:01
5.22
5.31
5.16
5.21
4.43
4.34
4.28
5.23
4.18
3.86
4.18
Oxygen.
12.88
12.47
9.67
11.14
13.33
10.61
10.91
3.25
2.52
3.44
16.01
4.54
11.65
2.12
Nitrogen.
1.84
1.58
1.93
2.09
1.33
1.57
1.24
1.13
1.28
Sulphur.
0.39
0.71
0.81
0.90
1.23
0.84
0.63
0.55
1.07
1.22
0.58
Ash.
1.03
2.20
1.35
5:00
1.96
1.43
0.75
1.20
3.32
7.18
2.28
8.60
10.53
2.25
Water.
11.29
9.52
10.45
3.23
0.79
0.68
0.78
3:06
Coke.
57.21
65.50
57.00
87.62
76.33
89.40
686
THE NON-METALLIC ELEMENTS
Locality.
Northumberland
"
""
""
""
Nottinghamshire
Blaina, S. Wales
"3
·
""
Epinal.
Charleroi .
Pas de Calais
Hungary
""
·
Sp. Gravity.
1.276
1.259
1.353
1.295
Carbon.
78.65
82.42
81.41
78.69
77.40
82.56
83.44
83.00
81.12
86.47
86.78
86.93
CAKING BITUMINOUS COALS.
Hydrogen. Oxygen. Nitrogen.
4.65
4.82
5.83
6:00
4.96
5.36
5.71
6.18
5.10
4.68
4.98
4:35
13.66
11.11
7.90
10.07
7.77
8.22
5.93
4.58
11.25
5:30
5.84
6.47
2.05
2.37
1.55
1.65
1.66
1.49
Sulphur.
0.55
0.86
0.74
1.51
0.92
0.75
0.81
0.75
0.86
Ash.
2:49
0.79
2:07
1.36
3.90
1.46
2.45
4:00
2.53
3.55
2:40
0.89
Water.
1.35
3.50
1.20
Coke.
66.70
63.18
63.60
84.43
77.05
78.85
COMPOSITION OF COALS
687
Wigan.
""
""
Ashton-under-Lyne
Tyneside
Mold, Wales.
Locality.
Scotland
""
,,
Swansea, S. Wales.
South Wales
""
""
Pennsylvania
39
""
•
Sp. Gravity.
1.317
1.276
1.319
1.348
1.392
1.462
Carbon.
84.07.
80.07
83.25
78.06
79.87
77.81
66.44
63.10
92.56
90.39
87.02
90.45
92.59
84.98
CANNEL COALS.
Hydrogen.
5.71
5.53
5.75
5.80
5.78
8.47
7.54
8.91
Oxygen. Nitrogen.
3.33
3.28
3.14
2:43
2.63
2.45
7.82
8.08
5:06
3.12
8.09
6.32
10.84
7.25
2.53
2.98
2.16
2.12
ANTHRACITE COAL.
2:45
1.61
1.15
1.85
1.36
0.83
0.90
0.92
1.22
Sulphur.
1.50
0.86
2.22
0.57
0.71
0.84
0.96
―――――――
0.91
0.67
Ash.
2.40
2.70
3.48
8.94
2.85
6.01
12.98
19-78
1.58
1.61
6.11
4.67
2.25
10.20
Water.
0.91
1.60
2.84
0.68
―――
2:00
Coke.
59.00
63.25
56.60
27.92
30.23
688
THE NON-METALLIC ELEMENTS
it possesses a brown instead of a black colour. When com-
pletely burnt it leaves a residue of about 20 per cent. of ash,
and when heated in a retort, about 70 per cent. volatilises partly
as gas, partly as more or less liquid or solid hydrocarbons.
These are known under the name of paraffin oils, and are
largely used for illuminating as well as for lubricating purposes,
and for the preparation of solid paraffin.
Brown Coal or Lignite belongs to a different and more recent
geological period than coal proper, being found in the tertiary
formation. It consists of the remains of trees and shrubs, as ash,
poplar, and others, which now exist on the surface of the earth.
It possesses a brown colour and often exhibits a characteristic
woody structure. Its specific gravity varies from 1:15 to 1:30.
Jet is a black variety of brown coal, compact in texture, and
taking a good polish. Hence it is largely used in jewellery.
Earthy Brown Coal is another brown friable material, some-
times forming layers in beds of lignite, but it is not a true coal,
inasmuch as a considerable portion of it is soluble in ether and
benzene, and often even in alcohol, whereas true coal is nearly,
if not quite, insoluble in these liquids.
Turf or peat is a material which is being constantly formed
by the decomposition of marsh plants, chiefly mosses, &c. It
always contains nitrogenous compounds, which are the cause of
the peculiar smell which it gives off on heating, is very rich
in ash, and after drying in the air still contains 15-20 per
cent. of water.
Yield of Coal in Great Britain.-The following table, taken
from the final report of the Royal Commission on Mining
Royalties (1893), shows the production of coal in each of the
districts named, giving the total produce in the year 1889.¹
1 For further information on the subject of coal, lying beyond the scope of this
work, we refer our readers to the following standard works :-Percy's Metallurgy:
Fuel. London, 1875; Hull On the Coal Fields of Great Britain; Statistics of
Coal, by R. C. Taylor. 1855; Ronalds and Richardson's Chemical Technology,
vol. i.; Ronalds On Fuel and its Applications; Jevons. The Coal Question ; Report
of the Royal Commission on Coal; Thorpe's Dictionary of Applied Chemistry.
OUTPUT OF COAL
689
Northumberland
Durham
Cumberland
Westmoreland
Lancashire.
Cheshire
Yorkshire.
·
North Staffordshire
Cannock Chase
South Staffordshire.
Warwickshire.
Worcestershire
Derbyshire
OUTPUT OF COAL IN 1889.
Nottinghamshire
Leicestershire
Shropshire..
Somersetshire
Gloucestershire:-
Bristol Field
Forest of Dean
Monmouth and South Wales
North Wales.
•
•
·
England and Wales
Scotland
Ireland.
Total
•
Total for United Kingdom
•
Tons.
8,794,005
30,307,177
1,740,737
22,327,968
21,976,027
4,712,010
11,819,766
18,012,378
710,490
876,254
504,847
854,967
28,064,235
2,895,499
153,596,360
153,596,360
23,217,163
103,201
176,916,724
45
690
THE NON-METALLIC ELEMENTS
CARBON AND HYDROGEN.
401 All the elements hitherto described combine with
hydrogen, but each of these elements possesses the power
of forming only a small number of hydrides. Carbon, on the
other hand, is distinguished from the foregoing elements, as
well as from all the others, by the fact that it is capable
of forming an extremely large number of hydrogen compounds.
These substances are called hydrocarbons, and most of them are
volatile bodies.
This peculiarity of carbon depends upon the fact that one
atom possesses the property of combining with another atom
of carbon, one or more of its four combining units being thus
saturated, whilst the remaining combining units are capable of
being saturated with hydrogen.
It has already been shown that other elements, such as
oxygen and sulphur, possess the same property, though in
a much smaller degree. Thus we know of no volatile com-
pound which contains more than seven atoms of sulphur or
of oxygen linked together in the same molecule. Disulphuryl
chloride, SOCl2, is the compound which contains the largest
number of atoms belonging to this group.
In the case of carbon such a limit has not, as yet, been found,
derivatives containing as many as sixty carbon atoms directly
united together, having been already obtained. The number of
hydrocarbons does not, however, depend merely upon the number
of carbon atoms which can occur combined with one another,
inasmuch as the atoms may saturate one another reciprocally
by the union of one, two, or three of their own combining
units. Notwithstanding this complexity of construction, the
hydrocarbons may be classed in certain groups, each one of
which can be represented by a general formula. Of these the
three simplest are :—
GROUP I
CnH2n+2.
Methane, CH,.
Ethane, CH
Propane, CH.
Butane, CH10
Pentane, CH12.
GROUP II.
CnH2n.
Ethene, CH4.
Propene, CH.
Butene, CH.
Pentene, C,H10-
GROUP III.
CnH₂-2.
―――
Ethine, C.H
Propine, CH.
Butine, CH
Pentine, CH.
CARBON AND HYDROGEN
691
The constitution of these compounds of carbon and hydrogen,
or hydrocarbons as they are usually termed, is best understood
by starting with the simplest, in which the four combining units
of a single carbon atom are saturated by hydrogen. This is the
well-known substance marsh gas, and its constitution is repre-
sented by the graphic formula
H-C
T
H
H H
If one atom of hydrogen in this compound be replaced by a
carbon atom, and the remaining valencies of the latter saturated
by hydrogen, we obtain a hydrocarbon having the composition
CH, the constitution of which is as follows:
H-C-H.
H H
ннн
1_d_d_d_
H
H C-C-H or CH3-CH3.
H
H
If, now, one of the hydrogen atoms in this hydrocarbon be re-
placed by a third carbon atom, and its remaining combining
units saturated with hydrogen, we obtain a new substance, C,H,,
its constitution being
H
-C-H or CH3. CH2. CH3.
The same process may again be carried out, but here it is not,
as in the previous cases, immaterial which hydrogen atom is
replaced by the added carbon atom, for these have not all an
equal value. An examination of the graphic formula given
above for CH, shows that two of the hydrogen atoms are com-
bined with a carbon atom which is itself attached to two other
carbon atoms, whilst the remaining six hydrogen atoms are
combined with a carbon atom attached to only one other carbon
692
THE NON-METALLIC ELEMENTS
atom. Hence according to the formula we can get the two
following compounds of the composition CH10
:-
H
H
нннн
H_d_d_d_
-C-H
HHH H
or
CH₂. CH₂. CH2. CH,
3
H
#_d_d_d_
ннн
H
(1)
CH.CH.CH.CH.CH,
(2)
CH.CH.CH
or
CH3
CH,. CH. CH,
CH3
H
and in fact two and only two compounds of this composition have
been experimentally obtained. Substances of this kind, which
have the same percentage composition but different physical
and chemical properties, are known as isomerides. Such cases
are occasionally met with in the compounds of the other ele-
ments, as for example in the case of the potassium sodium
sulphites (p. 373), but they are of far more frequent occurrence
among the carbon compounds. Thus three different hydro-
carbons having the formula CH₁₂ are known, their formulæ
being
12
CHS
C-H
(3)
CH3
CH-C-CH
CH;
and the number of possible isomerides rapidly increases with
the rise in the number of carbon atoms, no less than 799 different
isomerides having the composition CH being theoretically
possible.
26
All the hydrocarbons derived in this manner from marsh gas
have the general formula CnH2n+2, and are usually termed the
paraffins.
CONSTITUTION OF HYDROCARBONS
693
A different series of hydrocarbons, known as the olefines, and
having the general formula CnH2n, contains two carbon atoms
united together by two combining units, the simplest represen-
tative being ethylene, CH, the constitution of which is
represented by the formula
H
#c=
H
C=C
Propylene.
CH,.CH=CH₂
H
H
or CH2=CH2.
The hydrogen atoms of this hydrocarbon may be replaced by
other carbon atoms in exactly the same manner as described
under methane, giving rise to new hydrocarbons, of which the
following may be taken as examples :-
a-Butylene.
CH.CH,CH=CH₂
H
¿
Another series, somewhat similar to the foregoing, having the
general formula CnH₂n-2, contains two carbon atoms united
together by three combining units. The first member of this
series, known as acetylene, has the constitution
CH=CH.
B-Butylene.
CH..CH=CH.CH
In a fourth series of hydrocarbons the carbon atoms are united
together in such a manner that they form a closed chain. The
most important member benzene, contains a closed chain of six
carbon atoms, its constitution being represented as follows:-
H-C C-H
H-Ċ C-H
H
It will be observed that in this formula only three valencies.
of each carbon atom are represented as saturated; the manner
in which the remaining valencies are disposed is still a matter
of discussion.
694
THE NON-METALLIC ELEMENTS
Almost all the other compounds of carbon may be regarded
as derived from one or other of these numerous hydrocarbons;
thus, for example, chloroform, CHCl3, is marsh gas in which three
of the four hydrogen atoms have been replaced by chlorine, and
alcohol, CHO, is ethane, CH, in which one atom of hydrogen
has been replaced by the compound radical hydroxyl :-
―――――――――
CH,.CH3
Ethane.
CH3.CH2.OH
Alcohol.
Most of the substances occurring in the animal and vegetable
kingdoms belong to the group of carbon compounds, and in
addition to these an immense number of other derivatives con-
taining this element have been prepared artificially, so that the
total number of carbon compounds known is greater than that
of all the other elements put together. For this reason they
are separately treated of under the head of Organic Chemistry,
which is now usually defined as the Chemistry of the Hydrocarbons
and their derivatives. In this portion of the work only the
simpler hydrocarbons, containing one or two atoms of carbon,
will be considered.
METHANE, METHYL HYDRIDE, MARSH GAS, OR FIRE-DAMP.
CH₁
15.91.
=
402 This gas is found in the free state in nature, and its
occurrence was observed in early times. Thus Pliny mentions
the combustible gaseous emanations which occur in several dis-
tricts, and Basil Valentine remarks upon the outbreaks of flame
which occur in mines, and which are preceded by a suffocating
damp or vapour. He does not consider that this vapour is com-
bustible, but rather believes that the flame was emitted by the
rocks for the purpose of destroying this poisonous vapour.
Marsh gas, like some other combustible gases, was not dis-
tinguished from inflammable air, or hydrogen, until Volta in the
year 1776 showed that inflammable air, when burnt, required
only one-fourth of the volume of oxygen which was needed for
the complete combustion of marsh gas, and that in this latter
case alone was carbonic acid formed. In the year 1785 Ber-
thollet proved that this gas contains both carbon and hydrogen,
but it was at that time not distinguished from olefiant gas
METHANE
695
(or ethene, C₂H₁). In 1805 William Henry clearly pointed out
the difference between these two gases.
We have already seen that marsh gas occurs free in nature.
It is evolved, together with other hydrocarbons, in large quantities
in petroleum springs. The holy fire at Baku on the Caspian
Sea, which has been burning from the earliest historical times,
is due to marsh gas, mixed, according to Hess, with small
quantities of nitrogen, carbon dioxide, and the vapours of petro-
leum. The gas which is evolved from the mud volcanoes of
Bulganak in the Crimea has been shown by Bunsen ¹ to consist
of pure marsh gas. The gases which escape in large quantities from
the oil springs in Butler County, Pennsylvania, contain, according
to Sadtler's analyses,2 marsh gas and its homologues, together with
hydrogen. These gases are collected and carried by pipes to the
rolling mills at Pittsburg, a distance of fifteen miles, where the
gas is employed as a fuel.
Enormous quantities of marsh gas, or fire-damp, as it is
termed by the miners, are evolved in coal-pits, due in all proba-
bility to a slow decomposition of the coal. Reservoirs of this
gas in a highly compressed state are often met with pent up in
the crevices and cavities of the coal measures. Some beds of
coal are so saturated with gas that when they are cut it may be
heard oozing from every pore of the rock, and the coal is called
by the colliers singing coal; in other cases the gas escapes by
what are termed blowers, and the mixture of gases frequently
collects in the old workings or unventilated portions of the pit.
Not unfrequently fire-damp bursts forth in large quantities
from the seams of coal, or from the strata of clay which divide
them. This is the frequent cause of the terrible accidents which
sometimes, in spite of all care, will occur. The Lundhill colliery
explosion in 1857 was one of the most calamitous on record.
The sudden escape of gas from a blower in a neighbouring col-
liery is thus described: "The fire-clay of the floor of the seam
was seen to heave at different points along the face, and presently
large fractures were made in it, through which gas was ejected
with great violence, and with a sound very similar to the rushing
of steam at a high pressure from a boiler. After the explosion
at Lundhill, the pent-up gas still issued within the mine under
such pressure as to support a column of water thirty feet
1 Gasometry, p. 147.
2 Amer. Philos. Soc. 1876; see also Laurence Smith, Ann. Chim. Phys. [5],
8, 566.
696
THE NON-METALLIC ELEMENTS
high." The outburst of gas appears, sometimes at least, to
be connected with a rapid fall in the barometer, the reduced
atmospheric pressure enabling the gas to force its way out.¹
If he should escape from the effect of the explosion, the miner
has still to fear its result, inasmuch as the gas in exploding
renders ten times its own bulk of air unfit for respiration, and
the after-damp or vitiated atmosphere produced by the explo-
sion contains carbon dioxide sufficient to render it irrespirable.
Hence the difficulty of descending into the pit after the explo-
sion without proper precautions, or until a sufficient amount of
ventilation has been re-established. The only satisfactory means
of guarding against these sudden outbreaks in fiery pits is the
establishment of a thorough and perfect system of ventilation,
by means of which such an amount of air is brought into all
parts of the workings as to render the formation of the in-
flammable mixture difficult or impossible, even when a sudden
outbreak of gas occurs.
The escape of marsh gas at the surface of the ground in the
neighbourhood of the coal measures is frequently observed. This
was first noticed by Thomson, at Bedley, near Glasgow, where a
flame, once lighted, burnt for many weeks in succession. A similar
case has been noticed by Pauli near St. Helen's. The following
analyses by Graham show the composition of fire-damp :—2
Sp. Gr.
0.5802
Marsh gas. Nitrogen. Oxygen.
94.2 4.5 1.3
Five-quarter seam,
Gateshead colliery S
Bensham seam,
0.6327
0.6306
82.5 16.5
Another instance of the formation of methane by the slow
decomposition of vegetable matter, somewhat similar to that
taking place in the coal seams, occurs in ponds or marshes,
whence one of the names of the gas is derived. The gas-
bubbles which rise when a stagnant pool containing decom-
posing leaves and vegetable matter is stirred, consist essen-
tially of marsh gas, which is mixed with carbon dioxide and
nitrogen. The gas collected by Bunsen, in July 1848, from
a pond in the botanical gardens of Marburg, contained, after the
absorption of carbon dioxide by caustic potash, the following:-
Hebburn colliery
Killingworth colliery
Methane
Nitrogen
¹ Scott and Galloway, Proc. Roy. Soc. 20, 292.
-
0.6
1.0
48.5
51.5
2 Phil. Mag. 28, 437.
PREPARATION OF METHANE
697
Methane also invariably occurs amongst the products of the
dry distillation of organic bodies, and hence it is present in
very considerable quantities in coal gas.
403 Preparation.-(1) In order to prepare marsh gas an
intimate mixture of one part of dried acetate of soda with four
parts of soda-lime (a mixture of caustic soda and lime) is heated.
This is best accomplished in a tube of hard glass closed at one
end, and fitted with a delivery-tube at the other, or, in place of
this, an iron tube or a copper flask may be employed. In order
to prepare the gas as pure as possible, the mixture must only
be heated to the point at which the gas begins to be evolved,
but even with all care it is impossible to avoid the presence of
some free hydrogen and some ethylene. This latter impurity
may, however, be removed by passing the gas through a U-tube
containing pumice-stone soaked in strong sulphuric acid. In a
sample of the gas thus prepared and purified, Kolbe found 8
per cent. of hydrogen. The formation of methane from acetic
acid is shown by the following equation:-
1
C₂H₂O,Na+NaOH= Na₂CO₂+CH.
(2) Chemically pure methane is obtained from zinc methyl,
Zn(CH3)2, which is decomposed by water as follows:-2
Zn(CH3)2+2H₂O=Zn(OH)₂+2CH¸.
(3) The pure gas is most readily obtained by the action of
a zinc copper couple on a mixture of equal volumes of methyl
iodide and alcohol, the reaction being assisted by gentle
warming.3
(4) Marsh gas can be obtained synthetically by passing a
mixture of sulphuretted hydrogen and the vapour of carbon
bisulphide over red-hot copper :-4
2SH₂+CS₂+8Cu= CH₁+4Cu₂S.
(5) The same gas is likewise formed when a mixture of
carbon monoxide and hydrogen is exposed to the action of the
electric induction spark :-
5
CO+3H2=CH₁+H₂O.
1 Ausfuhrl. Lehrb. Org. Chemic, i. 275.
2 Frankland, Phil. Trans. 1852 [2], 417.
3 Gladstone and Tribe, Journ. Chem. Soc. 1884, i. 154.
4 Berthelot, Compt. Rend. 43, 454.
Brodie, Proc. Roy. Soc. 21, 245.
698
THE NON-METALLIC ELEMENTS
Properties.-Methane is a colourless odourless gas, which
has a specific gravity of 0.559; it is very sparingly soluble in
water, its solubility from 6° to 20° being obtained from the
equation:
c=0·05449-0·00118077+0·000010278ť².
It dissolves more readily in alcohol, the solubility between 2°
and 24° being found from the equation:
c=0·522586 − 0·0028655t+0·0000142ť².
-
The gas was first liquefied by Cailletet¹ in 1877; the liquid
boils at -155° to -156° under the ordinary pressure, and at -73.5°
under a pressure of 568 atmospheres, and has at-164° a
specific gravity of 0.415.3 It burns with a slightly luminous
flame, the illuminating power, when tested under the condi-
tions usually observed in testing coal-gas, being equivalent
to five candles as compared with an illuminating power of 15—
20 candles given by ordinary coal gas. When burnt in such a
manner that the temperature of the flame is very high, as in a
regenerative burner, the illuminating power is much greater.
When mixed with twice its volume of oxygen it explodes in
contact with a flame, the detonation being even more violent than
in the case of a mixture of hydrogen and oxygen. Its heat of
combustion per molecule in grams at 18° is 211,930 cal.
Methane is an extremely stable compound and when exposed
to a temperature at which hard glass softens is only de-
composed to a very slight extent. When the sparks from a
strong induction coil are passed through the gas it is partially
dissociated into carbon and hydrogen, acetylene being also
formed.
ETHYL HYDRIDE OR ETHANE, C,H
404 This gas is invariably present in the gaseous discharge
accompanying petroleum in the oil springs of Pennsylvania,
and is dissolved in considerable quantities in the liquid hydro-
carbons.5
= 29.82.
Preparation. (1) Ethane is readily obtained by treating ethyl
1 Jahresb. 1877, 221.
2 Wroblewski, Jahresb. 1884, 197.
3 Olszewski, Jahresb. 1887, 72.
4 Wright, Journ. Chem. Soc. 1885, i. 200.
5 Ronalds, Journ. Chem. Soc. 1851, 54.
ETHANE AND ETHYLENE
699
1
iodide with zinc and water under pressure at a temperature of
150°; thus:-
Zn+C₂H₂I+H₂O=ZnO+HI+C₂HË.
(2) The same gas is produced when a galvanic current is passed
through a concentrated solution of potassium acetate:-2
2C₂H¸O₂K+H₂O=C₂H¸+K₂CO¸+CO₂+H2.
Carbon dioxide and ethane are evolved at the negative pole,
whilst hydrogen is set free at the positive pole.
Properties.-Ethane is a colourless and odourless gas, slightly
soluble in water, but more soluble in alcohol. According to
Schickendantz, its coefficient of absorption in water is:
c=0·094556-0·0035324t+0·00006278t.²
It has a specific gravity of 1.036, and condenses to a liquid
under a pressure of 40 atmospheres at 4°. It burns with a
luminous flame, the illuminating power being about half that of
ethylene burnt under similar conditions. The heat of combus-
tion (for 1 mol. in grams) is 370,440 cal.
ETHYLENE, ETHENE, OR OLEFIANT GAS, C₂H.
Density 27.82.
=
3
1 Frankland, Journ. Chem. Soc. 1850, 263.
* Frankland, Journ. Chem. Soc. 1885, i. 237.
5 Nicholson's Journal, 1805.
405 This gas appears to have been discovered by Becher, who
obtained it by heating alcohol with sulphuric acid. His obser-
vations were, however, considered to be erroneous up to the time
of Priestley, who, in his Experiments and Observations on Air,
mentions that Ingenhouss had seen such a gas prepared by a
certain Enée in Amsterdam. The properties of olefiant gas
were accurately studied in the year 1795 by Deimann, Paets
van Troostwyk, Bondt, and Lauwerenburgh. These Dutch
chemists found that the gas obtained from alcohol and sulphuric
acid was totally different from ordinary inflammable air, and that
it contained both hydrogen and carbon. The difference between
marsh gas and olefiant gas was first pointed out by William
Henry in 1805, and his view of the composition of the gas was
borne out by the subsequent experiments of Dalton, Davy, and
Berzelius. In those days, marsh gas and olefiant gas were the
5
2 Kolbe, Annalen, 69, 257.
4 Crell. Ann. 1795.
1
700
THE NON-METALLIC ELEMENTS
only hydrocarbons known, so that their specific gravities being
very different, they were termed the light- and the heavy-
carburetted hydrogen respectively. In his History of
Chemistry¹ Thomas Thomson states that it was by the inves-
tigation of the chemical composition of these two gases that
Dalton was led to the recognition of the laws of combination in
multiple proportions, and to the conception of his atomic theory,
from noticing that for the same quantity of hydrogen heavy
carburetted hydrogen contains twice the quantity of carbon
that is contained in marsh gas.
Preparation. In order to prepare olefiant gas, alcohol is
heated with strong sulphuric acid. The method proposed by

b
a
患​得​建
​FIG. 188.
Twenty-five grams of
Erlenmeyer and Bunte 2 is the best.
alcohol and 150 grams of sulphuric acid are brought into a flask
of from two to three liters in capacity (see Fig. 188), and the
mixture heated till the evolution of gas begins. By means of
a funnel-tube, furnished with a stopcock, b, a mixture of equal
volumes of alcohol and sulphuric acid is then allowed to
drop into the flask. To purify the gas thus obtained it must be
first washed through concentrated sulphuric acid, and afterwards
through caustic soda, contained in the Woulff's bottles, c and d.
It may then be collected over water and preserved in a gas holder.
Properties.-Ethylene is a colourless gas, possessing a peculiar
2 Annalen, 168, 64.
1 Vol. ii. p. 291.
ACETYLENE
701
ethereal smell; it is only slightly soluble in water, but is more
so in alcohol; its solubility in water between 5° and 21° is
expressed by the following equation (Pauli):—
c=0·25629-000913631t+0.00018810812.
Its specific gravity is 0.9784.
It condenses under a pressure
of 45 atmospheres to a colourless liquid, which solidifies to a
crystalline mass at -181°, melts again at -169° and boils at
-105°. The liquid has a specific gravity of 0.335 at 8°.
Ethylene is readily inflammable, and burns with a brightly
luminous flame, the illuminating power being about 70 candles.¹
The heat of combustion (for 1 mol. in grams at 18°) is 333,350
cal. When mixed with three times its volume of oxygen and
ignited it explodes with extreme violence.
One very characteristic property of ethylene is its power of
uniting with an equal volume of chlorine to form a heavy
colourless liquid termed ethylene dichloride, C₂H,Cl₂, or, Dutch
liquid, it having been first observed by the four Dutch chemists
already named. From this property, indeed, the gas derives its
name. It was originally termed gaz huileux, but this name
was afterwards changed by Fourcroy to gaz oléfiant. When
brought in contact with strongly ozonised oxygen, ethylene de-
tonates very powerfully. In order safely to exhibit this property,
a current of the gas is allowed to pass through a wide tube
about 10 mm. in diameter, whilst ozonised oxygen is allowed to
enter into this by means of a narrow tube, which is inserted into
the wider tube to a distance of one centimeter. As soon as the ozo-
nised oxygen comes in contact with the olefiant gas detonation
occurs, usually accompanied with the formation of white fumes.2
ACETYLENE OR ETHINE, C₂H₂ = 25·82
406 This gas was discovered and its composition determined
in 1836, by Edmund Davy, who prepared it by treating with
water the black mass obtained in the manufacture of potassium.
The existence of this gas was afterwards observed by some other
chemists, but it was not until the year 1859 that Berthelot *
investigated it completely. Acetylene is produced by the
incomplete combustion of many volatile organic substances,
especially of ethylene, coal-gas, and other hydrocarbons, as well
1 Frankland, Journ. Chem. Soc. 1885, i. 237.
2 Houzeau and Renard, Compt. Rend. 76, 572.
3 Reports of British Association, 1836, p. 62.
4 Ann. Chim. Phys. [3], 57, 82.
T
702
THE NON-METALLIC ELEMENTS
as the vapours of alcohol, ether, &c. Acetylene is also formed
when the vapours of these organic liquids are passed through
red-hot tubes.
Acetylene is remarkable as being the only hydrocarbon
which has been obtained by the direct union of its elements.
These only combine together at the highest temperature which
can be artificially produced. In order to prepare acetylene in
this way the electric arc, obtained by the passage of a powerful
current between two poles of gas carbon is employed. These
carbon poles are fitted through apertures in a globular glass
vessel, through which a slow current of pure hydrogen is allowed
to pass.
Acetylene can also be prepared in any wished-for quantity
from ethylene dibromide. If this substance be heated with an
alcoholic solution of caustic potash, the following reactions take
place :-
4 2
(1) C₂H,Br₂+ KOH = C₂H¸Br+ KBr + H₂O.
(2) C₂H¸Br+KOH=C₂H₂+KBr+H₂O.
To remove any vapours of the very volatile bromethylene
which may be carried over from this operation, the gases evolved
from the boiling liquid are allowed to pass through a second
flask containing a boiling alcoholic solution of caustic potash.
Another method of preparation consists in allowing water to
drop on to barium carbide, which is prepared by heating to
redness a mixture of precipitated barium carbonate, magnesium
powder and gas carbon. The gas thus obtained only contains
2-3 per cent. of hydrogen and no appreciable quantities of other
hydrocarbons. Calcium carbide (p. 703) is also decomposed
by water with formation of pure acetylene.2
Properties.-Acetylene is a colourless difficultly condensible
gas, having a specific gravity of 0.92, with a very unpleasant
penetrating smell. It is usually stated that the smell observed
when a Bunsen burner "burns down" is due to the presence
of acetylene, but according to V. Meyer the odour of pure
acetylene is quite distinct from this.
At 18° it dissolves in its own volume of water, whilst alcohol
dissolves six times its volume of the gas. Acetylene burns with
a strongly luminous and smoky flame. It acts as a poison when
it comes in contact with the blood, combining with the hæmo-
globin.3 Like ethylene it combines directly with chlorine and
Moissan, Compt. Rend. 118, 501.
1 Maquenne, Compt. Rend. 115, 558.
3 Bistrow and Liebreich, Ber. 1, 220.
ACETYLENE
703
bromine forming with the latter element acetylene dibromide,
CH,Br₂, as well as the tetrabromide, CH,Bг.
Very characteristic derivatives of acetylene are the explosive
compounds which it forms with certain metals or metallic
salts. Thus, if the gas is allowed to pass over fused potassium,
hydrogen is evolved, and bodies having the composition C₂HK
and CK₂ are formed. Both of these substances are black
powders, which are decomposed in contact with water with
explosive violence, acetylene being reproduced. A similar
calcium compound, C₂Ca, is obtained by heating a mixture of
lime and sugar charcoal in the electric furnace :—¹
CaO + 3C = C,Ca + CO.
This substance, when treated with water, yields lime and acety-
lene. When the gas is passed through an ammoniacal solution of
a cuprous salt, a dark blood-red precipitate is formed, having
the composition Cu,C, or Cu,C,,H,O. This reaction is a very
characteristic one. It serves as a test for the presence of
acetylene, and is so delicate, that by its means part of a
milligram of acetylene can be with certainty detected (Berthelot).
For the purpose of exhibiting its action with cuprous chloride
the flame of a large Bunsen burner is allowed to burn down, the
air-openings at the bottom are then nearly closed, and when the
maximum amount of gas is burning, a large dry balloon is brought
over the tube of the burner, so that the incompletely burnt gas
is brought into the middle of the vessel. After some minutes
the balloon is removed and a few drops of ammoniacal solution
of cuprous chloride, Cu,Cl₂, are poured in, the balloon being
shaken so as to bring a thin film of the liquid over the whole
of the interior surface. This is then seen to be covered with a
coating of the red acetylene-copper compound.
If a current of acetylene be led into an ammoniacal solution
of a silver salt, C,Ag, or C,Ag₂H₂O separates out in the form of
a white precipitate. This body, like the copper compound,
explodes on heating or by percussion. On the addition of
acids to these compounds, acetylene is evolved. Hence they
may be employed for the separation of the gas from a gaseous
mixture, as well as for the purpose of obtaining it in a chemically
pure state; thus:-
C₂H¿Cu₂O+ 2HCl = C₂H₂+Cu₂Cl₂+H₂O.
2
¹ Moissan, Compt. Rend. 118, 501.
2 Blochmann, Ber. 7, 274; Keiser, Amer. Chem. Journ. 14, 285; Plimpton,
Proc. Chem. Soc. 1892, 109.
704
THE NON-METALLIC ELEMENTS
To determine the quantity of acetylene in coal-gas, or other
mixture of gases, these are passed through ammoniacal silver
nitrate, which gives a precipitate of silver acetylide mixed with
metallic silver. The former is converted by hydrochloric acid
into silver chloride, which may be separated by treatment with
ammonia, the filtrate precipitated with nitric acid, and the
silver chloride weighed; 1 gram. of silver chloride corresponds
to 0:09 gram, or 87 03 cc. of acetylene.¹
A simple method for preparing acetylene is as follows:-A
bent funnel (see Fig. 189) is placed over a burner, in which the

FIG. 189.
flame is burning down. Connected with the neck of the funnel
are two cylinders, a and b, containing an ammoniacal solution.
of nitrate of silver, and the products of the combustion are
drawn through the liquids contained in the cylinders by means
of an aspirator, c. By the addition of hydrochloric acid to the
precipitate. pure acetylene can be obtained, and the chloride
of silver which is precipitated only requires to be again dis-
solved in ammonia in order to fit it to be employed a second-
time for the same purpose.
If the compound C,Cu, be allowed to remain in contact with
zinc and aqueous ammonia, the nascent hydrogen evolved
1 Lewes, Journ. Chem. Soc. 1892, i. 324.
CARBON MONOXIDE
705
by the action of the zinc combines with the acetylene and forms
ethylene. Platinum-black when brought in contact with a
mixture of acetylene and hydrogen also brings about the same
combination, both ethylene and ethane being formed.¹
Acetylene is formed from its elements with absorption of heat,
and, like many other similar endothermic compounds, can be
made to undergo explosive decomposition when subjected to
the shock given by the explosion of fulminate of mercury.
(Cf. p. 734.)
CARBON AND THE HALOGENS.
407 Carbon combines with fluorine directly to form carbon
tetrafluoride, CF, the reaction taking place spontaneously at
the ordinary temperature with the less dense forms, whilst the
denser varieties must be heated to 50-100°. The remaining
halogens, however, do not combine directly with carbon, but
compounds of carbon with these elements can be prepared by
indirect methods. Thus the ultimate product of the action of
chlorine on methane is carbon tetrachloride, CC, the four
atoms of hydrogen being replaced by chlorine, which also com-
bines with the hydrogen set free, forming hydrochloric acid.
CH₁+4C12=CC¹¸+4HCl.
These compounds are described in the volumes relating to
Organic Chemistry.
CARBON AND OXYGEN.
408 Carbon forms two oxides, both of which are gaseous :-
Carbon monoxide, CO.
Carbon dioxide, CO
1 V. Wilde, Ber. 7, 353.
:-
CARBON MONOXIDE OR CARBONIC OXIDE, CO = 27.79
This compound, which is commonly known as carbonic
oxide, was first obtained by Lassone by heating zinc oxide
with charcoal. He found that a combustible gas was thus given
off which burned with a blue flame, and when mixed with air
did not explode, as was usually the case with inflammable air.
Lavoisier (1777) obtained the same gas by heating alum and
2 Mem. Paris Acad. 1776.
46
706
THE NON-METALLIC ELEMENTS
charcoal together, and found that on combustion it yielded
carbonic acid. In spite of these observations, the gas was for
a long time mistaken for hydrogen, and Priestley in 1796
showed that iron scale (oxide of iron), when heated with well-
calcined charcoal, gives out an inflammable air, whereas according
to Lavoisier's theory it ought only to give carbonic acid. This
fact was, in Priestley's opinion, opposed to the antiphlogistic
system, whilst it supported the view that the oxides contain
water, and that inflammable air is phlogisticated water, and this
was corroborated by the fact that when steam is led over red-
hot charcoal it is phlogisticated to inflammable air. This con-
clusion was one which upholders of the Lavoisierian system
found difficult to disprove, and they were driven to assume that
hydrogen was still contained even in the most strongly heated
charcoal; this fact, however, Priestley most satisfactorily proved
to be incorrect in his last work, The Doctrine of Phlogiston
Established, published in 1800.
In the same year Cruikshank was engaged with the exami-
nation of this same gas, which he obtained by heating carbon
with different metallic oxides. From its comparatively high
specific gravity he concluded that this gas was not a hydro-
carbon, as others had assumed it to be. When burnt with
oxygen it yielded no water, and nothing but an almost equal
volume of carbon dioxide, whilst the oxygen which was needed
for its combustion was less in volume than that contained in
the carbonic acid gas formed. Hence he concluded that it
must be an oxygen compound, and therefore gave to it the
name of "gaseous oxyde of carbone." Clément and Désormes
soon after confirmed Cruikshank's results; they determined the
composition of carbonic oxide more accurately than he had done,
and found that it is likewise formed when carbon dioxide is led
over red-hot charcoal.
409 Preparation.-Carbon monoxide can be prepared in
various ways. (1) It is formed when zinc oxide, ferric oxide
manganese dioxide, and many other oxides are heated with
charcoal; it is also formed when chalk (calcium carbonate)
magnesite (magnesium carbonate) and other carbonates are
heated with metallic zine or iron filings; the decomposition
which takes place in these cases is represented by the follow-
ing equations:-
ZuO+C=Zn + CO.
CaCO3+Zn-CaO+ZnO+CO.
CARBON MONOXIDE
707
(2) When carbon dioxide (carbonic acid gas) is passed through
a long tube filled with charcoal and heated to redness:-
C+CO,=2CO.
(3) When oxalic acid or an oxalate is heated with concen-
trated sulphuric acid, a mixture of equal volumes of carbon
monoxide and carbon dioxide is evolved; thus:-
C,O,H,=H,O+CO+CO,.
The carbon dioxide may readily be separated from the carbon
monoxide either by passing the mixed gases through a solution
of caustic soda or by collecting the mixture over water rendered
alkaline by this substance.
(4) Pure carbonic oxide is formed when formic acid or a
formate is heated with concentrated sulphuric acid:-
CH,O,=CO+H,O.
On the other hand, carbonic oxide can be converted into formic
acid by heating it with caustic potash, potassium formate being
produced (Berthelot):-
H
CO+ 0 =
K
COH O.
K
(5) Carbon monoxide can be readily prepared in quantity by
heating finely powdered potassium ferrocyanide with from 8 to
10 times its weight of strong sulphuric acid; in this reaction,
potassium sulphate, ammonium sulphate, and iron sulphate are
formed, the following equation representing the decomposi-
tion -
--
K¸FeC¸N¸+6H₂SO4 + 6H₂O=6CO + 2K2SO4 + 3(NH¸)₂SO₁
+ FeSO4.
The water required for the reaction is contained in the
ferrocyanide itself, which crystallizes with three molecules of
water, and also in the commercial acid, which invariably con-
tains some. According to Grimm and Ramdohr 2 it appears
that sulphur dioxide and carbon dioxide are evolved in the
beginning of the reaction, but afterwards pure carbon monoxide.
is given off.
(6) Carbonic oxide may be readily prepared by passing carbon
1 Fownes, Phil. Mag. 24, 21.
2 Annalen, 98, 127.
708
THE NON-METALLIC ELEMENTS
dioxide over zinc dust heated in a glass tube to rather less than
a red heat, the resulting gas being washed through caustic soda.¹
This reaction is sometimes made use of for the preparation of
the gas on the large scale.
Properties.-Carbon monoxide is a colourless, tasteless gas,
which possesses a peculiar though slight smell, and has a
specific gravity of 0.9678 (Cruikshank), 0·96702 (Leduc). The
relative density remains constant up to 1200°, but at 1690° the
gas partially decomposes with formation of carbon dioxide, free
carbon being deposited. It has been liquefied by Cailletet,
Wroblewski, and Olszewski; its critical temperature is — 139° 5,
the corresponding pressure being 355 atmospheres, it boils at
-190° and solidifies at -211° (Cailletet). In water it is only
very slightly soluble, its absorption coefficient, according to
Bunsen and Pauli, being obtained from the following equation-
c=
= 0.032874-0·00081632t+0.0000164212.
It is, however, easily soluble in an acid or ammoniacal solu-
tion of cuprous chloride, Cu,Cl₂.
410 Carbon monoxide easily burns in the air with a bright
blue flame, carbon dioxide being produced. The lambent flame
observed on the top of a large coal fire is due to the combustion
of this gas. When it is mixed with oxygen and ignited by the
application of a flame or by an electric spark, a violent explosion
occurs. If however the mixture of carbonic oxide and oxygen
be thoroughly freed from moisture by long-continued preserv-
ation over phosphorus pentoxide, it is found to be impossible to
produce explosion by passing an electric spark through the gas.
The addition of a trace of water-vapour or of gases, which,
by reaction with oxygen, are capable of producing water, such
as sulphuretted hydrogen, hydrogen, hydrocarbons, etc.,
immediately renders the mixture explosive. The combination
of carbon monoxide with oxygen therefore does not appear to
take place directly, and it is probable that the oxidation of
the gas is effected by means of the water-vapour present, which
acts as a carrier of oxygen, the following reactions taking place:
CO+ H2O = CO₂+H2.
2H₂+0₂ = 2H₂O.
1 Noack, Ber. 16, 75.
3 Meyer and Langer, Ber. 18, 134 c.
4 Compt. Rend. 99, 706; 100, 350; Monatsh. Chem. 6, 204.
2 Compt. Rend. 115, 1072.
CARBON MONOXIDE
709
This view of the chemical changes which occur when carbon
monoxide burns is supported by the fact that the rapidity of
explosion of a mixture of oxygen with carbon monoxide in a
tube one meter long is greater with a large quantity of aqueous
vapour than when only a trace is present.¹ This property of
carbon monoxide may be readily demonstrated by plunging a
flame of this gas burning in air into a jar of oxygen in which
some strong sulphuric acid has been shaken up; the flame is
at once extinguished.
When carbon is burned in a free supply of ordinary air or
oxygen, carbon dioxide is formed. If, however, every trace of
moisture is as far as possible removed both from the carbon and
from the oxygen, it is found that the carbon may be heated nearly
to redness without taking fire, the oxidation taking place without
the evolution of light, and that carbon monoxide is almost the
sole product. In order to obtain oxygen sufficiently dry for
experiments of this kind, it is necessary to pass the gas through
sulphuric acid and then allow it to remain for some weeks
in carefully closed flasks, containing a considerable amount of
phosphorus pentoxide.2
Carbon monoxide is also largely formed by the oxidation of
carbon when heated in air at a comparatively low temperature.
These facts render it probable that in the combustion of carbon
carbon monoxide is the first product, and is then oxidised to
carbon dioxide by the oxygen of the water-vapour present.
Carbon monoxide combines directly with metallic nickel and
iron to form very remarkable substances of the composition
Ni(CO), and Fe(CO). These substances readily decompose
when heated into the metal and carbon monoxide. They will
be described under the compounds of the metals.
Carbonic oxide is a very poisonous gas, inasmuch as it com-
bines with the hæmoglobin of the blood: small animals die
almost instantly when placed in the gas, and when even small
quantities are inhaled severe headache, giddiness, and insensi-
bility readily occur. The accidents, as well as suicides, which
occur from burning charcoal in a chauffer in a small room, are
due to the inhalation of this gas formed by incomplete com-
bustion, and deaths occurring from sleeping upon lime-kilns and
1 Dixon, Phil. Trans. 1884, ii. 617.
2 Baker, Journ. Chem. Soc. 1885, i. 349; Phil. Trans. 1888, A. 571.
3 Journ. Chem. Soc. 1890, i. 749; 1891, 1090; Compt. Rend. 113, 679; Proc.
Chem. Soc. 1891, 126.
710
THE NON-METALLIC ELEMENTS
brick-kilns are probably also produced by this gas. The
poisonous nature of the gas from red-hot charcoal was known
and expatiated upon as early as the year 1716, when F.
Hoffman published his work, Considerations on the Fatal Effects
of the Vapour from Burning Charcoal.
The remarkable poisonous action of carbonic oxide appears to
depend upon the fact that the whole dissolved oxygen is thereby
expelled, the blood acquiring a light, purple-red colour. The
absorption spectrum of the carbonic-oxide-hæmoglobin is dis-
tinguished by the fact that the bands between D and b are
situated nearer to b than is the case with oxyhæmoglobin (Hoppe-
Seyler), and that, unlike the latter, it remains unchanged in
presence of reducing agents. This test serves as a means of
detecting cases of poisoning with the gas.
Carbonic oxide may also be detected in small quantities in
the air by passing the air to be examined through blood and
then making a spectroscopic examination of the latter. The
gas is either estimated by explosion with oxygen or more
generally by absorption with acid cuprous chloride.
The composition of carbon monoxide is determined by mixing
the gas with oxygen, exploding the mixture, and measuring the
carbon dioxide formed. It is thus ascertained that one volume
of the gas unites with half a volume of oxygen to form one
volume of carbon dioxide. Now the latter is known to contain
its own volume of oxygen (p. 714), and it hence follows that
carbon monoxide contains half its own volume of oxygen. The
molecular weight of the gas is shown by its density to be about
27.79, and this amount of it therefore contains 15.88 of oxygen
and 11'91 of carbon, the formula of the gas being CO.
CARBON DIOXIDE OR CARBONIC ANHYDRIDE, CO₂ = 43.67.
411 This gas belongs to the class of acid-forming oxides for-
merly termed acids, and is still best known under its old name
of carbonic acid. It has already been stated in the historical
introduction that this gas was first distinguished from common
air by Van Helmont, who termed it gas sylvestre. He obtained
it by the action of acids on alkaline or calcareous substances, and
showed that it is also formed by the combustion of charcoal,
and in the fermentation and decay of carbonaceous matter, and
that it likewise occurs in the mineral water at Spa, in the
Grotto del Cane near Naples, and in other localities. Van
Helmont describes the suffocating action it exerts on animal
CARBON DIOXIDE
711
life as well as its effects in extinguishing flame. Fr. Hoffmann
made further observations on the gas contained in effervescing
mineral waters, and he states that this is frequently given
off in such quantity that when the water is enclosed in
bottles these are burst by the force of the gas. He also shows
that this substance, to which he gave the name of spiritus
mineralis, has the power of reddening certain blue vegetable
colouring matters, and hence he considers it to be a weak acid.
Although many other chemists investigated the properties of
this gas, it was not until the time of Black that it was dis-
tinctly shown to differ essentially from common air. Black
(1755) proved that this substance is a peculiar. constituent of
the carbonated or mild alkalis, being, in them, combined or
fixed in the solid state, whence it was termed by him fixed air.
In the year 1774 Bergman published a complete history of this
peculiar air, to which he gave the name acid of air, because
of its occurrence in the atmosphere. Its chemical nature was
first properly explained by Lavoisier, who showed that, whilst
mercuric oxide heated alone gives off pure oxygen gas, fixed air
is evolved when it is heated with carbon, proving that this
latter gas is an oxide of carbon.
Carbon dioxide is a body which is widely distributed in nature;
as we have already seen, it forms a small but constant and
essential constituent of the atmosphere; it is likewise invariably
contained in soil, being one of the chief products of the decay
of all organic substances. From the soil it is taken up by rain
and spring water, and it is to this substance that the latter, to a
great extent, owes its fresh and pleasant taste. It occurs in
chalybeate and acidulous waters in large quantities, whilst in
both ancient and modern volcanic districts it is emitted in very
large volumes from the fumeroles and rents in the ground, for
instance in the old craters in the Eifel, at Brohl on the Rhine,
as well as in the Auvergne, particularly in the neighbourhood of
Vichy and Hauterive, where the gas is actually employed for
the manufacture of white lead. Especially remarkable for the
evolution of this gas in very large quantities is the Poison
Valley in Java, which also is an old crater, and the Grotto del
Cane near Naples, which is of such a construction that the
heavy carbonic acid gas, entering from the fissures in the floor
of the cave, at a depth of from two to three feet below the
mouth of the cave, collects up to this depth, and small animals
such as dogs, when thrown into the cave, respiring the impure
712
THE NON-METALLIC ELEMENTS
air, fall down, whilst a man breathing the pure air above this
level is unaffected by the gas.
The carbonic acid contained in the air is derived from a
variety of sources; it is formed by the respiration of man and
animals, as well as in the act of combustion of organised
material, and in its decay and decomposition. The amount of
atmospheric carbonic acid varies between certain narrow limits,
but on an average reaches 3 volumes in 10,000 volumes of air.
In the presence of the sunlight, plants have the power, through
their leaves, of decomposing this carbonic acid, taking up the
carbon to form their own tissue, and eliminating the oxygen
gas; hence the amount of carbonic acid in the air does not
increase beyond the limits named (Saussure).
Carbon dioxide is an acid-forming oxide giving rise to a
series of salts termed the carbonates, many of which occur in
nature as minerals. Amongst these is especially to be men-
tioned calcium carbonate, CaCO3, which occurs in two distinct
crystalline forms as calc-spar and arragonite, whilst it is found
in a massive crystalline form in marble and limestone; calcium
carbonate also forms the chief constituent of the shells of
mollusca and foraminifera, the remains of which constitute the
chalk formation as well as the greater part of all the lime-
stones. The double carbonate of magnesium and calcium.
(MgCa)CO, also occurs in large masses as dolomite. Amongst
other naturally occurring carbonates may be mentioned mag-
nesite, MgCO; iron spar or spathic iron ore (FeMnCaMg)
CO; witherite, BaCO,; strontianite, SrCO,; and calamine,
ZnCO3.
412 Preparation.—(1) In order to prepare carbon dioxide a
carbonate, such as marble or chalk, is brought into a gas-
evolution flask, and dilute hydrochloric acid poured upon it,
when the gas is rapidly evolved with effervescence :-
CaCO3 + 2HCl = CO₂ + CaCl2 + H₂O.
2
The gas thus obtained invariably carries over small quantities
of hydrochloric acid vapour with it, from which it may be freed
by passing through a solution of bicarbonate of soda. In
order to obtain a constant stream of carbon dioxide the same
apparatus may be employed which was made use of for the
preparation of sulphuretted hydrogen gas (Fig. 100).
(2) Another method of obtaining a constant current is to
PROPERTIES OF CARBON DIOXIDE
713
pour concentrated sulphuric acid over chalk, and add a very
small quantity of water:-
CaCO3 + H₂SO₁ = CO₂ + CaSO4 + H₂O.
4
2
The dilute acid cannot be employed for this purpose, since
the calcium sulphate formed, which is soluble in the con-
centrated acid, does not dissolve in the dilute acid, and thus
prevents its further action on the chalk.
(3) The gas thus obtained from chalk possesses a peculiar
smell, which is due to the presence of small quantities of
volatile organic matter always contained in the chalk. In
order to prepare a very pure gas, sodium carbonate may be
decomposed with pure dilute sulphuric acid; thus:-
Na2CO3 + H₂SO₁ = CO₂ + Na₂SO4 + H₂O.
(4) Carbon dioxide may be obtained on a large scale for the
preparation of bicarbonate of soda, white lead, and other com-
mercial products by "burning" limestone, or even by the
combustion of charcoal or coke.
413 Properties.-Carbon dioxide is a colourless gas possessing
a slightly pungent smell and acid taste. It does not support
either combustion or respiration; hence a flame is extinguished
when plunged into the gas, and animals thrown in become
insensible and suffer death from suffocation. At the same time,
however, it does not exert a poisonous action on the animal
economy, as indeed may be gathered from the fact that it is
constantly taken into the lungs and emitted from them; it
destroys life, however, because it does not contain any free
oxygen. According to Berzelius, common air containing th
of its volume of carbon dioxide can be breathed without pro-
ducing any serious effects; but from Angus Smith's later
experiments¹ it appears that when air contains only 0·20 per
cent. by volume of this gas, its effect in lowering the action of
the pulse is rendered evident after the respiration has continued
for about an hour. It seems, therefore, premature to say that
the smallest increase of atmospheric carbonic acid may not be
productive of hurtful results. In close spaces inhabited by man
the quantity of carbon dioxide is naturally much larger than in
the open air. The oppressive feeling which respiration in such
air is apt to produce is not however in general due to the
presence of this gas, but rather to the volatile organic emana-
¹ Air and Rain, p. 209, “On Some Physiological Effects of Carbonic Acid."
714
THE NON-METALLIC ELEMENTS
tions, the presence of which is indicated by the unpleasant
smell observed on entering such rooms from the fresh air.
Carbon dioxide possesses, according to Regnault, a specific
gravity of 1·529, and hence, being much heavier than air, it
may be poured like water from one vessel to another; this
fact can be strikingly exhibited by bringing a lighted taper into
the vessel into which the carbon dioxide has been poured, when
it will be instantly extinguished. This property is made use of
to test the presence of the gas in old wells, cellars, and coal-pits,
where it frequently accumulates, and is termed choke damp.
Before the workman descends it is usual to lower down a
burning candle, and thus ascertain whether the air is pure
enough for respiration. Air containing 4 per cent. of carbon
dioxide extinguishes a candle-flame, but it will support respira-
tion for a short time.
Composition of Carbon Dioxide.-When carbon burns to the
dioxide, the volume of the gas which is formed is the same
as that of the oxygen which is needed to produce it. This fact
may readily be shown by help of the apparatus which is shown
in Fig. 108, under sulphur dioxide, the experiment being con-
ducted as there described, except that a small piece of freshly
heated charcoal is placed in the small combustion pan instead
of sulphur. The molecular weight of the gas, calculated from
its density, is 43'67, and this amount of it therefore contains
31.76 parts of oxygen and 11.91 parts of carbon, the molecular
formula being CO2.
Dumas and Stas¹ have accurately ascertained the atomic
weight of carbon by a series of careful experiments. For this pur-
pose they made use of the apparatus which is shown in Fig. 190,
in which they burnt diamond and purified graphite in a stream
of oxygen gas. The carbon dioxide formed was completely
absorbed by means of caustic potash, and the composition of
the carbon dioxide calculated from the weight of it formed,
together with the weight of carbon burnt. In order to carry
out this investigation it was necessary, in the first place, to in-
sure the absence of every trace of carbon dioxide from the oxygen
gas employed. The oxygen used was, therefore, collected in a
large Woulff's bottle-(a)-and preserved over water rendered
alkaline by caustic potash. The gas was driven out from this
gas-holder in the course of the experiment by means of a dilute
caustic potash solution (b). The oxygen then passed through a
1 Ann. Chim. Phys., [1] 40, 991.
ATOMIC WEIGHT OF CARBON
715
long wide tube filled to the
point (c) with pumice-stone
moistened with strong
caustic potash solution,
whilst at (d) it came in
contact with pieces of dry
solid caustic potash, and
at (e) with glass moisten-
ed with boiled sulphuric
acid. In order to insure
the absolute dryness of the
gas it passed through a U
tube, marked (f), filled
with pumice-stone moist-
ened with sulphuric acid.
This tube was weighed
before and after the ex-
periment. The pure dry
oxygen then passed into a
porcelain tube, which was
heated to redness in a
tube-furnace, the substance
undergoing combustion be-
ing placed at (g) in a
platinum boat. The boat
containing the substance
was accurately weighed
both before and after the
experiment, inasmuch as
even the purest diamond
and graphite always leave
on combustion traces of
inorganic ash, and this
must, of course, be sub-
tracted from the total
amount of substance taken.
The other end of the tube
contained, at (h), a quan-
tity of perfectly oxidised
copper scales, CuO, which
served for the purpose of
oxidizing to carbon dioxide

