A 59135 5
DUPL
ARTES
1817
VERITAS
LIBRARY
OF THE
UNIVERSITY OF MICHIGAN
inmala
TTEBOR
ROHAULT's
SYSTEM
OF
Natural Philofophy,
ILLUSTRATED WITH
Dr Samuel Clarke's NOTES
Taken moftly out of
Sir Iſaac Newton's Philoſophy.
With ADDITION S.
125 1620-1681
VOL. I.
Done into English by
OHN CLARKE, D. D. Prebendary of Canterbury,
and Chaplain in Ordinary to His Majesty.
LONDON.
Printed for JAMES KNAPTON, at the CROWN
in St. PAUL'S-CHURCH-YAR D. MDCCXXI
}
SION COLLEGE
LIBRARY.
SOLD BY ORDER OF THE
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1
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#
THE
Tranflator's Preface.
T
HE feveral Editions which this
Treatife has paſs'd through,
both in French and Latin,
are a füfficient Teftimony
how acceptable and uſeful
it has been to the World,
and a juſt Apology for my
tranflating it into English. I fhall not
therefore trouble the Reader with any par-
ticular Account either of the Excellency of
the Subject, the Abilities of the Author, or the
Method he has proceeded in, but refer them
all to be judged of by the Book it felf: Only
as to the Notes the Reader is defired to take
Notice, that therein is a full Anfwer to fuch
Objections made against the Author as feem
not to have any juft Foundation, and a great
many Things in Natural Philofophy, which
have been fince found out by the Pains and
Induſtry of later Philofophers, are here fè-
lected from the beſt Writers; and there are
allo
A 2
The TRANSLATOR'S PREFACE.
alſo ſeveral Things added out of the Obfer-
vations of the ancient Writers of Natural
Philofophy and Natural Hiftory, where they
feemed further to explain and illuftrate Mat-
ters. In all which, to avoid Repetition, Grati-
tude demands that the Reader fhould know
that there are a great many Things owing to
the learned and induftrious Dr. Laughton, and
to the Reverend Mr. Morgan. The former
of which communicated a great many Things
difperfed throughout the whole Book, and
corrected Abundance of Errours: And fix
whole Differtations are owing to the latter,
viz. Thoſe concerning The Laws of com-
municating Motion in elastick Bodies; The
Explication of the Forces of the mechanick
Powers, which are contained in this firft
Part, and thoſe concerning the Celerity with
which heavy Bodies defcend, the Motion
of Projectiles, the Motion of Pendulums,
and that concerning the Rainbow, which
are contained in the following Parts.
THE fourth Part of this Work is but
ſhort, and not very perfect; wherefore it is
thought more adviſeable to refer the Read-
er to later Writers of Anatomy who have
handled that Subject clearly and fully, than
to tranſcribe ſo many Particulars. I hope
the Whole will be agreeable and acceptable.
THE
THE
Author's PREFACE,
T
HE Treatifes of Natural Phi-
loſophy which have hitherto
been publiſhed, being pretty
much alike, both as to the
Matter of them, and the
Manner of handling them;
It is cafy for me to forcfee, that amongſt
thofe who read This, there will be a great
many who will be at firft furpriſed at the
great Difference there is between this Trea-
tife and others. To prevent therefore in
fome Meaſure this Surprize, and to give
what Satisfaction I can in this Matter, I
think my felf obliged to give an Account
of the Obfervations which I have made up-
on the Philoſophy of the Ancients, and of
the Method which I have taken in this
Work.
IN reflecting upon the different Effects of
Time, I have long fince obferved, how fa-
vourable it is to fome Things, which it
is continually advancing to Perfection, and
how pernicious it is to others, fo as to
ftrip them of thofe Beauties and Graces
which
A 3.
The AUTHOR's PREFACE:
which they had at their firft Rife; and I al-
ways concluded that Arts and Sciences can-
not be of the Number of theſe latter, but
that Time is ſo far from being prejudicial to
them, that on the other Hand it is very ad-
vantagious. For as a great Number of Per-
fons who cultivate the fame Art or Science
for feveral fucceeding Ages, add their own
Induſtry, and their new Light to the anci-
ent Discoveries of thofe who went before
them, it is impoffible but that ſuch an Art
or Science muft receive great Improvement,
and arrive nearer and nearer to its utmoſt
Perfection.
AND thus I faw that Mathematicks did
really increaſe by little and little in this
Manner; as it is cafy for any one to be con-
vinced of, who confiders only the vaft Pro-
grefs that hath been made by the great Ge-
nius's of our Time, who have excelled all
others in this Particular, and furmounted
fuch Difficulties as the moft Learned in for-
mer Ages confeffed they were not able to
folve. I faw alfo that moft Arts were per-
fected by Time; Workmen every Day find-
ing out a Multitude of curious Inventions,
which are not fo much efteemed as they
deſerve, becauſe they are very common,
and we do not enough take Notice of
them. Though amongſt thofe Engines which
are employed in making Things of common
Ufe, there is one that has been lately in-
vented, which has in it fo much Contri-
vance, that this fingle Thing deferves to be
morc
The AUTHOR's PREFACE.
more admired than all the Inventions of
Antiquity.
BUT when I came to confider Philofo-
phy, particularly Natural Philoſophy, I was
very much furprized to fee it fo barren as
not to have produced any Fruit, in fo much
that twenty Ages have paffed, without any
new Diſcovery made in it.
HOWEVER I could not perfuade my
felf, that the Study of Natural Things was
neglected, becauſe it was thought to be of
no Ufe; for Health has always been eſteem-
ed one of the chief Bleffings of Life, and
no one can be ignorant, that Phyfick, the
fole End of which is to maintain and re-
ftore Health, is built upon Natural Philo-
ſophy.
NOR could I ever perfuade my ſelf, that
thoſe who improved this Science were less
ingenious, than common Artiſts: For we
find by Experience that in Families where
there are a great many Children, when
they come to make Choice of their Profef-
fions, thofe of them which have the quick-
eft Genius, are appointed for Study, or vo-
luntarily incline themſelves to it; and thofe
only whofe Underſtanding is not ſo good,
apply themſelves to the mechanical Arts,
and are contented with their Lot.
HEREUPON I fufpected, that perhaps
the Knowledge of Natural Things was a-
bove the Reach of humane Understanding,
fo that it was in vain to labour to attain that
which is beyond our Capacity: But when I
confidered
A 4
The AUTHOR's PREFACE.
confidered the furprizing Things done by
ſome Philofophers of our own Age, who
within forty or fifty Years have found out
Things which were looked upon as moſt dif-
ficult, and which fome have doubted, whe-
ther ever they could be found out at all;
I immediately caft off this Sufpicion.
So that I was forced to conclude, that
the Manner of philofophizing, was the
Thing that had hitherto been miſtaken, and
that the Errors therein which have been intro-
duced, being fuch as no Body had any Hopes
of finding out a Remedy equal to, were
a certain Bar to hinder the Approaches to-
wards Truth. I fet my ſelf then to enquire
wherein the Manner of their treating Phi-
loſophy was defective; and after having ex-
amined with the greateſt Diligence poffible,
what the Method has been from the Schools
of the Athenians down to this very Time;
there feemed to me to be four Things blame-
able in this Matter.
First, THE too great Authority that hath
always been given to the Ancients in the
Schools: For befides that this prodigious
Difference which is put between them and
the Moderns, is without the leaft Foundati-
on; for Reafon is to be found in every
Place and every Age; it is certain that ſuch
a blind Submiffion to the Opinions of An-
tiquity, is the Caufe why Perfons of the
greatest Genius, receiving fuch Opinions for
true without confidering them, when per-
haps they may be falfe, have not an Op-
portunity
The AUTHOR's PREFACE.
portunity of knowing the contrary Opini-
ons, nor conſequently of finding out all
thofe other Truths that depend upon thoſe
which fo fatal a Prejudice has hindred them
from feeing. And further, this ſtrong Per-
fuafion of our being fo much inferiour to
the Ancients, caufes in us a Kind of Slug-
gifhnefs and Diffidence, which hinders us
from attempting to enquire into any Thing
at all. We imagine that Reafon is limited
at the Place where they ſtopped, and that
all is done that can be done humanely
ſpeaking, if we go as far as they went.
Thus the greateft Genius's contenting them-
felves with going over the Reaſonings of the
Ancients, don't exercife their own Reafon
at all; and though they be never fo capable
of finding out any Thing themſelves, they
contribute no more to the advancing Na-
tural Philoſophy, than if they had not med-
dled with it all.
I fay nothing in particular of that Vene-
ration which hath been paid to Ariftotle,
though fometimes it has rifen to fuch an Ex-
cefs, that to alledge that he ſaid fuch a Thing,
was fufficient to make any One not only
to doubt of what his Reaſon convinced him,
but even to condemn it. I fhall only make
this Obfervation; that the Imagination
which a great many have had, that he knew
all that could be known; and that all Sci-
ence was contained in his Books, hath
cauſed the greateſt Part of the beſt Philofo-
phers fince to apply themſelves in vain to
read
The AUTHOR'S PREFACE.
read his Works, to find out in them what
was not there, and what they might perhaps
otherwiſe have found out by their own In-
genuity. But if there have been ſome
who, not being quite fo zealous as others,
did not hope to reap fo very much Fruit
from reading him; yet it always happened
that the Defire of recommending themſelves
by explaining thofe Places which he left
obfcure (on Purpoſe, as fome think, or elſe
for Want of better Light) hath made them
imploy their whole Strength of Mind, and
all their leisure Time, to very little Purpoſe,
in writing Comments upon his Philofophy,
without promoting the Science at all: For
thoſe who have undertook to explain Ari-
ftotle; have underſtood him fo differently,
that there are an infinite Number of Places
which all the Schools are divided about
And if there be fome few in which they
have agreed, it is becauſe the Notions con-
tained in them were fo common, that ve-
ry few Perſons were ignorant of them. So
that they took more Pains to ſtudy Ariftotle
than they did to ftudy Nature, which per-
haps is not near fo myfterious as he. There
are a Multitude of Things which Nature
plainly declares to thofe who apply their
Mind thereto. But alas, this is not the Cu-
ſtom, we had rather hearken to Ariftotle and
the Ancients; and this is the Reafon why
we make fo little Progrefs.
ANOTHER Thing which hinders the
Progrefs of Natural Philofophy, is the Treat-
ing
The AUTHOR's PREFACE.
ing thereof in a Manner too metaphyfical;
and the Difputing about Queftions ſo abſtract
and general, that though all Philofophers
were agreed in their Notions of them,
yet they could not help to explain the
leaft particular Effect in Nature; whereas
every uſeful Science ought to deſcend im-
mediately to Particulars. For Inftance, what
good do thofe long and nice Diſputes do,
about the Divifibility of Matter? For
though it could not be accurately determi-
ned, whether it be infinitely divifible or
no; it would be fufficient to know, that
it can be divided into Parts fmall enough
to ferve for all Purpoſes that can be.
It is very uſeful, without doubt, to find
out the Nature of Motion in general. And
it may not be very improper to examine a
little whether it be well or ill defined thus,
The Act of a Being in Power, fo far forth
as it is in Power. But we ſhould not ſpend
too much Time in determining this, and
ſuch like Queſtions; I fhould rather think,
that after having confidered a little the true
Nature of Motion in general, we fhould
particularly and diftinctly examine all the
Properties of it, fo that what we affirm
concerning it, may be applied to fome
Ufe; In a Word, I think we fhould careful-
ly enquire into the Caufe why Matter pro-
duces fuch a particular Effect rather than
any other, and not accuſtom ourſelves to ſay
that it is the Effect of a certain Quality; for
from hence it is that we are led to give
Words
The AUTHOR's PREFACE.
f
Words inftead of Reaſons, and hence arifes
that fenfeleſs Vanity of thinking that we
know more than others, becauſe we know
Words which the common People don't
know, and which indeed have no deter-
minate Meaning. To fay the Truth; it
fhows a mean Spirit, and one that is foon
fatisfied; to believe that we know more of
Nature than other Men, becaufe we have
learn'd that there are occult Qualities, and
can give a general Anfwer to all Queſtions
propoſed to us concerning the different
Effects of Nature. For what Difference is
there in the Anfwer of a Plowman and a
Philofopher, if they are
are both
both asked,
whence is it, for Inftance, that the Load-
ftone attracts the Iron, and the one anfwers,
that he does not know the Reafon of it, and
the other fays, it is done by fome Vertue or
Occult Quality? Is not this in plain English,
to ſay the fame Thing in different Words?
and is it not evident, that all the Difference
there is betwixt them is only this, that the
one is fo honeft as to confefs his Ignorance,
and the other has the Vanity to endeavour
to conceal his?
A third Defect which I have found in the
Method of Philofophers, is, that ſome of
them are wholly for Reafoning, and depend
ſo much upon the Strength of their Argu-
ments (eſpecially if they be borrowed from
the Ancients) that they judge it ſuperfluous to
make any Experiments. Others on the
contrary, quite tired with fuch tedious Ar-
guments,
The AUTHOR's PREFACE.
guments, the greateſt Part of which are not
conclufive, or are nothing to the Purpoſe,
think every Thing ought to be reduced to
Experiment, and that there fhould be no Rea-
foning at all. But both theſe Extremes do e-
qually hinder the Progrefs of Natural Phi-
lofophy. For they who fall into the firft
of thefe Errors, hinder themſelves of the
beſt Means of finding out new Diſcoveries,
and of confirming their own Arguments
likewife; And they who fall into the fe-
cond, by depriving themſelves of the Li-
berty of drawing Conclufions, hinder the
Knowledge of a large Train of Truths,
which may many Times be deduced from
one fingle Experiment. Wherefore it can-
not but be very advantagious to mix Expe-
riments and Arguments together. For Rea-
foning perpetually, and upon fuch general
Things only as are ordinarily argued about,
without defcending to Particulars, is by no
Means the Way to attain any very exten-
five or very certain Knowledge: Thus we
fee the fame Things continually bandyed
about, and no new Difcoveries made; nay,
we are not very fure of the old ones, as
general as they are. We fee alſo that they
who confide moſt in thoſe Arguments which
they believe to be Ariftotle's, are in perpe-
tual Dispute, and that they contend for Opi-
nions which are directly contrary to one
another, without being able to convince
thoſe of the other Side by their Arguments.
And this plainly fhows how little Certain-
ty
The AUTHOR's PREFACE.
ty or Evidence there is in their bare Reafon-
ing.
EXPERIMENTS therefore are neceffary
to eſtabliſh Natural Philofophy; and this
was a Thing which Ariftotle was fo fully
convinced of, that the Reaſon why he
thought that very young Perfons fhould not
apply themſelves to the Study of Natural
Philofophy, was, becauſe at that Age they
are fo little acquainted with Things, as to
be unable to have made many Experi-
ments; and on the other Hand he was of
Opinion, that they were then moſt capable
of receiving Mathematicks, becauſe this
Science confifts of meer Reaſoning, of which
the Mind of Man is naturally capable, and
does not at all depend upon Experiments.
