THE WANDERING 
 OF ATOMS 
 
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THE STORY OF 
 THE WANDERINGS OF ATOMS 
 
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 A Piece of Coal. By E. A. MARTIN, F.G.S. 
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THE STORY OF THE 
 
 WANDERINGS OF ATOMS 
 
 ESPECIALLY THOSE OF CARBON 
 
 BY 
 
 M. M. PATTISON MUIR, M.A. 
 
 PBLLOW AND PR/ELECTOR IN CHEMISTRY OF GONVILLE AND CAIUS COLLEGE, 
 CAMBRIDGE 
 
 HODDER AND STOUGHTON 
 LONDON, NEW YORK, TORONTO 
 
AT 
 (At 
 

 PEEFACE. 
 
 IN attempting, in The Story of the Chemical Ele- 
 ments, to set forth some of the guiding con- 
 ceptions of chemistry, I gave a sketch of the 
 theory of the grained structure of matter. That 
 theory has put an instrument into the hands of 
 the chemist which has proved of signal service 
 in setting in order the connexions between the 
 compositions and the properties of bodies, and in 
 making possible and developing new chemical 
 industries. 
 
 To grasp the conceptions of the atom and the 
 molecule, and to be able to apply these in detail, 
 is demanded alike of the man who wishes to 
 follow the development of chemical science, and 
 of him who desires to be a successful manufac- 
 turer in any of the more recently established 
 branches of the chemical trades. And, to follow 
 some of those investigations which deal with the 
 notions of molecular structure and molecular 
 symmetry is an excellent exercise in accurate 
 
 452881 
 
 5 
 
6 PREFACE. 
 
 and imaginative reasoning, and in the use of 
 theories and hypotheses as instruments for 
 gaining knowledge. 
 
 As the ideas of the atom and the molecule 
 have been more fully applied in the elucidation 
 of the changes undergone by compounds of 
 carbon than elsewhere in chemistry, I have 
 confined myself in this volume to the com- 
 pounds of that element. 
 
 M. M. PATTISON MUIR. 
 
 OAMBEIDGB, August 1899. 
 
CONTENTS. 
 
 CHAP. JPAGB 
 
 I. INTRODUCTION ,- 9 
 
 H. A SURVEY OF THE COMPOSITION AND REACTIONS 
 
 OF THE COMPOUNDS OF CARBON ... 21 
 
 HI. AN OUTLINE OF THE CLASSIFICATION OF SOME OF 
 
 THE COMPOUNDS OF CARBON . . . . 44 
 
 IV. THE TWO OXIDES OF CARBON .... 48 
 V. MARSH GAS, AND CERTAIN COMPOUNDS DERIVED 
 
 THEREFROM 64 
 
 VI. ETHANE, AND SOME OF ITS DERIVATIVES . . 82 
 
 VH. ETHYLENE, GLYCERIN, AND TARTARIC ACID . . 98 
 THE. A FEW TECHNICAL APPLICATIONS OF COMPOUNDS 
 
 OF CARBON 114 
 
 IX. SUGARS, STARCHES, AND CELLULOSE . . . 129 
 
 X. BENZENE, AND SOME OF ITS ALLIED COMPOUNDS . 142 
 
 XI. CERTAIN CHEMICAL INDUSTRIES DEALING WITH 
 
 SUGARS, CELLULOSE, AND BENZENE COMPOUNDS 153 
 
 XIL ALIZARIN AND INDIGO ...... 165 
 
 XIH. THE ALKALOIDS AND ALBUMIN ... 174 
 
 XIV. SUMMARY AND CONCLUSION ..... 185 
 
 INDEX . 191 
 
THE STORY OF 
 
 THE WANDERINGS OF ATOMS ; 
 
 ESPECIALLY THOSE OF CARBON. 
 
 CHAPTER I. 
 
 INTRODUCTION. 
 
 IN The Story of the Chemical Elements mention 
 was made of certain changes in the combina- 
 tions and collocations of elementary substances 
 which are constantly occurring in plants and 
 animals. In this volume I shall endeavour to 
 tell portions of the chemical story of these trans- 
 mutations, and of transmutations like these. 
 The ripening of fruit is accompanied by the 
 change of starch into sugar, the disappearance 
 of acidic substances, the production of coloured 
 bodies which give the bloom to the ripening fruit, 
 and the formation of compounds which impart to 
 the ripened fruit its peculiar and agreeable aroma 
 and flavour. During the growth of certain kinds 
 of trees and shrubs there are produced various 
 gums, resins, and balsams; substances that are 
 valuable for their healing properties, or are 
 sought after because of the pleasant odours they 
 emit either in their natural state or when they 
 are burned. The products of the life-processes of 
 other plants are prized as medicinal agents ; such 
 are quinine, belladonna, and oil of wintergreen. 
 
10 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 Thsre Hra: plant* &nd trees whose leaves, or fruit, 
 contain compounds that are used as beverages, or 
 to afford solace to human beings : everyone drinks 
 a decoction of the compounds formed in the leaves 
 of the tea-plant, or in the seeds of the coffee- tree ; or 
 forgets some of his cares while inhaling the smoke 
 of tobacco. From the compounds produced in 
 growing plants are formed other substances which 
 find manifold uses ; in this class may be named 
 alcohol, chloroform, iodoform, ink, indigo, and 
 paper. 
 
 As examples of large industries that depend 
 upon the transformations of substances of vege- 
 table origin into materials useful to mankind, it 
 will suffice to mention the production of lubricat- 
 ing and burning oils, and paraffin-wax, the manu- 
 facture and the dyeing of cotton fabrics, the 
 making of aniline and alizarin colours, and such 
 processes as baking and biscuit-making. 
 
 A great part of the art of cooking consists in 
 bringing about changes in the combinations and 
 arrangements of the elementary substances of 
 which the flesh of animals is composed. If living 
 animals were not transformers of vegetable com- 
 pounds into other combinations of the elements 
 of these compounds, such industries as making 
 butter, cheese, leather, lanoline, Prussian blues, 
 and silk-stuffs, would have no existence. And, 
 think of the strange transformations which occur 
 when the food we consume is changed into the 
 substance of our bodies, and our very bodies serve 
 as material from which new tissues are con- 
 structed. The human being is constantly feed- 
 ing on himself. As Sir Thomas Browne puts it 
 
INTRODUCTION. 1 1 
 
 " We are what we all abhor, anthropophagi, and 
 cannibals, devourers not only of men, but of our- 
 selves ; and that not in an allegory but a positive 
 truth ; for all this mass of flesh which we behold 
 came in at our mouths ; this frame we look upon 
 hath been upon our trenchers ; in brief, we have 
 devoured ourselves." 
 
 Agriculture is very closely connected with the 
 selection and application of those kinds of food 
 which certain classes of plants are able to assimi- 
 late rapidly, and to transmute into compounds 
 which, in their turn, form the basis of foods for 
 animals and human beings. 
 
 There are departments of preventive medicine 
 which primarily depend on the formation of 
 mixtures of compounds wherein certain minute 
 organisms flourish, which organisms when injected 
 into a human being, or an animal, cause changes 
 such that the living being becomes immune, or 
 partially immune, to the attacks of certain 
 diseases. When an animal, or a plant, dies, 
 changes in the arrangements and combinations 
 of the elements continue ; many of the com- 
 pounds that are formed are harmful to living 
 human beings, and some of these compounds are 
 deadly poisons. But there are minute living 
 organisms that feed upon these harmful, or 
 deadly compounds, and produce other colloca- 
 tions of elements which are devoid of ill effects 
 on human beings. For instance, there are 
 bacteria which feed on the compounds contained 
 in sewage, destroying thereby the obnoxious sub- 
 stances in the sewage, and producing compounds 
 which are perfectly harmless. 
 
12 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 And finally, the chemist in his laboratory 
 brings about rearrangements of the elements 
 whereof compounds formed in living organisms 
 are composed, and thus produces a great array 
 of new bodies, many of which find important 
 applications in manufactures or in medicine. 
 
 In this volume I shall attempt to draw the 
 outlines of the chemical story of some of the 
 compounds formed in living organisms, and of 
 other compounds allied to these. 
 
 The object of chemistry is to elucidate the 
 changes of composition which compounds ex- 
 hibit, and to connect these with changes of pro- 
 perties. Many illustrations of the ways whereby 
 chemistry conducts the study of changes of com- 
 position and properties have been given in The 
 Story of the Chemical Elements. It is the business 
 of this volume to apply these methods to a 
 particular class of compounds, and to indicate 
 some of the general results that have been 
 obtained. 
 
 A brief statement of the more general results 
 that were gained by an examination of the com- 
 positions and the properties of definite kinds of 
 matter in the former volume will be given here ; 
 but it will be assumed that the reader has made 
 himself acquainted with these matters in some 
 detail. A compound is a definite kind of matter 
 which cannot be separated into portions unlike 
 one another, and unlike the original substance, 
 by any process of the nature of sifting or sorting. 
 The composition of a compound is stated in 
 chemistry by naming the elements into which 
 the compound can be resolved and by the union 
 
INTRODUCTION. 13 
 
 of which the compound can be produced, and 
 by indicating the quantity by weight of each 
 element that can be obtained from, or is used in 
 the formation of, some determinate weight of the 
 compound. So far as our knowledge goes at 
 present, an element is a substance all the particles 
 whereof, however small these particles may be, 
 are alike in every respect ; except, of course, that 
 one particle may be heavier, or lighter, than 
 another. 
 
 The properties of the product of the combina- 
 tion of two or more elements, which product is 
 called a compound, are unlike, and generally very 
 unlike, the properties of the elements by whose 
 union the compound is formed : but it is always 
 possible to tear the compound asunder, and to 
 obtain again the elements that seemed to lose 
 their identity in the act of producing the com- 
 pound. The information that is gained by the 
 study of the compositions of compounds is ex- 
 pressed by the chemical formulas which are 
 assigned to compounds. These formulae are 
 expressions of the compositions of compounds in 
 the terms of a special language. The formula of 
 a specified compound tells the number of com- 
 bining weights of each of the elements which 
 have combined to form a reacting weight of that 
 compound. It is possible to express the com- 
 position of a compound by a formula because it 
 is always true, and always exactly true, that all 
 elements react in the ratios of their combining weights, 
 or of whole multiples of their combining weights. 
 
 The study of the properties of elements and 
 compounds is more complex than the study of 
 
14 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 composition; and the results of the investiga- 
 tions concerning properties cannot be stated in 
 such generalised and yet perfectly exact terms as 
 those which express the results of the study 
 of composition. The chemical properties of an 
 element, or a compound, are those which the 
 body exhibits when it interacts with other 
 elements and compounds; these properties are, 
 therefore, better named reactions. The substances 
 which take part in a chemical reaction are 
 changed into others unlike themselves. For 
 instance any specified weight of the element 
 oxygen combines with one-eighth of its own 
 weight of the element hydrogen, the product is 
 water, and the weight of the product is exactly 
 the sum of the weights of the two elements 
 which have passed out of existence, as such, in 
 the act of producing this quantity of water; 
 these facts may be grouped together and called 
 a reaction of hydrogen, or they may be called a 
 reaction of oxygen, or they may be called a re- 
 action of water. 
 
 The study of the reactions of elements and 
 compounds, taken with the study of composition, 
 has enabled us to use such generalised terms as 
 adds, alkalis, salts, and the like. All compounds 
 which are called acids have one common reaction, 
 and one thing in common as regards their com- 
 position : they are all compounds of hydrogen, 
 and, in the presence of water, they all interact 
 with iron, lead, copper, zinc, or some other 
 element which is a metal, to exchange the whole 
 or a part of their hydrogen for metal. The 
 reactions of compounds are conditioned by the 
 
INTRODUCTION. 15 
 
 kinds of elements of which they are composed. 
 But there exist compounds of the same elements 
 which are wholly unlike one another in their 
 reactions : for instance, the compound whose 
 composition is expressed by the formula NH 3 is- 
 an alkali; but the compound of nitrogen and 
 hydrogen whose composition is expressed by the 
 formula N 3 H is an acid. It is evident that the 
 reactions of compounds are conditioned not only 
 by the kinds of elements, but also by the relative- 
 quantities of the elements, of which the com- 
 pounds consist. 
 
 Many of the differences that are observed 
 between the reactions of compounds are pictured 
 to himself by the chemist as accompaniments of 
 differences in the kinds, and differences in the 
 quantities, of the elements which compose these 
 compounds. But all differences in the reactions 
 of compounds cannot be connected with differ- 
 ences of composition, unless the word composition 
 is used with a more extended signification. There- 
 are compounds which shew very different re- 
 actions although equal weights of these com- 
 pounds are composed of the same quantities by 
 weight of the same elements. The case of the 
 two compounds urea and ammonium cyanate, 
 the composition of both of which is expressed 
 by the formula N^CH^O, was considered in 
 Chapter IX. of The Story of the Chemical Elements. 
 One hundred parts by weight of urea are composed 
 of 46f parts by weight of nitrogen, 26f parts by 
 weight of oxygen, 20 parts by weight of carbon, 
 and 6f parts by weight of hydrogen ; and one hun- 
 dred parts by weight of ammonium cyanate hava 
 
16 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 also exactly this composition. But the reactions 
 of these compounds are very different. In certain 
 cases, then, differences in the reactions of com- 
 pounds must be conditioned by some circum- 
 stance besides differences in the kinds and the 
 quantities of the constituent elements. The only 
 other conditioning circumstance that can be 
 thought of clearly is, differences in the arrange- 
 ments of the constituent elements. 
 
 The moment an attempt is made to apply the 
 conception of a definite arrangement of the 
 elements that form a compound so as to frame a 
 clear and consistent mental picture of the con- 
 nexions that are assumed to exist between this 
 arrangement and the reactions of the compound, 
 it is discovered that some theory of the structure 
 of matter must be adopted. The theory that is 
 always employed in chemistry is that which 
 likens the structure of matter to the structure of 
 a barrelful of apples or oranges, or to that of a 
 brick wall, or of a quantity of small shot. The 
 extremely minute particles whereof a quantity of 
 any definite kind of matter is composed, accord- 
 ing to this theory, are called molecules. The 
 properties of any element or compound are 
 asserted by the theory to be the properties of 
 the molecules of that body. Now, the fact that 
 a compound can be broken up into simpler com- 
 pounds and then into certain elements, is explic- 
 able in terms of the molecular theory only by 
 assuming the existence of particles of matter 
 weighing less than the molecule. The parts of 
 molecules are called atoms. The theory presents 
 the molecules of a body to our mental vision as 
 
INTRODUCTION. 17 
 
 collocations of atoms ; and it enables us to think 
 clearly of the properties of these molecules as 
 conditioned by three circumstances, firstly the 
 kinds of atoms that compose the molecules, 
 secondly the number of atoms of each kind, and 
 thirdly the arrangement of the atoms : or, as 
 Lucretius said, 1900 years ago, " it matters much 
 with what others, and in what positions, the same 
 first-beginnings of things are held in union, and 
 what motions they do mutually impart and re- 
 ceive. The same numbers of the same atoms 
 may be arranged in different ways ; the results 
 of the different arrangements will be molecules 
 whose reactions are not the same." * 
 
 The application of the molecular and atomic 
 theory to the problem of representing the 
 arrangement of the parts of molecules in such a 
 way as shall make it possible to connect chemi- 
 cal facts, clearly, consistently, and suggestively, 
 with the theoretical presentment of these facts, 
 necessitates the use of certain subsidiary hypo- 
 theses and certain conventions. The most im- 
 portant hypothesis is that which asserts the 
 atom-fixing power of each atom in a molecule to 
 be limited and definite. Facts like those re- 
 ferred to on p. 169, and the following pages, of 
 The Story of the Chemical Elements, can be inter- 
 preted in terms of the molecular and atomic 
 theory only by saying that, " there is a limit to 
 the number of atoms of any kind wherewith a 
 specified atom can enter into direct chemical 
 
 * For a more detailed discussion of the molecular and 
 atomic theory, the reader is referred to Chapter VIII. of 
 The Story of the Chemical Elements. 
 B 
 
18 THE STORY OF THE WANDERINGS OF ATOMS, 
 
 union to produce a molecular building which 
 does not fall to pieces." The hypothesis that 
 each atom in a molecule is able to link directly 
 to itself a certain limited number of other atoms 
 is forced on us by such facts as these : the exist- 
 ence of the molecule NH 3 and the impossibility 
 of forming a molecule composed of more than 
 three atoms of hydrogen and one atom of 
 nitrogen, the existence of the molecule H 2 and 
 the impossibility of forming a molecule composed 
 of more than two atoms of hydrogen and one 
 atom of oxygen, the existence of the molecule 
 CH 4 and the impossibility of forming a molecule 
 composed of more than four atoms of hydrogen 
 and one atom of carbon, and the existence of 
 the molecule HC1 and the impossibility of form- 
 ing a molecule composed of more than one atom 
 of hydrogen and one atom of chlorine. It is 
 customary to say that the atom-fixing power of 
 the carbon atom is twice that of the oxygen 
 atom, and is four times greater than the atom- 
 fixing power of the atom of chlorine. The 
 expression atom-fixing power of a specified atom 
 means the maximum number of atoms between 
 which and the specified atom direct action and 
 reaction occurs in any molecule : the atom-fixing 
 power of an atom is measured by the maximum 
 number of atoms of hydrogen with which the 
 specified atom combines to form a molecule. 
 The term valency of an atom is generally em- 
 ployed as synonymous with atom-fixing power. 
 The conventions adopted for expressing the 
 valencies of atoms are two ; the symbol of the 
 element is written with a Roman numeral above 
 
INTRODUCTION. 19 
 
 it, or straight lines equal in number to the 
 valency of the atom are attached to the symbol 
 of the element. 
 
 The molecule of a compound is thought of in 
 chemistry as a definite structure ; each atom is 
 pictured as in direct connexion with a limited 
 number of other atoms ; the reactions of the 
 molecule are regarded as dependent on the kinds 
 of atoms, the number of each kind of atoms, and 
 the way in which the atoms are linked together. 
 This is the only hypothesis which has been found 
 capable of bringing order into the enormous 
 array of chemical facts concerning the compo- 
 sitions and reactions of compounds. 
 
 Those formulae of compounds which represent 
 the structure or constitution of the molecules of 
 these compounds in the language of the hypothesis 
 of atom-linking are called structural or constitutional, 
 or sometimes rational, formula. For instance, two 
 compounds are known to be formed by the 
 union of four atoms of hydrogen with two atoms 
 of carbon and two atoms of chlorine ; the formula 
 C 2 H 4 C1 2 expresses the composition of both com- 
 pounds. To one compound is assigned the 
 constitutional formula 
 
 and to the other the 
 
 H T<r ci ' constitutional formula CI T9" CI - 
 
 H H H H 
 
 In one molecule, both atoms of chlorine are 
 represented as in direct union with the same 
 atom of carbon. In the other molecule, the chlorine 
 atoms are represented as directly linked to dif- 
 ferent atoms of carbon. These compounds are 
 
20 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 formed by different processes, and the reactions of 
 the two compounds differ ; the formulas are ex- 
 pressions of the reactions of formation, and the 
 reactions of decomposition, of the compounds 
 in a special and extremely symbolic language, 
 which has gradually grown as the necessity 
 has been felt for a suitable means of presenting 
 the facts of chemical composition and chemical 
 change. Each formula is a sentence in this 
 language ; it is not a realistic presentment of 
 the molecule itself. If we are to think clearly 
 about the properties of matter we must think 
 of matter as having a grained structure ; but 
 the conception the chemist has formed of a 
 molecule may be, nay almost certainly is, ex- 
 tremely unlike the actual structure of the minute 
 particles of any kind of matter. The very term 
 matter is only a convenient symbol. We must 
 bring together, compare, contrast, generalise the 
 facts regarding composition and reactions. The 
 first necessity is to express the facts in a clear, 
 descriptive, and suggestive language. The 
 chemical past is strewn with the fragments of 
 dead languages. The only mode of expression 
 which has been found capable of expansion and 
 modification is that which assumes the molecule 
 to be a definite structure of atoms, and enables 
 us to think clearly about this structure by 
 picturing it to ourselves as held together by 
 direct actions and reactions between the indi- 
 vidual atoms. As we proceed with the special 
 portion of the chemical story which concerns us 
 in this volume, we shall be convinced of the im- 
 possibility of making progress without a suitable 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 21 
 
 vehicle of expression ; and we shall recognise 
 both the elasticity and the imperfections of 
 that medium by means of which the facts of 
 chemical composition and change are presented. 
 
 CHAPTER II. 
 
 A SURVEY OF THE COMPOSITION AND REACTIONS 
 OF THE COMPOUNDS OF CARBON. 
 
 IF we consider the enormous number of living 
 things in the world, and, withdrawing our 
 thoughts from every other aspect of these living 
 organisms, we regard them only as active pro- 
 ducers of chemical compounds, we shall be over- 
 whelmed with the vastness of the problem that 
 is presented to the chemist. For a part of the 
 chemist's calling is to examine the changes in 
 the collocations of elements that are proceeding 
 every moment in living things, or by the agency 
 of living things, to sort and classify the com- 
 pounds that are produced, to compare and con- 
 trast the processes of change, and to endeavour 
 to express the knowledge thus accumulated in 
 the fewest possible general statements. More- 
 over the examination of the compounds formed 
 in organised beings, or by their aid, has led to 
 the discovery of a more numerous class of com- 
 pounds similar to these, although produced 
 without the help of living laboratories. These 
 compounds also must be analysed and synthes- 
 ised ; their reactions must be discovered and 
 chronicled ; and the resemblances and differences 
 between them must be elucidated. And then, 
 
22 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 the sum of knowledge expressed by the words 
 organic chemistry must be brought into vital 
 connexion with the other parts of the science of 
 chemistry; and the threads that bind this 
 portion of our knowledge of nature with the rest 
 of the domain of natural science must be disen- 
 tangled, and made ready for picking up at the 
 time when all parts of natural knowledge shall 
 be seen to be members of one body " fitly joined 
 together." 
 
 The first thing to be done in the chemical 
 examination of compounds is to find their com- 
 positions. An examination of the compositions 
 of the compounds produced in living organisms, 
 or by the help of these organisms, shows that 
 every one of them is a compound of the element 
 carbon. And those compounds which are allied 
 to the products of organisms, but are produced 
 in the laboratory, are found also to be com- 
 pounds of carbon. The elements with which 
 carbon is combined in by far the greater 
 number of the compounds referred to are these 
 three hydrogen, oxygen, and nitrogen. 
 
 Carbon is an element which exists in more 
 than one form. Diamond is almost pure car- 
 bon; graphite is a mixture of carbon with 
 other substances, and it is possible to remove 
 these substances and obtain pure graphite ; pure 
 amorphous carbon is formed by calcining cane 
 sugar in a covered vessel and treating the black 
 residue with various chemical reagents. If equal 
 weights of diamond, pure graphite, and pure 
 amorphous carbon are burnt in plenty of oxygen, 
 the only product is carbonic acid gas, and the 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 23 
 
 same weight of this gas is obtained in each case. 
 The compounds formed by combining carbon with 
 other elements are the same whether the carbon 
 used is diamond, graphite, or pure charcoal. 
 These facts oblige us to say that diamond, pure 
 graphite, and pure amorphous carbon are forms 
 or varieties of the same element. When diamond 
 is heated to an exceedingly high temperature, out 
 of contact with air or oxygen, it glows, swells, 
 and splits, and after cooling the surface resembles 
 graphite. Amorphous carbon has been changed 
 to the diamond form of the element by dissolving 
 it in cast-iron, melting the carburetted iron in an 
 electric furnace, and allowing it to fall into mer- 
 cury covered with a layer of water ; the spheres 
 of iron which form are found to contain very 
 minute transparent diamonds. 
 
 There are other elements which exist in 
 different forms ; for instance, arsenic, phos- 
 phorus, and oxygen. The differences between 
 the two varieties of arsenic are not very marked ; 
 one form is crystalline, the other is amorphous 
 (that is, without crystalline structure) : amor- 
 phous arsenic changes into crystalline arsenic 
 when it is heated for some time to a low red 
 heat, out of contact with air. Ordinary, or 
 yellow, phosphorus differs very much from red, 
 or amorphous, phosphorus. The yellow variety 
 is a soft, semi-transparent, colourless, crystalline, 
 solid, with a distinct smell ; it glows in the dark, 
 smokes when exposed to the air, and takes fire 
 very easily; it melts at 44 C. [111 F.], dis- 
 solves readily in bisulphide of carbon, and is 
 extremely poisonous. Red phosphorus is a dark 
 
24 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 carmine-coloured, opaque, powder without taste 
 or smell; it does not glow in the dark, and it 
 must be heated to about 250 C. [482 F.] before 
 it takes fire ; it does not melt at a red heat, nor 
 does it dissolve in bisulphide of carbon, and it is 
 not poisonous. The differences between these two 
 varieties of phosphorus are so great that one would 
 seem to be justified in saying the substances are 
 different kinds of matter. Nevertheless the pro- 
 ducts of the union of phosphorus with other 
 elements are the same whether these compounds 
 are formed from the yellow, or from the red, 
 variety : in other words, there is only one class 
 of compounds of phosphorus, and it is a matter 
 of complete indifference whether the compounds 
 are produced from yellow phosphorus, or from 
 red phosphorus, as a starting point. Moreover 
 equal weights of the two kinds of phosphorus 
 are changed into the same quantities by weight 
 of the same compounds ; for instance, if one grain 
 of either variety is burnt in plenty of oxygen, 
 2-29 grains of an amorphous, snow-like solid 
 called phosphorus pentoxide are produced. (The 
 change begins at the ordinary temperature of the 
 air when yellow phosphorus is used, but not till 
 a temperature approaching 500 F. is reached 
 when red phosphorus is employed.) Again if 
 one pound of yellow phosphorus is warmed in a 
 stream of chlorine, there are produced 4-435 Ibs. 
 of a clear, colourless, liquid, which fumes in the 
 air and gives off a vapour that attacks the eyes 
 in a most disagreeable way ; and the same weight 
 of the same liquid called phosphorus trichloride 
 is obtained by warming one pound of red phos- 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 25 
 
 phorus in chlorine. Finally, yellow phosphorus 
 is changed into red phosphorus by heating to 
 about 250 C. [482 F.] in a vessel filled with 
 carbonic acid gas, or nitrogen, or some other 
 gas which is without chemical action on phos- 
 phorus ; and the facts that red phosphorus 
 is the only product, and the weight of red 
 phosphorus formed is the same as the weight 
 of yellow phosphorus used, shew that nothing 
 has been added to, and that nothing has been 
 taken away from, the phosphorus in the pro- 
 cess. The reverse change, that is to say, the 
 change of red into yellow phosphorus, is effected 
 by heating the red variety to about 300" C. 
 [572 F.] in a vessel filled with an indifferent 
 gas ; the weight of yellow phosphorus obtained 
 is the same as the weight of red phosphorus 
 used. 
 
 The manufacture of safety-matches depends on 
 the facts that red phosphorus is not poisonous ; 
 that it can be produced by heating ordinary 
 phosphorus in a gas which does not react with 
 the phosphorus ; and that it cannot be ignited 
 by rubbing, unless the surface on which it is 
 rubbed contains substances rich in oxygen and 
 ready to give up oxygen to the phosphorus. The 
 heads of safety-matches are tipped with a mixture 
 of such compounds as chlorate of potash, chrom- 
 ate of potash, peroxide of lead, and sulphide of 
 lead ; and the rubbing surface on the box is a 
 mixture of red phosphorus and powdered glass, 
 sometimes containing antimony sulphide or man- 
 ganese peroxide, made into a paste with glue. 
 When the match-head is rubbed on the prepared 
 
26 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 surface the oxygen-containing compounds are 
 partially decomposed, oxygen is produced and 
 combines with some of the red phosphorus on the 
 rubbing-surface ; the act of combination produces 
 sufficient heat to cause the ignition of the com- 
 pounds of sulphur in the match-head, and the 
 flame is then passed on to the wooden stem of 
 the match. 
 
 The element oxygen exists in two modifica- 
 tions ; ordinary oxygen, and ozone ; both are 
 gases at the ordinary temperature of the air. 
 When oxygen is submitted to the action of the 
 silent electric discharge a portion of the oxygen 
 is changed into a substance which is a more 
 energetic oxidiser than oxygen. This substance 
 converts mercury, at the ordinary temperature, 
 into black oxide of mercury, a reaction which 
 is not accomplished by oxygen ; when brought 
 into contact with a solution of iodide of potas- 
 sium it produces oxide of potassium, iodine, and 
 oxygen, this also being a reaction which oxygen 
 does not effect. When oxygen which has been 
 submitted to the action of the silent electric dis- 
 charge is cooled to about minus 180 C. [minus 
 292 F.] a blue liquid is formed ; if this is allowed 
 to evaporate oxygen passes off as a gas, and the 
 modification of oxygen called ozone remains, 
 presenting the appearance of a very dark blue 
 liquid, and boiling at minus 106 C. [minus 214 
 F.]. Liquid oxygen boils at about minus 181 C. 
 [minus 294 F.]. Ozone is completely changed 
 into oxygen by heating to low redness ; the 
 weight of the oxygen obtained is equal to the 
 weight of the ozone used ; but the volume of the 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 27 
 
 oxygen is one and a half times as great as the 
 volume of the ozone. Ozone is one and a half 
 times heavier than oxygen, bulk for bulk ; and, 
 as a contraction of volume attends the formation 
 of ozone from oxygen, ozone may be called con- 
 densed oxygen. As both oxygen and ozone are 
 gases under the ordinary conditions of tempera- 
 ture and pressure, it is possible to find their 
 molecular weights (see The Story of the Chemical 
 Elements, pp. 156 and following}. The result is 
 that while a molecule of oxygen is 16 times 
 heavier than a molecule of hydrogen, a molecule 
 of ozone is 24 times heavier than a molecule of 
 the standard element, hydrogen. Now, as the 
 molecular weight of hydrogen is represented by 
 the number two (see Chemical Elements, pp. 160- 
 161), it follows that the molecular weight of 
 oxygen is 32, and the molecular weight of 
 ozone is 48. The atomic weight of oxygen is 
 16 (Chemical Elements, pp. 161-163). The mole- 
 cules of oxygen and ozone are composed of the 
 same kind of atoms, namely, atoms of oxygen ; 
 the molecular symbol of the former is 2 , and 
 the molecular symbol of the latter is 8 . 
 
 The consideration of oxygen and ozone, then, 
 shows that some of the properties of a molecule 
 composed of two atoms of the same kind may be 
 different from the properties of a molecule com- 
 posed of three atoms of the same kind as those 
 which formed what we may call the diatomic 
 molecule. This is a particular instance of the 
 phenomenon already noticed, that the reactions of 
 a molecule are conditioned, among other circum- 
 stances, by the number of the atoms which com- 
 
28 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 pose it. The present instance is especially in- 
 teresting, because the atoms which compose the 
 molecules of oxygen and ozone are all of the 
 same kind. 
 
 The molecular weights of red and yellow 
 phosphorus are not known with certainty, nor are 
 the molecular weights of the three varieties 01 
 carbon known; but, arguing from analogy, on 
 the basis of the similar phenomena presented by 
 oxygen and ozone, it is probable that the number 
 of atoms which compose the molecule of one 01 
 the modifications of phosphorus is different from 
 the number of atoms which compose the mole- 
 cule of the other modification of that element ; 
 and that the molecular weights of the three kinds 
 of carbon are different. 
 
 Carbon is rather an inert element ; it does not 
 very readily enter into union with other elements ; 
 for instance, combination with oxygen begins 
 only at about a red heat. Carbon belongs to the 
 class of non-metallic elements ; it is altogether 
 different in its chemical functions from such 
 elements as iron, copper, zinc, and lead. We 
 have then to consider the compositions and the 
 reactions of the compounds which are formed by 
 the union of this element carbon with other 
 elements, and more particularly with the three 
 elements hydrogen, oxygen, and nitrogen. 
 Hydrogen is the lightest known form of matter ; 
 it is a colourless gas, without smell or taste. 
 Hydrogen burns when a lighted match is brought 
 near it, provided air, or oxygen, is in contact 
 with the hydrogen ; the product of the burning 
 is water. The part played by hydrogen in com- 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 29 
 
 pounds is conditioned by the nature, and the 
 quantities, of the elements with which it is com- 
 bined. For instance, compounds of hydrogen 
 with comparatively large quantities of non- 
 metallic elements, such as sulphur, chlorine, and 
 oxygen, are acids ; whereas the compounds of 
 hydrogen with relatively large quantities of the 
 most markedly metallic elements, in addition 
 to oxygen, are alkalis. (Compare Chemical 
 Elements, pp. 81 to 90.) The elements oxygen 
 and nitrogen are colourless, odourless, tasteless, 
 gases. The former very readily combines with 
 most of the other elements to form oxides ; the 
 latter belongs to the class of inert substances. 
 The compounds which oxygen forms differ ex- 
 ceedingly in their chemical reactions ; some are 
 acidic, others are basic, and others are salts. 
 Most of the acids it is true are compounds of 
 oxygen, but in the class of compounds of oxygen 
 it is necessary to place the alkalis also. (Com- 
 pare Chemical Elements, pp. 109 to 113.) Nitrogen 
 forms a compound with hydrogen which is a 
 marked alkali; and it also forms a compound 
 with the same element which has the charac- 
 teristic reactions of an acid. Oxygen is pre- 
 eminently the supporter of combustion; in the 
 burning of coal, wood, coke, oil, wax, and other 
 sorts of fuel, it is the oxygen in the air which 
 combines with the elements, especially with the 
 carbon of the fuel, and produces the heat and 
 light that we feel and see. If the atmosphere 
 contained a relatively very large quantity of 
 oxygen, all processes of burning would proceed 
 with great rapidity and violence ; but the nitro- 
 
30 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 gen in the air amounts to nearly 80 per cent. 
 of the whole atmosphere, and this inert gaa 
 moderates the intensity, and prolongs the dura- 
 tion, of the processes of combustion. Air is 
 inhaled by all living animals, and changes which 
 are essentially of the nature of burnings proceed 
 in the tissues of animals; but along with the 
 active element oxygen, animals also inhale the 
 moderating element nitrogen, and the restraining 
 action of the latter prevents the too rapid des- 
 truction of the tissues of the living organisms. 
 At very high altitudes breathing becomes diffi- 
 cult; because the air at great heights is so 
 attenuated that the work which must be done 
 in inhaling a sufficient quantity of the gas which 
 feeds the combustion-processes that are necessary 
 accompaniments of the continuance of life is 
 much greater than the work which need be 
 done in order to obtain a sufficient supply of 
 oxygen at ordinary levels. On the other hand, 
 it has been found advantageous to supply 
 patients suffering from certain diseases with 
 pure oxygen, for inhalation, in place of ordinary 
 air ; because when a person is in a state of great 
 exhaustion he is not able to exert sufficient 
 muscular energy to enable him to gain oxygen 
 from ordinary air in sufficient quantity to carry 
 on those chemical changes that must be con- 
 tinued if life is to be prolonged. 
 
 These four elements, carbon, oxygen, hydrogen, 
 and nitrogen, combine in most diverse propor- 
 tions to form a vast multitude of compounds. 
 The composition of every one of these com- 
 pounds can be expressed by the chemical 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 31 
 
 formula C a 6 H c N d . In this formula C means 
 an atom of carbon, an atom of oxygen, H 
 an atom of hydrogen, and N an atom of nitro- 
 gen; and the small letters a, b, c, and d, re- 
 present whole numbers. As the atomic weights 
 of the four elements are these, carbon = 12, 
 oxygen =16, hydrogen = 1, and nitrogen = 14, 
 the formula also tells us that every compound 
 of these elements is composed of some whole 
 multiple of 12 parts by weight of carbon 
 united with some whole multiple of 16 parts 
 by weight of oxygen, some whole multiple of 
 one part by weight of hydrogen, and some whole 
 multiple of 14 parts by weight of nitrogen. 
 Some of the compounds we shall have to con- 
 sider are composed of only two, or three, of the 
 four elements carbon, oxygen, hydrogen, and 
 nitrogen. But carbon is a constituent of every 
 one of them. Carbon is the characteristic ele- 
 ment of the compounds which are produced by 
 the agency of living things, and of many com- 
 pounds that are allied to these. 
 