•
•
90
6.0
1月
​FIG. 190.
716
THE NON-METALLIC ELEMENTS
any monoxide which might be formed. The carbon dioxide then
passed through the tube (i) containing pumice-stone moistened
with sulphuric acid, and then into two Liebig's potash-bulbs
( and 7). In order to be certain that the whole of the
carbon dioxide was absorbed, and that no moist air passed
away from the potash-bulbs, the excess of oxygen was passed
through two tubes (m and n), the first of which contained
pumice-stone moistened with potash, and the last solid potash.
In this way five combustions of natural graphite were made,
four of artificial graphite, and five of diamond. As a mean of
the closely agreeing results, it was found that 800 parts of
oxygen combined with-
Natural Graphite.
299.94
Artificial Graphite.
299.95
Diamond.
300-02
The mean of these numbers is 299.97. Hence 2 atoms of
oxygen, or 31.76 parts by weight, combine with 119 parts by
weight of carbon. Similar numbers have been obtained by other
chemists, and the most accurate value is probably 1191. This
number is taken as the atomic weight of carbon, because it
is the least amount ever found in a molecule of one of its
compounds.
414 Liquefaction of Carbon Dioxide.-Carbon dioxide can be
liquefied both by cold and by pressure. Faraday was the first to
obtain this result by pouring some sulphuric acid into the closed
limb of a bent tube made of strong glass, whilst over it he
pushed a piece of platinum foil on which a lump of carbonate
of ammonia was placed. After closing the open end of the tube
the sulphuric acid was cautiously allowed to flow over the car-
bonate, and in this way the carbon dioxide which was evolved
was condensed by its own pressure. Liquid carbon dioxide pre-
pared in this way cannot be employed for further experiments,
as on endeavouring to open the tube it usually bursts with a
loud explosion. Gore, however, made an ingenious arrange-
ment in which the carbon dioxide is generated in a small bent
tube closed with a stopper of gutta-percha, to which is fixed a
small glass cup. In this way the solvent action of the liquid on
other bodies was first examined.
In order to condense carbon dioxide on a larger scale Thilorier?
constructed an apparatus which consisted of a strong cast-iron
cylinder, termed the generator, capable of being closed hermeti-
2 Thilorier, Annalen, 30, 122.
1 Phil. Trans. 1861, p. 83.
LIQUEFACTION OF CARBON DIOXIDE
717
cally, the original form of which is shown in Fig. 191. In
this cylinder he placed bicarbonate of soda, whilst sulphuric
acid was placed in a separate vessel lowered into the cylinder.
The cylinder was closed by means of the screws at the top,
and then inclined so that the sulphuric acid came in contact
with the bicarbonate of soda, evolving carbon dioxide in such
quantity that it was liquefied by its own pressure. The liquid
carbon dioxide was then separated from the sulphate of soda
formed at the same time, by distilling it into a similar cast-
iron vessel, termed the receiver. Owing to the brittle nature
and non-homogeneity of the cast-iron of which the cylinders
were made, and in consequence of the enormous pressure
which they had to withstand, it was feared that serious acci-

FIG. 191.
dents might occur in the production of the liquid according
to Thilorier's plan, and Hare suggested that the apparatus
should be made of wrought iron. Not long after this suggestion
had been made an accident occurred: one of Thilorier's cylinders
exploded with fearful force whilst it was being charged, and a
young chemist of the name of Hervey lost his life. The wrought-
iron apparatus, which was employed after this time, is shown in
Fig. 192, the construction having been much improved by Mareska
and Donny. Instead of wrought iron they employed a leaden
cylinder, surrounded by another one made of copper, this, in
its turn, being bound round with strong hoops of wrought iron.
In order to prepare liquid carbon dioxide with this apparatus,
1 Mém. cour. et mém. Savants étrang. Acad. Roy. Bruxelles, 18.
718
THE NON-METALLIC ELEMENTS
1
the requisite quantity of bicarbonate of soda is placed in the
generator, and a quantity of dilute sulphuric acid, sufficient for
its decomposition, is poured into the narrow copper cylinder.
The apparatus is closed with a strong screw-tap, and then
made to revolve on its axis so that the acid comes slowly in
contact with the carbonate. For the purpose of distilling the
carbon dioxide, which is formed by this decomposition, into the
horizontal receiver, a copper tube is screwed on, connecting the
one vessel with the other. When the screw-taps are opened the
FIG. 192.
liquid dioxide distils over, the receiver being kept cool and the
generator being placed, at the end of the operation, in hot water.
By repeating this operation a large quantity of liquid carbon
dioxide can easily be obtained.
The apparatus, however, which is now most commonly em-
ployed for the preparation of liquid carbon dioxide on the small
scale or for lecture purposes is that made by Natterer of Vienna
and Bianchi of Paris. In this apparatus, the general arrange-
ment of which is seen in Fig. 193, carbon dioxide, generated by
124 lbs. of bicarbonate of soda, 6 lbs. of water, and 1 lb. of oil of vitriol.
2 J. Pr. Chem. 35, 169.
LIQUEFACTION OF CARBON DIOXIDE
719

FIG. 193.
b
9
FIG. 194.
720
THE NON-METALLIC ELEMENTS
the action of dilute sulphuric acid on bicarbonate of soda, is con-
densed by means of a force-pump into a wrought-iron pear-shaped
vessel, seen in section in Fig. 194, furnished with suitable cocks,
t, Fig. 194, the air which was contained in the flask being first
of all displaced by the gas. During the operation the reservoir
(2) is surrounded by a freezing mixture contained in a vessel,
screwed on to the plate a b, Fig. 193.
415 Liquid carbon dioxide is now a commercial article and
sold in iron bottles containing 8 kg., being mostly prepared in
certain large breweries from the carbon dioxide evolved in the
process of fermentation. It is also made from the natural gas.
evolved from certain springs, especially at Burgbrohl on the
Rhine, and from the gas prepared from marble or sodium
bicarbonate. The liquefaction is accomplished by compressing
the gas by means of powerful pumps into recipients kept cool
by a stream of water. It is used for producing cold, as well as
high pressures, and therefore employed in the manufacture of
cast steel. It is also employed for beer-engines, in the manu-
facture of salicylic acid, and for the production of aërated
waters.
Liquid carbon dioxide is a colourless very mobile liquid
slightly soluble in water, upon the surface of which it swims.
Its specific gravity is 0·9951 at -10°, 0·9470 at 0°, and 0.8266
at 20°. These numbers show that liquid carbon dioxide
expands more upon heating than a gas, and its coefficient of
expansion is, therefore, larger than that of any known body.
The boiling-point of liquid carbon dioxide is 78.2° under a
pressure of 760 mm., and its tension at different temperatures
is given in the following table :-
Temperature.
25°
- 15
5
+ 5
Pressure in mm. of
mercury.
13007 02
17582:48
23441·34
30753.80
Temperature.
+15°
+ 25
+ 35
+45
-
Pressure in mm. of
mercury.
39646-86
50207:32
62447 30
76314.60
Liquid carbon dioxide conducts electricity badly; it does not
act as a solvent on most substances, does not redden dry litmus
paper, and possesses no very striking chemical properties (Gore).
When the stopcock of a vessel containing liquid carbon
dioxide is opened, a portion of the liquid which rushes out
solidifies in the form of white snow-like flakes (Thilorier).
This is caused by the absorption of heat, produced by the rapid
1 Andreef, Annalen, 111.
SOLID CARBON DIOXIDE
721
For the
For the purpose
evaporation of another portion of the liquid.
of obtaining larger quantities of this carbon dioxide snow, the
apparatus shown in Figs. 195 and 196, suggested by Natterer,
is employed. It consists of two brass cylinders, AB and CD, one
of which can be fixed inside the other, each cylinder being
fastened to a hollow handle.
The liquid carbon dioxide is
allowed to pass quickly through
the tube d (the position of
which inside the box is shown.
in Fig. 196), from the nozzle n,
Fig. 194, of the reservoir. A
portion of the liquid undergoes
evaporation and passes out as
gas through the fine holes in
the handles, whilst the larger
portion remains behind as a C
solid finely divided crystalline
mass, which is a very light sub-
stance, capable of being pressed
together like snow. It evapor-
ates much less quickly than the
liquid dioxide, because it is
much colder, and in addition it
has to take up the amount of
heat necessary for liquefaction.
Notwithstanding its excessive-
ly low temperature it may be
touched without danger, the
gas which it constantly emits
forming a non-conducting at-
mosphere round it. If, however,
it be pressed hard upon the
skin, solid carbon dioxide pro-
duces a blister exactly like that
produced by contact with a
red-hot body. This snow-like
mass may readily be pressed into cylindrical wooden moulds. The
cylinders thus obtained have a density of about 1.2, and require
a considerable time for evaporation in the air. When solid
carbon dioxide is mixed with ether, and the mixture brought
1 Landolt, Ber. 17, 309.
FIGS. 195 and 196.

A
B

D

a
47
722
THE NON-METALLIC ELEMENTS
into the vacuum of an air-pump, the temperature sinks to
-110°. A tube containing liquid carbon dioxide brought into
this solidifies to a transparent ice-like solid mass (Mitchell and
Faraday). When liquid ammonia is placed over sulphuric acid
in a vacuum and allowed to evaporate, such a diminution of tem-
perature is obtained that gaseous carbonic acid may be liquefied.
Exposed to a pressure of from 3 to 4 atmospheres, this solidifies,
and the transparent mass thus obtained, when pressed with a
glass rod, separates into cubical masses.¹ It is very remarkable
that solid carbon dioxide melts at 65°, that is at a temperature
lying much above its boiling point at the atmospheric pressure.
A process has been patented for carrying out the manufacture
of solid carbonic acid by a similar method on the large scale.
The solid material may be transported in bags or casks, or in the
iron chamber in which it is produced.2
-
In the paragraph (24) concerning the continuity of the
gaseous and liquid states of matter, it has been pointed out
that a certain temperature may be reached for any given gas,
above which this gas cannot be liquefied, however much the
pressure may be increased. This temperature is termed by
Andrews the critical point, that of carbon dioxide being
found by him to be 30 92°, whilst according to more recent
observations it is 31-35°, the pressure at this temperature, or
the critical pressure, being 729 atmospheres. In order to
determine this point, Andrews employed the apparatus shown
in Figs. 197 and 198. The dry and pure gas is contained in
the glass tube a b, which is closed at a and open at b, the gas
being shut in by the thread of mercury c. This tube is firmly
bedded in the brass end-piece d, and this, in its turn, is bolted
on to the strong copper tube which carries a second end-piece
at its lower extremity. Through this the steel screw e passes
packed with leather washers to render the cylinder perfectly
air-tight. The cylinder is completely filled with water, and
the pressure on the gas is increased up to 400 atmospheres by
turning the steel screw e into the water. The closed and
capillary end of the tube containing the compressed gas is
sometimes surrounded with a cylinder into which water of a
given temperature is brought. A second modification of the
apparatus is shown in Fig. 198. The capillary pressure-tube
is bent round so as to render it possible to place it in a freezing
1 Loir and Drion, Compt. Rend. 52, 748.
3 Phil. Trans. 1869, part ii., 575.
English Patent 13,684, 1891.
4 Amagat, Compt. Rend. 114, 1093.
LIQUID CARBON DIOXIDE
723
mixture under the receiver of an air-pump. The action both
of great pressure and great cold upon the condensed gas can
thus be examined,
с
If the screw be turned round when the gas possesses a
temperature below the critical point, liquid carbon dioxide is
formed when a certain pressure is reached, and a layer of this
liquid can be distinctly seen lying with the gas above it. If
the same experiment be repeated at a temperature above the
critical one, no liquid is
seen to form even when the
pressure is increased to 150
atmospheres. The volume.
gradually diminishes, no line
of demarcation or other
alteration in appearance can
be observed, and the tube
appears as empty as it did
before the gas was submitted
to pressure. If the tem-
perature be now lowered
below 31-35°, the whole
mass becomes liquid, and
when the pressure is dimin-
ished begins to boil, two
distinct layers of liquid and
gaseous matter being again
observed. Thus, whilst at all
temperatures below 31-35°
carbon dioxide gas cannot
be converted into a liquid
without a sudden condensa-
tion, at temperatures above
this point gaseous carbon
dioxide may, by the applica-
tion of great pressure and subsequent cooling to below the critical
point, be made to pass into a distinctly liquid condition without
undergoing any sudden change such as is observed in the case
of ordinary liquefaction.
FIGS. 197 and 198.
SA
416 Carbon dioxide is, a very stable body, requiring for its
decomposition an extremely high temperature. When passed
over pieces of porcelain in a porcelain tube, heated to a tempera-
ture of from 1200° to 1300°, it decomposes to a small extent
724
THE NON-METALLIC ELEMENTS
into carbon monoxide and oxygen (Deville); and the same
decomposition is brought about by the electric spark (Dalton
and Henry). In this way only a small portion of the gas
is
decomposed, as when a certain quantity of oxygen has been
formed this again combines with the carbon monoxide. Under
favourable circumstances the recombination of the gases, accom-
panied by the passage of a flame, can be actually observed to
take place periodically. If, however, hydrogen, or mercury, or
any oxidizable body be present, the whole of the carbon dioxide is
converted into monoxide (Saussure). This decomposition is best
shown by allowing the electric spark to pass by means of iron
poles through a measured volume of carbon dioxide; after the
action is completed, the volume of carbon monoxide formed is
found to be exactly equal to that of the dioxide taken, thus:-
1
2CO₂=2CO+02
the oxygen being completely absorbed by the metallic iron
(Buff and Hofmann).
A piece of burning magnesium wire or ribbon continues to
burn when plunged into carbon dioxide, the oxide of the metal
being formed and carbon liberated:-
CO₂+2Mg= 2MgO+C.
When carbon dioxide is passed over heated potassium or
sodium, the carbonates of these metals are formed, and carbon
is set free; thus :—
3CO₂+2K2=2K₂CO3 + C.
Liquid carbon dioxide is also attacked by the alkali metals
(Gore).
Carbon dioxide is soluble in water, its solubility beng re-
presented by the following equation:-
c = 1.7967-0-07761t+ 0.00164242,
or one vol. of water at 0° dissolves 1.7967 vols. of carbon dioxide.
5
10
1.4497
1.1847
1:0020
0.9014
15
20
""
..
""
99
""
"9
99
36
39
39
29
""
1 Hofmann, Ber. 23,
3303.
..
""
">
..
""
39
33
"
12
""
"9
21
CARBONIC ACID
725
It is more easily soluble in alcohol of specific gravity 0·792,
the coefficient of solubility being given by the following equa-
tion :-
c=432955-0·09395t + 0·00124t2.
When the pressure is much smaller than that of the atmo-
sphere, carbon dioxide follows Dalton and Henry's law of the
absorption of gases in water, but deviation is observed from this
law at higher pressures, increasing gradually as the pressure is
increased.
in atmospheres
1
5
10
15
20
25
30
The following table shows the volume of carbon dioxide,
measured at 0° and 760 mm. dissolved by one volume of water
at 0° (Column S), the pressure being given in the column (P).¹
P
S
Vols. at 0°
1.797
8.65
16.03
21.95
26.65
30.55
33.74
521P
S
1.797
1.730
1.603
1.463
1.332
1.222
1.124
S
P
If the law of Henry and Dalton were correct, the value of
would be constant. It appears from the table, however, that this
fraction gradually decreases in value, the solubility increasing
at a slower rate than the pressure.
CARBONIC ACID AND THE CARBONATES.
417 The aqueous solution of carbon dioxide contains in solu-
tion a dibasic acid, termed carbonic acid, CO(OH)2. This acid
does not exist in the pure concentrated state, and in this respect
resembles sulphurous acid and other acids whose corresponding
oxides are gaseous. Its existence is, however, ascertained by
the fact that the aqueous solution of carbon dioxide reddens.
litmus paper, whereas dry carbon dioxide, whether in the
gaseous or liquid state, produces no such action. It has also
been observed that if the water be saturated with carbon dioxide
under pressure, and if the pressure be then at once removed,
1 Wroblewski, Compt. Rend. 94, 1355.
726
THE NON-METALLIC ELEMENTS
the gas quickly makes its escape by effervescence, the bubbles
being so minute that the whole liquid, during the disengage-
ment of the gas, appears milky. If, however, the liquid be
allowed to remain in contact with the gas for some consider-
able time after saturation, before the pressure is removed, the
gas escapes, on removal of the pressure, in large bubbles which
adhere chiefly to the sides of the glass vessel in which the
liquid is contained. The difference may be explained by the
fact that, to begin with, the carbon dioxide is mechanically
dissolved in the water, but that after remaining in contact with
the water for some time an actual combination takes place
between these two substances, which may be represented by the
following equation:-
O=C=0 + H-O-H = 0=C
OH.
OH.
If a piece of porous substance like sugar or bread be brought
into a liquid, such as soda-water or champagne, which has been
saturated under pressure with carbon dioxide, and which has
been exposed to the air for some little time, so that the effer-
vescence due to the diminution of pressure has ceased, a strong
renewal of the effervescence is observed. Water which contains
a small quantity of common salt in solution dissolves carbon
dioxide more easily than common water. This depends on the
fact that a chemical decomposition takes place, a part of the
common salt, NaCl, being converted by the carbonic acid present
into acid sodium carbonate, NaHCO3, free hydrochloric acid,
HCl, being liberated. The presence of the latter acid may
easily be shown by the addition to the liquid of a small quantity
of ultramarine, this substance losing its blue colour in presence
of hydrochloric acid, whilst a solution either of common salt or
of carbonic acid fails to act upon it.
If a current of carbon dioxide be passed through a solution
of chloride of lead, an insoluble chlorocarbonate of lead is
formed¹:-
2 Pb
CI
CI
+CO,+H,O=
Pb
Pb
Cl
CO+ 2HCl.
CI
1 Hugo Müller, Journ. Chem. Soc. 1870, 37.
THE CARBONATES
727
In a similar way, when carbon dioxide is passed through a
solution of common sodium phosphate, acid sodium phosphate
and acid sodium carbonate are formed; thus:-
HNA,PO+H₂CO₂ = H₂NaPO4+HNaCO3.
Carbonic acid readily decomposes into water and carbon dioxide.
In consequence of this, litmus paper, which has been turned red
in the aqueous acid, becomes blue on drying. When carbon
dioxide gas is passed into a solution of blue litmus, this solution
becomes first violet and then of a wine-red colour; if this red
solution be then heated, carbon dioxide is evolved in large
bubbles, and after boiling for a few seconds the liquid again
becomes blue.
Although carbonic acid itself is such an extremely unstable
compound, the carbonates are very stable. Being a dibasic acid,
it forms two series of salts—namely, the normal and the acid
carbonates. When carbon dioxide is passed through a solution
of the hydroxide of an alkali-metal, a normal carbonate is first
formed; thus:-
2KOH+CO, K₂CO₂+H₂O.
These normal carbonates of the alkali-metals are soluble in
water, and have an alkaline reaction. When they are treated
with an excess of carbon dioxide, the solution becomes neutral,
and then contains an acid carbonate :-
-
=
K₂CO₂+CO₂+H₂O = 2HKCO.
2
If a solution of the hydroxides of the metals of the alkaline
earths, such as lime-water or baryta-water, be treated in a similar
way, a white precipitate of the normal carbonate of these metals
is obtained in the first instance, these salts being almost in-
soluble in water; thus:-
Ca(OH)2+CO₂ = CaCO3+H₂O.
If more carbon dioxide gas be passed through the milky solution
the precipitate dissolves, and the solution contains an acid
carbonate. These acid carbonates of the metals of the alkaline
earths are only known in solution. When their solutions are
boiled, carbon dioxide is given off, and the normal carbonate is
again precipitated. All the other normal carbonates are in-
soluble in water. A few, such as magnesium carbonate and
728
THE NON-METALLIC ELEMENTS
ferrous carbonate, dissolve like the carbonates of the alkaline
earth metals in an excess of carbonic acid. Carbonic acid being
a very weak acid, its salts are readily decomposed by the greater
number of the acids, when the solution is not too dilute, carbon
dioxide being evolved as gas. It may be readily recognised, as
it produces a white precipitate when brought in contact with
clear lime- or baryta-water.
418 In order to determine the quantity of carbon dioxide
contained in a carbonate, several methods may be employed.
It is easy to determine this quantity by
the loss of weight which the salt under-
goes when an acid is added, and for
this purpose the apparatus constructed
by Geissler, which is thus described
by Fresenius, is one of the best.
The apparatus, the construction of
which is shown in Fig. 199, consists
of three parts, A, B, and C. C is
ground into the neck of 4, so that it
may close air-tight, and yet admit of
its being readily removed for the pur-
pose of filling and emptying A. b c is
a glass tube, open at both ends, and
ground water-tight into C at the lower
end, c; it is kept in the proper position
by means of the movable cork, i. The
cork, e, must close air-tight, and so
must the tube, d, in the cork. The
weighed substance to be decomposed
is put into A, water is added to the
extent indicated in the engraving, and
the substance shaken towards the side
of the flask. C is now filled nearly to
the top with dilute nitric or hydro-
chloric acid, with the aid of a pipette, after having previously
moved the cork, i, upwards without raising b. The cork is
then again turned down, C' again inserted into A, B somewhat
more than half filled with concentrated sulphuric acid, and b
closed at the top, by placing over it a small piece of caoutchouc
tubing with a glass rod fitted into the other end. After weighing
the apparatus, the decomposition is effected by opening b a little
and thus causing acid to pass from C into A. The carbonic acid

C
b
FIG. 199.
B
GRAPHITIC ACID
729
passes through the bent tube h into the sulphuric acid, where it
is dried. It then leaves the apparatus through d. When the
decomposition is effected, A is gently heated, the stopper from b
removed, and the carbon dioxide still present sucked out at d.
The apparatus, when cold, is again weighed, and the difference
between the two weighings will be that of the carbon dioxide
expelled.
A more accurate method consists in passing the carbon
dioxide evolved by the decomposition of the substance by dilute
acids through a series of drying tubes and then absorbing it in
a weighed solution of potash, and thus ascertaining its weight
directly.
The method for the determination of the carbon dioxide con-
tained in the air has already been explained (p. 539).
In order to determine the quantity of this gas dissolved in
water, ammonia is added to a solution of barium chloride; this
is heated to the boiling point and filtered; 50cc. of this solution
are then brought into a flask of 300cc. capacity, and the water
under investigation added in measured quantity, the flask being
again stoppered up and allowed to stand for a considerable
length of time until the precipitated carbonate of barium has
separated out. The solution is then warmed, and the flask
again stoppered. The solution is allowed to become clear, the
clear liquid is poured off as completely as possible from the
precipitate, the flask is filled up with water which has been well
boiled, is again allowed to deposit, and the operation is repeated
several times. The precipitate is then brought upon a tared
filter and weighed, after being carefully dried at 110°. The
quantity of carbon dioxide can be readily calculated from the
weight of barium carbonate obtained.
GRAPHITIC ACID, CHO.
419 It has already been stated under the head of graphite
that in certain properties this substance differs remarkably from
the other modifications of carbon. Sir Benjamin Brodie ¹ has
shown that, when acted upon by certain oxidizing agents,
graphite is converted into a compact substance which contains
oxygen and hydrogen. In order to prepare graphitic acid, an
intimate mixture of one part of purified graphite and three parts
of potassium chlorate is treated with so much concentrated
1 Phil. Trans. 1859, p. 249.
730
THE NON-METALLIC ELEMENTS
nitric acid that the mass becomes liquid. It is then heated for
three or four days on a water-bath. The solid residue, after
having been washed with water and dried at 100°, is subjected
four or five times to a similar treatment, until no further change
is observed. Graphitic acid is a stable yellow substance existing
in thin microscopic crystals, which have the property of redden-
ing moistened blue litmus paper, and are slightly soluble in pure
and insoluble in acidified water. It is doubtful whether the
substance is in reality possessed of acid properties.
According to Berthelot¹ and Luzi 2 the products of oxidation
obtained from different specimens of graphite differ in several
important respects, and yield different series of derivatives.
22
When graphitic acid is heated, it swells up and yields a fine
powder consisting of pyrographitic oxide, CH₂O, a substance
which is almost entirely dissolved by the oxidizing mixture of
nitric acid and potassium chlorate, a little of the original
graphitic acid being regenerated. When heated with hydriodic
acid, graphitic acid is converted into hydrographitic acid,
which contains more hydrogen than graphitic acid itself, and
does not swell up and yield pyrographitic acid when heated.
Oxidizing agents reconvert it into graphitic acid. There is
still considerable doubt as to the composition of these sub-
stances, and their chemical nature as well as the relation of
the various derivatives to each other are but very imperfectly
understood.
Neither charcoal nor diamond yields similar compounds, and
Brodie believes that graphite may be considered to be a peculiar
radical, to which he gives the name of graphon. As already
mentioned, diamond is not attacked by the oxidizing mixture of
nitric acid and potassium chlorate, whilst ordinary charcoal is
converted into a brown mass, soluble in water. Berthelot has
made use of this property for the purpose of estimating the
quantity of charcoal, graphite, and diamond present in a
mixture. The finely powdered substance is treated by the
method described for the preparation of graphitic acid; care,
however, must be taken that not more than five grams of the
mixture are used at once, as otherwise explosions may take
place. In order to separate the diamond from the graphitic
acid, the residue is gently ignited, and again treated with the
oxidizing mixture. The process is repeated until the whole of
2 Ber. 24, 4085; 25, 1378.
1 Ann. Chem. Phys. [4] 19, 405.
3 Ann. Chim. Phys. [4], 19, 399.
CARBONYL CHLORIDE
731
the graphitic acid has disappeared, but any diamond which
may be present remains unaltered.
CARBON OXYCHLORIDE OR CARBONYL CHLORIDE, COC1, -98.17.
420 When equal volumes of dry chlorine and carbon mon-
oxide gas are brought together in the dark, no action takes.
place, but when the mixture is exposed to light combination
ensues, especially in the sunlight. J. Davy, who discovered
this compound in the year 1811, termed it phosgene gas (from
pos, light, and yevváw, I give rise to). Carbonyl chloride is,
at the ordinary temperature, a gas possessing a peculiar, very
unpleasant, pungent smell, and having a specific gravity of
3-4604 (Thomson). At a low temperature it can be condensed,
forming a colourless liquid, boiling at +8°1 and having a
specific gravity at 0° of 1432. The compound is decomposed
by water into hydrochloric acid and carbon dioxide; thus:-
COC1₂+H₂O=CO₂+2HCl.
For the purpose of preparing pure carbonyl chloride the
method proposed by Wilm and Wischin 2 is the best. The
mixed gases issuing at about the same rate are brought into a
large glass balloon having a capacity of about ten litres; from
this balloon the mixed gases pass into a second one, which, like
the first, is exposed to sunlight. It is best to employ a slight
excess of chlorine, this being afterwards got rid of by passing
the gas through a tube filled with lumps of metallic antimony.
The gas thus purified can be liquefied by passing into a tube
surrounded by ice, or, better, by a freezing mixture.
Carbonyl chloride is also formed when a mixture of twenty
parts of trichloromethane (chloroform), four hundred of sulphuric
acid, and fifty of potassium dichromate are heated together on a
water-bath (Emmerling and Lengyel), thus :-
2CHCl3+K₂Cr₂O7+5H₂SO¸=2COCI₂+
2KHSO₁+Cr₂(SO4)3+Cl₂+5H₂O.
Carbonyl chloride is also obtained when carbon tetrachloride
is acted upon by fuming sulphuric acid.³
1 Emmerling and Lengyel, Annalen, suppl. Band 7, 101.
2 Annalen, 147, 147.
3 Ber. 26, 1993.
732
THE NON-METALLIC ELEMENTS
It is the chloride of carbonic acid, just as sulphuryl chloride,
SO,Cl, is the chloride of sulphuric acid, and when treated
with ammonia is converted into the amide of carbonic acid,
known as carbamide or urea, CO(NH2)2 (p. 739).
Carbonyl chloride is largely used in the preparation of colour-
ing matters, for which purpose it is sometimes made by passing
equal volumes of carbon monoxide and chlorine over heated
animal charcoal; under these circumstances combination takes
place in the dark. Carbonyl chloride is also formed by the
spontaneous oxidation of chloroform when it is exposed to the
light in the presence of air. Chloroform which is to be used as
an anæsthetic should therefore be kept in the dark, and the
bottles containing it should be kept filled, as the presence of
carbonyl chloride in it renders it extremely dangerous if used
for inhalation,
CARBON OXYBROMIDE OR CARBONYL BROMIDE,
COBr₂ = 186.51.
421 When a mixture of carbon monoxide and bromine vapour
is exposed to the light, combination takes place slowly, but the
gas thus prepared retains a yellow colour. Potash decomposes
it with the formation of potassium carbonate and potassium
bromide.¹
Carbonyl bromide has also been obtained in the impure state
by the oxidation of bromoform.2
CARBON AND SULPHUR.
CARBON BISULPHIDE, CS₂ = 75'55.
422 This compound was accidentally discovered by Lampadius
in 1796, by heating pyrites with charcoal. In their investiga-
tion of carbonic oxide in the year 1802, Clément and Désormes
wished to ascertain whether charcoal invariably contained
combined hydrogen; they examined the action of sulphur on red-
hot charcoal, and obtained the same liquid which had been pre-
viously discovered by Lampadius. This liquid they first believed
to be a compound of hydrogen and sulphur, but they soon con-
vinced themselves that it only contained carbon and sulphur.
Notwithstanding these experiments, the nature of the compound
remained doubtful, until Vauquelin ascertained that its vapour,
2 Emmerling, Ber. 13, 874.
1
¹ Schiel, Annalen, suppl. Band 2, 311.
CARBON BISULPHIDE
733
Teor
400000
www
passed over red-hot metallic copper, is converted into carbon
and copper sulphide.
Carbon bisulphide is prepared on the large scale by passing
the vapour of sulphur over red-hot charcoal. For this purpose
a large upright cast-iron cylinder (Fig. 200), ten or twelve feet
long and one to two feet in diameter, is employed. This
cylinder is placed above a furnace and surrounded by brickwork,
and at the same time it is provided with a lid to admit of the
whole being filled with charcoal. A second opening (a), fur-
nished with a hopper, exists at the bottom of the cylinder, and
this serves to bring the sulphur into the apparatus. The
sulphur evaporates, and in the state of vapour combines with
the red-hot carbon, impure carbon bisulphide distilling over by