BUT on the contrary to reject entirely all
Reaſoning, in Order to do nothing but
make Experiments, is to run into another
Extremity much more prejudicial than the
former. For this is wholly to difcard Rea-
fon, and yield all up to Senfe, and to con-
tract our Knowledge into a very narrow
Compaís; for by Experiments we can come
to the Knowledge of grofs and fenfible
Things only. Wherefore if we would pro-
ceed rightly in our Enquiries into natural
Things, we muft of Neceflity mix thefe two
Means of Knowledge together and join
Reaſon with Experiments.
AND that we may the better fee the good
Effects of thefe two when joined together,
and the Ufe that may be made of them,
to
The AUTHOR's PREFACE.
to the Advantage of Natural Philoſophy,
we may obſerve that there are three Sorts
of Experiments. The firft is, to ſpeak pro-
perly, only the mere fimple ufing our Senfes ;
as when accidentally and without Deſign,
cafting our Eyes upon the Things around
us, we cannot help taking Notice of them,
without thinking of applying what we fee
to any Ufe. The fecond Sort is, when we
deliberately and defignedly make Tryal of
any Thing, without knowing or foreſeeing
what will come to paſs; As when, after the
Manner of Chymifts, we make Choice of
firſt one Subject and then another, and make
all the Tryals we can think of upon each of
them, and carefully remember what we
have at any Time found to fucceed, and
the Manner in which we arrived at any
certain Effect, in Order to apply the fame
Means another Time to produce the fame
Effect. We alfo make Experiments in this
fecond Way, when we go amongſt diffe-
rent Sorts of Workmen in Order to find
out the Myfteries of their Arts, as Glafs-
makers, Enamellers, Dyers, Goldsmiths, and
fuch as work different Sorts of Metals, and
to obferve how they prepare their Matters,
and how every one of them afterwards work
upon thoſe which belong to them. Laſtly,
The third Sort of Experiments are thofe
which are made in Confequence of fome Rea-
foning in order to diſcover whether it was just
or not As when after having confidered the
ordinary
The AUTHOR's PREFACE:
ordinary Effects of any particular Subject,
and formed a true Idea of the Nature of
it, that is, of That in it which makes it
capable of producing thofe Effects; we
come to know by our Reasoning, that if
what we believe concerning the Nature of
it be true, it must neceffarily be, that by
difpofing it after a certain Manner, a new
Effect will be produced, which we did not
before think of, and in Order to fee if this
Reaſoning holds good, we difpofe the Sub-
ject in ſuch a Manner as we believe it ought
to be difpofed in Order to produce fuch an
Effect.
Now it is very evident that this third
Sort of Experiments is of peculiar Ufe to
Philofophers, becauſe it difcovers to them
the Truth or Falfity of the Opinions which
they have conceived. And as to the two
foregoing ones, though they be not altoge-
ther fo excellent, yet they ought not to be
wholly rejected as of no Ufe to Natural
Philofophers: For befides that their Know-
ledge is continually enlarged by them, they
are alfo the Occafion of making the first
Conjectures concerning the Nature of thofe
Subjects which Natural Philofophers are con-
verfant about; and preferve them from
fome falfe Notions they might otherwife
perhaps have entertain'd. Thus, for In-
ftance, we might have concluded in gene-
ral, that Cold contracts and condenfes every
Thing, if we had not diſcovered by Chance or
other-
The AUTHOR's PREFACE.
otherwiſe, that there are Things which are
dilated by Cold.
THE fourth Defect that I obſerved in the
Method of Philofophers, is the neglecting Ma-
thematicks to that Degree, that the very firſt
Elements thereof are not fo much as taught
in their Schools. And yet, which I very much
wonder at, in the Divifion which they make
of a Body of Philoſophy, they never fail tỏ
make Mathematicks one Part of it.
Now this Part of Philofophy is perhaps
the moſt uſeful of all others, at leaft it is ca-
pable of being apply'd more Ways than all the
others: For befides that Mathematicks teach
us a very great Number of Truths which may
be of great Uſe to thoſe who know how
to apply them: They have this further very
confiderable Advantage, that by exercifing
the Mind in a Multitude of Demonſtrati-
ons, they form it by Degrees and accuſtom
it to difcern Truth from Falfehood infinitely
better, than all the Precepts of Logick with-
out Uſe can do. And thus they who ftu-
dy Mathematicks find themſelves perpetu-
ally convinced by fuch Arguments as it is
impoffible to refift, and learn infenfibly to
know Truth and to yield to Reafon; info-
much that if inſtead of neglecting them, as
is ufually done, it were an eſtabliſhed Cuf
tom, to make Children apply themſelves to
this Science at firft, and to improve them
in theſe Studies as much as we do in others;
it would be of vaft Ufe to hinder them
from contracting that invincible Obftinacy
b
in
The AUTHOR's PREFACE.
in their Opinions which we fee in the great-
eft Part of thoſe who have compleated their
Courſe of Philofophy; who probably would
not have fallen into fo pernicious a Tem-
per of Mind, if they had been accuſtomed
to, and familiar with convincing Truths; and
not ſeen thoſe who maintain in publick any
Doctrine whatever, continually triumph
over thoſe who endeavour to fupport the
contrary; fo that all Things feem to them
only mere Probabilities. They do not look
upon fudying as a Means to diſcover new
Truths, but only as a Piece of Wit to ex-
erciſe themſelves in, the only End of which
is fo to confound Truth with Falsehood by
Means of fome fubtle Diftinctions, that the
one or the other may be equally maintain-
ed, without ever being compelled by any
Reaſons to yield, let the Opinion they de-
fend be never fo extravagant. And indeed
this is the Event of all publick Disputes,
where very often Opinions directly contra-
ry to each other, are by Turns propofed
from the fame Chair, and equally triumph-
ed in, without making Matters at all clear
or eſtabliſhing any Truth thereby.
BUT the great Advantage that natural
Philofophers have from Mathematicks in
particular, is, that they are thereby accuft-
omed to the viewing of Figures, and ena-
bled to underftand the different Properties
of them. I know it is here objected by
fome, that we ought not to ftop at Fgures
becauſe they are not active. But though
they
The AUTHOR'S PREFACE.
they are not active in themſelves, yet it is
certain notwithſtanding that their Differ-
ences make Bodies which we put into Acti-
on capable of certain Effects, which other-
wife they could not have produced. Thus
a Knife by having an Edge fet upon it be-
comes capable of cutting, which before it
was not; and Workmens Tools, by their
different Figures, are fitted to produce thoſe
different Works which are made by the
Help of them. And if the Figures of Bo-
dies which come under our Senfes are fo
neceſſary to the Effects which they produce,
it is reaſonable to think that the moſt im-
perceptible Parts of Matter, feeing they have
every one a certain Figure, are alſo capable of
producing certain Effects in Proportion to
their Bigneſs, like thoſe which we ſee pro-
duced by the groffeft Bodies.
BUT not to enter too far into Particu-
lars concerning the great Ufe of Mathe-
maticks, Is it not enough to put us upon
applying ourſelves more to them than we
have hitherto done, to confider that 'tis by
their Means that the modern Philofophers
have diſcovered all that is excellent and pecu-
liar in natural Philoſophy? And alſo that it is
by the Help of Mathematicks, that the moft
celebrated Artifts in every Age have made
all thofe noble Diſcoveries, the Ufe of
which is ſo advantagious to us at this very
Time, and which make all the Variety of
Arts and all the Conveniences of Life. It
may be fome may think on the contrary,
b 2
that
The AUT HOR'S PREFACE.
that theſe very Artifts, the greateſt Part of
whom it is very probable have not much
applied themſelves to this Science, will juf
tifie it, that it is not ſo neceffary as I would
perfuade them. But here there are two
Things to be confidered: Firft, that as there
is a natural Logick in all Men, fo is there
alfo natural Mathematicks, which accord-
ing as their Genius's are difpofed, make them
more or less capable of Invention. Se-
condly, That if their Genius alone, con-
ducted only by natural Light, will carry
them fo far, we cannot but hope greater
Things from the fame Genius if the Study
of Mathematicks be added to its natural
Light, than if that Study be neglected.
And indeed all the Propofitions in Mathe-
maticks, are only fo many Truths, which
thofe, who apply themſelves to it, come to
the Knowledge of by good Senfe. And
they who find themſelves naturally difpofed
to it, do very ill to neglect what others
have before diſcovered: For it is the moſt cer-
tain Way of finding out any Thing new, to
know all that has been before found out by
others, and the Manner how it was found out.
HOWEVER, I don't put them upon
the Rank of Inventors who have met
with fomething by Chance which they did
not fearch after: As was the Cafe of that
Workman who by cooling on a fudden in
the Water a Piece of Steel which he had
heated red-hot, found it in a Moment ve-
ry much harder than it was before: It was
without
The AUTHOR's PREFACE.
without doubt a very lucky Thing to find
out this Way of tempering Steel; but the
Workman who had the good Fortune to
hit upon it, does not deferve the Name or
Title of an Inventor; as a great many o-
thers do who are not beholden to Chance
for the Glory of their Inventions: As for
Inftance, the Perfon who firft invented a
Fire-lock to a Gun; for it is certain that
this latter had the whole Engine in his Head,
if I may ſo ſpeak, before he made the leaft
Part of it, whereas the other found out the
Way of tempering Steel, by hitting upon
á Thing, as was before ſaid, by Chance,
which he did not fearch after.
Lastly, THAT Mathematicks are of ve-
ry great Ufe in the other Parts of Philo-
fophy, we need no other Teftimony than
that of the moſt celebrated ancient Philo-
fophers, who not only fpeak honourably
of them in their Writings, but do alfo
make uſe of them themſelves. It is fufficiently
known, that Plato caufed it to be written
over his School Door, That none but Geo-
metricians should enter in there. And they
who have taken the Pains to read over the
Works of Ariftotle, have taken Notice of
the feveral Applications he has made of Ma-
thematicks in many Places; fo that they
who do not underſtand the Elements at
leaft, have no great Reaſon to boaft of their
being able to underſtand the Writings of
this Philofopher.
b 3
THE
The AUTHOR'S PREFACE.
THE more I confider thefe four Defects
in the Method of Philofophers, the more I
find it impoffible to come to the Know-
ledge of philoſophical Truths, without cor-
recting them. And this does not appear to
me to be very difficult; for though I had
made fome Proficiency in Mathematicks,
and accuſtomed my felf to follow Reaſon
rather than Authority, yet I did not find
my felf fuch a Lover of my own Reaſon-
ings, as to neglect Experiments, nor ſo bent
upon Experiments, as not to fuffer my Rea-
fon to go beyond what they diſcovered.
BUT though this was fufficient to put mẹ
upon improving natural Philofophy, and to
make me hope that I might be able in fome
Meaſure to help forward the Progreſs of this
Science; yet I obferved a fifth Defect, not
in the Method of thoſe who ſtudy Philo-
fophy, but in that of a great many who
read their Works; which made me think,
that to publiſh any Thing upon natural Phi-
lofophy, was fo far from being any Advan-
tage, that it was but too much to expoſe
one's felf. For that Averfion which is ufu-
al againſt fuch Perfons, and that difagree-
able Manner in which thoſe who are un-
capable of finding out any Thing themſelves,
receive the Writings of fuch as attempt to
exceed what is common, often hazard the
Reputation of the Author. For fcarce can
a Philofopher prefent the Publick with any
Fruits of his Studies, but fome unknown
Perſon who has a Mind to fignalize him-
felf,
The AUTHOR's PREFACE.
felf, attacks them before he underſtands them.
And hence come thofe trifling Difcourfes
or Differtations, for the moſt Part anony-
mous ones, which never fail immediately
to appear, wherein are feldom any Thing
elfe but Reproaches and very low Jefts;
and not being able to overthrow Truths
that are fo firmly eſtabliſhed; they try to
turn them into Ridicule, by fhowing that
they are contrary to fome ancient Maxim
or popular Error, which tickle the Ears of
half-witted People, who are accuſtomed to
take Things without any Proof: And that
which is very remarkable here, is, that thefe
Writers for the moft Part attack the Works
of others only becauſe they think them con-
trary to Ariftotle; and yet becauſe they have
read nothing of this Philofopher but only
thofe Citations which they found in their
philofophical Lectures, it very often hap-
pens that the Thing which they thus at-
tempt to confute, is what Ariftotle himfelf
has faid in exprefs Terms. We may fafely
affirm, that the Ancients did more Juftice to
Men's Labours, and without doubt it was
in a good Meaſure owing to this, that Phi-
lofophy made fome Progrefs in the firft A-
ges of it; fo far were they from fuffering
thoſe who had made any new Difcoveries,
to be cried down at a Venture and without
any Reaſon; every Body knows that there
were publick Rewards appointed for fuch;
even to have fometimes Statues erected to
them; fo firmly were they perfuaded in
b 4
thofe
The AUTHOR'S PREFACE.
thofe Times that Honour contributed moft
to the Invention of Arts.
1
IT is true indeed, that this Maxim feems
to be revived and re-eſtabliſhed in our Age,
Yet though Princes have by their Authori-
ty approved and favoured Arts and Scien-
ces, the long Stiffness which they who
ftudied natural Philofophy have in fo ma-
ny Ages contracted, have fo accuſtomed
them to reft fatisfied with what they recei-
ved from their Predeceffors, that the very
propofing any new Thing, is enough to ren-
der both the Thing and him that propoſes it
odious. Now to take away the Foundati-
on or rather the Pretence of this Averfion,
fuch Perfons ought to know, that this Re-
proach of Novelty is generally a great De-
ceit: For if a Thing be true, it cannot be
new, becaufe nothing is fo ancient as Truth,
and it is the Diſcovery of the oppofite Er-
rour only that can be faid to be new. For
Want of rightly diftinguiſhing theſe two
Things, we often ſee ſome Perfons crying
out that we overthrow the Order of Na-
ture, when we only overthrow a falfe O-
pinion which they were prejudiced in. But
though fuch Sort of Perfons have not much
Reaſon on their Side, yet the Credit and
Authority which they may have over others,
is the Caufe of their Exclamations always
making an Impreffion upon the Minds of
a great many; and this muft ever be difa-
greeable to thofe who have no other De-
fign,
The AUTHOR's PREFACE.
fign, but to contribute to the Publick
Good.
WHAT а Vexation muſt it be to Dr Har-
vey, for Inftance, to ſee all his Life long,
how ill the Diſcovery he had made of the
Circulation of the Blood, was received;
the Motion of which was quite different
from what the Ancients thought? Surely
we cannot fhow too great an Acknowledg-
ment to a Man who had undeceived the
World of an ancient Errour, and by the
Truth which he eſtabliſhed, made us fee as
clear as the Day, that almoſt all the Theo-
ry of the Phyfick of the Ancients was falfe.
But how many Enemies has this Doctrine
got him inftead of Thanks? I folemnly de-
clare therefore, that upon feeing what Li-
berty is taken to oppofe the beft Things,
becauſe the Misfortune of Mens having al-
ways been ignorant of them, made them to
be thought new; I laid afide the Thoughts
of ever entertaining the Publick with any
Thing of my own, or what I learned from
the Works of fome modern Writers.
thus much I thought at leaft, that it was
not impoflible to advance a little further
than is generally done in the Knowledge of
Natural Things, if I carefully avoided fal-
ling into any of thofe Defects which I ob-
ferved in the Method this Study was in at
prefent. And indeed having spent fome
Years in reading the Ancients and Moderns,
but with a firm Refolution not to follow
them any further than I could fee the Rea-
But
fons
The AUTHOR'S PREFACE.