 The fact that each one of an exceedingly great 
 number of compounds is a combination of carbon 
 with one, or more, of three other elements might 
 make us expect to find some reactions belonging 
 to these compounds, as a class, which should dis- 
 tinguish them, broadly, from all other compounds. 
 The study of the compounds of carbon justifies 
 this expectation. Looked at broadly, these com- 
 pounds are characterised by an unwillingness to 
 enter into chemical reactions ; they are slow 
 to change; many of their interactions drag, 
 pause before they attain completion, and stop at 
 
32 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 resting-places on the road. For instance: all 
 compounds of carbon with the elements hydro- 
 gen and oxygen can be burnt, by heating in 
 oxygen, to produce water and carbonic acid gas ; 
 but if the processes of oxidation are conducted at, 
 or about, the ordinary temperature of the air, a 
 vast number of compounds can be produced many 
 of which are of more complex composition than 
 those which were the starting points of the reac- 
 tions. If alcohol is warmed 'slightly in an open 
 vessel, and a lighted taper is brought near the 
 surface of the liquid, the vapour of alcohol takes 
 fire, and the products of the burning are carbonic 
 acid gas and water ; but if oxygen is produced, 
 somewhat slowly, in contact with alcohol, by heat- 
 ing the alcohol with some mixture of compounds 
 which interact gradually to produce oxygen (for 
 instance, with Condy's fluid and oil of vitriol), a 
 substance called aldehyde is formed; and if this 
 compound is subjected to a very gradual process 
 of oxidation various compounds belonging to 
 the class of sugars are produced. The composi- 
 tion of alcohol is expressed by the formula C 2 H 6 ; 
 the formula C 2 H 4 expresses the composition of 
 aldehyde ; and most of the sugars belong either 
 to the class of compounds which have the com- 
 position C 6 H 12 O 6 , or to the class which has the 
 composition C 12 H 22 O n . There is a comparatively 
 simple compound of carbon and hydrogen called 
 acetylene ; the molecule of this compound is com- 
 posed of two atoms of carbon united with two 
 atoms of hydrogen the formula of the compound 
 is, therefore, C 2 H 2 . When this compound it is 
 a gas is passed through a hot tube, among the 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 33 
 
 products of the changes it undergoes is another, 
 more complex, compound of carbon and hydrogen 
 called benzene which has the composition C 6 H 6 . 
 If benzene is burnt in the presence of plenty of 
 air, or oxygen, water and carbonic acid gas, and 
 these compounds only, are produced. But if 
 benzene is gently warmed with aquafortis (nitric 
 acid), which is a compound that readily gives up 
 part of its oxygen to other bodies, an oily liquid, 
 smelling like oil of bitter almonds, is formed; 
 this oily liquid is called nitrobenzene ; its composi- 
 tion is C 6 H 5 N0 2 . Aniline, C 6 H 5 NH 2 > is made by 
 warming nitrobenzene with iron and acetic acid ; 
 and by treating aniline with suitable oxidis- 
 ing reagents, the complex compound rosaniline, 
 C 20 H 21 N 3 0, is formed. Kosaniline is a colour- 
 less, crystalline solid ; but it combines with acids 
 to form a series of brilliantly coloured salts many 
 of which are used for dyeing purposes. 
 
 These examples illustrate the statement that 
 the interactions of the compounds of carbon, as 
 a class, tend to the formation of compounds more 
 complex than those wherewith the reactions begin. 
 The reactions of compounds which do not contain 
 carbon when these bodies are contrasted with 
 the compounds of carbon they are called inorganic 
 compounds generally proceed at once to their 
 final goal ; these reactions are direct, decisive, 
 definite. The reactions of carbon compounds are 
 hesitating, easy-going, dilatory ; small changes in 
 the conditions of a reaction are often accompanied 
 by great changes in the compositions of the pro- 
 ducts ; the reactions readily wander off into side- 
 paths ; the most trivial excuses for meandering 
 
 
34 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 are accepted ; the mark of these chemical changes 
 is, unwillingness to be done. 
 
 A closer examination of the reactions of the 
 compounds of carbon enables us to say that the 
 atoms of this element are ready to combine with 
 one another, thereby producing groups of atoms 
 all atoms of carbon which hold together firmly, 
 and around which, as around nuclei or frame- 
 works, may be built less stable combinations of 
 other atoms. Such central nuclei of four, five, 
 six, or more atoms of carbon may be compared 
 to the square-set, massive, donjon of a castle ; and 
 the subsidiary groups of other atoms which are 
 attached to the central carbon nuclei may be 
 likened to the hall, kitchens, buttery, and living 
 rooms that are grouped around the central keep. 
 If the castle was attacked, the outworks were 
 carried first ; the keep resisted to the last. So, 
 when the molecule of a complicated carbon com- 
 pound is attacked by chemical reagents, the out- 
 lying atomic groups are disintegrated or changed ; 
 but in many cases the central nucleus of mutually 
 linked carbon atoms remains intact. 
 
 Chemists picture to themselves different modes 
 of linking of two or more atoms of carbon, in 
 order that they may be able to think clearly and 
 definitely about the connexions between the re- 
 actions of the carbon compounds and the com- 
 positions of their molecules. The existence of 
 the molecules CH 4 , CC1 4 , CBr 4 , CI 4 , CCl 3 Br, and 
 the failures that have attended every attempt to 
 isolate a molecule composed of more than four 
 atoms of hydrogen, chlorine, bromine, or iodine 
 united with a single atom of carbon, require us to 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 35 
 
 say that the atom of carbon is tetravalent, or, to 
 use another form of expression, that one atom of 
 carbon can link to itself directly not more than 
 four other atoms of any kind. Now it may 
 reasonably be supposed that a pair of carbon 
 atoms linked together may be able to hold directly 
 to themselves either six, four, or two, atoms of 
 hydrogen. The meaning of this supposition is 
 better realised by presenting it in chemical sym- 
 bols ; using the conventional representation of 
 atom-linking power by straight lines proceeding 
 from the symbol of carbon. The structural 
 formulae of the three hydrocarbon molecules, 
 each containing a pair of carbon atoms the 
 name hydrocarbon is given to all compounds of 
 hydrogen and carbon only are these : 
 
 (i.) H-C-C-H, (ii.) c=c, and (iii.) H - C=C - H. 
 
 H H H H 
 
 In the first case, one fourth of the atom-fixing 
 power of each atom of carbon is thought of as used 
 in holding together the two carbon atoms ; in the 
 second case, one half, and in the third case, three- 
 fourths, of the atom-fixing power of each carbon 
 atom is supposed to be employed in binding the 
 pair of carbon atoms together. The reactions of 
 a molecule which contains a pair of carbon atoms 
 linked in the manner that is represented by the 
 symbol C - C will not be the same as the 
 reactions of a molecule that contains a pair of 
 carbon atoms linked in the manner represented 
 by the symbol C = C; and the reactions of a 
 molecule containing a pair of carbon atoms linked 
 thus, O==C, will differ from those of both of 
 
36 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 the other molecules; and these statements will 
 hold good whatever be the atoms which are 
 united with the pair of atoms of carbon. Inas- 
 much as the hydrocarbon C 2 H 6 is called ethane, 
 the hydrocarbon C 2 H 4 is called ethylene, and the 
 hydrocarbon C 2 H 2 is called acetylene, it has 
 become customary to speak of the ethane-linking, 
 the ethylene-linking, and the acetylene-linking, of a 
 pair of carbon atoms, in different molecules ; one 
 also speaks of a pair of singly linked, or a pair of 
 doubly linked, or a pair of trebly linked, atoms of 
 carbon. 
 
 This language is highly symbolical. The 
 reader must guard himself against supposing that 
 the symbols C-C C = C and C = C are 
 intended to be presentments of the actual 
 arrangements of the atoms of carbon in different 
 molecules one is obliged to think of all the 
 parts of molecules as performing ordered move- 
 ments these symbols are only convenient and 
 workable ways of stating facts in the language of 
 a special hypothesis, which has itself grown out 
 of the application of a general theory of the 
 structure of matter to that class of natural occur- 
 rences called chemical reactions. One especially 
 important fact is told by the symbols we are 
 considering : six atoms of hydrogen or of other 
 monovalent elements can enter into direct com- 
 bination with the atomic group C - C ; whereas 
 only four atoms of hydrogen or of other monova- 
 lent elements can be directly combined with 
 the group C = C ; and only two atoms of hydro- 
 gen, etc., can be directly joined to the group 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 37 
 
 Other modes of linking carbon atoms are 
 conceivable ; and, in order to bring the facts 
 of organic chemistry into one point of view, for 
 the purpose of comparing and contrasting these 
 facts, other modes of linking atoms of carbon 
 must be presented in formulae and used as 
 working hypotheses. The most important of 
 these modes of linking is that which is spoken of 
 as the benzene-linking. The molecule of the 
 hydrocarbon called benzene is composed of six 
 atoms of carbon united with six atoms of hydro- 
 gen ; the formula C 6 H 6 , therefore, expresses the 
 composition of the molecule of this compound. A 
 study of the reactions of benzene leads to the 
 representation of the structure of the molecule of 
 this compound, in terms of the hypothesis of 
 
 MH 
 
 , C=C v 
 
 atom-linking, by the structural formula H <T JLH 
 
 nl 
 
 The atom of carbon is here, as always, taken to be 
 tetravalent, and the atom of hydrogen (as always) 
 to be monovalent. In the benzene-linking, the six 
 atoms of carbon are said to be alternately singly 
 and doubly linked. We shall understand more 
 fully what facts are sought to be conveyed by 
 this expression when we have considered the 
 reactions of benzene and its derivatives. Suffice 
 it to say here that the reactions of benzene are 
 altogether different from those of another hydro- 
 carbon which has the same molecular composition 
 as benzene. 
 
 As has been remarked already (p. 28) it is 
 probable that the molecular weights of the three 
 
38 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 varieties of the element carbon, namely, diamond, 
 graphite, and amorphous carbon, are not the 
 same ; that is to say, it is probable that the num- 
 ber of atoms of carbon bound together into a 
 group which moves about as a whole is different 
 in each case. But there is another possible way 
 of representing the existence of three different 
 carbon molecules in terms of the hypothesis of 
 atom-linking : the three molecules may contain 
 equal numbers of atoms of carbon, but these 
 atoms may be arranged differently in each 
 molecule; the molecule of one variety may be 
 represented by such a symbol as C - C - C - C, 
 the molecule of another variety by such a sym- 
 
 9"9, 
 
 such a symbol as c-c. Or the differences be- 
 * 
 
 bol as 9"9, and the molecule of the third form by 
 
 c 
 
 tween the three varieties of carbon may be 
 thought of as connected with differences in the 
 modes of linking of, say, a pair of carbon atoms : 
 if this hypothesis is adopted, then one molecule 
 may be represented by the symbol C C, another 
 by the symbol C = C, and the third by the sym- 
 bol 0=0. 
 
 The atom of hydrogen is monovalent, the 
 atom of oxygen is divalent, and the atom of 
 nitrogen is trivalent ; that is to say, an atom of 
 hydrogen can bind to itself directly only a single 
 other atom, an atom of oxygen can enter into 
 direct union with two other atoms, and an atom 
 of nitrogen can be linked directly with three 
 other atoms. Now, consider the bearing of these 
 
A SURVEY OF THE COMPOUNDS OF CAEBON. 39 
 
 statements on the question of the possibility of 
 forming compounds by adding outlying groups 
 of atoms to a pair of linked carbon atoms. Take 
 a pair of singly linked atoms of carbon ; C C. 
 It is possible, theoretically, to join to this group 
 six atoms of hydrogen, or of other monovalent 
 elements, but not more than six such atoms ; the 
 compounds C 2 H 6 , C 2 H 5 C1, C 2 H 4 Br 2 , etc., have 
 been isolated. But suppose five atoms of hydro- 
 gen and one atom of oxygen to be united to the 
 group C - C : as the oxygen atom is capable of 
 uniting directly with two other atoms, the pre- 
 sence of this atom brings with it the possibility 
 of adding another monovalent atom to the 
 aggregate ; the existence of such a molecule as 
 C 2 H 5 OH becomes possible. This possibility is 
 more clearly grasped if the structural formula is 
 
 H H 
 employed: H-C-C-O-H. The compound C 2 H 5 OH 
 
 H H 
 
 is alcohol. Now suppose that five atoms of hydro- 
 gen and one atom of nitrogen are combined with 
 the pair of singly linked carbon atoms : as the 
 atom of nitrogen can hold to itself directly three 
 other atoms, the entrance of this nitrogen atom 
 into the molecular building makes it possible, 
 theoretically, to add two more monovalent atoms 
 in order to complete the molecule. The struc- 
 tural formula of one such finished molecular 
 
 building is H -CC-M^. The compound C 2 H 5 NH 2 
 
 is called ethylamine. 
 
 These illustrations throw some light on the 
 statement, made on p. 34, that the atoms of car- 
 
40 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 bon tend to combine together to form what may 
 be called the foundations of molecular aggregates, 
 and that the molecular structures are completed 
 by building on these foundations. When com- 
 pounds whose molecules contain several atoms of 
 carbon are subjected to the action of moderately 
 active reagents, the tendency generally is towards 
 the occurrence of chemical changes in what may 
 be described as the outlying parts of the molecular 
 structures. For instance ; there is a compound 
 called dimethyl-benzene, the composition and the 
 reactions of which lead us to give it the structural 
 
 ! M ! 
 formula H V C -c' " VIC'-H . When this compound is 
 
 H' I *C-C* ! *H 
 
 : 6* i 
 
 submitted to the action of oxidising reagents, 
 that is, compounds which react to produce 
 oxygen, the two outlying CH 3 groups (repre- 
 sented in the formula to the right and left of 
 the dotted lines) are changed into groups which 
 contain oxygen, and whose composition is ex- 
 pressed by the structural formula |. _ M . The 
 
 compound produced is C 6 H 4 (C0 2 H) 2 ; it is 
 called phthalic acid. When this phthalic acid 
 is heated with lime, benzene (C 6 H 6 ) is obtained ; 
 that is to say, the two outlying C0 2 H groups 
 only are attacked, but the foundation of six 
 linked atoms of carbon remains unshaken. It 
 is only by the action of very energetic reagents, 
 aided by a high temperature, that the central 
 nucleus of six carbon atoms is broken down. 
 Another instance of the comparative ease with 
 
A SURVEY OF THE COMPOUNDS OF CAHBON. 41 
 
 which the side groups, or side chains as they are 
 generally called, of molecular structures built on 
 the foundation of six linked carbon atoms are 
 attacked by reagents, while the central nucleus 
 remains intact, is furnished by some reactions, 
 the starting point of which is a compound called 
 diazobenzene nitrate. The structural formula of 
 
 M 
 this compound is M c( -N-N-C-N ( ? . When this 
 
 Si * 
 
 compound is boiled with water, nitrogen is given 
 off, nitric acid (HN0 3 ) is formed and dissolves in 
 the water, and a compound called phenol (more 
 commonly known as carbolic acid), the composi- 
 tion of which is expressed by the formula 
 C 6 H 5 .OH, is also produced ; in other words, 
 the whole of the side chain N = N-N0 8 is re- 
 moved ; another and simpler side chain, OH, is 
 put in its place ; but the central nucleus is un- 
 touched. If phenol is treated with iodine, the 
 compound tri-iodophenol, CgH^.OH, is formed ; if 
 phenol is caused to react with nitric acid, three 
 compounds are formed, called nitrophenol, dinitro- 
 phenol, and trinitrophenol, and having the com- 
 positions C 6 H N0 2 .OH, C 6 H 3 .(N0 2 ) 2 .OH, and 
 C 6 H 2 .(N0 2 ) 3 .OH ; if phenol is heated with con- 
 centrated sulphuric acid a compound called 
 phenolsulphonic acid is formed, the composition 
 of which is C 6 H 4 .HS0 8 .OH ; lastly, the products 
 of distilling phenol with zinc dust are, zinc oxide 
 and benzene C 6 H 6 . In none of these reactions is 
 the central group of six linked carbon atoms 
 broken down ; the reactions consist in removing 
 
42 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 side chains and putting other groups in their 
 places, or in putting groups of atoms, or single 
 atoms, in the places of one or more of the atoms 
 of hydrogen which are attached to the founda- 
 tion of six linked atoms of carbon. 
 
 The examples of the reactions of carbon com- 
 pounds we have been considering shew that the 
 language wherein these reactions are recorded 
 and it is the only language that has been found 
 capable of expressing the facts intelligibly is 
 steeped in the conceptions of the molecular and 
 atomic theory. Unless a clear picture is kept in 
 one's mind of the molecule as an aggregate of 
 atoms arranged in an orderly manner, and held 
 together by actions and reactions between its 
 parts, the facts of organic chemistry become 
 merely a dust heap of disconnected details. 
 The conception of the fixed atom-linking powers 
 of the different kinds of atoms, and that of the 
 removal of a group of atoms from a molecule 
 and its replacement by another group, or by a 
 single atom, are perhaps the two most important 
 molecular and atomic conceptions to be grasped 
 clearly in trying to follow the element carbon in 
 its wanderings from compound to compound.* 
 
 The compounds of carbon are slow to enter 
 into deep-reaching chemical reactions ; they are 
 ready to form more complex compounds ; and 
 
 * I would request the reader to refer to pp. 120 to 141, 
 and 175 to 178, of The Story of the Chemical Elements for 
 an elementary treatment of the notion of atom-linking, 
 or atom-fixing, power ; and to pp. 169 to 175 of the same 
 book for a brief consideration of the notion of the com- 
 pound radicle, or atomic group. 
 
A SURVEY OF THE COMPOUNDS OF CARBON. 43 
 
 also to break down to simpler compounds, but 
 only to a limited extent ; they are characterised 
 by a kind of immobile plasticity ; they are at 
 once stable and unstable ; small alterations in 
 their conditions induce changes, but these 
 changes proceed leisurely with many stopping 
 places : while these statements are true, it is 
 also true that under the proper conditions the 
 compounds of carbon are decomposed with the 
 production of carbonic acid gas, and water, and 
 some simple nitrogenous 'compounds. The 
 compounds of carbon are evidently well suited 
 for the part they play in the life-processes of 
 plants and animals : their number is legion ; 
 their properties vary much ; every phase of the 
 life of an organism is marked by the production 
 or the disintegration of some of these com- 
 pounds ; changes in the conditions of the organ- 
 ism are accompanied by changes in the kind of 
 carbon compounds formed within it, and the 
 production of new compounds must tend to cause 
 a development, or a retrogression, of the organ- 
 ism in new directions. It was supposed at one 
 time that the carbon compounds which are 
 formed in living organisms could be formed only 
 by the agency of what was called " vital force " ; 
 but since thousands of such compounds have 
 been synthesised in the laboratory, that hypo- 
 thesis, which was essentially unscientific, inas- 
 much as it did not help research into natural 
 occurrences, has been abandoned. The hypo- 
 thesis of "vital force" does not aid us in 
 classifying and generalising the facts of organic 
 chemistry. It would be better not to employ 
 
44 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 the expression organic chemistry, were it not that 
 the more accurate phrase the chemistry of the 
 compounds of carbon is too cumbersome. At the 
 same time we ought not to forget that our know- 
 ledge of the compositions and reactions of the 
 compounds of carbon has not enabled us to 
 explain the phenomena of life. 
 
 CHAPTER III. 
 
 AN OUTLINE OF THE CLASSIFICATION OF SOME 
 OF THE COMPOUNDS OF CARBON. 
 
 THE purposes of classification are these: To 
 place together those objects which are like, and 
 to separate those which are unlike ; to discover 
 and set forth general expressions of the simi- 
 larities that are observed ; to aid the formation 
 of clear conceptions of the characters of the 
 objects classified ; and to help the memory to 
 retain these conceptions. 
 
 The object of this book will be better served 
 by attempting to classify a comparatively small 
 number of compounds of carbon, than by allow- 
 ing the attempt to range over the whole field. 
 By far the greater number of the compounds of 
 the element whose migrations we shall endeavour 
 to follow are composed of carbon united with 
 some, or all, of the three elements, hydrogen, 
 oxygen, and nitrogen : although I propose to 
 deal in the main only with compounds of these 
 four elements, it will be necessary to make 
 
CLASSIFICATION OF COMPOUNDS OF CARBON. 4 
 
 mention also of carbon compounds that contain 
 chlorine, bromine, or iodine. 
 
 I shall first of all consider, briefly, the two 
 oxides of carbon. 
 
 The other compounds of carbon to which 
 attention will be directed fall into two main 
 classes ; those that more or less resemble paraffin, 
 and those which on the whole are like benzene. 
 The name paraffinoid compounds is given to the 
 first class, and the name benzenoid com/pounds to 
 the second class. The terms fatty and aromatic 
 are much used, as synonymous with paraffinoid 
 and benzenoid; because the former compounds 
 include the common fats, and the latter include 
 the aromatic bodies that are found in plants. 
 The starting points from which the arrangements 
 of the two classes of compounds proceed are, the 
 hydrocarbon marsh gas for the first class, and 
 the hydrocarbon benzene for the second class. 
 
 The systematic name of marsh gas is methane. 
 From this compound, which is the simplest com- 
 bination of carbon and hydrogen its composition 
 and molecular weight are expressed by the for- 
 mula CH 4 are derived: first, chloromethane 
 (CH 3 C1) and chloroform (CHC1 3 ), then methylic 
 alcohol (CH 3 .OH) (commonly known as wood 
 spirit), then methylic aldehyde (CH 2 0), formic acid 
 (CH 2 2 ), and methylamine (CH 3 .NH 2 ). A simi- 
 lar series of compounds is derived from the 
 simplest hydrocarbon formed by the union of 
 hydrogen atoms with a pair of atoms of carbon : 
 ethane (C 2 H 6 ) forms chloro-ethane (CJLC1), 
 ethylic alcohol (C 2 H 5 OH), ethylic aldehyde (C 2 H 4 0), 
 ethylic ether ([C 2 H 6 ] 2 0), acetic acid (C 2 H 4 2 ), and 
 
46 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 ethylamine (C 2 H 5 .NH 2 ). Mention will be made of 
 certain other compounds belonging to the classes 
 of alcohols and acids ; and we shall also consider 
 a few compounds derived from acids, especially 
 certain ethereal salts. 
 
 The sugars, and the compounds allied to them 
 called starches or amyloses will be glanced at. 
 
 From benzene (C 6 H 6 ), which is the simplest 
 member of the class of aromatic compounds, is de- 
 rived phenol (C 6 ll..OE) t benzoic acid (C 6 H 5 .C0 2 H), 
 and salicylic acid (C 6 H 4 .OH.C0 2 H) ; also nitro- 
 benzene (C 6 H 5 .N0 2 ), and aniline (C 6 H 5 .NH 2 ). 
 The aromatic hydrocarbon anthracene (C^Hjg) 
 is connected with the alizarin colours. Lastly, it 
 will be advisable to consider, very slightly, the 
 alkaloids, and albumin. 
 
 The division of the compounds of carbon into 
 the two classes of fatty and aromatic compounds 
 rests, primarily, on differences of properties. An 
 examination of the reactions of members of each 
 class shews that the differences in properties are 
 accompanied by differences in composition, but 
 that the latter differences are rather dissimilari- 
 ties in molecular structure than in the relative 
 quantities of the elements in the molecules of the 
 two kinds of compounds. For instance ; there 
 are two hydrocarbons whose composition is ex- 
 pressed by the formula C 6 H 6 . The compositions 
 of these two compounds are identical, if the term 
 composition is employed in the ordinary restricted 
 sense. A molecule of one of the hydrocarbons 
 combines very readily and rapidly with eight 
 atoms of bromine to form a molecule of the com- 
 pound C 6 H 6 Br 8 ; the other hydrocarbon molecule 
 
CLASSIFICATION OF COMPOUNDS OF CARBON. 47 
 
 will not combine with more than six atoms of bro- 
 mine, and the formation of the molecule C 6 H 6 Br 6 
 is accomplished only under the influence of direct 
 sunshine and then very slowly. These and other 
 reactions of the two hydrocarbons shew that one 
 of them belongs to the class of fatty (or paraffinoid) 
 compounds, and the other to the class of aromatic 
 (or benzenoid) compounds. The only means we 
 have of intelligibly representing the differences 
 between the reactions of the two hydrocarbons 
 is by supposing that the six atoms of carbon are 
 arranged in essentially different ways in the two 
 molecules, C 6 H 6 . 
 
 The consideration of the processes by which 
 the changes are effected from the hydrocarbon 
 methane (CH 4 ) to formic acid (CH 2 2 ), or from 
 the hydrocarbon ethane (C 2 H 6 ) to acetic acid 
 (C 2 H 4 2 ), and the examination of the gradual modi- 
 fications of properties and reactions which accom- 
 pany these changes of composition, will furnish 
 examples of what was said in the last chapter 
 about the general character of the reactions of 
 the compounds of carbon. The many stages that 
 may be distinguished in the course of a chemical 
 change between carbon compounds, and the un- 
 willingness of these changes to rush to com- 
 pletion, will also be illustrated by following the 
 string of reactions that begins with benzene 
 (C 6 H 6 ), and, for our purpose, ends with salicylic 
 acid (C 6 H 4 .OH.C0 2 H). Such slight examina- 
 tions as we shall be able to make of one or two 
 sugars, of alizarin and the compounds allied to 
 alizarin, and of the alkaloids, will serve to 
 introduce the reader to the conception of mole- 
 
48 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 cular symmetry, and to give him a glimpse of the 
 vast field of inquiry that is opening as the in- 
 fluences of the presence of atoms of carbon, and 
 of their modes of linking with one another and 
 with other atoms, on the properties and the 
 functions of atomic aggregates, are being con- 
 nected with the special phenomena which are 
 exhibited by living organisms. 
 
 I hope also, to give illustrations of the employ- 
 ment in chemical industries of the reactions and 
 properties of the compounds whose classification 
 is being considered ; and to show how large and 
 extensive are many of the technical applications 
 of chemical changes which would certainly appear 
 to a superficial observer to be absolutely dis- 
 connected with anything that is " useful." 
 
 CHAPTER IV. 
 
 THE TWO OXIDES OF CARBON. 
 
 IF air is passed slowly through a heap of smoulder- 
 ing charcoal, coke, or coal, chemical changes take 
 place, and that compound of carbon which is the 
 chief product of these changes is composed of one 
 atom of carbon united with one atom of oxygen. 
 If a rapid current of air is driven over a small 
 quantity of burning charcoal, coke, or coal, 
 another oxide of carbon is produced, and this 
 compound is formed by the union of one atom 
 of carbon with two atoms of oxygen. The com- 
 position of the first compound is expressed by 
 
THE TWO OXIDES OF CARBON. 49 
 
 the formula CO, and that of the second com- 
 pound by the formula C0 2 . The former com- 
 pound is called carbon monoxide, the latter, carbon 
 dioxide or carbonic acid gas. Both compounds are 
 colourless gases ; both are poisonous : carbon 
 monoxide poisons by removing oxygen from the 
 blood, and forming a compound with a substance 
 called haemoglobin which is the main constituent 
 of the red corpuscles of the blood of vertebrate 
 animals; carbon dioxide, called choke-damp by 
 miners, poisons by cutting off the supply of 
 oxygen and causing suffocation. Carbon mon- 
 oxide takes fire when a light is brought near it, 
 if air or oxygen is present, and burns with a 
 pale blue-violet flame, producing carbon dioxide. 
 The dioxide is not combustible, and the presence 
 of a few parts of this gas in a hundred parts of 
 air stops the burning of a candle. Carbon 
 dioxide is about one and a half times heavier 
 than air, bulk for bulk ; if this gas is produced 
 in a closed place where the air is at rest it ac- 
 cumulates in the lower portions of the enclosed 
 space. Any specified volume of carbon mon- 
 oxide weighs very slightly less than an equal 
 volume of air. Carbon monoxide readily re- 
 moves oxygen from many compounds of that 
 element, forming carbon dioxide and some sub- 
 stance containing less oxygen than that from 
 which the oxygen has been removed; for in- 
 stance, most oxides of metals are deprived of 
 their oxygen by heating them in a stream of 
 carbon monoxide. Carbon dioxide, on the other 
 hand, does not combine with oxygen ; it has not 
 been found possible to cause more than two 
 D 
 
50 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 atoms of oxygen to combine with one atom of 
 carbon. It is customary to express these facts, 
 that carbon monoxide will combine directly with 
 oxygen (and also with certain other elements) and 
 that the dioxide will not combine directly with 
 oxygen (or other elements), by saying that car- 
 bon monoxide is an unsaturated compound, and 
 carbon dioxide is a saturated compound. Carbon 
 monoxide is very slightly soluble in water ; one 
 pint of water at about 50F. dissolves about '027 
 of a pint of the gas. The solution is neither 
 alkaline nor acid. Carbon dioxide is more 
 soluble in water than the monoxide; one pint 
 of water at 50F. dissolves about T185 pints 
 of the gas at the ordinary pressure of the air, 
 and at 68F. only about nine-tenths of a pint at 
 the atmospheric pressure. The weight of the 
 gas dissolved increases directly with the pres- 
 sure. In other words; while one litre of water 
 (about If pints) dissolves a quantity of carbon 
 dioxide weighing 2 '346 grams (about 36 J grains) 
 at 10C [50F.], when the pressure on the sur- 
 face of the water and the gas is the ordinary 
 pressure of the atmosphere (equal to about 14f 
 Ibs. on the square inch), the same volume of 
 water at the same temperature dissolves 2 '346 
 x 2 = 4'692 grams (about 72f grains) when the 
 pressure is doubled, and 2*346 x 3 = 7'038 grams 
 (about 109 grains) when the pressure is trebled. 
 If the pressure on a solution of 4 '692 grams of 
 carbon dioxide in one litre of water is reduced 
 to one half, 2 - 346 grams (that is, one half) of the 
 carbon dioxide escape ; and if the pressure on 
 a solution of 7 '038 grams of the gas in one litre 
 
THE TWO OXIDES OF CARBON. 51 
 
 of water is reduced to one third, 4'692 grams 
 (that is, two thirds) of the gas escape. A solu- 
 tion of carbon dioxide in water has feebly acidic 
 properties. 
 
 Soda water is made by forcing carbon dioxide 
 into water by increasing the pressure on the sur- 
 face of the gas and the water. It is customary to 
 bottle soda water at a pressure varying from 8 to 
 9 atmospheres ; that is to say, at a pressure from 
 8 to 9 times greater than the ordinary pressure 
 of the atmosphere. As the ordinary atmospheric 
 pressure is equal to about 14| Ibs. on the square 
 inch, soda water is bottled at a pressure equal to 
 from 14} x 8 == 118 Ibs., to 14} x 9 = 132f Ibs., on 
 the square inch. Now, one pint of water will 
 dissolve, at the ordinary temperature (say, 50F.), 
 approximately 166 grains of carbon dioxide when 
 the pressure is equal to that of 8 atmospheres 
 ( = 118 Ibs. on the square inch), and approxi- 
 mately 187 grains of carbon dioxide when the 
 pressure is equal to that of 9 atmospheres ( = 132} 
 Ibs. on the square inch); when the pressure is 
 reduced to the ordinary atmospheric pressure by 
 opening the bottle of soda water, about 130 grains 
 of carbon dioxide, equal to about If gallons of 
 the gas, will escape in the first case, and about 
 151 grains, equal to about 2 gallons of the gas, 
 will escape in the second case. 
 
 Both carbon monoxide and dioxide have been 
 liquefied, and both have been frozen to white, 
 snow-like solids, by subjecting them to pressure 
 at a very low temperature. If carbon dioxide 
 gas is compressed it refuses to become liquid 
 unless the temperature is 30'9C. [87'6F.], 
 
52 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 or lower than 30'9 e C. At that temperature, a 
 pressure 73 times greater than the ordinary 
 pressure of the atmosphere (that is, 14 j x 73 = 
 1076f Ibs. on the square inch) is required to 
 change gaseous into liquid carbon dioxide. At 
 lower temperatures less pressure is needed to 
 effect the change. The temperature 30'9C. 
 [87-6F.l is called the critical temperature of 
 carbon dioxide. There is a critical temperature 
 for each gas, that is, a temperature above which 
 the gas cannot be caused to become liquid by 
 compressing it. The critical temperatures of dif- 
 ferent gases are very different ; for instance, the 
 critical temperature of gaseous alcohol is about 
 240C. [464F.l and that of hydrogen is about 
 minus 234C. [minus 389F.J. This statement 
 means that gaseous alcohol can be liquefied by 
 pressure provided the temperature is lower than 
 464F.; but that if it is wished to liquefy hydrogen 
 by compressing it, the gas must be cooled to at 
 least 389 below the zero of Fahrenheit's scale. 
 
 Liquid carbon dioxide is a marketable com- 
 modity ; it is sold in strong steel bottles which 
 generally contain about 15 Ibs. of the liquid 
 = about 1 2 cub. feet of the gas at the ordinary 
 temperature of the air. Liquid carbon dioxide 
 is used for extinguishing fires, for propelling 
 torpedoes, for obtaining cold by allowing the 
 liquid to evaporate, for aerating water, and for 
 other purposes. Small spheroidal vessels of steel 
 containing liquid carbon dioxide are now sold, 
 under the name of " sparklets," for making soda, 
 water. One of the little vessels is placed in a 
 rubber washer at a short distance above the 
 
THE TWO OXIDES OF CARBON. 53 
 
 water in a strong glass bottle ; a sharp pin is 
 fixed in the middle of the washer, and the 
 " sparklet " is forced by a screw-cap on to this 
 pin which perforates the material of the little 
 vessel; the liquid carbon dioxide at once be- 
 comes gas, and this gas bubbles into the water 
 wherein it dissolves. 
 
 Enormous quantities of carbon dioxide exist in 
 the atmosphere. The total weight of this gas in 
 the air is probably about 3 billion tons ; more 
 accurately, 10,000 volumes of country-air contain 
 about 3 volumes of carbon dioxide. The quantity 
 of this gas in the air varies slightly in different 
 localities and at different seasons ; there is slightly 
 more in sea-air than in the air of country places, 
 and country-air contains a little more carbon 
 dioxide during the night than during the day. 
 The air of towns contains more carbon dioxide 
 than the air of country places : the average 
 amount in 10,000 volumes of the air in the streets 
 of London is 3 -8 volumes, in the streets of 
 Manchester, 4 volumes, and in the streets of 
 Glasgow, 5 volumes, while the average amount 
 in 10,000 volumes of country-air is 2'99 volumes. 
 Air taken from the tunnels of the Metropolitan 
 railway was found to contain from 14 to 15 vol- 
 umes of carbon dioxide in 10,000 volumes of the 
 air. As fogs interfere with the circulation of the 
 air and with the mixing of the various consti- 
 tuents by diffusion, it is to be expected that the 
 proportionate quantity of a gas which is being 
 poured every moment into the atmosphere of 
 every town, as a product of the burnings of fuel 
 and the breathings of men and animals, will be 
 
54 THE STORY OF THE WANDERINGS OF ATOMS, 
 
 very much altered by the occurrence of fogs. This 
 expectation is justified by measurements of the 
 quantity of carbon dioxide in town-air during 
 fogs ; the quantity of that gas sometimes amounts 
 to 10 volumes per 10,000 of foggy air. Inasmuch 
 as 100 volumes of the expired breath of human 
 beings contain about 4 volumes of carbon di- 
 oxide, it is evident that the atmosphere of rooms 
 wherein many people are congregated must soon 
 become rich in this poisonous gas, unless some 
 efficient method of ventilation is adopted to re- 
 move the carbon dioxide and replace it by fresh 
 air. A school-room, or lecture-room, is generally 
 supposed to be efficiently ventilated if the amount 
 of carbon dioxide does not exceed 6 or 8 volumes 
 per 10,000 volumes of air; in very many school- 
 rooms the quantity of carbon dioxide amounts to 
 10 volumes per 10,000; it frequently rises to 15 
 volumes, and in not a few cases to 20, or 25, or even 
 to 35 volumes per 10,000 volumes of air ; in some 
 schools in Austria as much as 55 volumes of this 
 gas have been found in 10,000 volumes of the air 
 of the rooms. In some cases, for instance in 
 schools at Aberdeen, Dundee, and Edinburgh, it 
 has been shown that the highest Government 
 grants, per scholar, are earned by those children 
 who attend the best ventilated schools ; in the 
 schools at Sheffield, on the other hand, no con- 
 nexion could be traced between the ventilation of 
 the schools and the amount of the grants earned. 
 Carbon dioxide is produced in vast quantities 
 by the burning of coal, coke, charcoal, oil, candles, 
 and all fuel and illuminants which contain carbon. 
 This gas is found in almost all natural waters ; 
 
THE TWO OXIDES OF CARBON. 55 
 
 it is one of the products of the putrefaction and 
 decay of animal and vegetable matter ; and it is 
 produced during the fermentation of sugar-con- 
 taining materials. When chalk is heated, it is 
 decomposed into lime and carbon dioxide. This 
 process is conducted, on a large scale, in lime- 
 kilns. A quantity of coal is thrown into the 
 kiln, then a cartload of chalk broken into pieces, 
 then more coal, then more chalk, and so on ; the 
 coal is ignited, and the heat produced causes the 
 separation of the chalk into carbon dioxide which 
 escapes at the top of the kiln, and lime which 
 sinks to the bottom. The process is generally 
 made continuous, by throwing more chalk and 
 fuel into the kiln at the top, while the lime is 
 removed at the bottom from time to time. The 
 reverse process to that which occurs in a lime- 
 kiln is accomplished by passing carbon dioxide 
 over lime ; the two compounds unite, and chalk 
 is formed. 
 