C
www
SAMA
4119
A
www
FIG. 200.
the tube (c), and collecting in the vessel (d) under water. The
tubes (e) serve as condensers, to separate the vapour of carbon
bisulphide from sulphuretted hydrogen formed during the re-
action, owing to the presence of hydrogen in the charcoal, the
sulphuretted hydrogen being absorbed by passing over the
layers of slaked lime contained in the purifier (f). In actual
work the yield of bisulphide is about 20 per cent. below the
theoretical amount. The crude substance invariably contains
sulphur in solution, from which, however, it may be separated
by distillation; but other sulphur compounds, which impart to
the crude material a most offensive odour, are also contained in
the distillate. In order to remove these impurities, different
processes are in use. The substance was formerly purified
by frequent re-distillation, over oil or fat, by which means
734
THE NON-METALLIC ELEMENTS
the disagreeably smelling compounds were held back. Another
means of purification employed is that of shaking the liquid
with mercury, and allowing it to remain for a long time in
contact with corrosive sublimate in the cold, and then distilling
it off white wax.
Pure carbon bisulphide is a colourless, mobile, strongly re-
fracting liquid, possessing a sweetish smell not unlike that of
ether or chloroform, having a specific gravity at 0° of 1.29232
(Thorpe) and boiling at 46°. It solidifies at -116°, remelting at
-110° (Wroblewski and Olszewski). When carbon combines
with sulphur to form carbon bisulphide, heat is absorbed which
is given out again when it is decomposed. Like many sub-
stances of this class, it can be caused to undergo explosive
decomposition, the heat thus evolved being sufficient to propa-
gate the explosion throughout the mass of the gas. When a
small quantity of fulminating mercury is exploded in a glass
tube containing vapour of carbon bisulphide, the vapour is
decomposed into carbon and sulphur, which are deposited on
the sides of the tube.¹ The same decomposition can be
brought about by the explosion of the yellowish-brown powder
which is formed by the action of sodium on carbon bisulphide,
and has the formula CS2.
Pure carbon bisulphide is exceedingly inflammable, taking
fire in the air at 149°, according to Frankland, and burning with
a bright blue flame. A mixture of one volume of carbon
bisulphide vapour and three volumes of oxygen detonates with
great violence on inflammation, so that great care is needed in
the manufacture, and frequent and severe explosions are caused
by its inflammability. A mixture of the vapour of carbon bisul-
phide and nitric oxide burns with a very bright blue flame, par-
ticularly rich in chemically active rays. Carbon bisulphide is
powerfully poisonous, and its vapour soon produces fatal effects
on small animals exposed to its action. It not only acts as a
poison when inhaled in large quantity, but it produces very
serious effects upon the nervous system when inhaled for a
considerable time even in very small amount.
The vapour of carbon bisulphide acts as a powerful anti-
putrescent, and Zöller 2 has shown that meat and other putrescible
bodies may be preserved fresh for a long time if kept in an
atmosphere containing the vapour of this compound.
Carbon bisulphide is largely used in the arts, especially in the
1 Thorpe, Journ. Chem. Soc. 1889, i. 220.
2 Ber. 10, 707.
THIOCARBONIC ACID
735
indiarubber and woollen manufactures, in the first case as a
solvent for the caoutchouc, and in the second as a means of
regaining the oil and fats with which the wool has to be treated.
As it dissolves iodine in large quantity, but does not dissolve
appreciably in water, it is employed for the purpose of determin-
ing the amount of moisture contained in commercial iodine. A
remarkably characteristic reaction of carbon bisulphide is that
it possesses the power of combining with triethylphosphine,
P(CH), a derivative of common alcohol, to form a solid
compound, crystallizing in magnificent red crystals, and having
the composition P(C₂H5),CS2.
5/3
Other compounds of carbon and sulphur have been described,
but these consist for the most part of amorphous brown sub-
stances, and are probably mixtures. A compound tricarbon
disulphide, CS2, has recently been described by Lengyel, ¹ who
prepared it by passing the vapour of carbon bisulphide over an
electric arc passing between carbon poles. It is a deep-red
liquid, which is slowly volatile at the ordinary temperature, the
vapour causing a copious flow of tears. It distils partly
unchanged under diminished pressure, but is partly converted
at the same time into a black solid amorphous modification
of the same composition. It combines with bromine to form a
yellow compound of the composition CS,Br, which has a not
unpleasant aromatic smell.
corresponding to
THIOCARBONIC ACID, H₂CS.
423 The salts of this acid, which is sometimes termed sul-
phocarbonic acid, are formed by processes analogous to those
by which the carbonates are produced. This is shown by the
reaction, discovered by Berzelius, in which carbon bisulphide
is brought in contact with a solution of the sulphide of an
alkali-metal; thus:-
CS₂+ Na2S Na₂CS3,
=
CO₂+ Na₂O
2
Na,CO3.
When alcohol is added to the solution thus obtained, the thio-
carbonate separates out in the form of a heavy liquid of a slightly
brown colour, which on the addition of cold dilute hydrochloric
acid decomposes into free thiocarbonic acid. This forms a
1 Ber. 26. 2960.
=
736
THE NON-METALLIC ELEMENTS
yellow oil, possessing a most disagreeable penetrating odour;
when slightly heated it is resolved into carbon bisulphide and
sulphuretted hydrogen.
The thiocarbonates of the alkali-metals and those of the
alkaline earths are soluble in water.
The soluble thiocarbonates give a brown precipitate with copper
salts, a yellow precipitate with dilute silver nitrate solution,
and a red precipitate on the addition of a lead salt. These
precipitates rapidly become black, owing to the formation of
the corresponding sulphides.
THIOCARBONYL CHLORIDE, CSCI₂ = 114·11.
424 This compound was obtained by Kolbe by acting for
some weeks with dry chlorine gas upon carbon bisulphide.
According to Carius, it can be easily obtained by heating
phosphorus pentachloride and carbon bisulphide together in
sealed tubes at 100°, when the following decomposition takes
place :-
PCI,+CS, PSC1, + CSCI
It is most readily obtained by acting on the compound CSCI,
described below, with tin and hydrochloric acid, and forms a
strongly smelling liquid which boils at 735°, has a specific
gravity of 1-5085 at 15˚, and is only slowly attacked by water. ³
3
The compound CSCI, is obtained by the action of chlorine on
carbon bisulphide in presence of a little iodine, and is a most
unpleasant-smelling liquid, boiling with slight decomposition at
149°.
=
CARBONYL SULPHIDE OR CARBON OXYSULPHIDE, COS=59.55.
4
425 This gas, discovered by Than, is formed when a mixture
of sulphur vapour and carbon monoxide is passed through a
moderately heated tube. It is also obtained by numerous other
reactions, such for example as the action of sulphur trioxide 5
or chlorosulphonic acid on carbon bisulphide. It is usually
prepared by the action of dilute acids on potassium thiocyanate,
1 Annalen, 45, 41. 2 Annalen, 112, 193.
4 Annalen Suppl. 5, 236.
6 Dewar and Cranston, Zeitsch. Chem. 1869, 734.
3 Klason, Ber. 20, 2378.
5 Armstrong, Ber. 2, 712.
CARBONYL SULPHIDE
737
the thiocyanic acid first formed splitting up into carbonyl
sulphide and ammonia-
HCNS + H,O = COS + NHg,
the latter combining with the excess of acid present. To
obtain the pure gas the following method is employed :-
-
To a cold mixture of five volumes of sulphuric acid and
four volumes of water such a quantity of potassium thiocyanate
(sulphocyanate of potassium) is added that the mass just re-
mains liquid. The evolution of gas, which commences without
heating, is regulated either by cooling or gently warming the
mixture, and by care a constant current of gas can thus
be obtained. The decomposition which takes place is as
follows:-
NCSK + H₂O + 2H₂SO₁ = COS + KHSO̟¸ + (NH¸)HSO.
The gas invariably contains the vapours of hydrocyanic acid,
HCN, and bisulphide of carbon. The first is removed by
passing the gas through a tube filled with cotton-wool which
has been rubbed in oxide of mercury, and the second by
passing it over pieces of cut caoutchouc contained in a tube
placed in a freezing mixture. According to Hofmann, the
vapour of carbon bisulphide cannot be thus completely re-
moved, but it may be got rid of by passing the gas over
cotton-wool saturated with an ethereal solution of triethyl-
phosphine, (CH),P.
Carbonyl sulphide is a colourless gas, having a specific
gravity of 2.1046, and a peculiarly resinous smell, at the same time
resembling that of sulphuretted hydrogen. It becomes liquid
at 0° under a pressure of 125 atmospheres, and the liquid when
poured out solidifies. It is very inflammable, taking fire even
when brought in contact with a red-hot splinter of wood, and
burning with a blue slightly luminous flame. A mixture of one
volume of the gas with one and a half volumes of oxygen in-
flames with slight explosion, burning with a bright blue flame;
if, however, it be mixed with seven volumes of air, the mixture
burns without explosion. A platinum wire heated to whiteness
by the electric current decomposes the gas completely, without
alteration of volume, into sulphur and carbon monoxide.
Carbonyl sulphide is soluble in water; one volume of the gas
dissolves in an equal volume of water, and imparts to the
solution its own peculiar smell and taste. The sulphur-
48
738
THE NON-METALLIC ELEMENTS
waters of Harkány and Parád in Hungary possess similar
properties, and probably contain, as other sulphur-waters may
do, carbonyl sulphide. The aqueous solution gradually decom-
poses with formation of sulphuretted hydrogen and carbon
dioxide thus:-
COS + H2O = SH₂ + CO₂
The gas is slowly absorbed by aqueous solutions of caustic
alkalis with formation of a carbonate and a sulphide :-
COS + 4KOH = K₂CO3 + K₂S + 2H₂O.
Alcoholic potash also absorbs it rapidly.
CARBAMIC ACID. COOH
NH₂
426 This acid, the monamide of carbonic acid, is not known
in the free state, although we are acquainted with its salts and
its ethereal salts. Ammonium carbamate is formed when dry
carbon dioxide is brought into contact with dry ammonia
(J. Davy, H. Rose) :-
CO2 + 2NH3
{ONH
It is best prepared by passing a rapid stream of the two
gases into well-cooled absolute alcohol. A deliquescent crystal-
line powder is formed, which smells of ammonia and is readily
soluble in water.1 Ammonium carbamate is also contained in
the commercial carbonate of ammonia, and can be obtained
from this source by allowing the commercial substance to stand
in contact with saturated aqueous ammonia for forty hours at
a temperature of from 20° to 25°; on cooling, the carbamate
separates out. The specific gravity of the vapour of ammonium
carbamate between 37° and 100° is 0.892 (Naumann); hence it
appears that this body is completely dissociated on evaporation
into carbon dioxide and ammonia. If the gaseous mixture thus
obtained be allowed to cool, ammonium carbamate is again
formed, but only slowly, owing to the fact that the combination
is not a direct one, an intramolecular change taking place.
When brought in contact with acids, ammonium carbamate
evolves carbon dioxide, whilst with alkalis it evolves ammonia.
=
со
1 Basarow, J. Pr. Chem. [2], 1, 283; Mente, Annalen, 248, 234.
2 Naumann, Annalen, 150, 1.
CARBAMIDE OR UREA
739
The sodium and potassium salts have also been prepared,
and also a calcium salt which has probably the constitution
NH,
{O.Ca(OH).
Carbaminic chloride or chloroformamide, NH.COCI, is pre-
pared by passing carbonyl chloride over ammonium chloride.
It usually exists as a very pungent-smelling liquid, but it may
be obtained in long prisms melting at 50°; the liquid boils at
61°-62°.
CARBAMIDE OR UREA, CO(NH2)2
427 Urea, which is largely contained in urine and in other
liquids of the animal body, was first described in the year 1773
by H. M. Rouelle as Extractum saponaceum urinæ. It was, how-
ever, more accurately investigated in the year 1790 by Fourcroy
and Vauquelin. The discovery by Wöhler, in the year 1828,¹
that when an aqueous solution of ammonium cyanate is heated,
this compound undergoes a molecular change and is converted
into urea, is one of the most important discoveries of modern
chemistry, inasmuch as it was the first case in which a compound
formed in the animal body was prepared from its inorganic
constituents:-
:-
NC(ONH)
CO(NH2)2
Urea is also obtained by the action of ammonia on carbonyl
chloride 2 :--
=
COC12+2NH₂ = CO(NH2)2 + 2HCl.
It was obtained first in this manner by John Davy, mixed
with ammonium chloride, in 1812, sixteen years before Wöhler's
synthesis, but he did not recognize its identity, although he
determined the relative volumes of the gases entering into
combination 3
(NH
со
ONH
4
It is likewise produced by heating ammonium carbamate or
common carbonate of ammonia to a temperature of from 130°
to 140° (Basarow):-
= со
( NH + H,O.
NH₂
2
1 Wöhler, Pogg. Ann [1828], 12, 253.
2 Natanson, Annalen, 98, 287; Neubauer, Annalen, 101, 342.
3 Phil. Trans. 1812; see also Matthews, Chem. News, 1890.
740
THE NON-METALLIC ELEMENTS
Urea is found in the urine of all mammalia, especially that of
the carnivora, and also in small quantities in that of birds and
reptiles, as well as in the excreta of the lower animals. It
forms an important constituent of the vitreous humour of the
eye,
and invariably occurs in small quantity in blood, and some-
times in perspiration and pus. In order to prepare urea from the
urine the liquid is evaporated down on a water-bath to dryness,
the residue treated with alcohol, the alcoholic solution evapo-
rated, and the residue again treated with absolute alcohol. On
evaporating this last solution, urea, slightly coloured with
extractive matter, remains behind.
A second method is to evaporate down urine to a syrupy
consistency, and then add pure nitric acid, when nitrate of urea
separates out as a crystalline powder, which is decomposed on
the addition of potassium carbonate, with formation of potassium
nitrate and free urea. These bodies can then be easily separated
by treatment with alcohol, in which the urea dissolves. A third
method is that recommended by Berzelius: a concentrated
solution of oxalic acid is added to the evaporated urine, when a
precipitate of the insoluble oxalate of urea is thrown down; this
is boiled with chalk, when the pure urea is left in solution.
The preparation of urea from ammonium cyanate is, however,
by far the best. For this purpose potassium cyanate is first
prepared by heating dried potassium ferrocyanide with potassium
bichromate in the manner described on p. 761. This may be
then dissolved in water, and an equivalent quantity of ammonium
sulphate added, or the requisite quantity of the latter may be
added to the mother liquors obtained in preparing the cyanate.
Potassium sulphate is formed, the greater part of which crys-
tallizes out, the small portion still remaining in solution being
got rid of by evaporation and subsequent crystallization. The
liquid is then evaporated to dryness, and the urea contained in
the dry residue is extracted with boiling alcohol, and may be
further purified by recrystallization from amyl alcohol.¹
Urea dissolves in its own weight of cold water and in every
proportion in boiling water. It also dissolves in five parts of
cold and one part of boiling alcohol, but is almost insoluble in
ether. It crystallizes in long striated quadratic prisms, and
possesses a cooling taste resembling that of nitre. On heating,
it melts at 132°, and decomposes when heated to a higher
point. Although urea does not possess an alkaline reaction, it
¹ Erdmann, Ber. 26, 2438.
SALTS OF UREA
741
――
combines easily with acids, and forms a series of salts which
crystallize well, and of which the following are the most
important:-
Hydrochloride of urca, CO(NH),HCl. This salt is formed
when dry hydrochloric acid acts upon urea. In consequence of
the heat which is evolved in the act of combination, the com-
pound appears in the form of a yellowish oil, but this crystal-
lizes on cooling. On addition of water it is resolved into
its constituents.
Nitrate of urea, CO(NH),NO₂H. This is a very charac-
teristic compound, being soluble in water but almost insoluble
in nitric acid. Hence if pure nitric acid be added to a toler-
ably concentrated solution of urea, a crystalline precipitate falls,
which on recrystallization from solution in hot water is deposited
in prismatic crystals.
Oxalate of urea, 2CO(NH2)2,C₂OH2. This salt separates out
in the form of monosymmetric tablets when a solution of urea
is mixed with a concentrated solution of oxalic acid.
Urea not only unites with acids but also with a variety of
salts and oxides. Thus, for instance, it forms with common
salt the compound CO(NH2)2 + NaCl + H₂O. This substance
crystallizes from aqueous solution in large glittering rhombic
prisms. Urea also forms similar compounds with certain other
chlorides and nitrates.
When treated with a solution of caustic potash in the cold,
it does not yield any ammonia, but on warming it is gradually
resolved into ammonia and carbon dioxide. When heated with
water above 100° it is also decomposed, and it is at once acted
upon by nitrous acid or anhydride, with liberation of the
nitrogen of both compounds in the free state and formation of
water if the solution be warm:-
CO(NH2)2 + N₂O3 = CO2 + 2N2 + 2H₂O.
·
In the cold ammonium carbonate is also formed¹:-
2CO(NH2)2 + N₂O₂ = (NH4)2CO3 + 2N2 + CO₂
3
Neutral potassium permanganate solution has no action on
urea in the cold, and very little at 100°, but the acidified solution
causes the rapid evolution of 1 volume of nitrogen and 2 volumes
of carbon dioxide at 100°. Solutions of urea evaporated with
silver nitrate yield ammonium nitrate and silver cyanate.
¹ Claus. Ber. 4, 1440.
742
THE NON-METALLIC ELEMENTS
428 In order to detect the presence of urea in a liquid it is
evaporated down on a water-bath, the residue extracted
with alcohol, the liquid evaporated, and a few drops of pure
nitric acid added to the residue. If urea is present the nitrate
of urea is precipitated, and this is recognised, inasmuch as it
consists of microscopic crystalline scales forming rhombic or
six-sided tables, having an angle of 82°, as measured by means
of the microgoniometer.
It has already been stated that urine when not sterilized de-
composes with separation of ammonium carbonate. This change
occurs in in presence of certain micro organisms, notably micro-
coccus urine and bacillus coli communes which can be separated
from the liquid by filtration. If the filter containing these
micro-organisms be now washed with water and dried at a
temperature not exceeding 40°, they retain their activity,
and if this paper be coloured with turmeric it may be used for
detecting the presence of urea in neutral liquids. In about ten
to fifteen minutes after the paper is moistened with a neutral
solution of urea a portion of the urea is converted into ammonium
carbonate, and the paper becomes of a brown tint. In this way
one part of urea can be detected in 10,000 of liquid.¹
Urea is the last product of the action of the atmospheric
oxygen on the nitrogenous components of the human body, and
the quantity which is excreted within a given time gives us a
measure of the changes which are going on; for although there
are other nitrogenous products contained in the urine, yet their
quantity is so small in comparison to that of the urea that in
most cases they may be disregarded. For the medical man
and the physiologist it is therefore of the greatest conse-
quence that a quick and accurate method should exist for
determining the amount of urea in the urine. One of the best
processes for this purpose is that proposed by Liebig. It de-
pends upon the fact that when mercuric nitrate is added to a
solution of urea, a white insoluble precipitate falls down,
possessing the composition 2CO(NH2)2+3HgO+Hg(NO3)2. It
is convenient that each cc. of the mercuric solution should pre-
cipitate 001 gram. of urea, and a solution of this strength is
obtained by dissolving 71:48 grams. of metallic mercury in
nitric acid, evaporating to drive off as much of the acid as
possible, and diluting the solution to one liter. Before this
solution is added to the urine it is necessary to precipitate the
phosphates and sulphates which the latter contains in solution. For
1 Musculus, Compt. Rend. 78, 132.
DETERMINATION OF UREA
743
this purpose two volumes of urine are mixed with one volume
of a mixture of equal volumes of baryta-solution and cold
saturated solution of barium nitrate. The standard mercury
solution, is then added by a burette to 15 cc. of the filtrate,
corresponding to 10 cc. of urine, until no further precipitate
occurs. The end of the reaction is easily ascertained by adding
a solution of sodium carbonate to a drop of the liquid, which
assumes a yellow colour as soon as a slight excess of mercury is
present. One cc. of the mercury solution corresponds to 0.01
gram. of urea.
According to Bunsen's method, the urine is heated with a
solution of barium chloride in dilute ammonia to a temperature
of 230°, and the barium carbonate which separates out is
weighed :-
CO(NH2)2 + BaCl2 + 2H₂O = BaCO3 + 2NH¸Cl.
A third method for the determination of urine, known as the
Davy-Knop method, depends on the fact that urea when
brought into contact with an alkaline hypochlorite or hypo-
bromite evolves pure nitrogen, the carbon dioxide which is
formed being absorbed by the free alkali :—
CO(NH2)2+3 NaOBr = CO2 + N₂ + 3NaBr + 2H₂O.
2
For this purpose a freshly prepared alkaline solution of sodium
hypobromite is prepared by dissolving bromine in excess of
caustic soda, and this is mixed with a solution of urea, the
nitrogen gas which is evolved by this reaction being collected
in a graduated tube. For the details of this process reference
must be made to the original papers.¹
NHOH
HYDROXYCARBAMIDE, CONH₂
429 For the preparation of this compound, a solution of hy-
droxylamine nitrate in absolute alcohol is cooled to -10°, and
to this a concentrated solution of potassium cyanate is added.
On standing, hydroxycarbamide separates out in white needles
which melt at 128-130°.
ISURETINE, CHON2..
This isomeride of urea is formed when an alcoholic solution
of hydroxylamine is warmed with a concentrated solution of
1 Hüfner, J. Pr. Chem. [2], 3; Russell and West, Journ. Chem. Soc. 1874, 249.
744
THE NON-METALLIC ELEMENTS
hydrocyanic acid to a temperature of from 40° to 50°. On
evaporating the solution isuretine separates out in the form
of long colourless rhombic crystals which have an alkaline re-
action, and melt at 105°. This substance forms with acids
a series of easily crystallizable salts, and its constitution is
represented by the following formula:-
NH₂.CH: N.OH.
CO
CO
430 This compound, discovered by Wiedemann,¹ is formed
when urea is heated for some time to 150-160°.
position which takes place is represented as follows:-
The decom-
NH₂
NH,
NH₂
NH₂
BIURET, CO₂HN3.
5
CO
=
CO
—
NH..
NH
+ NH3.
NH₂
When the residue is heated with water, cyanuric acid remains
behind, and biuret separates out on cooling the solution. This
may be purified by dissolving in hot water and reprecipitating
with dilute ammonia. Biuret forms long white needle-shaped
crystals, 100 parts requiring for their solution 6,493 parts of
water at 15°, whilst at 106°, the boiling-point of a saturated
solution, 222 parts only are needed.2
When a few drops of cupric sulphate and then an excess of
caustic soda are added to a solution of biuret in water, the
liquid assumes a colour varying from red to violet, according to
the quantity of copper salt added. By means of this reaction
it is easy to show the passage of urea into biuret; it is only
necessary to heat a small quantity of urea in a test-tube until
ammonia begins to be evolved, the melted mass being poured
into hot water and treated as described.
¹ Annalen, 68, 323.
2 Hofmann, Ber. 4, 262.
CARBONYLDIUREA
745
2 CO-
CARBONYLDIUREA, CH.NO.
431 When urea is heated to 100° with carbonyl chloride in a
sealed tube, this substance is formed; thus:-
{NH₂
NH, + COC,
=
CO
CO
(NH
{SNÍ
CO-
NH,
00 {HN {
NH₂
It is a white powder, which separates from boiling water as a
crystalline powder.
On heating biuret with carbonyl chloride a substance very
similar to the one now described is obtained, to which the name
of carbonyldi-biuret has been given,¹ and which has the follow-
ing constitution :—
CO(NH.CO.NH.CO.NH,),
OXYTHIOCARBAMIC ACID, CO NH₂
SH.
-CO + 2HCl.
432 Berthelot first obtained the ammonium salt of the above
acid. This salt separates out in crystals, when dry carbonyl
sulphide is brought in contact with ammonia. The acid itself
is not known.
When heated in a closed tube the ammonium salt is con-
verted into urea 2:-
1 E. Schmidt, J. Pr. Chem. [2], 5, 35.
2 Kretzschmar, J. Pr. Chem. [2], 7, 474.
3 Zeise, Annalen, 48, 95.
=
= H₂S + CO
SNH2.
{ NH2.
NH₂
SH
2
THIOCARBAMIC ACID, CS
433 The constitution of this compound, originally described
under the name of hydrothiosulphoprussic acid,³ was first pointed
out by Debus. The ammonium salt of this acid is formed by the
▲ Annalen, 73, 26.
746
THE NON-METALLIC ELEMENTS
union of carbon bisulphide with dry ammonia in presence of
absolute alcohol; the salt separates out, after some time, in
prismatic crystals. When hydrochloric acid is added to its
aqueous solution, free thiocarbamic acid separates out. This
substance is an oil at the ordinary temperature, but below 10° it
forms a crystalline mass. It has a smell resembling that of
sulphuretted hydrogen, possesses an acid reaction, and is easily
decomposed into sulphuretted hydrogen and thiocyanic acid,
NCSH.
THIO-UREA, OR THIOCARBAMIDE, CS(NH)2
434 Reynolds 2 was the first to prepare this analogue of urea.
It is formed by a reaction analogous to that by which common
urea is obtained. A molecular change takes place in ammonium
thiocyanate NCS(NH) when heated to 140°, corresponding to that
observed in the case of ammonium cyanate. At the same time
a quantity of guanidine thiocyanate is formed, while a portion
of the thiocyanate remains unchanged. In order to separate
these three bodies, the melted mass is treated with two-
thirds its weight of cold water, when the greater portion
of the thio-urea remains behind. This is then dried on
a porous plate, and purified by recrystallization from hot
water. According to Claus, it is not necessary to prepare am-
monium thiocyanate for this purpose, but a solution of the
crude salt obtained by dissolving carbon bisulphide in alcoholic
ammonia may be employed; this is quickly evaporated, until
ammonia, ammonium sulphide, and carbon bisulphide are abun-
dantly evolved, and then the residue treated as above described.
As long as thio-urea is not perfectly pure it crystallizes in
silky needles, but in the pure condition it forms magnificent
large rhombic prisms or thick tables. It dissolves in eleven parts
of cold water, and on heating with water to 140° is reconverted
into ammonium thiocyanate. Heated by itself to 170—180° it
is converted into guanidine thiocyanate and ammonium thio-
carbonate. Like common urea, thio-urea forms compounds with
acids, salts, and oxides. A characteristic compound is the nitrate
CS(NH)2, NO,H, which crystallizes exceedingly well.
Seleno-urea, CSe(NH2)2, has also been prepared.*
1 Mulder, J. Pr. Chem. 101, 401, and 103, 178. 2 Journ. Chem. Soc. 1855, 1.
8 J. Volhard, Ber. 7, 92, and J. Pr. Chem. [2], 9, 6.
4 Verneuil, Compt. Rend. 99, 1154.
CYANOGEN COMPOUNDS
747
CARBON AND NITROGEN.
CYANOGEN COMPOUNDS.
435 The history of these compounds commences with the dis-
covery of Prussian blue, made accidentally early in the eighteenth
century by a colour-maker of the name of Diesbach. The fact
was shortly afterwards communicated to the alchemist Dippel,
who ascertained the conditions under which the formation of the
colouring matter takes place. The method for preparing the
colour was, however, first published by Woodward, who states
that it was obtained by calcining the alkali obtained by heating.
equal parts of cream of tartar and saltpetre with ox-blood. The
residue of the calcination was then lixiviated, and green vitriol
and alum added to the solution, whereby a greenish precipitate
was thrown down, which, on treatment with hydrochloric acid,
yielded the blue colour.¹ About the same time John Brown
found that animal flesh could be employed instead of blood, and
Geoffroy, in 1725, showed that other animal matters could be
used for the same purpose.
Up to this time very remarkable opinions were held concerning
the composition of this colouring matter. Geoffroy, for example,
assumed that the colour was due to metallic iron which, owing
to the presence of the alum, was in an extremely fine state of
division. In 1752 Macquer found that Prussian blue could.
be manufactured without the use of alum, and that when the
colouring matter is boiled with an alkali, oxide of iron remaius
behind, and a peculiar body is formed which is found in
solution. To this substance (ferrocyanide of potassium) the
name of phlogisticated potash was given, and it was believed
that the iron contained in the Prussian blue was coloured
blue by a peculiar combustible substance. During the years
1782-5 Scheele occupied himself with the investigation of
this compound, and he showed that when phlogisticated
potash is distilled with sulphuric acid, a very volatile and
inflammable body is formed, which is soluble in water, and
possesses the property, when treated with alkalis and green
vitriol, of forming a very beautiful blue colour. To this body
the name of prussic acid was given. In 1787, the investigation
of this compound was taken up by Berthollet, who proved that
iron was contained in the prussiate of potash as an essential con-
stituent, together with alkali and prussic acid; and that prussic
1 Phil. Trans. 1724.
748
THE NON-METALLIC ELEMENTS
acid, the salts of which yield ammonia and carbonic acid as pro-
ducts of decomposition, consists of carbon, nitrogen, and hydrogen.
These results were afterwards confirmed by the investigations of
other chemists, but it is especially to the labours of Gay-Lussac
that we owe a clear statement of the true composition of
this acid and its salts. His researches are of great theoretical
interest, as it is in them that we find the fact, for the first time,
clearly pointed out, that a compound substance may exist which
is capable of acting in many respects as if it were an element.
Gay-Lussac¹ showed in 1811 that prussic acid is a compound
consisting of hydrogen combined with a radical containing
carbon and nitrogen, to which he gave the name of cyanogène
(from kúavos dark blue and yevváw I produce). He proved,
moreover, that the salts obtained by the action of prussic acid
upon a base are compounds of this cyanogène with metals. In
corroboration of this view, in 1815 he was able to show that this
radical cyanogen can be prepared and is capable of existing in
the free state.
By the name of compound radical we signify a group of atoms
which plays the part of a single atom, or, employing Liebig's
classical definition, we may say that cyanogen is a radical,
because
(1) "It is a never-varying constituent in a series of compounds:
(2) “It can be replaced in these compounds by other simple
bodies:
(3) "In its compounds with a simple body, this latter may be
easily separated or replaced by equivalent quantities of other
simple bodies."
"Of these three chief and characteristic conditions of a com-
pound radical, at least two must be fulfilled if the substance is
to be regarded as a true compound radical." 2
In addition to cyanogen, a number of other compound radicals
are known, some of which have already been mentioned. These
radicals are distinguished one from another, as the elements
are, by their quantivalence. Thus, nitrogen peroxide NO is
a monad radical occurring in nitric acid and in many of its
derivatives, and to it the name of nitroxyl has been given. Its
constitution is represented as follows:-
O
O
1 Annales de Chimie, 77, 128; 95, 136.
Annalen, 25, 3.
CYANOGEN GAS
749
indicating that the two atoms of dyad oxygen are connected
with the nitrogen each by two of the combining units of the
pentad atom of the latter. Hence, one combining unit of the
nitrogen remains free, and the group acts consequently as a
monad radical.
Sulphur dioxide or sulphuryl, SO2, serves as an instance of
a dyad radical. This radical is contained in sulphuric acid, and
a large number of compounds derived from it. It is termed a
dyad because, of the six combining units of the sulphur atom,
four only are saturated by combination within the molecule, and
its constitution may, therefore, be represented by the following
formula :-
0
S
Many phosphorus compounds contain a triad radical, phos-
phoryl PO, which, however, is not known in the free state. It
contains the pentad phosphorus atom combined with the dyad
oxygen atom, and its constitution can be represented as
follows:-
O=P
Cyanogen, CN, is a monad radical, as it contains the triad
atom of nitrogen and the tetrad atom of carbon; its constitution
is represented thus:-
Hydrocyanic acid, CyH.
Potassium cyanide, CyK.
Potassium cyanate, CyOK.
Free cyanogen, Cy2.
NEC-
The radical cyanogen, to which the symbol Cy. instead of
the formula CN is sometimes given, combines like chlorine
with hydrogen and the metals, and its compounds may, there-
fore, be compared with those of chlorine, to which they
have the strongest resemblance in their physical and chemical
properties:-
Hydrochloric acid, CIH.
Potassium chloride, CIK.
Potassium hypochlorite, CIOK.
Free chlorine, Cla.
CYANOGEN GAS, OR DICYANOGEN, CN, or Cy₂-
436 This gas, which was discovered by Gay-Lussac, is formed
when the cyanides of mercury, silver, or gold are heated. For
750
THE NON-METALLIC ELEMENTS
this purpose it is usual to employ mercuric cyanide, which
decomposes thus:-
Hg(CN), Hg + C₂N2
=
The mercury salt placed in a tube of hard glass fitted with
a cork and gas-delivery tube, or in a small hard glass retort, is
heated to dull redness, and the gas collected over mercury.
The heat of combination of mercuric cyanide is large, and a
very high temperature therefore is required to bring about
this decomposition. If mercuric chloride be mixed with the
cyanide, the cyanogen comes off at a lower temperature, as the
mercury then combines with the mercuric chloride, forming
mercurous chloride, and the whole reaction, represented by the
equation
Hg (CN)2 + HgCl₂ = (CN), + 2HgCl,
takes place with slight evolution of heat.
Cyanogen gas can also be obtained by other reactions
For instance, it is formed when oxalate of ammonium is heate
with phosphorus pentachloride; thus:—
C,O,(NH,), = C,Ng + 4H,O.
It is also formed by gently igniting an intimate mixture
of two parts of well dried ferrocyanide of potassium, Fe(CN), K,
with three parts of mercuric chloride. According to Ber-
zelius, however, the gas thus prepared contains free nitrogen,
and he recommends the employment of potassium cyanide in
place of the ferrocyanide. There is no doubt that in this
reaction mercuric cyanide is first obtained, and this is then
decomposed as already described.
Cyanogen is most readily obtained by gradually adding a
concentrated solution of potassium cyanide to a solution of
two parts of crystallized copper sulphate in four parts of water,
and then warming; cyanogen is evolved and cuprous cyanide
simultaneously separates out from the solution. If the cuprous
cyanide be washed by decantation, and then treated with ferric
chloride, the remainder of the cyanogen is evolved.³
Cyanogen gas is found in small quantities in the gases pro-
ceeding from the blast furnaces, and it is likewise formed when
a mixture of ammonia and coal-gas is burnt in a Bunsen
1 Kemp, Phil. Mag. 23, 179.
³ Jacquemin, Ann. Chim. Phys. [6], 6, 140.
2 Berzelius, Jahresb. 24, 84.
4 J. Pr. Chem. [1], 42, 145.
CYANOGEN GAS
751
burner. The following equation probably explains the reaction
which takes place in the latter case:—
4CO + 4NH3 + 0,= 2C₂N₂ + 6H₂O.
O2
Cyanogen is a colourless gas possessing a peculiar pungent
odour resembling that of peach kernels. It is poisonous, and
burns, when brought in contact with flame, with a charac-
teristic purple-mantled flame, with formation of carbon dioxide
and nitrogen. When cyanogen gas is mixed with an excess
of oxygen and an electric spark passed through the mixture,
an explosion occurs, and, on cooling, the gas possesses exactly
the same volume as before the explosion; thus:-
C₂N₂+ 20₂ = N₂ + 2CO₂.
2
Cyanogen is decomposed when subjected to the action of
electric sparks, carbon being deposited, and small quantities of
paracyanogen formed.
The specific gravity of the gas is 1.806 (Gay-Lussac); when
exposed to pressure or to cold, cyanogen condenses to a colour-
less liquid which boils at -207°, and possesses at 17° a specific
gravity of 0.866. When cooled to a still lower temperature the
liquid freezes, forming a crystalline mass which melts at -34-4°.2
According to Bunsen 3 the tension of cyanogen gas at different
temperatures is as follows:-
Temperature.
- 20.7°
-10
0
+10
15
20
Tension in atmospheres.
1
1.85
2.7
3.8
4.4
5.0
At the ordinary atmospheric temperature, one volume of
cyanogen dissolves in four volumes of water, and in twenty-
three volumes of alcohol. The aqueous solution soon becomes
turbid, with separation of a brown powder known as azulmic
acid, and the solution contains ammonium oxalate, together
with smaller quantities of ammonium carbonate, hydrocyanic
acid, and urea (Wöhler). The formation of these products of
decomposition will be explained later (Vol. III., Part ii., 2nd
1 Levoir, Jahresb. 76, 445. 2 Davy and Faraday, Phil. Trans. 1823, 196.
Pogg. Ann. 46, 101.
3
752
THE NON-METALLIC ELEMENTS
edition, pp. 139, 343). Cyanogen gas when passed over heated
potassium unites directly with the metal, forming potassium
cyanide.
Paracyanogen, (CN).-In the preparation of cyanogen from
mercuric cyanide a brown powder remains behind. This sub-
stance possesses the same percentage composition as cyanogen,
but its molecular weight is unknown; when it is heated in a
current of carbon dioxide or nitrogen it gradually volatilizes,
being converted into cyanogen gas.
HYDROCYANIC ACID, OR PRUSSIC ACID,¹ HCN.
437 We owe the discovery of hydrocyanic acid to Scheele in
1782. It was subsequently examined by Ittner,² and by Gay-
Lussac; the former of these chemists preparing it for the first
time in the anhydrous condition. For this purpose he heated
mercuric cyanide with hydrochloric acid in a flask, leading the
gas, which was evolved, through a long tube, half filled with
pieces of marble in order to retain any vapours of hydrochloric
acid which might be carried over, the second half of the tube
being filled with chloride of calcium for the purpose of absorbing
any aqueous vapour. The gas, thus dried, was passed into a
receiver placed in a freezing mixture, where it was condensed.
According to this method only two-thirds of the theoretical
amount of acid are obtained, as the mercuric chloride unites
with a portion of the prussic acid to form a compound which
is decomposed only at a high temperature. If, however, sal-
ammoniac be added, this salt combines with the mercuric
chloride, and the whole of the hydrocyanic acid is set free.*
Pure hydrocyanic acid is also obtained by leading sulphu-
retted hydrogen gas over dry mercuric cyanide heated to about.
30°. The salt is placed in a long horizontal tube of which the
front portion is filled with carbonate of lead for the purpose of
retaining the excess of sulphuretted hydrogen; as soon as this
begins to turn black the evolution of sulphuretted hydrogen is
stopped (Vauquelin).5
Hydrocyanic acid is generally prepared from ferrocyanide
of potassium, which, as Scheele showed, is decomposed by dilute
sulphuric acid. A process described by Wöhler enables
1 The name acide prussique was first used by Guyton de Morveau.
2 Beiträge z. Geschichte d. Blausaüre, 1809.
3 Ann. Chim. 78 128, and 95, 136.
4 Bussy and Buignet, Ann. Chim. Phys. [4], 3, 232. 5 Annalen, 73, 218.
HYDROCYANIC ACID
753
us to prepare any desired quantity, either of the aqueous
acid of different strengths, or of the anhydrous compound.
A cold mixture of 14 parts of water and 7 parts of concen-
trated sulphuric acid is poured upon 10 parts of coarsely
powdered ferrocyanide of potassium contained in a large retort
the neck of which is placed upwards in a slanting direction.
If the anhydrous acid be required, the vapour is allowed to pass
through cylinders or U-tubes filled with calcium chloride and
surrounded by water heated to 30°, after which the gas, thus
dried, is condensed by passing into a receiver surrounded by ice
or a freezing mixture. To prepare the aqueous acid the drying
apparatus may be omitted and the gas evolved from a flask as
shown in Fig. 201, passed through a Liebig's condenser, and then
FIG. 201.
The action
led into the requisite quantity of distilled water.
of the dilute sulphuric acid upon the prussiate of potash
is represented by the following equation:-
:-
2Fe(CN),K+3H₂SO₁ = 3K₂SO4 + Fe(CN),K,Fe + 6HCN.
As is seen from this equation, only half of the cyanogen con-
tained in the ferrocyanide is obtained as hydrocyanic acid, but
notwithstanding this loss the method is simpler and cheaper
than those previously described.
Amongst the various other methods which have been described
for the preparation of this acid, one proposed by Clarke may be
mentioned, inasmuch as by this method a dilute acid may readily
be prepared having an approximately known strength. For this
49
754
THE NON-METALLIC ELEMENTS
purpose nine parts of tartaric acid are dissolved in sixty parts of
water and the solution poured into a flask so that it is nearly
filled. Four parts of potassium cyanide are then added, and the
flask closed by means of a cork. The mixture is then well
shaken, and allowed to stand for some time until the whole of
the cream of tartar (acid tartrate of potassium) has separated
out, after which the dilute hydrocyanic acid is poured off. This
should contain 36 per cent. of hydrocyanic acid, and only a
small amount of cream of tartar in solution; the reaction being
represented by the following equation:-
KCN + C,H,O.=HCN + CH KO
6
6°
Hydrocyanic acid is also formed when a series of electric
sparks is passed through a mixture of acetylene and nitrogen;¹
thus:—
C₂H₂+N₂ = 2HCN.
2
It is also obtained by the passage of the silent discharge
through a mixture of cyanogen and hydrogen.2
438 Properties.-Pure anhydrous hydrocyanic acid is a colour-
less very mobile liquid possessing a characteristic smell resembling
that of bitter almonds, and producing when inhaled in small
quantities a peculiar irritation of the throat. Its specific gravity
is 0.7058 at 7° and 0·6969 at 18° (Gay-Lussac); it boils at 26.1°,
and forms a colourless vapour which possesses the specific
gravity 0-947. At 15° the liquid solidifies to a mass of colour-
less feathery crystals. If a drop of the liquid acid be brought
on a glass rod into the air, evaporation takes place so quickly,
and so much heat is thereby absorbed, that a portion of the
liquid freezes. Hydrocyanic acid is miscible in all proportions
with water, alcohol, and ether. A very singular phenomenon is
observed when hydrocyanic acid is mixed with water, inasmuch
as a diminution of temperature occurs without any increase of
the volume of the liquid; on the contrary, a diminution of
volume takes place, and this is the greatest when equal volumes
of water and acid are mixed. In this mixture the proportions
correspond to the relations, 3H,O + 2HCN, and, in this case,
the diminution in bulk which occurs is from 100 to 94 (Bussy
and Buignet). The anhydrous acid as well as its concentrated
aqueous solution is easily inflammable, and burns with a beauti-
ful violet flame. When not absolutely pure, it decomposes very
1 Berthelot, Compt. Rend. 77, 1141. 2 Boillot, Compt. Rend. 76, 1132.
HYDROCYANIC ACID
755
readily on exposure to light, with separation of a brown body,
and formation of ammonia, whilst, in the case of the pure dilute
acid, formic acid, ammonia, and other bodies are produced. The
tendency to decomposition is much diminished by the presence
of traces of mineral acid or of formic acid. Concentrated mineral
acids, as well as boiling alkalis, decompose hydrocyanic acid into
ammonia and formic acid, water being taken up; thus:-
HCN + 2H₂O = NH3 + HO.COH.
On the other hand, hydrocyanic acid and water are formed when
the ammonium salt of formic acid is quickly heated.
Pure hydrocyanic is one of the most powerful and rapid of
known poisons. When a small quantity of the vapour of the
pure substance is drawn into the lungs instant death ensues;
small quantities produce headache, giddiness, nausea, dyspnœa,
and palpitation.
A few drops brought into the eye of a dog kill it in thirty
seconds, whilst an internal dose of 0·05 grain is usually sufficient
to produce fatal effects upon the human subject, but cases have
been known in which 01 grain has been taken without death
ensuing. As antidotes, ammonia and chlorine water have been
proposed, and these appear to be efficacious, although we are
unable to explain their mode of action, for ammonia under
ordinary conditions only forms ammonium cyanide, and chlorine
cyanogen chloride, both of which are bodies as poisonous as
prussic acid itself. Prussic acid is used as a medicine, and is a
constituent of several officinal preparations, such as laurel water,
bitter-almond water, &c., which are obtained by distilling the
leaves of the common laurel, or bitter almonds with water.
These plants do not contain the prussic acid ready formed; but,
in common with most of the plants of the same family, contain
amygdalin, a complicated compound which, under certain cir-
cumstances, splits up into sugar, oil of bitter almonds, and
prussic acid.
439 To estimate the quantity of hydrocyanic acid contained
in these preparations, an excess of potash solution is added to a
measured or weighed quantity of the liquid, and then by means
of a burette a solution of nitrate of silver containing 6-3 gram.
in one liter is dropped in until a permanent precipitate appears.
Each cc. corresponds to two milligrams of anhydrous prussic acid.
In this reaction the double cyanide, AgCN + KCN, is formed,
which is not decomposed by alkalis, and is soluble in water.
756
THE NON-METALLIC ELEMENTS
Hence as soon as exactly half the quantity of prussic acid
present is converted into silver cyanide, one drop more of
the silver solution will produce a permanent precipitate of silver
cyanide.¹
To detect hydrocyanic acid, as in cases of poisoning, the
suspected matter is acidulated with tartaric acid, and the prussic
acid distilled off by means of a water-bath. The distillate is
made alkaline with caustic soda, and a mixture of a ferrous and
ferric salt (a solution of ferrous sulphate oxidized by exposure
to the air) is added, and then an excess of hydrochloric acid.
Prussian blue remains undissolved if prussic acid be present.
If the quantity contained be very small, the solution appears
first of a green colour, and, on standing, deposits dark blue
flakes. When dilute prussic acid is mixed with yellow ammo-
nium sulphide, and the liquid evaporated to dryness over a
water-bath, ammonium thiocyanate is formed. The presence of
this body is made known by means of ferric chloride, which pro-
duces in a solution of thiocyanate a deep blood-red coloration.
THE CYANIDES.
440 Hydrocyanic acid turns blue litmus paper red, but it is
so weak an acid that its soluble salts are decomposed by the
carbonic acid of the air, and they therefore smell of hydrocyanic
acid, and have an alkaline reaction even when their solution
contains excess of hydrocyanic acid.
Cyanides can be prepared in a variety of ways. Carbon and
nitrogen do not unite together even under the action of the
electric spark, but if these elements are heated in presence of
an alkali, a cyanide is formed. Thus, for instance, potassium
cyanide is obtained when nitrogen is led over a heated mixture
of carbon and potash :-
N₂+4C+K,CO₂ = 2KCN +3CƆ.
If, instead of potash, caustic baryta is taken, barium cyanide is
obtained :-
2
N,+3C + BaO = Ba (CN), + CO.
2
According to Kuhlman, ammonium cyanide is formed when
ammonia is passed over red-hot charcoal :-
*-
2NH,+C =NH,CN + HỌ.
1 Liebig, Annalen. 71, 102.
Liebig, Annalen, 61, 127.
THE CYANIDES
757
Cyanides are also easily formed when nitrogenous organic
bodies such as hoofs, clippings of hides, wool and blood, are
heated with an alkali such as potash. The potassium cyanide
thus obtained is prepared on the large scale, and forms the
starting-point of the cyanogen compounds. As this substance
is difficult to purify on account of its great solubility, it is con-
verted into ferrocyanide of potassium or yellow prussiate of
potash, which is used for the preparation of the other cyanogen
compounds.
The cyanides of the alkali metals, and those of the metals of
the alkaline earths, are soluble in water, and smell, as has
been already stated, of hydrocyanic acid, as they are decom-
posed by the atmospheric carbonic acid. They are, therefore,
just as poisonous as the free acid itself. The cyanides of the
other metals, with the exception of mercuric cyanide, are
insoluble in water. They dissolve, however, in the cyanides
of the alkali metals, with the formation of soluble double
cyanides. These double cyanides may be divided into two
distinct classes. The compounds of the first class are easily
decomposed by dilute acids with the formation of an insoluble
metallic cyanide, and free hydrocyanic acid; and possess all the
usual properties of the cyanides; of these, silver-potassium
cyanide and nickel-potassium cyanide may serve as examples.
=
A.CN.KCN +HNO, AgCN+HCN + KNO3.
Ni(CN)2. 2KCN + 2HCl = Ni(CN)2 + 2HCN + 2KCl.
The double cyanides of the second class, although containing,
like the others, two different metals, possess wholly distinct
properties. The best known of these are the yellow prussiate
of potash or ferrocyanide of potassium, Fe(CN), 4KCN, and
the red prussiate of potash or ferricyanide of potassium, Fe(CN),
3KCN. If to a solution of the first of these salts dilute hydro-
chloric acid be added, no hydrocyanic acid is evolved, but a
white crystalline precipitate is obtained having the composition
H,Fe(CN). This compound is a powerful acid, to which the
name of ferrocyanic acid has been given, and in which the four
atoms of hydrogen can be either partially or wholly replaced by
metals. In the same way the potassium can also be replaced
in the red prussiate of potash by hydrogen, and thus ferricyanic
acid, H,Fe(CN), can be obtained.
Similar double compounds are known, especially those of
cobalt, manganese, platinum, and of the metals allied to it.
758
THE NON-METALLIC ELEMENTS
In these compounds the second metal does not act as a base, but
has combined with the cyanogen to form an acid radical (for
example, Fe(CN)), which then unites with hydrogen and
metals in the same manner as other acid radicals such as
chlorine. (See also Vol. II., under Iron.)
441 Detection and determination of cyanogen and the cyanides.-
The detection of cyanogen in the cyanides, such as those of the
alkalis and alkaline earths, which are easily decomposed by
dilute acid, is simple enough. If the solution is not already
alkaline, a solution of caustic soda is added, and next, a few drops
of a solution of ferrous sulphate (green vitriol) which has been
partially oxidized by exposure to the air; an excess of hydro-
chloric acid is then added, when a precipitate of Prussian blue
is thrown down. In this reaction the ferrous salt in the
presence of caustic soda and a cyanide gives rise to the form-
ation of sodium ferrocyanide, which then undergoes the usual
reaction with the ferric salt which is present. Cyanides, which
do not evolve hydrocyanic acid on the addition of a dilute acid,
may be decomposed by fusing them with dry carbonate of soda.
The fused mass is then dissolved in water, and the filtered
liquid treated according to the above-mentioned process.
In the soluble cyanides, as well as in those which are de-
composed by dilute acids, the quantity of cyanogen can readily
be determined by precipitation with nitrate of silver. A precipi-
tate of cyanide of silver is obtained, which, after washing and
drying at 110°, is weighed. This method, however, cannot be
employed in the cases of mercuric cyanide, potassium ferro-
cyanide, and analogous substances, in which cases it is necessary
to obtain hydrocyanic acid from the compound by some ap-
propriate method, such as treatment with sulphuretted hydrogen,
distillation with dilute sulphuric acid, &c. Concerning the
further details of the qualitative and quantitative analysis of
such compounds we must refer to treatises on analytical
chemistry.
COMPOUNDS OF HYDROCYANIC ACID WITH THE HYDRACIDS OF
THE CHLORINE GROUP OF ELEMENTS.
442 Although hydrocyanic acid possesses weak acid properties,
yet it behaves in many respects as a weak base which, like
ammonia, is capable of direct union with certain acids. This
property depends upon the fact that its chemical constitution is
CYANOGEN CHLORIDE
759
analogous to that of ammonia, for hydrocyanic acid is formed
when ammonia is heated with chloroform under pressure;
thus:-
CHCI,+4NH₁ = NCH+3NH¸Cl.
3
According to this mode of formation, hydrocyanic acid may
be considered to be ammonia in which the three atoms of
hydrogen have been replaced by the triad radical methenyl CH,
whence it might be termed methenylamine.
Hydrocyanic acid forms two compounds with hydrochloric
acid. The monohydrochloride HCN,HCl is prepared by passing
hydrogen chloride into anhydrous hydrocyanic acid at 15° and
subsequent warming in a sealed tube to 35-40°. It is a
crystalline very hygroscopic substance, which is decomposed
by water into ammonium chloride and formic acid.¹ The
sesquihydrochloride 2HCN,3HCl is obtained by passing hydrogen
chloride into a mixture of anhydrous hydrocyanic acid and
ethyl acetate, and forms a very hygroscopic crystalline mass,
which yields with water the same products as the previous
compound. Similar derivatives have been obtained with
hydrobromic acid, having the composition HCN, HBr and
2HCN,3HBr respectively. The hydriodide HCN,HI has also
been prepared.
Hydrocyanic acid also combines with metallic chlorides to
form crystalline compounds, as
SbCl,+3HCN, SnCl4 + 2HCN and TiCl + 2HCN.
COMPOUNDS OF CYANOGEN WITH THE ELEMENTS OF THE
CHLORINE GROUP.
CYANOGEN CHLORIDE, CICN = 61.04.
443 Berthollet obtained this compound in 1787 by the action
of chlorine on hydrocyanic acid, and believed it to be oxy-
genated hydrocyanic acid. Its true composition was recognised
by Gay-Lussac in the year 1815. According to Wöhler this
compound is easily obtained by passing chlorine gas into a
saturated aqueous solution of mercuric cyanide containing some
of the solid salt, until the liquid is saturated with the gas, and
the air which remains above the liquid is replaced by chlorine.
3 Ann. Chim. Phys. 95, 200.
1 Gautier, Ann. Chim. Phys. [4], 17, 129.
2 Claisen and Matthews, Ber. 16, 309.
760
THE NON-METALLIC ELEMENTS
The vessel is then well closed and placed in a dark room till all
the mercuric chloride has dissolved, or all the chlorine has
combined. In order to remove any free chlorine which may
remain the solution is shaken up with mercury, and the liquid
heated in order to expel the cyanogen chloride, the volatile
product being condensed by passing through a bent tube plunged
into a freezing mixture.
According to Gautier¹ the liquid chloride is best obtained by
leading chlorine into a mixture of one part of hydrocyanic acid
and four parts of water contained in a retort or flask which is
connected with an inverted Liebig's condenser. As soon as
the liquid has attained a green colour, the current of chlorine
is stopped, and an excess of mercuric oxide and calcium chloride
is added to the liquid, which is well cooled down in a freezing
mixture. The chloride of cyanogen is next distilled off and
condensed in a well-cooled receiver.
At the ordinary temperature cyanogen chloride is a gas which
has a very penetrating odour and causes a copious flow of tears;
it is very readily condensed to a liquid, and freezes at -18°,
forming colourless prisms, which according to Regnault 2 melt
at -7°, the liquid formed boiling at 127°, whilst according to
Wurtz it melts at 12 to 15° and boils at 15.5°. Its vapour
density is 2:13. When allowed to stand it is partially
converted into the polymeric cyanuric chloride CNCl¸ (p. 766).
-
CYANOGEN BROMIDE, CNBr.
444 This compound, which was discovered in the year 1827
by Serullas, is formed by the action of bromine on hydrocyanic
acid, or on the metallic cyanides. If bromine is added drop by drop
to a well-cooled aqueous solution of potassium cyanide, crystals
separate out, which consist of a mixture of cyanogen bromide
and potassium bromide. When these crystals are heated to a
temperature of from 60° to 65°, cyanogen bromide sublimes in
the form of delicate transparent prisms, which soon pass into
the cubical form. It melts at 52° and boils at 61·3° under
750 mm. pressure,7 has an exceedingly pungent smell, and acts
very powerfully upon the eyes. It is also poisonous; one grain
1 Bull. Soc. Chim. [2], 403 2 Jahresb. 1863, 70. 3 Annalen, 79, 284.
4 Bull. Soc. Chim. [2], 4, 105.
5 Ann Chim. Phys. 34, 100.
6 Langlois, Ann. Chim. Phys. [3], 61, 482.
7 Mulder, Rec. Trav. Chim. 4, 151; 5, 85.
CYANIC ACID
761
dissolved in a small quantity of water brought into the œsophagus
of a rabbit produced instant death (Serullas). According to
Bineau the specific gravity of its vapour is 3607. When not
absolutely pure cyanogen bromide is converted at 130° to 140°
into cyanuric bromide, C₂NBr.¹
CYANOGEN IODIDE, CNI.
445 This compound was discovered by Davy in the year
1816,2 and is easily formed by the action of iodine on cyanide
of mercury and other metallic cyanides, whilst it frequently
occurs as an impurity in commercial iodine. According to
Liebig it can be best obtained by dissolving iodine in a warm
concentrated solution of potassium cyanide until the liquid
solidifies on cooling to a crystalline mass. Cyanogen iodide
forms long delicate white needles, easily soluble in alcohol and
ether, and from these solutions the compound crystallizes in
four-sided tables. It is sparingly soluble in water. It melts
at 146.5°, but begins to volatilize at a much lower temperature.
It has a smell similar to that of the bromide, and is equally
poisonous.
CYANIC ACID, NCOH.
446 Cyanic acid was first observed by Vauquelin in the year
1818, although it was first distinctly recognised as a peculiar
acid and its properties more exactly investigated by Wöhler in
1822. Its salts are formed when cyanogen gas is led into an
alkali; thus:-
C₂N₂+2KOH=CNOK+CNK + H₂O.
When potassium cyanide is melted in the presence of air, or,
better, with the addition of an easily reducible oxide or peroxide,
potassium cyanate is formed.
The most convenient method of preparing the cyanates is by
the action of potassium bichromate on potassium ferrocyanide.
For this purpose 200 grams of completely dehydrated potassium
ferrocyanide are mixed whilst still warm with 150 grams of
fused potassium bichromate, and the whole heated in an iron
dish. The black product is then quickly extracted with a
¹ Eghis, Zeitsch. Chem. [2], 5, 376; Ponamarew, Ber. 18, 3261.
2 Gill. Ann. 54, 384.
762
THE NON-METALLIC ELEMENTS
mixture of 800 cc. of 80 per cent. ethyl alcohol and 100 cc. of
methyl alcohol, potassium cyanate separating from the filtrate
on cooling.¹
Cyanates are readily decomposed by dilute acids, but the
cyanic acid at the same time takes up water, carbon dioxide
and ammonia being formed. Therefore on adding a dilute acid
to potassium cyanate effervescence takes place, carbon dioxide
being given off. This has a pungent smell, due to the presence
of a trace of cyanic acid. By acting on a cyanate with dry
hydrochloric acid, cyanic acid is set free. It at once combines,
however, with hydrochloric acid to form the compound HCl
NCOH, a colourless liquid, fuming in the air. It is impossible
to isolate cyanic acid from this compound, because it at once
changes into the polymeric cyanuric acid C,N(OH), (p. 766).
The only reaction by which cyanic acid can be obtained
is by the decomposition of cyanuric acid by heat, when it
is resolved into three molecules of cyanic acid, the vapours of
which must be condensed by means of a freezing mixture.
Cyanic acid is a colourless mobile liquid having a most pungent
smell. On taking the vessel containing it out of the freezing
mixture the liquid soon becomes turbid and hot, and with a
crackling noise, or if in large quantity with explosive ebulli-
tion, is soon converted into a white porcelain-like mass which is
a polymeric modification, called cyamelide, the molecular weight
of which is unknown. On heating cyamelide it is reconverted
into cyanic acid.
Of the cyanates the most interesting salt is the ammonium
cyanate, NCO(NH), obtained as a white crystalline mass by
mixing the vapour of dry cyanic acid with dry ammonia. The
freshly prepared aqueous solution gives the reactions of a cyanate
and of ammonia, but on standing for some time, or on heat-
ing, the ammonium cyanate is transformed into the isomeric
carbamide or urea, CO(NH2)2. The dry salt also undergoes the
same transformation on heating, and the change may also be
shown by heating a solution of potassium cyanate to which an
equivalent quantity of ammonium sulphate has been added.
THIOCYANIC ACID, NCSH.
447 This acid, to which the name of sulphocyanic acid has
also been given, was mentioned by Bucholz in 1798, but first
1 Bell, Chem. News, 32, 49; Erdmann, Ber. 26, 2438.
THIOCYANIC ACID
763
carefully examined by Porret in 1808. He obtained its potas-
sium salt by boiling a solution of potassium sulphide with
Prussian blue. Its quantitative composition was ascertained
by Berzelius in 1820.¹
-
Salts of this acid are formed by the direct union of sulphur
with a cyanide. Thus, for instance, potassium thiocyanate is
easily obtained by melting together potassium cyanide and
sulphur or dried potassium ferrocyanide with potassium
carbonate and sulphur. The ammonium salt is obtained by
warming hydrocyanic acid with yellow sulphide of ammonium;
thus:-
NH
NHS+NH,SCN.
NHS+HCN
Ammonium thiocyanate is easily obtained in large quantities
by warming a mixture of alcoholic concentrated ammonia and
bisulphide of carbon which has been allowed to stand for a con-
siderable time. Ammonium thiocarbonate and thiocarbamate
are formed, and these are decomposed on heating, with evolu-
tion of sulphuretted hydrogen (Millon); thus:-
SSNH
SNH
CS
CS {NH
SNÍ
=
NCS NH4+2H,S.
= NCS NH+H2S.
For this purpose it is best to take 350-400 grams of carbon
bisulphide, 600 grams of 95 per cent. alcohol, and 800 grams
of ammonia (sp.gr. 0912). As soon as the carbon bisulphide
has dissolved, the solution is concentrated, and on cooling the
salt separates out in crystals.2 If a solution of mercuric nitrate
be added to a solution of ammonium or potassium thiocyanate,
a heavy white precipitate of mercuric thiocyanate, (CNS),Hg,
is thrown down. This substance when dry is easily inflam-
mable, and burns with a pale sulphur flame, leaving a very
voluminous residue behind. This salt is used for the prepara-
tion of the so-called Pharaoh's Serpents.
Aqueous thiocyanic acid may be readily obtained by decom-
posing barium thiocyanate with dilute sulphuric acid. This
solution may be distilled under diminished pressure, the acid
coming over with the first portion, and if dried over calcium
chloride forms a yellowish oil which soon decomposes.3 On
2 Claus, Annalen, 179, 112.
1 Schweig. Journ. 31, 42.
3 Klason, J. Pr. Chem. [2], 36, 57.
764
THE NON-METALLIC ELEMENTS
boiling the aqueous solution, the greater part of the acid is
converted with absorption of water into carbonyl sulphide
and ammonia, whilst at the same time some hydrocyanic
acid is formed. Thiocyanic acid and its soluble salts are
coloured blood-red by ferric chloride, ferric thiocyanate being
formed. The soluble thiocyanates produce with silver nitrate a
precipitate of silver thiocyanate which is insoluble even in presence
of the stronger acids. A solution of potassium or ammonium
thiocyanate of known strength is, therefore, used for the
purpose of determining the quantity of silver contained in
a solution, by adding some nitric acid and ferric sulphate and
then dropping in a standard solution of ammonium thiocyanate
from a burette until the formation of a slight red colour shows
that all the silver has been thrown down.¹
CYANOGEN SULPHIDE, OR THIOCYANIC ANHYDRIDE, (CN),S.
448 In order to prepare this compound, silver thiocyanate is
added to an ethereal solution of iodide of cyanogen, the solution.
evaporated, and the residue treated with hot carbon bisulphide.*
If the solution be cooled to 0°, cyanogen sulphide separates
in rhombic tables, which possess a smell like that of cyanogen
iodide. They melt at 60°, but when heated above 30° they
begin to volatilize. They are very soluble in water, alcohol,
and ether, and are decomposed by caustic alkalis as follows:-
CN } S+2
K)
H
O
=
CN
KSS+
CNO+H₂O.
K
Cyanogen Selenide, (CN),Se, is obtained by adding dry silver
cyanide to a solution of selenium bromide in bisulphide of
carbon. It crystallizes in tables, which are decomposed by water
into selenium, selenious acid, and hydrocyanic acid.³
CYANAMIDE, CN.NH2.
449 This body was discovered by Bineau, who prepared it
by the action of dry ammonia on gaseous cyanogen chloride.
The solid mass thus obtained was supposed by him to be the
1 Volhard, J. Pr. Chem. [2], 9, 217. 2 Linnemann, Annalen, 70, 36.
3 Schneider, Pogg. Ann. 129, 364.
* Ann. Chim. Phys. [2], 67, 368, and 70, 251.
CYANAMIDE
765
H
chloride of cyanammonium, CICN(NH), but Clöez and Can-
nizzaro¹ showed in 1851 that this substance consists of a
mixture of ammonium chloride and cyanamide :-
――――――
CN
2 N}H+CNC1=NH,C1+N}H
Cyanamide is most readily obtained, however, by the action of
mercuric oxide upon a cold, not too dilute, solution of thio-
urea 2
:-
H
H
CS(NH2)2+HgO = CN(NH₂) + HgS + H₂O.
The mercuric oxide must be very finely levigated, or a dense
precipitated oxide may be employed; this is suspended in water
and gradually added to the solution of thio-urea; an excess of
the oxide must most carefully be avoided. The end of the
reaction may be easily recognised, a drop of the solution being
brought on to filter paper, and then a drop of ammoniacal
silver solution added; this produces a black spot as long
as thio-urea is present. As soon as all the thio-urea has
been decomposed the solution is filtered and evaporated down
on a water-bath, and the residue treated with ether; on evapora-
tion of the ethereal solution, pure cyanamide is left behind. It
forms small colourless crystals, melting at 40°, which deliquesce
on exposure to the air, and are volatile with steam. If a few
drops of nitric acid be added to its solution, cyanamide is trans-
formed into urea :—
CN.NH,+H,O=CO(NH,),.
Cyanamide acts as a weak base; the hydrochloride CN.NH2,
2HCl is obtained as an easily crystallizable powder when
hydrochloric acid gas is passed into a solution of cyanamide.
in anhydrous ether (Drechsel). The hydrogen of the cyanamide
may also be replaced by metals; thus, for instance, if an am-
moniacal silver solution be added to an aqueous solution of
cyanamide, an amorphous yellow precipitate, CN.NAg2, is
thrown down, which crystallizes from hot ammonia in micro-
scopic needles. In the same way, if one part of sodium be
dissolved in 15 parts of absolute alcohol, and to the cold
1 Compt. Rend. 32, 62.
2 Volhard, J. Pr. Chem. [2], 9, 6 ; and Drechsel, J. Pr. Chem. [2], 9, 284.
766
THE NON-METALLIC ELEMENTS
liquid an alcoholic solution of cyanamide be added, a light
crystalline powder falls down, possessing, according to Drechsel,
the composition CN.NH Na.
When cyanamide is strongly heated it undergoes polymerisa-
tion, first forming dicyanamide, C,N,H,, and finally cyanuramide,
C₂NH6.1
POLYMERISATION OF CYANOGEN COMPOUNDS.
450 Mention has frequently been made of the fact that many
of the cyanogen derivatives readily undergo polymerisation-that
is, are converted into compounds having the same percentage
composition but a higher molecular weight. Thus cyanogen
chloride readily passes into cyanuric chloride CNC, cyanic
acid into cyanuric acid C¸N(OH), and cyanamide into dicy-
anamide C,NH, and cyanuramide C,NH. These compounds
have very different properties from the cyanogen derivatives
from which they are produced, and contain in most cases a
nucleus of carbon and nitrogen atoms united together in such a
manner as to form a closed chain. Thus the cyanuric com-
2
4
N-C
pounds all contain the nucleus Ca
N, and from them
N-C
other substances containing the same nucleus can be prepared.
A very large number of compounds containing similar closed
chains of carbon and nitrogen atoms are now known, and they
form a very important division of the carbon compounds usually
considered under the head of Organic Chemistry; the poly-
merised cyanogen compounds will therefore be more fully
treated of together with these in a later volume.
GUANIDINE, CH,N。.
3
451 This powerful base was first obtained by Strecker2 by
acting with potassium chlorate and hydrochloric acid on guanine,
CH,NO, a compound contained in guano. It can also be ob-
tained by various other reactions. For instance, it is produced
by heating to 100° chloropicrin (nitrochloroform), CCI,NO,, a
substitution product of marsh gas, with an alcoholic solution
of ammonia; thus:-
3
CCI,NO2 + 3NH₂ = CH¿N¸.HCl + 2HCl + HNO。.
1 Annalen, 122, 22.
Hofmann, Zeitsch. Chem. [2], 2, 1073, and 4, 721.
2 Annalen, 118, 151.
GUANIDINE
767
As an excess of ammonia must be used, no nitrous acid is
liberated, water and free nitrogen being formed; for this reason
it is necessary to use strong glass tubes in order to withstand
the accumulated pressure.
Hydrochloride of guanidine is also formed when an alcoholic
solution of cyanamide is heated with sal-ammoniac¹:-
CN₂H₂+ NHCl = CN¸H¸Cl.
It is, however, not necessary to use pure cyanamide, but cyano-
gen chloride may be heated with alcoholic ammonia, or, still
better, cyanogen iodide may be employed, as this latter forms
iodide of ammonium, which is easily soluble in alcohol.2 When
ammonia acts upon carbonyl chloride urea is formed, and at
the same time small quantities of cyanuric acid, ammelide, and
guanidine.3
The best method, however, of preparing guanidine is from
ammonium thiocyanate, which must be heated for twenty
hours to a temperature of 190°, when the thio-urea which
is formed decomposes, with evolution of sulphuretted hydrogen
into cyanamide, and this is decomposed by the ammonium
thiocyanate still present; thus:-
CN(NH,)+CNSNH =CNH(NH,),CNSH.
A residue of guanidine thiocyanate is obtained, whilst large
quantities of ammonium thiocarbonate sublime.
3
Other guanidine salts can be obtained from the thiocyanate as
well as the free base. In order to prepare these, 100 grams of
the salt are dissolved in lukewarm water, and to the concentrated
solution 58 grams of pure potassium carbonate are added, the
solution evaporated to dryness, and the residue treated with
alcohol in order to dissolve the potassium thiocyanate, whilst
carbonate of guanidine, (CH¿N¸)2.CO,H2, remains behind, and
can be purified by recrystallization from aqueous solution. The
carbonate yields, on treatment with acids, the other guanidine
salts, of which the nitrate, CH,N.NO,H, crystallizing in tablets,
is remarkable from its slight solubility in water. The sulphate
forms crystals tolerably soluble in water, and yields the free
base when treated with baryta-water; the solution may then
be evaporated in a vacuum, and the base obtained in the form
2 Bannow, Ber. 4, 161.
5
1 Erlenmeyer, Zeitsch Chem. [2], 7, 28.
3 Bouchardat, Compt. Rend. 69, 961.
• Volhard, J. Pr. Chem. [2], 9, 6, and Delitsch, ibid. 1.
768
THE NON-METALLIC ELEMENTS
of a crystalline mass having a caustic taste and strongly
alkaline reaction, which on exposure to the air quickly absorbs
moisture and carbonic acid. When guanidine is heated with
dilute sulphuric acid, it is resolved into urea and ammonia ;
thus:-
C(NH)(NH,), + H,O = NHg + CO(NH,),.
3
A cold solution of potash decomposes guanidine thiocyanate in
a similar way; thus:-
CNH(NH), NSCH+KOH = CO(NH2)2+NCSK + NH₂.
These reactions, as well as all the modes of formation of
guanidine, show that it possesses the following constitution:-
NH₂
T
C-NH
NH..
Nitroguanidine, NH: C(NH₂)NH.NO, is obtained by adding
guanidine thiocyanate to concentrated sulphuric acid, then mixing
with fuming sulphuric acid, and after cooling adding fuming nitric
acid to the solution. When the resulting liquid is poured into
a large volume of water, nitroguanidine separates out, and after
recrystallization forms needles which melt at 230°, with evolu-
tion of ammonia.¹
Amidoguanidine, NH: C(NH)NH.NH,, is prepared by the
careful reduction of the preceding compound with zinc dust and
the theoretical quantity of dilute acetic acid, the whole being
carefully cooled. It is a crystalline compound soluble in water,
and is converted by boiling with acids or alkalis into hydrazine
(p. 470), for the preparation of which it is used.
PHOSPHORUS TRICYANIDE, P(CN)3.
452 In order to prepare this compound a mixture of phos-
phorus trichloride and dry silver cyanide is heated in closed
tubes for several hours to a temperature of 120°-140°. When
cold the tubes are opened, the excess of phosphorus trichloride
driven off by moderate heating, and the residue brought into
a tubulated retort, and heated in an oil-bath to 130°-140° in a
¹ Thiele, Annalen, 270, 1.
ARSENIC TRICYANIDE
769
stream of carbon dioxide. The phosphorus tricyanide sublimes
in long glistening white needles or thick plates. It takes fire
when slightly warmed, and burns in the air with a bright white
flame; it is decomposed by water into hydrocyanic and phos-
phorous acids;¹ thus:-
P(CN), + 3H₂O
P(OH)3 + 3HCN.
=
ARSENIC TRICYANIDE, AS(CN)3.
This substance is obtained by the action of finely divided
arsenic on cyanogen iodide in presence of carbon bisulphide.
It forms yellowish microscopic crystals, and is rapidly attacked
by atmospheric moisture, and almost instantaneously by water,
with formation of arsenious and hydrocyanic acids.²
BORON CARBIDE OR CARBON BORIDE, BC OR B.C
This is prepared by heating a mixture of boric anhydride and
carbon in an electrical furnace, and forms a graphite-like powder
which melts at a very high temperature, burns with difficulty in
oxygen, is insoluble in the usual solvents, but is decomposed by
fusion with alkalis.3
Another boride of carbon, having the composition CB, is
prepared by heating sixty-six parts of amorphous boron and
twelve parts of carbon, obtained from sugar, in an electrical
furnace, or by dissolving the two elements in iron, copper, or
silver at a very high temperature, and removing the metal by
treatment with aqua regia. It forms very hard black lustrous
crystals, having a density of 2:41, which are attacked by chlorine
below 1,000°, and by oxygen very slowly at that temperature;
like the previous compound, it is decomposed by fusion with
alkalis. The powder is sufficiently hard to cut the diamond,
but more slowly than diamond dust.4
COAL-GAS.
453 It was noticed so long ago as 1726 that when coal is
heated in a closed vessel an inflammable gas is evolved. In that
year Stephen Hales published his Vegetable Staticks, in which
1 Wehrhane and Hübner, Annalen, 128, 254, and 132, 277.
2 Guenez, Compt. Rend. 114, 1186.
3 Mühlhäusen, Zeit. Anorg. Chem. 5, 92. 4 Moissan, Compt. Rend. 118, 556.
50
770
THE NON-METALLIC ELEMENTS
he states that by the distillation of 128 grains of Newcastle
coal he obtained 180 cubic inches of an inflammable gas which
weighed 51 grains. Bishop Watson, in his Chemical Essays,
describes experiments made on coal-gas, and mentions that it
does not lose its illuminating power when it is passed through
water. The first to apply these facts practically to the manu-
facture of coal-gas was William Murdoch, a Scotchman living
at Redruth in Cornwall. He distilled coal in an iron retort,
and lighted his house with the gas which he thus manu-
factured. Murdoch was afterwards employed in the celebrated
engine works of Boulton and Watt at Soho near Birmingham.
Whilst there he improved his process for the manufacture of
gas, and in 1798 the Soho factory was for the first time lighted
with coal-gas. In 1802 there was a public display of gas illumina-
tion at Soho in honour of the Peace of Amiens; and during the
next three years Murdoch's process became so far perfected that
in 1805 the large cotton mill of Messrs. Phillips and Lee in
Manchester was lighted with gas.
The success of this undertaking attracted the attention of
several men of ability, especially Dr. William Henry and Mr.
Clegg. To the former we owe the first accurate investigation
of the chemical composition of coal-gas; to the latter we are in-
debted for many of the mechanical inventions still in use for its
preparation and purification, without which the present enormous
extension of the manufacture would have been impossible.
The streets of London were not lighted with gas until 1812,
and it was not introduced into Paris until after the peace of
1815.
When coal is subjected to dry distillation at a temperature of
a cherry-red heat, various products are formed. These may
be divided into three classes.
1. Coal-gas, a mixture of many gaseous compounds.
2. Coal-tar, a thick, oily, strongly smelling liquid.
3. Gas-liquor or Ammoniacal Liquor, an aqueous distillate
containing ammonium carbonate and sulphide, and other
products in solution.
The apparatus needed in the manufacture of coal-gas is: (1)
the retorts in which the distillation occurs; (2) the condensers
in which the liquid products of the distillation are condensed
and separated from the gaseous products; (3) the washers and
scrubbers in which the last portion of the liquid products and
the ammonia are removed; (4) the purifiers in which the remain-
COAL-GAS
771
FIG. 202.
ing gaseous impurities are removed; (5) the gas-holders in which
the gas is stored, and whence it is distributed through the gas-
mains and pipes to the place where it is to be burnt.