ſons of each of them; it appeared to me
that my Deſign was not entirely fruftrated.
But while I was thus inftructing my ſelf by
reading Books, and converfing with learn-
ed Men, and thoſe that were excellent in any
Art, I never laid afide the Ufe of my Rea-
fon, but confidered the feveral Subjects, and
endeavoured always to ground my Reaſons
upon mathematical Truths, and fure Expe-
riments. And fo good Succefs had I in
carrying on my Deſign, that a great many
of my Friends, whoſe Abilities all the World,
I faw, had a great Value for, adviſed me
to communicate it to others by publick Con-
ferences, or at leaſt by private Converſation.
I muſt ſay, that it was very difficult to per-
fuade my felf to this, becauſe I am diftruft-
ful of my ſelf, and do not think my ſelf
Oratour good enough to undertake to plead
the Caufe of Truth thus publickly. However
I fuffered my felf to be over-ruled; and
though I was fenfible I wanted a great ma-
ny Talents, yet I fubmitted to my Friends,
who affured me, that if the Things were plain-
ly propofed, and in a mathematical Way,
they would be acceptable at leaft to the beſt
Judges. And indeed their Advice fucceed-
ed: For thefe Conferences were not only
agrecable, but it was wifhed that the Sub-
jects had been put down in Writing. And
by confenting to this Opinion of my
Friends, I perceived that I had infenfibly
wrote a Book; and becauſe there were fo
many Copics of it about, that it was be-
come,
The AUTHOR's PREFACE.
come, as it were, publick, and a great many
Faults flipp'd in, I refolved to review it
more exactly, in Order to perfect it as much
as I could. They who read it over, will
eafily fee, that I have overlooked nothing
that is good in the Ancients.
I have taken all the general Notions
from Ariftotle, either for the eſtabliſhing
the Principles of natural Things, or the
chief Properties of them: And I have re-
jected a Vacuum and Atoms, or Epicurus's
indivifible Particles, which I think are
Things contrary to what is firmly eſtabliſh-
ed by Ariftotle; and I have learnt of him
to confider with the greateſt poffible Care
the different Bigneffes, Figures, and Moti-
ons of the infenfible Parts of which
fenfible Things are compofed. And this I
was the readier to do, becauſe all theſe
Things have a neceffary Connexion with,
and Relation to the Divifibility of Mat-
ter, which I acknowledge with Ariftotle,
who hardly refolves any particular Queſti-
on, without confidering the Bignefs, Fi-
gure, and Motion of the Parts of Bodies,
and the Pores which are between them.
But that which most of all determined me
to this Confideration, was, that though
there feems to me to be a juft
Ground to doubt of the Truth of fome
Qualities and Powers commonly afcribed
to fome Bodies, yet I do not think that
there is the fame Reaſon to doubt of their
being compofed of infenfible Parts, or
that
The AUTHOR's PREFACE.
that I can be deceived in affirming that all
thefe Parts have their particular Figure and
Bignefs.
BESIDES thofe Affiftances which I had
from the Ancients, I have alfo collected a
great many other Truths, from the moft
eminent modern Philofophers, whofe Names
you may find in their Places. But the Per-
fon whom I have moſt of all made Ufe of
in this Work, and whofe Name I have not
mentioned at all, to avoid perpetual Repe-'
tition, is the famous Cartes; whofe Merit,
by which he becomes more and more known
to all the Nations in Europe, as he has
long been to many of the principal States,
will draw a Confeflion from the whole
World, that France is at leaſt as happy in
producing and educating great Men in all
Sorts of Profeffions, as ancient Greece was,
I have divided this Work into four Parts.
The firft treats of natural Bodies in general,
and their principal Properties, fuch as Di-
vifibility, Motion and Reft, of Elements,
and of fenfible Qualities, and I have particu-
larly infifted upon explaining thofe which re-
late to Seeing. And I flatter my felf that
upon this fingle Subject I have collected
more Truths into eight or nine Chapters than
are contained in feveral large Volumes
which treat of Opticks, Dioptricks and Ca-
tropticks after the Manner of the Anci-
ents.
THE fecond. treats of the System of the
World, or of Cofmography, which I thought
might
The AUTHOR's PREFACE:
might prove more uſeful than the general
Queſtions that are ufually propoſed in the
common Books of natural Philofophy, which
are as it were Commentaries upon Ariftotle's
Books concerning the World. I have alfo
treated of the Nature of the Stars and
their Influences. And after having explain-
ed wherein Gravity and Levity confift
(which I could not ſpeak of in the firft Part,
not having premiſed what was neceffary,) I
conclude this Part with explaining the Flux
and Reflux of the Sea.
THE third Part is taken up in explain-
ing the Nature of the Earth and of ter-
reftrial Bodies, that is, of the Bodies con-
tained in it, or which furround it, as Air,
Water, Fire, Salts, Oyls, Metals, Mine-
rals, and Meteors.
Lastly, I have endeavoured in the fourth
Part to compriſe all that is hitherto, with
any Certainty, known of the Animal
Body.
ONE Thing perhaps will be obſerved in
the Method I have taken, viz. that I have
been pretty long and particular, in explain-
ing, in the first Part of this Book, all the fenfi-
ble Qualities, which Philofophers ufually
explain, and that but briefly, at the End of
their Treatifes of Philofophy, in which they
comment upon thefe Books of Aristotle's
concerning the Soul. The Reafon of which
is, becauſe this teaches us to know ourſelves,
and becauſe hereby we are feaſonably freed
from a popular Errour, and a Prejudice
which
The AUTHOR's PREFACE.
which we have entertained from our In
fancy, which I have known by Experience
a great many never to have been able to get
rid of, not even after they have gone through
their whole Courfe of Lectures, but have
brought back from the Schools thofe Ha-
bits they carried thither, viz. the afcribing
their own Senſations to the Objects which
cauſe them, and the confidering theſe Senſa-
tions as Qualities in the Objects.
FURTHER, you will not find a great
many Things in this whole Treatife con-
trary to Ariftotle; but you will find more
than I could wifh that are contrary to most
of the Commentators upon him: And be-
fides this, you will meet with a great ma-
ny Things, which neither Ariftotle nor his
Followers have treated of at all, which I
have however judged more uſeful than ma-
ny others which Philoſophers have wholly
imployed themſelves in. And in all this I
did not think it very ill in me to depart
from fome particular Notions, when I
found that thefe Notions were diſagreeable
to Truth.
BUT what has very much abated thoſe
Scruples which I had about this Matter, is,
that when I came to compare thofe Places in
this Treatife which are contrary to Arifto-
tle, with the Writings of the publick Pro-
feffors of his Philofophy, I could not find near
fo many in my own Works as in the Works
of others. And without enumerating the
Particulars, it is eafy to be fatisfied herein,
if
The AUTHOR's PREFACE.
"
if we do but confider, that there is fcarce
any Queſtion in Controverfy, but one half
of them draw Conclufions directly contra-
ry to the other half.
Whence it follows,
that we muſt neceffarily find in the Writings
of thoſe who profeſs to teach the Doctrines
of Ariftotle, as many Places againſt him as
for him.
BUT though all the Philofophers did a-
gree with each other and with Ariſtotle, I
don't fee that this Agreement of theirs
ought ro force me to be of their Opinions,.
nor that Philofophers can pretend that I
am obliged to follow them, in what I am
fully perfuaded and convinced they are in
the wrong of. For fince it is the Cuftom
with them to propofe the Matters which they
treat. of, in the Form of Queſtions, this
very doubting Manner of theirs fhows that
there is a perfect Liberty of taking that Side
which we think to be moſt reaſonable.
what Manner my good Intentions will be
received Time will fhow. However, I am
preparing a Latin Verfion for the Ufe of
Foreigners, with whom I hope to meet
with a favourable Reception.
In
CON.
The CONTENTS
€
O F
PART I.
HAP. 1. The Meaning of the Word Phyficks,
and the Manner of treating ſuch a Subječt.,
2. An Examination of the Notions that preceed
the Study of Natural Philofophy.
3. The Manner of applying Philofophy to particular
Subjects.
4. A Caution concerning Words.
5. The principal Axioms of Natural Philoſophy.
6. Of the Principles of Natural Things.
7. Of Matter.
8.
Some Corollaries of the foregoing Notion.
9. Of the Divifibility of Matter.
10. Of Motion and Reft.
11. Of the Continuation and Ceſſation of Motions
12. Of fuch Motions as are commonly afcribed to the
Fear of a Vacuum.
13. Of the Determination of Motion.
14. Of the Compofition of Motion and of its Deter
mination.
15. Of Reflexion and Refraction.
16. Of hard Bodies put into Liquors.
17. Of Accretion, Diminution, and Alteration.
18. Of Forms.
19. Of Elements according to the Opinion of the Ancients.
20. Of the Elements of the Chymifts.
21. Of the Elements of natural Things.
22. Of the Form of a Hard and of a Liquid Body,
or of Hardness and Liquidity.
23. Of Heat and Cold.
24. Of Taftes.
25. Of Smells.
26. Of Sound.
27. Of Light and Colours, and of Transparency, and
Opaken f's.
28. A Defcription of the Eye.
29. How
The CONTENTS.
29. How Vifion is commonly explained.
30. Of the Paffage of the Light through the Humours
of the Eye.
31. What we mean, when we say, that the Images of
the Objects are impreſſed upon the Organs of Sight.
32. How Vifion is performed.
33. Of Dioptricks.
34. Of Looking-Glaffes.
35. A Solution of fome Problems concerning Fifion.
"Ο
PART II.
F the Meaning of the Word Cofmography,
and the Ufefulness of the Science.
2. General Obfervations.
3. Conjectures how to explain the apparent Motions of
the Stars.
4. Of the principal Points, Lines and Circles, which
are imagined to be upon the Superficies of the World."
5. Of the chief Uses of the Circles of the Sphere of the
World.
6. Obfervations about the Sun's Motion.
7. Conjectures how to explain the Phenomena of the Sun.
8. Obfervations and Conjectures about the fixed Stars.
9. Obfervations about the Moon.
10. Conjectures whereby to explain the Phenomena of
the Moon.
11. Of Eclipfes.
12. Of the true Bignefs of the Earth, Moon, and
Sun, and of their Diſtance from each other.
13. Of the Phenomena of Mercury and Venus.
14. Conjectures for explaining the Phenomena of Mer-
cury and Venus.
15. Of the Phenomena of Mars, Jupiter, and Saturn,
16. Conjectures whereby to explain the Phenomena of
Mars, Jupiter and Saturn.
?
An
The CONTENTS.
An Explication of the Phænomena, upon Sup-
pofition that the Earth turns about its own Center
in Twenty-four Hours.
17. A Caution about the Poles and the Circles.
18. An Explication of the Sun's Phenomena.
19. An Explication of the apparent Motion of the fixed
Stars.
20. An Explication of the Motions of Mercury and
Venus.
21. An Explication of the Motion of Mars, Jupiter
and Saturn.
22. An Explication of the Moon's Motion,
23. Of the System of Tycho-Brahe.
24. Reflections upon the Hypothefes of Ptolemy, Co-
pernicus, and Tycho.
25. Of the Nature of the Stars.
26. Of Comets.
27. Of the Influences of the Stars, and of judicial
Aftrology.
28. Of Gravity and Levity.
29. Of the Flux and Reflux of the Sea.
PART III.
I.
2.
F the Earth.
Oof the Air.
3. Of Water.
4. Of Salt.
5. Of Mineral Oil.
6. Of Metals.
7. Of Minerals.
8. Of the Load-Stone.
9. Of fubterraneous Fires and Earthquakes.
10. Of Fountains.
11, Of Winds.
12. Of Mifts and Clouds.
13. Of Rain, Drizzle, Dew, and Evening Damps.
14. Of Snow, Hail, andHoar-Froft.
!
15. Of
The CONTENTS.
15. Of Honey-Dew, extraordinary Rain, and Manna.
16. Of Thunder, Lightning, and Thunderbolts.
17. Of the Rain-bow.
I.
2.
0
PART IV.
F the Things contained in this Fourth Part.
A general Defcription of the larger Parts
contained in a humane Body,
3. Of the Brain, Nerves, and Muſcles.
4. Of the Heart.
5. Of the Veins and Arteries.
6. Of the Lacteal and Lymphatick Veins.
7. Of the Tongue and falival Ducts.
8. Of the Lungs.
9. Of the Liver.
10. Of the Spleen.
11. Of the Kidneys and Bladder.
12. Of the Motion of the Blood.
13. Of the Pulfe, or Beating of the Heart and Arteries.
14. What Time the Blood circulates in.
15. Of natural Heat.
16. Of Nourishment and Growth.
17. Of the Animal Spirits, and of the Motion of the
Muſcles.
18. Of Refpiration.
19. Of Waking and Sleeping.
20. Of the Concoction of Meat.
21. Of the Motion of the Chyle.
22. How the Blood is made.
23. Of the Excrements.
24. Of Hunger and Thirſt.
25. Of Sickness and Health.
26. Of a Fever.
ER-
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TOERED TO 20 DO PA
PERSOA
ROHAULT's
ROHAULT's
SYSTEM
O F
Natural Philofophy.
है.
L
PART I
CHAP. I.
The Meaning of the Word Phyficks, and the Manner
of treating fuch a Subject.
T
HIS Word, Phyficks, ftrictly freaking, and I. The
according to the Etymology of it, fignifies the Word
Meaning of
no more than Natural; but we here ufe Phyficks.
it to fignify the Knowledge of natural
Things, that is, that Knowledge which
leads us to the Reafons and Caufes of eve-
ry Effect which Nature produces.
2. But becauſe we must first ftudy natural Philoſophy, 2. That it is
before we can be certain whether there be any fuch thing
needless to flop
at previous
as Phyficks or no; I fhould not proceed in a proper Me- Questions.
thod, if I fhould here undertake to refolve this Difficulty.
B
I fhall
[N]
Part I.
ROHAULT's SYSTEM
5. That the
I fhall not therefore at all infift on this, nor any other
Queſtions which are commonly called previous ones.
had better at firft remain in fome kind of Doubt about
theſe fort of Queſtions; but fuch a Doubt only, as ought
not in the leaſt to hinder us from ufing our utmoſt En-
deavours to acquire this Knowledge, and to obtain the End
propoſed, without neglecting any Thing, that may ferve to
illuftrate the Truth, and explain the Effects of Nature.
3. One Thing we ought particularly to take notice of,
Notions of the and that is, that all they who apply themſelves to the Stu-
Antients may
be injurious, dy of Natural Philofophy, are not Perfons utterly ignorant;
for by their Converfation with learned Men, by reading of
Books, by Experiments, and particular Obſervations, their
Minds are filled with variety of Notions. But becauſe, per-
haps, we have given too much Credit to the Reports of
others, or perhaps have not throughly examined what we
have received by our own Senfes, or have impoſed upon
our felves by falfe Reaſoning; therefore we are not to
think, that there is any great Advantage to be had from
that Knowledge which is got by thefe Means: On the
contrary, it may be very injurious, becauſe the Errors im-
bibed in our tender Age, before we could make a right
Uſe of our Reaſon, may caufe us to fall into ftill greater
4. That they
examined.
ones.