 Pure carbon dioxide is usually prepared by 
 decomposing pure calcium carbonate (chalk and 
 marble are more or less pure calcium carbonate) 
 by diluted hydrochloric acid, washing the gas by 
 passing it through water, drying it by causing it 
 to bubble through concentrated sulphuric acid, 
 and collecting it in dry vessels. Pure carbon 
 monoxide may be prepared by burning a large 
 quantity of pure carbon in a slow stream of 
 oxygen, and passing the gaseous products of 
 the reaction, in a slow current, through a solu- 
 tion of potash which absorbs the carbon dioxide 
 but not the monoxide ; the gas that is not 
 absorbed by the potash solution is washed, 
 
56 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 dried, and collected. The monoxide is more 
 generally prepared by decomposing oxalic acid 
 by hot concentrated sulphuric acid, passing the 
 gases produced, which consist of the two oxides 
 of carbon, through several bottles containing a 
 solution of potash, and then washing and drying 
 that gas which is not absorbed by the potash. 
 
 Carbon dioxide can be decomposed into its 
 elements by passing the gas over burning mag- 
 nesium ; the oxygen of the dioxide combines 
 with the magnesium, forming magnesia, and 
 carbon appears as a black solid. A process 
 somewhat like this is brought about by the 
 green parts of living plants aided by the sun- 
 light. . Plants absorb carbon dioxide, from the 
 air and the rain, and separate it into its con- 
 stituent elements ; much of the oxygen which is 
 thus produced is breathed out by the plants into 
 the atmosphere, while almost the whole of the 
 carbon is retained and used by the plants in 
 building up their tissues. It is not to be sup- 
 posed that the living plant actually separates 
 the carbon dioxide into carbon and oxygen, and 
 then brings about the combination of the separ- 
 ated carbon with other elements; rather, the 
 compounds in the green parts of the plant, aided 
 by sunshine and moisture, react with the carbon 
 dioxide which the plant has absorbed, and the 
 products of these reactions are, various more or 
 less complex carbon compounds which remain in 
 the plant, and oxygen which is returned to the 
 atmosphere. It is also to be remembered that 
 only a portion, although a large portion, of the 
 oxygen of the absorbed carbon dioxide is sent 
 
THE TWO OXIDES OF CARBON. 57 
 
 back into the atmosphere ; some of it is used 
 by the plant, just as the carbon is used, in 
 building up its tissues. These processes occur 
 under the influence of sunshine ; the reactions 
 which take place in the living plant during the 
 night are broadly similar to those that occur in 
 a living animal, and consist, in the main, in 
 changes wherein carbon dioxide is produced and 
 exhaled by the plant. 
 
 When a stream of carbon dioxide is passed 
 through a heap of red hot charcoal, or coke, 
 coal, or other carbonaceous fuel, half of the 
 oxygen of the dioxide is taken away by the 
 hot carbon, and carbon monoxide is produced. 
 The changes of composition that occur may 
 be expressed in a chemical equation thus : 
 C + C0 2 = 2CO. The gas which passes away 
 from the upper surface of the heap of burning 
 fuel consists chiefly of carbon monoxide, mixed, 
 perhaps, with a little of the dioxide which has 
 escaped the deoxidising action of the carbon of 
 the fuel; if the gas issues from the surface of 
 the burning fuel into the air, the carbon mon- 
 oxide in the hot gas combines with oxygen in 
 the air and is burnt to carbon dioxide. The 
 change that takes place at the surface of the 
 burning heap of fuel may be expressed in an 
 equation thus : CO + = C0 2 . At the begin- 
 ning of this chapter it was stated that carbon 
 dioxide is produced by passing plenty of air, 
 or oxygen, into burning charcoal, coke, or coal. 
 Eemembering these reactions, let us consider the 
 chemical changes that take place in a coal fire 
 when the coal has become red hot. Air is con* 
 
58 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 stantly entering at the bottom and sides of the 
 grate : some of the oxygen in the air seizes the 
 carbon in the outer and lower layers of coal and 
 combines with it to produce carbon dioxide ; as 
 this carbon dioxide passes through the glowing 
 coal in the inner part of the grate it is deprived 
 of half of its oxygen by the red hot carbon, and 
 carbon monoxide is formed ; finally, as the car- 
 bon monoxide leaves the surface of the burning 
 fuel it mixes with plenty of air, and as both the 
 air and the monoxide are very hot, the carbon 
 monoxide is burnt to carbon dioxide which 
 passes by the chimney into the air outside the 
 house. When a large fire is burning in a grate, 
 and the coal has become red hot throughout, and 
 has ceased to give off smoke and little puffs of 
 gas which take fire and burn with some bril- 
 liancy, a pale bluish flame may be seen playing 
 fitfully over the upper surface of the mass of 
 glowing fuel ; this flame is the accompaniment 
 of the burning of carbon monoxide to dioxide 
 that is taking place where the monoxide, pro- 
 duced in the centre of the fuel, meets the 
 oxygen in the air of the chimney. 
 
 I wish to direct attention to one other reaction 
 whereby a mixture of the two oxides of carbon 
 is produced. When steam is passed over such 
 carbonaceous materials as coke, anthracite coal, 
 or charcoal, the steam is robbed of its oxygen, 
 and hydrogen and oxides of carbon are formed. 
 By regulating the temperature, it is possible to 
 produce either carbon dioxide or carbon monoxide, 
 or a mixture of these gases. The principal re- 
 action that occurs between steam and hot carbon 
 
THE TWO OXIDES OF CARBON. 59 
 
 at temperatures lower than 500C. [932 F.] is 
 expressed by the following chemical equation : 
 2H 2 O + C = COn-f 4H ; at a temperature towards 
 1000C [1832F.] the same quantity of steam 
 reacts with twice as much carbon as at the lower 
 temperatures, and the products are hydrogen and 
 carbon monoxide (2H 2 O + 2C = 2CO + 4H.) As 
 both carbon monoxide and hydrogen are gases 
 that are easily burnt, and as the burning of these 
 gases is accompanied by the production of much 
 heat but very little light, an inflammable gas of 
 great heating power, but of no use as an illu- 
 minant, may be produced by passing steam 
 over extremely hot carbon. The product of 
 this reaction is known as water-gas. The 
 changes that occur when water-gag is burnt 
 in air are these: (i) 4H + 20 = 2H 2 0, (ii) 
 2CO + 20 = 2C0 2 . The heating power of water- 
 gas is rather less than half that of an equal 
 volume of coal-gas. On the other hand, the 
 volume of air required for the complete com- 
 bustion of water-gas is less than one third of 
 that required for the perfect burning of an equal 
 volume of coal-gas, and the flame of water-gas is 
 absolutely free from smoke ; for these reasons, 
 water-gas is very useful for obtaining high tem- 
 peratures when a ' clear ' heat is required, as, for 
 instance, in welding metals, burning porcelain, 
 and melting glass. Some years ago it was 
 noticed that water-gas which had been kept 
 compressed in steel* cylinders for some time 
 emitted much light when it was burnt, and that 
 iron was deposited from the burning gas. In- 
 vestigation proved that part of the carbon 
 
00 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 monoxide in the water-gas had combined with 
 some of the iron of the cylinders, produc- 
 ing a volatile compound of iron and carbon 
 monoxide, and that this compound was decom- 
 posed when the gas was burnt, with the deposition 
 of particles of oxide of iron which became red hot 
 and evolved much light. 
 
 The discovery that carbon monoxide combines 
 with iron to form a volatile substance was pre- 
 ceded by the discovery of a volatile compound 
 of carbon monoxide and nickel. When finely 
 powdered oxide of nickel is heated to bright 
 redness in a stream of hydrogen, the oxygen is 
 removed and very finely divided nickel remains ; 
 if this nickel is allowed to cool gradually in 
 contact with carbon monoxide a gaseous com- 
 pound of the nickel with the monoxide is pro- 
 duced; and when this gaseous compound is 
 heated strongly it is decomposed into nickel, 
 which is deposited as a lustrous solid, and carbon 
 monoxide which passes away as a gas. These 
 reactions suggested a new method for extracting 
 nickel from its ores. Most nickel ores contain 
 the metal combined with arsenic, or sulphur; 
 when these ores are roasted in air nickel oxide 
 is produced ; if the oxide is then deoxidised by 
 heating in a stream of hydrogen, the metal 
 thus formed is allowed to cool in a slow current 
 of carbon monoxide, and the gaseous compound 
 of the nickel and the carbon monoxide is heated, 
 pure nickel is obtained. This process has been 
 used on the large scale recently ; it is simpler 
 and more expeditious than the older methods for 
 extracting nickel from its ores ; and, considering 
 
THE TWO OXIDES OF CARBON. 61 
 
 the large quantity of nickel now used for nickel 
 plating, the new process is likely to come into 
 favour. 
 
 Mention has been made of the deoxidising action 
 of carbon monoxide. When this gas is passed over 
 red hot oxide of copper, the oxygen of the oxide 
 is removed, and carbon dioxide and copper are 
 produced. Many other oxides of metals are re- 
 duced, or deoxidised, by heating them in contact 
 with carbon monoxide, and the metals are 
 obtained. The deoxidising action of carbon 
 monoxide is made use of, on a very large scale, 
 in the manufacture of iron from ironstone in 
 blast furnaces. Charges of iron oxide and coke 
 are thrown into the furnace, which resembles an 
 enormous lime-kiln, the fuel is ignited, and hot 
 air is blown in near the bottom of the furnace ; 
 the carbon of the coke is burnt by the oxygen of 
 the air, chiefly to carbon dioxide ; the carbon 
 dioxide is robbed of half of its oxygen by the hot 
 carbon, with which it comes into contact at a 
 little distance above the point where the air-blast 
 is sent in, and carbon monoxide is produced ; the 
 carbon monoxide reacts with the oxide of iron to 
 produce carbon dioxide and iron ; the iron sinks, 
 and melts, and is run from the furnace into 
 moulds ; the ascending carbon dioxide is reduced 
 to monoxide by contact with fresh layers of hot 
 carbon ; more iron oxide is deprived of its 
 oxygen, and more carbon dioxide is formed ; the 
 dioxide is again reduced to monoxide which 
 reacts once more with oxide of iron ; finally, 
 carbon monoxide, mixed with some dioxide, 
 passes off at the mouth of the furnace. Many 
 
62 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 other reactions occur in the blast furnace ; 
 for our purpose it is sufficient to notice one of 
 these. The hot carbon of the coke and the oxide 
 of iron interact to produce iron, and carbon 
 monoxide ; so that, besides what is formed by 
 the combustion of the fuel, a supply of the 
 deoxidising agent is produced by the direct action 
 of the fuel on the oxide of iron. 
 
 In this chapter we have followed a few of 
 those wanderings of carbon wherein the element 
 does not venture very far afield. The partner of 
 carbon in the journeyings which we have noticed 
 has been the element oxygen. We have seen an 
 atom of carbon joining company with a pair of 
 atoms of oxygen, and the triplet swinging off 
 into the atmosphere. We have seen the meeting 
 of the molecule of carbon dioxide with a molecule 
 of lime, and the production, by their union, of a 
 molecule of chalk. We have followed the splitting 
 of this molecule of chalk, in a lime kiln, into 
 the two simpler molecules which coalesced to 
 form it, and the return to the atmosphere of the 
 carbonic acid gas which had been for a time im- 
 prisoned in the soil by reason of its temporary 
 union with lime. We have pictured to ourselves 
 the seizure of the molecule of carbon dioxide by a 
 plant, the dissolution, by the combined action of 
 the green parts of the plant and the sunshine, of 
 the partnership of the atoms of carbon and 
 oxygen, and the incorporation of the atom of 
 carbon whose wanderings we have been following 
 into the substance of the plant. It is not difficult 
 to imagine the consumption of the plant by an 
 animal, the union in the body of the animal of 
 
THE TWO OXIDES OF CARBON. 63 
 
 the carbon atom with two atoms of oxygen, and 
 the escape into the atmosphere of the molecule of 
 carbonic acid gas thus formed : nor would it be 
 less easy to follow this molecule as it is carried, 
 by the inrushing stream of air, into a brightly 
 burning fire of coals, to see one of the atoms of 
 oxygen torn from the other atoms of the molecule 
 by a red hot atom of carbon, and to behold the 
 restoration to the molecule of carbon monoxide 
 of another atom of oxygen, in place of that which 
 the carbon removed, at the moment when the 
 molecule leaves the glowing fire and comes into 
 contact with the oxygen in the cooler air. 
 Instead of following the molecule of carbon 
 dioxide into a coal-fire, we might picture its 
 entry into a blast furnace ; and, after half of its 
 oxygen had been removed by one of the crowd of 
 rushing carbon atoms which it encountered 
 there, we might be spectators of the seizure by 
 the half satisfied molecule of carbon monoxide 
 of an atom of oxygen which had long been 
 quietly united with iron, and the formation, once 
 more, of a molecule of carbonic acid gas. And, 
 finally, we may follow the molecule of carbonic 
 acid gas as it is again captured, and torn asunder, 
 by a growing plant ; and then, passing over tens 
 of thousands of years, we may be mental spec- 
 tators of the digging from the earth of a piece 
 of coal that has been produced by the very 
 gradual decay, in the absence of air and light, 
 of the plant which long ago had built into its 
 structure the atom of carbon whose migrations 
 we have been witnessing. In this piece of coal 
 we recover our atom of carbon unworn and un- 
 
64 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 changed, in every respect the same as when it 
 started on its wanderings it may be a million 
 years ago. 
 
 CHAPTER V. 
 
 MARSH GAS, AND CERTAIN COMPOUNDS 
 DERIVED THEREFROM. 
 
 IF the mud at the bottom of a pool which con- 
 tains the remains of dead plants is stirred, 
 bubbles of gas rise to the surface. If a wide- 
 mouthed bottle is filled with water, and inverted 
 in the pool so that the rising bubbles of gas pass 
 into the bottle, the water is driven out of the 
 bottle, and its place is taken by the gas. Suppose 
 that a bottle has been filled with this gas, and 
 that a lighted taper is brought near the mouth 
 of the bottle ; the gas takes fire, and burns with 
 a tolerably luminous flame. Now let another 
 bottleful of this marsh gas be mixed with its own 
 volume of the gaseous element chlorine (which is 
 a yellow, very suffocating, gas obtained from 
 hydrochloric acid, or from bleaching powder), 
 and let the mixture be placed in sunlight ; a re- 
 action proceeds slowly, and after some hours both 
 the marsh gas and the chlorine have disappeared, 
 and in their place have been produced hydro- 
 chloric acid gas, and a pleasantly smelling gaseous 
 compound, called chloromethane, which burns with 
 a green-edged flame when a lighted taper is 
 brought near it. Now let some of this chloro- 
 methane be dissolved in an aqueous solution of 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 65 
 
 caustic potash, let the liquid be boiled for several 
 days (in an apparatus so arranged that any 
 volatile products of the reaction are condensed and 
 caused to flow back into the liquid), and let the 
 contents of the vessel then be distilled ; a colour- 
 less, mobile liquid with a spirituous odour, boil- 
 ing at about 64 C. [147 F.], is obtained. This 
 liquid is called methylic alcohol. If this alcohol is 
 oxidised very slowly and carefully, a portion of 
 it is converted into formic aldehyde ; and if the 
 oxidation is carried farther, a transparent, colour- 
 less liquid, with a sour taste, called formic acid, 
 is produced. If methylic alcohol is heated with 
 salammoniac, a colourless gas is formed which 
 smells like ammonia. This gas is called methyl- 
 amine. 
 
 We have now to examine these excursions of 
 carbon, from the compound called marsh gas to 
 that named methylamine. The compositions of 
 the compounds of carbon with which we have to 
 deal are expressed by the following formulae : 
 marsh gas, CEL ; chloromethane, CH 3 C1 ; me- 
 thylic alcohol, CH 4 ; formic aldehyde, CH 2 ; 
 formic acid, CH 2 2 ; and methylamine, CH 5 N. 
 We shall find our main interest in this series of 
 compounds in an attempt to trace the relations 
 between the reactions of these bodies, and then 
 to express, or at least to suggest, these rela- 
 tions of function by formulae which exhibit 
 relations of molecular structure. Marsh gas 
 the systematic name of this compound is methane 
 is chlorinated in sunshine ; one of the products 
 of this reaction (for several compounds are pro- 
 duced) is kept in contact with a boiling, aqueous 
 E 
 
(56 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 solution of potash for some days ; a portion of 
 the methylic alcohol that is slowly formed is 
 caused to react, very gradually, with oxygen ; 
 the formic aldehyde which is the product of this 
 change is oxidised to acetic acid ; finally, the rest 
 of the methylic alcohol is boiled with salam- 
 moniac, and methylamine is obtained. The 
 following chemical equations present statements 
 of the changes of composition which occur in 
 these five reactions : 
 
 (i) CH 4 + 2 C1 = CH 3 C1 + HC1. 
 (ii) CH 3 
 (iii) 
 
 (iv) CH 2 + = CH 2 2 . 
 (v) CH 4 + NH 4 C1 = CH 5 
 
 What are the characteristic chemical pro- 
 perties of the six compounds beginning with 
 CH 4 and finishing with CH 5 N ? The chief re- 
 action of chloromethane (CH 3 C1) which interests 
 us at present is that with potash solution which 
 has been mentioned already, and is represented 
 in equation (ii) above. The mere presentation 
 of the compositions of methane and methylic 
 alcohol by the formula CH 4 and CH 4 seems to 
 show that the statement made in former chapters 
 that an atom of carbon never directly unites with 
 more than four other atoms, is not true. But, is 
 the atom of carbon in the molecule CH 4 in 
 direct union with all the other atoms of the mole- 
 cule 1 The form of this question must be changed 
 before an answer can be hoped for by experi- 
 mental inquiry. The question must be put in 
 some such words as these : are the reactions of 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 67 
 
 methylic alcohol in keeping with the hypothesis 
 that the atom of carbon is directly linked to all 
 the other atoms in a molecule of this compound ? 
 
 The metal potassium (caustic potash is a com- 
 pound of this metal with oxygen and hydrogen) 
 dissolves in methylic alcohol, forming the 
 compound CH 3 KO ; however great a quantity of 
 potassium is added, no other compound is pro- 
 duced. This reaction is expressed in molecular 
 and atomic language by saying that an atom 
 of the metal potassium turns out one atom, and 
 only one atom, of hydrogen from the molecule of 
 methylic alcohol ; hence, there is some difference 
 of function between one of the hydrogen atoms 
 and the other three hydrogen atoms in the mole- 
 cule CH 4 0. When hydrochloric acid gas (HC1) is 
 passed into methylic alcohol, in the presence 
 of some substance which very readily combines 
 with water but does not interact with methylic 
 alcohol, chloromethane and water are formed ; 
 the following equation expresses the change : 
 CH 4 + HC1 = CH 3 C1 + H 2 0. 
 
 In this reaction, the atom of oxygen in a mole- 
 cule of methylic alcohol has been removed along 
 with one atom of hydrogen, and one atom of 
 chlorine has been put in the place of the oxygen 
 and hydrogen atoms taken away. Here again a 
 difference is shewn between the part played, in 
 the molecule, by one of the atoms of hydrogen 
 and the parts played by the other three hydrogen 
 atoms. In both of the reactions described, three 
 atoms of hydrogen remain in combination with 
 the atom of carbon in the molecule CH 4 0; in 
 one reaction an atom of hydrogen is taken out of 
 
68 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 the molecule (in exchange for an atom of potas- 
 sium), and in the other reaction an atom of 
 hydrogen and also the atom of oxygen are 
 removed (in exchange for an atom of chlorine). 
 We do not know what are the relations to one 
 another of the atoms in the molecule CH 4 O ; but, 
 using one of the rough, crude, symbols with 
 which we are compelled to work at present, we 
 can represent the inter-atomic relations in this 
 molecule by a structural formula, which certainly 
 advances our knowledge, because it is an instru- 
 ment that helps us to conduct further inquiries. 
 The structural formula suggested for the molecule 
 of methylic alcohol by the reactions that have 
 
 H 
 been described is H-C-O-H. This formula repre- 
 
 H 
 
 sents one atom of hydrogen in direct union with 
 the only atom of oxygen in the molecule ; it 
 represents the other three atoms of hydrogen in 
 direct union with the carbon atom of the mole- 
 cule, and as all related in the same way to the 
 rest of the molecule. The structural formulae of 
 the products of these reactions that have been 
 described would be these : 
 
 ? ? 
 
 H-C-O-K *d H-C-CI 
 H H 
 
 What we know of the reactions of other com- 
 pounds of potassium, and of other compounds of 
 chlorine, is in keeping with the assertion implied 
 in these two formulas that an atom of potassium, 
 and an atom of chlorine, directly links to itself a 
 single other atom. 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 69 
 
 Let us make another hypothesis. Let it be 
 assumed that the four atoms of hydrogen and also 
 the atom of oxygen are directly linked to the 
 atom of carbon in the molecule of methylic 
 alcohol : the structural formula of this molecule 
 
 will then be H-C'-O . 
 
 H 
 
 But if this is a correct translation into a 
 language we can use as an instrument of thought, 
 of the relations of the atoms that form the molecule 
 CH 4 0, why is there a difference between the 
 function of one of the hydrogen atoms and the 
 functions of the other three, and why cannot the 
 atom of oxygen be removed, and replaced by 
 chlorine, without carrying with it one of the atoms 
 of hydrogen ? Moreover, the symbol which 
 represents the atom of carbon in direct union 
 with all the other atoms of the molecule CH 4 O, 
 runs counter to what we know regarding the 
 atom-fixing power of this element. If it is 
 possible for a single atom of carbon to link 
 directly to itself five other atoms, then we must 
 rebuild the whole edifice of organic chemistry ; 
 wondering the while at the sight of a stable and 
 stately building resting on nothing. 
 
 We must come back, then, to the symbol 
 HgC.OH, which is a less cumbrous form of the 
 
 symbol H-C-O-H, as a working representation of 
 
 the structure of the molecule of methylic alcohol. 
 Is this structural formula in keeping with the 
 changes that occur when methylic alcohol is 
 oxidised 1 To answer this question, let us look 
 
70 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 at the processes whereby methylic alcohol is 
 changed, first to formic aldehyde (CH 2 0), and 
 then to formic acid (CH 2 2 ), and at the reactions 
 of these two compounds. The first step of the 
 change may be accomplished by passing a current 
 of air charged with vapour of methylic alcohol 
 over finely divided platinum, and then cooling the 
 gases ; in this way a solution of formic aldehyde 
 in methylic alcohol is obtained. Finely divided 
 platinum absorbs large volumes of many gases ; 
 in the present case it absorbs oxygen from the 
 air, and also the vapour of methylic alcohol, and 
 brings these two into such close contact that a 
 chemical change occurs. Formic aldehyde re- 
 moves oxygen from many compounds of that 
 element, and is thus converted into formic acid, 
 which is a colourless liquid with the sour taste 
 and corrosive action on the skin that characterise 
 the acids as a class. Formic acid, CH 2 O 2 , reacts 
 with metals to form salts. All these salts 
 called formates contain one atom of carbon, one 
 atom of hydrogen, and two atoms of oxygen, or 
 some whole multiple of one atom of carbon, one 
 of hydrogen, and two of oxygen, in their mole- 
 cules ; in other words, they all consist of metal 
 united with the group of atoms CH0 2 , or with 
 2CH0 2 , or, generally, with 7iCH0 2 , where n is 
 a whole number (not greater than 4). The 
 composition of the formates may be expressed 
 by saying that only one atom of hydrogen in the 
 molecule of formic acid (CH 2 2 ) can be replaced 
 by an atom of a metal. But this is equivalent to 
 the statement that one of the two atoms of 
 hydrogen in the molecule CH 2 2 performs a 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 71 
 
 function different from that of the other atom ; 
 one of the atoms of hydrogen seems to be more 
 firmly held to the carbon atom than the other 
 atom of hydrogen is held. We found the struc- 
 
 H 
 
 tural formula H-C-O-H to be a fair expression of 
 
 H 
 the reactions of methylic alcohol. The structural 
 
 . H-C-O-H ,. _ 
 
 formula expresses the reactions 01 
 
 formic acid ; and the formula H " < J"" M expresses 
 
 o 
 
 the reactions of formates, where M is an atom of 
 such a metal as potassium, sodium, or silver. 
 In these formulae one, and only one, atom of 
 hydrogen is represented in direct union with the 
 atom of carbon. The formulse are in keeping 
 with reactions by which the acid and its salts 
 are prepared ; for instance, potassium formate is 
 produced by passing carbon monoxide (CO) over 
 moist caustic potash (KOH), and the acid is 
 formed by the action of an electric discharge on 
 a mixture of carbon dioxide and hydrogen. 
 Both of these reactions are in keeping with the 
 formulse given above : thus, 
 
 C=O + K.O.H = H-C-O-K; ando=C = O + H-H = H-C'O-H. 
 O O 
 
 Moreover, formic acid readily decomposes to 
 carbon dioxide and hydrogen; and it is very 
 easily oxidised to carbon dioxide and water. 
 These two reactions find simple expressions by 
 
72 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 using the structural formula given to the acid : 
 thus, 
 
 H-C-O-H = 
 
 O 
 H-C-O-H +0 = O=C=0 + H-O-H. 
 
 O 
 
 If, then, the reactions of formic acid are sug- 
 gested by picturing the arrangement of the 
 atoms in a molecule of this compound by the 
 
 formula II : what structural formula will 
 o 
 
 best present the reactions of formic aldehyde in 
 terms of the arrangement of the atoms of the 
 molecule CH 2 ? The molecule of formic alde- 
 hyde contains an atom of oxygen less than the 
 molecule of formic acid ; is it the atom of oxygen 
 in direct union with the carbon atom only, or 
 the atom of oxygen in direct union with atoms of 
 carbon and hydrogen, which is added to the mole- 
 cule when the aldehyde is oxidised to the acid ? 
 In other words ; which of the two formulae 
 H-C-H or H-C-O-H 
 
 o 
 
 better expresses the reactions of formic aldehyde ? 
 We found that methylic alcohol reacts as if the 
 atoms were arranged in a manner which can be 
 roughly represented by the structural formula 
 H 3 C H ; we also found that when this 
 compound is heated with hydrochloric acid, the 
 oxygen atom and an atom of hydrogen are re- 
 moved, their place is taken by an atom of 
 chlorine, and the compound H 3 C.C1 is produced. 
 Now, if the arrangement of the atoms in the 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 73 
 
 molecule of formic aldehyde is that pictured by 
 the formula H - - - H, one might reasonably 
 expect that hydrochloric acid would react with 
 this compound (if it reacts at all) to form 
 H - C - Cl. From the reactions of other com- 
 pounds we learn that, when oxygen only is 
 removed from a carbon compound and its place 
 is taken by chlorine, two atoms of chlorine al- 
 ways take the place of a single atom of oxygen ; 
 
 H-f*-H 
 
 hence, if formic aldehyde has the formula u > 
 we might expect hydrochloric acid to react (if it 
 
 H - f**H 
 
 reacts at all) to produce the compound n 
 
 CI 2 
 
 Neither oxygen alone, nor oxygen and hydrogen, 
 can be removed from formic aldehyde by the 
 action of hydrochloric acid ; but if pentachloride 
 of phosphorus (PC1 5 ) is employed and this is a 
 compound which reacts with many compounds 
 of carbon to put chlorine in the place of oxygen, 
 or of oxygen and hydrogen oxygen only is 
 removed, and two atoms of chlorine are put in 
 the place of the atom of oxygen which is taken 
 away from the molecule of formic aldehyde. In 
 other words, we do not obtain H C Cl, but 
 
 H "<? Ia Hence, we conclude that the structural 
 formula to be assigned to formic aldehyde is H ~S" H . 
 
 The relations which an examination of their 
 reactions shews to exist between the three 
 compounds methylic alcohol, formic aldehyde, 
 and formic acid, may, then, be conceived as 
 
74 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 relations between the arrangements of the atoms 
 which form the molecules of these compounds 
 by using the following formulae : 
 
 H 
 H-C-O-H H-p-H and H .Q- O -H 
 
 Methylic alcohol. Formic aldehyde. Formic acid. 
 
 There remains the compound methylamine, 
 CH 5 N. What are the relations of this com- 
 pound to methane, methylic alcohol, formic 
 aldehyde, and formic acid ? Methylamine is a 
 colourless gas, which becomes a liquid at a tem- 
 perature considerably under the freezing point 
 of water ; it smells like ammonia ; like ammonia, 
 it is very soluble in water, forming a solution 
 which is very caustic (as a solution of ammonia 
 is); this solution precipitates the hydroxides of 
 many metals from solutions of salts of these 
 metals, a reaction that is brought about also by 
 a solution of ammonia. Methylamine combines 
 with acids and forms salt-like compounds; and 
 in each case the salt-like compound is composed 
 of the methylamine and the acid, and not only of 
 certain constituent portions of the methylamine 
 and the acid. Ammonia also combines with 
 acids, producing salts that are composed of 
 one, two, or more molecules of ammonia united 
 to one, two, or more molecules of the reacting 
 acid. The reactions of methylamine are, evi- 
 dently, like the reactions of ammonia. Now, 
 similarity of reactions accompanies similarity of 
 composition ; using the word composition to 
 include molecular structure. Hence the com- 
 positions of methylamine and ammonia are, pro- 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 75 
 
 bably, similar. When the formulae of the two 
 compounds are placed side by side, NH 3 and 
 CH 5 N, no marked likeness is forced on our 
 notice. Let us turn back to the structural 
 formulae which were found good working repre- 
 sentations of the reactions of methane (or marsh 
 gas), chloromethane, and methylic alcohol ; these 
 formulae are 
 
 H * * 
 
 H-C-H H-C-CI wd H-C-O-H 
 
 & A A 
 
 The second compound is derived from the first 
 by replacing an atom of hydrogen by an atom of 
 chlorine, and the third from the first by replacing 
 an atom of hydrogen by an atom of oxygen and 
 an atom of hydrogen directly linked together ; 
 the molecules of chloromethane and methylie 
 alcohol are represented as each containing an 
 atom of carbon directly united with three atoms 
 of hydrogen. Now, suppose that the reaction 
 between methylic alcohol and sal ammoniac, 
 whereby methylamine is formed, is stated, in 
 structural formulas, in this way 
 
 H tf H 
 
 H-C-0-H+ HH 4 Cl - H-C-<;;+ H 2 + HCl 
 ft ft 
 
 then, the molecule of methylamine, also, is repre- 
 sented as containing an atom of carbon directly 
 joined to three atoms of hydrogen ; and, farther, 
 the molecule of methylamine is represented as 
 derived from a molecule of ammonia by replacing 
 an atom of hydrogen by the group of atoms 
 CH 3 . The relation which is here asserted to 
 exist between the molecular structures of am- 
 
76 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 monia and methylamine becomes more evident 
 if the two formulae are compared : 
 
 Ammonia. Methylamine. 
 
 But is the structural formula given to methy- 
 lamine in keeping with other reactions of this 
 compound ? When hydriodic acid (HI) reacts 
 with ammonia, a compound of the two (called 
 ammonium iodide) is produced ; the reaction 
 may be thus expressed in an equation, 
 
 = NH 3 .HI. 
 
 There is a compound called iodomethane, similar 
 to chloromethane, the composition of which is 
 given by the formula (H 3 C)I ; it is a compound 
 of an atom of iodine with the group of atoms 
 CH 3 (the bracket is used to shew this clearly). 
 If this compound reacted with ammonia what 
 should we expect to be formed ? I think we 
 might reasonably expect this reaction to occur, 
 
 (H 3 C)I + NH 3 = NH 3 .(H 3 C)I. 
 
 That is, we might reasonably expect a com- 
 pound to be produced similar to that which is 
 produced by the combination of HI and NH 3 . 
 In place of using HI we are using (CH 3 )I and, 
 therefore, in place of getting NH 3 .HI we expect 
 to get NH 3 .(CH 3 )L Iodomethane and ammonia 
 combine to form NCH 6 I. But is NCfl 6 I pro- 
 perly represented by the structural formula 
 NH 3 .(CH 3 )I? Turn back to the compound of 
 ammonia and hydriodic acid, NH 3 HI : when 
 this compound is heated with potash, there are 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 77 
 
 produced ammonia, iodide of potassium, and 
 water; thus, 
 
 NH 3 .HI + KOH -= NH 3 T KI + H 2 0. 
 If the compound NCHJ, formed by the union 
 of iodomethane [(CH S )IJ and ammonia (NH 3 ), is 
 properly represented by the formula NH 3 .(CH 3 )I, 
 then we should expect potash to react with it 
 and produce methylamine [(CH 3 )NH 2 ], iodide of 
 potassium, and water ; thus, 
 
 NH 3 .(H 3 C)I + KOH = NH 2 (CH 3 ) + KI + H 2 0. 
 Potash does react with the compound formed 
 by the union of iodomethane and ammonia, and 
 the products of the reaction are methylamine, 
 iodide of potassium, and water. 
 
 Putting the whole of these reactions together, 
 we are evidently justified in asserting that the 
 reactions of methylamine are expressed by the 
 
 H V 
 
 structural formula >-C-H. 
 
 H H 
 
 The relations which are shewn to exist 
 between the six compounds methane, chloro- 
 methane, methylic alcohol, methylamine, formic 
 aldehyde, and formic acid, by a study of their 
 reactions, are summarised by the structural 
 formulae which are given to the molecules of 
 these compounds, provided one is acquainted 
 with the language in which these formulas are 
 expressed. The structural formulae, in abbrevi- 
 ated form, are these : 
 
 H 3 C.H H 8 C.CI H 8 C.OH H g C.NH a H.C.H and H.C.OH 
 
 O O 
 
 Methane. Chloro- Methylic Methyl- Formic Formic 
 
 methane. alcohoL amine. aldehyde. acid. 
 