FIG. 204.
FIG. 206.
FIG. 203.

d

FIG. 205.

FIG. 207.
454 The Retorts originally introduced by Clegg in 1812
were made of iron, but these soon gave place to retorts made of
fire-clay or fire-brick. Several common forms are shown in
772
THE NON-METALLIC ELEMENTS
Figs. 202, 203, 204. At one time the retorts were always closed
at one end, and this is still very often the case, especially in
smaller works, but in the larger works longer retorts open at
each end are now usually employed, these being known as
through-retorts. The first-named retorts vary from 8-10 feet
G
0
H
g

H
FIG. 208.
H
G
d
in length, 14-18 inches in breadth, and 12-16 inches in
height, and contain a charge of from 1-3 cwt. of coal.
An iron mouthpiece, shown in section in Fig. 205 and in end
elevation in Fig. 206, is attached to the open end by means of
bolts, and when the retort has been charged the end of the
mouthpiece is closed by an iron lid (Fig. 207), held in position by
means of a holdfast and screw. An upright wide tube (d), cast
COAL-GAS
773
H
the
on to the mouthpiece, carries away the gases, which pass up
vertical ascension pipes H H H H, Fig. 208, and thence by the dip
pipes n n, Fig. 209, into the horizontal hydraulic main G G,
Fig. 208. The retorts are arranged so that a number, varying
usually from five to ten, are heated by means of one furnace, a
bed of five retorts being shown in elevation in Fig. 208 and in

E
n
R
d
B
HERMESAARED
FIG. 209.
section in Fig. 209. The through-retorts are similar in shape,
breadth, and height to those already described, but are double
the length, and contain a correspondingly greater weight of
charge. Mouthpieces, ascension pipes, &c., are attached in this
case to each end of the retort, so that the gases formed can pass
out at either end. Such retorts are frequently capable of car-
774
THE NON-METALLIC ELEMENTS
bonising 24 cwt. of coal, and yielding 12,000 cubic feet of gas,
per twenty-four hours.
455 The Condensing Apparatus.-Large quantities of vola-
tile products make their way from the retorts into the hydraulic
main as gases, but are there condensed into liquids. The end
of the upright pipe from each retort (n, Fig. 209), termed the
dip-pipe, passes for about one to three inches under the surface
of the liquid in the hydraulic main; so that the gas as it is
evolved can readily pass, but all entrance of air into the main
when the retort is opened is prevented. The tarry products as
well as the ammoniacal liquor, collecting in the hydraulic main,
run off when they reach a certain height by a pipe which com-
municates with the tar-well, where all the liquid products of
the distillation are allowed to collect.
Owing to the several water-joints which are necessary in
various parts of the apparatus, the back pressure on the gas in
the retorts is very considerable, and much loss of gas ensues
owing to the necessarily porous nature of the fire-clay retort.
To obviate this loss, and also to avoid the decomposition of the
dense hydrocarbons into gas-carbon and marsh gas which takes
place when the coal is distilled under pressure, an exhauster
worked by an engine is employed to draw out the gas from
the retorts, and pass it on through the condensers and purifiers,
so that in the hydraulic main the pressure is always less than
that of the atmosphere; care must however be taken that the
vacuum is not so great that air is drawn into the hydraulic
main through the dip-pipe when the retort lids are opened for
drawing the coke or charging with fresh coal.
The gas after leaving the retort houses passes through the
atmospheric condensers, which consist generally of a series of
iron pipes arranged either vertically or horizontally. An ex-
ample of a vertical condenser is shown in Figs. 210, 211. The
gas in passing through the pipes is gradually reduced to the
atmospheric temperature, and the more readily condensable
hydro-carbons, &c., separate out in the form of tar; a consider-
able quantity of water is likewise condensed which dissolves a
portion of the ammonia, sulphuretted hydrogen, carbonic acid,
and other impurities present in the gas, yielding ammoniacal
liquor. These fall to the lowest portion of the apparatus, and
then pass away through siphons into the tar-well. After
leaving the condensers the gas passes to the exhausters, and
from this point onwards is under pressure instead of vacuum,
COAL-GAS
775
1
and is forced through the remainder of the apparatus as this
throws a considerable amount of back pressure.
FIG. 210.
456 A considerable quantity of tarry matter is still retained
in suspension in the gas, and to remove this and the remainder
of the ammonia it is passed either through washers or scrubbers,
or through both these successively. Numerous varieties of
washers are employed, but the principle of almost all of these


m
80
A
FIG. 211.
is the same, and consists in dividing the gas up into a number
of small streams and forcing these through water or the
weak liquor which separates out in the hydraulic main. By
this means the tarry matter and most of the ammonia are
removed, and the solution of ammonia simultaneously combines
with a portion of the sulphuretted hydrogen, carbonic acid, and
carbon bisulphide in the gas. To completely remove the
TIL
776
THE NON-METALLIC ELEMENTS
ammonia, the gas is passed through the scrubbers, which
consist of iron towers filled with coke down which water is
allowed to trickle, thus exposing a very large wetted surface to
the gas. The construction of these scrubbers is shown in
Fig. 212, the gas entering at the bottom of the tower and
passing away from the top, whilst the liquor formed falls to the
bottom, and thence by means of siphons to the storage well.

L
FIG. 212.
L
457 Purification.-After leaving the scrubbers the gas still
contains the following impurities: sulphuretted hydrogen, 1-2
per cent. or 600-1,200 grains per 100 cubic feet; carbonic acid,
1-3 per cent. or 800-2,400 grains per 100 cubic feet; carbon bi-
sulphide, 15-50 grains per 100 cubic feet, and about 7-8 grains
per 100 cubic feet of other sulphur compounds. Carbonic acid
COAL-GAS
777