4. Wherefore if we would proceed regularly, we muſt
•ght to be re- lay afide all our old Prejudices, and reject them as falſe; not
that we are immediately to embrace the contrary Opini-
ons as true, but only fo to diſpoſe our Minds, as to give
Credit only to thoſe Things which we have throughly
examined; and to begin natural Philofophy at the very Be-
ginning. But feeing this is a very difficult Task, and it
is hard to bring our felves to it, becauſe we eafily per-
fwade our felves, that amongst the Errors that have private-
ly crept in, there have been alfo a great many Truths,
which ought by no means to be rejected; we will there-
fore go in the common Method, and retaining as ma-
my of our antient Opinions as we can, we will endeavour
to lighten that Burden which cannot but be very heavy.
And we muſt be very unreaſonable indeed, if we will not
review our old Notions, and ſubmit them to a freſh Ex-
amination.
CHAP.
Chap. 2 of NATURAL PHILOSOPHY,
CHA P. II.
An Examination of the Notions that precede the
Study of Natural Philofophy.
Heads.
T
HE Notions which precede the Study of Natural 1.The VV bol
Philoſophy, may be reduced to two general Heads. of natural
For firft, we know that there are Things really exifting in Philosophy.
may be com
the World; and from hence we think we know, at least prehended
in part, what they are. Theſe two Confiderations are prin- under twe
cipally to be attended to, that our propoſed Examination
may be as univerfal as poffible. Let us firft fee what Mo-
tives there are to induce us to believe, that there are certain
Things really exifting in the World; and then let us fee what
Reaſon we have to believe them to be fuch as we judge
them to be.
2. How we
2. And to begin with our own felves; we know by ex-
perience, that we are capable of diverfe Thoughts, which come by the
Knowledge of
cannot be in us, but they must be perceived. The Idea
our own Ex-
of Existence is one of theſe Thoughts; and our natural Rea- iftence.
fon teaches, that Nothing can have no Properties, and that
what thinks; muſt exift. Hence it is plain how we come
by the Knowledge of our own Existence. For every Man
muft neceffarily reafon in this manner: I think; that which
thinks muſt of neceffity exift; therefore I exiſt.
3.That our
s
Mind is
two really di-
3. A Man who comes to the Knowledge of his Exift-
ence in this manner, knows himſelf only to be fomething
that exifts, the Idea of which does not include Extenfion Jooner than
in it. It is true, he may have an Idea of a Thing extended our Body, and
into Length, Breadth, and Height; but becauſe this Idea that these are
does not at all include Thought in it, the Thing that thinks, fing Things,
and the Thing that is extended, are to be looked upon as
two Things really different from each other; and there is
no Reaſon hitherto for fuch a Perfon to think himſelf an
extended Thing. And becauſe That which thinks, which
is in us, which we know before all other Things, which
we imagine not to be extended, is what we call our Soul
or Spirit, and That which we conceive to be extended in
Length, Breadth, and Height, and to which we imagine
Thought does not belong, is what we call our Body; it is
evident, that our Soul or Spirit is known to us fooner than
our Body.
В 2
4. As
ROHAULT's SYSTEM
Part I.
>
7.
4. That we
4. As to thoſe Bodies of which the world is compofed,
have no other
knowledge of
(amongſt which our own is to be reckoned) it is certain
the Existence we cannot know that they exiſt, but by the different Ways
of thoſe Bo- of Knowledge which are in us; and in order to know if we
the World is have made a right Uſe of them, we will here confider each
sompofed, but of them diſtinctly.
dies of which
by the diffe-
rent ways of
5. The different Ways of Knowledge that are in us, may
knowing that all be reduced to thefe Four: viz. Perception, Fudgement,
are in us. Reafon, and Senfation.
5. What thefe
Ways of
are.
6. What is
6. By Perception is meant fimple Apprehenfion, or the
Knowledge fimple Idea which we have of Things, without affirming
or denying any thing concerning them; whether this Idea
meant by Per- raiſes any Image in our Minds, and fo is called Imagination,
ception or I- or raifes no Image, and fo has only the general- Name of
magination.
Perception given to it. Thus when we hear the Word Tree,
the Idea which we then form in our Minds, is an Imagina-
tion; but when we fpeak of a Thing which cannot be
reprefented by any Image, as of "Doubtfulness; the Idea
which we then have, is only fimple Perception.
7. What is
meant by
Fadgement.
Reaſon.
7. Fudgement is the joining or disjoining of two Things by
the Mind, when, according to the different manner of its
conceiving them, it affims or denies the one or the other.
Thus when we ſay, that the Earth is round, we join toge-
ther the two Things which we underſtand by the Words
Earth and Roundneſs, and this is called Judgement: So alfo
when we ſay that the Earth is not round, that is, disjoin
thofe Words; this is alfo called Judgement.
8. What is 8. Reafon is a Judgement that depends upon a former
meant by
Judgement. For Example: After I have judged, that no
even Number can be compounded of five odd Numbers, and al-
ſo, that the Number Twenty is an even Number, and thence
conclude, that the Number Twenty cannot be divided into
five odd Numbers; this is called, Reafoning.
9. What is 9. Senfation, is Touching, Smelling, Tafting, Hearing and
meant by Sen- Seeing.
fation.
10. That Per-
;
10. Firft, it is evident, that the bare Perception of a Thing
ception alone is not fufficient to convince us that the Thing it felf exifts
is not a fuffi- for Inftance, becauſe I can conceive a Triangle, it does by
cient Aff
rance of the no means from thence follow, that a Triangle exifts.
Exifience of 11. It is certain alſo, that our Judgement alone is not fuf-
any Thing ficient to convince us of the Exiftence of any Thing. For
does Fudge- though we cannot help paffing our Judgement upon many
Things; for Inftance, That if two Things be equal to a Third,
fully convince
ss of the Ex- they are equal to each other; that if Equals be added to Equals,
iftence of their Sums will be equal, &c. notwithstanding which, we
Things. do not certainly know, that any Things that are equal or
11. Neither
ment alone
une-
}
Chap. 2. of NATURAL PHILOSOPHY.
5
unequal exiſt, and the Truth of our Fudgement agrees only
to the Things that may poffibly exiſt.
12. That
not convince
without our
12. We may alfo reafon infinitely various ways; and by
this means all the Mathematical Truths are difcovered, Reafon does
which are fo different from one another, and from the us that any
Principles from which they are deduced: But becauſe the Thing exifts
Confequences have a ſtrict Relation to the Antecedents, and felves.
can contain no more in them than they; and we have al-
ready ſeen that our Judgement does not prove that any Thing
exifts; it follows, that our Reasoning proves no more than
this, that Things without us may poffibly exift.
13, The Ex-
iftence of God
13. However, there is one Exception to this Rule, and
that is, God: For whoever has the Idea of Him, may by may be pro-
Reafon be affured of his Exiſtence, if he be confidered as ved by Rea-
a Being every way perfect, and if Éxiſtence be owned to ſon.
be a Perfection. But I fhall not here enter into the Particulars
of this Demonſtration; the Dignity of this Subject merits
to be treated of particularly by it felf.
ufe of our Sen-
14. But fince we are here fpeaking only of natural Things, 14. That we
and our Perception, Judgement, and Reafon alone do not ought to make
prove their Exiſtence, we muſt certainly have recourſe to fes to prove,
our Senfes before we can judge that they exiſt. And we
cannot know whether our Senfes do fufficiently prove this, out us exiſt.
nor in what manner they prove it, unlefs we first define
what we mean by Senfation.
15. Long Cuſtom makes us many times reaſon with ſo
much eaſe and readineſs, that very often, Reafon and Sen-
fation go together, when we think that Senfation only is
concerned: Wherefore that we may not confound the one
with the other, and fo be led into Error, let us examine
this Matter in other Perfons. Let us fuppofe a Man juft
born, and that he was in an extraordinary manner endued
with the Judgement and Prudence of a grown Perfon; and,
that we may examine only one Senſe at a time, let us fup-
pofe that his Eyes are not yet open, and, that he is put
into a Place, where there is no kind of Smell or Noiſe.
that the
Things with-
to
15. The Way
know di-
findly what
Senfation is.
ample in a
16. Now in order to find out what the Senfe of Feeling 16. Au Ex-
is; let this Man's Arm be prick'd with a Needle. It is Needle.
manifeft, that he will feel the fame fort of Pain that we
feel, when at any time we are pricked with a Needle,
becauſe we ſuppoſe him to be fuch a fort of a Man as we
There is one Exception,) How the
Idea of God proves his Exiſtence,
See Cartes. Princip. Part. 1. Artic.
14. and Regis Metaphyf. Lib. I.
Part. 1. Cap. 5. But this is too nice
and fubtle an Argument; that drawn
from the Variety, Beauty, Order, and
Difpofition of the Creation, does
much more fully and ftrongly infer a
God.
B 3
are:
6
17. That we
feel the prick
ing, and no-
thing else.
18. This Ex-
ample teaches
+
ROHAULT's SYSTEM · Part I.
•
are Now abitractedly from any Judgement or Reaſoning,
it is evident, that Senfation in this Man is nothing elfe,
but the being affected with a certain Pain, which belongs
to himſelf only. So that if any Perſon were fo weak as
to believe, that a like Pain was in the Needle, we ſhould
certainly know for all that, that it was not the very Pain
which the Man by Senſation felt.
17. Let us make fome Reflection here: In the Senfation
now mentioned, there are four Things obfervable: Firft,
A Man capable of Senfation Secondly, A Needle, or the
Object that raiſes the Senfation: Thirdly, The Action of a
Needle upon the Body, in which it produces fome Change:
Lastly, The Effect of the Action of the Needle, and of the
Paffion of the Body, namely, the Pricking, or the Pain.
Now fince 'tis this Laft only that is known, we muſt con-
clude, that this Senfation not being attended with any
Judgement or Reafon, is nothing else but a confufed Per-
ception arifing from the new State of the Mind, which does
not any way make known to us this new State, nor the
external Object which caufes it, and is the Occafion of the
Senfation.
18. From what has been faid of that Pain which is cau-
as mhat the fed by a Needle, it is eafy to apprehend the fame thing of
Senfations of the other Sort of Senfations, fuch as Feeling, Tafting, and
Feeling, Taft- Smelling. For fuppofe the naked Arm of the forementi-
ing and Smel-
ling are.
oned Perfon to be lightly touched with a Feather, or
any other foft Thing, fuppofe a red-hot Coal, or a Piece of
Ice to be laid on any part of his Body; fuppofe a Drop of
Wine poured on his Tongue, or a Rof, or any other fweet-
Smelling Thing put to him; we can eafily understand, that
the Tickling, the Heat, the Cold, the Taste, and the Smell,
which this Man perceives, are all within himſelf, and be-
long to him in the fame manner as the Pain did.
19. Ariftotle
19. And fince there is no Reaſon why we ſhould think
had good Rea- differently of the Senfations of Hearing and Seeing than of
fon to affirm,
that Senfati- the others, we may look upon it as certain, that Sound, and
en and Paf Light, and Colours, are as much in us as Pain or Tickling.
fion were the Wherefore we may fay with Ariftotle, that all Senfation is
Same.
a kind of Paſſion, and when we have any Senfation, what-
ever fort it be, we know very well what the Objects raiſe
in us, but we don't know what they are in themfelves.
1. Ariftot. de Anima. Lib. 2. cap.
5. Senfation confifts in being put into
Motion, and is a fort of Paffion, as
was faid before ; for there seems to be
I
fome Change or Alteration made in
us, and again, chap. 11. Senfation is
a fort of Paffion.
20. But
Chap. 2. of NATURAL PHILOSOPHY.
7
20. But this is not the general Opinion of Mankind, 20. Avul-
who, on the contrary, are apt to think, that the Sound gar Error.
which they hear, is in the Air, or in the founding Body as
they call it; fo alſo that the Light and Colours which they
fee, are in the Flame or the Tapestry which they look upon;
and the Reaſon of it is this, becauſe we do not feel Sounds
and Light, and Colours within our felves, as we do Pain
and Tickling, but aſcribe them to external Things; and be-
fides, the Colours which we fee, oftentimes feem to be
much bigger than our ſelves.
21. But to ſhow that thefe Reaſons are not of 21. The com-
any
mon Notion
Weight, we need only confider, that very often we have
refuted by
a Perception of a Multitude of Things, which we think many Expe-
are without us, and are a great deal bigger than our ſelves,
when at the fame time there really is nothing without us,
that is the Cauſe of that Perception.
riments.
22. First, In Dreams we very often hear Sounds, and 22. 1. Expe-
fee Colours, in the fame manner as if we were awake, and riment.
we afcribe thoſe Sounds and Colours to external Objects;
and we imagine thofe Colours to be much larger than our
felves; though there is indeed nothing without us, to which
they can truly be aſcribed.
23. Secondly, Perfons in a Phrenfie, or in a violent 23.II. Ex-
Fever, fee alfo Things without them, which really are periment.
not fo.
24. Thirdly, We often hear a Ringing in our Ears, or a 24. III. Ex-
certain Sound which we judge to be at a great distance, periment.
when the Cauſe of it is very near us.
25. Fourthly, A Candle, or any other fmall Object, at a little 25. IV. Ex-
diſtance, appears double to a Perſon in Drink; or if we periment.
prefs the Corner of our Eye with our Finger; ſo that
there will then appear to be two Objects, when we certain-
ly know, that there really is but one.
1. Sound, and Light, and Colours,
&c.) In order to account for theſe
Prejudices, we may obferve, 1. That
Pain and Tickling do much more
ftrongly affect us, and make a greater
Change in the State of the Mind, than
Sound, and Light, and Colours; fo
that they are fooner and more eaſily
taken notice of, and imagined to be-
long to us, and to be in us. 2. When
Sound, and Light, and Colours, are
at first perceived, there is always
fomething before us, that acts upon
us, and to which we afcribe them:
But Pain and Tickling often ariſe
I
B 4
|
from an invifible Alteration of the
ſmall Particles of the Body, that is,
from a Cauſe at firſt unknown to us:
Therefore we are a long while uſed to
look upon theſe as fomething in us;
'till there appears to be fomething
without us, to which they may be
afcribed; and afterwards, when we
do fometimes experience, that they
proceed from various external Things,
we are ftill apt to think, that they
are not in thofe external Things, buc
in our felves, becauſe we have been
ufed to think ſo.
26. Fifthly,
}
8
ROHAULT''s SYSTEM Part I,
1
1
}
26. V. Ex-
periment.
27. There is
markable in
rrent.
26. Fifthly, If in the Dark we wink with our Eyes up-
on the Flame of a Candle at a little diftance, we fhall ima-
gine, that we fee Rays of Light, which feem to ftream
from the Flame upwards and downwards in the Air; and
yet there is no doubt,. but that thofe Rays arife from the
Senfation of him that perceives them, and that out of
him they are nothing; if we confider, that other Perfons.
who look upon the Candle at the fame time, do not fee
them; and the Perfon himſelf who fees them when he
winks, ceaſes to fee them the Moment that he opens his
Eyes, and looks more intently.
up-
27. We fhall be more fully fatisfied, that theſe Rays are
Something re- not in the Place that we imagine them to be, by this
this Experi- Confideration; If they were there, it would follow, that
on putting a dark Body between the Eye and the Place where
they appear to be, they muft immediately vanish; but they
do not vaniſh, but on the contrary are feen ftill, only a little
nearer, viz. between the Eye and the dark Body that interpo-
fes. But that which is moft obfervable in this Experiment,
is, that if the dark Body be raiſed by little and little, as if the
lower Rays were intended wholly to be hidden by its In-
terpofition, they will be ſtill feen, when the upper ones
wholly diſappear; which could not be, if the Rays were
really in the Place which they feem to be in.