78 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 Looking at the reactions of tn"e compounds in 
 this series, in a broad and general way, it is 
 noticed that a difference between the functions 
 of the atoms of hydrogen in the molecule of 
 any one of the compounds distinctly begins 
 when methylic alcohol is reached ; one, and 
 only one, of the hydrogen atoms in the mole- 
 cule HgC.OH can be replaced by the metal 
 potassium. A comparison of some of the re- 
 actions of methylic alcohol and formic acid is 
 instructive. The molecules of both of these 
 compounds are represented as containing the 
 group of atoms OH. The hydrogen atom 
 of this group in formic acid can he replaced by 
 a metal when the acid reacts with the hydro- 
 oxide of a metal; for instance, H.CO.OH + 
 KOH = H.CO.OK + H 2 0, and H.CO.OH + NaOH 
 = H.CO.ONa + H 2 0. But the hydroxides of 
 metals do not react with methylic alcohol ; 
 the hydrogen atom of the group - OH in the 
 molecule of this compound can be replaced, it 
 is true, by potassium or sodium (but not by 
 metals in general), but only by causing the 
 alcohol to react with the metal itself in place 
 of using the hydroxide of the metal. 
 
 The hydrogen atom in the group - OH in the 
 molecule of formic acid is generally said to be 
 acidic hydrogen ; because a characteristic reaction 
 of acids, as a class, is, that part or all of their 
 hydrogen can be replaced by metals by causing 
 them to react with hydroxides of metals. If we 
 examine the two formulae H 3 C.OH and H.CO.OH 
 more closely, we see that the carbon atom to 
 which the group -OH is directly attached is 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 79 
 
 in direct union with an atom of oxygen in the 
 molecule of formic acid, but in direct union with 
 three atoms of hydrogen in the molecule of 
 methylic alcohol. Oxygen is a very decided 
 non-metallic element; in many respects, hydro- 
 gen is a metal. As we proceed we shall find 
 that when, in a molecule of a compound, an atom 
 of hydrogen is attached, directly or not, to an 
 atom of carbon which also holds directly to itself 
 atoms of a very distinctly non-metallic element, 
 that atom of hydrogen can be replaced by metal 
 in very many cases, by reactions between the 
 compound and metallic hydroxides. 
 
 The linking of the markedly non-metallic 
 atoms to the carbon atom impresses acidic 
 functions on the atom of hydrogen which is 
 in union with the carbon atom. The meaning 
 of this statement will be clearer, and it will 
 be possible to give a more definite expression 
 to the statement, when we have become ac- 
 quainted with the reactions of a number of com- 
 pounds. Neither of the atoms of hydrogen in 
 the molecule of formic aldehyde (H.CHO) has 
 acidic functions ; yet the carbon atom is re- 
 presented, in the structural formula, as in direct 
 union with an atom of oxygen. But there are 
 two atoms of the non-metallic oxygen in the 
 molecule of formic acid, and only one in that of 
 formic aldehyde; and one of the oxygen atoms 
 in the molecule of the acid is directly linked 
 both to the carbon atom and also to the atom 
 of hydrogen which is acidic in its reactions. 
 
 But, as I have said, more knowledge of the 
 reactions of individual compounds, and more 
 
80 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 acquaintance with the use of structural for- 
 mulae as expressions of reactions, are required 
 before we can profitably approach, except in 
 the most general way, questions concerning the 
 influence, on the functions of this or that atom 
 in a molecule, of the nature of the other atoms, 
 and the arrangement of all the atoms, in the 
 molecule. 
 
 Methylic alcohol is known commercially as 
 Wood Spirit. A very brief account of the manu- 
 facture of wood spirit may not be out of place 
 here. When wood is heated to a high tempera- 
 ture in an iron retort, many different products 
 are obtained ; charcoal and ash remain in the 
 retort ; hydrogen, carbon monoxide and dioxide, 
 marsh gas, and many other carbon compounds, 
 pass off as gases ; and the liquid portion of the 
 distillate contains, among other substances, 
 water, tar, benzene, pyroligneous acid, and 
 wood spirit. The last named substance is crude 
 methylic alcohol. The pure alcohol is obtained 
 by adding lime to the liquid part of the product 
 of the distillation of wood, then distilling again, 
 and collecting that portion of the distillate which 
 is lighter than water, adding a little sulphuric 
 acid to this distillate and once more distilling, 
 then adding chloride of calcium which combines 
 with the methylic alcohol and forms a solid sub- 
 stance; this solid compound is then separated, 
 and decomposed by distilling it with water, when 
 the alcohol passes over at a temperature much 
 below the boiling point of water. Lime is added 
 to the crude distillate in order that lime salts of 
 
MARSH GAS AND SOME ALLIED COMPOUNDS. 81 
 
 the acids which are present may be formed; 
 when the liquid is then distilled these lime salts 
 remain in the retort. The distillate is shaken 
 with sulphuric acid, in order that salts of the 
 alkaline compounds present may be formed, 
 which salts will remain behind when the next 
 distillation-process is conducted. 
 
 These processes, whereby methyl alcohol is 
 obtained from a liquid which contains many 
 other compounds, are examples of the methods 
 generally used for such a purpose. The com- 
 pounds that are not wanted are eliminated, 
 one by one, by causing them to form new 
 combinations, which differ so much in their 
 physical properties from the compound that is 
 to be obtained that they may be separated from 
 that compound by making use of these differ- 
 ences. If the desired compound cannot be 
 separated completely by such processes, it is 
 caused to combine with some other substance 
 to form a new compound, which can be purified, 
 and then decomposed with the separation of the 
 wished-for compound. 
 
 There are two other compounds derived from 
 marsh gas which demand our attention for a 
 moment. When marsh gas is mixed with three 
 times its volume of chlorine, and the mixture is 
 exposed to sunlight, the main product of the re- 
 action which occurs is a compound that has the 
 composition CHC1 3 . This compound is called 
 chloroform. A similar combination of carbon, 
 hydrogen, and iodine, which has the composition 
 CHI 3 , and is called iodoform, is produced by 
 heating chloroform with hydriodic acid (HI), 
 F 
 
82 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 and also by other reactions. Both chloroform 
 and iodoform may be prepared from ordinary 
 alcohol ; the former by heating the alcohol with 
 bleaching powder, the latter by causing the 
 alcohol to react with potash and iodine. The 
 formulae of the two compounds CHC1 3 and 
 CHI 3 suggest the names trichloromethane and 
 tri-iodomethane ; and these are the names by 
 which the compounds are designated in a 
 systematic method of nomenclature. Chloro- 
 form is a liquid with a sweetish taste and a 
 somewhat spirituous odour ; iodofonn is a yellow, 
 crystalline solid, with a very incisive and per- 
 sistent smell. Chloroform was discovered by 
 Liebig in 1831. About 1847 Sir James Simp- 
 son made use of it as an anaesthetic (that is, 
 a substance which produces loss of sensibility) in 
 surgical operations ; and since that time it has 
 been very much employed for rendering patients 
 insensible to pain during operations. Iodoform 
 is much used as an antiseptic (that is, a substance 
 which prevents putrefaction) in the treatment 
 of wounds. 
 
 CHAPTER VI. 
 
 ETHANE AND SOME OF ITS DERIVATIVES. 
 
 THE series of compounds to be considered in this 
 chapter is very similar to the series beginning 
 with marsh gas and ending with methylamine, that 
 was noticed in Chapter V. Among the gases 
 which arise from natural petroleum is a hydro- 
 carbon whose composition is expressed by the 
 
ETHANE, AND SOME OF ITS DERIVATIVES. 83 
 
 formula C 2 H 6 . This compound is called ethane. 
 Treatment of ethane with chlorine produces 
 chloro-ethane, C 2 H 5 C1; ethylic alcohol, C 2 H 6 0, is 
 formed by the reaction of a boiling solution of 
 caustic potash with chloro-ethane ; ethylic alcohol 
 can be oxidised, first to acetic aldehyde, CgH^O, 
 and then to acetic acid, C 2 H 4 2 ; and ethylamine, 
 C 2 H r N, is produced by heating ethylic alcohol 
 with a concentrated watery solution of ammonia. 
 The processes whereby the compounds from 
 chloro-ethane to ethylamine are formed from the 
 hydrocarbon ethane, are evidently very like the 
 processes whereby the corresponding series of 
 compounds, from chloromethane to methylamine, 
 is produced from the hydrocarbon methane ) 
 moreover, the chemical properties of any com- 
 pound in the first series very closely resemble 
 the chemical properties of the corresponding 
 compound in the second series. Hence we should 
 expect a detailed examination of the reactions of 
 the compounds in the ethane series to shew that 
 the arrangements of the atoms in the molecules of 
 these compounds are similar to the arrangements 
 of the atoms in the molecules of the compounds 
 in the methane series. Assuming the atomic 
 arrangements to be similar in the molecules of 
 the compounds of the two series, we have the 
 following structural formulae : 
 
 de'rivaTv'e. Alcoho1 ' Amine ' ^^ Acid ' 
 
 Methane Series 
 
 H*C HLC.CI HsC.OH H.C.NHj H.CH H.C.OH 
 
 o o 
 
 Ethane Series 
 
 HiC.CH* HsC.CHiCI HaC.CHs.OH HsC.CHs.NHs HaC.CH H.C.C.OH 
 
 O O 
 
84 THE STORY OF THE WANDERINGS OF ATOMS, 
 
 A detailed study of the reactions of ethylic 
 alcohol, ethylamine, acetic aldehyde, and acetic 
 acid confirms the justness of the formulas assigned 
 to these compounds. It is not necessary to 
 enumerate even the main results of this study ; 
 let us rather select a few reactions of the com- 
 pounds in the ethane series. Ethylic alcohol 
 reacts with the metal potassium, or the metal 
 sodium, to produce the compound C 2 H 5 KO (or 
 C 2 H 5 NaO), and hydrogen: only one of the six 
 hydrogen atoms in the molecule of the alcohol 
 can be replaced by potassium or sodium; five 
 atoms of hydrogen always remain in combination 
 with the whole of the carbon and the whole of 
 the oxygen of the molecule C 2 H 6 0. Acetic 
 aldehyde is very ready to remove oxygen from 
 many compounds of that element, and, by com- 
 bining with the oxygen so removed, to be changed 
 to acetic acid. Acetic acid is monobasic ; that is, 
 it forms only one silver salt, one potassium salt, 
 and one sodium salt : or, to put this reaction into 
 other words, the composition of every compound 
 which is produced by replacing hydrogen in acetic 
 acid by a metal can be expressed by one or other 
 of these formulae, H 3 C.CO.OM', (H 3 C.CO.O) 2 M", 
 (H 3 C.CO.O) 3 M'", or(H 3 C.CO.) 4 M""; where M' is 
 an atom of such a metal as potassium, M" is an 
 atom of such a metal as copper, M"' is an atom 
 of such a metal as iron, and M"" is an atom of 
 such a metal as tin. Ethylamine is a colourless 
 liquid, smelling like ammonia : it is very soluble 
 in water, and the solution reacts very similarly to 
 ammonia solution; for instance, precipitates of 
 the hydroxides of iron, copper, lead, tin, and 
 
ETHANE, AND SOME OF ITS DERIVATIVES. 85 
 
 many other metals, are formed when a solu- 
 tion of ethylamine is added to solutions of salts 
 of these metals, just as the same hydroxides 
 are precipitated from the same salts by using 
 a solution of ammonia. As methylic alcohol 
 (H 3 C.OH) reacts with hydrochloric acid gas, in 
 the presence of some substance which readily 
 combines with water, to produce chloromethane 
 (H 3 C.C1), so ethylic alcohol (H 3 C.CH 2 .OH) reacts 
 under similar conditions to produce chloro-ethane 
 (HoC.CH 2 Cl). 
 
 Let us look a little more closely at the struc- 
 tural formulae which express the reactions of 
 methylic and ethylic alcohols. Let us write 
 these formulae in full, thus : 
 
 H-C-O-H tnd H-C-C-0-H 
 ft H H 
 
 In each molecule an atom of carbon is repre- 
 sented in direct union with an atom of oxygen 
 which is directly linked to an atom of hydrogen, 
 and the same atom of carbon is represented 
 in direct union with two atoms of hydrogen ; in 
 the molecule of methylic alcohol this carbon 
 atom is shewn as directly joined to a third atom 
 of hydrogen, while in the molecule of ethylic 
 alcohol it is shewn as directly joined to the group 
 of atoms CH 3 . Looking at the formulae in this 
 way, we may express them thus : H.CH 2 OH 
 and H 3 C.CH 2 OH. Both molecules contain the 
 atomic group CH^OH. Now it is customary to 
 say that this group of atoms is the characteristic 
 group of the molecules of a great many compounds 
 all of which are alcohols. All these alcohols have 
 
86 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 certain common properties; for instance, they 
 all oxidise to aldehydes eachaldehyde containing 
 the same number of atoms of carbon and oxygen 
 as the alcohol contains, and two atoms of hydro- 
 gen less than the alcohol contains and these 
 aldehydes then oxidise to acids, which contain the 
 same number of atoms of carbon as the parent 
 alcoHols, but one atom of oxygen more, and two 
 atoms of hydrogen less, than the parent alcohols. 
 The fact that these alcohols have many chemical 
 properties in common, is connected with the 
 atomic structure of their molecules by saying that 
 these molecules all contain the group CH 2 OH. 
 
 I ask the reader to pay especial attention 
 to this statement, the molecules of all the alcohols of 
 a certain class contain the atomic group CH 2 OH, and 
 to consider it carefully in the light of the re- 
 actions of methylic and ethylic alcohols. The 
 statement summarises many reactions, and sug- 
 gests other reactions; but it does these things 
 only to the person who understands the special 
 language of the molecular and atomic theory. 
 
 As another illustration of the use of this very 
 convenient phrase, certain molecules contain a 
 wmmon atomic group, let us look a little closely at 
 the structural formulae which summarise the 
 reactions of formic and acetic aldehydes. These 
 
 H 
 
 formulas are H-^-H < H-C-C-H O r, more shortly, 
 
 H.CHO and H S CCHO. 
 
 The atomic group common to the two mole- 
 cules is HCO, or more fully, f M . There are 
 many other aldehydes which behave similarly to 
 
ETHANE, AND SOME OF ITS DERIVATIVES. 87 
 
 methylic and ethylic aldehydes, under similar 
 conditions : all are easily oxidised to acids by 
 combining with an atom of oxygen ; all remove 
 oxygen from many other compounds, and are, 
 therefore, said to act as reducing (or deoxidising) 
 agents. The reactions of these aldehydes are 
 summed up in the one structural formula, 
 R.CHO; where E, represents an atom of hydro- 
 gen in the case of methylic aldehyde (which is the 
 first aldehyde of the series), or a group of atoms 
 of carbon and hydrogen in all cases except that 
 of methylic aldehyde. E-eadiness to remove 
 oxygen from compounds which do not insist 
 on retaining all their oxygen is a reaction common 
 to certain compounds ; the molecules of these 
 compounds are thought of as containing an 
 arrangement of carbon, hydrogen, and oxygen 
 atoms which is presented in intelligible form by 
 
 the symbol ~ H . Now turn to the structural for- 
 mulae of formic and acetic acids, H OH "<*H*C.COH . 
 and compare these with the structural formulas of 
 the two aldehydes H * H '"" H ' C | H . The molecule of 
 
 formic acid is represented as containing the 
 group H.CO ; whereas the molecule of acetic acid 
 is not represented as containing this group. But 
 HCO is the characteristic aldehydic group : there- 
 fore, if the formula given to formic acid is a truthful 
 representation (in its own language) of the re- 
 actions of that acid, we should expect formic acid 
 to act as a remover of oxygen from compounds 
 which are fairly easily deprived of their oxygen, 
 just as aldehydes act as removers of oxygen ; and 
 
88 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 we should not expect acetic acid to act as an 
 energetic deoxidising agent. These expectations 
 are fulfilled when the reactions of the two acids 
 with fairly easily deoxidised compounds are 
 examined. For instance, a cold solution of per- 
 manganate of potassium '(Condy's fluid) is 
 deoxidised by formic acid, but not by acetic 
 acid; gold, platinum, and palladium are pre- 
 cipitated from solutions of their salts by formic 
 acid (that is, the formic acid decomposes the salts 
 of these metals and combines with the oxygen in 
 them), but not by acetic acid; and an alkaline 
 solution of a salt of copper is reduced by formic 
 acid, but not by acetic acid. 
 
 This comparison of the structural formulae of 
 formic and acetic acids, and of these formulae 
 with those of two aldehydes, shews how sugges- 
 tive of reactions these formulas are, and, therefore, 
 how much they may help to advance the study 
 of the connexions between composition and pro- 
 perties the elucidation of which connexions is 
 the goal of chemical inquiry. 
 
 When ethylic alcohol is heated with nearly 
 double its weight of sulphuric acid, in a flask 
 connected with a condensing apparatus, and more 
 ethylic alcohol is allowed to drop into the hot 
 mixture, a colourless, mobile, very inflammable, 
 liquid collects in the receiver. This liquid is ether. 
 
 When a mixture of ethylic alcohol and acetic 
 acid is heated, and the gaseous product of the 
 reaction is condensed, a colourless, very fragrant, 
 liquid is obtained. This liquid is ethylic acetate. 
 
 What relations, of properties, and of composi- 
 tion, exist between these two compounds, ether 
 
ETHANE, AND SOME OF ITS DERIVATIVES. 89 
 
 and ethylic acetate, and ethylic alcohol from 
 which they are prepared ? The composition of 
 ether is given by the formula C 4 H 10 0, and that 
 of ethylic acetate by the formula C 4 H 8 2 . These 
 formulae do not suggest any definite relations 
 between the two compounds and ethylic alcohol 
 (C 2 H 6 0). Let us look at the reactions of the two 
 compounds. 
 
 When ether is treated with hydrochloric acid 
 gas, chloro-ethane (C 2 H 5 C1) is formed ; if the 
 quantity of ether which reacts, and the quantity of 
 chloro-ethane which is produced, are determined, 
 the interaction is found to be expressed by the 
 chemical equation C 4 H 10 + 2HC1 = 2C 2 H 5 C1 + 
 H 2 0. The atom of oxygen is removed from the 
 molecule C 4 H 10 0, and two atoms of chlorine are 
 put in its place; but instead of one molecule, 
 having the composition C 4 H 10 C1 2 , being formed, 
 two molecules are produced each having the 
 composition C 2 H 5 C1. Now we found before 
 (p. 67) that methylic alcohol and hydrochloric 
 acid react to produce chloromethane, the re- 
 action being represented in structural formulae 
 thus, 
 
 H.CH 2 .OH + H.C1 = H.CH 2 .C1 + H.O.H. 
 
 We also noticed (p. 73) that formic aldehyde re- 
 acts with phosphorus pentachloride, which is 
 a compound that acts like hydrochloric acid 
 but more energetically than that acid, to form 
 CH 2 C1 2 ; thus, 
 
 H.C.H H.C.H 
 
 II + PCI = 'I + POCIs (oxychloride of phosphorus). 
 O CI 2 
 
"90 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 Looked at in the light of these reactions, 
 the interaction of hydrochloric acid and ether 
 suggests that the oxygen atom in the molecule of 
 ether is in direct union with a carbon atom, or 
 with carbon atoms, and is not linked to an atom 
 of hydrogen which is again linked to an atom of 
 carbon. 
 
 Let us turn to another reaction of ether. 
 Ether and water act on one another to produce 
 ethylic alcohol ; the change is presented in an 
 equation in this way, 
 
 Remembering that the reaction of ether with 
 hydrochloric acid suggests the direct linkage of 
 the oxygen atom in the molecule of ether to 
 carbon, and recalling the structural formula of 
 ethylic alcohol, and the close relationship which 
 evidently exists between that alcohol and ether, 
 we try the following scheme as a possible re- 
 presentation of the change that occurs when 
 ^ther reacts with water to form ethylic alcohol : 
 
 H H 
 
 HaC-C-O-C-CHa + H-O-H = 
 i i 
 
 H H 
 
 H H 
 
 HaC-C-O-H + HsC-C-O-H. 
 i i 
 
 H H 
 
 If this presentation of the changes is a correct 
 translation of the process into the language we 
 .are trying to learn, then ether is an oxide of the 
 atomic group H 3 C.CH 2 ; and ethylic alcohol is a 
 compound of that group with oxygen and hydro- 
 
ETHANE, AND SOME OF ITS DERIVATIVES. 91 
 
 gen, that is to say, it is an hydroxide of the same 
 atomic group. The name ethyl is given to the 
 group of atoms H 3 C.CH 2 which is supposed to 
 exist in the molecules of ether and ethylic 
 alcohol; supposed to exist in these molecules., 
 let it be remembered, because of the reactions 
 of the two compounds. Let us employ the 
 symbol Et to represent the group ethyl ; then the 
 formulae of ether and ethylic alcohol are repre- 
 sented by these symbols : Et.O.Et and Et.OH. 
 We may call these two compounds ethyl oxide 
 and ethyl hydroxide, when we wish to emphasise 
 the conception we have formed of the relations 
 between them. By using these names we imply 
 that the compounds are similar to such inorganic 
 compounds as potassium oxide and potassium 
 hydroxide; K.O.K and K.OH. Attention might 
 be drawn to other reactions of ether, but I think 
 it will be sufficient to say that these reactions are 
 quite in keeping with the representation of this 
 compound as ethylic oxide. 
 
 But what are the relations of ethylic acetate, 
 C 4 H 8 2 , to ethylic alcohol, and to acetic acid, 
 the two compounds which react to produce the 
 acetate? One of the two compounds which 
 react to produce ethylic acetate is an acid, and 
 we have learned that the other compound, 
 ethylic alcohol, is like potassium hydroxide 
 in its reactions. Now what is the ordinary 
 reaction between an acid and a metallic hy- 
 droxide 1 Sulphuric acid and sodium hydroxide 
 interact to produce sodium sulphate, which is 
 a salt, and water; nitric acid and potassium 
 hydroxide interact to form potassium nitrate, 
 
92 THE STORY OF THE WANDERINGS OF ATOMS.* 
 
 which is a salt, and water; hydrochloric acid 
 and barium hydroxide interact to produce the 
 salt barium chloride, and water ; acetic acid and 
 lead hydroxide interact to produce the salt lead 
 acetate, and water. The ordinary products of 
 the reaction between an acid and a metallic 
 hydroxide are a salt, formed of the metal of the 
 hydroxide and the radicle of the acid, and 
 water. It is probable then that acetic acid 
 and ethyl hydroxide will react to produce a 
 salt and water; and we should expect the salt 
 to be composed of the radicle ethyl, which plays 
 the part of a metal in the hydroxide ethylic 
 alcohol, and the radicle of acetic acid. But 
 what is the composition of the radicle of acetic 
 acid? This question is answered by tabulating 
 the compositions of some salts of acetic acid 
 these salts are called acetates and comparing 
 these compositions with that of acetic acid. The 
 following table presents data sufficient for our 
 purpose. 
 
 Acetic acid. C 2 H 4 O 2. 
 
 Acetates. CaH.-.OzNa. CzHsC^K. (CzHsOz^Ba. (C 2 H 3 O 2 ) 2 Pb. 
 
 Sodium Potassium Barium Lead 
 
 acetate. acetate. acetate. acetate. 
 
 One, and only one, of the four atoms of hydro- 
 gen in the molecule C 2 H 4 2 is replaced by metal 
 when acetates are formed : the radicle of acetic 
 acid has the composition C 2 H 3 0. We should, 
 then, expect the reaction between acetic acid and 
 ethyl hydroxide to result in the formation of a 
 salt, composed of the radicle or atomic group ethyl 
 (C 2 H 5 ) in combination with the radicle of acetic 
 acid, C 2 H 8 O. Moreover, as ethyl hydroxide 
 
ETHANE, AND SOME OF IT3 DERIVATIVES. 93 
 
 (C 2 HgOH) is similar to potassium hydroxide 
 (KOH), we should expect the salt which is 
 formed, according to the hypothesis that is guid- 
 ing us, by the reaction of acetic acid with ethyl 
 hydroxide, to have a composition analogous to 
 the composition of potassium acetate ; that is to 
 say, we should expect the composition of the salt 
 to be expressed by the formula C 2 H 3 2 (C 2 H 6 ). 
 Now the formula which expresses what we may 
 call the empirical composition of the compound 
 in question that is, the composition without 
 taking into account what may be the structure of 
 the molecule of the compound is C 4 H 8 0. The 
 symbol C occurs four times, the symbol H occurs 
 eight times, and the symbol O occurs twice, in 
 the formula C 2 H 2 (C 2 H 5 ) ; in other words, 
 C 2 H 3 2 (C 2 H 5 ) = C 4 B[ 8 2 . The product of the re- 
 action between acetic acid and ethylic alcohol 
 (ethyl hydroxide) is then, probably, the salt ethyl 
 acetate, C 2 H 3 2 (C 2 H 5 ). A closer examination of 
 the reactions of this substance shows that it be- 
 longs to the class of salts ; and a quantitative study 
 of the change that occurs when acetic acid and 
 ethylic alcohol react proves that this change is 
 represented, correctly, by the equation 
 
 C 2 H 3 2 .H + C 2 H 5 .OH = C 2 H 3 2 .C 2 H 5 + H 2 0. 
 
 Alcohols are hydroxides of certain atomic groups, 
 or radicles ; and ethers are oxides of the same 
 atomic groups. The salts that are formed by the 
 replacement of the acidic hydrogen of acids by 
 these atomic groups are called ethereal salts. 
 Ethylic acetate is the first example we have 
 had of an ethereal salt. 
 
94 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 The reaction which occurs when an ethereal 
 salt is heated with a solution of caustic potash 
 or caustic soda, is one of considerable import- 
 ance. Let us consider this reaction for a moment, 
 and let us first of all see what change takes place 
 when a metallic salt reacts with a solution of 
 caustic potash. Let lead acetate be taken as the 
 example of a metallic salt. The reaction between 
 lead acetate and caustic potash solution results in 
 the formation of lead hydroxide and potassium 
 acetate ; the change is expressed thus in an equa- 
 tion 
 
 (C 2 H 3 2 ) 2 Pb + 2KOH = Pb(OH) 2 + 2(C 2 H 3 2 .K). 
 
 This is a special case of the change that generally 
 occurs when caustic potash, or soda, reacts with 
 a metallic salt ; what may be called the normal 
 products of this reaction are, a hydroxide of the 
 metal of the salt, and a salt of potassium (or 
 sodium) composed of that metal united with the 
 acidic radicle of the original salt. Now when the 
 ethereal salt ethylic acetate reacts with a solution 
 of caustic potash (if it does react), we should expect 
 the products to be, hydroxide of ethyl and potas- 
 sium acetate ; that is, we should expect the change 
 to be expressed by this equation 
 
 C 2 H 3 2 .C 2 H 5 + KOH = C 2 H 5 .OH + C 2 H 3 2 .K. 
 
 And this is indeed what happens. The reaction 
 proceeds slowly : in order to change the whole of 
 the ethylic acetate, considerably more potash, in 
 solution, must be used than is expressed in the 
 equation ; the reacting compounds must be kept 
 in contact, at a high temperature, for a long time ; 
 
ETHANE, AND SOME OF ITS DERIVATIVES. 95 
 
 and the ethylic alcohol that is produced must be 
 removed, by distillation, from time to time as the 
 change proceeds. We shall have other examples, 
 of the reaction between ethereal salts and caustic 
 potash, or soda, especially when we are consider- 
 ing the processes of making soaps (in chapters- 
 VII. and VIII.). This reaction produces alcohols, 
 and potassium (or sodium) salts of the acids of 
 the ethereal salts employed. 
 
 In this chapter, and in chapter V. we have been 
 concerned with two similar series of compounds 
 of carbon. Each series begins with a hydro- 
 carbon whose composition is given by the formula 
 C n H 2n + 2 ; by the action of chlorine on the hydro- 
 carbon a chloro-derivative is obtained which has the 
 composition C n H 2ll+1 Cl ; this chloro-derivative of 
 the hydrocarbon reacts slowly with a watery 
 solution of caustic potash to form an alcohol^ 
 C n H 2n+1 .OH; the alcohol reacts with ammonia, 
 or with ammonium chloride, to produce an amine r 
 C^H^+j.NHg the alcohol also reacts with oxidis- 
 ing agents to yield, first an aldehyde, C n H 2n O, and 
 then an add, C n H 2n 2 . In the second series, we 
 noticed, also, that the alcohol reacts with sul- 
 phuric acid to produce an ether, (CJB^.jJoO ; and 
 that the acid and the alcohol interact to form an 
 ethereal salt, C n H 2n _ 1 2 .C n H 2n+1 . It is possible, 
 by appropriate reactions, to obtain an ether from 
 the alcohol of the first series, and also an ethylic 
 salt of the acid of that series ; the compositions 
 of these two compounds are given by the formulae, 
 (CH 3 ) 2 for methylic ether, and CH0 2 .C 2 H5 for 
 ethylic formate. The relations of composition 
 of the members of the two series are perhaps 
 
96 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 more fully expressed by using ths following gen- 
 eral formulas : 
 
 General Formula. Examples. 
 
 HYDROCARBON. CnH2n+2 CH4 (methane; and C2He (ethane). 
 
 CHLORO-DKRIVATIVE. CnH2n+lCl CHsCl (chloromethane) and CaHsCl 
 
 (chloro-ethane). 
 AMINB. CnHan+i.NHa CHsNHa (methylamine) and CaHsNHa 
 
 (ethylamine). 
 ALCOHOL. CnH2n+l.CHaOH H. CHaOH (methylic alcohol) and 
 
 H 3 C.CH20H (ethylic alcohol). 
 ALDEHYDE. CnH2n+i.CHO H.CHO (formic aldehyde) and HsC.CHO 
 
 (acetic aldehyde). 
 ACID. CnH2n+l.CO.OH H.CO. OH (formic acid) and HsC.CO.OH 
 
 (acetic acid). 
 ETHEREAL SALT. H.CO.OCCaHs) (ethyl formate) and 
 
 C n H a +l.CO.O(CmH2m+l) H3C.CO.O(C2Hs) (ethyl acetate). 
 
 ETHEK. (CnHan+l)aO (CHs)aO (methyl ether) and (CaHs)aO 
 
 (ethyl ether). 
 
 Note. In the general formula n . o in the cases of methylic alcohol, formic 
 aldehyde, formic acid, and ethyl formate. 
 
 The examination of the compounds in these 
 two series has brought with it many examples of 
 the use of the conception of the compound radicle, 
 or atomic group which holds together throughout 
 many reactions and impresses certain common 
 properties on all the molecules of which it forms 
 a part. The study of the two series of com- 
 pounds has also furnished illustrations of the 
 meaning of the term substitution ; we have had 
 cases of the substitution of one atom by another, 
 for instance in the passage from CH 4 to CH 3 C1 
 and from C 2 H 6 to C 2 H 5 C1 ; we have had cases of 
 the substitution of an atom by an atomic group, 
 for instance in the passage from CH 3 C1, or 
 C 2 H 5 C1, to CH 3 .OH, or C 2 H 5 .OH ; and we have 
 had cases of the substitution of one atomic group 
 by another, for instance in the passage from 
 CH 3 .OH, or C 2 H 5 .OH, to CH 3 .NH 2 , or C 2 H 5 .NH 2 . 
 
 The attempt made in this, and the previous 
 
ETHANE, AND SOME OF ITS DERIVATIVES. 97 
 
 chapter to connect the reactions of certain com- 
 pounds of carbon with the compositions of these 
 compounds has shewn, I hope, how impossible it 
 is to elucidate the relations of reactions to com- 
 position without the help of some definite theory 
 of the structure of matter, and the aid of not 
 a few subsidiary hypotheses and conventions 
 whereby the application of the theory is made 
 practicable to chemical phenomena. 
 
 Finally, the general description of the features 
 of the chemical changes that occur between 
 compounds of carbon, given in Chapter II. (pp. 
 31-34), has, I think, been justified by what we 
 have learned of the changes in the two series of 
 compounds derived, respectively, from marsh 
 gas and ethane. In some respects, the hydro- 
 carbons may be regarded as representatives 
 among carbon compounds of the metals of in- 
 organic chemistry. When one desires to form 
 a salt of a metal, it is sufficient to dissolve the 
 metal in the appropriate acid and to evaporate 
 the solution. But in order to make a salt from 
 a hydrocarbon, it is necessary to prepare a chloro- 
 derivative of the hydrocarbon, from this to pass 
 to the alcohol, and then, through the aldehyde, to 
 the acid; having obtained the acid and the 
 alcohol, it is still necessary to cause these to 
 react in order to obtain the wished-for salt. To 
 prepare an oxide of a metal, it is generally suffi- 
 cient to burn the metal in oxygen ; but between 
 the hydrocarbon and the oxide, which is an 
 ether, how many stages there are ! The alcohol 
 is obtained after two operations, and a third pro- 
 cess of change is required before the oxide appears. 
 
98 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 In Chapter VIII. I shall give a short account 
 of the technical applications of some of the com- 
 pounds whose reactions and compositions have 
 formed the subject-matter of Chapters V. and 
 VI. 
 
 CHAPTER VII. 
 
 ETHYLENE, GLYCERIN, AND TARTARIO ACID. 
 
 ONE of the many hydrocarbons present in 
 ordinary coal-gas is called ethylene, and has the 
 composition C 2 H 4 . This compound is generally 
 prepared in the laboratory by heating ethylic 
 alcohol with about four times its volume of con- 
 centrated sulphuric acid; the principal reaction 
 that occurs is represented by the equation 
 C 2 H 6 + H 2 S0 4 = H 2 S0 4 .H 2 + C 2 H 4 . Ethylene 
 is a colourless gas, slightly lighter than air ; 
 when cooled a little below the freezing point of 
 water, and subjected to a pressure of about 44 
 atmospheres (about 650 Ibs. on the square inch), 
 the gas changes to a colourless liquid which boils 
 at minus 103C. [minus 153T.J. 
 
 Ethylene is easily burnt in the presence of 
 air. When the gas is mixed with a little air 
 and burnt, the main products of the combus- 
 tion are carbon dioxide, water, and carbon. 
 Under these conditions the gas burns with a 
 luminous flame ; and the luminosity is caused, 
 chiefly, by the minute solid particles of carbon 
 becoming red hot and radiating light. If ethy- 
 lene is mixed with somewhat more than three 
 times its own volume of oxygen (equal to more 
 
ETHYLENE, GLYCERIN, AND TARTARIC ACID. 99 
 
 than fifteen volumes of air) and the mixture is 
 burnt, only carbon dioxide and water are formed, 
 and the flame is almost non-luminous. Part of 
 the luminosity of an ordinary flame of coal-gas is 
 due to the limited combustion of the ethylene 
 which the gas contains. When coal-gas issues 
 from a burner, admixture with air occurs at 
 the edges of the stream of gas ; and when a 
 lighted taper is brought near the burner chemi- 
 cal change begins where the air and the gas are 
 in contact ; the combustion of the ethylene, and 
 the other hydrocarbons in the gas, is, therefore, 
 not complete ; solid particles of carbon are pro- 
 duced, and these are heated to so high a tem- 
 perature that they give out much light. If 
 coal-gas is mixed with a large quantity of air 
 and the mixture is ignited, the combustion of 
 the ethylene, and other hydrocarbons, is practi- 
 cally complete, and all the products are gaseous. 
 The flame of such a mixture is extremely hot but 
 almost free from light. The Bunsen-lamp, which 
 is employed in all laboratories for getting high 
 temperatures, is an extremely simple contrivance 
 for securing the admixture with coal-gas of suffi- 
 cient air to insure the complete combustion of 
 the carbon compounds in the gas. The apparatus 
 consists of an ordinary gas-burner over which is 
 fitted an iron tube, about 3 inches long and f inch 
 diameter, having a couple of holes (about J inch 
 diameter) pierced near its lower end. Air flows 
 in through the holes, and the mixture of gas and 
 air is ignited at the upper end of the tube. The 
 principle of the Bunsen-lamp is applied to the 
 construction of gas-stoves. Sometimes the stove 
 
100 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 consists of several small upright tubes, into each 
 of which gas and air are admitted at the bottom, 
 and the mixture is burnt at the top. Sometimes 
 there is one rather wide tube pierced with many 
 small holes or slits ; this tube is attached to the 
 gas-supply, and there is a large opening, near 
 where the gas enters, to admit air ; the wide 
 tube becomes filled with a mixture of gas and 
 air, and this mixture is burnt as it issues from 
 the small holes or slits. If too much air is 
 mixed with the gas in a Bunsen-burner, or a 
 gas-stove, the temperature of the flame is de- 
 creased by the superfluous air ; if too little air 
 is admitted the combustion of the carbon com- 
 pounds of the gas is not complete, and the flame 
 is smoky. 
 