acts injuriously on the gas, inasmuch as it seriously deteriorates
its illuminating power, and the sulphur compounds when burnt
give rise to the formation of sulphur dioxide, and eventually of
sulphuric acid, which acts prejudicially on metal and leather
work exposed to its influence. In order to remove these
SHOUDE
C
B
m
m
bet
FIG. 213.
impurities as far as possible the gas is passed through layers of
various solid absorbents which are placed in trays in large cast-
iron boxes, generally termed the purifiers, the construction of
which is shown in Figs. 213, 214.
FIG. 214.
B
C
mi
D
n
D
G
G
Where no attempt is made to remove the sulphur compounds
other than sulphuretted hydrogen, the gas is first passed
through a series of boxes (usually three or four) containing
hydrated oxide of iron, Fe2O3xH,O, which absorbs the sulphur-
etted hydrogen, forming a sulphide of iron as follows:-
Fe2O3,H2O+ 3H2S = Fe₂S, + 4H₂O.
778
THE NON-METALLIC ELEMENTS
As soon as the gas passing the last box is found to contain
sulphuretted hydrogen, the first box of the series is charged
with fresh oxide of iron, and this then made the last of the
series. The gas, after being thus freed from sulphuretted
hydrogen, is passed through another series of purifiers contain-
ing layers of moist slaked lime which absorbs the carbon
dioxide forming calcium carbonate.
When the "spent" oxide of iron is allowed to remain in a
moist condition exposed to the air it gradually undergoes oxida-
tion, and is reconverted into ferric oxide with separation of
sulphur-
2Fe2S3 + 30₂ = 2FeO3 + 3S₂
2
and may then be again placed in the boxes to remove sulphur-
etted hydrogen, and this process of conversion into sulphide
and revivification continued until the spent oxide contains 50-
60 per cent. of free sulphur, when it is sold to the sulphuric
acid manufacturer.
If a quantity of oxygen, equal to about half that of the
sulphuretted hydrogen in the crude gas, be admitted into the
purifiers, the above reaction goes on in the boxes simultane-
ously with the formation of sulphide of iron, so that a por-
tion of the spent oxide is revivified as fast as it is formed,
and the boxes then last for a much greater length of time
without changing, thus effecting a considerable saving in the
cost of labour. It is immaterial for the purpose whether the
oxygen be added in the pure state or in the form of air, but
when the latter is used the inert nitrogen added to the gas
causes some diminution of the illuminating power, and therefore
necessitates the use of a certain amount of richer coal to main-
tain the standard illuminating power. On the other hand, air
is obtained without cost, whilst the use of oxygen necessitates
the employment of a separate plant for preparing it; each gas
manager has therefore to decide which plan will be the less
costly under his conditions of working. It must further be
remembered that it is almost impossible to keep air entirely
out of the gas, and there is always a certain amount, varying
considerably in different works, already in the gas at the inlet
to the purifiers, and the average amount of this should be
determined before deciding the amount of air or oxygen it is
necessary to add for revivification.
As already stated, the above process does not affect any of the
COAL-GAS
779
sulphur compounds other than sulphuretted hydrogen, the
carbon bisulphide, &c., being allowed to remain in the gas. In
many works, however, these impurities are also removed as far as
possible, although the complete elimination has not yet been
effected on the large scale. In certain towns a limit is placed on
the amount of sulphur which may be present in the gas supplied
from the works; thus in London the maximum allowed in
winter is 22 grains per 100 cubic feet, and in summer 17 grains,
and the average amount actually found in London gas is generally
about 10-15 grains.
To carry out the more complete purification one of two
processes is now usually employed. In the first, known as
the Beckton system, the gas passes first through two boxes con-
taining moist slaked lime which absorbs the carbonic acid; the
sulphuretted hydrogen is also absorbed, but as the carbon dioxide.
comes forward it decomposes the calcium sulphide which had
been formed, and the sulphuretted hydrogen is driven forward
into the next two boxes, which contain oxide of iron, where it is
completely absorbed. The gas next passes through two lime
boxes which have previously been saturated with gas contain-
ing sulphuretted hydrogen but free from carbon dioxide, and
therefore contain chiefly sulphides of calcium. This acts upon
the carbon bisulphide contained in the gas, forming probably
among other compounds calcium thiocarbonate, CaCS; the
reaction is however very complicated, and at present very incom-
pletely understood. The gas then passes through boxes contain-
ing Weldon mud, which consists chiefly of a compound of lime
and manganese dioxide, and this absorbs any traces of sul-
phuretted hydrogen which may have passed the other purifiers.
In the second, or all-lime system, the gas is passed through
a set of boxes, three or four in number, containing slaked lime
throughout; the first two boxes absorb the greater portion of
the carbon dioxide, whilst the sulphuretted hydrogen is ab-
sorbed mainly in the last two boxes, and the sulphides of
calcium formed act simultaneously on the carbon bisulphide in
the gas. To prevent the passage of any traces of sulphuretted
hydrogen, the gas after leaving the lime purifiers passes through
"catch boxes" containing oxide of iron.
66
The great objection to the last-named system is that the
I spent lime" removed from the boxes has scarcely any com-
mercial value, and possesses an extremely objectionable odour,
due chiefly to the presence in it of calcium sulphide, which
780
THE NON-METALLIC ELEMENTS
is decomposed by the carbon dioxide and moisture in the air,
giving off sulphuretted hydrogen. It has been found, however,
that the addition of a small quantity of oxygen or air to the
gas at the inlet to the purifiers renders the spent lime almost
odourless, and at the same time assists the purification, the
charges lasting for a longer period without changing, and the
process having at least as great an effect on the carbon bisul-
phide as the former plan, provided the amount of oxygen be not
excessive. If, however, too much oxygen be added to the gas,
the carbon bisulphide is practically unacted upon, as the sul-
phides are oxidised by the excess of oxygen before they have an
opportunity of acting on the bisulphide. Further, if an excess
of oxygen is accidentally admitted to purifiers which have been
previously absorbing the bisulphide, this latter is rapidly given
off again, and the gas may then be found to contain far more
sulphur at the outlet than at the inlet of the purifiers.
After leaving the purifiers the gas passes through the meter
to the gas-holders, where it is stored and distributed.
458 The illuminating power and also the yield of gas per ton
vary very much with the nature of the coal employed in the
manufacture, and also with the conditions under which the
distillation takes place. Cannel coal yields the largest
quantity of gas, possessing a high illuminating power, whilst
ordinary bituminous coal heated in the same manner may yield
nearly as much gas, but its illuminating power is greatly
inferior to that obtained from cannel. Bituminous coals are,
as a rule, used as the chief basis, but as the illuminating power
of the gas they yield is not, for the most part, sufficiently high,
a certain proportion of cannel is added, in order to maintain
the standard illuminating power, which varies very much
in different localities. Of late years, however, the price of
cannel having greatly increased, many attempts have been
made to find a cheaper enriching agent, and several processes
are now successfully at work. In many cases the enriching
agent is oil-gas, obtained by the dry distillation of the heavier
hydrocarbon oils (p. 788); in others the vapour from the most
volatile portion of petroleum, boiling below 100°, is mixed with
the gas as it passes from the purifiers to the holders. Another
plan consists in preparing water-gas by a separate plant (p. 790),
and carburetting this by mixing it with the vapour of oil, and
strongly heating the whole, which is then passed in the requisite
proportions into the coal-gas.
BUNSEN'S PHOTOMETER
781
459 Determination of the illuminating power of coal-gas.-
The commercial value of coal-gas depends of course chiefly
upon its illuminating power, and it is of great importance to
have a ready method of determining the latter. At the present
time coal-gas is largely used for heating as well as illuminating
purposes, and its heat of combustion is, therefore, also of im-
portance, but it has been found that within certain limits the
illuminating power and calorific value are very nearly propor-
tional to one another, and that, therefore, from a determination.
of the one the value of the other may be approximately judged.
In determining the illuminating power, it is first necessary to
fix upon the standard of light to be employed, and in this
country a sperm candle of definite construction and consuming
120 grains of sperm (7.79 grams) per hour is employed. This
standard is not altogether satisfactory, but hitherto no other
standard has been found to meet with general approval.
To compare the value of the coal-gas flame and that of the
candle, Bunsen's bar photometer, or some modification thereof,
is employed, in which advantage is taken of the fact that the
intensity of the illumination from a luminous point is inversely
proportional to the square of the distance of the illuminated
surface from that point. All that is necessary is to ascertain
the distances at which the candle and standard gas flame both
produce the same illuminating effect, the illuminating power
being then calculated from these observa-
tions. In Bunsen's photometer the candles
and gas flame are fixed at the end of a
graduated bar, usually sixty inches in length,
and the illuminated surface consists of a
diaphragm of paper, Fig. 215, which, with
the exception of a small circle in the
centre, has been painted with a solution of
spermaceti in benzene, thus rendering the
disc, with the exception of the central portion,
transparent. If, therefore, the disc is more
strongly illuminated on one side than the other, a difference
is observed between the two portions of the disc, but when
both sides are equally illuminated this difference disappears.
The disc is fixed at right angles to the bar in a carriage which
can be moved in either direction along the bar, and at the
back of the box containing the disc are placed two inclined
mirrors which enable the observer looking at the diaphragm
B
FIG. 215.
782
THE NON-METALLIC ELEMENTS
in a direction at right angles to the bar to see simultaneously
a reflection of both sides of the disc. The disc carriage is then
moved along the bar until the point is found at which the
whole surface of the disc on both sides appears to be equally
illuminated.
In making the test two candles are usually employed, as
these do not vary so much as a single candle. They are fixed
in a balance in order that the rate at which the sperm is con-
sumed may be accurately ascertained, as this very rarely takes
place at the specified rate of 120 grains an hour, and a correction
must be made accordingly. The gas burnt is passed through
an experimental meter, which is so regulated that the gas is
consumed at the rate of five cubic feet an hour, this being the
rate agreed upon at present as the standard rate of consumption.
The temperature and height of the barometer are read off at the
time of testing, and a correction made, the standard tem-
perature being taken as 60° F. and the standard pressure as
thirty inches of mercury. As a rule a test is allowed to extend
over ten minutes, and the average of the readings taken, these
being made once each minute.
The illuminating power of the same gas varies very much
with the nature of the burner employed, and it is therefore
necessary to use a standard burner. The same burner cannot
however be universally employed, as a burner which gives good
results with the quality of gas in one town may give less
satisfactory results with the higher or lower quality in other
places, and the particular burner to be used must be decided in
each locality according to the local circumstances. In London,
where the statutory quality of the gas is sixteen candles when
burnt at the rate of five feet per hour, the burner employed is
an Argand, known as the London Argand, in which the number
of holes in the steatite burner, height and width of the glass
chimney, &c., are accurately defined. This burner is so con-
structed that the requisite amount of air for consuming sixteen-
candle gas at the rate of five feet per hour is admitted; and
if, therefore, this burner is used for determining the illuminating
power of gas of either a decidedly lower or higher quality, the
result obtained is too low, for in the first case the inferior quality
of gas, containing less heavy hydrocarbons, requires less air for
its combustion, and the flame is, therefore, cooled and its illumin-
ating power reduced by the excess of air drawn into the burner
above the requirements of the flame. With the higher quality of
COMPOSITION OF COAL-GAS
783
gas the air supply is insufficient, and this again causes a reduction
in the illuminating power. Correct results may, however, be
obtained with this burner by burning the gas at such a rate that
the height of the flame in the chimney is about three inches, as
it is with 16-candle gas at 5 feet per hour, and then making a
correction for the consumption of gas. Thus, in one case a
poor quality of gas gave an illuminating power of 17:30 candles
with a consumption of 65 feet per hour, the true illuminating
power calculated for the 5-feet rate being therefore 13:19
candles, whilst when burnt at the rate of 5 feet per hour the
same gas only showed an illuminating power of 9:35 candles.¹
Another means of testing the quality of coal-gas is
Lowe's jet photometer, which depends upon the fact that the
pressure required to give a fixed height of flame in gas issuing
from a small jet varies inversely as the illuminating power.
This test is only of an approximate nature, but is very valuable
in the gas manufacture, as by its means the quality of the gas
passing through the apparatus can be roughly ascertained at
any moment.
460 Composition of Coal-gas.-Purified coal-gas is a mixture
of combustible gases and vapours, which may be divided into
two classes, viz.: (1) Those which burn with a luminous flame.
and are termed the illuminating constituents; (2) those which
burn with a non-luminous or scarcely luminous flame, and are
termed the diluents.
The most important of the first class, which are also termed
heavy hydrocarbons, are:-
Ethylene, C₂HË,
Propylene, CH¸,
Butylene, CHg,
C,H₂,
Acetylene,
Allylene,
Benzene-vapour, CH
CH₁
To the second class belong hydrogen, carbon monoxide, and
marsh gas. In addition to these constituents, coal-gas usually
contains small quantities of atmospheric nitrogen and oxygen as
well as the small quantities of sulphur compounds, which, as
already mentioned, cannot be altogether removed.
The influence of the composition of the coal upon that of the
gas obtained by its distillation is seen in the following table
showing the composition of gas from bituminous and from
cannel coal.
1 A Board of Trade Committee is at present (1894) sitting to consider the
question of the standards of light; their report is shortly expected.
784
THE NON-METALLIC ELEMENTS
•
Hydrogen
Marsh gas
Carbon monoxide
Olefines
Nitrogen
Carbon dioxide
Oxygen
Sulphuretted hydrogen
·
·
•
COAL-GAS FROM CANNEL COAL.
Manchester Manchester
Gas..
Gas.
(Bunsen & (C. R. A.
Roscoe.) Wright.)
Rochdale
Gas.
(C. R. A.
Wright.)
London
Gas.
(Frank-
land.)
45.58
52.71
53:44
35.94
34.90
31.05
29.87 41.99
6.64 4.47 5.86 10:07
6:46 11.19 1083 10.81
2.46
3.67 0.58
•
•
Hydrogen
Marsh gas
Carbon monoxide
Olefines
Nitrogen
Carbon dioxide
COAL-GAS FROM BITUMINOUS COAL.
•
·
0.29
Oxygen
Carbon bisulphide.
100.00 100.00 100.00 100.00
London Gas. Heidelberg Gas.
(Frankland.) (Landolt.)
a
b
50.05 51-24 4400
32.87 35.28 38:40
12.89
7:40 4.73
3.87
3.56
7.27
2.24
0.32 0.28
1.19
___
4.23
0.37
London.
S. Metro-
politan Gas,
(Lewes.)
0.15
0.79
0.96
0:02
10000 10000 10000 100.00
57·08
33:991
2.63
4:38
461 The influence of the temperature of the retorts on the
composition of coal-gas is clearly shown by experiments made
by Dr. Henry, who collected and analysed samples of coal-gas
at various stages of the process of manufacture. The following
table gives the results of these experiments made on a large
scale, in five different gas-works, showing that during the first
hour the specific gravity of the issuing gas was high, whilst
that of the gas coming off during the fifth hour became much
lower, and at the end of the operation fell to a still lower
1 Saturated hydrocarbons, i.e. marsh gas and its homologues.
COMPOSITION OF COAL-GAS
785
point. This diminution of density is accompanied by an altera-
tion in the percentage chemical composition as is shown in the
table :-
0.633 8.3
70.8
1st hour.
5th hour.
0.500 21.3
56.0
10th hour 0.345 60.0 20.0
•
•
·
·
Specific
Gravity.
Hydro- Marsh Ole- Carbon Nitro-
gen.
Gas. fines. monoxide gen.
Similar experients made by Erdmann and Kornhardt gave
the following numbers for the specific gravity of the gas issuing
at different periods of the manufacture :-
1
3
4
Erdmann 0.6 0.52 0.43 0.37 0.37
Kornhardt. 0·416 0·397 0.353 0.282 0.240
Hours after commencement-
2
Hours after commencement
Sulphuretted hydrogen
Carbon dioxide
Unsaturated hydrocarbons
Oxygen
Carbon monoxide
·
Hydrogen.
Saturated hydrocarbons
Nitrogen (by difference) .
•
12.3
7:0
0.0
·
The following table shows the composition of the gas evolved
from a Derbyshire gas coal at different periods after charging.
The samples were aspirated from the ascension pipe and drawn
through dilute sulphuric acid to remove tarry matters and
ammonia. The temperature of the retort was about 950° C.
.
.
1282
hour.
5
11
hours.
5.8
2.7
11.0
4.7
10.0 10.0
6
0.30 Sp. Gr.
―――
""
""
21 31 5
hours. hours. hours.
3.8
3.1
3:0
2.8
2.8 2.1
1.2
2.6 2.3 1.7
3.6 2.4 0.0
0.0 00 trace
8.65 5.2
0.0
0.0
4:35 5:0
4.9 4.5 3.8
29.8
37.5
42.2 46.2 60.8
49.7
42:05 394 37.5 26.3
07 4.35 4.5 5:0 6.2
51
786
THE NON-METALLIC ELEMENTS
The volume of gas yielded by a given sample of coal depends
also upon the temperature at which the distillation occurs.
If the distillation be conducted at too low a temperature,
the volume of gaseous products obtained is small and the
quantity of liquid products known by the name of gas-tar is
large, whereas if the temperature be raised too high, or if the
distillation be conducted too slowly and continued for too long a
time, a gas having little or no illuminating power and containing
large quantities of non-combustible nitrogen is given off. In
practice the retorts are usually maintained at a cherry-red heat,
the temperature being about 900-980 °C. or 1650°-1800° F.;
the different varieties of coal yield under these conditions 8,000
-12,000 cubic feet of gas per ton.
462 Chemical analysis of Coal gas.-To carry out the analysis
of coal-gas a measured volume of the gas is subjected to the action
of various absorbents in one of the many forms of apparatus
which have been devised for this purpose, and the amount
absorbed in each case measured. Carbonic acid is absorbed by
treatment with caustic potash solution, and the illuminat-
ing hydrocarbons by bromine or fuming sulphuric acid,¹ the
oxygen by phosphorus or pyrogallic acid, and the carbonic
oxide by a hydrochloric acid or ammoniacal solution of cuprous
chloride. After absorbing these, the residue consists of methane,
hydrogen, and nitrogen, which are determined by exploding a
mixture of a known volume of the residual gas with air or
oxygen. The contraction observed after the explosion is due to
the condensation of the water formed from combustion of the
hydrogen and marsh gas. After the explosion the gases are
treated with caustic potash which absorbs the carbon dioxide
formed by the combustion of the marsh gas. From these
observations the amount of hydrogen and marsh gas can be
ascertained as follows:-
Marsh gas in combustion yields its own volume of carbon
dioxide:
CH₁+20=CO₁₂+2H₂O,
hence the volume of carbon dioxide observed represents the
amount of marsh gas in the volume taken. The contraction
1 Fuming sulphuric acid cannot be used for absorbing the unsaturated hydro-
carbons, unless the oxygen has been previously removed, as some of the latter is
simultaneously absorbed. When phosphorus is used for absorbing the oxygen, the
unsaturated hydrocarbons must be first removed, as they render the phosphorus
inactive, so that in their presence bromine must be used as the absorbent.
WOOD-GAS
787
due to the formation of water is due partly to the hydrogen and
partly to the marsh gas. The latter in combustion combines
with twice its volume of oxygen, which must therefore be
deducted from the total contraction after the explosion in order
to obtain the amount of contraction due to combustion of the
hydrogen. Two volumes of hydrogen unite with one volume of
oxygen, and therefore the total amount of hydrogen present is
equal to two-thirds of the contraction thus obtained. The
nitrogen is found by difference.
In making the analysis in this manner it is assumed that the
only hydrocarbon present in the gas after treatment with the
various absorbents is methane. Traces of the higher homo-
logues of methane are however frequently present, and these
yield more than their own volume of carbon dioxide on explo-
sion, which renders the calculation of the percentages of
methane, hydrogen, and nitrogen incorrect. In the case of coal-
gas the amount of higher saturated hydrocarbons is usually so
small that the error may for ordinary purposes be disregarded,
but in the case of oil-gas (p. 788) the quantity of these is
much greater, and the above method cannot be adopted. In
order to absorb these hydrocarbons the gas is treated with
paraffin oil, which has been previously heated on the water-bath
for an hour. This absorbs all the higher homologues of methane
and some of the methane itself; the residue is then analysed in
the manner described above, and the volume of methane found,
added to that absorbed by the paraffin, gives the total volume of
saturated hydrocarbons.
In carrying out the analysis of illuminating gases a large
number of precautions must be taken to obtain reliable results.
For these and other details reference must be made to works on
gas analysis.¹
WOOD-GAS.
463 In countries where wood is cheap and coal dear the
former material is distilled for the purpose of yielding an
illuminating gas. The first to propose the use of wood for
this purpose was the French engineer, Le Bon, at the end of last
century. His proposal was, however, not carried out, inasmuch
¹ Bunsen's Gasometry (Roscoe's Translation); Hempel's Methods of Gas Analysis
(Macmillan & Co.); Winkler and Lunge's Technical Gas Analysis (Gurney &
Jackson); see also Lewes, Journ. Soc. Chem. Ind. 1891, 407.
788
THE NON-METALLIC ELEMENTS
as the gas thus obtained did not possess a sufficient illuminating
power, consisting of a mixture of hydrogen, carbon monoxide,
carbon dioxide, and a little marsh gas. With these gaseous
products a large quantity of easily condensable liquid oils distils
over, and in 1849 Pettenkofer showed that if these oils are
exposed to a high temperature they are partially decomposed
into heavy gaseous hydrocarbons. This observation was practi-
cally applied by Rüdinger, who in this way succeeded in pre-
paring a gas from wood which could be used for illuminating
purposes, and many towns in Germany and in Switzerland are
now lighted with gas thus made from wood.
The retorts used in this manufacture are made of cast iron,
and when their temperature is raised to a red heat, they are
completely filled with wood. The products of decomposition
are rapidly evolved, and the vapours of the oils coming into
contact with the red-hot sides of the retort are decomposed with
formation of the necessary quantity of the heavy hydrocarbons.
Wood-gas contains no volatile sulphur compounds, but large
quantities of carbon dioxide, which is got rid of by purification
by lime.
The following numbers give the average composition of wood-
gas:-
:-
Per cent.
Heavy hydrocarbons (ethylene and its homologues) 106 to 65
Light
(marsh gas
) 353 to 94
41.7 to 18.7
61.8 to 22:3
29
•
Hydrogen
Carbon monoxide.
""
""
•
Wood-tar consists of a mixture of a variety of substances,
amongst which creosote is the most important. The watery
distillate does not contain much ammonia in solution, but a
number of organic products, such as acetone, wood-spirit (methyl
alcohol), and acetic acid are present and are recovered on the
large scale.
OIL-GAS.
464 The preparation of illuminating gas by the dry distillation
of oil was first carried out about the year 1815, the gas being
compressed in strong reservoirs, so as to allow of its being moved
from place to place, being therefore known as "portable gas."
The compressed gas always deposited a considerable quantity of
OIL-GAS
789
liquid hydrocarbons, which were investigated about the year 1820
by Faraday, who was thus led to the discovery of benzene. The
manufacture of oil-gas however was soon discontinued as it was
unable to compete with the cheaper coal-gas. Since about the
year 1870 the manufacture has been revived, owing chiefly to
the demand for gas of high illuminating power for railway
trains, where a small volume of the gas is required to last for
a long period. It is also used to a considerable extent for
lighthouses.
The oil employed is either a shale oil or a mineral oil having
a specific gravity of 0.82-088, and its distillation is usually
carried out by one of two processes, known as the Pintsch and
Pope processes respectively. In the former two cast-iron retorts
one above the other are employed, both being maintained at a
bright cherry-red heat; the oil is allowed to drop at the rate
of twelve gallons per hour on to a plate in the upper retort,
where the non-volatile impurities remain, whilst the volatile
portions pass through the lower retort where they are further
carbonised. The purification of the gas is carried out in the
same manner as with coal-gas, but is of a simpler nature inas-
much as the amount of sulphur compounds in the gas is small.
Details of the Pope process have not been published; it is stated
that the oil drops first into the lower retort, and the vapours
then pass through the upper retort, but in other respects the
two processes are probably similar. For lighting trains, etc.,
the gas is stored in reservoirs beneath the carriages under a
pressure of about five atmospheres. The tar obtained from oil
contains a comparatively small amount of benzene; it is there-
fore of little or no commercial value, and is usually employed
for heating the retorts.¹
Owing to the rise in the price of cannel coal, oil-gas is now
frequently used in its place for enriching coal-gas. In what is
known as the "Peebles" process the retorts are heated only to
a low redness, and the oil added in a fairly large stream; the gas
evolved contains large quantities of undecomposed oil vapours,
which separate in the liquid state in the condensers and after
mixing with a certain proportion of fresh oil again pass back
into the retort.2 In this manner the formation of any liquid
residuals is avoided, the sole products being a permanent gas of
very high illuminating power and a dense black coke which
1 Ayres, Proc. Inst. Civil Eng. 93, 298; see also Thorpe's Dict. vol. ii. p. 213.
2 Journ. Gas Lighting, 1894, i. 1050.
790
THE NON-METALLIC ELEMENTS
remains behind in the retort. The gas is then mixed with the
coal-gas in the proportion requisite to bring the latter up to the
standard illuminating power.
WATER-GAS.
465 When steam is passed over coke or charcoal at a tempera-
ture of about 500° it is converted into hydrogen and carbon
dioxide-
2H2O + C = CO2 + 2H2.
whilst at 1000° the products are hydrogen and carbon mon-
oxide.
C + H2O = CO + H2.
At intermediate temperatures both reactions take place, but
above 600° the product consists chiefly of carbon monoxide and
hydrogen.
The mixture of gases thus prepared is known as water-gas, and
is at the present time manufactured on the large scale especially
in the United States, where it is used for both lighting and
heating purposes. In the former case it is usually "carburetted"
by mixing it with the vapour from hydrocarbon oils, and raising
the whole to a high temperature. Carburetted water-gas is also
used in this country to some extent for enriching coal-gas, but
in the United States it forms two-thirds of the whole supply of
illuminating gas.
For the manufacture of uncarburetted water-gas two cupolas
are usually employed, one of which forms the "generator" and
the other the "superheater." The former is filled with coke or
anthracite, and after the fuel has been fired, is raised to in-
candescence by an air blast. In this process a gas is evolved,
consisting of nitrogen, hydrogen, carbon monoxide, and dioxide,
and known as generator or producer gas. This passes through the
superheater, which is filled with firebrick checker-work, where
it parts with its sensible heat, and the generator gas may then
be employed for heating boilers, etc. As soon as the generator
has attained a sufficiently high temperature the air blast is
stopped and replaced by one of steam, which is raised to a high
temperature by passing through the superheater. The water-
gas then comes off rapidly, but as heat is absorbed in the re-
action the temperature of the generator gradually becomes
lower. When the temperature is so low that the proportion of
WATER-GAS
791
carbon dioxide becomes considerable, the steam supply is shut
off and the air blast substituted, which, when the temperature
is sufficiently high, is again replaced by steam, this cycle of
operations being maintained, with the necessary intervals for
charging with fresh fuel and clearing out the non-volatile pro-
ducts from the generator.
It will be seen that the process as described is intermittent,
and various forms of apparatus have been devised in which the
coke or anthracite is heated by an external furnace, but hitherto
it has been found that the first named is the cheaper and more
reliable plan.
Water-gas has about two-fifths of the heat of combustion of
London coal-gas, but as it contains such a small proportion of
hydrocarbons, it requires a very much smaller volume of air for
its complete combustion, and, therefore, in burning gives rise to
a very high temperature. It has on this account been used in
steel smelting. The gas known as "Dowson gas," which is
largely used for driving gas engines, is prepared by passing air
and steam simultaneously into a generator filled with coke or
anthracite, and consists, therefore, of a mixture of producer and
water-gas.
For the preparation of carburetted water-gas, two cupolas are
employed in addition to the generator, both of which are filled
with firebrick checker work, the additional cupola being termed
the "carburetter." The generator is worked in the usual
manner, and the producer gas passed through the carburetter
and superheater, air being added so as to bring about its com-
bustion within the apparatus, and raise the brickwork to a very
high temperature. The air blast of the generator is then re-
placed by one of steam, and the water-gas passed through the
carburetter into which the requisite quantity of oil is allowed to
flow; the oil vapours mix with the hot water-gas, and both pass
together through the superheater which renders the gas per-
manent. As soon as the temperature falls too low the steam is
replaced by air, the flow of oil stopped, and the whole apparatus
raised to a high temperature as before, and this cycle of opera-
tions repeated. The purification is carried out in the same
manner as with coal-gas.¹
¹ Journ. Gas Lighting, 1894, i. 908; Thorpe's Dict. vol. ii. p. 217.
792
THE NON-METALLIC ELEMENTS
THE NATURE OF FLAME.
466 The first statement of the nature of flame with which
we are acquainted occurs in the works of van Helmont, who
regarded it as burning smoke, but scarcely recognised the part
played by the atmosphere in the phenomenon. Hooke shortly
afterwards speaks of "that transient shining body which we call
flame" as "nothing but the parts of the oyl rarified and raised
by heat into the form of a vapour or smoak, the free air that
encompasseth this vapour keepeth it into a cylindrical form, and
by its dissolving property preyeth upon those parts of it that
are outwards producing the light which we observe; but
those parts which rise from the wick which are in the middle
...
FIG. 216.
are not turned to shining flame till they rise towards the top of
the cone, where the free air can reach and so dissolve them.
With the help of a piece of glass anyone will plainly perceive
that all the middle of the cone of flame neither shines nor
burns, but only the outward superficies thereof that is con-
tiguous to the free and unsatiated air." A century later
Lavoisier confirmed and extended Hooke's views, and showed
that flame was due to the combination of the components of
gaseous substances with the oxygen of the air, the gases being
raised to incandescence by the intensity of the action.
1
The simplest flames with which we are acquainted are those
of hydrogen and carbonic oxide burning in air or oxygen. The
¹ Lampas, published 1677.
STRUCTURE OF FLAME
793
flame of either of these gases burning from the end of a tube
appears as an incandescent cone, which on examination proves
to be hollow, the incandescence only taking place when the gas
has mixed by diffusion with air. The hollow nature of these,
and indeed of all flames, may be readily shown in various ways.
(1) A bent glass tube may be brought into the centre of the
flame, when the unburnt gases will pass up the tube and may be
ignited at the other end (Fig. 216). (2) The head of a match
may be thrust quickly into the centre of the flame and held
there for some time without the phosphorus catching fire, whilst
the wood will be charred and may even take fire where it is in
contact with the hot outer sheath of the cone. (3) A thin
platinum wire held horizontally in the flame is seen to glow at

-b
d
a
-C
FIG. 217.
b
...d
a
C
two points where it comes in contact with the outer zones in
which the combustion is going on, whilst between them it
remains cool.
With gases which yield more than one product of combustion
the phenomena become more complex. Thus, for example, the
flames of cyanogen and sulphuretted hydrogen burning in air
are found to consist of two sharply defined cones, possessing
different colours. Matters become still more complex in the
case of the flames with which we are most familiar, namely,
those obtained from a burning candle or from gaseous hydro-
carbons. In these flames four distinct regions, first defined by
Berzelius, are usually distinguished: (a) the dark central
region, (b) the yellow region, (c) the blue region, (d) the
794
THE NON-METALLIC ELEMENTS
faintly luminous region, which are clearly shown in Fig.
217.
The dark region consists of a zone of unburnt gases, in which,
however, chemical changes are going on owing to the action of
the heated sheath of gases surrounding it.
The yellow region, which is as a rule the largest, and gives off
by far the greatest amount of light, is known in common parlance
as the luminous portion. The first theory as to the cause of the
great luminosity of this zone is due to Davy, who believed it
to be caused by the decomposition of the hydrocarbons with
separation of solid particles of carbon, which increase in a high
degree the intensity of the light, being raised to a very high
temperature, first by the strongly heated gases around it and
then by its own combustion. Numerous facts appear to favour
this view. Thus we know that the light emitted by glowing
solids is much more intense than that given by glowing gases
at the same temperature, as may readily be shown by holding
a piece of platinum wire in the faintly luminous hydrogen flame,
when it becomes heated to whiteness. Further, if we hold a
sheet of paper a short time horizontally in a candle flame a
black ring of soot is deposited. Davy's theory, therefore, rapidly
obtained universal acceptance, and remained unchallenged until
about 1868.
467 About this time Frankland noticed that the flame of a
candle burning on the summit of Mont Blanc emits much less
light than when burning in the valley at Chamounix, although
the rate of combustion is the same in both cases, and was led to
examine the effect of pressure on the luminosity of flames. He
found that hydrogen burns in oxygen with a luminous flame
under a pressure of 20 atmospheres, and from these and other
experiments 2 he concludes that dense gases and vapours become
luminous at a much lower temperature than the same gases in
a more rarefied condition. The luminosity of the yellow region
of a hydrocarbon flame he regards as due not to the separation
of carbon but to the formation of dense hydrocarbons, which
then burn with a luminous flame.
3
Further researches have shown, however, that solid particles
1 On the Safety Lamp for Miners, with some Researches on Flame, 1818; Phil.
Trans. 1817, pp. 45 and 47.
2 Journ. Gas Lighting, March, 1861; Phil. Trans. 1861, p. 629.
3 Heumann, Phil. Mag. 1877, 1, 89, 366; Hilgard, Annalen, 92, 129;
Landolt, Pogg. Ann. 99, 389; Soret, Phil. Mag. 1875, 50; Burch, Nature, 31,
272; Stokes, Nature, 44, 263; 45, 133; Proc. Chem. Soc. 1892, 22.
NATURE OF FLAME
795
are undoubtedly present in the flame, and these must of ne-
cessity emit a large quantity of light at such a high tempera-
ture. Frankland's views cannot therefore be taken as a complete
explanation of the cause of the luminosity, but it is probable
that both the incandescence of the solid particles of carbon and
of dense hydrocarbons are together the chief causes of the
phenomenon.
1
The manner in which the separation of carbon is brought
about is still a matter of discussion. It was first supposed that
the separation was due to the fact that in the presence of an
insufficient amount of air for complete combustion the hydrogen
only of the hydrocarbons combines with the oxygen, leaving the
carbon in the elementary condition. This theory is usually
supposed to have been due to Davy, but it is nowhere definitely
stated in his published works. The experiments of other ob-
servers have shown that when hydrocarbons are burnt with
an insufficient supply of oxygen, the chief product is carbon
monoxide, and that considerable quantities of hydrogen are
liberated. Hence the conclusion has been drawn that the
separation of carbon in the flame is brought about not by the
preferential combustion of hydrogen, but by the action of the
heat from the outer sheath of burning gases on the hydrocar-
bons, which causes them to split up into their constituent
elements. Armstrong 2 has, however, pointed out that the final
state of affairs, as indicated by analysis of the products, is
probably the resultant of several successive or simultaneous
chemical actions, and that the data at present at our disposal
are not sufficient to enable us to draw the conclusion that the
oxidation of the hydrocarbons or the separation of carbon or
of hydrogen from them takes place in any one way.
Lewes has shown by the analysis of the gases taken from a
coal-gas flame at different heights that the unsaturated hydro-
carbons decrease very slowly in the dark portion of the flame,
but quickly disappear in the luminous zone. The nature of the
unsaturated hydrocarbons, however, undergoes a considerable
alteration in the non-luminous zone, the amount of acetylene
increasing very rapidly and forming 70 per cent. of the total
¹ Dalton, New System, Part II. 442 (1810); Kersten, J Pr. Chem. (1), 84,
310; Smithells and Ingle, Journ. Chem. Soc. 1892, i. 204; Lean and Bone,
Journ. Chem. Soc. 1892, i. 873.
2 Proc. Chem. Soc. 1892, 23.
796
THE NON-METALLIC ELEMENTS
unsaturated hydrocarbons when the top of the non-luminous
cone is reached. He, therefore, concludes that the first action.
of heat on the mixed hydrocarbons is to convert them into
acetylene, which, when the temperature is sufficiently high, is
dissociated into its elements. If, however, the flame be cooled
by allowing it to impinge against a cold platinum dish, the
temperature does not become sufficiently high to bring about the
decomposition of the acetylene, and this burns as such, with a
non-luminous flame, whilst on heating the platinum dish on the
inside the luminosity reappears (Heumann). In confirmation
of this supposition, Lewes, as well as previous investigators,
has shown that large quantities of acetylene are formed by the
action of heat on hydrocarbons such as methane and ethylene.¹

FIG. 218.
C
d
FIG. 219.
468 The blue portion of the flame is the least in extent of
any of the four divisions, and is probably caused by the com-
bustion of hydrocarbons which have become mixed with a
sufficient quantity of air to allow them to burn with a scarcely
luminous flame; it is probable that the combustion is incom-
plete, and that the reaction going on here corresponds with that
taking place in the inner cone of the flame of the Bunsen
burner (Smithells). The faintly luminous sheath is the region
of complete combustion in which those substances which have
been incompletely oxidised in the other portions of the flame,
chiefly hydrogen and carbonic oxide, are finally converted into
1 Journ. Chem. Soc. 1892, i. 322; Proc. Roy. Soc. 55, 90.
NATURE OF FLAME
797
water and carbon dioxide. This may be regarded as correspond-
ing with the outer cone of the Bunsen burner flame.
469 When a certain amount of air is mixed with coal-gas or
any other hydrocarbon before burning, the flame becomes non-
luminous, and burns with a pale blue perfectly smokeless flame,
which has a two-coned structure, the inner cone having a pale
blue colour and the outer a still paler shade of the same colour.
The most familiar example of this flame is seen in the Bunsen
gas-lamp, now so universally employed for heating purposes.
In this burner the gas emerges from a central jet (Fig. 219), and
passing unburnt up the tube (e, e, Fig. 218), draws air with it
through the holes (c, d), the mixture burning with a pale blue or
bluish smokeless flame. If the airholes be closed, the gas burns
with the ordinary smoky flame. The first explanation given of
these facts was, that owing to the presence of a considerable
quantity of oxygen in the interior of the flame the whole of the car-
bon is enabled to burn at once, so that no solid particles separate
in the flame. It was, however, soon found that a non-luminous
flame is also obtained by mixing the gas with indifferent gases
such as nitrogen and carbon dioxide, or with combustible gases
such as hydrogen and carbonic oxide. The volume of these
gases required to bring about the change to a non-luminous
flame varies considerably in each case, as is shown by the
experiments of Lewes,¹ embodied in the following table, coal-
gas of 16-candle power being employed as the illuminating
gas:-
Volume of gas required to render 1 vol. of coal-gas non-luminous.
1 vol. of gas requires 05 vol. of oxygen.
1.26
2.27
2:30
5.11
""
""
""
""
""
""
در
""
""
""
""
""
""
""
""
12:4
د.
""
""
""
""
carbon dioxide.
air.
nitrogen.
carbonic oxide.
hydrogen.
The temperature of the flames in each case shows considerable
variation, as is seen in the subjoined table, the temperatures
being ascertained by means of the platinum and platinum-
rhodium couple devised by Le Chatelier, one end of which
1 Journ. Chem. Soc. 1892, i. 332.
798
THE NON-METALLIC ELEMENTS
was introduced into the different parts of the flame through the
tube from which the gas was burning:
Point in flame.
Flame rendered non-luminous by
Air.
Half inch above burner.
1 inch above burner
Tip of inner cone.
Centre of outer cone.
Tip of outer cone.
Side of outer cone, level
with tip of inner cone 1333
54°
175
1090
1533
1175
Nitrogen.
30°
111
444
999
1151
1236
Carbon
dioxide.
35°
70
393
770
951
970
Luminous
flame from
Bunsen.
135°
421
913
1328
7281
1236
From these results, as well as those of Heumann, it appears
that oxidation, dilution, and cooling all help to bring about the
destruction of luminosity in the flame. The conclusions from
these and other experiments are summed up by Lewes as
follows:-
The various actions which lead to the loss of luminosity
are-
(1) The chemical activity of the oxygen introduced in the air,
which causes loss of luminosity by burning up the molecules of
hydrocarbons before in their diluted condition they can form
acetylene.
(2) The diluting influence of the nitrogen, which increases
the temperature necessary for the formation of acetylene from
the hydrocarbons, whilst if any be formed a higher temperature
is necessary for its decomposition. In this way diluents alone
will render a flame non-luminous, and in the normal Bunsen
flame nitrogen acts in this way until the hydrocarbons have
been destroyed by oxidation.
(3) The cooling influence of the air introduced, which is able
to add to the general result, although the cooling is less than
the increase of temperature brought about by the more rapid
oxidation.
(4) In a normal Bunsen flame the nitrogen and the oxygen
are of about equal importance in bringing about non-luminosity,
Probably incorrect owing to unsteadiness of flame at this point.
2 Annalen, 181, 129; 182, 1; 183, 102; 184, 206.
NATURE OF FLAME
799
but if the quantity of air be increased the oxidation becomes
the principal factor, and the nitrogen practically ceases to exert
any influence.
a
470 When the quantity of air mixing with the gas in
Bunsen burner is greater than that required to destroy lumin-
osity the mixture burns much less quietly and the inner cone
assumes a green colour; if more air be gradually added a point
is reached at which the mixture becomes so explosive that the
flame passes down the tube and ignites the gas issuing from the
central jet at the bottom of the burner.
This continues to burn, but as it is
unable at that point to obtain sufficient
air for its complete combustion, large
quantities of acetylene and other un-
saturated hydrocarbons are formed,
giving rise to the well-known un-
pleasant odour.
On a close examination of the phe-
nomena which take place when air is
gradually added to coal-gas or other
hydrocarbon burning from the end of
a glass tube, Smithells observed that
the inner cone becomes greener and
smaller, but that when the mixture
becomes sufficiently explosive the flame
does not pass down the tube as a
whole, but that the green inner cone
detaches itself from the outer one and
passes down the tube. If the tube
be constricted lower down, the des-
cending cone is arrested at that point,
the speed of the gases being there
greater, and it continues to burn there whilst the outer cone re-
mains in its former position. Smithells has devised an ingenious
arrangement in which this separation is readily effected, repre-
sented in Fig. 220A and 220B. It consists of two concentric tubes,
a and b, the former being wider than the other, and maintained in
a concentric position by the india-rubber collar c, and the brass
guide d. The outer tube can be slid up and down the inner
one as desired. The apparatus is placed over a Bunsen burner,
and on turning on the gas and applying a light, the usual
Bunsen flame is obtained at the top of the outer tube. If the
FIG. 220A. FIG. 220B.
800
THE NON-METALLIC ELEMENTS
quantity of air be now gradually increased, a point is reached at
which the inner portion of the flame descends until it reaches
the orifice of the inner tube where it is arrested, the speed of
the gas being greater at that point. By sliding the outer tube
upwards, the two cones may be separated any desired distance, as
shown in Fig. 220A, or the outer tube may be lowered until the
whole flame burns in the usual manner from the inner tube as seen
in Fig. 220B. The gases in the space between the cones consist
chiefly of carbon dioxide, water vapour, carbonic oxide and hy-
drogen, together sometimes with small quantities of hydrocarbons.
The flame of a burning mixture of cyanogen and air can also
be separated into two cones in a similar manner, the inner one
having a cherry-red and the outer a blue-gray colour. The
interconal gases were found to contain nitrogen, carbon monoxide
and carbon dioxide, the two latter gases being present almost
exactly in the proportion of 2 vols. of the former to 1 of the
latter. Mixtures of air with sulphuretted hydrogen and with
carbon bisulphide can also be separated into two cones in the
same way, and with care the same may be done when air is
gradually added to hydrogen and carbon monoxide, each of
which, when burning alone, yields, as already stated, a single-
coned flame.¹
SILICON OR SILICIUM.
Si = 28.2.
471 This element is, next to oxygen, the chief constituent of
the solid earth's crust. It always occurs combined with oxygen
in the form of silicon dioxide, or silica, SiO,, known in the
crystalline condition as tridymite, quartz, and the various kinds
of sand and sandstone, and in the amorphous condition as opal,
flint, &c. This substance combined with bases forms a large and
important class of minerals termed the silicates, which occur
very largely in many geological formations. For instance,
granite and the allied primitive rocks contain between 20 and
36 per cent. of silicon.
Minerals rich in silica were used in ancient times by reason
of their hardness for the purpose of glass-making, and Becher
believed that they contained a peculiar kind of earth which he
termed terra vitrescibilis. In the seventeenth century it was.
discovered that this glassy earth undergoes no alteration when
1 Smithells, Journ. Chem. Soc. 8921, i. 204; Nature, 49, 86, 149, 198;
Armstrong, Nature, 49, 100, 171; Newth, Nature, 49, 171.
SILICON
801
heated by itself, but that when brought in contact with
certain other bodies, it can be made to form a fusible glass.
This substance was for a long time supposed to be the essen-
tial principle of all earths, but it was found that it differed from
them inasmuch as it has no power of neutralizing acids; and
Tachenius, in the year 1660, noticed that it possesses acid
rather than alkaline properties, since it combines with alkalis.
The true nature of silica was then unknown, but Lavoisier, in
his chemical nomenclature, anticipates that the time may pro-
bably soon arrive when this substance will be recognized as
a compound body. After Davy's discovery of the compound
nature of the alkalis and alkaline earths in the year 1808,
silica, which was then classed amongst the earths, was supposed
to possess a similar constitution.
Berzelius, in the year 1810, first obtained impure silicon by
fusing together iron, carbon, and silica; and in 1823 he de-
scribed the following method for obtaining this element¹ in the
pure state. Ten parts of dry potassium silico-fluoride mixed
with eight to nine parts of metallic potassium are heated to
redness in an iron tube. The following decomposition takes
place:-
:-
K,SiF + 4K = 6KF + Si.
Instead of potassium, sodium may be employed. As soon as
the violent reaction which takes place is ended, the mass is
allowed to cool, and then treated with cold, and afterwards with
hot water, until all the potassium fluoride is dissolved. The
residual silicon is found in the form of an amorphous brown.
powder, which may likewise be obtained by passing the vapour
of silicon tetrachloride through a red-hot tube over sodium,2 or
silicon tetrafluoride over heated potassium.
An impure form of silicon, which may be advantageously
used for the preparation of many of its derivatives, can be
rapidly obtained by heating 40 grm. of dry powdered white sand
with 10 grm. of magnesium powder in a wide test-tube. The
reaction is a tolerably vigorous one, and if carried out with
precipitated silica is accompanied by a brilliant flash of light.3
Prepared by any of these processes, silicon is a dark brown
amorphous powder which when heated in the air easily takes fire,
burning to the dioxide, SiO2; the latter frequently fuses round
the particles of the silicon, leaving a portion of this substance
1 Pogg. Ann. 1, 169.
2 H. Deville, Ann. Chim. Phys. [3], 49, 68.
3 Gattermann, Ber. 22, 186.
52
802
THE NON-METALLIC ELEMENTS
unburnt in the centre of the mass. Amorphous silicon is not
attacked by sulphuric or nitric acid, but it readily dissolves in
aqueous hydrofluoric acid, and in hot concentrated solutions of
the alkalis. When it is heated to redness in absence of air, it
becomes denser, and assumes a graphitic appearance, after which
it oxidizes much less readily on heating, and is insoluble in
hydrofluoric acid.
Silicon may also be obtained crystallized either in six-sided
plates or long needles, and these forms are sometimes dis-
tinguished as graphitoidal and adamantine silicon. The crystals
of both forms are made up of regular octahedra, and they do
not differ markedly in either physical or chemical properties.
The crystalline modification of silicon is prepared by heating
metallic aluminium with from twenty to forty times its weight
of potassium silico-fluoride in a Hessian crucible to the melting-
point of silver. A regulus is thus obtained, and this when
treated with hydrochloric acid and then with hydrofluoric
acid, leaves the silicon in the form of black shining six-sided
tabular crystals resembling graphite in their appearance, which
have a specific gravity of 2:49¹ and will scratch glass. The
reaction is thus represented :-
3K,SiF+4A1 = 6KF + 4A1F3 + 3Si.
Crystallized silicon can also be prepared by passing a slow current
of silicon tetrachloride over aluminium previously melted in an
atmosphere of hydrogen :-
3SiCl4 + 4Al = 4AlCl3 + 3Si.
The silicon formed dissolves in the excess of fused aluminium
until the metal is saturated with it, and on cooling, the silicon
separates out in the form of long needle-shaped crystals which
consist of aggregations of octahedra and tetrahedra of an iron-
grey colour and a reddish lustre.
Crystalline silicon can be best prepared by throwing a mixture
of thirty parts of potassium silico-fluoride, forty parts of granu-
lated zinc, and eight parts of finely-divided sodium into a red-hot
crucible which is kept for some time at a heat just below the
boiling-point of zinc. The regulus is then treated successively
with hydrochloric acid, boiling nitric acid, and hydrofluoric acid,
when dark glittering octahedral crystals are found to remain
1 Wöhler, Annalen, 97, 266.
PROPERTIES OF SILICON
803
behind. If the temperature be raised above the boiling-point of
zinc, the silicon melts and may be cast in sticks.¹
If the vapour of silicon tetrachloride be passed through a
porcelain tube heated to redness and containing silicon, that
substance becomes denser and assumes a light iron-grey colour.
In this process a portion of the silicon appears to be volatilized,
inasmuch as needle-shaped crystals are found in the further
portion of the tube, the formation of which is due to the pro-
duction and decomposition of a lower chloride.
Crystallized silicon is not attacked by any acids with the
exception of a mixture of nitric and hydrofluoric acids. When
strongly heated in oxygen it oxidizes only slowly and super-
ficially. Heated however to redness in carbon dioxide, it burns
to form its oxide, whilst carbon monoxide is produced. It
dissolves in hot caustic potash or soda with evolution of hydrogen
and formation of the corresponding silicate; thus―
Si + 2KOH + H2O = K2SiO + 2H.
Crystalline silicon is also formed when the amorphous variety
is melted, the change being accompanied by the evolution of
8,059 units of heat per atom.2 The specific heat of silicon
varies with the temperature considerably, but becomes constant
at about 232°, and is then equal to 0.203, whilst at 22° it is
01697 (Weber).3 When strongly heated in an electric furnace
silicon volatilizes and condenses in small spheres mixed with a
grey powder and a little silica.*
The Atomic Weight of Silicon.-Berzelius determined the
atomic weight of silicon by the oxidation of the element and
by the analysis of barium silico-fluoride, but did not obtain
satisfactory results. The analysis of the chloride carried out by
Dumas and Schiel led to the value 27.9, whilst Pelouze obtained
a slightly higher number; finally, the conversion of the bromide
into the oxide by decomposition with water has given the
number 28.2.5
1 Ann. Chim. Phys. [3], 63, 26; 67, 435.
2 Troost and Hautefeuille, Ann. Chim. Phys. [5], 9, 76.
3 Pogg. Ann. 154, 367.
4 Moissan, Compt. Rend. 116, 1429.
Thorpe and Young, Journ. Chem. Soc. 1887, i. 576.
804
THE NON-METALLIC ELEMENTS
SILICON AND HYDROGEN.
SILICON HYDRIDE, OR SILICO-METHANE, SiH, = 32.2.
472 This substance, which was discovered by Buff and
Wöhler in 1857, is obtained by acting with hydrochloric acid
upon an alloy of silicon and magnesium, prepared by fusing
together forty parts of anhydrous magnesium chloride with a
mixture of thirty-five parts of sodium silico-fluoride, ten parts of
common salt, and twenty parts of sodium. The dark-coloured
mass which is thus obtained evolves silicon hydride when brought
in contact with acidulated water. For this purpose the magne-
sium silicide is brought into the bottle (Fig. 221), and the latter
then completely filled with cold water from which all the air has

FIG. 221.
been expelled by boiling. A wide gas-delivery tube, also com-
pletely filled with the same water, is connected with the bottle,
and hydrochloric acid is poured down a funnel tube passing
through the cork to the bottom of the bottle. The evolution
soon begins, and the gas must be collected over water free from
air. Every bubble of the gas thus obtained takes fire sponta-
neously when brought in contact with the air, burning brilliantly
with formation of a cloud of silica which escapes in the form of
a ring similar to that produced by phosphuretted hydrogen. If
a jar be filled with the gas over water, and then opened in the air,
the gas also takes fire spontaneously, burning with a luminous
flame and depositing a brown film of amorphous silicon in con-
sequence of the limited supply of oxygen. The gas thus pre-
pared always contains free hydrogen, and this depends, according
to Wöhler, upon the fact that the black mass contains two
SILICON HYDRIDE
805
magnesium compounds, by the decomposition of one of which
the pure silicon hydride is evolved, whilst the other yields
hydrogen and hydrated silica.
Pure silico-methane can be obtained by acting with sodium on
triethyl silicoformate, a compound which will be subsequently
described (Friedel and Ladenburg). In this reaction ethyl
silicate and silico-methane are formed, thus :-
3Si(OC,H),+SiH.
4Si (OC₂H),H
5/3
=
The sodium employed in this reaction remains unacted on,
and the part it plays is not understood. The colourless gas thus
obtained does not take fire at the ordinary temperature, but it
does so when slightly warmed or when mixed with hydrogen.
If the gas be collected over mercury and the bubbles as they
emerge from the surface of the mercury be brought in contact
with a heated knife-blade, they take fire, and the mercury soon
becomes sufficiently heated to enable the bubbles to take fire
spontaneously. Silicon hydride decomposes at a red heat into
amorphous silicon and hydrogen, the volume of the hydrogen
gas being twice that of the compound gas taken, and when the
gas is passed through a heated narrow tube an opaque mirror
of silicon is deposited. When decomposed with caustic potash,
one volume of the gas yields four volumes of hydrogen :—
K2SiO3 + 4H2.
The gas may also be obtained by passing silicon fluoride over
heated magnesium and then treating the mass with acids.2
Silicon hydride takes fire when brought into chlorine gas, with
formation of silicon tetrachloride and hydrochloric acid. It con-
denses to a liquid under a pressure of 100 atmospheres at -1°,
of 70 atmospheres at -7°, and of 50 atmospheres at -11°; its
critical temperature is probably about 0°.3
SiH + H₂O + 2KOH
4
=
1 Annalen, 143, 124.
3 Ogier, Compt. Rend. 88, 236.
When silicon hydride is submitted to the action of the electric
current a yellow substance of the formula SiH, is deposited,
which burns when heated in the air or in chlorine.¹
2 Warren, Chem. News, 58, 210.
4 Ogier, Compt. Rend. 89, 1068.
806
THE NON-METALLIC ELEMENTS
SILICON AND FLUORINE.
SILICON TETRAFLUORIDE, SiF, = 103.8.
473 This gas was first observed by Scheele in the year 1771.
It was also obtained by Priestley, and was afterwards examined
by Gay-Lussac and Thénard in 1808, and by J. Davy in 1812.
We are however indebted to Berzelius for the most accurate
investigation of this compound, carried out in the year 1823.¹
In order to prepare this gas, white sand or powdered glass is
heated with fluor-spar and concentrated sulphuric acid :
2CaF₂+2H2SO4 + SiO₂ = SiF, + 2CaSO4 + 2H,O.
In this operation it is necessary that an excess of sulphuric
acid should be employed in order to absorb the water which is
formed in the reaction and which would otherwise decompose
the gas.2
-
Silicon tetrafluoride is a colourless gas, fuming strongly in the
air, possessing a highly pungent odour like that of hydrochloric
acid, and condensing to a colourless liquid under a pressure
of 9 atmospheres; or when exposed to a temperature of — 160°.
According to Olszewski the tetrafluoride freezes at 102°
and volatilizes without liquefying. The specific gravity of the
gas according to the experiments of J. Davy is 3:57. It is
incombustible, and is decomposed by water with separation of
gelatinous silica. Fused sodium takes fire when brought into
the gas and burns with a red flame. Dry ammonia combines
with the gas, forming a white crystalline body, having the
composition SiF,2NH,, which is decomposed by water. Three
volumes of the gas unite with two of phosphuretted hydrogen
at - 22° and 53 atmospheres to form lustrous crystals of an
unstable compound.
--
SILICO-FLUORIC ACID OR HYDROFLUOSILICIC ACID, H,SiF
1 Pogg. Ann. 1, 169.
3 Monatsh. 5, 127.
474 When silicon tetrafluoride is led into water the following
decomposition takes place :-
―
3SiF¸ + 4H₂O = 2H₂SiF¸ + Si(OH).
6'
2 Faraday, Phil. Trans. 1845, 155.
• Besson, Compt. Rend. 110, 80.
SILICON TETRAFLUORIDE
807
up,
The silicic acid separates out in theform of a gelatinous mass
and in order to prevent the gas delivery-tube, by which the
tetrafluoride is passed into the water, from becoming stopped
the end of this tube is allowed to dip under mercury as seen
in Fig. 222. As soon as the mass begins to become thick it
must be frequently stirred up, otherwise channels are formed
through which the gas can escape into the air without coming
into contact with the liquid. The thick jelly is pressed through
a linen filter and the filtrate concentrated at a low temperature.