28. VI. Ex-
periment.
29.
VII. Ex-
periment.
30.VIII.Ex-
periment.
31. A Dif-
ficulty which
arifes from
-the common
Custom of
Speaking.
28. Sixthly, We fee the Colours through a triangular
Glafs Prifim, very bright, and exactly like the Colours
in the Rainbow; theſe we certainly know are not where
they appear to be.
29. Of this kind are the Experiments of Looking-Glaf-
fes and Multiplying-Glaffes, which reprefent Objects to us,
where we are fure they are not.
30. We must not here omit an Experiment of thoſe
Perfons who have loft any of their Limbs, an Arm, or
a Leg, who, many Months, and fometimes many Years
after they are cured, feel frequent Prickings, and other
Senfations, which they cannot help judging to be with-
out them, viz. in thofe Places where their Fingers or
Toes would have been, if they had not been cut off.
This Judgement is evidently a Miftake, it being certain, that
this Senfation is within themfelves, and not where they
take it to be.
31. This Experiment, together with all the foregoing
ones, plainly fhow, that we have within our felves the
Senfations of many Things, which we cannot help think-
ing are without us, though they really are not; and were
it not for the common Way of Speaking, which is the u-
fual
Chap. 2 of NATURAL PHILOSOPHY.
fual Reafon given, we ought wholly to lay afide that vul-
gar Notion, which we have entertained in our Minds
from our Infancy, viz. that they are without us. For
(may any one fay) as he who touches a Stick, has reaſon
to believe, that the Stick is fomething without him that
touches it; fo when any one fays, that he fees a Colour,
he has Reaſon to ſay, that the Colour which he fees, is
fomething different from him that fees it, and belongs to
the Object.
-
The com
plained.
32. But it is eafy to get clear of this Difficulty, if we ob- 32.
ferve, that all Languages do not afford equal Plenty of men way of
Words upon every Subject. Thus for Example, in the peaking ex-
Latin Tongue, the Word Animal is uſed to exprefs
the Kind, under which the whole Species of Animals is
contained; the Words Man and Horfe, are uſed to figni-
fy thoſe Species; and the Words Peter and Paul, Buce-
phalus and Bayard, to fignify the Individuals of thoſe Spe-
cies: But the Cafe is different in the prefent Subject; we
ufe indeed in our Language the Word Senfation, by which
we underſtand, in general, every Perception which we have
by the means of Bodies; we have alfo the Words Feeling,
Tafting, Smelling, and Hearing, to fignify the particular Spe-
cies of thoſe Senfations; but if we would defcend to any
thing ftill more particular; we then want Words, and are
forced to make ufe of a general Name, with which we
only joyn fome other Word, to determine its Signification:
Whence it follows, that when we fay, for Example, that
we feel the Heat, or that we fee the Colour, if we forbear
Reaſoning about them, and attend only to the bare Sen-
fation; the Feeling ought no otherwife to be diftinguiſhed
from the Heat, nor the Seeing from the Colour, than in
any Species, the Genus is diftinguished from the Diffe-
rence: For the Colour and the Heat are Senfations which
belong to our own felves only, and are nothing more than
our own Perceptions.
1. The Genus is diftinguished, &c.)
The Author's Meaning is this, that
many People are led into Error, by
the Forms of ſpeaking; as when by
reafon of the Fewnefs of Words, our
Meaning cannot be expreffed but by
more Words than one ; thus when we
fay, that we fee Redness, or feel Heat;
they fo underſtand it, as if by one
of thefe Words we intended to figni-
fy the Senfation it felf, and by the
other, to fignify fomething without
us, which is the Cauſe of that Sen-
fation. Now if what we call feeing
|
Redness, and feeling Heat could be ex-
preffed by one word, as Pain, which
is the fame Thing as feeling Pain, or
Tickling, which is the fame as feel-
ing Tickling, are exprelled by one
Word; we ſhould eaſily apprehend,
that the Redness which we perceive
by our Sight, and the Heat which
we perceive by our Feeling, are no
more without us, than the Pain which
we feel when our Arm is pricked
with a Needle; or the Tickling,when
it is touched lightly with a Feather.
33. Though
ROHAULT's SYSTEM
Part I.
33. The Con-
formity there
is betwixt
33. Though I have been already too long in fhowing
that what we perceive fimply by Sight, is wholly within our
Sight and felves; I would yet make appear the entire Conformity
Feeling.
there is betwixt Seeing and Feeling. Let us confider then,
that when an Object of Feeling affects the Body but lightly,
it raiſes in us indeed a real Senfation, but it is fo weak
an one, that it is gone as foon as the Object ceaſes
to touch the Organ of Senfation; fo likewife, if the Ob-
ject of Light be weak, it is no fooner removed from our
Eyes, but we ceaſe to fee it. And as an Object of Feel-
ing, which ſtrikes us with a greater Force, excites a Sen-
fation, which remains after it is feparated from the Organ;
in the ſame manner alfo, a very ftrong Object of Sight,
raiſes a bright Senfation, which continues for fome time,
though we do not look upon it, but turn our Head ano-
ther way. Thus if any one looks full upon the Sun, and
immediately goes into a dark Place, he will fee the Sun
there, and fome Sparklings of it.
34. That me
have made
uſe
34. From what has been faid concerning our Senfes, and
the Manner of Senfation, fince it is evident, that they make
ufe of feveral known to us only what is in us, and belongs to us; it is
Means of
Knowledge, alfo as certain, that they are not alone fufficient to prove
in order to be to us, that any Thing at all exifts without us which does
Things exift not belong to us; and this having been already ſhown of
every particular Means of Knowledge, we muft neceffarily
conclude, that we have made ufe of feveral of thofe Means
in order to be convinced that Things do exift without
convinced that
without us.
35. The Me-
thod which we
proceed in.
36. The Ex-
iftence of
us.
35. The Method we feem to have proceeded in, is this.
Firſt, Senfation: Next, we obferve, That this Senfation is
fometimes in our own Power, and fometimes not: Whence
we infer, that we our felves are not the fole Caufe of our
own Senfations; that we contribute fomething towards them,
but not fo much, but that we depend alſo upon ſome o-
ther Cauſe; and fo we begin to fee, that we do not exiſt
alone, but that there are many other Beings exifting toge-
ther with us in the World.
36. Whoever acknowledges this Truth, muft confefs,
Things cogni- that he has been in an Error fo long as he thought that the
zable by our Exiftence of Things without him was proved by his Sen-
Senfes, is
made known
1
to as princi-
1.There are many other Beings,&c.
pally by Rea- But even this does not feem fuffici
fouing.
ently to demonſtrate, that corporeal
Things exift and indeed it does
not feem capable of a ſtrict Demon-
firation. Šee Malbranch. Annot.
Chap. 10. Book. 1. of his Search after
|
Truth. We must acquiefce in this;
That God has not created us in fuch
a manner,that every Judgement which
we make of Things exifting without
us, ſhould be inevitably falfe. See
Cartes. Princip, part 2. Artic.1.
fes?
Chap. 2. of NATURAL PHILOSOPHY.
fes; for all that theſe can do, is only to be the Occafion of
knowing them; and it is chiefly from Reafoning that we
are affured of their Exiſtence.
1
there are ma-
37. In the fame manner as we conclude from one fin- 37. How we
gle Senfation, that one Thing exiſts; we conclude alfo from know that
different Sorts of Senfations, that there are different Things nyforts of Bo-
exifting; all which, becauſe we imagine them to be ex- dies exifting.
tended in Length, Breadth, and Thickness, we call
Bodies.
38. How we
come to the
Knowledge of
our own Body
38. Amongſt theſe Bodies, there is one which we con-
fider differently from the reft, and are obliged, in a fpeci-
al manner, to look upon as our own; not only becauſe it
is always prefent with us, but alfo, becaufe, when any in particular.
Alteration is made in it by other Things, it caufes certain
Senfations in us; and on the other hand, certain Thoughts
in us, produce certain Alterations in that. Thus if I will
to move my Arm, it is prefently moved; but if I will
to move another Body, that will not be put into motion
by my Will alone.
that there arE
us, as me
39. We may further obferve, that after the foregoing 39. We are
Reflections have convinced us that our Body is compo- not to think
fed of many different Parts, fome of which are the Or- as many
gans of Senfation; the different Senſations we have, are Things exift-
no longer a certain Proof of the Exiftence of a Number ing without
of Things without us: For there is juft Reaſon to fufpect, have differens
that the fame Object may raiſe different Senfations in us, Senfations.
by acting upon different Organs; and therefore though the
Fire by affecting our Eyes when it is at a great diftance,
raiſes the Senfation of Light; and when it is near, raiſes
the Senfation of Heat by affecting our Hands; yet we
cannot from hence collect the Exiſtence of more than
one Object.
40.
There is another Miſtake contrary to this, which it is
eafy to fall into, and therefore ought to be avoided. For
does it not feem reaſonable to determine with Affurance
the Exiſtence of many Things, without any danger of be-
ing deceived, if. in making uſe of but one Senſe, and em-
ploying it in but one manner only, it repreſents to us ma-
ny Objects at the fame time? Now that we may not be
deceived here alfo, we ought to confider the Medium
through which the Action of the Object is tranſmitted;
for Example, a multiplying Glafs makes us fee many Ob-
jects at once, when there is only one that really affects
our Eyes; which fhows, that here alfo we may be de-
ceived.
41. Theſe
40. A Pre-
order to be
certain of s
Number of
cartion, in
Things.
12
Part I.
ROHAULT''s SYSTEM
41. The Sig 4L. Theſe two Obfervations teach us, that we ought
nification of not to judge rafhly, nor at firft Sight, that a Number of
which we give Things exift: However, after having taken all the Precau-
the Names
to many
Things.
tions requifite, when we are once plainly and fully con-
vinced of their Exiſtence, by Means of the different Sen-
ſations which they raiſe in us; we cannot help arguing from
the Act to the Power, as Philofophers call it, which is
very natural to all Perfons; and thence concluding, that
thofe Things have within them a Power to affect our
Senſes: And hence it is, that we give Names to thoſe
Things, fignifying fuch different Powers. Thus a Body
which raiſes Heat in us, we call a hot Body; and the bare
Power of raiſing this Senfation in us, we call the Heat of
this Body.
42. Whence it is plain, that they are deceived, who, bẹ-
42. A Mif-
ake about the fore they have ftudied Philofophy, underſtand thefe Words
Signification in a larger Senfe than was faid before; for example,
of Words.
who, when we mention the Heat of the Fire, imagine
preſently, that there is fomething, I know not what,
in the Fire, like that Heat which the Fire raiſes in us;
for the giving of a mere Name only to a Thing unknown
does not at all make that Thing known to us.
43. Another
Miftake.
43. They alfo deceive themíelves as foolishly, though
to appearance they are more acute, who, in order to
prove that there is in the Fire fomething, I know not what,
like that Heat which it excites in us, bid us go near it
and try: Now, though we a thouſand times go near it,
nay, though we were ſcorched by it, all that this de-
monftrates, is only what the Fire does to us, and not
what it is in it felf. When we ſpeak therefore of the
Heat, or Cold, or Smells, or Sounds, or Light, or Colours of
Bodies, to ſay, that they are really Things which are properly
Objects of our Senfes, is a great Miftake. For he who
fays
this, muſt imagine, that we come to the Know-
ledge of them by bare Senſation only, which is abfo-
lutely falfe.
#
СНАР.
Chap. 3. of NATURAL PHILOSOPHY.
CHA P. III.
The Manner of applying Philofophy to particular
Subjects.
HE Obfervation which we have now made, is of, 1. W may
have no Pre-
fo great Importance, that it alone fhows us the true
judices in
Method of Philofophy on particular Subjects: For from Philofophy.
hence we learn, that in order to find out what the Nature
of any Thing is, we are to ſearch for fome one Particular
in it, that will account for all the Effects which Experi-
ence ſhows us it is capable of producing. Thus, if we
would know what the Heat of the Fire is, we muſt en-
deavour to find out fome particular Thing, by means of
which, it is capable of producing in us that Sort of Tick-
ling, or pleaſant agreeable Heat which we feel at a little
diftance from it; and that Sort of Pain, or fcorching Heat,
which we feel when we approach too near it; and the
fame Thing muft alfo explain to us, how the Fire comes
to rarify fome Bodies, and to harden others, and to diffolve
others: In a word, it muſt explain all the Effects that Fire
produces. And in order to this, we are principally to
guard against any Prejudices we may have entertained con-
cerning it; and not immediately to imaging that there is
in the Fire the fame kind of Heat, whether pleaſant or
fcorching, which we feel, when at a diftance, or near to it.
For indeed, there is no more reaſon to attribute fuch fort
of Heat to the Fire, than there is to afcribe the fame
fort of Pain to the Needle, which we feel when we are
prick'd by it; and as he would without all doubt be de-
ceived, who ſhould aſcribe the fame Pain that we feel to
the Needle; and would after this, labour to no purpoſe,
in trying to find out the Nature of it; fo likewife would
it be in vain, after having aſcribed to the Fire that fort
of Heat which we our felves feel upon that Oscafion, to
attempt philofophically to explain the Nature of Fire;
for nothing folid can be built upon fo bad a Foundation,
nothing but Conjectures and Chimera's.
are to be al-
2. What is now faid of Heat, may be applied to all 2. When o
other Things: And by this Rule, every Thing hereafter is Conjectures
to be examined, If that which we fix upon, to explain lowed, and
the particular Nature of any Thing, do not account clear- when not,
ly and plainly for every Property of that Thing, or if it
be
evidently contradicted by any one Experiment; then
We
ROHAULT's SYSTEM
Part I.
}
3. We must
for the most
part be con-
tent with
Probability.
A
Conjecture
may be allow-
ed very pro-
bable.
per-
we are to look upon our Conjecture as falfe; but if it
fectly agrees with all the Properties of the Thing, then
we may efteem it well grounded, and it may pafs for ve
ry probable.
3. Thus we muſt content our felves for the moſt part,
to find out how Things may be, without pretending to come
to a certain Knowledge and Determination of what they
really are; for there may poffibly be different Caufes ca-
pable of producing the fame Effect, which we have no
Means of explaining.
4. When a 4. Now as he that undertakes to decypher a Letter,
finds out an Alphabet fo much the more probable, as it
anfwers to the Words with the feweft Suppofitions; fo
we may affirm of that Conjecture concerning the Nature
of any Thing, that it is the more probable, by how much
the more fimple it is, by how much the fewer Properties
were had in view, and by how much the more Properties,
different from each other, can be explained by it. Thus,
for Example; if having taken notice only of four Proper-
ties of a Thing, we form fuch a Notion of it, that the
Conjecture we make to explain them, will hold as ftrong
for twenty Properties which we find to be in it; it is cer-
tain, that theſe are fo many Proofs that our Conjecture
is very good.