 We have learnt, in chapters V. and VI., that 
 both marsh gas (CH 4 ) and ethane (C 2 H 6 ) react 
 with chlorine to produce compounds containing 
 chlorine in place of part of the hydrogen of the 
 marsh gas or the ethane. The reaction of ethyl- 
 ene (C 2 H 4 ) with chlorine is not similar to the 
 reactions of the two hydrocarbons we have con- 
 sidered. Ethylene combines with chlorine slowly, 
 in diffused daylight, to form an oily liquid, with 
 a sweetish odour, called ethylene chloride. The re- 
 action is expressed thus in an equation ; C 2 H 4 
 + 201 = C 2 H 4 C1 2 . The chlorine does not remove 
 hydrogen from the molecule of ethylene, as it 
 does from the molecules of marsh gas and 
 ethane ; but the chlorine adds itself to the 
 molecule of ethylene. Bromine reacts with ethyl- 
 ene in a way exactly similar to that wherein 
 chlorine reacts ; a compound of ethylene and 
 
ETHYLENE, GLYCERIN, AND TARTABIO ACID. 101 
 
 bromine is formed, called ethylene bromide, and 
 having the composition C 2 H 4 Br 2 . It is cus- 
 tomary to distinguish such compounds as CH 3 C1 
 and C 2 H 5 C1 (chloromethane and chloro-ethane), 
 that are produced by the substitution of an atom 
 (or atoms) in one molecule by another atom (or 
 by other atoms), from such compounds as C 2 H 4 C1 2 
 and C 2 H 4 Br 2 (ethylene chloride and ethylene 
 bromide) that are produced by the addition of 
 an atom, or atoms, to a molecule of a compound. 
 Compounds of the former class are called substi- 
 tution-compounds, and compounds of the latter 
 class are named additive compounds. It is also 
 customary to speak of compounds, like marsh 
 gas and ethane, which always react to form 
 substitution-compounds as saturated and com- 
 pounds, like ethylene, which react to produce 
 additive compounds as unsaturated. These four 
 terms are convenient aids towards remembering 
 the general reactions of carbon compounds. A 
 hydrocarbon is being examined : it is found to 
 combine readily with chlorine, and to form a 
 compound composed of chlorine added on to the 
 whole of the molecule of the hydrocarbon ; the 
 hydrocarbon is placed in the class of unsaturated 
 compounds ; and many of its other reactions are 
 known, because all unsaturated compounds ex- 
 hibit certain common reactions. 
 
 Ethylene chloride (C 2 H 4 C1 2 ) and ethylene 
 bromide (C 2 H 4 Br 2 ) are oily liquids. In the year 
 1795, four Dutch chemists named Deiman, 
 Ptets van Troostuyk, Bondt, and Lauweren- 
 burgh discovered a new gas, produced by the 
 action of oil of vitriol on common alcohol ; they 
 
102 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 found that the gas was composed of carbon and 
 hydrogen only ; and they noticed that an oily 
 liquid was formed when this gas was brought 
 into contact with the substance now called 
 chlorine. The Dutch chemists gave the name 
 gaz huileux (oily gas) to the new compound, a 
 name that was afterwards changed to gaz oUfiant 
 (oil-forming gas). The oily compound formed 
 by the union of this gas with chlorine was 
 known for a long time by the names Dutch liquid 
 and oil of the Dutchmen ; these names have dis- 
 appeared, but the compound which is produced 
 by the reaction of oil of vitriol and alcohol is 
 still commonly spoken of as olefiant gas. 
 
 Ethylene chloride and ethylene bromide react 
 gradually with caustic potash solution to form a 
 compound which belongs to the class of alcohols. 
 The composition of this compound is 2 H 4 (OH) 2 ; 
 because of its sweet taste it is called glycol. (The 
 termination -ol is common to the names of all 
 compounds which are alcohols). The reactions 
 of ethylene are expressed, in terms of the atomic 
 and molecular theory and the hypothesis of 
 
 H H 
 
 atom-linking, by the structural formula c=c 
 
 H H 
 
 or more shortly H 2 C = CH 2 . The reaction be- 
 tween ethylene and chlorine, and that between 
 ethylene chloride and caustic potash, are pre- 
 sented by the following schemes : 
 
 (i.) B 2 C = CH 2 + CL = OlHjO-OBLCL 
 (ii.) C1H 2 C - CH 2 01 + 2KOH = 
 
 HO.H 2 C-CH 2 .OH + 2KC1. 
 I do not intend to discuss the reactions of 
 
ETHYLENE, GLYCERIN, AND TARTARIC ACID. 103 
 
 the alcohol, C 2 H 4 (OH) 2 , derived from ethylene ; 
 I merely wish to note its existence, and the 
 method of its formation. 
 
 If the compositions of the two hydrocarbons 
 methane and ethane, CH 4 and C 2 H 6 , are com- 
 pared, it is seen that both are expressed by 
 the formula C n H2n +2 ; that is to say, the 
 number of hydrogen atoms in either mole- 
 cule is equal to twice the number of atoms of 
 carbon in the molecule plus two. These com- 
 pounds are the first and second members respec- 
 tively of a series of hydrocarbons all of which 
 have the common composition C n H 2n+2 . The 
 series is known as the paraffins, because of their 
 indifference to the action of chemical reagents 
 (parum affinis = not much affinity). The third 
 member of the series of paraffins (n = 3) is C 3 H 8 . 
 This hydrocarbon, called propane the names of 
 all the paraffins end in -ane is found in the 
 gases given off from certain petroleum springs 
 in America. Propane reacts with bromine to 
 form various substitution-compounds ; one of 
 these, called tribromopropane, has the composition 
 C 3 H 5 Br 3 ; when this compound is boiled with 
 caustic potash the following reaction occurs : 
 C 3 H 5 Br 3 + 3KOH = C 3 H 5 (OH) 3 + 3KBr. 
 
 The compound C 3 H 5 (OH) 3 is an alcohol : it is 
 generally known by the name glycerin but it is 
 better called glycerol, because the syllable -ol 
 is the characteristic termination of the names of 
 those compounds which are alcohols. Just as 
 the atomic group C 2 H 5 , that is present in tbe 
 molecules of ethylic alcohol, ethylic ether, and 
 ethylic acetate, is called ethyl, so the atomic 
 
104 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 group CgHg, is called glyceryl. As we spoke of 
 ethylic alcohol by the name ethyl hydroxide, so we 
 may speak of glycerin as glyceryl hydroxide. It 
 is to be noticed that while a molecule of ethyl 
 hydroxide is composed of the atomic group 
 ethyl combined with one atom of oxygen and 
 one atom of hydrogen, a molecule of glyceryl 
 hydroxide is composed of the atomic group 
 glyceryl combined with three atoms of oxygen 
 and three atoms of hydrogen. As ethyl hydroxide 
 (C 2 H 5 .OH) is analogous, in composition and in 
 many reactions, to potassium hydroxide (KOH) 
 and sodium hydroxide (NaOH), so glyceryl hy- 
 droxide is similar, in composition and in some 
 reactions, to ferric hyroxide [Fe(OH) 3 ], and 
 aluminium hydroxide [A1(OH) 3 1. 
 
 When tallow, linseed oil, palm oil, whale oil, 
 or any one of several other oils, is boiled with 
 caustic soda, or potash, or with slaked lime, 
 glycerin is formed along with a compound which 
 is a soap. The main constituents of fats and 
 oils are compounds of glyceryl with the radicles 
 of certain acids that are often classed together 
 under the name fatty acids. These compounds 
 are ethereal salts ; they belong to the same class 
 of compounds as ethylic acetate. Turn back for 
 a moment to the reaction (considered on p. 91) 
 between ethylic alcohol and acetic acid whereby 
 ethylic acetate and water are formed, and to the 
 reaction between ethylic acetate and caustic 
 potash (considered on p. 94) whereby ethyJic 
 alcohol and potassium acetate are produced. 
 These reactions find their expressions in the 
 equations 
 
ETHYLENE, GLYCERIN, AND TARTARIO ACID. 105 
 
 (i.) C 2 H 5 .OH + C 2 H 3 2 .H - C 2 H 3 2 .C 2 H 5 + H 2 0, 
 
 Ethylic alcohol. Acetic acid. Ethylic acetate. 
 
 (ii.) C 2 H 8 2 .C 2 H 5 + KOH = C 2 H 5 .OH + C 2 H 3 2 .K. 
 
 Ethylic acetate. Potash. Ethylic alcohol. Potassium 
 
 acetate. 
 
 Remembering that the radicle ethyl (C 2 HA 
 which combines with one atomic group OH, 
 replaces one atom of hydrogen in an acid to 
 form an ethylic salt, we might expect that the 
 radicle glyceryl (C 3 H 5 ), which combines with 
 three atomic groups OH, would replace three 
 atoms of hydrogen in an acid to form a glyceryl 
 salt. Examination of the glyceryl salts shews 
 that this expectation is correct. Glyceryl acetate 
 is C 3 H, (C 2 H 3 2 ) 3 , glyceryl nitrate is C 3 H 5 (N0 3 ) 3 , 
 glyceryl chloride is C 3 H 5 C1 3 , and so on. These 
 salts may be produced by causing glycerin 
 [C 3 H 5 (OH) 3 ] to react with the appropriate acid : 
 acetic, nitric, and hydrochloric acids are mono- 
 basic, in other words, a molecule of any one of 
 these acids contains only one atom of hydrogen 
 which can be replaced by metals or by such 
 atomic groups as ethyl or glyceryl; therefore, 
 when glycerin reacts with one of these acids to 
 form a glyceryl salt, three molecules of the acid 
 1 react with one molecule of glycerin. The reac- 
 tion, in the case of nitric acid for instance, is 
 expressed in an equation in this way 
 
 C 8 H 6 (OH) 8 + 3HN0 3 = C 3 H 6 (N0 8 ) 3 + 3H 2 0. 
 
 The chief constituent of tallow is glyceryl stearate, 
 of linseed oil glyceryl oleate, and of palm oil 
 glyceryl palmitate. Stearic, oleic, and palmitic 
 
106 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 acids are monobasic acids; their compositions 
 are given by the formulae 
 
 Stearic acid CLjIIggOo.!!. 
 Oleic acid C l8 H. 3l O*.l3.. 
 Palmitic acid C 16 H3i2- H - 
 
 And the glyceryl salts of these acids are 
 
 Glyceryl stearate (CjgHggO^gCgHg. 
 Glyceryl oleate (C 18 H 3 A) 3 C 3 IL. 
 Glyceryl palmitate (C 16 H 31 2 ) 3 C 3 H 5 . 
 
 The reaction that occurs when one of these salts 
 is boiled with caustic potash solution is exactly 
 similar to that which occurs when ethylic 
 acetate is boiled with caustic potash solution. 
 Selecting glyceryl stearate, the main constituent 
 of tallow, we express the reaction by the follow- 
 ing equation : 
 
 (C 18 H 35 2 ) 3 C 8 H 6 + 3KOH - 3(C 18 H S5 2 -K) + 
 C 8 H 6 (OH) 3 . 
 
 The equations which express the reactions of 
 caustic potash with the other glyceryl salts are 
 similar to this. If caustic soda is used, glycerin 
 and the sodium salt of the acid of the glyceryl 
 salt are formed. The sodium salts, and the 
 potassium salts, of stearic, palmitic, and oleic 
 acids, and of certain other acids analogous to 
 these, are soaps: the sodium salts are called 
 hard soaps, and the potassium salts are called 
 soft soaps. 
 
 The process that takes place when the glyceryl 
 salt of a fatty acid reacts with potash, or soda, 
 to fora? glycerin and a potassium, or sodium, salt 
 
ETHYLENE, GLYCERIN, AND TARTARIC ACID. 107 
 
 of the fatty acid, which salt is a soap, is called 
 saponification. It is customary to apply this 
 term to all cases of the reaction of potash, or 
 soda (or other alkali), with an ethereal salt ; for 
 instance, the reaction between ethylic acetate 
 and potash is a particular case of saponification, 
 although potassium acetate has not the physical 
 properties of a soap. 
 
 Among the many acids that are found in the 
 juices of fruits there is one which demands our 
 attention at this point in our attempt to follow 
 some of the wanderings of carbon. That com- 
 pound is tartaric add. This acid, or one of its 
 salts, is found in grape-juice, potatoes, Jerusalem 
 artichokes, the berries of the mountain-ash, pine- 
 apples, the juice of beetroots, and in many other 
 plants. As the juice of grapes ferments, a reddish 
 solid is deposited on the sides and bottoms of the 
 vessels which contain the fermenting liquid. This 
 solid matter consists chiefly of potassium tartrate 
 mixed with colouring matters from the grape- 
 juice. The crude potassium tartrate is dissolved 
 in water, the solution is filtered, evaporated, and 
 allowed to cool : the crystals of potassium tartrate 
 that separate from the cold liquid are dissolved 
 in boiling water, and chalk is added to this 
 liquid; a reaction occurs whereby calcium tar- 
 trate and potassium carbonate are produced, the 
 latter dissolves in the water, and most of the 
 calcium tartrate remains undissolved as a white 
 solid substance. The calcium tartrate is washed, 
 and then decomposed by the proper quantity of 
 dilute sulphuric acid; tartaric acid and calcium 
 sulphate are formed ; the acicl dissolves and the 
 
108 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 calcium sulphate is insoluble. The solution is 
 filtered, and evaporated ; and crystals of tartaric 
 acid form in the cooling liquid. Tartaric acid 
 forms white crystals, the composition of which 
 is expressed by the formula C 4 H 6 6 . 
 
 The property of tartaric acid to which I ask 
 the special attention of the reader is a physical 
 rather than a chemical property. The property 
 is this : a solution of tartaric acid in water 
 rotates the plane of polarisation of a ray of light in 
 the same direction as that in which the hands 01 
 a watch turn when the watch is looked at in the 
 ordinary way. What is the plane of polarisation 
 of a ray of light ? And what is meant by the 
 rotation of this plane ? Common light is sup- 
 posed to consist of vibrations of a something 
 called " ether " ; some of these vibrations of 
 the ether are thought of as taking place in 
 one plane, and some in a plane at right angles 
 to the first. Under certain conditions, for in- 
 stance when a ray of light is reflected from a 
 bright surface, one set of vibrations is stopped. 
 Such a ray of light is said to be plane-polarised \ 
 all the vibrations are now occurring at right 
 angles to a certain plane, which is called the plane 
 of polarisation. Some substances do not allow a 
 ray of plane-polarised light to pass through them 
 without modifying the vibrations of the ray. The 
 general result of these modifications is this : 
 before the ray of plane-polarised light enters the 
 substance that modifies it we may suppose all 
 the vibrations to be taking place in a direction 
 at right angles to the surface of the earth ; when 
 the ray leaves the modifying substance it has 
 
ETHYLENE, GLYCERIN, AND TARTARIC ACID. 109 
 
 been separated into two rays, the vibration* in 
 which take place in different planes, acd as 
 these rays move with different velocities the 
 effect of the modifying substance is equivalent 
 to the turning round, or rotation, of the plan 
 of polarisation of the ray, either in the same 
 direction as that in which the hands of a watch 
 move, or in the direction opposite to that in 
 which the hands of a watch move, when the 
 watch is looked at in the ordinary way. Those 
 substances which produce a rotation of the plane 
 of polarisation of a ray of light are said to be 
 optically active ; all other substances are said to 
 be optically inactive. Substances which cause the 
 rotation of the plane of polarisation in the same 
 direction as that in which the hands of a watch 
 move or, we may say, substances which cause 
 watch-like rotation of the plane of polarisation 
 are said to be dextro-rotatory ; and substances 
 which cause the rotation of the plane of polarisa- 
 tion in a direction opposite to that in which the 
 hands of a watch move are called laew-rotatory 
 bodies. 
 
 Tartaric acid, in solution, is an optically 
 active compound. Tartaric acid, in solution, 
 is an optically inactive compound. Tartaric 
 acid, in solution, is a dextro-rotatory compound. 
 Tartaric acid, in solution, is a laevo-rotatory 
 compound. These four statements are true. In 
 other words there are four tartaric acids. The 
 molecule of each of these acids has the com- 
 position given by the formula C 4 H 6 6 . Evi- 
 dently we must inquire somewhat closely into 
 the reactions of the tartaric acids in order that 
 
110 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 we may connect the differences in their optica 1 
 activities with differences of molecular siructtab. 
 Ethylene (C 2 H 4 ) combines readily with bromine 
 (see p. 101) to form ethylene bromide (C 2 H 4 Br 3 ) ; 
 by boiling this compound with a solution 'of 
 potassium cyanide (KCN) in alcohol, a com- 
 pound called ethylene cyanide is produced ; when 
 this compound, which has the composition 
 C 2 H 4 (CN) 2 , is boiled with water it is slowly 
 changed to ammonia, and an acid whose com- 
 position is C 2 H 4 (CO.OH) 2 , called succinic acid. 
 Succinic acid reacts with bromine to form 
 dibromo-succinic acid, C 2 H 2 Bi' 2 (CO.OH) 2 , and this 
 acid is transformed into one of the four tar- 
 taric acids by boiling with water. 
 
 Inasmuch as ethylene can be formed from 
 carbon and hydrogen, the series of reactions 
 that begins with ethylene and ends with tar- 
 taric acid presents an example of the synthesis, 
 in the laboratory, of a compound which is one of 
 the characteristic products of living plants. 
 
 When the reactions whereby tartaric acid is 
 synthesised from ethylene are considered in 
 the light of other reactions that are similar 
 to them, they lead to the structural formula 
 C 2 H 2 (OH) 2 (CO.OH) 2 for the tartaric acid that is 
 thus obtained. The other reactions of this acid are 
 altogether in keeping with this formula. More- 
 over all the purely chemical reactions of the acid 
 are suggested by the following representation of 
 the lin kings of the atoms and atomic groups in 
 the molecule : 
 
 v 
 
 -CO OH 
 0> 
 
ETHYLENE, GLYCERIN, AND TARTARIC ACID. Ill 
 
 Eafc the reactions of the other three tartaric 
 acids also can be interpreted into the language 
 of molecular structure, only by supposing that 
 Uie arrangement of the atoms and atomic groups 
 in the molecule of each of them is that which is 
 presented by the above formula. We are then 
 face to face with a new phenomenon. Four 
 compounds exist having the same composition; 
 the molecular weights of the four compounds are 
 identical, and the four molecules are composed of 
 the same numbers of the same atoms ; moreover, 
 the reactions of the four compounds are so very 
 similar that we are obliged to picture the arrange- 
 ments of the atoms and atomic groups, that form 
 the four molecules, by the same structural for- 
 mula. Nevertheless the four compounds are 
 not identical ; they differ in their crystalline 
 forms ; two of them have the same solubilities 
 in water, but the solubilities of the other two 
 acids are different ; their melting points are not 
 the same ; and they differ very widely in their 
 optical activities. Although the chemical re- 
 actions of the four acids are very much alike, 
 there are some differences. As regards optical 
 activity : the acid prepared from succinic acid is 
 inactive, but it can be resolved into equal weights 
 of the dextro-rotatory and the laevo-rotatory acid ; 
 the fourth variety of tartaric acid is optically in- 
 active and cannot be resolved into the optically 
 active acids. At this point we must inquire what 
 is meant by resolving the optically inactive acid into 
 the dextro-rotatory and laevo-rotatory varieties 
 of tartaric acid. The resolvable inactive tartaric 
 acid is known by the name racemic acid. If a 
 
112 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 solution of this acid in water is divided into two 
 equal parts, if one half is neutralised by ammonia 
 and the other half by soda, and if the two neutral 
 liquids are mixed and the mixture is allowed to 
 evaporate, crystals of ammonium-sodium racemate 
 (NaNH 4 .C 4 H 4 O 6 ) are obtained. It is possible to 
 separate these crystals, by hand-picking, into two 
 sets, the difference between the crystals being the 
 same as the difference between an object and its 
 reflection in a mirror ; a crystal of one kind is 
 the reflected image of a crystal of the other 
 kind ; or, one may say, a crystal of one kind 
 bears to a crystal of the other kind the same 
 relation that a right-handed glove bears to a 
 left-handed glove. By decomposing the crystals 
 of one kind by the proper quantity of sulphuric 
 acid, dextro-rotatory tartaric acid is obtained ; 
 and an equal quantity of laevo-rotatory tartaric 
 acid is formed by decomposing the crystals of 
 the other kind by sulphuric acid. The separa- 
 tion of inactive racemic acid into dextro-rotatory 
 and laevo-rotatory tartaric acids can also be 
 effected by other methods. There is no process 
 known whereby the non-resolvable inactive tartaric 
 acid (generally called inactive tartaric acid) can be 
 separated, or resolved, into optically active acids. 
 There must be some differences in the struc- 
 tures of the molecules of the four tartaric acids ; 
 but the differences must be of a kind that is not 
 expressed by the structural formulae we have 
 been using hitherto. In considering the structu- 
 ral formulas we have employed as aids to forming 
 clear conceptions regarding the connexions 
 between the reactions of compounds and the 
 
ETHYLENE, GLYCERIN, AND TARTARIC ACID. 113 
 
 arrangements of the atoms, and atomic groups, 
 that form the molecules of these compounds, the 
 careful reader will probably have observed that 
 these formulae make use of a certain convention 
 which is palpably incorrect. The parts of a 
 molecule must be arranged in three dimensions 
 in space ; the molecule must have length, breadth, 
 and thickness. But our structural formulae have 
 represented the parts of molecules as arranged 
 only in two dimensions in space ; these formulae 
 present molecules as having length and breadth, 
 but no thickness ; they make us think of the 
 atoms, and atomic groups, as arranged all in one 
 plane. May it be that the differences between 
 the optical activities of the four tartaric acids are 
 connected with different spatial arrangements of 
 the same atoms, and groups of atoms, that form 
 the molecules of these four compounds? In 
 order to test this suggestion, it is necessary to 
 adopt some convention, with the help of which 
 we may form a clear working hypothesis regarding 
 the connexions we are supposing to exist between 
 the arrangements of the parts of molecules in 
 three dimensions in space, and the properties of 
 these molecules. At present I content myself 
 with observing that a working hypothesis has 
 been made, that formulae have been constructed 
 which enable us to connect the properties of many 
 molecules with a spatial arrangement of their 
 parts which can be thought about clearly, and 
 that this hypothesis and these formulae have led 
 to many new conceptions of the greatest interest, 
 and to new and fascinating suggestions, concern- 
 ing the structure of those particles which are so 
 
 H 
 
114 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 minute that the smallest piece of matter one can 
 see by the help of a first-rate microscope is about 
 one hundred million times larger than one of 
 them. I defer the consideration of this subject 
 until I come to speak of the sugars, in Chapter 
 
 CHAPTER VIII. 
 
 A FEW TECHNICAL APPLICATIONS OF 
 COMPOUNDS OF CARBON. 
 
 THE hydrocarbons methane (CHJ, ethane (C 2 H 6 ), 
 and ethylene (C 2 H 4 ) are constituents of coal-gas. 
 In the last chapter (p. 99), I drew attention to 
 the conditions under which coal-gas burns with a 
 luminous flame, and the conditions which destroy 
 the luminosity, while they increase the tempera- 
 ture, of the flame of ordinary gas. When coal- 
 gas is to be used for giving light, it must not be 
 allowed to mix freely with air before it is 
 ignited \ when it is to be used as a source of heat, 
 the gas must be mixed with about fifteen times 
 its volume of air before it is burnt. But what is 
 the composition of coal-gas ; and how is it manu- 
 factured 1 Let us briefly consider these questions. 
 If coal is strongly heated in an iron tube, closed at 
 one end, and fitted at the other end with a cork 
 through which passes a glass tube, gas will be 
 given off at the open end of the glass tube, and 
 this gas will take fire when a lighted taper is 
 brought near it. If the glass tube is bent down- 
 wards and passed through a cork which fits into 
 
TECHNICAL USES OF CARBON COMPOUNDS. 115 
 
 the mouth of a glass bottle, and another tube 
 also passes through this cork into the air, tar and 
 a watery liquid will collect in the bottle, and 
 inflammable gas will issue from the open end of 
 the exit-tube from the bottle. This experiment 
 illustrates part of the process which takes place 
 in a manufactory of coal-gas. Coal, usually a 
 kind called cannel coal (because a piece of it burns 
 like a candle), is placed in fire-clay retorts which 
 are strongly heated in a furnace ; the products 
 are cooled, whereby tar, and water holding 
 ammonia in solution, are condensed ; the gas is 
 washed (to remove ammonia), passed through 
 purifiers (to remove carbonic acid gas, compounds 
 of sulphur, etc.), and stored in gas-holders, from 
 which it is sent, through the gas-mains, to the 
 places where it is to be burnt. One ton of 
 average cannel coal yields about 11,000 cubic 
 feet of gas, about 13 gallons of gas-liquor 
 (that is, tar and watery solution of ammonia), and 
 about half a ton of coke which remains in the 
 retorts. The number of compounds found in the 
 products of the distillation of coal in a closed 
 space is very great : these compounds comprise 
 about 50 hydrocarbons ; about a dozen com- 
 pounds of carbon, hydrogen, and oxygen ; about 
 15 compounds of carbon, hydrogen, and nitrogen ; 
 9 or 10 carbon compounds that contain sulphur ; 
 and the three elementary substances, carbon, 
 hydrogen, and nitrogen. After the products of 
 distillation have been cooled, by passing through 
 condensers, and the tar and ammoniacal liquor 
 have thus been removed, the gas contains 
 ammonia and various compounds of ammonia, 
 
116 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 carbonic acid, sulphuretted hydrogen, carbon bi- 
 sulphide, and small quantities of other compounds 
 of sulphur, besides the bodies which form the 
 mixture that is called coal-gas ; most of the 
 ammonia and the compounds of ammonia are 
 removed by causing the gas to pass upwards 
 through a tower containing coke, or broken 
 flints, down which water trickles in thin streams ; 
 the greater portion of the carbonic acid, the 
 whole of the sulphuretted hydrogen, and part of 
 the carbon bisulphide, are removed by passing 
 the gas through slaked lime, or through hydrated 
 oxide of iron and then through slaked lime. 
 The gas that now remains is a mixture of hydro- 
 gen, methane, ethylene, and other hydro-carbons, 
 carbon monoxide, a little carbon dioxide, nitro- 
 gen, oxygen, and small quantities of gaseous 
 sulphur compounds. The constituents of coal-gas 
 to which the luminosity of the flame of the gas is 
 chiefly due are the hydrocarbons other than 
 ethylene and methane. Part of the luminosity 
 is certainly caused by the glowing particles of 
 carbon produced by the decomposition of ethylene 
 (and to a smaller extent, of methane) in the 
 flame ; but the other hydrocarbons, although 
 they are present only in small quantities, are 
 much richer in carbon than marsh gas and 
 ethylene, and they burn with a highly luminous 
 flame. No light is obtained by the burning of 
 the hydrogen and the carbon monoxide in coal- 
 gas ; but the combustion of these gases produces 
 much heat, part of which is used in decomposing 
 the hydrocarbons with the formation of particles 
 of carbon. The nitrogen, oxygen, and carbonic 
 
TECHNICAL USES OF CARBON COMPOUNDS. 117 
 
 acid in coal-gas diminish the luminosity of the 
 flame by dilution and cooling. The most ob- 
 jectionable impurities in coal-gas are carbon 
 bisulphide and other sulphur compounds ; when 
 the gas is burnt, these compounds are oxidised 
 to sulphurous and sulphuric acid, substances 
 which are very injurious to health, and also to 
 books and furniture. The coal-gas supplied in 
 London must be perfectly free from sulphuretted 
 hydrogen, and the quantity of sulphur in the 
 other compounds of that element which are 
 allowed to be present must not exceed 17 J grains 
 per 100 cubic feet of gas. 
 
 Among the products of the destructive distil- 
 lation of coal are various hydrocarbons, belonging 
 to the paraffin series, which are solids at ordinary 
 temperatures. The composition of all the par- 
 affins is given by the formula C M H 2n+2 (compare 
 p. 103). The compound in which n= 14 (C 14 H 3 g) 
 melts at 4'5C. [ = 40F.] ; this compound, and all 
 the paraffins higher in the series than this, are 
 solids at the ordinary temperature. The solid, 
 wax-like, substance that is known commercially 
 as paraffin is a mixture of the higher hydro- 
 carbons of the paraffin series. The liquid known 
 as paraffin oil is a mixture of many hydrocar- 
 bons; the American oils consist chiefly of par- 
 affins (CvHgn+g), while Eussian oils generally 
 contain also hydrocarbons of the composition 
 C^H^, and sometimes hydrocarbons belonging to 
 the series C n H 2n _g. The manufacture of paraffin- 
 wax and paraffin-oil from shale is a process re- 
 sembling that of the manufacture of coal-gas, 
 in that both processes are based on the destruc- 
 
118 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 tive distillation of carbonaceous minerals. When 
 coaly shale is heated in a retort to low redness it 
 is decomposed with the production of (i.) a gas 
 which is not liquified at ordinary temperatures, 
 (ii.) water holding a little ammonia in solution, 
 (iii.) a thick oil which solidifies at the ordinary 
 temperature, and (iv.) coke which remains in the 
 retort. The gas is generally burnt, and the heat 
 so produced is used in distilling fresh quantities 
 of shale. The ammoniacal liquor is- neutral- 
 ised by acid, and the ammonia salt that is pro- 
 duced is used in other manufactures or as a 
 manure. The oil is distilled, whereby it is 
 separated into five portions: (i.) naphtha, (ii.) 
 burning oil, (iii.) light mineral oil, (iv.) lubri- 
 cating oil, and (v.) paraffin-wax. Naphtha is the 
 lightest and most volatile portion of the liquid 
 products of the distillation of shale ; it is gene- 
 rally separated, by re-distillation, into portions 
 which have different specific gravities and boiling 
 points, and are known by such names as gasoline, 
 benzine, and petroleum ether. Burning oil is a mix- 
 ture of hydrocarbons, from 30 to 80 per cent, of 
 which belong to the olefine (or ethylene) series. 
 Mineral oil and lubricating oil consist almost en- 
 tirely of olefines ; and paraffin-wax is a mixture 
 of the higher (solid) members of the paraffin 
 hydrocarbons. The district between Edinburgh 
 and Glasgow is the seat of the manufacture of 
 paraffin oils and solid paraffin from shaly coals. 
 One ton of the shale which is distilled yields from 
 20 to 40 gallons of oil. In 1890, about 65 
 million gallons of crude oil were obtained ; and 
 this yielded about 2 million gallons of naphtha, 
 
TECHNICAL USES OF CARBON COMPOUNDS. 119 
 
 about 19 million gallons of burning oil, about 
 10 million gallons of lubricating oil, and about 
 20,000 tons of solid paraffin. 
 
 In America and Russia, enormous quantities 
 of oil are obtained by boring wells in the surface 
 of the earth. These wells differ greatly in depth ; 
 in some places oil is found after boring to a depth 
 of 40 or 50 feet, in other places the oil wells are 
 nearly 3000 feet deep. Newly opened wells are 
 generally "gushers " ; that is to say, the pressure 
 of the gases that are produced in the earth, along 
 with the oil, is sufficient to cause the oil to flow, 
 or in some cases to spout, from the openings of the 
 wells. Some American newly opened " gushers " 
 have poured forth as much as 8000 or 9000 bar- 
 rels of oil per day ; but the yield generally falls 
 off rapidly, and in most cases it becomes necessary 
 after a time to use pumps to raise the oil to the 
 surface. One of the Russian wells spouted so 
 violently that the outflow of oil was uncontrol- 
 lable for three or four months ; a column of oil 
 from 100 to 300 feet high issued from the mouth 
 of the well ; the surrounding country was inun- 
 dated ; the workshops were nearly buried in the 
 sand that was ejected with the oil ; and from 100 
 to 200 million gallons of oil were lost. The 
 Russian well ejected a column of oil and sand 400 
 feet high, and on windy-days the oil-spray was 
 carried to a distance of eight miles. 
 
 The American and Russian crude oils are gene- 
 rally called petroleum. The processes whereby the 
 petroleum is separated into naphtha, burning oil, 
 lubricating oil, and paraffin-wax are essentially 
 the same as those used for the separation of shale- 
 
120 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 oil into portions; they consist of distillation, 
 washing with acid and alkali successively, and 
 re-distillation. About 1000 million gallons of 
 crude petroleum were produced in the United 
 States in 1890, and about 76 million gallons 
 were exported in that year. The quantity of 
 crude petroleum exported from the Baku dis- 
 trict in Russia amounted to about 3 million 
 tons in 1890. 
 
 The oil-wells in America and Russia emit in- 
 flammable gases, besides those liquid compounds 
 that form the petroleum of commerce. In the 
 oil-bearing districts of Russia, especially in the 
 neighbourhood of Baku, those inflammable gases 
 have issued from the earth from time immemorial. 
 Long ago, we do not know when or by whom, 
 the issuing gas was ignited, and the eternal fire 
 continued to burn for a great many centuries. 
 An account of these fires was given, in 1754, by 
 Mr Hanway, who was sent from England to 
 arrange the conditions of a trade in oil from 
 Baku to India via the Caspian Sea. Mr Hanway 
 says : 
 
 "What the Guebers, or Fire- Worshippers, call the 
 Everlasting Fire is a phenomenon of a very extra- 
 ordinary nature. The object of devotion lies about ten 
 English miles north-east by east from the city of Baku, 
 on a dry rocky land. There are several ancient temples 
 built with stone, supposed to have been all dedicated to 
 fire. Amongst others is a little temple at which the 
 Indians now worship. Here are generally forty or fifty 
 of these poor devotees, who come on a pilgrimage from 
 their own country. A little way from the temple is a 
 low cleft of a rock, in which there is a horizontal gap, 
 two feet from the ground, nearly six long, and about 
 three wide, out of which issues a constant flame, in 
 
TECHNICAL USES OF CARBON COMPOUNDS. 121 
 
 colour and gentleness not unlike a lamp that burns 
 with spirits, only more pure. When the wind blows, it 
 rises sometimes eight feet high, but much lower in still 
 weather. They do not perceive that the flame makes any 
 impression on the rock. This also the Indians worship, 
 and say it cannot be resisted, but if extinguished will 
 rise in another place. The earth round the place, for 
 about two miles, has this surprising property, that by 
 taking up two or three inches of the surface and applying 
 a live coal, the part which is so uncovered immediately 
 takes fire, almost before the coal touches the earth ; the 
 flame makes the soil hot, but does not consume it, nor 
 affect what is near it with any degree of heat. A Any 
 quantity of this earth carried to another place does not 
 produce this effect. ... If a cane or tube even of paper 
 be set about two inches in the ground, confined and closed 
 with earth below, and the top of it touched with a live 
 coal, and blown upon, immediately a flame issues without 
 hurting either the cane or paper, provided the edges be 
 covered with clay ; and this method they use for light in 
 their houses, which have only the earth for their floor ; 
 three or four of these lighted canes will boil water in a 
 pot, and thus they dress their victuals. . . . Lime is 
 burnt to great perfection by means of this phenomenon." 
 
 Gibbon tells us (in The Decline and Fall) that 
 in 624 A.D. Heraclius wintered 70 miles south of 
 Baku, and that he 
 
 "Signalised the zeal and revenge of a Christian em- 
 peror. At his command the soldiers extinguished the 
 fire and destroyed the temples of the Magi." 
 