FIG. 222.
The same acid is formed when silica is dissolved in hydro-
fluoric acid, and also when silicon fluoride is passed into concen-
trated hydrofluoric acid, the solution in this case depositing
crystals of the formula H,SiF+ 2H2O,¹ which melt at 19°.
The saturated solution forms a very acid, fuming, colourless
liquid, which may be evaporated down in platinum vessels
without leaving any residue, as, on boiling, it decomposes into
silicon tetrafluoride and hydrofluoric acid. The specific gravity
of the aqueous hydrofluosilicic acid is seen in the following
table.2
1 Kessler, Compt. Rend. 90, 1285.
2 Stolba, J. Pr. Chem. [1], 90, 193.
808
THE NON-METALLIC ELEMENTS
Per cent.
0.5
1.0
1.5
2:0
5.0
10.0
15
20
25
30
Specific gravity.
1.0040
1.0080
1:0120
1.0161
1 0407
1.0834
1.1281
1.1748
1.2235
1.2742
This acid forms salts which are termed the silico-fluorides.
Most of these are soluble in water, the exceptions being the
lithium salt, Li,SiF, the sodium salt, Na,SiF, the potas-
sium salt, K,SiF, the barium salt, BaSiF, the calcium salt,
CaSiF, and the yttrium salt, YSiFe, which are more or less
difficultly soluble. One part of the barium salt dissolves in
3,802 parts of cold water.¹ Hence this acid is used as a
re-agent for barium salts and for the separation of this metal
from strontium. The soluble silico-fluorides possess an acid
reaction, and a bitter taste. They all decompose on heating
into a fluoride and a silicate.
A subfluoride of silicon, the exact composition of which is
unknown, is said to be formed when silicon fluoride is passed
over melted silicon and then suddenly cooled. It is deposited
as a white volatile powder, which contains less fluorine than the
tetrafluoride and reduces potassium permanganate solution.2
SILICON AND CHLORINE.
SILICON TETRACHLORIDE, SiC), = 168·96.
475 This compound, discovered by Berzelius in the year 1823,
is obtained by the direct union of its elements or by passing a
current of dry chlorine over a strongly heated mixture of silica
and charcoal, obtained by mixing silica and oil in the form of
small balls, igniting these in a covered crucible, and then placing
them in a porcelain tube (a b Fig. 223) heated to whiteness. The
reaction which takes place is as follows:-
SiO2 + 2C + 2Cl2
1 Fresenius, Annalen, 59, 120.
2 Troost and Hautefeuille, Ann. Chim. Phys. [5], 7, 463.
=
SiCl,+2CO.
4
SILICON TETRACHLORIDE
809
A
The escaping vapours and gases pass through an absorption
tube (c) surrounded by a freezing mixture, and the product is
separated from the excess of absorbed chlorine by shaking it up
with metallic mercury and subsequent distillation.
It may be readily prepared by passing chlorine over the mass
of crude silicon obtained by Gattermann's method, heated at
300°-310° (p. 801).
Silicon tetrachloride is an acrid colourless liquid, fuming in
the air, having a specific gravity at 0° of 152408 and boiling
at 59.57° (Thorpe). The specific gravity of its vapour, according
to Dumas, is 5'937. When thrown into water, silicon tetrachlo-

FIG. 223.
b
=
ride is at once decomposed, hydrochloric and silicic acids being
formed, and the latter deposited as a gelatinous mass.
It reacts with six molecules of ammonia to form a white amor-
phous mass, and forms an unstable crystalline compound when
compressed with phosphuretted hydrogen at a low temperature.¹
SILICON TRICHLORIDE, Si,Cl
267.54.
476 Friedel obtained this compound, along with a small
quantity of a spontaneously inflammable liquid 2 which probably
1
Besson, Compt. Rend. 110, 240.
2 Annalen, 203, 254.
810
THE NON-METALLIC ELEMENTS
has the composition SiCl,, by gently heating the corresponding
tri-iodide with mercuric chloride.¹ The same compound was
obtained by Troost and Hautefeuille 2 by passing the vapour
of silicon tetrachloride over silicon heated in a porcelain tube to
whiteness, and by Gattermann in the preparation of the tetra-
chloride from crude silicon.3 It is a colourless liquid solidifying
at 1° and boiling at 146°, having a specific gravity at 0° of
1.58. The specific gravity of its vapour is 97. It fumes
strongly in the air, and when heated it takes fire. At a tem-
perature of 350° it begins to decompose, and the amount of
decomposition increases with the temperature up to 800°, when
it is completely dissociated into the tetrachloride and silicon.
--
If the temperature of the vapour be quickly raised beyond
1000° no such dissociation is, on the contrary, observed. We
have therefore here to do with a substance possessing the re-
markable property of being stable at temperatures below 350°
and above 1000°, and dissociating at intermediate temperatures.
This explains the singular fact observed by Troost and Haute-
feuille, that if the vapour of silicon tetrachloride be passed
over silicon heated to above 1000° in a porcelain tube, the silicon
is transported from the heated to the cooled part of the tube.
This is not due to the volatilization of the silicon, for no such
change is observed when this body is heated in an atmosphere
of hydrogen, but is to be explained by the alternate formation
(at a high temperature) and dissociation (at a lower temperature)
of the trichloride.
It unites with ten molecules of ammonia to form a white
substance which loses ammonia at 100°. Phosphuretted
hydrogen is immediately reduced by it with formation of solid
hydrogen phosphide.*
also
A chloride of the formula SigCl, boiling at 210°-215°, is
formed by the action of chlorine on crude silicon. Water
decomposes it with formation of a white powder H¿Si¸О5.
SILICO-CHLOROFORM OR TRICHLORSILICO-METHANE,
SiHCl=134-77.
477 This body was first obtained in the impure state by
Wöhler and Buff by heating silicon in a current of dry hydro-
chloric acid gas at a temperature just below red heat, and may
1 Compt. Rend. 73, 1011.
3 Ber. 27, 1945.
2 Ann. Chim. Phys. [5], 7, 461.
Besson, Compt. Rend. 110, 516.
SILICON TETRABROMIDE
811
:-
readily be prepared in this way at a temperature of 450°-500°
from the crude silicon obtained by Gattermann's method
(p. 801). In order to prepare the pure compound, the crude
product thus obtained is condensed in a tube surrounded by a
freezing mixture, and the silico-chloroform separated by sub-
sequent distillation from the tetrachloride formed at the same
time. It is a colourless, mobile, strongly-smelling liquid which
fumes on exposure to the air and boils at 34°.
It is very
inflammable and burns with a green mantled flame, evolving
white clouds of silica. When a hot glass rod is brought into a
mixture of this body and air, the mixture burns with explosion
(Gattermann). Water readily decomposes this substance in the
cold, and a white powder is precipitated to which the name
silico-formic anhydride, Si,H,O,, has been given (Friedel and
Ladenburg):
2
2SiHCl,+3H,0 = 6HCl + Si,H₂O3.
2
This body is very unstable, and is decomposed by dilute ammonia
into silicic acid and hydrogen.
SILICON TETRABROMIDE, SiBr₁ = 345 64.
478 This compound, discovered in the year 1831 by Serullas,¹
is obtained by a reaction similar to that employed for the pre-
paration of the chloride. On leading bromine vapour over a
heated mixture of carbon and silica, best obtained by mixing
gelatinous silica with lamp-black and syrup of cane-sugar,
drying and igniting,2 a volatile distillate is obtained, and this is
freed from an excess of bromine by shaking up with mercury
and subsequent distillation. It may also be prepared directly
from crude silicon, which is heated in a current of bromine.
vapour (Gattermann).
The tetrabromide is a colourless heavy liquid, having a specific
gravity of 2813, boiling at 153°, and solidifying at 13° to a
crystalline mass. When brought in contact with water it is
decomposed into hydrobromic and silicic acids.
It forms a white amorphous compound, SiBr, + 7NHg, with
ammonia, which is decomposed by water; and seems to form
an unstable compound with phosphuretted hydrogen at a high
pressure.³
1 Ann. Chim. Phys. [2], 48, 87.
Reynolds, Journ. Chem. Soc. 1887, 590.
3 Besson, Compt. Rend. 110, 240.
812
THE NON-METALLIC ELEMENTS
SILICON TRIBROMIDE, Si,Br
= 532.86.
This colourless and crystalline compound is formed when the
corresponding iodine compound is treated with bromine in the
presence of carbon bisulphide. Large crystalline tablets are
formed, which melt on heating, and may be distilled at 240°
without undergoing decomposition.¹
SILICON BROMOFORM, SiH Brg
= 267.28.
This substance is formed by the action of hydrogen bromide
on silicon,2 and may be prepared in this way from crude silicon
(Gattermann).
It boils at 115°-117° (Gattermann),3 109°-111° (Besson),+
and is spontaneously inflammable in the air. In its chemical
relationships it resembles silico-chloroform.
SILICON AND IODINE.
SILICON TETRA-IODIDE, Sil
531.84.
479 Friedel obtained the tetra-iodide by the direct com-
bination of the two elements. For this purpose he volatilized
iodine in a stream of dry carbon dioxide, and led the mixed
gases over heated silicon;5 the iodide may also be prepared in
this way from crude silicon (Gattermann).
The tetra-iodide is a colourless crystalline mass which is
deposited in the form of regular octahedra from solution in
carbon bisulphide. Its melting-point is 120 5° and its boiling-
point 290°. Heated in the air it takes fire and burns with a
reddish flame. It is decomposed in presence of water into
hydriodic and silicic acids.
=
SILICON TRI-IODIDE, Si,I= 811-86.
6
This compound was likewise first obtained by Friedel and
Ladenburg by heating the tetra-iodide with finely-divided
silver to a temperature of 280°:
1 Annalen, 203, 253.
3 Ber. 22, 193.
* Annalen, 149, 96.
2SiI+2Ag = Si₂I。 + 2AgI.
2 Wöhler and Buff, Annalen, 104, 99.
4 Compt. Rend. 112, 530.
6 Annalen, 203, 247.
SILICON AND IODINE
813
It crystallizes from carbon bisulphide in splendid colourless
hexagonal prisms or rhombohedra, which fume on exposure to
the air, and, owing to the absorption of moisture, change to a
white mass with formation of silicic and hydriodic acids. It
melts when heated, but decomposes with the formation of
Sil, and a lower iodide. Ice-cold water decomposes it, and also
the trichloride, without evolution of hydrogen, but with formation
of a white substance having the composition Si,O,H2, to which
the name of silico-oxalic acid has been given because it possesses
a composition corresponding with that of oxalic acid, C₂OH₂.
This substance is, however, decomposed even by weak bases
with evolution of hydrogen and formation of silicic acid accord-
ing to the equation:-
H.Si₂O + 4KOH= 2K,SiO, + 2H₂O + H₂.
2
When heated or rubbed it decomposes with a feeble explosion.¹
SILICO-IODOFORM, SIHI,
For the purpose of preparing this compound, a mixture of
hydrogen and hydriodic acid is passed over silicon heated just
to redness. It is a colourless strongly-refractive liquid boiling
at 220° and having a specific gravity at 0° of 3·362. It is
decomposed by water in a manner corresponding to the chlorine
compound (Friedel).
= 406.93.
MIXED HALOGEN DERIVATIVES OF SILICON.
480 When a mixture of hydrogen bromide with silicon chlo-
ride vapour is passed through a red-hot glass tube, a mixture of
the chlorobromides of silicon is formed. Bromotrichlorosilicon,
SiCl, Br, is also formed by the action of bromine on silicon.
chloroform at 100°, or on silicon chlorosulphide at the ordinary
temperature.3
The chloro-iodides and bromo-iodides of silicon are obtained
by similar reactions. They are all decomposed by water and
form solid compounds with ammonia. Their physical properties
are given in the annexed table:-
1 Gattermann, Ber. 27, 1243.
2 Besson, Compt. Rend. 112, 788.
3 Friedel, Annalen, 143, 118; 145, 185.
814
THE NON-METALLIC ELEMENTS
Formula.
SiCl, Br
SiCl, Br₂
¹ SiClBrg
2 SiCl,I
2 SiCl. I,
2 SiCII,
3
•
·
3 SiBr₂I
3 Si Br,La
3 SiBrig.
•
·
Boiling-point.
80°
103°-105°
(126°-128° (B)
140°-141° (R) í
113°-114°
172°
234°-237°
192°
230°-231°
255°
Melting-
point.
--
-39°
2°
14°
38°
53°
SILICON AND OXYGEN.
Combines with
55NH,
5 NH,
11 NH3
5.5 NH,
5 NH,
SILICON DIOXIDE OR SILICA, SiO
481 Silicon forms only this one oxide, which is an extremely
important constituent of our planet. It is found not only in
the mineral but also in the vegetable and animal kingdoms,
existing in large quantity in the glassy straw of the cereals
and of bamboos, and in the feathers of certain birds, which
have been found to contain as much as 40 per cent. of this
substance. Vast deposits of pure silica in a very fine state
of division occur in various parts of Germany, especially in
Hanover and near Berlin. This consists of the scales of
extinct diatomaceæ, and is termed kiesel-guhr. Large quanti-
ties of this substance are now used for a variety of purposes,
especially for the preparation of dynamite, for filtering, and as a
non-conducting medium for packing steam-pipes. The minute
and beautifully-formed spicules of the spongidæ and radiolariæ
also consist of pure silica. In the mineral kingdom it is found
in three distinct forms :-
= 59.96.
1 Reynolds, Journ. Chem. Soc. 1887, 590.
2 Besson, Compt. Rend. 112, 611, 1314.
3 Friedel, Ber. 2, 60.
• Besson, Compt. Kend. 112, 1447.
(a) Quartz is the most important form of silica. It crystal-
lizes in the hexagonal system (trapezohedrally tetartohedral)
forming usually combinations of the prism with the rhombo-
SILICON AND OXYGEN
815
Ce
hedron; three of the most usual forms are shown in Figs. 224,
225, and 226. The relation of its axes is 1:10999. It has a
specific gravity of 265 and a hardness of 7. The purest form of
quartz is known as rock crystal. It is usually colourless and
transparent. Sometimes, however, quartz is coloured by the
presence of traces of oxides of manganese, which give it a violet
tint, and it is then termed amethyst quartz. Other varieties
containing more or less impurity are known as milk quartz,
--
FIG. 224.
- Pi
+R
F
FIG. 225.
+R
с
a
ap
FIG. 227.
rose quartz, and smoky quartz. Crystalline quartz also occurs
in large masses, forming whole mountain ranges as quartzose
rock. It forms one of the chief constituents of granite, gneiss,
and syenite, whilst sand and sandstone consist of an impure
variety of quartz.
a
P
∞P
1 Pogg. Ann. 133, 507, and 135, 437.
2 Lasaulx, Jahrb. Min. 1878, p. 408.
FIG. 226.
(b) Tridymite is a second crystalline variety of silica dis-
covered by G. v. Rath¹ in the trachytic porphyry found at
Pachuca in Mexico. This form of silica occurs in many other
similar rocks, and is found in considerable quantity in the
trachyte of Stenzelberg in the Siebengebirge. Tridymite cry-
stallizes in the asymmetric system, and forms six-sided tablets,
which are combinations of a prism with pyramids, Fig. 227. The
816
THE NON-METALLIC ELEMENTS
ratio of the axes is 0.5812: 1:11040, and the whole form
approaches very closely to one belonging to the rhombic system.
It has the specific gravity 2:3, and the same hardness as quartz.
A very characteristic property of this mineral is that it generally
occurs in trillings (Fig. 228), from which its name has been
b
a
a
b
2
a
a
a
b
a
a
FIG. 228.
derived. These are generally found grown together in twins,
as in Fig. 229.
Identical with tridymite is the mineral asmanite which has
been found in certain meteorites.
a
a
FIG. 229.
b
a
b
3
(c) Amorphous Silica occurs in nature containing more or less
water as opal. This substance is either colourless or variously
coloured, possesses a vitreous fracture, and has a specific gravity
of 2.2. It not unfrequently happens that crystals of tridymite
are found in the non-transparent portions of this opal, and
these remain as an insoluble residue when the opal is treated
SILICA
817
with caustic potash (G. Rose). Chalcedony, agate, and flint
are intimate mixtures of amorphous silica with quartz or
tridymite.
Hydrated amorphous silica is formed by passing silicon tetra-
fluoride into water. When the gelatinous mass is washed with
water, dried, and ignited, pure finely divided amorphous silica
remains behind as a white very mobile powder. Amorphous
silica may be obtained in the same form by decomposing a
silicate of an alkali-metal with an acid. For this purpose one
part of quartz, flint, or white sand, is fused with four parts
of sodium carbonate. If flint or quartz be employed it must
previously be reduced to powder, and this is easily effected by
heating the mass to redness and then quickly plunging it into
cold water, after which it becomes friable and can readily be
powdered. The fused mass is then heated with hydrochloric
acid, when it becomes gelatinous, an alkali chloride and
silicic acid being formed. The gelatinous mass is afterwards
evaporated to dryness on the water bath, and the residue
moistened with strong hydrochloric acid, in order to dissolve
any oxide of iron or other oxides, whilst the silica remains
undissolved in the anhydrous state as a light white powder,
which only requires washing and drying to render it perfectly
pure. Amorphous silica has a specific gravity of 2.2. If, how-
ever, it be heated strongly for a long time its specific gravity
increases, inasmuch as it is then transformed into tridymite.¹
Rock crystal undergoes the same change when the perfectly
pure material is finely powdered and heated to 2,000°, its specific
gravity diminishing from 26 to 2.3.2
Silica melts in the oxy-hydrogen flame to a colourless glass
which may be drawn out in threads. It slowly volatilizes when
maintained at a high temperature in a wind furnace for some
time, and can readily be made to boil when heated in the
electric furnace.*
In all three conditions silica is insoluble in water, and also in all
acids except hydrofluoric, in which it readily dissolves. Crystal-
lized silica is as a rule scarcely attacked by alkalis, whilst the amor-
phous form is soluble, especially when in a fine state of division, but
loses this property to a large extent when ignited. The amor-
phous variety, especially if it contains water, also dissolves in
1 Rammelsberg, Ber. 5, 1006.
3 Cramer, Zeit. Angew. Chem. 1892, 484.
4 Moissan, Compt. Rend. 116, 1122.
2 G. Rose, Ber. 2, 338.
53
818
THE NON-METALLIC ELEMENTS
alkaline carbonates. Hence silica is found in many spring waters,
especially those of the hot Icelandic springs, which on exposure
to air deposit this substance, the alkali silicates being decom-
posed by the atmospheric carbonic acid, while silica and an
alkaline carbonate are formed. The occurrence of silica in
these springs was observed so long ago as 1794 by Black, but
even before that date Bergman had shown that small quantities
of silica were contained in solution in the water of many
springs.
When an alkaline solution of silica is heated in a sealed tube
the glass is attacked and an acid silicate is formed from which
silica separates out on cooling. If the temperature at which
the deposition occurs be above 180°, the silica separates out as
quartz; if below this point, it crystallizes out as tridymite;
whilst at the ordinary temperature of the air it separates in
the form of a hydrated amorphous mass.¹
The various forms of silica are employed in a variety of
technical processes, many of which will be hereafter described,
especially its application to the manufacture of glass and
porcelain. The coloured varieties of silica are also largely used
as gems and for other ornamental purposes. It has recently
been found possible to colour the natural agates artificially.
Thus brown or yellow agates or chalcedonies when strongly
heated are changed into ruby carnelians, the yellow oxide of
iron being thereby changed into the red anhydrous oxide. Many
agates and chalcedonies are permeable to liquids, and in this way
they may be artificially coloured. This fact was known to the
ancients and made use of in darkening the colour of agates.
Thus Pliny describes that in Arabia, agates occur which having
been boiled in honey for seven days and nights become marked
by veins, striæ or spots, and are thus rendered much more
valuable as ornaments, the boiling in honey having the object
of freeing them from all earthy and impure materials. Pliny
was evidently only acquainted with half the mode of procedure,
the object being to saturate the stone with honey, which by
heating was carbonized, and thus brown or black streaks were
produced. This process was kept very secret by the Roman
lapidaries, and for centuries they came to the valley of the Nahe
(where formerly rich agate-quarries existed, and which is still
the chief seat of the agate industry) to buy up badly-coloured
stones and to colour them at home. Not very long ago a trades-
1 Maschke, Pogg. Ann. 145, 549, and 146, 90.
SILICIC ACID
819
man from Idar was imprisoned in Paris for debt, and met there
a "Romaner," who told him the secret, which consists of soak-
ing the stones in sulphuric acid after they have been boiled in
honey, the sugar being thus carbonized and the agate coloured
dark.¹ Agate is also largely used for making the agate mortars
so necessary for the chemist, and rock crystal is employed for
preparing the unalterable weights which he uses in his most
accurate investigations.
SILICIC ACID.
482 Silica belongs to the class of acid-forming oxides, and we
should therefore expect the hydrated acid to have either the
formula Si(OH), or SiO(OH)2.
Orthosilicic acid, Si(OH), is just as little known as is sulphurous
acid, H₂SO,, or carbonic acid, H₂CO,, since, like these acids, it
appears to have a great tendency to split up into water and the
acid-forming oxide. If a solution of an alkaline silicate, termed
soluble glass, be acidified with hydrochloric acid, a portion of
the silicic acid separates out as a gelatinous mass, whilst another
portion remains in solution. If, on the other hand, the solu-
tion is sufficiently dilute, no precipitate will occur, all the silicic
acid remaining dissolved. This liquid contains silicic acid,
hydrochloric acid, and common salt in solution. In order to
separate the two latter compounds from the former, the liquid is
brought into a flat drum, the bottom of which consists of parch-
ment paper, and this "dialyser" containing the liquid is allowed
to swim on the surface of a large volume of water. The sodium
chloride and the excess of hydrochloric acid pass through the
membrane, whilst a clear aqueous solution of silicic acid remains
behind.
This mode of separation was termed "dialysis" by its dis-
coverer, Graham.2 This chemist first pointed out that sub-
stances which crystallize, hence termed "crystalloids," have the
power of passing in solution through a porous membrane, whilst
substances, such as gum and glue, which form jellies and
are termed "colloids," are unable to pass through a porous
diaphragm or septum such as parchment paper. In this
way an aqueous solution of pure silicic acid may be obtained
which contains 5 per cent. of silica, and this may be concen-
1 Ber. über die Entw. Chem. Ind. p. 293.
2 Phil. Trans. 1861, p. 204.
820
THE NON-METALLIC ELEMENTS
trated by boiling it in a flask until it reaches a strength of 14
per cent. When heated in an open vessel, such as an evaporat-
ing basin, it is apt to gelatinize round the edge, after which the
whole solidifies. The solution of silicic acid thus prepared has a
feebly acid reaction and is colourless, limpid, and tasteless. On
standing for a few days the solution gelatinizes to a transparent
jelly. This coagulation is retarded by the presence of a few
drops of hydrochloric acid, or of caustic alkali, but it is brought
about by even the smallest traces of an alkaline carbonate.
If the clear solution be allowed to evaporate in a vacuum
at 15° a transparent glasslike mass remains behind, which,
when dried over sulphuric acid, possesses approximately the
formula, H2SiO3 = SiO2 + H₂O. This has been termed meta-
silicic acid. By drying the gelatinous silicic acid at the
ordinary temperature, it was at one time supposed that
hydrates of a constant composition could be obtained, and it
was believed that the different kinds of opals, which usually
contain water, consisted of hydrates of well-defined composition
corresponding with the different hydrates of phosphoric acid.
Further investigation, however, has shown that the quantity
of water contained in the artificial, as well as in the natural
amorphous silica, varies within very considerable limits, whilst
the water can be partially driven off at low temperatures, so
that it is now supposed to be mechanically and not chemically
combined. In other words, these hydrates must be considered
to be very loose compounds of silica and water.
THE SILICATES.
483 Although no solid hydrate of silicic acid possessing a
constant composition is known, we are acquainted with an ex-
tremely large number of the salts of silicic acid termed silicates,
of which by far the largest proportion occur in nature as distinct
mineral species. Of great theoretical interest also are the volatile
organic silicates or silicic ethers, which correspond in formula
with the hypothetical modifications of silicic acid, the hydrogen
atoms of the latter being replaced by monovalent organic
radicals. Our knowledge of the chemical nature of the mineral
silicates is very limited, and does not, as a rule, extend beyond
the empirical or simplest formula of the compound. Many of
the naturally occurring silicates contain water, which may be
either present as water of crystallization, which is lost at a
THE SILICATES
821
comparatively low temperature, or as water of constitution, in
which case it is only lost on strong ignition.
According to Groth, nearly all the silicates fall into one or
other of the classes contained in the following table, a few
examples of each class being given. The small Roman
numerals placed over the symbols denote the number of atoms
of hydrogen replaced by the metal or group in question. In
many minerals some of the metals are partially displaced by
equivalent amounts of other isomorphous elements.
I. Derivatives of orthosilicic acid, H,SiO..
Ethyl orthosilicate
Sodium silicate
Augite
Leucite
Talc
·
·
Potassium metasilicate
Wollastonite
Enstatite
Olivine
Phenakite.
Dioptase
Muscovite (potash mica).
Common garnet
II. Derivatives of metasilicic acid, HSiO..
Ethyl metasilicate
·
•
Andalusite
Calamine.
Serpentine
•
•
•
·
•
•
•
.
•
•
Mg₂SiO4.
Be₂SiO4.
. H,CuSiO.
iii
•
·
•
•
·
•
·
•
·
(C₂H) SiO4.
Na,SiO4.
·
ii
CaMg(SiO3)2
KAI(SiO2)2
iii
1 ii
. H₂Mg(SiO3)4.
ii iii
Be,Al(SiO3)
·
Emerald
III. Derivatives of disilicic acid, H,Si,O,, and of the polysilicic
acid, H.Si,Og, derived from this and metasilicic acid.
Orthoclase (felspar)
Petalite
(Li; Na)¸Al¸(Si₂O5)15º
IV. Basic silicates, derived from ortho- and metasilicic acids.
iii
Äl(AIO)SiO.
.
KH₂Al(SiO4)3.
Al,Ca,(SiO4)3.
•
iii
(C‚̸),SiO..
KoSiO3.
CaSiOg
MgSiO3.
ii ii
3.
i
1ii
(K; Na) AlSiOg.
1ii
. (ZnOH),SiO3.
ii
i
Mg₂(MgOH)H¸(SiO)2
1 Tabellarische Uebersicht der Mineralien (Vienna, 1882).
822
THE NON-METALLIC ELEMENTS
V. Hydrated silicates.
(a) Zeolites, containing water of crystallization:
Na,Al(AIO)(SiO3)3+2H₂O.
NaAl(SiO3)2 + H₂O.
(b) Containing varying amounts of water loosely combined:
AlSiO + 5H,O.
Allophane
Halloysite
AlSi₂O, + 4H₂O.
Natrolite
Analcime
•
·
·
•
Many silicates have been prepared artificially either by the
fusion of the constituents in the proper proportions with or
without a flux, or by heating them together with water at a high
temperature under pressure. Thus felspar, mica, olivine, and
garnet have all been prepared artificially, and these researches
have thrown much light on the study of the igneous rocks in
which these minerals occur.¹
The silicates of the alkali-metals are the only ones soluble
in water. These form the so-called soluble glass, which dis-
solves the more readily the larger the quantity of alkali it
contains. Helmont was acquainted with this property of the
silicates, and was aware that acids precipitated silicic acid from
such an alkaline solution.
Of the silicates insoluble in water, some, and especially those
which are hydrated silicates containing water in combination,
are easily attacked by hydrochloric acid. In this case the silicic
acid separates out as a gelatinous mass.
In order to decompose the silicates which are unattacked
by acids, it is necessary to fuse them with an alkali carbonate
and afterwards to decompose the fused mass with hydrochloric
acid.
SILICON OXYCHLORIDE, Si,ClO = 283·42.
484 When the vapour of silicon tetrachloride is passed over
felspar, AlKSi,Og, heated in a porcelain tube to whiteness,
potassium chloride and silicon oxychloride are formed, and this
compound is also obtained when a heated mixture of chlorine
with one fifth to one half of its volume of oxygen is passed
over crystallized silicon at about 800°.2 It is a colourless
1 For further information on this point see Fuchs, Die künstlichen Mineralien
(Harlem, 1870). Fouqué and Lévy, Synthèse des Minéraux et des Roches (Paris,
1882). Bourgeois, Reproduction artificielle des Minéraux (Paris, 1884). Traube,
Ber. 26, 2735.
2 Troost and Hautefeuille, Bull. Soc. [2], 35, 360.
SILICON AND SULPHUR
823
fuming liquid boiling at 137°, and is easily decomposed by
water into silicic and hydrochloric acids. Its vapour density
is 10.05 corresponding with the formula given. By the
further action of this oxychloride on heated felspar, or, better,
by repeatedly passing a mixture of oxygen and the oxychloride
through a red-hot tube filled with pieces of porcelain, the
following series of oxychlorides has been obtained by Troost
and Hautefeuille :-¹
Si,OC110
Si,O,Cl
Sig010C112
(Si₂OgCl2)n
(Si₂O,Cl₂)
Boiling point.
152° to 154°
198° to 202°
about 300°
above 400°
440°.
در
The vapour density of the first of these corresponds with half
the molecular weight required for the formula given. The
molecular weights of the second and third have been ascertained
by a determination of their vapour densities, whilst those of the
last two compounds are unknown, but are doubtless multiples of
the simplest ratio.
SILICON AND SULPHUR.
SILICON DISULPHIDE, SIS2.
485 A mixture of silica and carbon, such as is used in the
preparation of silicon tetrachloride, is employed for the prepara-
tion of this compound. The vapour of carbon bisulphide being
led over this mixture heated to whiteness, long silky needles
are formed, which burn in the air to form silica and sulphur
dioxide and are decomposed by water into sulphuretted hydro-
gen and silicic acid. This last substance is thus obtained in
the form of bright shining crystals when the sulphide is exposed
to the action of moist air.2
An orange-yellow coloured volatile sub-sulphide, SiS, and an
oxysulphide of silicon, SiSO, are said to be formed by the action
of sulphuretted hydrogen on silicon at a white heat.3
1 Ann. Chim. Phys. [5], 7, 453.
2 Frémy, Ann. Chim. Phys. [3], 38, 314.
3 Colson, Bull. Soc. Chim. [2], 38, 56.
824
THE NON-METALLIC ELEMENTS
SILICON CHLOROHYDROSULPHIDE, SiCl,SH.
This compound was discovered by Pierre, but its exact
composition was determined by Friedel and Ladenburg.2 In
order to prepare it a mixture of sulphuretted hydrogen and
the vapour of silicon tetrachloride is passed through a red-
hot porcelain tube :-
SiCl + HS SiCl₂HS + HCl.
It is a colourless liquid which boils at 96°, and fumes in tne
air, water decomposing the compound into hydrochloric and
silicic acids, sulphuretted hydrogen and sulphur. Its vapour
possesses a specific gravity of 5·78. ´
=
SILICON AND NITROGEN.
SILICON NITRIDE.
486 When silicon is strongly heated in nitrogen gas, a white
amorphous substance, known as silicon nitride, is produced. If
pure nitrogen is employed, the nitride formed has, according to
Schützenberger, the composition Si,N, and is insoluble in
hydrofluoric acid. When the experiment is carried out, as it
was by Deville and Wöhler, by heating silicon in a crucible
placed inside another which was packed with carbon to pre-
vent the access of oxygen, the product contains silicon carbo-
nitride, Si,C,N, silicon carbide, SiC, silicon carboxide, SiCO,
and a nitride of the formula Si,N, which dissolves in hydro-
fluoric acid with formation of ammonium silicofluoride.5
A white amorphous powder was also obtained by Deville and
Wöhler by the action of ammonia on silicon chloride, which,
according to Gatterman,7 contains a substance of the compo-
sition SiN,H. According to Schützenberger and Colson the
product of the reaction, after ignition in a current of hydrogen,
has the composition Si N Cl₂, and when ignited in a current of
ammonia yields a substance of the formula HS,N, which is
soluble in hydrofluoric acid and evolves ammonia when treated
with caustic potash.8
1 Ann. Chim. Phys [3], 24, 286.
3 Compt. Rend. 89, 644; 92, 1508.
3 Compt. Rend. 114, 1089.
7 Ber. 22, 1, 94.
2 Ann. Chim. Phys. [4], 27, 416.
4 Annalen, 110, 248.
6 Annalen, 104, 256.
8 Compt. Rend. 92, 1511.
SILICON AND CARBON
825
SILICON AND CARBON.
SILICON CARBIDE, SIC.
487 Silicon carbide is formed when a mixture of coke, sand
and salt is fused in an electric furnace by means of a carbon
terminal,¹ and may be obtained pure by fusing silicon with the
requisite amount of carbon in the electric furnace. It is also
formed by the action of carbon on iron silicide and by the
union of the vapours of carbon and silicon in the electric
furnace, needles of the carbide being deposited (Moissan).
The crystals when pure are colourless or sapphire blue and are
usually found as six-sided plates, but when formed by the com-
bination of the two vapours they are prismatic. Their specific
gravity is 312 and they are hard enough to scratch rubies.
Silicon carbide is not oxidized by oxygen at 1000° and is not
attacked by sulphur vapour, by fused potassium nitrate, or by
any acid. It is completely decomposed by chlorine at 1200°
and on fusion with lead chromate it is gradually oxidized, whilst
fused caustic potash slowly converts it into potassium carbonate
and silicate.
The crude material, obtained by the method first described,
is known as carborundum and is used as a cutting and polishing
agent.
SILICON CARBOXIDE, SICO.
When silicon is heated to whiteness in an atmosphere of
carbon dioxide, a greenish white mass is left, which after treat-
ment with hydrofluoric acid has the composition (SiCO),.
3Si + 2CO₂ = SiO, + 2SiCO.
2
This compound is not acted on by alkalis and is not affected by
being heated in oxygen, but is oxidized by a mixture of oxide
and chromate of lead. At a lower temperature silicon carbide
is formed.3
Amorphous substances of the empirical formula, SiCO,
Si,C₂O, and Si,CO₂ have also been obtained by analogous
methods.4
¹ Mühlhäuser, Zeit. Anorg. Chem. 5, 105.
2 Moissan, Compt. Rend. 117, 425.
3 Schützenberger and Colson, Compt. Rend, 92, 1508; 114, 1087.
4 Colson, Bull. Soc. Chem. [2]. 38, 56.
826
THE NON-METALLIC ELEMENTS
CONSTITUTION OF THE VOLATILE SILICON COMPOUNDS.
488 Silicon forms a series of compounds which are volatile, and
the molecular weight of which can, therefore, be readily ascer-
tained. When this has been determined, it appears that the
chemical constitution of these bodies, that is to say, the mode
in which the different atoms are arranged in the molecule, can
be most simply expressed on the assumption that silicon is a
tetravalent element. The following graphic formulæ show the
constitution of the most important of these compounds :-
Silicon tetrachloride,
Cl
Cl-Si-Cl
_c1
CL
Silicon trichloride,
CI
Cl-Si-Cl
Cl-Si-Cl
Cl.
Silicon oxychloride,
CI
Cl-Si-Cl
Cl-Si-Cl
Cl.
Silicon chlorhydrosulphide,
CI
Cl-Si-S-H
di
CRYSTALLOGRAPHY
827
CRYSTALLOGRAPHY.
489 When a chemical substance passes from the gaseous or
the liquid state into that of a solid, it generally assumes a
definite geometrical form, and is said to crystallize.
A crystal is a solid body, formed in this way, and bounded by
plane surfaces. As a rule, every chemical substance in the
solid state possesses a distinct form in which it crystallizes, and
by which it can be distinguished.
The occurrence of various mineral substances in distinct
crystalline forms was noticed by the ancients, and they gave the
name crystal (púστaλλos, ice) to one of these, viz., to quartz or
rock-crystal, because they believed that this body owed its for-
mation to the effect of cold. Geber was aware that crystals
can be obtained artificially by the evaporation of a solution of
a salt. He describes the production of several chemical com-
pounds in the crystalline condition, and shows how they may
be purified by recrystallization. Many years, however, elapsed
before this property of matter was regarded as anything more
than an unimportant one. Libavius, it is true, asserted in the
year 1597, that the nature of the saline components of a mineral-
water could be ascertained by an examination of the crystalline
forms of the salts left on the evaporation of the water. But
so imperfect were the views regarding the formation of crystals
that Lemery classed crystals simply according to their thickness,
believing that this is dependent upon the size of the ultimate
particles of the acids contained in the salts. In the year 1703,
Stahl pointed out that the compounds obtained by the action.
of acids upon sea-salt crystallized in forms different from those
assumed by the corresponding compounds of potash, and hence
he concluded that sea-salt contains a peculiar alkali, distinct
from the common alkali potash. Gulielmini appears to have held
much more rational views than Lemery concerning the formation
of crystals. In his Dissertatio de Salibus, published in 1707,
he asserts that the smallest particles possess definite crystalline
828
CRYSTALLOGRAPHY
forms, and that the differences in form which we observe between
the crystals of alum, nitre, and sea-salt, are caused by similar
differences existing in the forms of the smallest particles.
It is, however, to Haüy (1743-1822) that we are indebted
for the foundations of the science of crystallography. Haüy was
the first to point out the fact that every crystalline substance
possesses certain definite and characteristic forms in which it
crystallizes, and that all these different forms can be derived from
one fundamental form which can be ascertained by measure-
ment of the angles of the crystal. Upon this principle he
founded in 1801 his celebrated system of the classification of
minerals.
General Characteristics of Crystals.-A crystal of any sub-
stance is characterised, in the first place, by its special
geometrical form, and in the second place, by its physical
properties, both of which we must suppose to be due to the
special mode of arrangement of the particles of which the crys-
tal is built up. In an amorphous, or non-crystalline, substance
the physical properties are as a rule the same in every direction
throughout its mass, whilst in a crystalline substance this may
not be, and usually is not, the case. Thus, for example, under
ordinary conditions light travels through a piece of glass at the
same rate in whatever direction it may happen to pass, whilst
in a crystal of quartz or calc-spar the rate is different in different
directions. The physical properties are even more character-
istic of a crystalline substance than the geometrical form which
it assumes, and can be recognised in fragments of the substance
in which the characteristic crystalline form may be quite un-
recognisable.
Symmetry of Crystals.-Any plane which divides a crystal
into two parts which are geometrically similar, so that one part
bears the same relation to the other as it would to its image in
a plane mirror placed in the same position as the dividing plane,
is called a plane of symmetry, whilst a direction at right angles
to this plane is known as an axis of symmetry. Thus in Fig. 230,
representing a double pyramid with a square base, there are
shown three planes of symmetry, ABCD, AECF, BEDF, each of
which divides the figure into two symmetrical halves, and three
axes of symmetry EF, AC, and BD. These three planes have
not however the same degree of symmetry. If the whole figure
be rotated about the axis EF through an angle of 90°, the axis
AC will take the position BD, and a figure precisely similar to
SYMMETRY OF CRYSTALS
829
the original will be obtained. If, however, the rotation be
made about BD this will not be the case; the axis EF will take
the direction AC, but, since it is not equal to AC, the resulting
figure will be different from the original, and in order to obtain
a similar figure the rotation must be continued through an
additional angle of 90°. The axes AC and BD are said to be
equivalent, and the plane ABCD which contains two equivalent
axes is called a principal plane of symmetry, whilst the planes
AECF and BEDF are termed secondary planes of symmetry.
The corresponding axes are similarly distinguished. It has,
moreover, been found that every plane of geometrical symmetry
of a crystal is also a plane of physical symmetry, the two parts
B
E
FIG. 230.
into which the crystal is divided being not only geometrical but
also physical counterparts.
Classification of Crystals.-The classification of crystals is
based on the degree of symmetry which they possess, this being
determined by the number of principal and secondary planes of
symmetry which characterise them. All the possible forms of
crystals are comprised in the following six systems :-
Name of System.
Regular or Cubic.
Degree of Symmetry.
1. Three principal planes, cut-
ting one another at 90°.
Six secondary planes, cutting
one another at 60°,
1
⚫830
CRYSTALLOGRAPHY
Degree of Symmetry.
2. One principal plane.
(a) Six secondary planes, cut-
ting one another at 30°.
(b) Four secondary planes,
cutting one another at
45°.
3. No principal plane.
(a) Three secondary planes,
cutting one another at
90°.
(b) One secondary plane.
(e) No secondary plane.
0
0
0
FIG. 231.
Parts of a Crystal, Faces, Edges, and Angles.-The plane sur-
faces by which crystals are bounded are termed faces. The
straight lines formed by the intersection of two contiguous
∞0∞
∞0∞
Name of System.
Hexagonal.
Quadratic.
Rhombic.
Monosymmetric.
Asymmetric.
FIG. 232.
8
O
da000
O
FIG. 233.
GOOD
faces are termed edges. Angles are made by the incidence of
the bounding surfaces or faces, and are sometimes termed solid
angles or summits, and are distinguished as three-faced, four-
faced, &c., according to the number of faces by which they are
formed.
Similar faces are those which resemble each other in form
and have a similar relative position. Dissimilar faces are those
which are unequal, and occupy different positions. The crystals
represented by Figs. 231 and 232 are bounded by similar faces;
that shown in Fig. 233 is bounded by dissimilar faces. In Fig.
233 the four-sided faces lying between four trianglar faces are
similar, and these latter are also similar.
Simple and Complex Forms.-A crystal which is bounded
PARTS OF A CRYSTAL
831
entirely by similar faces is termed a simple form. A form in
which dissimilar faces occur is termed a complex form, or a
combination. Every complex form is made up of two or more
simple forms. This will be understood if we imagine one
set of its similar faces to be extended until all dissimilar
faces have disappeared. Thus, if the triangular faces in Fig.
8
∞0∞
P
P
FIG. 234.
a P
0
FIG. 236.
0
233 be extended until they inclose space, the form will become
that of the octohedron (Fig. 231); whereas a similar extension in
the square faces gives rise to a cube (Fig. 232).
Faces are said to be dominant when they are the largest
present in a given combination, or when their simplest form
most closely resembles that of the combination. Subordinate or
∞ P
T117
808
FIG. 235.
OP
OP
∞P
&0&
FIG. 237.
A
secondary faces, on the other hand, are the smallest ones seen in
a combination, or those which are relatively unimportant, and do
not determine the form of the combination. Figs. 233, 234, and
235 represent combinations of the octohedron and the cube. In
Fig. 233 the faces of both forms are equally developed; in Fig.
234 the octohedral faces are dominant; in Fig. 235 those of the
cube.
832
CRYSTALLOGRAPHY
The simple dominant form is termed the fundamental form ;
simple forms are again distinguished as closed and unclosed
forms. The first of these classes includes all those whose faces,
or faces produced, inclose space. Such a form is shown for the
faces marked P, Fig. 236, which will, if produced, give rise to the
double six-sided pyramid. The second class is made up of forms
which cannot thus inclose space, and an example of such a
form is seen in Fig. 237, in which the faces marked Pare
arranged in directions parallel to each other, and cannot inclose
space. Fig. 237 is a crystal made up of two open forms, the
faces P, and the faces OP.
Crystallographic Axes and Symbols. For the sake of conveni-
ence it is usual to select three (or in the hexagonal system four)
lines intersecting in a point as axes about which the faces of
the crystal are disposed. Whenever possible, axes of symmetry
are chosen for this purpose, and are termed the crystallographic
axes. Thus, in the regular system, which is characterised by
three principal planes of symmetry, the three principal axes of
symmetry are taken as the crystallographic axes, these being all
at right angles to each other; whilst in the rhombic system,
which has no principal but three secondary planes of symmetry,
the secondary axes, which are also at right angles, are chosen.
In order to briefly express the position of any face with refer-
ence to these axes, the lengths cut off by this face from the
three axes are expressed in terms of the lengths, cut off by a
face of one of the simple forms of the crystal, the simplest
pyramid, each face of which cuts all the three axes, being
generally taken. The ratio of the lengths thus cut off from the
axes by this fundamental face is known as the axial ratio.
In the regular system the simplest pyramid, the regular
octohedron (Fig. 231) cuts all the axes at equal distances from
their point of intersection, since the three axes are interchange-
able, and hence the crystal made up of such faces is known by
the formula a: a:a (Weiss), or still more shortly O (contracted
from octohedron; Naumann). A crystal of which each face cuts
one of these equal axes at the distance a, a second at 4a, and the
third at 2a, is similarly known as, a: 4a 2a or more shortly
402, the numbers 4 and 2 being known as the parameters of
the face.
Another system of symbols, known as Miller's index system,
is also largely used. The fundamental face (0) is represente l
by the formula (111), whilst the position of any other face is
given by indices which are the reciprocals of the parameters.
Thus in the case of the face a: 4a: 2a, the parameters are
1:4:2, and the indices therefore 1:1/4: 1/2, or, removing
fractions (412). In the rhombic system the face of the simplest
pyramid cuts the three axes at different distances a : b: c, con-
tracted to P (pyramid); a face cutting the axis a at the distance
2a, the axis b as before, and the axis c at 2c, would be expressed
as 2a: b: 2c (Weiss); 2P2 (Naumann); 121 (Miller).
If a face is parallel to one of the axes its parameter is infinity
(∞), and its index the reciprocal of this or zero.
It has been found experimentally that only faces of which the
indices may be expressed by rational numbers occur on any
crystal.
HEMIHEDRAL FORMS
Holohedral, Hemihedral, Tetartohedral, and Hemimorphous
Forms.-A crystal which is bounded by all the faces required by
Olen
+8
FIG. 238.
833
C
FIG. 239.
the symmetry of the system to which it belongs is said to be
holohedral. Thus in the regular system, the octohedron is a
holohedral form, because, starting with one face cutting all
three axes at equal distances, the symmetry of the system
requires seven other faces fulfilling the same condition, making
in all the eight faces of the octohedron. In many cases, how-
ever, only one half of these faces is present on a crystal, which
is then said to be hemihedral. Thus, if the four faces afd, fbc,
bga, and gcd of the octohedron in Fig. 239 be extended, a closed
form (Fig. 238), known as the regular tetrahedron, is obtained,
whilst the extension of the remaining four faces gives rise to a
similar figure (Fig. 240). These two forms, although identical
in shape, are distinguished as positive and negative, and may
occur in combination with other forms either alone or together.
Figs. 241 and 242 represent combinations of the cube with a
positive tetrahedron.
"
19
54
834
CRYSTALLOGRAPHY
When the two tetrahedra are equally developed on the same
crystal the form produced is identical with the original octo-
hedron.
When only one quarter of the faces of the holohedral form
occurs, the crystal is said to be tetartohedral.
Hemihedral crystals always have a smaller degree of geo-
AUX
FIG. 241.
FIG. 240.
FIG. 242
metrical symmetry than the holohedral forms of the same
system; the physical symmetry, however, remains unaltered.
In other cases certain faces appear at one end of an axis of
symmetry but not at the other, the crystal being thus made up
of two dissimilar portions (Fig. 308). Such crystals are said to
be hemimorphous, and exhibit remarkable physical differences at
the two dissimilar ends.
FIG. 243.
FIG. 244.
Single Crystals and Twin Crystals, or Macles.-Holohedral
crystals having their faces symmetrically arranged about
their axes are termed single crystals in contradistinction to
those which, although formed according to a certain law, are not
subject to the same symmetrical arrangement. This latter class
are called twin crystals. In one portion of such crystals the
axes lie in a different position from that which they occupy in
TWIN CRYSTALS
835
FIG. 245.
another portion, and the twin may be supposed to be derived
from the single or normal crystal by the latter having been cut
into two portions parallel to a certain plane, usually parallel to
a possible face of the crystal, and one part having been turned
Found on the other through a given angle. Fig. 243 represents