5. When a
Conjecture is
Such as may
F
5. And indeed there may be fo many, and fo very dif
ferent Properties in the fame Thing, that we fhall find it
be allowed for very difficult to believe, that they can be explained two
different ways. In which Cafe, our Conjecture is not
only to be looked upon as highly probable, but we have
Reason to believe it to be the very Truth.
a Truth.
6. We ought
not too easily
to part with
that is well
grounded.
:
6. Laſtly, To prevent any Scruples that may afterwards
arife, we muft confider, that, if our Conjecture be other
a Conjecture wife well grounded, it does not lofe its Probability, be-
cauſe we cannot upon the Spot explain by it a Property,
which appears from fome riew Experiment, or which we
did not before think of: For it is one Thing to know cer-
tainly, that a Conjecture is contrary to Experience; and
another Thing, not to fee how it agrees to it; for though
we do not at all fee the Agreement, it does not from thence
follow, that it is repugnant. And it may be, though we
don't fee it to Day, we may fee it to Morrow; or others
who can ſee further than we, may at one time or other dif
cover it. Thus, as we fhall fee* afterwards, Teleſcopes
which were not in ufe till our Days, have confirmed the
Hypothefis of Copernicus, concerning the Motion of Venus and
Mercury, which feemed not very well to agree with the dif
ferent Magnitude of Venus at different times.
* Part II.
Chap. 14.
Artic. 7.
:
CHAP
Chap.4. of NATURAL PHILOSOPHY.
5.
SINC
CHA P. IV.
A Caution concerning Words.
INCE we are accuſtomed to connect our Thoughts 1. That we
with our Words, and oftentimes attend more to the ought to accid
Words, whofe
Words than to the Things fignified by them; that we Meaning we
may not for the future be led into Miftake by Words, we don't under-
ſhall not make uſe of any here, nor have regard to any, frand.
whoſe Meaning we do not clearly underſtand. Where-
fore in this Treatife we fhall wholly neglect fuch fpecious.
Words as Antiperiftafis, Sympathy, Antipathy, a Defire of
Union, Contrariety, and the like. And as we do not uſe
them our felves, fo we ſhall have no regard to them from
others, unleſs they tell us, very clearly and diſtinctly, what
they mean by them, and how we are to underſtand
them.
Left therefore we ſhould fall into that Fault which we
condemn in others, we fhall here define the Terms of Art,
which, after the Example of moſt Philofophers, we thall
make uſe of.
2. The Word Being fignifies only that which is cr exifts;
for that which does not exift, is indeed nothing. For if
any Thing be to exift next Year, we may affirm, that at
preſent it is nothing, and it is only the Idea which we
have of it, that is any Thing.
2. What is
meant by
Being.
3. What is
Subftance.
3. We underſtand by Subftance here a Thing which we
conceive to ſubſiſt of it felf, independent of any other meant by
created Thing: Thus a Piece of Wax is a Subftance, be-
'cauſe we conceive it to fubfift of itſelf, independent of
any other created Thing.
4. Obferve here, that I don't fay abfolutely, that a Sub-
ſtance is a Thing which fubfifts of it felf; but that it is
a Thing which we conceive to fubfift of it felf, which I fay
on purpoſe to make this Definition of uſe. For though
I know very well, that our Conceptions or Imaginations.
lay no Neceffity upon the Things themselves, yet they are
neceffary towards our judging of them, becauſe we know
Things from our Idea's only, and we ought always to judge
according to our Thoughts.
4. That me
ought to judge
of Things ac-
cording to our
Ideas of them.
5. We call that a Mode, or Manner of Exifting, or an 5. VVhat is
Accident, which we conceive neceffarily to depend upon meant by #
fome Subſtance. Thus, becauſe we cannot poffibly con-
ceive the Roundness of a Globule of Wax to fubfift with-
out
Mode.
1
I
· Part I.
· RÖHAULT'S SYSTEM
out the Wax, therefore we call it a Mode or Manner of
exifting, or an Accident.
6. That a 6. From whence it follows, that a Mode, or an Acci-
Mode mot dent, cannot be transferred from that Subftance which is
be transferred
from one Sub- the Subject of it, to any other Subftance; for if it could,
ject to ano- it would not then have depended entirely upon the firſt
Subſtance when it was in it, which is abfurd.
thera
7. What is
meant by a
Quality.
8. That the
has not a de-
but is howe-
ver uſeful.
7. By the World Quality we mean that, by which a
Thing is denominated fuch; Thus that in the Fire, what-
ever it be, which has a Power to raiſe the Senfation of
Heat in us, we call a Quality of the Fire, becauſe it is
from this that the Fire is faid to be hot.
8. That which is to be feared here, and which hath
word Quality made fome over-fcrupulous Perfons with that this Word
terminate were never uſed, but wholly fuppreffed, is, that fome Men
Signification, foolishly think, that they are very knowing, if they can
but apply this Word, and ſome other of the like Sort, to
exprefs a Thing which they do not at all underſtand.
However, I cannot agree to them, but think it fufficient,
if we do not uſe it in a bad Senfe. For it ſeems to me
(as it did formerly to Ariſtotle) to be very properly uſed
for that in general, whatever it be, which we conceive
to belong to a Subject, and on the account of which, we
give a particular Name to it. Thus, until we clearly and.
diftinctly underſtand what the Heat of the Fire is, we may
call it a Quality of the Fire.
9. What is
meant by the
Words Fer-
sy.
9. The Words Vertue or Faculty, in any Subject, figni-
fy in general, the Power which a Thing has to produce
the or Facul- fome Effect in another Thing. Thus what we juſt now
called a Quality, upon this Account, that the Fire is from
thence denominated hot; may alſo be called a Vertue of
the Fire, if we confider, that it is from this, though we
know not what it is, that the Fire can heat any Thing.
10. What
10. The Effence of a Thing, is that which it principal-
the Effence of ly is, or that which conſtitutes the Nature of it, and by
a Thing is.
which it is what it is: Thus the Effence of a right-lined
Triangle confifts in this, that it is a Figure terminated by
three right Lines. From whence it is evident, that al-
lowing the Effence of a Thing, is allowing the Thing it
felf; and on the contrary, taking away the Effence, is ta-
king away the Thing it ſelf.
11. What
the effential
Property of
Thing is.
a
II. We call that an effential Property of a Thing, which
we conceive fo to belong to the Thing, that it is the
neceffary Confequence of its Effence: Thus, that any two
Sides together, are longer than the Third; and that the
! three Angles are equal to two right ones, are Properties
that
Chap. 4. of NATURAL PHILOSOPHY.
17
}
that belong to the Effence of a Triangle; becauſe theſe ſo
belong to it, that they are a neceffary Confequence of a
Figure's being terminated by three right Lines. So like-
wife it is the effential Property of a right-angled Triangle,.,
to have the Square of the Side oppoſed to the right Angle,
equal to the Squares of the two other Sides; becaufe
this fo belongs to this Sort of Triangle, that it neceſſari-
ly follows from its being right angled.
12. We call that an accidental Property of a Thing, or
in general an Accident, which we do not think neceffary
to it; or which fo belongs to it, that it might have been
without it, and yet not ceaſed to have been what it was:
Thus the Blackneſs in a Triangle is an Accident, becauſe
this Colour is not neceffary to a Triangle; and it may
be not Black without ceafing to be a Triangle.
13. The Production of Something which before was
not, we call Generation; thus we fay Fire is generated,
when we fee Fire where the Wood was before; fo like-
wife we fay a Chicken is generated, when we fee a Chicken
in the room of an Egg.
is a
14. When a Thing is deſtroyed, or ceaſes to be what
it was before, we call it Corruption; thus we fay it is
Corruption of the Wood, when we fee the Wood no lon-
ger, but only the Fire in the Place of it; And in the fame
manner we fay an Egg is corrupted, when we fee the Egg
no longer, but a Chicken in its Place.
15. A Thing is faid to be altered, when it has under-
gone fome Change, but not fo great a Change as for us
not to know it again, or to have a new Name given to
it: Thus when a Piece of Iron, which was before cold,
is made hot, it is faid to be altered; for this Change is not
fo great, but we know it to be Iron ftill, and do not
give a new Name to it. We muſt take particular Notice
here, that the Alteration must make but a moderate
Change; for if it be fo great, that we cannot know the
Thing thus changed, we do not then fay that it is altered,
but that it is corrupted.
12. What
the accidental
Thing is.
Property of a
13. What is
meant by the
Word Gene-
ration.
14. What is
meant by that
of Corruption.
15.
What is
meant by the
Word Alts
ration.
16. By the first Principles of natural Things, we under- 16. What is
ſtand, that which is first, and moft fimple in them, or that meant by the
of which they are originally compofed, and beyond which ples of nat
first Princi-
they cannot be reduced. Thus, the first Principles of a ral Things,
Chicken, are thofe Things which are united together to
compofe a Chicken, and which are fo fimple, that they
themſelves are void of all Compofition.
C
17. Now
18
Part I.
ROHAULT'S SYSTEM
17. That the
17. Now I do not pretend that the foregoing Derini-
forementioned tions contain any fecret Things in them, nor do I defign
Terms fignify
no more than they fhould paſs for Things very fublime, as fome Philo-
is contained fophers have done; but on the contrary, my principal De-
in the Defini-
tion of them. fign in laying them down here, was no other than to ex-
plain the Meaning of the Terms which I have defined ſo
diſtinctly, that no one might be deceived, in putting any
other Senſe upon them more enlarged or reftrained; and
to do it in fuch a manner, that no Fictions might be
made out of them.
18. A Canti-
18. I fhall here add one Caution about Words, and it
on about the is this, That though thoſe which we call Nouns fuftantive
Meaning of were invented to fignify Subftances; and Adjectives and
fome Nouns
Subftantives. Verbs properly fignify only Qualities or Modes, or Man-
ners of exifting or acting; yet there are a great many
Words, which in Grammar pafs for Nouns fubftantive;
whofe Signification is the fame as that of Verbs. Thus
when we fay that a Walk is wholeſome, we mean no more
than that it is whole fome to walk.
19. An Er-
from want of
attending
hereto.
19. For want of attending to this Rule, the Generality
Tour arifing of young Men, when they begin to ſtudy, take the Things
fignified by theſe Sort of Nouns fubftantives, to be real
Beings, and imagine them to have a particular Exiſtence,
and by this Means fill the World with Scholaftick Entities
and rational Entities, which they are many times fo poſ-
feffed with, that they become incapable, all their Lives after,
of applying themfelves to any Thing that is folid and
fubftantial.
The Forn-
dation of na-
CHAP. V.
The principal Axioms of Natural Philoſophy.
AF
FTER having explained the principal Terms made
uſe of in natural Philofophy; I fhall now lay down
tural Philofo- fome important Truths, which are felf-evident, and which,
being the Foundation of all Philofophical Truths, are con-
fequently the principal Axioms of Philoſophy.
phy.
2. Axiom I.
2. The firft is, that Nothing, or that which has no Ex-
istence, has no Properties. Thus we cannot fay that No-
thing is hot, or cold, can be divided, or has Parts, &c.
Therefore where we know there is any Property, whate-
ver it be, there we may affirm, that there is fome Thing,
fome real Being.
3. Se-
Chap. 5. of NATURAL PHILOSOPHY.
19
3. Secondly, It is impoffibie that Something hould be made 3. Axiom II.
of abfolute Nothing; or that mere Nothing can become any
Thing. This Axiom is a neceffary Confequence of the
foregoing one, and proves it felf to them who grant that.
For if Nothing can be made Something, it would follow,
contrary to the preceeding Axiom, that Nothing has fome
Property: Which is abfurd.
4. When I faid that it is impoffible for Something to be
made of Nothing, I exprefsly added the Word Abfolute,
becauſe I do not at all doubt, any more than any other
Perſon, that a Thing may be made out of what has no-
thing of that Thing in it, or to fpeak more clearly, may
be made out of that which is not that Thing. Thus for
Example: No one can doubt, but that Bread may be
made of Water and Meal, which are not yet Bread.
4. In what
Senſe it may
be faid, that
any Thing is
made of No-
thing.
5. Thirdly, No Thing or Substance can be wholly annihi- 5.Axiom III.
lated; that is, fo ceaſe to be, that there shall remain nothing
at all of it. Indeed; when any thing wholly diſappears,
we eaſily apprehend, that it ceaſes to be the Thing that it
was, in order to become fome new Thing: Thus we ea-
fily apprehend, that Corn ceaſes to be Corn, in order to
become Meal, and that every Part of the Meal may be
ftill divided into other Parts, fo fmall that they may be
utterly imperceptible; but how that which is Something,
can become abfolutely Nothing, this is utterly uncon-
ceivable.
6. Fourthly, Every Effect presupposes fome Caufe. This 6. Axiom IV.
is fo generally allowed by all the World, that the dulleſt
of all, are led to admire certain Effects, for that very
Reaſon, becauſe they are perfwaded that they proceed
from a Cauſe, and that this Cauſe is wholly unknown to
them. If this was not a very true Axiom, we ſhould not
fo much wonder at that moſt known Property of a Load-
ſtone for Example; but reft fatisfied, with knowing only
that the Iron does really approach the Loadſtone, without
withing for any Thing further.
7. Fifthly, Which is a Confequence of the foregoing 7. Axiom V.
Axiom; If we our felves are not the Caufe of any Effe&t, it
must neceffarily depend upon fome other Caufe. Thus, if I
know certainly, that a particular Effect which is within
my own felf, does not depend upon me; I certainly con-
clude, that it depends upon fome other Caufe.
8. Sixthly, Every Thing, as much as it can, endeavours to 8. Axiom VI.
continue in that State in which it is. Thus, if any Thing
be fquare, it will continue always fquare, and will never
of its own felf become round, or any other Figure. This
C 2
is
ROHAULT'S SYSTEM
Part I.
9. AxiomVII.
10. Axiom
VIII.
II. That
ny more Ax-
is what others mean, when they fay, that Nothing tends to
the deſtroying of it ſelf.
9. From whence it follows, Seventhly; That every Al-
teration is made by fome external Caufe. Thus if we fee a
Flower in a Garden very freſh in the Morning, and in the
Evening find it withered; we conclude, that either the Sun,
or the Wind, or perhaps fome Perfons roughly handling of
it, have cauſed this Change, and though we could not at
all gueſs what it was that had made this Change; yet we
fhould aſcribe it to fome Caufe.
10. Eighthly, Every Alteration is always proportio-
nable to the Force of the Agent which caufes it. So that
the Thing which is altered continues, as much as it can,
in its firft State. Thus if a Body, which moves flowly,
comes upon another Body at reft, and puſhes it before it,
we cannot think that it can move this latter Body ſwifter
than it goes it ſelf.
11. There are yet more Axioms which I fhall afterwards
there are ma- draw many Conclufions from; but becauſe they are not
fo general as thefe, I fhall content my felf with men-
tioning them, when I have occafion to make uſe of
them.
10ms.
12. That
Things are
of in their
Batural
State.