 The process whereby acetic acid (C 2 H 4 2 ) is 
 obtained from ethylic alcohol (C 2 H 6 0) has been 
 sketched in Chapter VI. (p. 83). The conver- 
 sion of ethylic alcohol, contained in certain fer- 
 mented fruit-juices, into acetic acid is the main 
 chemical change that occurs in making vinegar. 
 Mere contact of alcohol, or an alcoholic liquor, 
 with oxygen does not suffice to effect the oxida- 
 
122 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 tion of the alcohol to acetic acid ; but if a small 
 quantity of the minute fungus called mycoderma 
 aceti is present, the oxidation proceeds. This 
 little plant grows in a liquid which contains 
 albuminous bodies and certain mineral salts, pro- 
 vided there is free access of air ; and if the liquid 
 also contains 10 per cent., or less than 10 per 
 cent., of alcohol, the pjant absorbs the alcohol 
 slowly, and causes its oxidation to acetic acid 
 by the oxygen which is taken from the air by 
 the growing fungus. As the germs of this 
 acetifying fungus are always present in the 
 air, wine or beer soon begins to turn sour when 
 it is kept in an open vessel. The most favour- 
 able conditions for the acetification of an alcoholic 
 liquor are these : the presence in the liquid of 
 plenty of food for the fungus ; the presence of 
 not more than 10 per cent., nor less than 4 per 
 cent., of alcohol ; the exposure of a large surface 
 of the liquid to the free access of air ; and the 
 maintenance of the temperature at about 25 C. 
 [ = 77 F.]. There are two processes whereby 
 vinegar is manufactured. In the older process, 
 the alcoholic liquid generally poor wine about 
 a year old, or a fermented infusion of malt is 
 kept for many days in large casks, made of 
 beech- wood, which have been soaked in vinegar. 
 In the quick vinegar process, an alcoholic liquid 
 is caused to trickle, in very thin streams, over 
 beechwood shavings, or purified charcoal, which 
 have been " soured " by immersion in hot vinegar, 
 while a current of air passes through the shavings, 
 or the charcoal, in the direction opposite to that 
 taken by the spray of alcoholic liquid. 
 
TECHNICAL USES OF CARBON COMPOUNDS. 123 
 
 The casks employed in the slow process contain 
 from 50 to 100 gallons of liquid apiece ; each is 
 pierced by two holes, one to admit air, and the 
 other for pouring in, or withdrawing, liquid. The 
 casks are filled to one-third with vinegar, and the 
 temperature is kept at about 25 C. [ = 77 F.] ; 
 after eight days about ten pints of wine are 
 added, and after another eight days about ten 
 pints more, and so on, until the casks are two- 
 thirds full. When fourteen days have passed, 
 a portion of the liquid is drawn off, and more 
 wine is poured in. Each cask produces annually 
 a quantity of vinegar equal to about twice its 
 own capacity. The casks are thoroughly cleaned 
 about once in six years. It is said that a good 
 cask will last for five and twenty years. The 
 manufacture of malt vinegar is very much like 
 that of wine vinegar. Sometimes the casks are 
 placed in rows in the open air ; in such a case 
 the operation generally begins in the spring, and 
 is finished in about three months. 
 
 The liquors that are acetified by the quick 
 process are generally prepared by mixing poor 
 brandies with water and some vinegar, and add- 
 ing bran or rye to give food to the vinegar fungus. 
 Sometimes diluted brandy, or whiskey, is mixed 
 with fermented infusion of malt ; in other cases 
 an infusion of barley-meal and wheat-meal is fer- 
 mented ; molasses or honey is added occasionally 
 to give a rich colour to the vinegar. The acetifi- 
 cation is conducted in large vats, which are fur- 
 nished with perforated false bottoms placed about 
 18 inches above the true bottoms, and fitted near 
 the tops with wooden discs which are pierced by 
 
124 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 a great many very small holes ; there are also a 
 few holes sloping downwards in the sides of each 
 vat, below the false bottom. The space between 
 the false bottom and the upper disc is nearly filled 
 with beech-wood shavings, or with pieces of char- 
 coal which have been freed from saline impurities 
 by soaking in acid and washing. Through each 
 hole in the upper disc is suspended a thread of 
 twisted cotton yarn, the lower end of which 
 touches the shavings, or the charcoal. The shav- 
 ings (or charcoal) are soaked in hot vinegar for 
 a day or two. The liquor is poured on to the 
 upper disc, and trickles slowly down the twisted 
 threads and then through the shavings, or the 
 charcoal ; when the shavings have become coated 
 with the vinegar fungus, the oxidation of the 
 alcohol in the liquor proceeds fairly rapidly, 
 and the temperature rises in the interior of 
 the vat to about 37 C. [ = 98 F.] : one effect 
 of this heating is to cause a current of air to 
 enter, by the holes in the sides of the vat 
 beneath the false bottom, and, in passing up- 
 wards, to come in contact with the descending 
 thin stream of liquid, and thus to aid the oxida- 
 tion of the alcohol in the liquid. The vinegar is 
 drawn off by a tap placed at about an inch above 
 the bottom of the vat. If the alcoholic liquor 
 contains about 4 per cent, of alcohol, the acetifi- 
 cation is complete when the liquid reaches the 
 bottom of the vat ; liquids richer in alcohol must 
 be passed through the apparatus two or three 
 times. 
 
 Besides acetic acid, which is the main product 
 of the chemical changes that occur in making 
 
TECHNICAL USES OF CARBON COMPOUNDS. 125 
 
 vinegar, there are formed small quantities of 
 ethylic acetate, and some other ethereal salts ; it 
 is to the presence of these compounds that the 
 odour of vinegar is chiefly due. The quantity 
 of acetic acid should never be less than 5 per 
 cent, in genuine vinegar, but wine vinegar some- 
 times contains as much as 12 per cent. Besides 
 acetic acid, vinegar contains small quantities of 
 alcohol, sugar, and various substances extracted 
 from malt or wine-juice, in addition to chlorides, 
 acetates, sulphates, and phosphates, of various 
 metals, the principal of which are sodium, potas- 
 sium, and calcium. It is legal to add one part 
 of sulphuric acid to 1000 parts of vinegar. 
 
 An artificial vinegar is prepared by mixing 
 acetic acid with water, and adding a little burnt 
 sugar, and a trace of ethylic acetate to give an 
 agreeable odour. 
 
 The only other technical application of com- 
 pounds of carbon which I shall notice in this 
 chapter is that wherein the ethereal salts, 
 glyceryl stearate, palmitate, and oleate, are de- 
 composed, by alkalis, to produce soaps and 
 glycerin. We have already considered the 
 chemical changes that occur when caustic potash, 
 or soda, is boiled with an ethereal salt ; the pro- 
 ducts are a potassium, or sodium, salt of the 
 acid the radicle whereof formed part of the 
 ethereal salt used, and an alcohol, that is a 
 hydroxide of the ethereal radicle of the salt that 
 has been decomposed. (For a detailed account 
 of these changes see the last chapter, pp. 94, 95.) 
 The reactions that occur when glyceryl stearate, 
 palmitate, or oleate, is decomposed by steam are 
 
126 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 very much like those which take place when the 
 decomposition is effected by caustic potash; only, 
 in place of obtaining potassium stearate, palmi- 
 tate, or oleate, we obtain stearic, palmitic, or 
 ole'ic acid. When glyceryl stearate is used, the 
 reaction may be expressed thus : 
 
 C 3 H 6 (0 18 H 35 2 ) 3 + 3H( 
 C 3 H 6 (OH) 3 + 3(H.C 18 H 35 2 ). 
 
 Glycerin. Stearic acid. 
 
 For the sake of comparison, the reaction between 
 glyceryl stearate and a boiling solution of potash 
 is repeated here : 
 
 C3H 5 (C 18 H 35 2 ) 3 + 3KOH 
 C 3 H 6 (OH) 3 + 3(K.C 18 H 35 2 ). 
 
 Glycerin. Potassium stearate. 
 
 The decomposition of the glyceryl salts of 
 palmitic, stearic, ole'ic, and other acids, for the 
 purpose of making soaps, is conducted in large 
 metal vats, holding from 10 to 40 tons of 
 material ; the fatty matter is melted, weak soda 
 ley (solution of caustic soda) is run in, and the 
 whole is heated to boiling by steam ; after a time 
 more concentrated soda ley is added, and boiling 
 is continued until the fat has been saponified; 
 common salt, or a concentrated solution of com- 
 mon salt, is then added, because soap is in- 
 soluble in concentrated brine ; as the contents of 
 the vat cool, the soap separates to the top, as 
 a curd, and the glycerin dissolves in the lower 
 watery layer. The curd is separated, and boiled 
 with successive quantities of soda ley until it has 
 a distinctly alkaline taste; the curd is again 
 allowed to separate, and is then run into cooling 
 
TECHNICAL USES OF CARBON COMPOUNDS. 127 
 
 frames made of iron or wood. The semi-fluid 
 material is often mixed with scents, antiseptic 
 substances, or such salts as silicate of soda or 
 sulphate of soda; the presence of these salts 
 gives a finer texture to the soap, and also enables 
 it to take up a large quantity of water and yet 
 remain solid. Sometimes the soap is 'fitted.' In 
 this process the soap is heated by means of wet 
 steam, and then allowed to rest for some days, 
 when a separation occurs into three layers : the 
 upper layer is a frothy soap known as l fob' ; the 
 lowest layer contains various impurities, and 
 water, mixed perhaps with more or less caustic 
 soda; and the middle layer consists of the 'neat' 
 soap, which is run into cooling frames. A fitted 
 soap may contain 30 or 40 per cent, of water, 
 but it is almost quite free from caustic soda ; curd 
 soap, on the other hand, often contains consider- 
 able quantities of alkali. If such fats as linseed, 
 poppy, or hempseed, oil are boiled with just 
 sufficient potash ley to complete the process of 
 saponification, and the whole mass is allowed to 
 cool, a soft, homogeneous jelly is produced which 
 contains the whole of the glycerin formed in the 
 saponification ; such a soap is very soft and dis- 
 solves easily in water ; it generally, however, 
 contains free alkali. 
 
 Some of the finest toilet soaps are obtained by 
 drying carefully made soap, then dissolving it in 
 alcohol, and distilling off the spirit; the residue 
 is more or less transparent ; as the soap sets it is 
 mixed with a little glycerin. 
 
 Soap is fairly soluble in water, but it is in- 
 soluble in water containing a certain, not very 
 
128 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 large, quantity of saline matter. As soaps made 
 from cocoanut oil, and palm-kernel oil, are more 
 soluble in a watery solution of saline matter than 
 other soaps, the products of saponifying these 
 oils by potash are sold as marine soaps, because 
 they give a lather with salt water. If soap 
 made in this way is mixed with such salts as 
 silicate or sulphate of soda, it is possible to add 
 a very large quantity of water and yet to obtain 
 a firm mass ; marine soaps sometimes contain 80 
 to 85 per cent, of water and salts, and only 15 to 
 20 per cent, of genuine soap. 
 
 A theoretically perfect soap consists of potas- 
 sium, or sodium, salts of certain fatty acids, mixed 
 together. Contact with water breaks up the soap 
 into salts which contain more of the acidic radicle 
 relatively to the quantity of potassium or sodium 
 present, and some potash, or soda ; and the de- 
 tergent value of soap is largely due to the small 
 quantity of alkali thus produced. If the water 
 which comes into contact with soap contains 
 chalk, or gypsum, or carbonate or sulphate of 
 magnesium, then a reaction occurs between the 
 soap and the salt in the water, whereby a cal- 
 cium (or magnesium) salt of the acidic radicle of 
 the soap, and a potassium (or sodium) salt of the 
 acidic radicle of the salt that is present in the 
 water, are produced. Supposing the soap to 
 consist of potassium stearate only, and the water 
 to contain calcium carbonate (chalk), then the 
 reaction may be thus expressed in an equation 
 
 2(K.C 18 H 35 2 ) + CaC0 3 = Ca(C 18 H 35 2 ) 2 
 
 Potassium Calcium Calcium Potassium 
 
 stearate. carbonate. stearate. carbonate. 
 
SUGARS, STARCHES, AND CELLULOSE. 129 
 
 When the reaction between the soap and the 
 salts in the water is finished, but not until then, 
 the ordinary action between a soap and pure 
 water begins, and a lather is produced. All 
 " hard " waters contain chalk, or gypsum, or sul- 
 phate or carbonate of magnesium, in solution ; 
 hence, in washing with a hard water, a com- 
 paratively large quantity of soap must be used 
 before a lather is produced. 
 
 CHAPTER IX. 
 
 SUGARS, STARCHES, AND CELLULOSE. 
 
 THE juices of the sugar-cane, beetroot, certain 
 palms, the maple, and the sorghum, contain a 
 compound of carbon, hydrogen, and oxygen, 
 which has the composition C 12 H 22 O n . All 
 plants contain starch, C 6 H 10 5 . And the main 
 constituent of all vegetable tissues is cellulose, 
 C 6 H 10 5 . A sugar, whose composition is ex- 
 pressed by the formula C 6 H 10 6 , is found in the 
 juices of fruits, and in honey. 
 
 Cane-sugar, or saccharose (C 12 H 22 O n ) is pre- 
 pared by crushing sugar-cane, or beetroots, 
 neutralising the juice by lime, adding sulphurous 
 acid (to prevent fermentation), evaporating, and 
 crystallising. The raw sugar is refined by dis- 
 solving it in water, filtering, removing colouring 
 matter by means of animal charcoal, evaporating 
 in vacuum-pans, and separating the solid sugar 
 from the syrup by centrifugal machines. Starch 
 I 
 
130 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 is obtained by steeping potatoes in water, wash- 
 ing, rasping, and straining, and allowing the 
 starch to settle; the starch is then washed, 
 drained, and dried. Rice-starch is manufactured 
 by macerating rice with a dilute solution of soda 
 or potash, whereby gluten is removed, draining, 
 washing, grinding, and sifting; in some processes 
 the gluten is removed by a process of fermenta- 
 tion, followed by washing and treatment with 
 much diluted acid. The variety of fruit-sugar 
 known as glucose (C 6 H 12 6 ) is generally manu- 
 factured from the starch of sago, maize, or rice, 
 by heating with dilute sulphuric acid, neutral- 
 ising by chalk, separating from calcium sulphate, 
 decolourising by animal charcoal, and evaporat- 
 ing to a syrup. Cellulose is made from vegetable 
 tissues by treatment with alkali and some weak 
 oxidiser; for instance, by treating cotton with 
 bleaching powder. 
 
 These four compounds, cane-sugar, fruit-sugar, 
 starch, and cellulose, belong to the class of com- 
 pounds called carbohydrates. The formula of each 
 contains six atoms, or a whole multiple of six 
 atoms, of carbon, and always twice as many 
 atoms of hydrogen as of oxygen ; in other words, 
 the compositions of the compounds are expressed 
 by the formula 7iC 6 .mH 2 0. Although the weights 
 of hydrogen and oxygen in these compounds are 
 in the same ratio as in water, the substances are 
 not compounds of carbon with water (as the name 
 carbohydrate implies); their reactions negative 
 this view of their constitution. 
 
 Cane-sugar is the most important representative 
 of the group of carbohydrates called saccharoses, 
 
SUGARS, STARCHES, AND CELLULOSE. 131 
 
 all of which have the composition C 12 H 22 lr 
 Fruit-sugar belongs to the group of glucoses, 
 C 6 H 12 Og. And starch and cellulose are amyloses, 
 C 6 H 10 5 . These formulae are the simplest ex- 
 pressions that can be given of the compositions 
 and some of the reactions of the four compounds, 
 but they are not necessarily molecular formulae ; 
 the formulae which tell the numbers of atoms of 
 carbon, hydrogen, and oxygen, in the molecules 
 of the compounds may be multiples of these 
 simplest expressions. There is, however, good 
 reason to regard the formulae C 12 H 22 O n and 
 C 6 H 12 6 , given to cane-sugar and fruit-sugar, 
 respectively, as molecular formulae. 
 
 The examination of the reactions of the glu- 
 coses has been carried much further than that of 
 the reactions of the saccharoses or of the amy- 
 loses. There are at least eight sugars to all of 
 which the formula C 6 H 12 6 must be given ; more- 
 over the reactions of these eight sugars shew that 
 the structural formula CH 2 OH.(CHOH) 4 .HCO 
 must be assigned to each of them. Now it is 
 not possible to form eight modifications of this 
 expression if the assumption is made that the 
 atoms are arranged in only two dimensions in 
 space. It has been noticed already (see pp. 112, 
 113) that the hypothesis that molecules have 
 only length and breadth and no thickness served 
 admirably to group together many facts con- 
 cerning the reactions of carbon compounds, but 
 that the hypothesis broke down in some cases, for 
 instance in the case of the four tartaric acids (see 
 pp. Ill, 112). We have now another set of facts 
 which refuse to be brought into order by the use 
 
132 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 of the hypothesis that worked so well for a time. 
 We are forced to attempt to form expressions 
 which shall represent the atoms of carbon, hydro- 
 gen and oxygen as arranged in three dimensions 
 in space in those groups which are the molecules 
 of the various glucoses. 
 
 It is of course impossible to form realistic 
 presentments of the tri-dimensional arrange- 
 ments of the atoms in molecules ; all we can 
 do is to endeavour to construct an hypothesis 
 which shall be the framework wherein our 
 knowledge of the observed reactions of the 
 compounds under consideration may be set, 
 shall bind that knowledge into a consistent 
 whole, and shall indicate the directions wherein 
 new attacks, likely to prove successful, may be 
 made on the problem of the connexions between 
 the compositions and the reactions of molecules. 
 
 The glucoses resemble some of the tartaric 
 acids in one respect ; they are optically active 
 compounds when dissolved in water: solutions 
 of some of them rotate the plane of polarisation 
 of a ray of light to the right hand, and solutions 
 of some are laevo-rotatory. An aqueous solution 
 of cane sugar is dextro-rotatory ; ordinary starch 
 which has been dried in the air is insoluble in 
 water, but there is a variety of starch called 
 soluble starch, and an aqueous solution of that 
 compound rotates the plane of polarisation of 
 a ray of light in the direction opposite to that 
 in which the hands of a watch move ; cellulose 
 is insoluble in water. 
 
 The hypothesis that is used in framing tri- 
 dimensional formulae for such compounds as the 
 
SUGARS, STARCHES, AND CELLULOSE. 133 
 
 glucoses and the tartaric acids rests on the con- 
 ception of the asymmetric carbon atom. 
 
 Let an atom of carbon be in direct union with 
 four other atoms, two of which are identical, and 
 the two others are also identical but different 
 from the first pair ; the composition of such a 
 molecule will be expressed by the symbol 
 CR^RgRg. The compound CH 2 C1 2 (dichloro- 
 methane) is an example of this arrangement. 
 Now it is possible to assign two different for- 
 mulae to this molecule, using the ordinary 
 hypothesis that the atoms are arranged in 
 two dimensions in space; these formulae are 
 
 ci ci 
 
 H C-H and H-c-ci In one arrangement each 
 
 CI H 
 
 hydrogen atom has a chlorine atom on either 
 side of it ; in the other arrangement each 
 hydrogen atom has for its immediate neigh- 
 bours an atom of chlorine and an atom of hy- 
 drogen. But observed facts tell that molecules 
 of this composition never exist in more than 
 one modification : there is only one dichloro- 
 methane, not two dichloromethanes as there 
 ought to be if the ordinary view of the arrange- 
 ment of atoms were sufficient. We want then to 
 picture the arrangement in space of a molecule 
 composed of an atom of carbon united to four 
 other atoms, or atomic groups, in such a way as 
 shall bring our theoretical conception of this 
 arrangement into keeping with the observed 
 facts. Now suppose the carbon atom to be 
 placed in the centre of a regular tetrahedron, 
 
134 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 and each of the four other atoms to be placed 
 at one of the summits of the tetrahedron ; we 
 have the arrangement pictured thus : 
 
 R R 
 
 where each E stands for an atom, or atomic 
 group, in direct union with the atom of carbon 
 supposed to be in the centre of the tetrahedron. 
 Suppose the four atoms, or groups, in connexion 
 with the carbon atom to be different (suppose 
 them, for instance, to be an atom of hydrogen, 
 an atom of chlorine, an atom of bromine, and 
 an atom of iodine) ; then the arrangement would 
 be pictured by one of the two following figures, 
 and the image of this arrangement in a mirror 
 would be represented by the other figure : 
 
 R 3 
 
 We have here two arrangements like a right- 
 handed and a left-handed glove. Just as it is 
 impossible to lay a pair of gloves together, both 
 palms upward, or both backs upward, without 
 bringing the thumbs on different sides, so it is 
 impossible to lay one of the arrangements shewn 
 in the figures on the other so that R x shall be 
 superposed on R p R 2 on R 2 , R 3 on R 3 , and R 4 
 on R 4 . The properties of one of the molecules 
 pictured by, say, the first of the two figures will 
 differ from the properties of the molecule pictured 
 by the other figure. 
 
SUGARS, STARCHES, AND CELLULOSE. 135 
 
 When two compounds have the same composi- 
 tion but different properties, and the two mole- 
 cules are composed of the same numbers of the 
 same atoms, one compound is said to be an 
 isomeride of the other; and the existence of two 
 (or more than two) such compounds is said to be 
 a case of isomerism (from two Greek words signi- 
 fying equal parts). In the case before us, the 
 compound symbolised by one of the tetrahedral 
 figures is called a geometrical isomeride, or some- 
 times a mirror-isomeride, of the other. 
 
 Now consider the case of an atom of carbon in 
 direct union with four other atoms, or groups of 
 atoms, two of which are the same. Representing 
 these atoms, or groups, by RjRjR^ and R 3 , we 
 can picture the tetrahedral arrangement of all the 
 atoms by one of the following figures, and the 
 mirror- image of that arrangement by the other 
 figure : 
 
 R, RI R; R, 
 
 One of these figures can be superposed on the 
 other : take the second figure, turn it round so 
 that the Rj at the top of the figure is brought 
 where the R 2 was before the figure was turned 
 round ; then the result is identical with the first 
 figure. In such a case as this, that is in a mole- 
 cule composed of an atom of carbon united with 
 four other atoms (or groups) two of which are 
 the same, isomerism cannot occur if the hypo- 
 thesis we are working on is a satisfactory method 
 of symbolising facts. There is no case known of 
 
136 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 the existence of more than one modification of a 
 compound whose molecular composition is ex- 
 pressed by the symbol CRJ^RgRg. There are 
 cases known of isomerism shewn by compounds 
 whose composition is expressed by the symbol 
 
 123 4. 
 
 Turn back for a moment to the two figures 
 placed side by side on p. 134. The fact that 
 neither figure is superposable on the other is 
 sometimes expressed by saying there is no plane 
 of symmetry in either figure. It is this notion of 
 symmetry which underlies the expression, I am 
 now trying to explain, the asymmetric carbon atom. 
 If a single atom of carbon is in direct union with 
 four other atoms, or atomic groups, all of which 
 are different, then a compound is produced which 
 can exist in two modifications ; or, it would be 
 more correct to say, two compounds are possible 
 both having the same composition. But if a 
 single atom of carbon is in direct union with four 
 other atoms (or groups), two, or three, of which 
 are the same, then the compound that is formed 
 does not exist in more than one modification. 
 The only way we have been able to think clearly 
 about these facts (the reader should notice that 
 it is almost impossible to state the bare facts 
 except in terms of a theory of the structure of 
 matter) is by picturing to ourselves the arrange- 
 ment of the atoms of the two isomeric molecules, 
 CRjRgRgR^ as like a regular tetrahedron with 
 the atom of carbon in the centre, and each atom, 
 or atomic group, at one of the summits. And as 
 this is an unsymmetrical arrangement, inasmuch 
 as the mirror-image of this arrangement cannot 
 
SUGARS. STARCHES, AND CELLULOSE. 137 
 
 be superposed on the original configuration, the 
 atom of carbon, which is thought of as the 
 central pivot whereon the other atoms are hung, 
 is spoken of as an asymmetric atom. In some 
 cases then we think of the existence of two, or 
 more than two, compounds of carbon with the 
 same composition and the same molecular weight 
 as dependent on the presence of asymmetric 
 carbon atoms in the molecules of these com- 
 pounds ; and by the expression, an asymmetric 
 carbon atom, we mean an atom of carbon in direct 
 union with four atoms (or atomic groups) no one 
 of which is the same as any other. The only 
 way we have at present of connecting the ex- 
 hibition of isomerism with the presence of asym- 
 metric carbon atoms in molecules is by likening 
 
 figures, with an asymmetric carbon atom in the 
 centre of each, and the four atoms, or groups, 
 that are in direct union with this asymmetric 
 atom at the four summits of each figure. Of 
 course we are sure that the molecules are not 
 really tetrahedral arrangements with an atom of 
 carbon in the centre, for we know that a molecule 
 must be an exceedingly complicated structure, 
 and that the parts of every molecule must be 
 performing regulated movements. Nevertheless 
 we must frame some conception of the arrange- 
 ment of the parts of molecules, and in order 
 to form a conception which shall be useful in 
 furthering exact knowledge of the connexions 
 between composition and properties, we must 
 employ a tool the trick of which we have learned 
 and can use. The only tool that has been found 
 
138 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 suitable for the work to be done is that whose 
 mechanism I have been trying to describe. 
 
 The reactions of every compound which is 
 optically active in solution (that is to say, which 
 rotates the plane of polarisation of a ray of light) 
 indicate the existence of at least one atom of 
 carbon in the molecule in direct union with four 
 different atoms, or groups of atoms. The ex- 
 hibition of optical activity by a compound in 
 solution seems then to be associated with the 
 presence of asymmetric carbon atoms in the 
 molecule of the compound. We found (Chapter 
 VII., p. 109) that four tartaric acids exist, that 
 the. reactions of all are expressed by the formula 
 C 2 H 2 (OH) 2 (C0 2 H) 2 , that one of these acids is 
 dextrorotatory in solution, another is laevorota- 
 tory, another is optically inactive but can be 
 resolved into equal weights of the dextrorotatory 
 and the laevorotatory acids, and that the fourth 
 is optically inactive in solution and cannot be 
 resolved into the active modifications. Writing 
 the formula C 2 H 2 (OH) 2 (C0 2 H) 2 in full, we have 
 the following expression : 
 
 H H 
 CO H-C - e'-CO H 
 
 2 i i 2 
 
 OH OH 
 
 The molecule of tartaric acid contains two 
 atoms of carbon each of which is in direct union 
 with four different atoms or atomic groups, namely, 
 with H, (OH), (C0 2 H), and (C.H.OH.C0 2 H) ; 
 that is, the molecule contains two asymmetric 
 carbon atoms (the symbols of these atoms are 
 printed in italics in the formula). Now if this 
 iormula must be assigned to each of the four 
 
SUGARS, STARCHES, AND CELLULOSE. 139 
 
 tartaric acids, it is evident that a compound may 
 be optically inactive although its molecule con- 
 tains two asymmetric carbon atoms, and that an 
 optically active compound may have the same 
 composition as another which is inactive and, 
 like the active compound, contains a pair of 
 asymmetric carbon atoms. 
 
 To attempt to follow in detail the working out 
 of the hypothesis of geometrical isomerism as it 
 is applied to the cases of the four tartaric acids 
 would be out of place in this book ; nevertheless 
 a slight outline of the application of the hypo- 
 thesis may be given. The conception that is 
 formed of the structure of the molecule of tartaric 
 acid is that of two tetradedra with one summit 
 of one joined to one of the other; one asym- 
 metrical carbon atom is thought of as placed at 
 the centre of each tetrahedron, and the atomic 
 groups are supposed to be placed at the remain- 
 ing six summits. Two of the possible arrange- 
 ments of those groups are shown in the following 
 figures : 
 
 HO.OC 
 
 One of these figures bears to the other the 
 same relation as an object bears to its image in 
 a mirror. If one of these molecules is dextro- 
 
140 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 rotatory the other will be laevorotatory. If we 
 suppose a dextrorotatory molecule joined to a 
 laevorotatory molecule the right-handed optical 
 activity of one will be neutralised by the left- 
 handed activity of the other ; the compound 
 molecule will be optically inactive, but resolvable 
 into a dextrorotatory and a laevorotatory mole- 
 cule. The existence of two optically active, and 
 one inactive but resolvable, modification of tar- 
 taric acid is thus in keeping with the hypothesis. 
 It is also possible to represent the molecule of 
 tartaric acid by such a modification of the figures 
 given above that one-half of the molecule shall be 
 the mirror-image of the other half. One half of 
 such an arrangement would be dextrorotatory 
 and the other half laevorotatory ; the dextroro- 
 tatory part would neutralise the other, and the 
 whole molecule would be optically inactive ; but 
 this modification of tartaric acid would not be 
 resolvable into two optically active acids, because 
 splitting a molecule into parts is the same thing 
 as changing the substance into other substances 
 altogether different from it. 
 
 The hypothesis which rests on the conception 
 of the asymmetric carbon atom has been applied 
 to the glucoses, and has been found sufficient to 
 bring the reactions of this class of sugars under 
 one general principle, to elucidate the relations 
 which experiments have shown to exist between 
 these compounds, and to indicate many reactions 
 which have afterwards found experimental veri- 
 fication. 
 
 We have learned in this chapter that the influ- 
 ence of carbon atoms on the properties of the 
 
SUGARS, STARCHES, AND CELLULOSE. 141 
 
 molecules whereof they form parts are profoundly 
 modified by the relations which exist between 
 these atoms and the other atoms that, with them, 
 constitute the molecules. In all its wanderings 
 carbon preserves a certain sameness of character, 
 but it is enormously influenced by its companions. 
 A very small change in the spatial arrangement 
 of a few atoms of which carbon is one is some- 
 times accompanied by a great change in the 
 properties of the whole group of atoms. The 
 facts concerning the properties of such compounds 
 as the tartaric acids and the glucoses drive home 
 the importance of the study of the finer relations 
 between the parts of compounds. No amount of 
 examination of the elements that form tartaric 
 acid, or glucose, could in the least prepare us to 
 expect that any collocation of these elements 
 should possess the properties which characterise 
 the compounds that are formed by their union. 
 The composition of every compound is absolutely 
 unalterable ; the properties of every element, 
 taken by itself, are fixed and unchangeable ; yet 
 join together the same elements, and the same 
 quantities of the same elements, and you pro- 
 duce compounds in many cases very unlike one 
 another. As investigation advances, the problem 
 of the connexions between composition and pro- 
 perties becomes finer and more elusive, until we 
 come to bodies which are identical so far as we 
 can test them in the laboratory but produce com- 
 pletely different effects on living organisms. 
 
CHAPTER X. 
 
 BENZENE, AND SOME OF ITS ALLIED COMPOUNDS. 
 
 IN Chapter III. it was said that most of the 
 compounds of carbon belong to one or other of 
 two main classes ; those which bear a general 
 resemblance to the paraffins and are derived from 
 these hydrocarbons, and those which are derived 
 from the hydrocarbon benzene and are related to 
 that compound in their reactions. Hitherto we 
 have been concerned with pamffinoid compounds ; 
 in this chapter we shall have to deal with a few 
 benzenoid compounds. The common fats are in- 
 cluded in the first class of compounds, and all 
 the members of that class are often called fatty 
 compounds ; inasmuch as those substances which 
 give aromatic odours to certain plants belong to 
 the benzenoid class, the name aromatic compounds 
 is commonly applied to all the members of that 
 class. 
 
 A vast number of compounds is derived from 
 the hydrocarbon benzene ; in this chapter we 
 shall consider a few of these. Benzene is com- 
 posed of carbon and hydrogen united in the 
 proportion of 9 2 '3 per cent, of carbon to 7 '7 per 
 cent, of hydrogen : as the atomic weight of carbon 
 is 12, and that of hydrogen is 1, it follows that 
 there is the same number of atoms of carbon as 
 
 of hydrogen in the molecule of benzene [ 9 ^ 3 = 7 '7 ; 
 = 7'7]. The molecular weight of this compound 
 is found to be 78; hence the molecule is com- 
 
 142 
 
BENZENE, AND ITS ALLIED COMPOUNDS. 143 
 
 posed of 6 atoms of carbon united to 6 atoms 
 of hydrogen [(6 x 12) + (6 x 1) = 78], and the 
 molecular formula of the compound is C 6 H 6 . 
 No aromatic compound is known containing less 
 than 6 atoms of carbon in its molecule. 
 
 Benzene can be obtained, by a somewhat in- 
 direct process, from gum benzoin ; as benzene is 
 an oily substance, the name benzole (probably 
 abbreviated from benzoin oleum) was originally 
 given to the hydrocarbon, and this is the name 
 by which the compound is generally known in 
 commerce both in this country and abroad. 
 Benzene is manufactured from coal-tar, by re- 
 peatedly distilling the lower boiling portions 
 until a liquid boiling at about 80 C. [=176 R] 
 is obtained. The hydrocarbon is a clear, colour- 
 less, limpid liquid, somewhat lighter than water 
 bulk for bulk ; it freezes to colourless crystals 
 at a temperature a little higher than that of the 
 freezing point of water. 
 
 When benzene is mixed with concentrated 
 nitric acid, an oil smelling like oil of bitter 
 almonds is produced; this oil has the composi- 
 tion C 6 H 5 .NO 2 , and is called nitrobenzene. By 
 subjecting this compound to the action of iron 
 filings and acetic acid it is converted into aniline, 
 C 6 H 5 NH 2 , from which is obtained a series of 
 compounds called the aniline colours. Another 
 compound called phenol, or often carbolic acid, is 
 obtained from benzene by a series of reactions ; 
 this compound has the composition C 6 ELOH. 
 One of the products of the oxidation of ben- 
 zene is benzoic acid C 6 H 5 .C0 2 H; and from this 
 acid is obtained another named salicylic acid, 
 
144 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 C 6 H 4 .OH.C0 2 H. I propose to consider these 
 compounds briefly ; always with the object of 
 tracing connexions between the compositions 
 and the properties of definite kinds of matter. 
 
 There are three statements concerning the 
 reactions of benzene and its derivatives which 
 must be insisted on. (1) In almost all of its 
 reactions benzene acts as a saturated compound ; 
 that is to say, it shews very little liking for the 
 addition of other elements to itself, it is generally 
 ready to exchange hydrogen for other elements. 
 For instance, it is easy to form C 6 H 5 C1 or 
 C 6 H 4 C1 2 , C 6 H 5 .N0 2 or C 6 H 4 (N0 2 ) 2 , from C 6 H 6 ; 
 but the formation of C 6 H 6 C1 6 proceeds only 
 very slowly and under the influence of sunshine, 
 and when this compound is produced it fairly 
 readily changes to C 6 H 3 C1 3 and hydrochloric 
 acid (HC1). (2) The general result of the action 
 of reagents on the derivatives of benzene is to 
 produce some compound, or compounds, contain- 
 ing six atoms of carbon in the molecule ; such 
 compounds break down into simpler bodies only 
 by the prolonged action of energetic reagents. 
 (3) The compound C 6 H 5 C1 is called monochloro- 
 lenzene, and the compound C 6 H 4 C1 2 dichloro-ben- 
 zene ; there is only one monochloro - benzene, 
 but there are three different dichloro-benzenes. 
 There are also three dibromo-benzenes (CgH 4 Br 2 ), 
 three dinitro-benzenes [C 6 H 4 (N0 2 ) 2 ], and gener- 
 ally three isomeric compounds of the composition 
 C 6 H 4 R 2 where R is an atom, or an atomic group, 
 capable of replacing one atom of hydrogen in a 
 molecule. 
 