FIG. 247.
FIG. 246.
such a twin crystal or hemitrope, often found in magnetic iron
ore, and obtained by turning one half of an octohedron (Fig.
244) through an angle of 180° on the other half. Fig. 245
exhibits a twin form often observed in gypsum.
These twin forms are distinguished from single crystals by
FIG. 248.
the occurrence in them of re-entering angles, as shown in the
figures. Intersecting twin crystals, as opposed to twins by contact,
such as that shown in Fig. 246, frequently occur in fluorspar
and in sal-ammoniac.
Perfect Crystals and Imperfect or Distorted Crystals.-It
generally happens that crystals found in nature are distorted
836
CRYSTALLOGRAPHY
or irregularly developed. For instance, crystals of quartz usually
occur in the forms shown in Figs. 247, 248, and 249, which
apparently bear no relation to the regular and much rarer form
of quartz seen in Fig. 250. There is, however, no difficulty
in ascertaining the simple form of these crystals in spite of
FIG. 249.
FIG. 251.
& P
P
∞ P
P
8
FIG. 252.
P
FIG. 250.
this apparent want of conformity due to the increase of certain
faces and the decrease of others, and in spite even of the
fact that only half of the complete crystal is often seen, as in
Fig. 248.
If the angles which the similar faces in the perfect and the
imperfect crystals make with one another be carefully measured,
we find that the corresponding angles in the distorted forms
are always identical with those in the perfect crystal. Thus,
for instance, the contiguous faces of the prism cut each other
at a constant angle of 120°, whilst each face of the pyramid cuts
the next face at an angle of 130° 44'.
MEASUREMENT OF CRYSTALS
837
Similar distorted growths are observed in the case of many
artificial crystals. These are produced by the undue develop-
ment in some special direction, owing to the alteration of
external circumstances, of the perfect crystal which is first
formed. Thus crystals of alum, which are octohedral, appear
not unfrequently in the form shown in Fig. 251, which is
derived, as is seen in Fig. 252, by an irregular growth of the
octohedron.
In describing or drawing the crystalline form of any chemical
substance, the ideal forms only are considered. All irregulari-
ties and distortions are ignored, and the corresponding faces of
the crystal are supposed to be placed at equal distances from
the origin.
Artificial Growth of Crystals.-The small crystals which are
first deposited from a solution are usually perfect. If these
are placed in a concentrated solution of the substance, and
carefully turned every day so that all the faces of the crystal
are equally exposed to the action of the solution, large and
perfectly developed crystals can be obtained. The same end
is attained by hanging a small and perfect crystal in the
saturated solution suspended by a fine hair. The solution
gradually evaporates, and the crystal grows symmetrically.
Perfect crystals of alum of large dimensions can in this way
readily be grown.
Determination of Crystalline Forms and Axial Ratios.—The
angles which the faces of a crystal make with one another
serve as the data from which the crystalline form is determined,
for it is by the measurement of these angles that the relative
length and the mutual inclination of the axes can be ascertained
by calculation.
The instruments used for this purpose are the hand gonio-
meter (Fig. 253) and the reflecting goniometer (Figs. 254 and
255). The first of these instruments was made in the last
century by Carangeot of Paris, for the use of the French
crystallographer Romé de l'Isle. It can only be used for the
measurement of tolerably large crystals, and consists of a divided
semicircle, a, b, d, to which two metallic rules are adapted.
One of these (km) is fixed, the other (gh) is movable round.
an axis (c) placed at the centre of the semicircle. The crystal
to be measured is placed between the rules (gh) and (km), so
that the edges of these rules may both be at right angles to the
line of intersection of the two faces whose angular distance is to
838
CRYSTALLOGRAPHY
be measured. The angle is then read off on the divided circle.
This instrument, though very useful, and indeed necessary for
large crystals, is quite inapplicable to small ones, and in any
case cannot yield very accurate measurements.
120 110
100
FIG. 253.
B
To obviate these difficulties the reflecting goniometer was
invented by Wollaston in the year 1809. It is arranged on the
following principle. The divided circle gh (Fig. 254) carries a
moveable axis or arm upon which the crystal to be examined is
FIG. 254.
fixed. This crystal, shown in section in a bed, fixed on the arm
by means of some wax, must now be placed in such a position
that the edge (c), the angle over which (b c d) has to be measured,
is placed exactly parallel with the axis of the instrument.
MEASUREMENT OF CRYSTALS
839
P
OFE
M
Having placed the goniometer opposite a window, the eye of
the observer being at (0), the reflection of one rib of the window
in one bright face of the crystal is noticed. The divided circle
is now moved round, the crystal moving with it, until the
second face (de) of the crystal comes round to the position
formerly occupied by the first (cb), as ascertained by the re-
appearance of the rib of the window frame. The angle through
which the crystal has been turned is evidently the supplement
a
y
K
-
E 470
480
150

140
130
120
1910
100
90
80
70
60
40
FIG. 255.
R
C
G
H
ST
I
of the required angle, so that if the pointer (p) stood at 0° to
begin with, and after the circle was turned at 2, the angle of
the crystal is 180°-°. A simple form of reflecting goniometer
is seen in Fig. 255. The crystal (a) is fastened with wax upon
the end of the moveable rod (oo), which can be bent so as
to enable the crystal to be properly adjusted. The screw (G)
serves to turn the divided circle (E), so that 180° on the circle
is made to coincide with the zero point on the vernier (R).
840
CRYSTALLOGRAPHY
When this is done the screw (U) is tightened, and thus the
circle is held fast.¹
Cleavage of Crystals.-The cohesion of a crystal is as a rule
less in one direction than in another. This direction is de-
pendent upon the special form of the crystal, and is termed
the cleavage. This facility of breaking more readily in one
direction than another is well seen when calc-spar or rock salt
is broken. The planes of cleavage in a crystal can be ascertained
by means of a chisel and hammer, or by the help of a strong
knife.
I. THE REGULAR, CUBIC, OR ISOMETRIC SYSTEM.
490 This system is characterised by three principal planes
of symmetry, all at right angles, and six secondary planes of
symmetry cutting one another at an angle of 60°.
The three principal axes of symmetry are chosen as the
crystallographic axes, and they are therefore all equal and at
right angles to each other.
O
0
O
FIG. 256.
808
∞O∞
FIG. 257.
*0*
The simplest form of the system is the regular octohedron
(Fig. 256). Each of its eight faces cuts the three axes at an
equal distance from the origin. This distance being (a) the
general formula for this form is a : a: a (Weiss), abbreviated to
the symbol 0 (Naumann). Many substances crystallize in octo-
hedra, amongst others spinelle, mercury, magnetic oxide of iron,
alum, and lead nitrate.
The next simple form is the cube (Fig. 257). Each face of
the cube cuts one axis at the distance (a) and lies parallel to
1 For a description of a much more accurate instrument Groth's Physikalische
Krystallographie may be consulted.
THE REGULAR SYSTEM
841
0
the other two axes.
The formula for this form is oo a: a: ∞ a,
or ∞o 0 ∞. The combinations of octohedron and cube have
been already mentioned (Figs. 233, 234, and 235). Sodium
chloride, potassium iodide, and fluor-spar are substances which
crystallize in cubes, whilst galena, sulphide of silver, and lead
nitrate occur in various combinations of octohedron and cube,
∞0
FIG. 258.
€0
0ܣ
∞0
80
∞0
FIG. 260.
O
∞0
which are produced by a truncation of the six summits of the
octohedron to a greater or less degree.
The third simple form is the rhombic dodecahedron (Fig. 258).
The formula for this form is a: a: o a, or abbreviated ∞ 0, as
each face of this crystal cuts two of the axes at the same distance
from the origin, whilst it is parallel to the third axis. The
0∞
∞α
FIG. 259.
∞0
808
0ܣ
ܣܘܣ
80
FIG. 261.
Oa
0
000
rhombic dodecahedron combines with the octohedron by re-
placing each of its twelve edges, as is shown in Fig. 259. The
combination in which the dodecahedron is dominant is seen
in Fig. 260. Fig. 261 indicates the form obtained by the
combination of cube and dodecahedron. The following sub-
stances crystallize in dodecahedra and its combinations: garnet,
phosphorus, cuprous oxide (Fig. 259), magnetic oxide of iron
842
CRYSTALLOGRAPHY
(Fig. 260), and alum when deposited from an alkaline solution
(Fig. 261).
Each of the simple crystals which we have described is in-
capable of assuming more than one form. Other crystalline
forms, belonging to the regular system, occur which can exist in
several modifications. The first of this class of forms is the
202
202/202
202 202
FIG. 262.
0
Icositetrahedron (Fig. 262). The formula of this form is a : ma: ma,
m0m. This signifies that if a face cuts one axis at the distance
(a) it cuts the two other axes at a different distance, m times
(a), when the face is produced. The form a: 2a: 2a, or 202
(Fig. 262) occurs frequently. This means that the distance at
which two axes cut the face produced is twice as great as that
FIG. 264.
∞0∞
202
FIG. 265.
50S
202
303
∞0
FIG. 263.
∞0
202
202
∞0
202
0
FIG. 266.
at which the third axis cuts it. Fig. 263 has the formula
a: 3a: 3a, and other forms such as a:a: a are also found,
though not so frequently. Combinations of the icositetrahedron
with the foregoing forms are seen in Figs. 264, 265, and 266.
The simple form (Fig. 262) occurs in silver glance. The com-
bination (Fig. 264) has been observed in spinelle; Fig. 265
occurs in analcime, and Fig. 266 is often noticed in garnet.
THE REGULAR SYSTEM
843
20
The Triakisoctohedron, or pyramid octohedron (Fig. 267),
is represented by the formula a: a: ma, or m0. The value of
m is generally 2. The simple form does not occur frequently,
but it is often seen in combination. If a four-sided pyramid
replace each side of the cube we obtain the Tetrakishexahedron
(Fig. 268). Each face in this form cuts an axis at the distance
FIG. 267.
30
FIG. 270.
20
FIG. 268.
800
∞0∞
B
(8
FIG. 269.
(a) from the centre; the second axis at the distance (ma); and
the third axis at the distance co. Hence the general formula
for this is a ma:coa, abbreviated into m0 co. The more
common forms are 20 ∞ and 30 ∞. The first of these occurs
in gold and copper (Fig. 268), whilst the second form is found
FIG. 271.
xO∞
80)
xOx
in fluor-spar, when it usually occurs with the cube, as seen in
Fig. 269.
The last holohedral form of the regular system is the Hexa-
kisoctohedron (Fig. 270), or the forty-eight sided figure. This
form is represented by a : ma: na, or m 0 n. Of these, the forms
30, and 402 most frequently occur, although generally met
with in combinations. The form 302 is found in garnet; 4 0 2
2
844
CRYSTALLOGRAPHY
in fluor-spar. A combination of this form with the cube is seen
in Fig. 271.
The hexakisoctohedron, m 0 n, is the most complicated of all
the holohedral forms of the regular system, and the other six
forms may be represented as special cases of this more general
expression. This is seen, if special values are given to m and n;
thus, m=n; m = 1; m = ∞o, &c. The relations between this and
the other forms of the regular system are shown by the following
diagram:-
*0.
Ola
-------
10/2
FIG. 273.
FIG. 275.
m0
mOn
Hemihedral forms of the Regular System. The tetra-
hedron (Fig. 238) is one of the most important of these
0n
FIG. 272.
m0m
∞
0
BOB
0109
010
FIG. 274.
+91 9
FIG. 276.
Oog
forms. This is derived by the extension of the alternate
faces of the octohedron until the other faces disappear. The
formula of the tetrahedron is
0
2'
and the two forms (p. 833)
THE REGULAR SYSTEM
845
D
are further distinguished by the signs (Fig. 238) and
(Fig. 240). A combination of the two tetrahedra is seen in
Fig. 273; Figs. 274 and 275 are combinations of the tetrahedron.
and cube, and Fig. 276 is a tetrahedron on which the faces of
the rhombic dodecahedron appear. The following substances
crystallize in tetrahedra: sodium sulphantimoniate, sodium
+202
FIG. 277.
FIG. 279.
D
B-202
m0m
2
FIG. 278.
chlorate, cuprous chloride, and fahl-ore. Combinations of two
tetrahedra are frequently seen in boracite. In this case the two
tetrahedra are equally developed, so that the crystal appears to
be an octohedron, but it can be distinguished from this by the
fact that four of its faces are bright, whilst the other four
(B
gla
D
FIG. 280.
-
are non-reflecting. The triakistetrahedron is derived from the
icositetrahedron, and therefore exists in several modifications.
and each form can have a
The general formula
is
positive or negative position. These occur as complete forms,
and also in combination, as in the case of fahl-ore (Figs. 277
and 278).
846
CRYSTALLOGRAPHY
The hemihedral forms of the triakisoctohedron are termed
deltoid-dodecahedra (Figs. 279 and 280). Their general formula
m0
is and they may be either positive or negative.
2
D
+50%
FIG. 281.
+
A
The hexakisoctohedron in like manner yields hemihedral
forms, termed Hexakistetrahedra (Figs. 281 and 282). These
FIG. 283.
30%
2
2000
2
D
FIG. 285.
-50%
FIG. 282.
forms are found in combination in fahl-ore and boracite, and
alone in diamond.
303 B
YOU
THE
Έ
FIG. 286.
FIG. 284.
+20
∞O∞
FIG. 287.
Another class of hemihedral forms is also derived from the
hexakisoctohedron. These are termed dyakisdedecahedra, or
THE HEXAGONAL SYSTEM
847
trapezoid dicositetrahedra (Figs. 283 and 284). In order to dis-
tinguish these forms from those of the hexakistetrahedron the
faces are represented by the same general formula, but bracketed
thus,
They occur in cobalt glance and iron pyrites.
The pentagonal dodecahedron is the hemihedral form of the tetra-
m0 x
kishexahedron, and therefore has the formula
The form
(mon).
2
20 x
2
occurs most frequently in pyrites and cobalt glance (Figs.
285 and 286). The combination shown in Fig. 287 is also
found in the same substance.
In addition to the forms already mentioned, another hemi-
hedral form, the pentagonal icositetrahedron, and a tetartohedral
form, the tetrahedral pentagondodecahedron, are also derived
from the hexakisoctohedron.
P
II. THE HEXAGONAL SYSTEM.
491 This system is characterised by one principal plane of
symmetry and six secondary planes of symmetry making angles
of 30° with one another. The principal axis of symmetry is
chosen as the principal crystallographic axis, and is placed
P
FIG. 288.
5
FIG. 289.
OP
op
∞P
FIG. 290.
xP
vertically; three of the secondary axes of symmetry, which lie
in a plane perpendicular to the vertical axis being taken as
secondary crystallographic axes. These are all equal, but are
not equal to the vertical axis in length, and meet in a point
making angles of 60°.
Each face of the simplest pyramid (Fig. 289) cuts the prin-
cipal or vertical axis and two of the secondary axes, but runs
848
CRYSTALLOGRAPHY
parallel to the third. The formula for the pyramid is there-
fore a: aac, in which the letter c refers to the principal
axis, or abbreviated P. The vertical axis of an acute pyramid
is of course longer than the secondary axes, whilst that of an
obtuse pyramid is shorter. Thus, for example, the relative
length of the axes in the case of the acute pyramid of quartz is
11:1, whilst in the case of the obtuse pyramid of beryl it is
0.498: 1.
If, in the pyramid, the length of the principal axis be reduced
to 0, a face known as OP will be produced, which is parallel to
the plane containing the three secondary axes, whilst if the
length of this same principal axis be increased indefinitely, the
pyramid becomes an open six-sided prism (Fig. 290), the symbol
of which is a a: ac, or P. This prism may occur
FIG. 291.
28
along with the face OP in the closed combination shown in
Fig. 290, 0P, ∞ P.
The prisms and pyramids, of which each face cuts two of the
secondary axes at equal distances, and is parallel to the third,
so that the secondary axes pass through the angles (Figs. 289,
290), are termed forms of the first order. A second series of
similar forms occurs in this system, which may be derived from
the forms of the first order by rotating them through an angle
of 30°, so that the secondary axes cut the centre of the six
sides of the base of the pyramid. Such prisms and pyramids
are termed forms of the second order. Fig. 291 exhibits the
relation between these two series of forms. It will be seen that
in the pyramid of the second order which is there represented
in section, each face cuts one of the secondary axes at the
THE HEXAGONAL SYSTEM
849
distance a, and the other two axes at the distance 2a. The
formula of the pyramid of the second order is accordingly
2a:a: 2a c, or P2, whilst that of the corresponding prism of
the second order is 2a: a : 2a, ∞ c, or ∞ P2.
In the same way faces may occur which cut the vertical
axis at a greater or less distance than c from the centre, so that
the general formula for the pyramid of the first order is mP,
whilst that of pyramids of the second order is mP2. In the
case of the combination of several pyramids in which the value
of m varies, these values, in accordance with the general law
(p. 833), exhibit a simple ratio, such as 1: 2, 1: 3, 1: 5, etc.
Combinations of pyramids of the first and second orders also
occur; when they are equally developed, the combination assumes
FIG. 292.
the form of a symmetrical twelve-sided pyramid, the formula
for which is a na: pa: mc, abbreviated into mPn (Fig. 292).
This is in reality the most general form possible in the system
from which all the other forms may be derived. Thus when
n=1 and p = ∞, it becomes an hexagonal pyramid of the first
order, whilst when po it becomes a pyramid of the second
order.
The closed holohedral forms of the hexagonal system do not
occur frequently. On the other hand, combinations such as
shown in Fig. 293 are found in quartz; Fig. 294 exhibits a
form found in calc-spar, and Fig. 295 a combination seen in
apatite.
The hemihedral forms in the hexagonal system are
numerous and important, occurring even more frequently than
55
850
CRYSTALLOGRAPHY
the holohedral forms. The most important of these are the
rhombohedra (Fig. 296), which are derived from the hexagonal
x P
P
& P
1
P
R
FIG. 293.
P.
P
+R
FIG. 296.
+R
20 P
pyramid by extending the alternate faces until they produce a
closed form (Fig. 297). These faces have the symbol or R.
P
FIG. 298.
OP
OP
FIG. 294.
Calc-spar, iron-spar, and sodium nitrate occur in the form of
simple rhombohedra. The rhombohedron, like the tetrahedron,
-R
P
-R
-R
FIG. 299.
FIG. 295.

FIG. 297.
2R
+R
+R
-21
R
FIG. 300.
+R
R
may occupy two positions according as the one or the other set
of alternate faces of the six-sided pyramid is extended. These
THE HEXAGONAL SYSTEM
851
are shown in Figs. 298 and 299; the two rhombohedra are
distinguished by the signs + and -. Combinations of the rhom-
bohedron also frequently occur.
One of these is represented by
Fig. 300, which exhibits a combination of several positive and
R
+4 R.
+4R
+4 R
FIG. 301.
a P
R
-P
R
R
FIG. 302.
........
P
∞P?
∞0P2
FIG. 303.
P2
negative rhombohedra, occurring in chabasite. Fig. 301 repre-
sents a combination of two rhombohedra which are both
positive, but differ in axial ratio, a form which occurs in calc-
spar. A combination of the rhombohedron with
FIG. 304.
a prism of the first order is seen in Fig. 302;
this form has also been observed in calc-spar.
A combination of a rhombohedron with a
prism of the second order is shown in Fig. 303.
This has been observed in dioptase.
A second hemihedral form of this system is
the scalenohedron. This is obtained from the
symmetrical twelve-sided pyramid, by extend-
ing the alternate pairs of planes, thus giving
rise to a positive (+) and negative (-)
scalenohedron. If we suppose a rhombohedron
to be placed within a scalenohedron, it is
termed the inscribed rhombohedron, and the
lateral edges of both forms will be seen to
have a similar position with regard to the
axes. It is clear that the scalenohedron may
be supposed to be derived from such a rhom-
bohedron by an elongation of the axis to the
distance n, lines being drawn from the end of
this axis to the lateral solid angles of the rhombohedron, as seen in
Fig. 304. Although in this way an infinite number of scaleno-
hedra may be formed, we find that only those forms actually
852
CRYSTALLOGRAPHY
exist in which the length of the primary axis bears some simple
ratio to that of the inscribed rhombohedron. The general
symbol for the scalenohedron is mR". The axis in the case
of the commonest scalenohedron occurring in calcite is three
o P
P
P
Pu
a P
❤.....
FIG. 305.
FIG. 306.
times as long as that of the inscribed rhombohedron, hence
the symbol for this scalenohedron is + R³.
Tetartohedral Forms.-In addition to some other forms of
hemihedry, a number of tetartohedral forms occur in this
system. Such forms, shown in Figs. 306 and 307, occur in the
R
R
2R R
FIG. 308.
FIG. 307.
R
P₂
case of quartz. Two forms, known as trigonal trapezohedra, are
possible, both derived from the twelve-sided pyramid, and these
two are not congruent, so that when they occur as in Figs. 306
and 307 on different crystals one of these appears as the
THE QUADRATIC SYSTEM
853
reflected image of the other, or one is right-handed and the
other left-handed.
Some cases of hemimorphism also occur in this system,
a well-known instance of which is seen in crystals of tourmaline,
seen in Fig. 308.
III. THE QUADRATIC, TETRAGONAL, OR PRISMATIC SYSTEM.
492 As in the hexagonal system, the principal axis of sym-
metry is chosen as the principal crystallographic axis, whilst two
P P
P
of the secondary axes of symmetry serve as the other two crys-
tallographic axes. These two are in one plane, at right angles
P
FIG. 310.
FIG. 309.
P
P
FIG. 311.
P
P "
P
FIG. 312.
to each other and to the principal axis,
and equal to each other but not to
the principal axis. This system differs
from the hexagonal in only possessing
four secondary planes of symmetry
making angles of 45°.
The most general form of the system
854
CRYSTALLOGRAPHY
is the ditetragonal pyramid (Fig. 309), which has the symbol
nPm, and from which all the other forms may be derived. It
Po
P.
FIG. 313.
P/P
. P. P
P
P
PA
FIG. 316.
OP
P
P
has an octagonal base, and is bounded by sixteen triangular
faces. The various forms are related much in the same way as
in the hexagonal system.
FIG. 320.
OP
FIG. 314.
∞ Poo
∞P
FIG. 317.
Pa
@P
P/ P
∞ P
P
co PocoP
FIG. 321.
op
FIG. 318.
The simplest form of this system is that of the four-sided
pyramid with a square base. This may be either of the first
order, a ac or P, or of the second order, a: ∞ a: c or P∞,
∞ P∞
FIG. 315.
P
P
Poo
∞ P∞
PPP
FIG. 319.
P
P
P
FIG. 322.
derived from the former by rotating it through 45°. These
square pyramids are distinguished as acute and obtuse (Figs.
310 and 311). When several of these square pyramids occur
THE QUADRATIC SYSTEM
855
P
together in a combination, the lengths of the various principal
axes, in accordance with the general law (p. 833), stand in
a simple ratio to one another. This is seen in Fig. 312,
which represents a crystal of nickel sulphate. From this it
will be seen that the principal axis of the pyramid P is
exactly twice as long as that of the pyramid P. A combina-
P
P
P
OP
+P
FIG. 323.
P
4P2
P
P
P
FIG. 326.
ᎭᏢ
P
P
OP
P
P
P
P
P
FIG. 324.
P
FIG. 327.
P
∞ Poo
Pr
P
tion of pyramids of the first and second order is shown in Fig.
313. This form is likewise found in nickel sulphate. When
the principal axis equals 0, the basal terminal planes OP make
their appearance, and when it is infinitely prolonged we obtain
the faces of the prism of the first order, P, or those of the
secondary prism, P. See Figs. 314 and 315.
Combinations belonging to the quadratic system are seen in
P
FIG. 325.
FIG. 328.
Figs. 316 and 317; the first of these is found in zircon, the second
in crystals of calcium copper acetate. Fig. 320 represents another
common form of this system, the crystals of potassium ferro-
cyanide. The usual form of tin-stone (stannic oxide) is seen in
Fig. 321, that of mellite in Fig. 322. Figs. 323 to 325 exhibit
the more complicated forms of nickel sulphate.
A remarkable form of the quadratic system occurs in the
856
CRYSTALLOGRAPHY
mineral leucite. This is a combination of P with 4P2, their
forms being equally developed (Fig. 326). This form can only
with difficulty be distinguished from that of the regular
icositetrahedron (202). So close indeed is the resemblance of
these forms that leucite was long supposed to crystallize in this
form of the cubic system, and the name leucitohedron was given
to it. We owe the discovery of the true crystalline form of
the mineral to Vom Rath. The combination, Fig. 327, clearly
shows the quadratic form, inasmuch as small faces of a third
pyramid as well as those of a prism occur. The twin crystal
FIG. 329.
FIG. 330.
∞P
P
FIG. 331.
(Fig. 328) occurring in the case of leucite also shows that the
form is a quadratic one.
Hemihedral forms can be derived from the quadratic prism by
the same process as that by which the tetrahedron is derived
from the octohedron. The forms thus obtained are termed
quadratic sphenoids. These can occur in two positions, shown
in Figs. 329 and 330; and they are distinguished from the
regular tetrahedra inasmuch as each face is a scalene and not
an equilateral triangle. They are not often found in nature;
Fig. 331 shows a combination which occurs in the case of
Epsom salts and of zinc vitriol.
1 Pogg. Ann. Ergänz. Bd. 6, 198,
THE RHOMBIC SYSTEM
857
-
IV. THE RHOMBIC SYSTEM,
493 This system is characterised by three secondary planes of
symmetry all at right angles. The three secondary axes of
symmetry are chosen as the crystallographic axes, and they are
all unequal and at right angles. It is usual to take as the
principal axis that which lies in the direction in which the
crystal is most fully developed or in which most modifications
are observed. This is called (c), and placed vertically. In the
following figures the longer of the other two axes, termed the
macrodiagonal (b), runs from back to front of the observer, and
the shorter, the brachydiagonal (c), from right to left. The
arrangement of the horizontal axes is purely conventional, and
is frequently the reverse of that just described.
The fundamental form of this system is the rhombic pyramid
Fig. 332, having the formula a: b: c or P. Secondary pyramids.
also occur, and these are distinguished by the
signs or placed over the P according as the
length of the intercept on the macro- or brachy-
diagonal is increased. The amount of this altera-
tion is represented by a number placed after the
letter P, so that the symbol Pn denotes a pyramid
the faces of which cut the principal axis and
the brachydiagonal at the normal distances c and
a, and the macrodiagonal at a distance of n times
b; whilst Pn signifies that the brachydiagonal is
cut at a distance of n times a.
P P
P
P
FIG. 332.
Faces also occur which would cut the principal
axis at a distance which is some multiple of the normal length c,
and this is represented by a letter placed before the symbol
P. Thus the symbol 3P2 signifies a form each face of which
cuts the macrodiagonal at the distance b, the principal axis at a
distance of 3 times c, and the brachydiagonal at a distance of
twice a from the point of intersection of the axes.
In addition to these series of pyramids, a number of prisms
are also possible, the faces of which cut two of the axes and are
Farallel to the third. If the faces are parallel to the principal
axis, the form is known as a vertical prism P, if to the macro-
or brachy-diagonal as a macro- or brachy-dome, P or P∞, these
last two being nothing more than horizontal prisms (Figs. 334
and 338).
858
CRYSTALLOGRAPHY
Faces which are parallel to two of the axes but cut the
third are known as terminal planes or pinacoids. Of these
there are three, OP, usually called the basal plane, P∞ or
the macropinacoid, and P the brachypinacoid (Fig. 339).
OC
P P
P P
FIG. 333.
PP
8
FIG. 336.
""
A form termed the right rhombic prism is formed by the com-
bination of the three terminal planes, and is found in nature in
crystals of anhydrite, CaSO,
""
These various forms are summarised with their symbols in
the following table, in which a = brachydiagonal, b = the macro-
diagonal, and c = the vertical axis.
""
P P
دو
P ∞P
P
PPP
39
FIG. 337.
""
PP
FIG. 334.
"?
""
®d®
8
∞P
Rhombic Pyramids.
ab me, abbreviated into mP = primary rhombic pyramid.
měn = brachypyramid.
na: b: mc
a: nb: mc
mPn = macropyramid.
Poo
P
0
∞ PP
P
∞ P
FIG. 335.
FIG. 338.
Vertical Prisms.
a: b:c, abbreviated into a P = rhombic prism (primary).
x Pn = brachyprism.
na: b:xc
a: nb:xc
Pn macroprism.
=
toř‰o
FIG. 339.
P
THE RHOMBIC SYSTEM
859
Domes or Horizontal Prisms.
aab: mc, abbreviated into mĎ x = brachydome.
a:ab: mc
mPx macrodome.
a:b:xc
a:xb:xc
xaxb:c, abbreviated into
2
›
Pa P ∞ Pa
ca
""
FIG. 340.
""
در
P
P
Terminal Planes.
""
x P2
""
Sulphur is one of the most important substances crystallizing
in this system. It occurs in rhombic pyramids and in numerous
combinations. Fig. 332 exhibits the fundamental form of
Pp
">
Ĭ'on
ОР
FIG. 341.
a Px
xPx = {
=
DI2
(basal terminal plane
or pinacoid.
brachyterminal plane
or pinacoid.
macroterminal plane
or pinacoid.
قدر
P
P2P P2
P.
FIG. 342.
P
sulphur, in which the ratio of the axes a:b: c = 0·811: 1:1·898,
and Figs. 333 and 334 various combinations which are found on
crystals of this element. Zinc sulphate is found in the forms
represented in Figs. 335, 336, 337. Barium formate, crystal-
lizing in a combination of a prism with a dome, is shown in
Fig. 338. Fig. 339 represents a crystal of uranium nitrate,
possessing two basal terminal faces. Potassium sulphate crystal-
lizes in several interesting forms. Of these, Figs. 340 and 341
may at once be recognized as belonging to the rhombic system,
whilst Figs. 342 and 343 might at first sight be mistaken for
hexagonal forms.
The most important hemihedral forms of the rhombic system
are the rhombic sphenoids. The faces of the two forms of these
860
CRYSTALLOGRAPHY
sphenoids (shown in combination Figs. 344 and 345) are equal in
number, and exactly similar in form, but when the + and forms
occur on separate crystals, they are developed on opposite sides, so
that the one may be regarded as a reflected image of the other.
Fig. 344 is the double sodium and ammonium salt of ordinary
tartaric acid, which is termed dextro-tartaric acid, from its hav-
ing the power of turning the plane of polarization to the right.
r2
∞ P2
P
FIG. 343.
2 P
ܣܧܣ
P2P
P.
FIG. 344.
2 P
-
op
∞∞∞∞P
FIG. 345.
P3 P
Fig. 345 is a salt of the same bases combined with a very similar
acid, differing only from ordinary tartaric acid in its power of
rotating the plane of polarization to the left. Another in-
teresting example of a rhombic sphenoid is found in the ordinary
crystals of magnesium sulphate, MgSO4 + 7H₂O. In these the
domes at the summits of the vertical prism are placed in opposite
directions.
V. THE MONOCLINIC OR MONOSYMMETRIC SYSTEM.
494 This system has only one plane of symmetry, and there-
fore only one axis of symmetry. This is chosen as one of the
crystallographic axes, and is known as the axis of symmetry, or
more generally as the orthodiagonal. This axis is supplemented
by two others which lie in the plane of symmetry, and are,
therefore, at right angles to the orthodiagonal. These
intersect at an angle (B) which is not a right angle, and one
of them is usually referred to as the primary axis, the other
being known as the clinodiagonal. The position of the crystal,
like that of crystals of the rhombic system, is purely a matter of
convention. In the following figures the orthodiagonal runs from
back to front of the observer, so that the plane of symmetry
is parallel to the plane of the paper, whilst the clinodiagonal
runs from right to left. Many authors, on the other hand, make
THE MONOSYMMETRIC SYSTEM
861
the orthodiagonal run from left to right, and place the primary
axis vertical. The symbols referring to the clinodiagonal are
distinguished by being enclosed in parentheses. The complete
description of each form of this system includes the axial ratio
and, in addition, the angle of intersection of the primary axis with
the clinodiagonal. Thus, for instance, the data of the crystals of
felspar are a b: c 1519: 1: 0844; B 63° 53'.
=
=
The fundamental form of this system, since there is only one
plane of symmetry, consists of a hemipyramid made up of four faces.
Thus in Fig. 346, in which the plane of symmetry is parallel to
the plane of the paper, the two upper right-hand faces and the
two lower left-hand faces (marked +) belong to one of these
hemipyramids, whilst the other two pairs of faces (marked —)
belong to the other. The complete octohedron shown in the
figure is, therefore, made up of two crystallographically distinct
forms, which are known as +P and P. Each of these two
hemipyramids may of course occur by itself in combination with
other forms, so that very frequently only four pyramid faces are
found on complex crystals of this system (Figs. 348, 349). The
pinacoids, prisms, and domes, which are known as the ortho- and
clino-domes, are derived from the pyramids in precisely the same
way as in the rhombic system. The prism and the clinodome
comprise four faces, whilst the orthodome, owing to the presence
of only one plane of symmetry, consists of only two faces, and
is really a hemidome + P or - P. The following figures
represent the forms of a variety of well-known chemical com-
pounds crystallizing in this system.
-P
-P
-P/ +P
# D#
[xx] P
P
∞PP
-P
FIG. 346.
FIG. 349.
FIG. 347.
op
FIG. 348.
OP
Figs. 347, 348, and 349 give the crystalline form of sodium
acetate; Fig. 350 that of cane-sugar; Fig. 351 that of nickel
potassium sulphate; Fig. 352, ferrous sulphate (green vitriol),
FeSO4+7H₂O. In this latter form the primary axis cuts the
862
CRYSTALLOGRAPHY
clinodiagonal at an angle of 75° 40′, and the relation of the axes
a b c = 0.8476:1:1.267. Gypsum, CaSO4+2H2O, fre-
quently occurs in fine monosymmetric crystals. The form of
OP
OP
[P]
P
FED BE
P
P
[Poo]
FIG. 350.
IP
PPP
FIG. 353.
FIG. 351.
gypsum is shown in Figs. 353 and 354; the relation of these
axes is as a:b:c=14504: 1:06003, with an inclination of
80° 57'.
A
+2Po
FIG. 354.
P
P
+P
Bo
Some substances, such as felspar, crystallize in forms which
are apparently very different (Figs. 355 and 356) owing to the
greater or less development of certain faces. Crystals of Glauber's
+P
ор
-P
OP
-P Poo +P
8
+P
FIG. 357.
aP
口味
​[2oPoo]
OP
+Po
P/ P
FIG. 356.
-Poo
FIG. 358.
∞ Po
FIG. 352.
∞P
P
[P] P
FIG. 355.
salt and oxalic acid are often found developed in the direction
of the orthodiagonal (Figs. 357 and 358).
Hemimorphous forms are not uncommon in this system, only
+Pa
Po
THE ASYMMETRIC SYSTEM
833
two faces of the clinodome being found, both of them on the
same side of the plane of symmetry. Dextro-tartaric acid not
unfrequently crystallizes in the simple form Fig. 358. More
usually, however, the four-faced angles at the front are replaced
by a dome (Fig. 360), whilst in the case of the laevo-tartaric
acid the corresponding angles at the back are similarly replaced,
as shown in Fig. 361.
Poo
600
PPPo
FIG. 360.
FIG. 359.
FIG. 361.
FIG. 362.
It is a singular fact that hemimorphous forms of this kind
generally occur in the case of bodies which are optically active-
that is, which possess the power of rotating the plane of polari-
zation of light. They are accordingly found (Fig. 362) in the
case of cane-sugar, which also possesses this optical property.
VI. THE TRICLINIC OR ASYMMETRIC SYSTEM.
495 This system is entirely without any plane of symmetry, and
the choice of axes is, therefore, quite arbitrary. Any one face of
the crystal is chosen as the pyramid face, and three unequal axes
cutting one another at angles a, B, y are then adopted. One of
these is known as the primary axis, the other two being dis-
tinguished as brachy- and macro-diagonals. Five values have
to be given in order completely to determine an asymmetric form,
viz. ab:c, the relative length of the axes, and the three acute
angles (a, B, y) which they form with each other.
The possible forms belonging to this system correspond closely
with the forms of the rhombic system. In addition to the
faces P of the fundamental pyramid, the following faces of
secondary pyramids occur, m P, Pn, Pn, m Pn, m Pn, together
with the surfaces of the prisms, «P, Pn, × Pn. Then we
864
CRYSTALLOGRAPHY
have the domes, m Px, m йx, and the basal end-faces, 0 P,
αΡα, α῏α, αΡα.
Since there is no plane of symmetry, each crystallographically
complete form consists of a pair of parallel faces, so that an
entire octohedron would be made up of four independent
pyramids, and each prism or dome of two independent forms.
FIG. 363.
SPI
FIG. 365.
Pa
P
„Ěæ
FIG. 366.
To
These forms having similar and parallel faces are represented by
accentuated letters. Thus P' represents a face of the funda-
mental form, which occurs in the upper front of the crystal to
the right, whereas the similar and parallel face P is one which
occurs in the lower part of the crystal to the left. Thus the
symbol P indicates the two faces of the prism x P, one of
1
P
P
B
01'
FIG. 364.
Po
-8
P
.P.
FIG. 367.
which occurs above to the left; m‚Ï' a denotes the faces of a
brachydiagonal dome which lie above to the right and below to
the left.
The direction in which crystals belonging to the triclinic
system are placed is a purely conventional one. Thus Fig. 363
may in the first place be regarded as a combination of the forms
×Р¤, ×Þ×, 0 P; or secondly as 'P, «P',, 0 P.
ISOMORPHISM
865
The following figures exhibit some commonly occurring
asymmetric forms:-
Copper sulphate, CuSO4+5H₂O, Fig. 364. Calcium thio-
sulphate, Fig. 365. Albite, Figs. 366 and 367.
Other common substances crystallizing in this system are
axinite, boric acid, cupric selenate, manganese sulphate, potas-
sium anhydrochromate, and sodium succinate.
ISOMORPHISM.
496 In his celebrated system of classifying minerals devised
in 1801, Haüy propounded the principle that a difference in the
primary form of two crystals invariably indicates a difference of
chemical composition, whilst identity of form, except in crystals
of the regular system, equally implies identity of composition.
It was however soon found that his principle could not be
applied in all its generality, for Leblanc had shown so long ago
as 1787 that crystals of an identical form could be obtained
from solutions containing sulphate of copper and sulphate of
iron mixed in very different proportions. He likewise states
that crystals of alum, a sulphate of aluminium and potassium,
may frequently be found to contain a considerable quantity
of iron, although no alteration in the crystalline form can be
noticed. Vauquelin, too, showed in 1797 that common potash
alum may contain large quantities of ammonia, and yet the
crystalline form of the substance does not undergo any change.
It was also well known that many minerals which are identical
in crystalline form may possess a very different chemical com-
position. Thus in certain specimens of red silver ore, arsenic is
present as an essential constituent, whilst in others antimony
takes its place. In like manner, the common garnet, crystalliz-
ing in the regular system, sometimes contains much iron and
little aluminium, and sometimes large quantities of aluminium
and little iron. Berthollet considered facts like these to be
in accordance with his views on chemical combination, but
Proust explained them by supposing that we have here to
deal not with chemical compounds but rather with mechanical
mixtures.
In the year 1816 Gay-Lussac made the remarkable observa-
tion that when a crystal of common potash alum is hung up in
a saturated solution of ammonia alum it grows exactly as if it
had been placed in the solution from which it was originally
56
866
CRYSTALLOGRAPHY
obtained. From this fact he drew the conclusion that the
molecules of these two alums possess the same form. Later on,
in 1819, Beudant noticed that if solutions containing two of the
following salts, sulphate of zinc, sulphate of iron, or sulphate of
copper, be crystallized, the crystals which are deposited always
possess the form of one of these salts, although they contain a
considerable quantity of the other salt, which, when crystallized
by itself, possesses a totally different form.
In order to explain these and similar well-recognised facts,
Haüy threw out the notion that certain bodies possess the power
of crystallization to such a degree that even when present in
small quantities they compel other bodies to adopt their crystal-
line form.
Clear light was thrown on this subject by the researches of
Mitscherlich, the results of which were communicated to the
Berlin Academy in 1819. Mitscherlich showed that the com-
pounds of various elements possessing a similar constitution
have also identical crystalline form, as ascertained by the
measurement of their angles. The first substances examined
by Mitscherlich were the arsenates and phosphates of sodium
potassium, and ammonium. He showed that the crystals of the
phosphate and arsenate of the same metal not only contain the
same quantity of water of crystallization, and crystallize in the
same form, but that when the two salts are mixed in varying
quantities the crystals which such a solution deposits are of the
same form as those obtained from solutions of the pure salts,
whilst the proportion of each ingredient found in the crystal
deposited from the mixed solution varies according to the propor-
tions in which the ingredients were mixed. Hence Mitscherlich
concluded that analogous elements or groups of elements can
replace one another in compounds without any alteration of
crystalline form. Such substances are said to be isomorphous
(loos, equal to, μoppń, shape). A large number of other com-
pounds were shown by Mitscherlich to conform to the above
law. Subsequent investigations have, however, proved that the
angles of isomorphous crystals are not absolutely identical, but
that each substance differs slightly from the other in this
respect. When several of such substances are crystallized from
solution together, the angles of the crystal deposited are the
mean of those of the pure substances.
497 The following minerals serve as an excellent illustration
of the law of Isomorphism :-
ISOMORPHISM
867
{F.
Apatite, Ca.(PO4)2 + Ca₂ { PO
Pyromorphite, Pb(PO4)2 + Pb₂ { Cl.
(PO
4
Mimetesite,
Pb¸ (AsO,)½ + Pb₂ { PO
2
Cl.
(VO▲
Pb. (VO4)2 + Pb₂ Cl
Vanadinite,
The isomorphous chemical elements in these minerals are:-
(1) Phosphorus, Arsenic, Vanadium. (2) Calcium and Lead.
(3) Chlorine and Fluorine.
LP P
∞o P
FIG. 369.
They all crystallize in hexagonal prisms, seen in Figs. 369 to
371, derived from the fundamental form P. (Fig. 368). The
values of the terminal solid angle a, the lateral solid angle ẞ,
P
2P2
Pa
∞P
∞. P
DOO
2Pa
FIG. 370.
P
·
FIG. 368.
•
C.
0.7321
Apatite
Pyromorphite
Mimetesite
0.7362
0.7392
Vanadinite . 0-7270
•
and the length of the primary axis (c), that of the secondary
axes being taken as the unit, are found to be :-
a.
142° 20'
142° 15'
142° 70'
142° 30'
PPPP
P
P
FIG. 371.
B.
80° 25'
80° 44'
80° 58'
80° 0'
Not unfrequently both chlorine and fluorine occur in the
same specimen of apatite; and phosphorus and arsenic, not
unfrequently, replace one another in pyromorphite.
868
CRYSTALLOGRAPHY
Another well-marked case is that of the rhombohedral
carbonates of the magnesium class of metals, calcium, mag-
nesium, iron, zinc, and manganese. These minerals all crys-
tallize in similar rhombohedra, and the several metals can
replace one another in varying proportions without any change
of form occurring. It has, however, been observed that the
angles of the different rhombohedra are not exactly equal, but
that the different members of the series vary in this respect by
one or two degrees; thus:-
Angle on
terinl. edge.
CaCO3.
. 105° 5'
Calcium carbonate, or calc-spar,
Calcium magnesium carbonate, or dolomite, (CaMg)CO,106° 15′
Magnesium carbonate, or magnesite, MgCO. . 107° 25'
Ferrous carbonate, or spathic iron ore, FeCO. . 107° 0′
Zinc carbonate, or calamine,
ZnCO,. . 107° 40'
Manganese carbonate, or diallogite,
MnCO. . 106° 5'
The replacement in minerals of varying proportions of
the different isomorphous compounds is well illustrated by
the following percentage analysis of spathic iron ore:—
=
Ferrous oxide,
Manganous oxide,
Lime,
Magnesia,
Carbonic acid,
FeO.
MnO.
CaO.
MgO.
CO2.
=======
•
.
•
•
·
•
•
•
45.55
12.50
1.57
1.80
38.58
If we now divide the percentage of oxide by its combining
45.55
weight thus for ferrous oxide, etc., etc., we obtain the
71.48
following numbers respecting the proportion between the number
of equivalents of the several constituents present:-
FeO 06372, MnO 01773, CaO 0.0282, MgO= 0.0448, or
summing these we have 08875. This is, however, very nearly
the proportion which must be present in order to unite with
38.58
=0·8833 equivalents of CO,, the difference being owing to
43.67
errors of experiment. In other words, the number of molecules
the basic oxides present is the same as that of the carbon
di
; hence the formula for this spathic iron ore is (Fe, Mn,
100.00
ISOMORPHISM
869
Ca, Mg)CO,, signifying that the relative quantities of the
metals in question present are indeterminate, but are in the
aggregate such as are needed to combine with CO2. Hence we
have the following as the composition of the mineral:-
Ferrous carbonate, FeCO3
Manganese carbonate, MnCO,
Calcium carbonate, CaCO3
Magnesium carbonate, MgCO,
Zn
Ag
·
.
•
Fahl-ore offers another striking example of isomorphism.
The simple formula for this mineral is 2Cu,S+SbSg, but
usually part of the copper is replaced by iron, zinc, silver, or
mercury, and some of the antimony by arsenic. This is seen
by the following analysis:—
Sulphur, S.. 25.48÷ 31.82 =
= 0.801
Antimony, Sb 17.761194 = 0·149
Arsenic, As 1155 744 = 0·155)
}
Copper,
Cu
30-73÷ 628 = 0·489
Iron,
Fe
142 556 = 0·025
Zinc,
2.53÷ 65
= 0.039
Silver,
10.53107·13 = 0·098
•
•
.
.
100.00
•
=
•
73.39
20.24
2.80
3.95
100-38
S=0.801
(Sb, As)=0.304
(Cu, Fe, Zn, Ag) = 0·651
0.304
0.651
If the several percentage weights be divided by the corre-
sponding atomic weights, and if the resulting quotients of the
isomorphous constituents be added together, the following
result is obtained :—
These numbers have the ratio 52:2:42, or allowing for
experimental error, 5:2:4, and the formula of Fahl-ore is
therefore :-
---
2[(Cu,Fe,Zn, Ag),S]+(Sb,As),Sg.
870
CRYSTALLOGRAPHY
DIMORPHISM AND TRIMORPHISM.
498 Even before Haüy started the idea that bodies which
crystallize in different forms differ also in chemical composition,
Vauquelin had noticed that titanic acid occurs as rutile and
anatase, two minerals possessing distinct crystalline forms. In
like manner, Klaproth pointed out that hexagonal calc-spar is
the same chemical compound as rhombic arragonite. These
exceptions to Haüy's law were then explained by the presence
in the compound of some impurity which has the power of
altering the crystalline form. Thus, arragonite was found to
contain small quantities of strontium carbonate, a mineral
which is found crystallized in the same form, and this small
proportion was supposed to exert so powerful an action as to
compel the calcium carbonate to assume a rhombic form.
In 1821, Mitscherlich proved that this property of crystallizing
in two distinct forms is common to many bodies both elementary
and compound, and he termed such bodies Dimorphous,
Other substances, again, are capable of existing in three
distinct crystalline forms. These Mitscherlich termed Tri-
morphous substances. If, lastly, two substances exhibit a double
isomorphism, they are said to be Isodimorphous. The trioxides
of arsenic and antimony serve as a striking example of isodi-
morphism. For a long time these compounds were only known
to occur in two forms which were not isomorphous, and this
was the more remarkable as their elementary constituents
exhibit such a close analogy. It was afterwards found that
arsenious oxide, As,O, which usually crystallizes in regular
octohedra, is occasionally met with in rhombic crystals, exactly
identical in form with those in which antimonious oxide, Sb,Og,
commonly occurs in nature. A mineral consisting of this
latter oxide, and called senarmontite, was next discovered.
The crystals of this are octohedral, so that the isodimorphism
of these substances is now completely proved, especially since
it has been found possible to produce the octohedral crystals of
antimonious oxide artificially.
Dimorphous and trimorphous bodies are not only distinguished
by their difference in crystalline form. The other physical
properties, such as specific gravity, hardness, refractive power,
etc., are all different. One of the best examples of a trimorphous
compound is titanium dioxide, TiO2. The commonest form of
this compound is rutile. This mineral crystallizes in the quad-
DIMORPHISM AND TRIMORPHISM
871
ratic system with the combinations P, ≈ P, P x, and x P x; the
relations of the axes are a: b=1:06442; its specific gravity
is 4.2494. The second form of titanium dioxide is anatase,
Fig. 372; it likewise crystallizes in the quadratic system,
but the relation of the axes is in this case quite different
P P
P P
FIG. 372.
from that in the case of rutile, viz. a : b=1:1.777. Anatase
crystallizes as a rule in the fundamental form P. The specific
gravity of anatase is 3.826. The third form is brookite; it
crystallizes in flat rhombic prisms, and possesses a specific
gravity of 4-22.
THERMAL AND OPTICAL RELATIONS OF CRYSTALS.
499 Crystals which belong to the regular system being
equally developed in all directions expand when heated equally
in every direction. On the other hand, crystals belonging to
the hexagonal and quadratic systems expand differently in the
directions of the primary and of the secondary axes. This is
clearly seen in the following table, which gives the coefficients
of expansion of substances crystallizing in these two systems.¹
Primary axis.
Quadratic System.-Tinstone . . 0.0004860
Zircon. . 0.0006264
Hexagonal System.-Quartz
. . 0·0008073
Tourmaline 0.0009369
Secondary axis.
0.0004526
0.0011054
0.0015147
0.0007732
Crystals belonging to the other systems possess coefficients
of expansion which differ for each of the three directions of the
1 Pfaff, Pogg. Ann. 104, 171.
872
CRYSTALLOGRAPHY
axes. Hence whilst the angles of substances crystallizing in the
regular system remain constant under change of temperature,
those of crystals belonging to the other systems undergo small
deviations with alteration of temperature.
The same relation is exhibited by crystals with regard to the
conduction of heat as holds good in the case of expansion. The
conducting power of crystals of the regular system is the same
in all directions. Those belonging to the quadratic and hexa-
gonal systems conduct equally in two directions, and unequally
in the third, whilst crystals belonging to the other systems con-
duct differently in every direction. This difference in the con-
ducting power of crystals can be well shown by covering a face
of the crystals with a thin coating of wax, allowing the wax to
solidify and then bringing the point of a hot needle or other
pointed hot body against the wax coating. If the crystal conduct
equally, the wax will melt in a circle of which the hot point is
the centre. If the conduction be unequal, the melted wax will
be seen to assume an oval form.
Pyro-electric Action of Crystals.-Certain hemimorphons or
tetartohedral crystalline forms when heated exhibit a peculiar
development of electricity, one end of the crystal, or fragment of
crystal, becoming negatively electrified, whilst the other end ex-
hibits positive electricity. Amongst these crystals, tourmaline
exhibits this property in a very high degree. Boracite, cane-
sugar, topaz, and silicate of zinc are crystals which exhibit pyro-
electrical reactions.
500 Optical Properties of Crystals.-The relations of crystals to
light are of great importance, as enabling us to determine crys-
talline forms in cases in which the usual methods either give
uncertain results or fail entirely. Transparent crystals belonging
to the regular system exert no peculiar action on a ray of light;
they behave in this respect like glass or any other amorphous
substance. The incident ray gives rise to one refracted ray, and
crystal possesses one refractive index.¹
Crystals belonging to the hexagonal and quadratic systems
are doubly refractive. Each incident ray separates into two re-
fracted rays, one being bent more out of its course than the other.
This phenomenon of double refiaction is, however, only observed
1 Cases have indeed been observed by Brewster in which a crystal belonging to
the cubic system exhibits double refraction, but this is due to unequal tension in
the mass of the crystal and resembles the case of double refraction by unequally
heated or compressed glass.
OPTICAL PROPERTIES OF CRYSTALS
873
when the incident ray falls on the crystal in such a direction as
to make an angle with the primary axis. If the ray pass into
the crystal parallel to this axis, no double refraction is observed.
Crystals belonging to the hexagonal and quadratic systems are
accordingly called uniaxial crystals, whilst those of the rhombic
and the inclined systems are termed liaxial crystals, because
they possess two directions in which the ray of light may fall
without producing the effect of double refraction.
Polarization by Absorption in Crystalline Media.-Doubly re-
fracting crystals, especially tourmaline, have the power of only
allowing rays of light to pass through them when those rays are
polarized in a given direction with regard to their optic axes.
Thus, if a ray be allowed to pass through a plate of tourmaline
cut with its faces parallel to the optic axis (the axis c of the
hexagonal system), it will refract doubly, but the ordinary ray
will be completely absorbed, and only the extraordinary ray will
C
g
h
FIG. 373.
go
с
FIG. 374.
pass through. This ray is polarized in a plane perpendicular to
the axis of the crystal, and so that a second plate of tourmaline
cut in a similar way and placed with its axis in a direction at
right angles to that of the first will not allow any light to pass,
as in Fig. 374, though when the two axes are placed parallel to
each other the polarized ray will be transmitted by both crystals,
as in Fig. 373. The first crystal used to polarize the ray is
termed the polarizer, whilst the second, used to examine the
direction of the polarization, etc., is termed the analyzer.
The property of circular polarization is possessed not only by
such as solution of
crystals but also by many organic liquids,
sugar, tartaric acid, and many essential oils. In these substances
the plane of polarization is rotated sometimes to the right, some-
times to the left hand. The specific rotary power of any sub-
stance is the angle through which a column of the substance
of unit length rotates the plane of polarization of a beam of
polarized light.
874
WEIGHTS AND MEASURES