22
12. But before we proceed any further; as my De-
bere treated fign is to treat of natural Things, and to explain as well
the Cauſes by the Effects, as the Effects by the Cauſes
that I may not go beyond the Limits of my Subject, but
contain my ſelf within the Bounds of the Science I treat
of; I expreſsly declare, that my Defign is to confider
Things in their ordinary and natural State, and that I pre-
tend not to ſay, or determine, what they are, or may be,
in an extraordinary or preternatural State: Becauſe, I think,
it is great Raſhneſs to undertake to determine, how far
the Power of God can extend it felf, whom I acknow-
ledge to be the Author of every Thing in the World,
and who, I believe, can make a Multitude of Things.
above the Capacity of humane Underſtanding.
13. That we
ought not to
fay, that there
is any Thing
which God
cannot do.
13. Wherefore I will never venture to affirm, that
there is any Thing impoffible with God; and inſtead of
fpeaking in fuch a manner, which is too common amongſt
Philofophers, I will content my felf, with only faying, that
fuch a Thing is not of the Number of thofe Things which
I know he can do.
1 Swifter than it goes it felf.) Un-
lefs it be endued with an elaftick
Force, which is to be underſtood
as an Addition of new Force. See
below, Chap. xi. Art, 6.
14. And
Chap. 6. of NATURAL PHILOSOPHY.
be too inqui
14. And above all Things, I particularly guard my felf 14. That we
2gainſt enquiring into the Mysteries of Faith, and attempt- ought not to
ing to explain what is obfcure therein; becauſe I am fitive into
firmly perfwaded, that that which God Almighty would Myfteries.
have to be a Myftery to the Ignorant and Unlearned, he
would have to be fo likewife to the moſt exalted Genius,
and to them who think themſelves much greater Philo-
fophers than I am.
CHAP. VI.
Of the Principles of Natural Things.
IN of
ter.
N order to know what the Principles are, of which 1.0f Mat-
natural Things are compofed, we may take one par-
ticular Effect for a Rule, and examine that; as for Exam-
ple, what is done, when the Wood is converted into Fire:
For by this Means, it will be eafy to judge, what paffes in
other Productions of Nature; and this will, as it were,
lead us by the Hand, and help us to diſcover what natu-
ral Principles are, and how many there are of them. Firſt
then, becauſe, according to the Maxims before eſtabliſh-
ed, it is impoffible to conceive the Wood to be wholly
annihilated, or the Fire to be made out of abfolute No-
thing, therefore we muſt think, that there is Something
which before belonged to the Wood, which now belongs
to the Fire, and is therefore common to them both. Now
this, whatever it be, that fubfifts under theſe two Forms,
we call Matter, as others call it; fo that Matter is one of
the Principles of natural Things.
2. Secondly, We apprehend alſo, that there muſt necef- 2. Of Form.
farily be fomething elfe added to Matter, which makes it
to be Wood and not Fire, or to be Fire and not Wood;
and whatever this be, which does not cauſe Matter to
exiſt, but only to exist in that manner, we call it the
Form; and this we reckon another Principle of natural
Things.
3. Ariftotle obferved, that though a Thing could not 3. That Pri-
be made abfolutely out of Nothing, it might however be vation ought
to preceed the
made out of what was not that Thing. Thus a Chicken Generation of
may be made out of that which is not now a Chicken;
fo that the Non-exiſtence of a Thing ivhich he calls
Privation, muſt immediately preceed the Generation of
ЁЗ
it :
A
Thing.
22
Part I
ROHAULT'S SYSTEM
it: From whence he concludes, that there are three
Principles of natural Things, Privation, Matter, and
Form.
4. That Pri 4. But by making Privation a Principle, the Word
vation ought Principle becomes ambiguous, and quite another Mean-
led a Princi- ing is given to it, than when we faid of Matter and Form,
not to be cal-
ple.
5. That there
are only two
Principles,
viz. Matter
and Form.
6. That it is
neceffary
rightly to un-
derstand
what Matter
and Form are.
that they were the Principles of natural Things; for it is
certain, that Privation is not at all a Thing, nor does it
go to the Compofition of any Thing.
5. Befide, there is no Reafon to make a particular Myf-
tery of this Word Privation; for there is no Body but
knows what it means; and fince it is of no ufe to explain
natural Things by, we conclude, that there is but two
Principles of natural Things, viz. Matter, and Form.
6. But we have not yet made any great Advances in
the Knowledge of the Things of Nature: For, he is ve-
ry far from underſtanding the Nature of Fire, who knows
only thus much, that Matter is neceffary to the Compo-
fition of it, that is, it has fomething, we know not what,
in common with other Things; and that a Form is alſo ne-
ceffary to it, that is, another Something, we know not what,
which gives that particular Existence to the Fire; for, as
was obſerved before, a Thing that is unknown, does not
become known, by giving a Name to it; we muſt there-
fore confider more diftinctly, what Matter and Form par-
ticularly are. We will begin with Matter, and try to find
out what that is, which we call we don't know what, which
is common to all the Things in Nature.
1. The Me-
thod of find-
ing out what
Matter is.
CHAP. VII,
Of Matter.
INCE there are but three Things neceffary to a per-
fect Underſtanding of any Thing, viz. ics Effence, its
Properties, and its Accidents, that we may comprehend
fully what Matter is, we muſt diſtinctly explain what the
Effence of it confifts in, what the Properties of it are, and
what Accidents it is capable of; in order to which, we
have no more to do, but to examine all that we conceive
any way to belong to material Things, confidered as ma-
terial, that is to belong to Matter; and then exactly to
diftinguish its Effence, from its Properties, and Accidents.
3. Now
Chap. 7. of NATURAL PHILOSOPHY.
23
dents which
2. Now according to this Method, if we confider, that 2. The Acci
though we do not perfectly underſtand what Hardness, Li- Belong to
quidity, Heat, Cold, Heaviness, Lightness, Tafte, Smell, Sound, Matter.
Light, Colour, Transparency, Opacity, and the like, are;
yet we underſtand enough of them, to know, that they
are none of them infeparable from Matter, that is, it may
exiſt without any, of them, (for we fee that fome materi-
al Things are without Hardness, fome without Liquidity,
fome without Heat, and fome without Cold, and fo of the
reft,) wherefore we fay, that the Effence of Matter does
not confift in any of thefe Things, but that theſe are ac-
cidental only.
to Matter.
V
3. But when we confider Matter as extended into Length, 3. That Ex-
tenfion is not
Breadth, and Thickness; as having Parts, and thofe Parts accidental
having fome Figure, and that they are impenetrable, we do
not judge in the fame manner of thefe, nor think them
mere Accidents of Matter, For,, as to Extenfion, it is
certain, that we cannot feparate the Idea of that, from
any Matter whatſoever; becauſe if Extenfion does not go
along with it, we immediately loſe the Idea of Matter, in
the fame manner as the Idea of a Triangle vaniſhes, if
we ceaſe to have in our Minds the Image of a Figure ter-
minated by three Lines.
4 As to the Parts of Matter, we apprehend them to
belong to it ſo neceffarily, that we cannot imagine any
Portion of it fo fmall, be it the fmalleft we can conceive,
but that if it be put upon a plain Superficies, we muſt
think at the fame time, that it touches it in one
Part, and does not touch it in another; that is, this ſmall
Portion of Matter, confifts of Parts.
4. To have
Parts, is not`,
accidental to
Matter.
accidental to
5. With respect to Figure, though it be nothing elfe 5. That Fi
but the Difpofition of the extreme Parts of a Body, and gure is not
perhaps we cannot determine the particular Figure of a Matter.
particular Body; it is however manifeft, that we cannot
conceive any Body, be it ever fo great, or ever ſo ſmall,
but at the fame time we conceive it to have fome
Figure.
6. Lastly, With regard to Impenetrability, fince a cer-
tain Portion of Matter, fuppofe a cubic Foot, has all that
is neceffary to fuch a Magnitude, we cannot conceive how
another cubic Foot can be added to it, without making
two cubic Feet: For fuppofe any one would reduce them
to one cubic Foot by Penetration, this would not be fo
much reducing them to one cubic Foot, as it would be
deſtroying the firſt Suppofition; whence we are led to
think, that the Parts of Matter are in their own Nature
impenetrable.
7. Now
C +
L
6. That Im-
penetrability
cident of
Matter.
is not an ac-
:
!
2
24
ROHAULT's SYSTEM. Part I.
7. Of the Ef-
Sential Pro-
perties of
Matter.
8. What the
Effence of
Matter con-
fifts in.
9. In what a
natural Phi-
to acknow-
ledge the Ef
fence and ef
Sential Fro-
perties of
Matter to
Lonfift.
7. Now this being fo, we muſt fay, that Extenfion, Di
vifibility, Figure, and Impenetrability, are, at leaſt, effential
Properties of Matter, becauſe they always go along with
it, and cannot be feparated from it; and theſe being all
that we conceive to belong to Matter neceffarily, for we
know of nothing more, we are affured, that the Effence
of Matter confifts in one of theſe.
8. And becauſe we conceive Extenfion before the other
Three, and becauſe we cannot conceive the other Three,
without firſt ſuppoling Extenfion, I
we ought to think
that Extenfion is that in which the Effence of Matter
confifts.
9. If it fhould be here objected; That God could make
lofopher ought Something to be the Effence of Matter, which neither we,
nor any Man living, can underſtand what it is; we can
make no other Anfwer, but only this; that God, being
Lord of all Things, might create them according to, his
own Will; for we do not pretend to determine by our
Reaſon, that which Reaſon cannot come at. Wherefore
leaving fuch Sort of Queſtions to be treated of by thofe,
who are of a higher Profeffion than that of mere natural.
Philofophy, and who carry their Views far beyond what
Reaſon can do; we fhall contain our felves within the
Limits which that preſcribes, without invading the Ter-.
ritories of others; and conclude from that Knowledge
which we have by Reaſon, that the Effence of Matter con-
fifts in Extenfion, becauſe that is what we first perceive
in it, and from which every Property of Matter is derived,
and upon which it depends.
1
I.
|
We ought to think Extenfi- feftly flow, may be more truly called
on, &c.) It does no more feem to the Effence of Matter.
follow from hence; that, becauſe we
conceive Extenfion before any other
Properties of Matter, and that thoſe
Properties can't be conceived to ex-
ift, without first conceiving Extenfi-
on; therefore Extenfion is the Ef-
fence of Matter; than it follows from
hence, that Existence is conceived be-
fore all other Properties of Matter,
and therefore Existence is the Effence
of Matter. But fince Extenfion is a
more general Word, and compre-
hends more under it than material
Things, it ſhould feem, that that im-
penetrable Solidity which belongs to
all Matter, and to Matter only, and
from which all its Properties mani-
But further, if Extenfion were the
Effence of Matter, and fo Matter
the fame as Space it felf; it would
follow, that Matter is infinite, and
neceffarily eternal, and could nei-
ther have been created, nor be redu-
ced to nothing; which is very ab-
furd. Befide, it evidently appears
from Gravity, as fhall be afterwards
explained, and from the Motion of
Comets, and from the Vibrations
of Pendulums, that Space it felf is
not Matter. Wherefore not Exten-
fion, but folld Extenfion, impenetra-
ble, which is endued with a Power of
refifting, may (as was before faid) bẹ
more truly called the Eſſence of Matter.
10. Fur-
Chap.
25
of NATURAL PHILOSOPHY.
¿
a mere Mode.
1. Further, that we may carry our Knowledge as far 10. That Ex-
as the Light of Nature will permit, let us confider that tension is not
the Idea of Extenfion is fo far from depending upon any
created Thing, that we can scarce get it out of our
Minds, when we try to imagine Nothing, which we be-
lieve was before the Creation of the World; which
ſhows that it does not depend upon created Things, that
it is not a Confequence nor a Property of them, much
lefs is it an Accident or Mode of exifting, but a true
Subſtance.
agrecable to
11. It is generally believed, that this is very different 11. That this
from the Opinion of Ariftotle, becauſe he fays in his Me- Notion is no
taphyficks, that Matter is not a Thing that can any way the greatest
anfwer to Queſtions which relate to Effence, Quantity, or part of them,
Quality; and indeed, that it is not a certain determinate who call
Thing, This the Ariftotelians, for the moſt part, fo in- the Disciples
terpret, that they would have us think that Matter is not of Ariftotle.
at all extended, nor has any Exiſtence.
themſelves
contrary to it.
12. But Ariſtotle ſeems in this Place to ſpeak of Mat- 12. That his
ter in general; for he exprefsly diftinguiſhes between Ex- Opinion is not
tenfion and Quantity, as every one ought, becauſe we
can conceive the one without the other. Thus, for Ex-
ample, a Surveyor of Land conceives at firft Sight, that
a Field is extended, but he does not know the Quantity
of it, till after he has meaſured it. Now in this Senſe of
the Word Matter, there is no Inconfiftency in ſaying,
that it may be extended, and yet not be any Thing that
will anſwer to thoſe Queſtions which Ariftotle there enu-
merates; for thoſe Queſtions are to be underſtood only of
Matter under fome particular Form: Thus we cannot
fay of Matter in general, that it is Hot or Cold, that it
contains a certain Number of Feet, or that it is fuch a
particular Thing, as Gold, or Wood, or Marble; any
more than we can fay of an Animal in general, that
it is a Horſe, and not a Dog, or any other particular
Species.
That it is
✡y but Rea-
13. But be this as it will, if Ariftotle was not of this 13.
Opinion, as many of his Interpreters think he was not; not Author:-
we ſhall make no Difficulty in this Matter, to differ from fon, which
him; becauſe we do not govern our felves by Authority, ought to be
when we endeavour to eſtabliſh Things upon Reafon. the Judge of
And there ſeems to me no Reaſon to fay, that Matter,
which is the common Subject of all Things, has it felf no
Exiſtence; for there is no Difference betwixt Non-Ex-
iftence and Nothing, or having no Properties.
14. Some
Truth.
26
Part I.
ROHAULT''s SYSTEM
14. That Ex-
tenſion in
cannot be a
Mode.
14. Some Ariftotelians, who may be fatisfied with this
Anfwer, will perhaps find fault with me, becauſe I call
Length,
Breadth, and Extenfion in Length, Breadth, and Thickneſs, a Sub-
Thickness, ſtance, and not a mere Mode or Accident, as they do.
Thus, for Example, when we ſpeak of the Extenfion of
a Table, they understand that the Extenfion is a Mode,
and the Table the Subftance of it. But it is eaſy to make
appear, that this is a Miſtake ariſing from the Manner of
Speaking; and is altogether as grofs, as it would be, in fpeak-
ing of the City of Rome, to imagine, that theſe were two
different Things, one the Mode, and the other the Subſtance.
But to clear all Difficulty in this Matter, we must ob-
ſerve, that it is of the Nature of a Substance to be able to
exift without its Mode, on the other hand, The Nature of
a Mode is, not to be able to exift without that Subftance of
which it is the Mode. For it is evident, I that the whole
Extenfion of the Table can fubfift without being a Table,
but on the contrary, there can be no Table without Ex-
tenfion. Wherefore, fo far ought we to be from ſaying,
that Extenfion is a Mode of which the Table is the Sub-
ftance, that we ought to fay, on the contrary, that Ex-
tenfion is the Subſtance, and the Form of the Table
the Mode.
15. Whence
it is that na-
aral Philofo-
phy has been
hitherto fo
barren.
15. Lastly, They who deny Extenfion to be the Effence
of Matter, cannot diftintly tell us what they mean by
Matter, nor in what its Effence confifts; and they lay
down ſo obſcure a Thing for a Principle, that it is im-
poffible to draw any Confequences from it, that can en-
lighten our Minds, or ferve to clear up any Truth.