 How are these facts to be suggested in a 
 
BENZENE, AND ITS ALLIED COMPOUNDS. 145 
 
 structural formula for benzene 1 How are we to 
 represent the arrangement of six carbon atoms 
 and six hydrogen atoms so that the facts concern- 
 ing the reactions of the molecule C 6 H 6 may be im- 
 plicitly contained in the formula which expresses 
 the supposed arrangement 1 Nearly forty years 
 ago (in March, 1865) the/ German naturalist 
 Kekule* was living in London. He had been 
 thinking eagerly for some time about the structure 
 of the molecules of the benzene compounds. As 
 he was riding, one day, on the top of a Clapham 
 omnibus he saw in his mind's eye the arrange- 
 ment of the atoms in the molecule C 6 Hg; the 
 problem was solved. Kekule' pictured to himself 
 six atoms of carbon arranged so that each was 
 in direct union with two other atoms of carbon 
 and with one atom of hydrogen. The simplest 
 way of presenting this arrangement on a plane 
 surface, using the ordinary conventions, is per- 
 haps the following : 
 
 Kekule" preferred to think of the six carbon atoms 
 as forming an hexagonal figure ; thus 
 
 And since that time chemists have been accus- 
 tomed to speak of the hexagon-formula of benzene. 
 The six atoms of carbon are thought of as 
 forming a compact, stable nucleus, and the six 
 atoms of hydrogen as attached to this nucleus 
 K 
 
146 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 each to one of the six carbon atoms. The 
 derivatives of benzene are represented as formed 
 by removing one, or more, atoms of hydrogen 
 from the molecule C 6 H 6 , and putting other atoms, 
 and groups of atoms, in their places, the six 
 carbon nucleus remaining intact. The general 
 action of reagents on these derivatives of benzene 
 is pictured as resulting in the breaking off of 
 some, or all, of the side chains, without (in most 
 cases) decomposing the nucleus of six carbon 
 atoms. If six atoms of chlorine are added to the 
 molecule C 6 H 6 , it should not be possible to add 
 any more chlorine; because the molecule of 
 benzene hexachloride would have this structure : 
 
 and experience tells that an atom of carbon 
 cannot hold directly to itself more than four 
 other atoms of any kind in a molecule. Finally, 
 consider the structure of the isomeric dichloro- 
 benzenes which ought to exist if Kekule's hypo- 
 thesis is a sufficient translation of the facts into 
 the language of molecular structure. There may 
 be three, but not more than three, dichloro-ben- 
 zenes ; and the formulae of these three isomeric 
 compounds are : 
 
 XCI -CCl .CCI 
 
 -eAa .A- ./Y 
 
 3G .IE ^ 
 
 Y r ? <? 
 
 In the first of these structures the chlorine 
 
BENZENE, AND ITS ALLIED COMPOUNDS. 147 
 
 atoms are attached to contiguous carbon atoms ; in 
 the second, one atom of carbon comes between the 
 two carbon atoms which hold chlorine atoms 
 bound to themselves ; and in the third structure, 
 two carbon atoms intervene between the carbons 
 which are attached to atoms of chlorine. Any 
 other arrangement of C 6 H 4 C1 2 in terms of Kekule^s 
 hypothesis, would be identical with one or other 
 of these three. For, suppose the carbon atoms 
 are numbered from the top of the hexagon in the 
 direction wherein the hands of a watch move ; 
 we have this arrangement : 
 
 Then, calling the first C 6 H 4 C1 2 1:2 dichloro-ben- 
 zene, the second 1:3, and the third 1:4 dichloro- 
 benzene, inspection of the formulae will shew 
 that if the chlorine atoms were attached to 
 the carbon atoms 1 and 6, the compound so pro- 
 duced would be the same as that formed by 
 attaching the chlorine atoms to the carbon atoms 
 1 and 2, and that the position 1:5 is the same 
 as 1:3. 
 
 Kekule's formula for benzene then summarises, 
 in a special language, the chief reactions of this 
 compound and its derivatives ; and investigation 
 has shown that the formula suggested by Kekule' 
 has been most fruitful in suggesting lines of 
 advance by pursuing which much accurate know- 
 ledge has been gained not only concerning ben- 
 zene compounds, but also concerning the main 
 
148 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 problem of Chemistry, which, as has been said so 
 often, is to elucidate the connexions between 
 composition and properties. 
 
 It should be noted that the hexagon-formula 
 does not pay any heed to the arrangement of the 
 atoms in three dimensions in space. The formula, 
 which is a sentence in a language, has proved 
 sufficiently elastic to include many facts dis- 
 covered since it was constructed; and when it 
 was found necessary to adopt an expression for 
 the structure of certain benzene compounds, 
 which includes in itself the conception of the 
 tri-dimensional arrangement of atoms, Kekule"'s 
 formula was not abandoned, but only modified. 
 Even a slight study of the meaning and purpose 
 of the benzene hexagon-formula will enable the 
 reader to realise to some extent what can be done 
 by the use of hypotheses in natural science, and 
 how absolutely necessary it is to frame and con- 
 stantly employ hypotheses if advance is to be 
 made in the accurate investigation of natural 
 occurrences. Incidentally, it may be added that 
 Kekul6's ride on the Clapham 'bus has been 
 worth millions of pounds to chemical manu- 
 facturers. 
 
 The relations between benzene, phenol, nitro- 
 benzene, aniline, benzoic acid, and salicylic acid 
 are suggested by the following structural for- 
 mulae : 
 
 .C.CO.H CCO.H 
 
 Benzent. Phenol . Nitro Anil- Benzole Salicylic 
 
BENZENE, AND ITS ALLIED COMPOUNDS. 149 
 
 Let us endeavour to read something of the 
 meanings of one or two of these formulae. 
 
 So far as composition is concerned, the relation 
 of phenol to benzene is similar to that of 
 methylic alcohol to methane, and to that of 
 ethylic alcohol to ethane. Methane and methylic 
 alcohol were briefly examined in Chap. V. (pp. 65- 
 69), and ethane and ethylic alcohol in Chap. 
 VI. (pp. 83-85). The substitution of the atomic 
 group OH for one of the hydrogen atoms in the 
 molecule of either of the hydrocarbons CH 4 or 
 C 2 Hg is accompanied by a change from the pro- 
 perties of a hydrocarbon to those of an alcohol. 
 Benzene is a hydrocarbon, and the change of com- 
 position that occurs when benzene becomes 
 phenol consists in the substitution of the atomic 
 group OH for one of the hydrogen atoms in the 
 molecule of benzene; we might then expect 
 phenol to be an alcohol. Experiments shew that 
 some of the reactions of phenol are the reactions 
 of an alcohol, but that other reactions are those 
 of an acid. The compositions of methylic 
 alcohol, ethylic alcohol, and phenol are all ex- 
 pressed by the formula B.OH ; in the first com- 
 pound R = CH 3 , in the second E C 2 H 6 , and in 
 the third K = C 6 H 6 : the fact that the third com- 
 pound is an acid, as well as an alcohol in some 
 reactions, must be connected with the composi- 
 tion of the group of atoms C 6 H 6 . We express 
 our conception of the structures of the molecules 
 CH 8 .OH and C 2 H 5 .OH by the formulae 
 
 H-C-O-H and H 8 = C'C-O'H 
 Ha Ha 
 
150 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 The arrangement of the atoms of carbon and 
 hydrogen (leaving out of sight the group OH) in 
 the molecules is thought of as very different 
 from that of the atoms of carbon and hydrogen 
 in the molecule C 6 H 6 .OH. That the difference 
 between the properties of phenol and those of 
 ethylic alcohol is to be connected with the 
 arrangement of the atoms of carbon and hydro- 
 gen, other than those of the group OH, rather 
 than with the numbers of these atoms, is made 
 clear by the fact that the sixth hydrocarbon of 
 the paraffin series, C 6 H 14 , gives a derivative 
 C 6 H 13 .OH which is a true alcohol and shews no 
 acidic reactions. In phenol we have another 
 example of the proposition that the functions of 
 this or that atom in a molecule are dependent, 
 among other conditions, on the arrangement of 
 all the atoms which form the molecule. 
 
 It is because of its distinctly acidic character 
 that phenol is so often called carbolic acid. 
 
 The production of nitrobenzene (C 6 H 5 .N0 2 ) 
 from benzene, and of aniline (C 6 H 5 NH 2 ) from 
 nitrobenzene, will be glanced at in the next 
 chapter. 
 
 An examination of the formulae given to ben- 
 zene and salicylic acid shews that to pass from 
 the first of these compounds to the second it is 
 necessary to remove two atoms of hydrogen from 
 the molecule of benzene and to substitute there- 
 for the atomic groups OH and CO. OH. This 
 process is effected thus : in the first place phenol 
 is made from benzene; then sodium phenylate 
 (CLHg.ONa) is prepared; then this substance 
 (which is a solid) is heated in a stream of carbon 
 
BENZENE, AND ITS ALLIED COMPOUNDS. 151 
 
 dioxide to a temperature of about 130 C. 
 [ = 266 F.l, when the following reaction occurs : 
 
 C 6 H 6 .ONa + C0 2 -C 6 H 4 .OH.C0 2 Na. ; 
 lastly the sodium salicylate thus obtained is 
 decomposed by the proper quantity of a solution 
 of hydrochloric acid, whereby a solution of 
 sodium chloride and crystals of salicylic acid are 
 produced. This is the method by which salicylic 
 acid is manufactured. A salt of salicylic acid is 
 contained in oil of winter-green, and what is 
 called 'natural salicylic acid* is obtained from 
 this salt. 
 
 From time to time differences have been 
 noticed in the actions of natural and artificial 
 salicylic acid when administered medicinally. 
 There ought to be no differences ; for the com- 
 positions of the two things are identical. Now 
 if the reader will consider the following struc- 
 tural formulae he will see that the hexagon 
 formula for benzene provides for the exist- 
 ence of three isomeric acids of the composition 
 C 6 H 4 .OH.C0 2 H : 
 
 These acids must have different reactions, and 
 almost certainly different therapeutic effects. 
 The reactions of both natural and artificial 
 salicylic acid shew that the first formula is the 
 formula for that acid. Careful experiments have 
 shewn that when sodium phenylate is heated in 
 
152 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 carbon dioxide to about 200 0. [ = about 390 F.] 
 a certain amount of the acid represented by the 
 third formula is produced. Hence unless the 
 temperature is carefully regulated in the pre- 
 paration of salicylic acid, the product is a mixture 
 of this acid with one of its isomerides. Deter- 
 minations of the percentage of each element in 
 the product could not detect the presence of the 
 isomeric acid ; and therefore it would be easy to 
 pass as pure salicylic acid a substance which was 
 really a mixture of that acid with another com- 
 pound. 
 
 In this chapter we have again had examples of 
 the very great influence which is exerted on the 
 functions performed by carbon in the compounds 
 whereof it forms a part by the relations between 
 it and the other elements wherewith it may be 
 united. There is no element which is so much 
 affected by its companions as carbon ; in all the 
 wanderings of the atoms of carbon, it matters 
 much what these companions are, how many of 
 this or that kind there are, and whether they 
 crowd round the carbon atoms or stand apart 
 from them, whether they surround the carbon 
 atoms in solid phalanxes or draw themselves out 
 in scattered chains. 
 
CHAPTER XL 
 
 CF>RTAIN CHEMICAL INDUSTRIES DEALING WITH 
 SUGARS, CELLULOSE, AND BENZENE COM- 
 POUNDS. 
 
 THE chief changes of composition that occur in 
 the preparation of beer, of whiskey, and of wines 
 are changes that begin with sugars and end with 
 alcohols and ethereal salts. 
 
 The main stages in beer-making are, the con- 
 version of barley into malt, the extraction by 
 water of the soluble constituents of malt, fermen- 
 tation, and clearing. Barley is moistened and 
 allowed to germinate slightly ; during this pro- 
 cess a substance is formed, called diastase, which, 
 acting in the presence of water, converts the 
 starch of the barley into dextrin, and that variety 
 of glucose called maltose, and also slightly alters 
 the compositions of a group of nitrogen-con- 
 taining compounds in the barley so that the 
 products dissolve in water. The composition of 
 dextrin is the same as that of starch (G 6 H 10 6 ) ; 
 but, unlike starch, dextrin easily dissolves in 
 water. As maltose belongs to the group of the 
 glucoses, its composition is expressed by the 
 formula C 6 H 12 Og. The germination is stopped 
 by heating the barley in a kiln ; the product is 
 called malt. The malt is crushed, and treated 
 with water at about 60 to 70 C. [= 140 to 
 158 F.]; the liquor obtained by this process of 
 mashing is called wwt. The wort contains 
 glucose, dextrin, soluble nitrogenous compounds, 
 
 153 
 
154 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 and some mineral salts. Experience shows that 
 the changes from starch and insoluble nitro- 
 genous compounds to dextrin, glucose, and soluble 
 nitrogen-containing substances, proceed most 
 favourably at the temperature of 64 to 67 C. 
 [= 147 to 152 F.]. The wort is then boiled 
 with hops ; certain substances which give a 
 peculiar flavour to the liquid are extracted from 
 the hops by the boiling wort. The liquid is 
 now rapidly cooled ; if this is not done changes 
 occur in the constituents of the wort, and various 
 acids are formed that render the beer undrink- 
 able. During the cooling, air finds its way into 
 the wort, and the oxygen in this air aids the 
 process of fermentation to which the wort is now 
 subjected. The fermentation that is, the con- 
 version of glucose into alcohol and carbonic acid 
 gas is accomplished by adding yeast to the wort, 
 and maintaining a suitable temperature. In 
 English breweries fermentation is conducted at 
 about 15-5 to 21 C. [= 60 to 70 F.]; in 
 German breweries, at about 12 to 15 C., or 
 sometimes at temperatures as low as 6 to 8 C. 
 [53 to 59 F., or 43 to 46'5 F.]. When fer- 
 mentation occurs at 60 to 70 F., carbonic acid 
 gas escapes rapidly from the liquor, and particles 
 of yeast are carried towards the top of the fer- 
 menting wort ; when the process is conducted at 
 43 to 53 F. the carbonic acid gas forms slowly, 
 and the yeast sinks to the bottom of the liquor. 
 The two processes are called high fermentation 
 and low fermentation, respectively. As has been 
 said, high fermentation is practised in England, 
 and low fermentation in Germany. The fer- 
 
INDUSTRIES DEALING WITH SUGARS, ETC. 155 
 
 mented liquor is run off into cleansing tanks, 
 where various insoluble substances gradually 
 settle, and the yeast is skimmed off from the 
 surface ; after a second process of settling and 
 cleansing, the beer is run into barrels, and these 
 are sent into the market. 
 
 The following figures present the quantities 
 of the chief constituents in different kinds of 
 beer : 
 
 Burton Scotch Lager DM4M . Bavarian 
 
 Ale. Ale. Beer. forter - draught-beer 
 
 Per cent, of Water . . 79'6 81'5 90'8 86-3 90'28 
 
 Alcohol . . 5-9 8-5 3'7 6'9 8'8 
 
 Carbonic 
 
 acid . . .. 0-15 0'22 0'16 014 
 
 Sugar*. . A 
 Dextrin . . 
 
 Ash. . . .[- 14-5 9-85 5'28 6'64 6'8 
 
 Nitrogenous I 
 compounds./ 
 
 * The quantity of sugar in beer rarely amounts to more than a half 
 per cent. ; porter may contain from 1*5 to 2 per cent. 
 
 Unless beer is made with great care the finished 
 product is subject to various diseases. These 
 diseases consist in the production of compounds 
 that give a disagreeable taste and odour to the 
 beer. The chief causes of these obnoxious changes 
 are impurities in the yeast employed for effecting 
 the fermentation. Yeast is the name given to a 
 group of very simple plants of a low order ; these 
 plants grow in wort, feeding on some of the con- 
 stituents of the wort, and producing, during their 
 growth, alcohol, carbonic acid, and traces of other 
 compounds. Among the constituents of ordinary 
 yeast are certain minute funguses, collectively 
 known as wild yeast, which change alcohol into 
 acetic acid, and glucose into lactic acid and butyric 
 acid, and effect other transmutations, the pro- 
 
156 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 ducts of which are more or less objectionable to 
 the beer-drinker. The cure for the disease of 
 beer is, therefore, the very careful selection of 
 yeast free from those wild varieties which bring 
 about the undesirable chemical changes. Un- 
 fortunately, it is impossible to detect wild yeast 
 in samples of the fungus microscopically ; but if 
 the samples are grown under very definite con- 
 ditions, and a microscopic examination is made 
 periodically of the growing organisms, it is 
 possible to distinguish the wild yeasts from those 
 which produce only alcoholic fermentation, for 
 the manner of growth is different in the different 
 kinds of yeast. 
 
 Whiskey is, or ought to be, made from malt. 
 The object of the distiller is to convert the whole 
 of the starch of the barley into glucose, and the 
 whole of the glucose into alcohol. The wort is 
 subjected to a brisk fermentation under condi- 
 tions which ensure the transformation of the 
 whole of the sugar into alcohol; and sugar is 
 sometimes added to the wort, so that the fer- 
 mented liquor may be richer in alcohol than it 
 would be if only what may be called the natural 
 sugar of the wort were acted on by the yeast. 
 The fermentation of wort for making whiskey 
 should be conducted at as low a temperature as 
 possible, else fusel oil is produced. This name is 
 given to a group of alcohols, the chief of which is 
 amylic alcohol (the fifth alcohol of the ethylic 
 series = C 5 H n .OH); these alcohols are very intoxi- 
 cating and distinctly poisonous. As whiskey 
 matures the fusel oil (some of which is present in 
 all newly-made whiskey) slowly reacts with the 
 
INDUSTRIES DEALING WITH SUGARS, ETC. 157 
 
 small quantities of acids that fermented liquors 
 always contain, and forms ethereal salts (compare 
 Chap. VI., p. 93); and the bouquet of well- 
 matured whiskey is chiefly caused by these 
 ethereal salts. The fermented wort is distilled, 
 and the distillate is distilled again; a liquid is 
 thus obtained containing from 61 to 77 per cent, 
 of alcohol ; this is generally diluted so as to con- 
 tain about 55 per cent, of alcohol, and bonded at 
 this strength. Irish whiskey is usually bonded at 
 about 64 per cent, alcoholic strength. 
 
 The strength of whiskey, brandy, rum, and 
 other spirits, is very generally stated as so much 
 ' overproof,' or so much ' underproof ' ; for 
 instance, rum is generally imported at the 
 strength of twenty overproof. In former times 
 the strength of spirit, that is, the quantity of 
 alcohol in the spirit, used to be roughly tested 
 by pouring the spirit on to gunpowder, and 
 igniting the vapour; if the burning vapour set 
 fire to the powder the spirit was said to be over- 
 proof; if the powder remained unfired the spirit 
 was said to be underproof. Hence arose the name 
 proof spirit ; spirit of such alcoholic strength that 
 it would just fire gunpowder when it was poured 
 on the powder and the vapour was ignited. As 
 more accurate methods of estimating alcohol in 
 liquids were worked out, it was found that proof 
 spirit contained approximately 50 per cent, of 
 alcohol and 50 per cent, of water. Proof spirit is 
 now defined as a mixture of 49*24 parts by weight 
 of alcohol with 50 '7 6 parts by weight of water. 
 A sample of whiskey containing 60 per cent, of 
 alcohol, by weight, contains approximately 10 per 
 
158 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 cent, more alcohol than proof spirit ; as one part 
 of alcohol is approximately equal to two parts of 
 proof spirit, that whiskey would be described as 
 twenty overproof. 
 
 Methylated spirit consists of nine parts rectified 
 spirits of wine (containing about 84 or 85 per 
 cent, alcohol) mixed with one part methylic 
 alcohol (for a brief account of methylic alcohol, 
 or wood spirit, see Chapter V., p. 80). The 
 Inland Eevenue authorities have allowed this 
 mixture to be bought and sold duty free, under 
 certain definite restrictions, since 1855. The 
 small quantity of methylic alcohol interferes very 
 slightly with the purposes for which methylated 
 spirit is used (for dissolving resins, for burning 
 in lamps, and in certain manufactures), it would 
 be extremely difficult to separate the ethylic 
 alcohol from the wood spirit in the mixture, and 
 the presence of the wood spirit is supposed to 
 make the liquid undrinkable. Of late years the 
 Board of Inland Revenue have added a small 
 quantity of oil to methylated spirit ; if the spirit 
 is poured into water it forms a milky liquid with 
 a nasty smell. 
 
 Wine is the fermented juice of grapes. The 
 fermentation is generally conducted in open vats, 
 in summer, at about 10 to 12 or 14 C. [ = 50 
 to 57 F.], and the process occupies from 10 
 to 14 days. The air always contains sufficient 
 yeast-cells to bring about the change of the 
 sugar of the juice into alcohol and carbonic 
 acid. 
 
 The carbonic acid escapes ; the liquid is racked 
 off into casks where it stands for some time, and 
 
INDUSTRIES DEALING WITH SUGARS, ETC. 159 
 
 it is then transferred to other casks where it 
 matures. Many chemical changes occur as wine 
 matures ; there is a slow transformation of sugar 
 into alcohol, various nitrogenous compounds and 
 certain salts (especially tartrate of potassium) are 
 precipitated, traces of acids are formed and react 
 with the alcohols present (for fermented grape- 
 juice always contains several alcohols besides 
 ordinary, or ethylic, alcohol) to produce ethereal 
 salts to the presence of which the bouquet of wine 
 is chiefly due ; and a great many other changes 
 of composition, which have not been fully eluci- 
 dated, take place. 
 
 Wine is a very complicated liquid ; it contains 
 ethylic alcohol and traces of other alcohols, one 
 of which is glycerin ; acids, especially malic, 
 tannic, tartaric acids, and succinic acid; very 
 small quantities of different ethereal salts ; nitro- 
 genous compounds ; colouring matters ; and in- 
 organic salts, especially phosphates and chlorides- 
 of potash, soda, lime, and magnesia. The 
 quantity of alcohol in natural wines (that is, 
 wines to which alcohol has not been added after 
 fermentation, nor sugar beyond that contained in 
 the grape-juice before fermentation) varies from, 
 about 6 to about 12 per cent.; the amount of 
 acid is from 0*3 to 0'7 per cent.; the ethereal 
 salts amount to a few hundredths of a per cent, 
 of the wine ; most of the natural wines of France 
 and the Khine are practically free from sugar, 
 sherry contains about 2 per cent., port about 4 
 to 5 per cent., and in such sweet wines as Tokay 
 the sugar may amount to 25 per cent.; there may 
 be about T V n s of a per cent, of mineral salts ; 
 
160 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 and from 2 to 3 per cent, of extractive matter, 
 including nitrogenous compounds. 
 
 Champagne is bottled before the fermentation 
 is finished ; the carbonic acid which is produced 
 as fermentation proceeds in the bottled wine, can- 
 not escape, and dissolves in the liquid ; when the 
 cork is withdrawn the carbonic acid passes into 
 the air and makes the wine effervesce (compare 
 the remarks on soda water in Chapter IV. p. 51). 
 A great deal of liquid is manufactured to be sold 
 as champagne, by adding sugar to inferior white 
 wine, and pumping in carbonic acid gas under 
 pressure. 
 
 We must now briefly consider the changes that 
 are turned to account in the manufacture of 
 various explosives from cellulose. As we learned 
 in the last chapter, cellulose is the main con- 
 stituent of all vegetable tissues, and it has the 
 same composition as starch. The simplest 
 formula that expresses the composition of this 
 compound is C 6 H 10 6 , but it is almost certain 
 that the molecule of cellulose contains a fairly 
 large multiple of six atoms of carbon, ten atoms of 
 hydrogen, and five atoms of oxygen. 
 
 If cotton wool, which is approximately pure 
 cellulose, is steeped in a mixture of concentrated 
 nitric and sulphuric acids, a change occurs which 
 may be expressed approximately by the follow- 
 ing equation : 
 
 C 6 H 10 6 + 3HN0 8 + H 2 SO, = C 6 H r (N0 2 ) 3 6 + 
 3H 2 O.H 2 S0 4 
 
 Three atoms of hydrogen in the cellulose mole- 
 
INDUSTRIES DEALING WITH SUGARS, ETC. 161 
 
 cule are replaced by three atomic groups, each 
 of which consists of an atom of nitrogen joined 
 to a couple of atoms of oxygen ; the hydrogen 
 taken out of the cellulose molecule combines with 
 the rest of the oxygen of the nitric acid, and 
 the water that is so produced is seized by the 
 sulphuric acid wherewith it combines. The 
 nitrated product of this reaction [C 6 H 7 (N0 2 ) 3 5 ] 
 is called trinitrocellulose ; for all technical pur- 
 poses it is known as gun-cotton. 
 
 When gun-cotton is dried and ignited in the 
 air it burns rapidly and is entirely changed into 
 gases ; when it is fired in a closed space, although 
 it cannot obtain oxygen from the air, it contains 
 in itself sufficient oxygen to convert the whole of 
 the carbon in it to carbon monoxide and carbon 
 dioxide and a considerable quantity of the 
 hydrogen to steam. The other products of com- 
 bustion under these conditions are hydrogen and 
 nitrogen ; so that even in the absence of oxygen 
 outside itself, gun-cotton is wholly changed to 
 gaseous substances when it is burnt, and the pro- 
 cess of burning takes place rapidly. The volume 
 of the gases produced by exploding gun-cotton, 
 after allowing the steam to condense, is about 
 750 times the volume occupied by the material 
 before the explosion. If gun-cotton is very 
 strongly compressed while wet a compact solid 
 is produced containing about 15 per cent, of 
 water ; this substance is quite uninflammable, 
 and may be handled and moved about with per- 
 fect safety. Dry gun-cotton is easily exploded 
 by firing a small quantity of a detonator in 
 contact with it, such as fulminate of mercury 
 
 L 
 
162 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 contained in a thin iron metal case, and the explo- 
 sion spreads through the mass of gun-cotton 
 with enormous rapidity. Moist, compressed, gun- 
 cotton containing 15 per cent, of water cannot 
 be exploded by a detonator unless a very large 
 charge is employed ; but if a small quantity of 
 dry gun-cotton is brought into contact with the 
 moist sample, and the dry substance is exploded 
 by a detonator, the explosion spreads through 
 the whole mass of the moist gun-cotton which is 
 thereby completely and rapidly changed to 
 gaseous substances. 
 
 It is evident then that gun-cotton possesses 
 many of the properties of an ideal explosive ; it 
 is not very difficult to manufacture ; under con- 
 ditions which are easily attained it may be 
 handled, carried, and stored, with complete 
 safety ; it can be exploded easily ; the explosion 
 spreads rapidly throughout the whole of the 
 material, and produces an enormous volume of 
 gaseous substances without the smallest quantity 
 of any solid matter. Gun-cotton is the principal 
 explosive used in the army and navy for effecting 
 demolitions on land, for submarine mines, and 
 for torpedoes. 
 
 It may be well to mention here another ex- 
 plosive, nitroglycerin, which is allied to gun- 
 cotton chemically. This compound is made by 
 treating glycerin [C 3 H 5 (OH) 3 ] with concen- 
 trated nitric and sulphuric acids ; the change 
 may be expressed, approximately, thus : 
 
 C 3 H 5 (OH) 3 + 3HN0 3 + HjS0 4 = C 3 H 5 (ON0 2 ) 3 
 3 H 2 O.H 2 S0 4 . 
 
INDUSTRIES DEALING WITH SUGARS, ETC. 163 
 
 Nitroglycerin is a highly explosive oil, danger- 
 ous to handle and to carry. Dynamite consists 
 of a very fine naturally occurring silica soaked in 
 nitroglycerin ; the silica dilutes the nitrogly- 
 cerin, so that the mixture can be safely handled 
 and transported, and the solid dynamite is more 
 convenient than the liquid nitroglycerin. Other 
 diluents of nitroglycerin are employed, and 
 most of the smokeless powders that are used at 
 present are mixtures of nitroglycerin, or nitro- 
 glycerin and gun-cotton, with diluting materials 
 which render the substances safe and convenient 
 to handle, and also moderate the velocity of the 
 explosion that occurs when the powder is fired 
 by a detonator. 
 
 Let us now turn for a moment to some of those 
 changes of composition and properties occurring 
 among derivatives of benzene which are the bases 
 of chemical industries. I will ask the reader's 
 attention more particularly to the manufacture 
 of aniline and aniline-colours. 
 
 Aniline, C 6 H 5 .NH 2 , is made from nitrobenzene ; 
 and nitrobenzene, C 6 H 5 .N0 2 , is made from ben- 
 zene, which is one of the substances present in 
 coal-tar. Hence the name coal-tar colours often 
 applied to the colouring matters derived from 
 aniline. Benzene is treated with a mixture of con- 
 centrated nitric and sulphuric acids, and the oily 
 nitrobenzene so produced is thoroughly washed 
 with water, and run into a still containing 
 iron (in the form of scrapings from soft cast- 
 ings), and either acetic or hydrochloric acid, 
 steam being blown into the still as the nitro- 
 
164 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 benzene runs in. A reaction begins at once; 
 water, aniline, and unchanged nitrobenzene distil 
 over ; more iron is added to the contents of the 
 still, and after some time the distillate consists 
 of only aniline and water. The aniline, which 
 slowly separates from the water, is drawn off 
 and redistilled. The principal reactions which 
 occur in the manufacture of aniline from benzene 
 may be expressed thus : 
 
 (i.) CLH 6 + HN0 3 + H 2 S0 4 = C 6 H 5 .N0 2 + 
 
 H 2 O.H 2 S0 4 . 
 (ii.) C 6 H 5 .N0 2 -t- 6 H (produced by the reaction 
 
 of iron and acid) = C 6 H 5 .NH 2 + 2H 2 0. 
 
 From aniline, which is a colourless oil, has 
 been produced a vast number of derivatives and 
 related compounds that are brilliantly coloured. 
 For instance, the oxidation of commercial aniline, 
 containing the compound C^H^NH^ (called tolui- 
 dine), produces magenta-red or fuchsme. The con- 
 stitutions of most of the aniline colours are very 
 complicated. Some of them are substituted anil- 
 ines, that is, compounds derived from C 6 H 5 .NH 2 , 
 by removing atoms of hydrogen and putting 
 different more or less complex atomic groups in 
 their place ; for instance, a yellow colouring matter 
 is produced by treating aniline with the gas formed 
 by warming nitric acid with starch ; this yellow 
 body is derived from aniline by removing an atom 
 of hydrogen from the molecule, C 6 H 6 .NH 2 , and 
 putting in its place the group of atoms C 6 H 5 .N 2 . 
 This yellow substance then becomes the starting 
 point from which a series of coloured compounds 
 is derived. 
 
ALIZARIN AND INDIGO. 165 
 
 It is impossible to go into the constitution of 
 the aniline colours without possessing a very 
 minute acquaintance with the facts of organic 
 chemistry ; it must suffice to say here that the 
 whole of the aniline-colour industry is a direct out- 
 come of conceptions regarding the arrangements of 
 atoms in molecules, and the influences exerted 
 by the compositions and the relative positions of 
 various atomic groups on the properties of the 
 molecules into which they enter. 
 
 CHAPTER XII. 
 
 ALIZARIN AND INDIGO. 
 
 WE are told that, in times of war and when certain 
 religious ceremonies were being performed, our 
 ancestors used to dye their skins blue with the 
 juice of a plant called woad ; to-day we manu- 
 facture the blue dye-stuff of the woad in our 
 laboratories. For centuries large tracts of country 
 in the more southern parts of Europe were de- 
 voted to the growth of the plant whose roots were 
 used, under the name madder, for dyeing cloth 
 various shades of red : I do not suppose that there 
 is to-day more than a few acres of land in Europe 
 where this plant is cultivated, for one laboratory 
 now produces more of the dyeing compound of 
 madder, in a month, than was obtained in a year 
 from the plants grown on a thousand acres. 
 
 The compound which gives its dyeing pro- 
 perties to woad is the same as that we call 
 
166 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 indigo; the compound contained in madder is 
 named alizarin. The name indigo is applied by 
 Pliny to a pigment which came from India. 
 The name alizarin is formed from the word 
 alizari, the commercial name of madder in the 
 Levant ; this word is said to be derived from the 
 Arabic azara, to press or squeeze, so that alizari 
 would mean the pressed extract. 
 
 The composition of alizarin is expressed by the 
 formula C U H 8 4 . When the vapour of this com- 
 pound is passed over heated zinc dust a hydro- 
 carbon is obtained, which has the composition 
 C 14 H 10 , and is called anthracene. This hydrocarbon 
 is one of the many constituents of coal-tar ; as 
 coal-tar is produced in enormous quantities as a 
 bye-product in making gas, there is here an 
 almost unlimited source of alizarin, could a 
 workable method be found for reversing the 
 change that occurs by heating alizarin with zinc 
 dust, and so producing alizarin from anthracene. 
 
 It was known, in the early sixties, that certain 
 aromatic hydrocarbons were converted into com- 
 pounds called quinones by the action of such 
 oxidising agents as a mixture of sulphuric acid 
 and bichromate of potash; in every case, the 
 molecule of the quinone contained two atoms of 
 oxygen in place of two atoms of hydrogen 
 removed from the molecule of the hydrocarbon. 
 It was also known that, in many cases, the 
 hydrocarbon from which the quinone was ob- 
 tained was produced by heating the quinone 
 with zinc dust. In 1862 a compound was ob- 
 tained by oxidising the hydrocarbon anthracene ; 
 the composition of anthracene is C 14 H 10 ,.and the 
 
ALIZARIN AND INDIGO. 167 
 
 composition of this product of its oxidation is 
 C 14 H 8 2 . The two chemists Graebe and Lieber- 
 mann were busy in these days studying the 
 relations of alizarin to other compounds. They 
 made the guess that the compound C 14 H 8 2 was 
 the quinone of anthracene ; and that alizarin 
 (C 14 H 8 4 ) was a derivative of anthraquinone re- 
 lated to that compound in the manner expressed 
 by the formulae 
 
 C 14 H 8 2 anthraquinone, 
 O 14 H 6 (OH) 2 2 alizarin. 
 
 The facts then were these : anthracene, C 14 H 10 , 
 could be oxidised without much difficulty to the 
 compound C 14 H 8 2 ; alizarin had the composition 
 C 14 H 8 4 , and could be converted into anthracene 
 by heating with zinc dust; several aromatic 
 hydrocarbons could be oxidised to compounds 
 containing the same number of atoms of carbon 
 as the parent hydrocarbon, but two atoms of 
 hydrogen less than that compound, and contain- 
 ing also a couple of atoms of oxygen; these 
 products of oxidation of aromatic hydrocarbons 
 were named quinones. Graebe and Liebermann 
 assumed the compound C 14 H 8 2 to be the 
 quinone of anthracene, and alizarin to be the 
 dihydroxyl derivative of this quinone (the atomic 
 group OH is call hydroxyl). One could begin 
 with anthracene and make what was probably its 
 quinone ; one could start from alizarin and arrive 
 at anthracene ; there was one step untaken, that 
 from the supposed quinone to alizarin. Experi- 
 ments on the reactions of alizarin shewed that 
 two of the eight atoms of hydrogen in the 
 
168 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 molecule of this compound were closely held to 
 two of the four atoms of oxygen, and that the 
 remaining six hydrogen atoms, and the remaining 
 two oxygen atoms, were not directly joined to 
 one another. (This is of course a statement of 
 the results of experiments in the extremely 
 symbolic language of the hypothesis of atom- 
 linking.) In other words, one part of Graebe 
 and Liebermann's hypothesis was justified. But 
 how were two atoms of hydrogen to be removed 
 from the molecule C 14 H 8 2 and two atomic 
 groups OH to be put in their place? Many 
 aromatic compounds were known to exchange 
 atoms of hydrogen for an equal number of atoms 
 of bromine when treated with bromine ; and it was 
 also known that such bromo-derivatives frequently 
 exchanged their bromine atoms for an equal 
 number of OH groups when fused with caustic 
 potash. Graebe and Liebermann, therefore, 
 treated the supposed anthraquinone (C 14 H 8 2 ) 
 with the quantity of bromine calculated for the 
 equation 
 
 C 14 H 8 2 + 4Br = C 14 H 6 Br 2 2 + 2HBr. 
 
 The product had the composition C 14 H 6 Br 2 2 . 
 They then fused this compound with caustic 
 potash, expecting the reaction 
 
 C 14 H 6 Br 2 2 + 2KOH = C 14 H 6 (OH) 2 2 + 2KBr 
 
 to occur. The product had the composition 
 C 14 H 8 4 , and the properties of alizarin. 
 
 The preparation of alizarin from anthracene 
 was completed, and alizarin was proved to be 
 dihydroxyl-anthraquinone. 
 
 But this method of making alizarin could not 
 
ALIZARIN AND INDIGO. 169 
 
 be commercially successful, because bromine is 
 very expensive, and very troublesome to mani- 
 pulate. About this time (1866-68) Perkin, and 
 independently of him Graebe and Leibermann 
 also, applied to anthraquinone another reaction 
 whereby atoms of hydrogen can be removed 
 from the molecules of many aromatic compounds 
 and their place taken by the atomic group OH. 
 When benzene is heated with concentrated sul- 
 phuric acid a compound is produced which has 
 the composition C 6 H 6 .HS0 3 and is called benzene 
 sulphonic acid. 
 