By Act of Parliament (27 and 28 Vict. cap. 117, 29th July, 1864) the use of the Metrical System of Weights and Measures is rendered legal.
The weight of the Kilogram is settled by this Act to be equal to 15432 3488 English grains.
COMPARISON OF THE METRICAL WITH THE COMMON MEASURES.
BY DR. WARREN DE LA RUE.
MEASURES OF LENGTH.
Millimeter
Centimeter
Decimeter
Meter
Decameter
Hectometer
Kilometer
Myriometer.
1 Inch
1 Foot
=
=
In English Inches.
Centiare or square meter
Are or 100 square meters
Hectare or 10,000 square meters
0·03937
0.39371
3.93708
39:37079
393 70790
3937 07900
39370-79000
393707 90000
2:539954 Centimeters.
3 0479449 Decimeters.
In English Square
Feet.
10-7642993
1076-4299342
107642-9934183
In English Feet
= 12 Inches.
1 Square Inch = 6.4513669 Square Centimeters.
=
1 Square Foot 9-2899683 Square Decimeters.
0.0032809
0.0328090
0:3280899
3.2808992
32.8089920
328-0899200
3280 8992000
32808 9920000
MEASURES OF SURFACE.
In English Square
Yards 9 Square Feet.
In English Yards
= 3 Feet.
1.1960333
119 6033260
11960 3326020
0.0010936
0.0109363
0.1093633
1.0936331
10.9363310
109-3633100
1093 6331000
10936 3310000
1 Yard
1 Mile
=
In English Poles
= 272-25 Square Feet.
In English Fathoms
= 6 Feet.
0.0395383
3.9538290
395-3828959
0.0005468
0.0054682
0.0546816
0.5468165
5:4681655
54-6816550
0 91438348 Meter.
1 6093149
546 8165500
5468 1655000
Kilometer.
In English Roods
= 10,890 Square Feet.
0.000988457
0.098845724
9.884572398
In English Miles
= 1,760 Yards.
0.0000006
0.0000062
0.0000621
0.0006214
0.0062138
0.0621382
0.6213824
6.2138244
In English Acres
=43,560 Square Feet.
0.0002471143
0.0247114310
2:4711430996
1 Square Yard = 0.83609715 Square Meter or Centiare.
1 Acre
= 0·404671021 Hectare.
WEIGHTS AND MEASURES
875
Milliliter, or cubic centimeter
Centiliter, or 10 cubic centimeters
Deciliter, or 100 cubic centimeters
Liter, or cubic decimeter
Decaliter, or centistere
Hectoliter, or decistere
Kiloliter, or stere, or cubic meter
Myrioliter, or decastere
Milligram
Centigram
Decigram
Gram
•
Decagram
Hectogram
Kilogram
Myriogram
1 Cubic Inch = 16 3861759 Cubic Centimeters.
1 Grain
1 Troy oz.
=
=
MEASURES OF CAPACITY.
In Cubic Feet
= 1,728 Cubic Inches.
In Cubic Inches.
0.061027
0.610271
6.102705
61.027052
610-270515
6102.705152
61027 051519
610270 515194
In English Grains.
0.015432
0.154323
1.543235
15.432349
0.064798950 Gram.
31.103496 Gram.
154 323488
1543 234880
15432 348800
154323 488000
0.0000353
0.0003532
0.0035317
0.0353166
0.3531658
3.5316581
35-3165807
353-1658074
1 Gallon = 4.543457969 Liters.
MEASURES OF WEIGHT.
In Trov Ounces
= 480 Grains
In Pints = 34.65923
Cubic Inches.
0.000032
0.000322
0.003215
0.032151
0.321507
3.215073
32.150727
321-507267
0.001761
0.017608
0.176077
1.760773
17.607734
176 077341
1760-773414
17607-734140
In Avoirdupois Lbs.
= 7,000 Grains.
0.0000022
0.0000220
0 0002205
In Gallons = 8 Fints
=277-27384
Cubic Inches.
1 Cubic Foot = 28-3153119 Cubic Decimeters.
0.0022046
0.0220462
0.2204621
2.2046213
22.0462126
0.00022010
0.00220097
0.02200967
0.22009668
2.20096677
22 00966767
220 09667675
2200 96676750
In Cwts. 112 Lbs.
= 734,000 Grains.
1 Lb. Avd. =
1 Cwt.
0.00000002
0.00000020
0.00000197
0.00001968
0.00019684
In Bushels = 8 Gallons
=2218 19075
Cubic Inches.
0.00196841
0.01968412
0.19684118
0.000027512
0.000275121
0.002751208
0.027512085
0.275120846
2 751208459
27 512084594
275 120845937
0.45359265 Kilog.
= 50 80237689 Kilog.
Tons 20 Cwt.
= 15,620,000 Grains.
0.000000001
0·000000010
0-000000098
0·000000984
0.000009842
0.000098421
0.000984206
0-009842059
ADDENDUM.
P... According to Colefax (Journ. Chem. Soc. 1892, 1083),
the equation given does not represent the change which occurs.
The primary products of the reaction are a sulphate and a
tetrathionate; any trithionate formed is the product of a
secondary change between sulphite and tetrathionate.
INDEX
A.
ABSOLUTE temperatures, 62
Absorptiometer, Bunsen's, 286
Acetylene, 693,, 701; metallic deriva-
tives of, 703; preparation of, 702;
properties of, 702
Acid-forming oxides, 234
Acids, 235
INDEX
After-damp, 696
Agate, 817
Agricola, 9
Air, 523; analysis of, 530; bacteriology
of, 546; composition of, 528, 531;
liquefaction of, 528; mean tempera-
ture of, 527; weight of, 523, 527
Albertus Magnus, 6
Alchemy, 5-10
Alcohol, 694
Alkaline waters, 297
Allophane, 822
Allotropic modifications of the
elements, 56
Amethyst quartz, 815
Amidoguanidine, 768
Amidophosphoric acid, 610
Amidosulphonic acid, 520
Ammonia, 452; aqueous solution of,
462; atmospheric, 542; combination
with acids, 469; composition of,
463; detection and estimation of,
469; liquefaction of, 457; occurrence
of, 452; percentage in aqueous solu-
tions of different specific gravity,
463; preparation of, 455; properties
of, 456 solubility in water, 460;
sulphonic acids of, 519; synthesis
of, 454
Ammoniacal liquor, 454, 770
Ammonium carbamate, 738
Ammonium salts, 470
Analcime, 822
Analysis, definition of, 86; qualitative,
87; quantitative, 87
Andalusite, 821
Animal charcoal, 678
Anthracite, 683
Antozone, 243
Apatite, 867
Aqua regia, 497
Aqueous solutions, properties of dilute,
116; vapour, tension of, 276
Aquinas, Thomas, 6
Arnold Villanovanus, 6
Arsenic, atomic weight of, 616; detec
tion of, in cases of poisoning, 633;
Marsh's test for, 637; occurrence
of, 612; preparation of, 614; pro-
perties of, 615; Reinsch's test for,
640
Arsenic acid, 628
Arsenic di-iodide, 621
Arsenic disulphide, 630
Arsenic pentafluoride, 619
Arsenic pentasulphide, 632
Arsenic pentiodide, 621
Arsenic pentoxide, 627
Arsenic selenosulphide, 633
Arsenic thioselenide, 633
Arsenic tribromide, 620
Arsenic trichloride, 619
Arsenic tricyanide, 769
Arsenic trifluoride, 619
Arsenic tri-iodide, 621
Arsenic trioxide, 622
Arsenic trisulphide, 630
Arsenic vapour, molecular weight of,
615
Arsenic, white, 622
Arsenious acid, 625
Arsenious oxide, 622
A:seniuretted hydrogen, 616
Aisine, 616
Asmanite, 816
Asymmetric system of crystallography,
863
Atmolysis, 67
Atmosphere, the, 523; composition of,
528; ozone in, 240; weight of, 523,
527
Atmospheric ammonia, 542; carbonic
acid, 538; moisture, 541; organic
matter, 544; pressure, 526
Atomic theory, 34, 95
880
INDEX
Atomic weight, definition of, 100
Atomic weights, 52, 96
Atomic weights, Dalton's tables of, 37
Augite, 821
Avogadro's hypothesis, 39, 98
Azoimide, 472
Azulmic acid, 751
Basic oxides, 234
Becher, 13
Benzene, 693
BACON, Roger, 6
Bacteriological examination of water,
300
Bacteriology of air, 546
Balance, the, 56
Bergman, 32
Berthollet, 33
Berzelius, 38
B.
Biaxial crystals, 873
Bituminous coal, 684
Biuret, 744
Black, 15
Bleaching, 317, 369
Bleaching powder, 319
Boart, 667
Boghead coal, 684
Borates, 655
Boric acid, 650; spectrum of, 654;
sources of, 650, 653
Boron, 640; adamantine, 642; amor-
phous, 641; atomic weight of,
643; crystallized, 642; occurrence of,
640; preparation of, 641
Boron ammoniochloride, 649
Boron carbide, 769
Boron hydride, 645
Boron nitride, 657
Boron pentasulphide, 656
Boron phosphide, 658
Boron phosphoiodide, 658
Boron tribromide, 649
Boron trichloride, 647
Boron trifluoride, 645
Boron trifluoride, hydrate of, 646
Boron tri-iodide, 649
Boron trioxide, 650
Boron trisulphide, 656
Boyle, 10
Boyle's law, 59; deviations from, 70
Brimstone, 340
Bromic acid, 331
Bromide of nitrogen, 480
Bromides, 197
Bromine, atomic weight of, 193;
detection and estimation of, 198;
occurrence of, 188; preparation of,
189; properties of, 191
Bromine hydrate, 192
Bromine monochloride, 198
Brown coal, 688
C.
CALAMINE, 821
Calorie, definition of, 230
Cannel coal, 684
Carbamic acid, 738
Carbamide, 739
Carbaminic chloride, 739
Carbon, 658; allotropic modifications of,
658; amorphous, 673; atomic weight
of, 689; halogen derivatives of, 705
Carbon bisulphide, 732
Carbon boride, 769
Carbon dioxide, 710; composition of,
714; critical point of, 722; deter-
mination of, 728; liquefaction of,
716; occurrence of, 711; preparation
of, 712; properties of, 713; proper-
ties of liquefied, 720; solidification
of, 720
Carbon monoxide, 705; poisonous
action of, 709; preparation of, 706;
properties of, 708
Carbon oxybromide, 732
Carbon oxychloride, 731
Carbon oxysulphide, 736
Carbon tetrachloride, 705
Carbon tetrafluoride, 705
Carbonado, 660
Carbonated waters, 297
Carbonates, 724
Carbonic acid, 725;
atmospheric,
538; and the carbonates, Black's
researches on, 15
Carbonic acid gas, 710
Carbonic anhydride, 710
Carbonic oxide, 705
Carbonyl bromide, 732
Carbonyl chloride, 731
Carbonyl-dibiuret, 745
Carbonyl-diurea, 745
Carbonyl sulphide, 736
Carborundum, 825
Carburetted hydrogen, heavy, 700;
light, 700
Carburetted water gas, 791
Cavendish, 15-22
Chalcedony, 817
Chalybeate waters, 297
Charles's law, 60
Chemical action, illustrations of, 42
Chemical combination, laws of, 86
Chemical decomposition, 42
Chemical equations, 102
Chemical formulæ, 98
Chemical nomenclature, 118
Chemical symbols, 100
Chemistry, derivation of the term, 4;
general principles of, 41
Charcoal, 674; absorption of gases
by, 680; animal, 678; employment
in decolourising and deodourising,
678; manufacture of, 675; proper-
ties of, 676; wood, 674
INDEX
881
Chlorates, 323
Chlorhydric acid, 168
Chloric acid, 322
Chloride of nitrogen, 475
Chlorides, 185
Chlorine, 153; allotropic modification
of, 163; atomic weight of, 188;
bleaching power of, 184; detection
and estimation of, 186; explosion
with hydrogen, 169; occurrence of,
153; preparation of, 154; properties
of, 159; solubility of, 168
Chlorine hydrate, 165
Chlorine monoxide, 315
Chlorine peroxide, 320
Chlorites, 319
Chloroform, 694
Chloroformamide, 739
Chlorosulphonic acid, 404
Chlorous acid, 319
Circular polarisation, 873
Coal, 682; analysis of, 685-687;
bituminous, 684; Boghead, 684;
brown, 688; cannel, 684; parrot,
684; yield of, in Great Britain, 688
Coal gas, analysis of, 786; composition
of, 783; condensing apparatus
employed in the manufacture of,
774; determination of illuminating
power of, 780; history of, 769;
manufacture of, 770; purification of,
776; scrubbers, 775; retorts and
retort settings for the manufacture
of, 770; washers, 775
Coal-mines, explosions in, 695
Coal tar, 770
Coke, 682
Colloids, 819
Combination by volume, 97; by weight,
87; chémical, 42; heat of, 230
Combination in definite and unalter-
able quantities, discussion of, by
Berthollet and Proust, 33
Combination of gases by volume,
Gay-Lussac's observations of, 38
Combining weights, 93
Combustion, Lavoisier's views of, 27
Compound, definition of the term, 51
Compound radical, 470
Compound radical, definition of, 748
Constitutional formulæ, 128
Critical point of liquefaction of gases,
73
Critical pressure of gases, 73
Cryohydrates, 281
Crystalline form, determination of,
837
Crystallization, 279; water of, 103,
279
Crystallographic axes, 832
Crystallographic symbols, 832
Crystallography, 827; asymmetric
system of, 863; cubic system of,
840; early ideas of, 827; hexagonal
system of, 847; isometric system of,
840; monoclinic system of, 860;
monosymmetric system of, 860;
prismatic system of, 853; quadratic
system of, 853; regular system of,
840; rhombic system of, 857;
tetragonal system of, 853; triclinic
system of, 863
Crystalloids, 819
Crystals, angles of, 830; artificial
growth of, 837; axes of, 832; biaxial,
873; classification of, 829; double
refraction of, 872; edges of, 830;
faces of, 830; general characteristics
of, 828; hemihedral, 833; hemi-
morphous, 833; hemitropic, 835;
holohedral, 833; optical relations of,
872; polarization of light by, 873;
predominant faces of, 831; pyro-
electric action of, 872; secondary faces
of, 831; simple and complex forms
of, 830; specific rotatory power of,
873; symmetry of, 328; tetartohe-
dral, 833; thermal relations of, 871;
uniaxial, 873
Cubic system of crystallography, 840
Cyamelide, 762
Cyanamide, 764
Cyanic acid, 761
Cyanides, 756
Cyanogen compounds, history of, 747
Cyanogen gas, 749
Cyanogen bromide, 760
Cyanogen chloride, 759
Cyanogen iodide, 761
Cyanogen selenide, 764
Cyanogen sulphide, 764
Cyanuric derivatives, 766
D.
DALTON, 34-38
Dalton's law of gases, 60; deviations
from, 70
Dalton's law of partial pressures, 284
Davy lamp, 228
Davy, Sir Humphry, 38
Deliquescence, 280
Density of gases, 103
Dialysis, 819
Diamide, 470
Diamidophosphoric acid, 610
Diamond, 658; artificial production of,
661; sources of, 660
Dicyanamide, 766
Dicyanogen, 749
Diffusion of gases, 63; illustrations of,
67
Dihydroxylaminesulphonic acid, 521
Di imidodiphosphaminic acid, 611
Di-imidodiphosphoric acid, 611
Dilute solutions, properties of, 111
Dimetaphosphates, 597
57
882
INDEX
Dimorphism, 870
Dioptase, 821
Diperiodates, 336
Diphosphoric acid, 601
Dissociation, 109
406
Dissociation, electrolytic, 116
Distilled water, 290
Distorted crystals, 835
Disulphuryl chloride,
Dithionic acid, 414
Dithiophosphoric acid, 607
Double cyanides, 757
Double refraction, 872
Dowson gas, 791
Drummond light, 268
Dulong and Petit's law of specific
heats, 39
E.
Elixir vitæ, 8
Emerald, 821
Enstatite, 821
EARTH'S crust, composition of, 55
"Eau de Javelles," 315
Efflorescence, 280
Effusion of gases, 70
Electrolytes, 116
Electrolytic dissociation, 116
Elements, allotropic modifications of,
56; Aristotelian doctrine of, 3;
atomic weights of, 52; Boyle's view
of, 11; classification of, 53; com-
bining weights of, 93; definition of,
51; distribution of, 54; equivalents
of, 92; Geber's views of, 7; list of,
52
F Equations, chemical, 102
Equivalents, 92
Ethane, 698
Ethene, 699
Ethine, 701
Ethylene, 693, 699; preparation of,
700; properties of, 701
Ethyl hydride, 698
Euchlorine, 322
Explosions in coal mines, 695
Explosion in gases, phenomena of,
265
F.
FELSPAR, 821
Ferricyanic acid, 757
Ferrocyanic acid, 757
Fire-damp, 695
Flame, Davy's researches on, 793, 794;
dissection of, 799; Frankland's re-
searches on, 794; Lewes' researches
on, 795, 797; luminous, rendered
non-luminous by addition of other
gases, 797; nature of, 792; oxy-
hydrogen, 267; Smithells' researches
on, 799; structure of, 792
Flame of Bunsen burner, 797
Flame of burning hydrocarbons, 793
Flint, 817
Fluoboric acid, 646
Fluorides, 152
Fluorine, atomic weight of, 147; de-
tection of, 151; isolation of, 143:
occurrence of, 142; properties of,
146
100; constitu-
Formulæ, chemical,
tional, 128
Freezing mixtures, 282
Freezing point, depression of, by dis-
solved substances, 114
Fuming sulphuric acid, 402
G..
GARNET, 821
Gas-carbon, 674
Gas-liquor, 454, 770
Gaseous and liquid states, continuity
of, 72
Gases, absorption of by water, 284;
critical point of, 73; critical pres-
sure of, 73; density of, 103; deter-
mination of molecular weight, 105;
diffusion of, 63; effusion of, 70; ex-
plosion of, 265; kinetic theory of, 60;
fiquefaction of, 74; molecular weight
of, 99; properties of, 59; relation of
volume to pressure, 59; relation of
volume to temperature, 60; transpi-
ration of, 66; van Helmont's inves
tigations of, 9
Gay-Lussac, 38
Gay-Lussac's law, 60
Gay-Lussac tower, employment in sul-
phuric acid manufacture, 389
Geber, researches of, 5, 7
Generator gas, 790
Glauber, 10
Glover's tower, employment in sulphuric
acid manufacture, 385
Goniometer, 837
Graphite, 667; artificial production of,
668; properties of, 668, 670; uses
of, 671
Graphitic acid, 729
Graphitite, 670
Graphon, 730
Guanidine, 766
H.
HALLOYSITE, 822
Halogens, the, 142
Hard water, 298
Hartshorn, spirits of, 153
Heat of combination, 230
INDEX
883
Heat of liquidity, 273
Heavy carburetted hydrogen, 700
Hemihedral crystals, 833
Hemimorphous crystals, 833
Hemitropic crystals, 835
Hexagonal system of crystallography,
847
Hexametaphosphates, 598
Hexathionic acid, 422
Higgins, 34
Holohedral crystals, 833
Hooke, 12
Humboldt, 39
Hydrated oxides, 234
Hydrazine, 470
471
Hydrazine hydrate,
Hydrazoic acid, 472
Hydriodic acid, 206; preparation of,
206 properties of, 208
Hydrobromic acid, 193; preparation of,
193; properties of, 196
Hydrocarbons, constitution of, 690
Hydrochloric acid, 168; composition
of, 175; formation of, 169; manu-
facture of, 179; occurrence of, 168;
preparation of, 173; properties of,
174; properties of aqueous solutions
of, 118
Hydrocyanic acid, 752; detection and
estimation of, 756, 758; preparation
of, 752; properties of, 754
ydrocyanic acid hydriodide, 759
ydrocyanic acid hydrobromide, 759
ydrocyanic acid hydrochloride, 759
ydrofluoric acid, 147; preparation of,
148; properties of, 150; vapour
density of, 150
Hydrofluosilicic acid, 806
Hydrogen, 129; absorption of, by
metals, 136; experiments with, 139;
explosion of, with chlorine, 169; ex-
plosion of, with oxygen, 261; lique-
faction of, 135; occurrence of, 129;
preparation of, 129; properties of,
134; spectrum of, 139
Hydrogen arsenide, gaseous, 616; solid,
618
Hydrogen bromide, 193
Hydrogen chloride, 168
Hydrogen dioxide, 308; detection and
atimation of, 312; preparation of,
Hydrogen
Hydrogen monoxide, 244
Hydrogen peroxide, 308
Hydrogen persulphide, 355
Hydrogen phosphide, gaseous
liquid, 570; solid, 573
Hydrogen selenide, 427
Hydrogen sulphate, 377
Hydrogen telluride, 439
IVAS
566;
Hydrogenium, 138
Hydrographitic acid, 730
Hydrosulphurous acid, 408
Hydrothiosulphoprussic acid, 745
Hydroxides, 234
Hydroxycarbamide, 743
Hydroxylamine, 475
Hydroxylaminedisulphonic acid, 520
Hydroxylaminesulphonic acid, 521
Hygrometry, 542
Hypobromous acid, 329
Hypochlorous acid, 317
Hypochlorous anhydride, 315
Hypoiodous acid, 332
Hyponitrous acid, 504
Hypophosphites, 582
Hypophosphoric acid, 586
Hypophosphorous acid, 581
Hyposulphites, 408
Hyposulphurous acid, 408
I.
ICE, artificial production of, 458; crys-
talline form of, 274; melting point
of, 273
Ignition, temperature of, 228
Imidodiphosphoric acid, 611
Imidodisulphonates, 520
Imidosulphurylamide, 522
Indestructibility of matter, 47
Induction, photochemical, 172
Introduction, historieal, 3
Iodates, 333
Iodic acid, 332
Iodide of nitrogen, 480
Iodides, 209
Iodine, 199; atomic weight of, 206;
detection and estimation of, 211;
occurrence of, 199; preparation of,
199; properties of, 203
Iodine bromide, 214
Iodine fluoride, 212
Iodine monochloride, 213
Iodine pentoxide, 332
Iodine trichloride, 213
Isometric system of crystallography,
840
Isomorphism, 39, 865
Isuretine, 743
JET, 688
KIESELGUHR, 814
Kirwan, 32, 34
J.
K.
884
INDEX
LAMP-BLACK, 673
Laughing-gas, 503
L.
Lavoisier, 24-42
Law of partial pressures, 284
Laws of chemical combination, 86
Lefebre, 10
Lemery, 10
Leucite, 821
Libavius, 9
Light carburetted hydrogen, 700
Lignite, 688
Lime light, 268
Liquefaction of gases, 74
Liquefied gases, tension of, 74
Liquid and gaseous states, continuity
of, 72
Liquidity, heat of, 273
Liquids, boiling points of, 85
Liquor ammoniæ, 462
Lucifer matches, 564
Lully, Raymond, 6
M.
MACLES, 834
Marggraf, 15
Marsh-gas, 694
Marsh's test for arsenic, 637
Matches, 565
Matter, different states of, 41
Matter, indestructibility of, 47
Mayow, 12
Mesoperiodates, 336
Metaboric acid, 654
Metals, transmutation of, 5
Metaperiodates, 336
Metaphosphoric acid, 591, 596
Metarsenates, 629
Metasilicic acid, 820
Metathioarsenates, 632
Methane, 694; preparation of, 697;
properties of, 698
Methenylamine, 759
Methyl hydride, 694
Metrical system, comparison of, with
English measures, 874, 875
Microbes in the atmosphere, 546
Milk of sulphur, 345
Milk quartz, 815
Mimetisite, 867
Mineral waters, 296
Mitscherlich, 39
Moisture, atmospheric, 541
Molecular motion, 61
Molecular weight, determination of, in
solution, 111
Molecular weight, experimental deter-
mination of, 104
Molecular weight of gases, 99
Molecule, definition of the term. 98
Molecules, size of, 97; velocity of, 61, 63
Monoclinic system of crystallography,
860
Monometaphosphates, 597
Monosymmetric system of crystallo-
graphy, 860
Monothiophosphoric acid, 607
Muriatic acid, 168
Muscovite, 821
N.
NATROLITE, 822
Natural waters, 289
Nitric acid, 482; action of metals on,
494; detection and estimation of.
495; formation of, 482; hydrates of,
488; manufacture of, 488; prepara-
tion of, 483; properties of, 486:
properties of aqueous solutions of,
486
Nitric anhydride, 497
Nitric oxide, 505
Nitrilosulphonates, 519
Nitrilotrimetaphosphoric acid, 612
Nitrites, interaction with sulphites,
522
Nitrogen, 446; assimilation of, by
plants, 451; combustion of, 451;
discovery of, 446; liquefaction of,
450; preparation of, 447; properties
of, 450; spectrum of, 451
Nitrogen bromide, 480
Nitrogen chloride, 477
Nitrogen dioxide, 505
Nitrogen iodide, 480
Nitrogen monoxide, 499
Nitrogen pentoxide, 497
Nitrogen peroxide, 512
Nitrogen selenide, 516
Nitrogen sulphide, 516
Nitrogen tetroxide, 512
Nitrogen trioxide, 508
Nitroguanidine, 768
Nitrometer, Lunge's, 496
Nitrosohydroxylaminesulphonic acid,
521
Nitrosulphonic acid, 516
Nitrosulphonic anhydride, 499, 518
Nitrosulphonic chloride, 5:7
Nitrosyl bromide, 511
Nitrosyl chloride, 510
Nitrosyl tribromide 511
Nitrous acid, 509
Nitrous anhydride, 508
Nitrous oxide, 499
Nitroxypyrosulphuric acid, 518
Nomenclature, chemical, 116
OCCLUSION, 137
Oil gas, 788
0.
INDEX
885
Olefiant gas, 699
Oletines, 693
Olivine, 821
Opal, 816
Optical relations of crystals, 872
Organic constituents of water, 300
Organic matter, atmospheric, 544
Organic substances, first artificial pre-
paration of, 40
Orpiment, 630
Ortho-arsenates, 628
Orthoboric acid, 650
Orthoclase, 821
Orthophosphoric acid, 591, 592
Orthosilicic acid, 819
Orthothioarsenates, 632
Osmotic pressure, 112
Oxides, 234
Oxy-acids of chlorine, constitution of,
328
Oxygen, 214; atomic weight of, 255;
discovery of, 15; explosion of, with
hydrogen, 261; occurrence of, 214;
preparation of, 215; properties of,
222; separation of, from nitrogen, 289
Oxy-hydrogen flame, 267
Oxynitrosulphonic anhydride, 518
Oxythiocarbamic acid, 745
Ozone, 235; atmospheric, 240; mole-
cular formula of, 239; production
of, 235; properties of, 242
P.
PARACELSUS, 9
Paracyanogen, 752
Paraffins, 692
Paraperiodates, 336
Parasulphatammon, 520
Parrot coal, 684
Peat, 688
Pentathionic acid, 419
Perbromic acid, 331
Perchlorates, 327
Perchloric acid, 324; hydrates of, 327
Periodates, 335
Periodic acid, 335; hydrates of, 335
Peroxides, 234
Persulphuric acid, 410
Petalite, 821
Pharaoh's seients, 763
Phenakite, 820
Phlogistic theory, 13, 23, 31
Phosgene gas, 731
Phospham, 610
Phosphamide, 610
Phosphamidic acid, 610
Phosphine, 566
Phosphites, 585
Phosphonium bromide, 570
Phosphonium iodide, 570
Phosphoric acid, 590; determination
of, 601
Phosphoric acids, constitution of, 591;
poisonous action of, 592
Phosphorous acid, 584
Phosphorous anhydride, 583
Phosphorous diamide, 609
Phosphorous oxide, 583
Phosphorus, 549; allotropic modifica-
tions of, 554; amorphous, 561;
black, 564; detection of, 559; dis
covery of, 549; luminosity of, 556;
manufacture of, 551; metallic, 563;
molecular weight of, 555; occur.ence
of, 550; octohedral, 554; proje ties
of, 554; poisonous action of, 560;
red, 561; rhombohedral, 563
Phosphorus bromonitride, 612
Phosphorus chlorobromide, 579
Phosphorus chloronitride, 612
Phosphorus di-iodide, 580
Phosphorus oxy bromide, 604
Phosphorus oxy bromochloride, 605
Phosphorus oxychloride, 602
Phosphorus oxyfluoride, 602
Phosphorus pentabromide, 579
Phosphorus pentachloride, 575
Phosphorus pentafluoride, 573
Phosphorus pentasulphide, 606
Phosphorus pentiodide, 580
Phosphorus pentoxide, 587
Phosphorus suboxide, 581
Phosphorus sulphides, 606
Phosphorus sulphoxide, 607
Phosphorus tetroxide, 586
Phosphorus tribromide, 578
Phosphorus trichloride, 574
Phosphorus tricyanide, 768
Phosphorus trifluoride, 573
Phosphorus trifluorodibromide, 579
Phosphorus trifluorodichloride, 578
Phosphorus tri-iodide, 580
Phosphoryl bromide, 604
Phosphoryl bromochloride, 605
Phosphoryl chloride, 602
Phosphoryl fluoride, 602
Phosphoryl nitride, 611
Phosphoryl triamide, 612
Phosphuretted hydrogen, 567
Photochemical induction, 172
Photometer, 781
Plastic sulphur, 345
Plumbago, 667
Polarization, circular, 873; in crystal-
line media, 873
Portable gas, 788
Potash mica, 821
Potassium hydroxylaminedisulphonate,
520
Potassium imidodisulphonate, 520
Potassium nitrilosulphonate, 519
Potassium sulphazinate, 521
Pott, 15
Priestley, 15-22
Prismatic system of crystallography,
853
886
INDEX
Producer gas, 790
Proust, 33
Prussic acid, 752
Pyro-arsenates, 629
Pyroboric acid, 654
Pyro-electric action of crystals, 872
Pyrographitic acid, 730
Pyrographitic anhydride, 730
Pyromorphite, 867
Pyrophosphaminic acids, 611
Pyrophosphoric acid, 591, 599
Pyrophosphoryl chloride, 604
Pyrophosphoryl thiobromide, 609
Pyrothioarsenates, 632
Q.
QUADRATIC system of crystallography,
853
Quartz, 814; coloured varieties of, 815
R.
RADICALS, Compound, 470
Rain-water, 295
•
S
Raoult's method of determining mole-
cular weights, 114
Realgar, 630
Red phosphorus, 561
Regelation, 274
Regular system of crystallography, 840
Reinsch's test for arsenic, 640
Rhombic system of crystallography, 857
Richter, 32
River-water, 305
Rose quartz, 816
Ruby sulphur, 630
SAFETY LAMP, 228
Sal ammoniac, 453
Saline waters, 297
Salts, 235
Sand, 815
Sceptical Chymist (Boyle), 10
Scheele, 22
Sea-water, 306
Selenic acid, 433
Selenious acid, 431
Selenium, 423; action of light on, 426;
atomic weight of, 427; preparation
of, 424; properties of, 425
Selenium chlorobromide, 430
Selenium dioxide, 431
Selenium fluoride, 431
Selenium moniodide, 430
Selenium monobromide, 429
Selenium monochloride, 428
Selenium oxychloride, 433
Selenium tetrabromide, 430
Selenium tetrachloride, 429
Selenium tetriodide, 430
Seleniuretted hydrogen, 427
Selenosulphur trioxide, 435
Selenosulphuric acid, 435
Selenotrithionic acid, 436
Selenourea, 746
Selenyl bromide, 433
Selenyl chlori le, 433
Serpentine, 821
Silica, 814
Silica, amorphous, 816
Silicates, 820; artificial production of,
822; classification of, 821
Silicic acid, 819
Silicious waters, 297
Silico-bromoform, 812
Silico-chloroform, 810
Silicofluoric acid, 896
Silicofluorides, 808
Silicoformic anhydride, 811
Silico-iodoform, 813
Silicomethane, 804
Silicon, 800; amorphous, 801; atomic
weight of, 803; crystallized, 802;
mixed halogen derivatives of, 813;
occurrence of, 800; preparation of,
801; properties of, 801
Silicon carbide, 825
Silicon carbonitride, 824
Silicon carboxide, 825
Silicon chlorohydrosulphide, 824
Silicon dioxide, 814
Silicon disulphide, 823
Silicon hydride, 804
Silicon nitride, 824
Silicon oxychloride, 822
Silicon oxysulphi te, 823
Silicon subsulphide, 823
Silicon tetrabromide, 811
Silicon tetrachloride, 808
Silicon tetrafluoride, 806
Silicon tetra-iodide, 812
Silicon tribromide, 812
Silicon trichloride, 810
Silicon tri-iodide, 812
Silico-oxalic acid, 813
Smoky quartz, 815
Soap test for hardness of water, 299
Soda-bleach, 313
Soft water, 298'
Solutions, electrolyti
283; properties of, zoz
Specific heats, Dulong and Petit's law
of, 39
Specific rotatory power, 873
Spirits of hartshorn, 453
Springs, thermal, 296
Spring-water, 296
S ahl, 13
Steam, latent heat of, 275; volumetric
composition of, 253
Suffioni, 651
Sulphamide, 523
INDEX
887
-
Sulphatammon, 520
Sulphates, 400
Sulphazinates, 521
Sulphazotinates, 522
Sulphimide, 523
Sulphites, 372
Sulphites, interaction with nitrites,
522
Sulphites, isomerism of, 373
Sulpho-arsenates, 632
Sulpho-arsenites, 631
Sulphocyanic acid, 762
Sulphoselenoxytetrachloride, 436
Sulphur, 336
a-Sulphur, 342
B-Sulphur, 343
7-Sulphur, 345
8-Sulphur, 347
Sulphur, allotropic modifications of,
342; atomic weight of, 349; colloi-
dal, 346; crystalline forms of, 342,
detection and determination of, 348;
flowers of, 340, 346; manufacture of,
338; milk of, 345; occurrence of,
336; properties of, 342; plastic,
345; recovery of, from waste pro-
ducts, 341; spectrum of, 348
Sulphur, vapour, molecular weight of,
347
Sulphur dichloride, 360
Sulphur dioxide, 364; bleaching action
of, 369; detection and estimation of,
370; liquefaction of, 368; prepara-
tion of, 366; properties of, 367
Sulphur diphosphide, 605
Sulphur fluoride, 363
Sulphur heptoxide, 410
Sulphur hexiodide, 363
Sulphur moniodide, 362
Sulphur monobromide, 362
Sulphur monochloride, 359
Sulphur oxytetrachloride, 406
Sulphur sesquioxide, 407
Sulphur subiodide, 363
Sulphur tetrabromide, 362
Sulphur tetrachloride, 360
Sulphur tetraphosphide, 605
Sulphur trioxide, 374
Sulphuretted hydrogen, 349; action of
on metals, 355; composition of, 353;
preparation of, 350; properties of,
352
Sulphuretted waters, 297
Sulphuric acid, 377; action of, on
metals, 399; action of water on, 398;
fuming, 402; history of, 377; hy-
drates of, 399; manufacture of, 381;
properties of, 397; purification of,
396; rectification of, 391; specific
gravity of aqueous solutions of, 401;
theory of the formation of, 379
Sulphuric anhydride, 374
Sulphurous acid, 371
Sulphurous anhydride, 364
Sulphuryl bromide, 407
Sulphuryl chloride, 405
Sulphuryl hydroxychloride, 404
Symbols, chemical, 100
Synthesis, definition of, 86
T.
TALC, 821
Tellurates, 445
Telluretted hydrogen, 439
Tellurium, 436; atomic weight of, 438;
preparation of, 437; properties of,
438
Tellurium dibromide, 441
Tellurium dichloride, 440
Tellurium di-iodide, 441
Tellurium dioxide, 442
Tellurium disulphide, 445
Tellurium hydride, 439
Tellurium monoxide, 442
Tellurium oxybromide, 445
Tellurium oxychloride, 445
Tellurium sulphate, 442
Tellurium sulphoxide, 446
Tellurium tetrabromide, 441
Tellurium tetrachloride, 440
Tellurium tetrafluoride, 442
Tellurium tetriodide, 441
Tellurium trioxide, 443
Tellurous acid, 443
Temperature, absolute zero of, 62
Tension of aqueous vapour, 276
Tetartohedral crystals, 833
Tetragonal system of crystallography,
853
Tetrametaphosphates, 598
Tetraphosphoric acid, 601
Tetraphosphorus trisulphide, 606
Tetrathionic acid, 418
Thermal relations of crystals, 871
Thermal springs, 296
Thermo-chemistry, 233
Thioarsenates, 632
Thioarsenites, 631
Thiocarbamic acid, 745
Thiocarbamide, 746
Thiocarbonic acid, 735
Thiocarbonyl chloride, 736
Thiocyanic acid, 762
Thiocyanic anhydride, 764
Thionyl bromide, 374
Thionyl chloride, 373
Thiophosphoric acid, 607
Thiophosphoryl bromide, 608
Thiophosphoryl chloride, 608
Thiophosphoryl fluoride, 608
Thiosulphuric acid, 412
Thiourea, 746
Thomson, 38
Torricellian vacuum, 524
Transmutation of metals, 5
Transpiration of gases, 66
888
INDEX
Tricarbon disulphide, 735
Trichlorosilicomethane, 810
Triclinic system of crystallography, 863
Tridymite, 815
Trimetaphosphates, 598
Trimorphism, 870
Triphosphorus hexasulphide, 606
Trithionic acid, 416
Trithiophosphoric acid, 607
Turf, 688
Twin crystals, 834
U.
UNIAXIAL crystals, 873
Urea, 739; detection and estimation of,
742; preparation of, 740; properties
of, 740; synthesis of, 739
Urea hydrochloride, 741
Urea nitrate, 741
Urea oxalate, 741
V.
VACUUM, Torricellian, 524
Valency, 127
Valentine, Basil, 8
Van Helmont, 9
Vanadinite, 867
Vapour density, determination of, 106
Vapour pressure, diminution of, by dis-
solved substances, 115
Ventilation, 545
Volume, combination by, 97
W.
WACKENRODER's solution, 420, 422
Water, 244; absorption of gases by,
284; alkaline, 297; analysis of, 300;
bacteriological examination of, 300;
carbonated, 297; Cavendish's investi-
gations of, 20; chalybeate, 297;
composition of, by volume, 250; com-
position of, by weight, 255; dis-
covery of the composition of, 20, 244;
dissolved gases in, 291; distillation
of, 290; electrolytic analysis of, 251;
eudiometric synthesis of, 246; ex-
pansion and contraction of, 270;
freezing point of, 273; hard, 208;
latent heat of, 272; mineral, 296;
natural, 289; organic constituents of,
300; permanent hardness of, 299;
point of maximum density of, 271;
properties of, 270; purification of,
290; rain, 295; river, 305; saline,
297; sea, 306; silicious, 297; soft,
289; solvent action of, 278; specific
gravity of, at different temperatures,
272; spring, 296; sulphuretted, 297;
synthesis of, by volume, 250; tem-
porary hardness of, 299
Water-gas, 790; carburetted, 791
Water of crystallization, 103, 279;
volume occupied by, 281
Weight, combination by, 87
Weights of elements in compounds,
relations of, 87-95
White arsenic, 622
Willis, 10
Wollaston, 38
Wollastonite, 821
Wood-charcoal, 674
Wood-gas, 787
Wood-tar, 788
Z.
ZEOLITES, 822
Zosimus of Panopolis, 4

Reb
GENERAL LIBRARY,
UNIV. OF MICH
80 NOV 1898
BOOK CARD
AUTHOR Roscoe
TITLE
SIGNATURE
•Treatise on chemistry.
Vol. I
H.M. Trimble
Medical Library
UNIVERSITY OF MICHIGAN
CHEMICAL LIBRARY
Q47
3/0
R793
ISS'D
3 9015 07709 8104
Madical Libreny
RETD
2671727)