 (C 6 H 6 + H 2 S0 4 = C 6 H 5 .HS0 3 + H 2 0) j 
 
 and when this acid is fused with caustic potash, 
 phenol (C 6 H 5 .OH) is formed. 
 
 Anthraquinone was heated with concentrated 
 sulphuric acid for a little time, but no reaction 
 occurred ; very concentrated acid was used, and 
 the heating was continued for a long time; at 
 last a reaction began, and a compound was ob- 
 tained whose composition and reactions shewed 
 it to be anthraquinone disulphonic acid. The 
 change may be expressed thus : 
 
 C U H 8 2 + 2H 2 S0 4 = C 14 H 6 (HS0 3 ) 2 2 + 2H 2 0. 
 
 This acid (which was a solid body) was then 
 fused with caustic potash ; the product was 
 alizarin : 
 
 C 14 H 6 (HS0 3 ) 2 2 + 2KOH = C 14 H 6 (OH) 2 + 
 2KHS0 3 . 
 
 The results of many investigations into the re- 
 
1 70 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 actions and relations of alizarin find expression 
 in the structural formula 
 
 The hydrogen and oxygen atoms, and the two 
 atomic groups OH, are thought of as attached to 
 three benzene nuclei, with two carbon atoms 
 belonging to both the first and the second 
 nucleus, and two common to the second and the 
 third. But it is not possible to analyse this 
 formula and elucidate its meaning without a 
 more thorough knowledge of organic chemistry 
 than is to be looked for on the part of readers of 
 such a book as this. There are nine possible 
 isomerides of alizarin; but alizarin is the only 
 one which can be used as a colouring material. 
 It is a somewhat remarkable coincidence that the 
 result of the first attempt to prepare dihydroxyl 
 anthraquinone artificially should have been the 
 formation of that one isomeride which was of any 
 value as a dye-stuff. 
 
 Thirty years ago, about half a million tons of 
 madder were sent into the market annually, and 
 about half of that was grown in France. The 
 amount of madder exported from France ten 
 years ago was a few hundred tons. The dis- 
 covery of a cheap method for making alizarin, 
 taken with the discovery of the aniline colours, 
 entirely revolutionised the trade in dyeing 
 materials. In his report on the Exhibition of 
 1862, Hofmann wrote: 
 
ALIZARIN AND INDIGO. 171 
 
 "England will, beyond question, at no distant day, 
 become herself the greatest colour-producing country in 
 the world ; nay, by the strangest of revolutions, she may, 
 ere long, send her coal-derived blues to indigo-growing 
 India, her tar-distilled crimsons to cochineal- producing 
 Mexico, and her fossil substitutes for quercitron and 
 safflower to China, Japan, and the other countries whence 
 these articles are now derived." 
 
 Hofmann was both right and wrong. Coal- 
 derived blues and tar-distilled colours of every 
 shade are sent all over the world ; but they are 
 not sent by England. Germany is the great 
 colour-producing country of the world to-day. 
 The artificial production of dyeing compounds is 
 a triumph of the systematic study of the order 
 which reigns in the domain of the almost in- 
 finitely minute; but regarded from the artistic 
 standpoint that triumph has been a calamity to 
 mankind. 
 
 Indigo, a substance that has been used as a 
 pigment, and also for dyeing, since very early 
 times, is obtained from many plants most of 
 which nourish in tropical regions. Indigo is 
 imported chiefly from India, where it is manu- 
 factured from the plants of indigofera tinctoria, a 
 perennial that is cultivated as an annual. The 
 plants are steeped in water in large vats ; fer- 
 mentation proceeds for several hours ; the yellow 
 liquid is run off into other vats wherein it is 
 agitated by paddles until the colour changes to 
 dark blue and a blue sediment is deposited ; this 
 sediment is thoroughly washed with boiling 
 water, drained on canvas filters, pressed in 
 shallow wooden frames, and cut into cubes which 
 
172 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 are dried in open-air sheds. The compound 
 which gives its colour to indigo can be separated 
 by a series of operations ; it is a dark-blue crystal- 
 line powder with a bronzy lustre ; the compound 
 is called indigotin, and its composition and mole- 
 cular weight are expressed by the formula 
 C 16 H 10 N 2 2 . 
 
 When indigotin is subjected to the action of 
 reagents that remove oxygen from compounds, 
 an alkali also being present, it combines with 
 two atoms of hydrogen and forms an almost 
 white compound called indigo-white, C 16 H 12 N 2 2 , 
 which dissolves in the alkali that is present. On 
 exposing this yellowish liquid to the air, oxygen 
 is slowly absorbed and the blue compound, 
 indigotin, is reproduced as a solid. It is on 
 these reactions that the dyeing of silk, wool, and 
 cotton, with indigo is based. An indigo-vat is 
 prepared by mixing indigo with water, an alkali 
 (commonly slaked lime), and some deoxidiser 
 such as green vitriol (sulphate of iron), or zinc 
 powder, or hyposulphite of soda. The indigotin 
 is reduced to indigo-white and this goes into 
 solution ; the goods are steeped in the nearly 
 colourless liquid, and then exposed to the air; 
 indigotin is slowly formed, by the action of 
 atmospheric oxygen, and being deposited in the 
 fibres of the cloth it remains firmly attached 
 thereto. Indigo-vats are sometimes prepared by 
 mixing the indigotin with water, lime, and 
 materials which bring about fermentation; the 
 result of the fermentation is indigo-white. 
 
 Indigotin has been made artificially by several 
 processes, each of which involves a complex series 
 
ALIZARIN AND INDIGO. 173 
 
 of reactions. All that may be done here is to 
 give the merest sketch of the oldest of these 
 transformations. The study of the relations of 
 indigotin to other compounds was undertaken by 
 Baeyer about the year 1865, and the synthesis of 
 the compound was effected by him after experi- 
 ments which lasted more or less continuously for 
 thirteen years. First it was shewn that indigotin 
 (C 16 H 1Q N 2 2 ) is oxidised to isatin, C 8 H 5 N0 2 ; 
 then, in attempting to re-convert isatin into 
 indigotin, various compounds were discovered 
 intermediate between these two, more especially 
 oxindol, C 8 H r NO ; then oxindol was changed into 
 isatin ; and then isatin was deoxidised to indigotin 
 (2C 8 H 5 N0 2 + 4H = C 16 H 10 N 2 O 2 + 2H 2 0) ; finally 
 oxindol was prepared from an acid (called amido- 
 phenylacetic acid), which itself was made from 
 phenol, a compound that is found in large 
 quantities in coal-tar. It was only necessary 
 then to separate phenol from coal-tar, from the 
 phenol to prepare the acid, from this to make 
 oxindol, and then to convert oxindol into isatin, 
 and isatin into indigotin. 
 
 Many other methods for making indigotin have 
 been worked out, some of them considerably 
 simpler than the original method of Baeyer. 
 Artificial indigo is now manufactured in large 
 quantities in Germany ; and the trade in natural 
 indigo is rapidly disappearing. 
 
CHAPTER XIII. 
 
 THE ALKALOIDS AND ALBUMIN. 
 
 A GREAT many compounds capable of forming 
 salts by combining with acids, and having marked 
 toxicological effects, have been obtained from 
 plants. These compounds are classed together 
 under the name alkaloids, because they resemble 
 alkalis in some of their reactions. All the alka- 
 loids except three are compounds of carbon with 
 nitrogen, hydrogen, and oxygen ; the three ex- 
 ceptions are coniine (the alkaloid of hemlock), 
 nicotine (the alkaloid of tobacco), and sparteine 
 (the alkaloid of the common broom) ; these three 
 alkaloids are composed of carbon, nitrogen, and 
 hydrogen only. Among the commoner alkaloids 
 may be named quinine and cinchonine, obtained 
 (with more than twenty other alkaloids) from 
 Peruvian bark ; theobromine, from cocoa ; caffeine, 
 from coffee; theine, from tea; nicotine, from 
 tobacco ; morphine and narcotine, obtained (with 
 about fifteen other alkaloids) from opium ; and 
 strychnine and brucine from nux vomica. 
 
 The formulae which express the compositions of 
 the alkaloids are generally rather complex ; such 
 as, C 20 H 24 N 2 2 for quinine, C 21 H 22 N 2 2 for 
 strychnine, and C^HjgNOg for morphine. The 
 study of the reactions of the alkaloids, and their 
 relations to other compounds whose chemical 
 properties have been expressed in constitutional 
 formulas, has not yet advanced very far ; although 
 a vast number of observations and experiments 
 
 174 
 
THE ALKALOIDS AND ALBUMIN. 175 
 
 has been made. A few of the alkaloids have been 
 built up from simpler materials ; for instance, 
 coniine, atropine (the alkaloid of deadly night- 
 shade), caffeine, and theobromine. In working 
 on the reactions of atropine, a compound was 
 discovered closely related to that alkaloid, and 
 possessing, more pronouncedly than atropine, the 
 property of dilating the pupil of the eye. The 
 alkaloid cocaine (prepared from coca leaves) is 
 used as a local anaesthetic in smaller surgical 
 operations, especially in operations on the eye : 
 it seems certain that this alkaloid will be syn- 
 thesised very soon ; indeed a compound has been 
 obtained with exactly the same composition as 
 cocaine but without any physiological action ; 
 when some alterations have been effected in the 
 spatial arrangement of the atomic groups in the 
 molecule of this inactive isomeride, we shall have 
 artificially made cocaine in the hands of the 
 oculists. After that will come the synthesis of 
 quinine and other fever-allaying compounds. A 
 distinguished chemist has told us that when he 
 asked the purpose of the large buildings he saw 
 being erected in a German chemical works, some 
 years ago, he was told : "These are our future 
 quinine works." Already at least two drugs that 
 are most beneficial in reducing the temperature 
 of the body in fever-cases have been manufactured 
 artificially ; these are antipyrine and phenacetin. 
 The molecule of antipyrine is built up step by 
 step from benzene and acetic acid ; the reactions 
 that occur in the process of synthesis, taken with 
 those which are noticed when antipyrine is de- 
 composed into simpler compounds, lead to the 
 
176 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 representation of the intramolecular arrangement 
 of the atoms in this compound by the formula 
 
 CHs 
 
 Antipyrine has all the chemical properties of an 
 alkaloid ; it is an example of a body of this class 
 made in the laboratory, but not yet manufactured, 
 so far as we know, by a living plant. 
 
 The synthetical and analytical reactions of 
 phenacetin are expressed in the language of atom- 
 linking by the comparatively simple formula 
 C.OAH, 
 
 JH.CO.CH, 
 
 One of the hydrogen atoms of a benzene mole- 
 cule is replaced by the atomic group (O.C 2 H 5 ), 
 and another by the group (NH.CO.CH 3 ). 
 
 Although the constitution of quinine is not yet 
 fully worked out, it is certain that this alkaloid 
 is closely allied to two compounds named quinoline 
 and pyridine which are found in coal-tar. Quino- 
 line has the composition C 9 H 7 N, and pyridine 
 the composition C 5 H 5 N. The reactions of pyri- 
 dine find their expression in the constitutional 
 formula 
 
THE ALKALOIDS AND ALBUMIN. 177 
 
 The reactions of quinoline are expressed by a 
 formula which represents the molecule of that 
 compound as composed of a benzene molecule 
 with two hydrogen atoms removed, and this 
 residue joined to a pyridine molecule from which 
 a pair of hydrogen atoms has been taken away ; 
 thus 
 
 It is probable that the molecule of quinine 
 is formed by adding another pyridine ring to the 
 molecule of quinoline, and substituting various 
 atomic groups for some of the hydrogen atoms in 
 this arrangement. Such a formula as the follow- 
 ing would express this conception of the structure 
 of the molecule of quinine. 
 
 C.C 6 H 18 
 
 A general similarity may be detected in the 
 formulae whereby we picture to ourselves the 
 connexions between the reactions and the mole- 
 cular structures of the alkaloids that have been 
 studied most thoroughly. There seems to be a 
 foundation, either of a benzene nucleus, or a 
 benzene nucleus intimately linked to a similar 
 nitrogen - containing group of carbon and hy- 
 drogen atoms, and attached to this foundation 
 there are certain side chains composed in part of 
 
 M 
 
178 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 the atomic groups that occur in the molecules of 
 the ethylic alcohols and ethers (compare Chapter 
 VI. p. 95). 
 
 Chemists are only feeling their way towards a 
 knowledge of the relations of the alkaloids those 
 products of the activity of living plants to 
 other simpler bodies, towards a knowledge so 
 exact as to be expressed in the clear, descriptive, 
 and suggestive language of molecular structure. 
 The method whereby that accurate knowledge is 
 being gained is no new method : it consists in 
 examining the reactions of this or that compound 
 under definite conditions, and thereby finding 
 other bodies to which the compound is related, 
 both in that the compound is obtained from some 
 of these other bodies, and some of them are 
 obtained from the compound. The reactions of 
 such a substance as quinine are exceedingly 
 many ; but most of the recorded reactions seem 
 to throw little light on the structure of the 
 molecule of the substance. I have said that the 
 method employed in trying to elucidate the 
 structure of the molecules of such complicated 
 bodies as quinine and other alkaloids is the 
 method which has been so fruitful when applied 
 to bodies of less molecular complexity ; never- 
 theless, it may be necessary to devise new ways 
 of applying the method when one is dealing with 
 compounds which are the balanced products of 
 many most delicate transformations. As the pro- 
 blems of intramolecular arrangement become 
 finer and more subtile the plan of attack must 
 be marked by greater mobility and more finesse. 
 
 We have become acquainted with many facts 
 
THE ALKALOIDS AND ALBUMIN. 179 
 
 concerning composition and properties which can 
 be reduced to order, and thought of clearly as 
 related one to another, at present at any rate, 
 only by stating them in terms of the molecular 
 and atomic theory : no mechanism has yet 
 been devised whereby we can definitely connect 
 changes in the properties, with changes in the 
 compositions, of compounds, except that of the 
 molecule and the atom. And we have more and 
 more come to think of the molecule as a system 
 in equilibrium because of the mutual actions and 
 reactions between its parts ; as a system of atoms, 
 and groups of atoms, wherein the function per- 
 formed by one part is modified by the functions 
 of all the other parts. The conception of mole- 
 cular symmetry is one of the leading guides 
 to-day to those who are trying to throw more 
 light on the influence of composition on the 
 properties of compounds, and especially com- 
 pounds of carbon. A certain atomic group is 
 introduced into a molecule of a carbon com- 
 pound ; the product has marked dyeing powers ; 
 it dyes a deep yellow : another atomic group 
 is introduced in place of what may be called the 
 yellow group of atoms ; the product dyes blue. 
 It is wished to prepare a compound which shall 
 dye purple ; this is effected by balancing the 
 influence of the yellow group of atoms by such 
 a number of blue groups as to produce a mole- 
 cule which is a purple dye-stuff. The series of ex- 
 periments is repeated but under somewhat different 
 conditions, perhaps the temperature whereat the 
 changes are effected is higher than it was in the 
 first series of experiments ; the product has the 
 
180 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 same composition as the purple-dyeing compound, 
 but it does not dye at all. A nother carbon com- 
 pound is known allied to that from which the sub- 
 stances that dyed yellow, blue, and purple were 
 obtained ; the yellow group of atoms is intro- 
 duced into the molecule of this compound, and 
 the result is a body that dyes a pale, feeble, and 
 undesirable yellow. Evidently, then, it is not 
 accurate to speak of a certain group of atoms as 
 a yellow-dyeing group, and of another group as 
 a blue-dyeing group : whether tinctorial effects 
 do or do not accompany the presence of these 
 groups in molecules of carbon compounds, and 
 what the tinctorial effects are, evidently depend 
 on the positions of the groups in a molecule 
 relatively to other groups and atoms, and also on 
 what are the compositions of these other groups, 
 and on the nature of the other atoms. It is only 
 when the yellow group, or the blue group, is 
 introduced into a molecule of appropriate struc- 
 ture, and is placed in an appropriate position in 
 that molecule, that the new molecule dyes yellow 
 or blue : it is only when the yellow group is 
 balanced by the blue group, by reason of the 
 relative positions of these groups, and this 
 balancing is effected in a molecule wherein the 
 arrangement and also the compositions of the 
 other parts are such as to exert a certain influ- 
 ence on the functions of the two dyeing groups, 
 it is only then that the new molecule dyes 
 purple. A vague general conception of mole- 
 cular symmetry would be of no use in practical 
 science ; it is because this conception is trans- 
 formed by the hypotheses of atom-linking and 
 
THE ALKALOIDS AND ALBUMIN. 181 
 
 definite atom-fixing power into a working instru- 
 ment of research that it has become so important 
 in advancing the knowledge of the interdepend- 
 ence of composition and properties. 
 
 In Chapter IX. I attempted to sketch, in out- 
 line, the application to the tartaric acids and 
 certain sugars of the conception of the asym- 
 metric carbon atom. That hypothesis, and the 
 formulae which arise from it, pictured the differ- 
 ences between the structures of the tartaric acids 
 as the difference between an object and its re- 
 flection in a mirror. If some of the figures given 
 in Chapter IX. are considered (see especially 
 p. 139), the reader will see that a change in the 
 structure, and hence in the properties, of such 
 a compound as tartaric acid might be caused by 
 the semi- rotation of one of the tetrahedra, that 
 is, one of the groups of a carbon atom attached to 
 four different atoms and atomic groups ; for the 
 partial rotation would alter the positions of the 
 atoms and groups belonging to the rotated 
 tetrahedron relatively to the atoms and groups 
 of the other, unmoved tetrahedron. Changes in 
 the structure, and the properties, of compounds 
 that exhibit geometrical isomerism (see p. 135) 
 might theoretically occur with very small changes 
 of external conditions. Now experiments show 
 that many geometrically isomeric compounds are 
 very ready to undergo small changes which do 
 not alter the percentage compositions or the 
 molecular weights of the compounds. There is 
 one especially interesting example of such 
 changes. The existence of two isomerides of 
 a certain compound was asserted by a German 
 
182 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 chemist of recognised accuracy and ability ; the 
 existence of one of these isomerides was denied 
 by another chemist of equal capacity and equal 
 accuracy. One of the chemists said he had 
 obtained both forms of the compound ; the other 
 chemist replied that he could obtain one form 
 only. If the two forms of the compound existed, 
 it was necessary to think of them as geometrical 
 isomerides, as one the mirror-image of the other. 
 After a good deal of controversy each worker 
 published the exact details of his experiments ; 
 one had worked on a bench exposed to full sun- 
 shine, the other in a part of the laboratory 
 screened from the direct rays of the sun. The 
 shade-loving chemist repeated the experiments in 
 the sunshine, and obtained two modifications of 
 the compound. The interpretation given by the 
 hypothesis of geometrical isomerism is simple ; 
 under the influence of the direct sunshine a semi- 
 rotation of parts of some of the molecules of the 
 compound had occurred ; when the sunshine was 
 excluded all the molecules were geometrically 
 identical. 
 
 This example showshowfinearesome of theprob- 
 lems concerning the connexions of properties with 
 structure, and how delicate the instruments must 
 be whereby these problems are to be solved. And 
 as we approach those substances which seem to be 
 very intimately associated with the maintenance of 
 life in animals and plants, the problems become yet 
 finer and more difficult of solution. Albumin is 
 the name given to a class of compounds of carbon, 
 hydrogen, oxygen, nitrogen, and sulphur which 
 form a section of a larger class of compounds of 
 
THE ALKALOIDS AND ALBUMIN. 183 
 
 these five elements named proteids. Proteids are 
 always present in the protoplasm of living, 
 active, cells. The percentage composition of 
 different proteids varies within not very wide 
 limits. Analyses of the same proteid, for in- 
 stance, of egg albumin, show distinct differences ; 
 this may be because the specimens analysed 
 contained varying quantities of impurities, or 
 because the substance we call albumin is a 
 mixture of different compounds, or because the 
 compositions of the proteids in a living organism 
 are constantly changing. If the third hypothesis 
 is adopted, then the proteids cannot be classed 
 among chemical compounds, if the term compound 
 is employed with its ordinary signification. But, 
 one may say, if a substance is not a compound, if 
 it has not a definite, unchangeable composition, it 
 is a mixture ; and if proteids are mixtures, the 
 chemist is not concerned with them, at least not 
 until they have been separated into their con- 
 stituent compounds. In the study of natural 
 occurrences it is well constantly to remind one- 
 self that " in nature there are no boundary lines, 
 however necessary it may be for us to draw 
 them " ; and " in nature everything is distinct, 
 yet nothing defined into absolute independent 
 singleness." A change in the arrangement of a 
 group of atoms so small that we can only liken 
 the product of that change to the image of an 
 object reflected in a mirror is accompanied by a 
 very distinct change of properties. May we not 
 refine a little more ? May we not picture to 
 ourselves a complex collocation of atoms con- 
 stantly giving up a few atoms to the substances 
 
184 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 which environ it, and constantly assimilating 
 some of the atoms which compose the materials 
 of its environment ? May we not speak of that 
 complex atomic group as existing only as long as 
 it undergoes change of this kind, as exhibiting 
 its characteristic properties only while it is in a 
 state of flux ; as being, only when it is becom- 
 ing ; as existent, only as it is ceasing to exist ; 
 as finding itself only by losing itself ? The diffi- 
 culty is to translate such loose conceptions as 
 these into a working hypothesis for use in the 
 laboratory. We cannot do this to-day, we may 
 do it to-morrow. 
 
 Inorganic chemistry the chemistry of the 
 metals and of mineral compounds presents 
 phenomena not wholly unlike those set before us 
 by the chemistry of the constituents of living 
 protoplasm. The presence of a minute trace of 
 phosphorus enormously modifies the properties of 
 steel ; and the same specimen of steel changes its 
 properties very markedly when it has been used 
 for some time. Sodium may be distilled in 
 oxygen without a trace of a compound being 
 formed, if both substances are perfectly dry ; but 
 if there be a very minute trace of water present, 
 combination occurs violently. Electric sparks 
 may be passed through a perfectly dry mixture 
 of carbon monoxide and oxygen gases, and no 
 change occurs ; let an infinitesimal quantity of 
 water be added, the carbon monoxide and oxygen 
 disappear, and carbonic acid gas is formed in 
 their place. 
 
 Should the student of natural science succeed 
 in removing the boundary line that has been 
 
SUMMARY AND CONCLUSION. 185 
 
 drawn between the phenomena of living matter 
 and the phenomena of non-living matter, the 
 removal will deepen our realisation of the unity 
 of nature, and increase our feeling of delightful 
 wonder. 
 
 CHAPTER XIV. 
 
 SUMMARY AND CONCLUSION. 
 
 BEFORE the facts about the compositions of 
 compounds had been generalised in the laws of 
 combination, the knowledge of the composition 
 of a body gained by analysis was expressed by 
 stating the quantity by weight of each element 
 in one hundred parts of the compound analysed. 
 These were the days of facts, and facts only. 
 When a theory of the structure of matter had 
 become a light to the feet of the investigator, the 
 results of the analyses of compounds were ex- 
 pressed in general statements, and the laws of 
 chemical combination brought order into the 
 chaos of facts. The compositions of compounds 
 were now expressed in terms of the number of 
 combining weights of each element in that 
 quantity of a compound which reacted with other 
 compounds to produce new substances. And 
 then the introduction of the illuminating con- 
 ceptions of the atom and the molecule enabled 
 an exact meaning to be given to the indefinite 
 expression that quantity of a compound which reacts 
 with other compounds to produce new substances. 
 
186 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 A clear picture could now be formed of the 
 mechanism of chemical changes : the chemist 
 could see, mentally, the clashing of molecules, 
 the disintegration of these minute particles, and 
 the re-arrangement of the parts, the atoms, in 
 new collocations which are new molecules. The 
 atomic and molecular theory is the only guide 
 that has been found equal to the task of lead- 
 ing the chemist through the maze of facts, and 
 especially the facts concerning the compositions 
 and properties of compounds of carbon. 
 
 I have asked the reader of this book to follow 
 the wanderings of certain atoms and to pay heed 
 to some very striking changes of properties which 
 accompany the association of these minute bodies 
 with other particles of like magnitude with them- 
 selves. I have tried to show that a vast number 
 of facts about changes of composition and changes 
 of properties are brought into due order and are 
 related to one another by the hypothesis that the 
 properties of those collocations of atoms we call 
 molecules are conditioned by the nature, the 
 number, and the arrangement, of the individual 
 atoms, and that very great changes in the pro- 
 perties of molecules may be effected by very 
 small alterations in the grouping of the atoms 
 whereof the molecules are composed. I have 
 -sketched the treatment by which the conception 
 of molecular structure has been made into a 
 potent instrument for framing general expressions 
 of the similarities and dissimilarities between 
 reactions, for forming clear pictures of the ways 
 wherein composition and properties are linked 
 together, and for helping the memory to retain 
 
SUMMARY AND CONCLUSION. 187 
 
 the results of the experimental investigations of 
 chemical changes. 
 
 In applying the conception of molecular 
 structure to express observed relations between 
 properties and composition, the parts of molecules 
 were represented, for a time, as arranged in two 
 dimensions in space ; and it was found possible 
 for many years to present all the observed facts 
 in terms of this admittedly imperfect hypothesis. 
 But reactions and properties of compounds were 
 observed which could not be translated into two- 
 dimensional formulae; and so chemists were 
 obliged to refine their methods of presenting 
 experimental results in structural formulae. The 
 consideration of the three-dimensional formulae 
 that were fashioned to present the finer differ- 
 ences between isomeric compounds suggested the 
 possibility of changing one isomeride into another 
 without effecting any disintegration of the mole- 
 cule. A change so small that it could be likened 
 to giving a half-turn to one of a pair of tetra- 
 hedra joined at one summit of each was found 
 often to be sufficient to alter some of the 
 physical properties of a compound. And as 
 chemists are beginning to see into the condi- 
 tions of existence of those chemical substances 
 which are very intimately connected with living 
 cells, it is evident that their conceptions of 
 molecular structure must be refined much 
 more. 
 
 At one time differences of properties were 
 associated with the removal of one or more 
 atoms from a molecule, and the wandering of 
 these atoms into other molecules. Then the 
 
188 THE STORY OF THE WANDERINGS OF ATOMS. 
 
 migration of an atom, or a group of atoms, 
 from one position to another in the same 
 molecule was recognised as carrying with it a 
 change in the properties of the molecule. Then 
 it was found necessary to admit that changes of 
 properties may occur if a slight twist is given to 
 one half of a molecule, the two halves of which 
 are counterparts of one another. And already 
 chemists see that their conceptions of molecular 
 structure, and changes of properties as accom- 
 paniments of atomic wanderings, must be made 
 much finer and more delicate if they are to 
 embody the facts that are being accumulated 
 in their laboratories. As, formerly, it was 
 found possible to present many facts in terms 
 of an hypothesis which was admittedly quite 
 insufficient to explain the facts the hypothesis, 
 namely, that the parts of molecules are arranged 
 in two dimensions in space ; so, now, even the 
 most refined formulae, whereby slight differences 
 of molecular structure are presented in a con- 
 sistent and suggestive manner, wilfully ignore 
 a matter of fundamental importance. All struc- 
 tural formulae represent the parts of molecules as 
 fixed relatively to one another ; but we are sure 
 that these parts are constantly in motion. We 
 simplify knowingly, that the problem may be 
 made amenable to accurate treatment. It is 
 impossible to attack the problems of natural 
 science otherwise than by simplifying them : 
 we cannot give an exact and complete state- 
 ment, much less a complete explanation, of 
 any natural occurrence ; it is literally true 
 that 
 
SUMMARY AND CONCLUSION. 189 
 
 11 1 hold you here, root and all, in my hand, 
 Little flower but if I could understand 
 What you are, root and all, and all in all, 
 I should know what God and man is." 
 
 We cannot yet devise formulae which shall 
 present the molecule as a system whose parts 
 are constantly performing regulated movements ; 
 therefore, we make shift to do with formulae 
 which picture these parts as fixed relatively to 
 one another. And this crude device works well 
 on the whole ; for, whatever may be the motions 
 of the parts of molecules, certain relations are 
 always maintained between the parts ; and it is 
 the mutual relations of the atoms, and the groups 
 of atoms, that form the subject of chemical in- 
 vestigation. 
 
 The atoms are always wandering ; but as long 
 as their excursions follow a certain sequence, and 
 are confined with a certain limit, no change occurs 
 in the properties of the molecule. One atom, or 
 a group of atoms, may wander beyond its normal 
 limits, and its relations to the rest of the mole- 
 cule may become permanently altered. An atom, 
 or an atomic group, may wander outside the 
 molecule, and become part of another molecule. 
 These are the kinds of the wanderings of atoms. 
 To distinguish these wanderings, to express them 
 in lucid and suggestive language, and to connect 
 some of them with definite changes in the pro- 
 perties of molecules, is the business of chemical 
 science. 
 
INDEX. 
 
 Acetic acid, 83, 84, 87. 
 
 Acetic aldehyde, 83, 84, 87. 
 
 Acidic hydrogen, 78. 
 
 Acids, reactions of, 14. 
 
 Additive compounds, 101. 
 
 Albumin, 182. 
 
 Alcohols, 93. 
 
 Aldehydes, 87. 
 
 Alizarin, 166, 170. 
 
 Alkaloids, 174. 
 
 Amyloses, 131. 
 
 Aniline, 143, 148, 163, 164. 
 
 Anthracene, 166. 
 
 Anthraquinone, 167. 
 
 Antipyrine, 175. 
 
 Aromatic compounds of 
 carbon, 45. 
 
 Asymmetric carbon atoms, 
 133, 136. 
 
 Atom-fixing power, 18. 
 
 Atomic and molecular theory, 
 application of to represent 
 molecular structure, 17. 
 
 Atomic groups in molecules, 
 85, 86. 
 
 Atoms, 16. 
 
 Atoms of carbon, arrange- 
 ment of, in nuclei, 34, 
 40. 
 
 Atoms of carbon, benzene 
 linking of, 37. 
 
 A.toms of carbon, various 
 modes of linking, 34, 36. 
 
 Beer, 153, 155. 
 Benzene, 142, 144, 145. 
 190 
 
 Benzenoid compounds, 45, 
 
 142. 
 
 Benzoic acid, 143, 148. 
 Bunsen lamp, 99. 
 
 Cane sugar, 129. 
 Carbohydrates, 130. 
 Carbolic acid, 143, 148. 
 Carbon, classification of some 
 
 compounds of, 44. 
 Carbon, general reactions of 
 
 compounds of, 31, 42, 45, 
 
 97. 
 
 Carbon, properties of, 22, 28. 
 Carbon, survey of compounds 
 
 of, 21. 
 Carbon, the two oxides of, 
 
 48. 
 
 Carbon dioxide, 49, 55. 
 Carbon dioxide, action of hot 
 
 charcoal on, 57. 
 Carbon dioxide, action of 
 
 plants on, 56. 
 Carbon dioxide in the air, 
 
 53. 
 
 Carbon monoxide, 49, 55. 
 Carbon monoxide, action of, 
 
 in blast furnaces, 61. 
 Carbon monoxide, compound 
 
 of nickel with, 60. 
 Cellulose, 130. 
 Chalk, effect of heat on, 55. 
 Champagne, 160. 
 Chemistry, object of, 12. 
 Chloro-ethane, 83. 
 Chloroform, 81. 
 
INDEX. 
 
 191 
 
 Chloromethane, 65. 
 Coal-gas, 114. 
 Coal tar colours, 163. 
 Combustion, 29. 
 Compound, meaning of term, 
 
 12. 
 
 Compound radicle, 96. 
 Compounds, composition of, 
 
 13. 
 
 Compounds, formulae of, 30. 
 Critical temperature, 52. 
 
 Dextrin, 153. 
 Diastase, 153. 
 Dynamite, 163. 
 
 Element, different forms of 
 
 the same, 23. 
 Element, meaning of the 
 
 term, 13. 
 Elements, arrangement of, 
 
 in compounds, 16. 
 Ethane, 82. 
 Ethane, compounds derived 
 
 from, 83. 
 Ether, 88, 90. 
 Ethereal salts, 93. 
 Ethers, 93. 
 Ethyl, 91. 
 Ethylamine, 83. 
 Ethylene, 98. 
 Ethylene bromide, 101. 
 Ethylene chloride, 101. 
 Ethylic acetate, 88, 91. 
 Ethylic alcohol, 83, 84. 
 
 Fatty compounds of carbon, 
 
 45. 
 
 Fermentation, 154. 
 Flame of coal-gas, 98. 
 Fogs, effect of, on carbon 
 
 dioxide in the air, 53. 
 Formic acid, 70, 87. 
 Formic aldehyde, 70. 
 Formulae, chemical, 13, 31. 
 Formulas, rational, 19. 
 
 Formulae, tri-dimensional, 
 
 113, 132, 136. 
 Fuel, chemical changes in 
 
 burning, 57. 
 
 Geometrical isomerism, 135, 
 
 181. 
 
 Glucoses, 131. 
 Glycerin, 103. 
 Gun-cotton, 161. 
 
 Hard water, 129. 
 
 Hexagon formula of benzene, 
 
 145. 
 Hydrogen, functions of, in 
 
 compounds, 29. 
 
 Indigo, 166, 171, 172. 
 Indigotin, 172. 
 lodoform, 81. 
 Isomerism, 135. 
 
 Lime-making, 55. 
 
 Madder, 165. 
 
 Malt, 153. 
 
 Marsh gas, 64. 
 
 Marsh gas, compounds de- 
 rived from, 65. 
 
 Methylamine, 74. 
 
 Methylated spirit, 158. 
 
 Methylic alcohol, 66. 
 
 Mirror-isomerism, 185, 181. 
 
 Molecule, 16. 
 
 Molecules, properties of, de- 
 pendent on number and 
 arrangement of atoms in, 
 60, 140, 178, 181. 
 
 Nickel, separation of, from 
 
 ores, 60. 
 
 Nitrobenzene, 143, 148. 
 Nitroglycerin, 162. 
 
 Olefiant gas, 102. 
 Oleic acid, 106. 
 
192 
 
 INDEX. 
 
 Optically active compounds, 
 
 109. 
 
 Oxides of carbon, 48. 
 Oxygen, two modifications 
 
 of, 26. 
 Ozone, 26. 
 
 Palmitic acid, 106. 
 
 Paraffin, 117. 
 
 Paraffinoid compounds, 45. 
 
 Paraffins, 103. 
 
 Petroleum, 119. 
 
 Phenacetin, 175. 
 
 Phenol, 143, 148, 149. 
 
 Phosphorus, 23. 
 
 Plane of polarisation of light, 
 
 108. 
 
 Proof spirit, 157. 
 Propane, 103. 
 Proteids, 183. 
 Pyridine, 176. 
 
 Quinine, 177. 
 Quinoline, 177. 
 Quinones, 166. 
 
 Racemic acid, 111. 
 Reactions, chemical, 14. 
 
 Safety matches, 25. 
 
 Salicylic acid, 143, 148, 150. 
 
 Saponifi cation, 107. 
 
 Saturated and unsaturated 
 compounds, 50, 101. 
 
 Soap, 104, 106, 125, 128. 
 
 Soda water, 51. 
 
 "Sparklets, "53. 
 
 Starch, 129. 
 
 Steam, action of hot char- 
 coal on, 58. 
 
 Stearic acid, 106. 
 
 Substitution, 96. 
 
 Substitution compounds, 101. 
 
 Tallow, 105. 
 
 Tartaric acid, 107, 109, 110, 
 138. 
 
 Valency, 18. 
 
 Valency, methods of repre- 
 senting, 35, 
 Vinegar, 43. 
 
 Water-gas, 59. 
 Whiskey, 156. 
 Wine, 158. 
 Wood spirit, 80. 
 Wort, 153. 
 
 Yeast, 155. 
 
 TURNBULL AND SPEARS, PRINTERS EDINBURGH. 
 
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