Wherefore we need not be furprized, that their Philo-
fophy is fo barren, and that it is not capable of explain-
ing the ſmalleſt Effect in Nature. Let us now ſee if the
fame may be affirmed of the Principle which we have
maintained.
1. That the whole Extenfion of the
Table, &c) Yes, if neither the Table,
nor the Matter it ſelf, or Subſtance of
the Table exiſted. This Inftance
therefore does not prove, that Exten-
fion is that Subftance or Matter of
the Table, but that there muſt necef-
[farily be fome Subſtance ſubſiſting un-
der the Form of the Table, which is it
ſelf extended; which extended Sub-
ſtance is not Extenfion it felf, but
fubfifts in Extenfion or extended
Space.
1
· CHAP.
Chap. 8., of NATURAL PHILOSOPHY.
27
FR
CHAP. VIII.
Some Corollaries of the foregoing Notion.
that impoffible
For
there should
but
be what the
Philofophers
call a Vacu-
um.
ROM what we have now laid down concerning the 1. That it is
Effence of Matter, we infer in the firſt place,
what the Philofophers call a Vacuum cannot poffibly be:
by a Vacuum they mean a Space void of all Matter;
by Space (or Extenfion) we mean the fame Thing
Matter; and to ask if there can be any Space without
1. That what Philofophers call a
Vacuum, &c.) This it confiftently e-
nough faid by him, who affirms the
Effence of Matter to be Extenfion :
But it is very evident from Gravity,
(which thall afterwards be briefly ex-
plained) that there muſt not only be
à Vacuum in Nature, but that it is
the far greateſt Part.
*
Befides, a Vacuum, as I faid now,
is demonftrated from the Motion of
Comets. For fince the Comets are car-
ried with a continual Motion through
the Heavenly Spaces, from every Part,
and all Ways, and to all Parts (in
Orbits which cut the Orbits of the
Planets tranfverfely every way) it is
evident from thence, that the Heavenly
Spaces, muſt be void of any fenfible
Refftance, and confequently of any
fenfible Matter. Newt. Optic. p.310.
See alfo the Notes on Part II. chap.
75,26.
as
projectile Bodies is infinitely diminif
ed, by the infinite Divifion of the
Parts of the Fluid; (Princip.Book II.
Prop. 38. Corol. 2.) For on the con-
trary, it is evident, that the Refift-
ance can be but a very little diminish-
ed, by the Divifion of the Parts of the
Fluid (Ibid. Frop. 40. Corul. 3.) For,
the refifting Forces of all Fluids are
very nearly as their Denfities. For why
fhould not the fime Quantity of Mar-
ter, make the fame Refiftance, whe-
ther it be divided into a great many
very fmall Parts, or into a few large
ones? Wherefore, if there were no
Vacuum, it would follow, that a Bo-
dy moved in Air, or in a Place out
of which the Air is exhaufted, would
meet with as much Difficulty, as if
it were moved in Quick-filver
which is contrary to Experience, and
therefore it is evident, that there is a
Vacuum in Nature, and (as was faid
before) it is much the greateſt Part.
This is ftill further evident from
the Vibrations of Pendulums, for they Since therefore the Effence of
meet with no Refiftance in Spaces, Matter does not confift in Extenfion,
out of which the Air is exhaufted, but in impenetrable Solidity, we muſt
wherefore it is plain, there is no fen- fay, that the whole World is made
fible Matter in thoſe Spaces, nor in up of folid Bodies which move in a
the occult Pores of the Bodies them- Vacuum. And we need not fear, that
felves. The Fiction of Cartes, that the Phænomena of Nature fhould not
the Smalness of his fubtil Matter is the be fo well explained thereby; for the
Reafon why the Refiftance is infenfi- Explication of thofe Phænomena
ble, for a ſmall Body ſtriking againſt which feem chiefly to depend upon
a large one, cannot move it in the a Plenum, viz. The Barometer, the
leaft, nor hinder its, Motion, but is Flux and Reflux of the Sea, the Mo-
reflected with the Whole of its own tions of the Stars, and of Light, theſe
Motion; this is very weak, and con- can be more eaſily and fully explain-
trary both to Reafon and Experience. ed upon other Principles (as fhall be
For the famous Sir Ifaac Newton has ſhown hereafter;) but as to the other
demonftrated, that the Denfity of fiu- Phænomena of Nature, which de-
id Mediums is pretty nearly in propor-pend upon Caufes not fo general, the
tion to their Refiftance (Opt. p. 311.) Explication of them is the fame in
and that they are very much mista our Syftem as in that of Cartes.
ken, who think that the Refiftance of
Matter,
?
28
Part I.
ROHAULT's SYSTEM
2. What the
"Matter, is the fame as to ask, if there can be any Matter with-
out Matter, which is a manifeſt Contradiction. And it fig-
nifies nothing to fay, that we can conceive a Space, in
which we fuppofe there is no Light, Colour, Hardneſs,
Heat, Weight, in a Word, in which we fuppofe there is
not any one Quality that we can imagine; for when
this is done, and all theſe Things denied of Extenſion,
it is the Accidents only that are taken away from the
Thing, whofe real Effence is at the fame time fuppofed.
2. And here we ſhall not trouble our felves to give an
Confequence Anfwer to any one who fhould put the following Queſtion
to us; Whether God could not by his Omnipotence
annihilate the make a Vacuum, by annihilating all the Air in a Room,
and hindring any more from coming in its Place? For, as
we faid before, it does not belong to us to, determine How
far the Power of God can extend it felf. But if the Que-
ſtion be a little altered, and we be only asked, what we
conceive would follow, if God ſhould annihilate all the Air
in a Room, and not ſuffer any other to enter in its Place?
We thould return for Anſwer, not concerning our felves
with what would come to pafs without the Room, that
the Walls would approach one another ſo near, that there
would remain no Space betwixt them.
would be, if
God should
Air in a
Room.
3. That the
making
3. Perhaps it may be urged by fome, that the Walls of
Difpofition of a Room exiſt independent of what is contained between
the Walls in them, and confequently that they might continue in the
Room, depend State they were, without approaching one another, though
pon the Ex- what is between them were annihilated. To which I
tenfion of the
Matter that anfwer, that it is very true, that the Existence of the
is contained Walls does not depend upon what is contained between
between them. them, but the State they are in, or the Diſpoſition of
4. What is
meant by
Place.
them, in order to compofe a Room, this depends upon
Extenſion, or fome Matter which is between them, and
confequently, this Extenfion cannot be deftroyed with-
out deſtroying the Difpofition which the Walls were in
before, though not the Walls themſelves.
I
4. Secondly, We are to underſtand that internal Place,
or the Space which any Body poffeffes, does not at all differ
from the Body it felf. And therefore when we fay a
Body changes its Place, we mean its external Place, that
is, with regard to the Superficies of other Bodies with which
it is furrounded, to the different Parts of which, it may be
differently applied.
I. Does not at all differ, &c.)
This indeed is not true; but it makes
no difference as to the Explication
of the Phænomena of Nature. For
the true Definition of Place. See
the Notes on Chap. x. Art. 2.
5. Thirdly,
Chap. 8. of NATURAL PHILOSOPHY.
29
>
5. Thirdly, When a Body appears to take up more 5. How Bo-
Room than it did before, without our perceiving any Mat- dies are rare-
fyed and con-
ter to be added to it, which is what we call Rarefaction, denfed.
we ſhall conclude that fome very fubtile Matter has
entered into it, and diſtended its Parts. So likewiſe, when
a Body appears to take up lefs Room than it did before,
without our perceiving any thing to be taken from it,
which is what we call Condenſation, we fhall think that
fome imperceptible Matter is gone out of its Pores, and
that by this means its Parts approach nearer each other.
For fince Extenfion and Matter are to us the fame Thing,
we cannot conceive that a Body fhould appear more or
lefs extended, let the Manner be what it will, but that it
muſt have more or lefs Matter.
6. And this does not hinder, but that we may fay with 6. In what
Ariftotle, that a rare Body is that, which has but a little Senfe it is,
that we fay
Matter, and paffeffes a large Space, and a denfe Body, is that a rare-
that which poffeffes a fmall Space, and has a great deal fyed Body ac-
of Matter, or which is the fame. Thing, that a rarefyed thing, and a
Body does not acquire any new Matter, nor a conden- condensed Bo
quires no-
fed Body loſe any of its own. For this imperceptible dy lofes no-
Matter which we fpeak of, ought to be confidered as a
Thing that is foreign, and which does not at all belong to
the Body it enters into, or comes out of, when it is ra-
refyed or condenſed. Thus when Pafte is turned into
Bread, it is rarefyed before, and while it is baking, yet
we don't fay, becauſe of this, that we have more Bread
than we had Pafte; though it is vifible, that a great deal
of Air is got into thoſe large Spaces which we call the
Eyes of the Bread, becaufe, what is thus got in, is not
what we call Bread: So alſo when we prefs the Crumb
of the Bread in our Hand, and bring it to a lefs Com-
país, though we are fure that a great deal of Air is
fqueezed out of it, yet we don't ſay that there is lefs
Crumb than there was before, becauſe there remains yet
all that we call Crumb, and the Air which went out of
ir, did not belong to it.
thing.
Chefùnt
7. What we have now faid about Rarefaction, may be
7. Whence
thought perhaps hardly to agree with what we experi- it is that a
ence in a Chefnut, which, when put upon the Fire, burfs pen
burſts with a Noife; for it may perhaps be imagined, the Fire.
1. That fome very fubtile Matter,
&c.) When any Body is rarefyed,
it is often very manifeft, that its
Parts are diftended by the Entrance
of the Air, or fome more fubtile
Matter. But this does not follow
from a Plenum, but either from the
Liquidness, or from an elaſtick Force,
or from Gravity and Preffure, or
from fome accidental Motion in that
fubtile Matter which enters into the
Pores of the rarefyed Body.
that
1
ROHAULT'S SYSTEM
Part I
-:
g. That the
that the fubtile Matter which enters through the Pores
of the Husk of the Chefnut, may come out with the fame
eafe as it enters in, without breaking, or making any.
Noife. But this Difficulty is eafily refolved, if we con-
fider, that it is not the foreign Matter that enters in, and
comes out of the Chefnut, which is the immediate Caufe
of the Noife; but the more grofs Parts of the Chelnut
it felf, which are torn in Pieces, and put in fuch Motion, by
the fubtile Matter which enters the Pores like fo many
little Wedges, that they break the Husk with a Noiſe.
}
זי
I
8. Fourthly, We conclude, 2 that the World is indefinite,
World is in- becaufe at how great diſtance foever we fet its Bounds, it
definite.
is impoffible for us not to imagine Extenfion to be till
beyond. Now Extenfion and Matter, being, as was faid
before, the fame Thing; we have no Notion of the
World's being fo big, but we can imagine it to be ſtill
bigger.
9. That it is
impoffible
9. Fifthly, It is evident, that though we can fee no
Reaſon why there may not be many Bodies like to our
Should be ma- Earth, and capable of containing many Animal
that there
ny Worlds.
10. That the
as that
does; yet it is impoffible 3 that there should be many
Worlds; for this, in which we are, poffeffes all that Space
which we are able to conceive.
10. Sixthly, Becauſe the Idea we have of the Extenſion
Matter of the of the Heavens is the fame as that of the Extenfion of
Heavens, and
of the Bodies Things here below, we ought to think 4 that they are of
the fame Kind, and it is no Objection against this, to fay,
Earth, are of that the Extenfion or Matter of the Heavens is brighter,
the Same
upon this
Kind.
and not fo mutable as that of Things here below, becauſe
this Difference regards only the Accidents of Matter and
not the Effence of it.
1. By the fubtile Matter, &c.) Or
rather by the included Air, which is
very much rarefyed by the Heat, and
tears the Chefnut in pieces.
2. That the World is indefinite-
&c.) From the Hypothefis of. a Ple-
num, it muft neceffarily follow, that
the World is really and truly infinite,
nay, that it is uncreated and eternal,
(as was faid before.) But fince it
evident, that Extenfion may exift
without Matter, whether the mate-
rial World be infinite or no, fuch is
the Shortness of humane Underſtand-
ing, that it cannot certainly be
known: Therefore it may very well
be called indefinite ftill.
"
3. That there fhould be many
Worlds, &c.) It is evident, that
there may be many Earths like this
G be of ours, that there may alfo be
many Syftems of Stars and Planets
difperfed through the vaſt Immenſity
of Space; but whether there be a
Plenum or no, the whole Univerſe,
which may properly be called the
Worlds can of Necefity be but one.
4. That they are of the fame Kind,
&c.) This is equally true, whatever
be the Effence of Matter.
11. Lastly,
Chap. 8. of NATURAL PHILOSOPHY.
3 x
contain an e-
11. Lastly, We cannot affirm, that a Veffel filled with 11. That two
Lead contains more Matter than if it were filled with equal Bulks
Wax, though it be heavier; for Heaviness is not effential qualQuantity
to Matter, but only Extenfion, which we ſuppoſe to be of Matter.
equal in them both.
Matter may
12. That Notion alone which we have eſtabliſhed con- 12. That the
cerning the Effence of Matter, has been the only Princi- Properties of
ple we have made uſe of, to anſwer all the foregoing Que- make a Dif-
ftions with fo much Eaſe; whence there is Room to be- covery of ma-
lieve, that we may with the fame Eafe give a fatisfactory ny other
Anſwer to many more, if we reaſon in the fame manner
about any of its Properties: The firft that offers it felf is
Divifibility, which is the more copious, becauſe all its Va-
riety of rigures depend upon it.
1. Contains more Matter, &c.) This
is aboutely falfe, as fhall be fully
demonftrated afterwards, when we
come to difcourfe of the Nature of
Gravity.
Truths.
་
CHA P. IX.
Of the Divifibility of Matter.
HEN
EN
ble.
we confider a determinate Portion of Mat- 1. That Mat-
ter without Prejudice, and compare it with other ter is diviji-
Porcions of Matter with which it is encompaffed, we ea-
fily conceive that its particular Exiftence is wholly inde-
pendent of thoſe that are near it, and that it does not
ceaſe to be what it is, by being joined or united to other
Portions of Matter; the firft Portion of Matter there-
fore is feparable from thoſe with which it is united, and
this ſhows the Divifibility of Matter; and the Poffibility
of having its Parts divided into ftill leffer Particles.
and
rus's Atems,
2. Indeed, when we confider the Power of God, 2. of Epicu
his abfolute Dominion over all Things that are in the and that they
World, we cannot doubt, but that he is able to make are really di-
certain Parts of Matter of ſuch a Nature, that there is no visible.
Being in the Univerſe capable of dividing them; whence it
would follow, that theſe Parts would not at all differ from
thofe little Bodies, which Epicurus calls Atoms: But this
Property of not being capable of being divided by any
external Being, is arbitrary, and not, built upon any na-
tural Principle, but only upon a mere Suppofition, which
does not alter their real Nature; and therefore we may,
notwithſtanding this, hold it for certain, that all Matter
is
32.
Part I.
ROHAULT's SYSTEM
3.That Mat-
ble in all
Points that
ed.
is divifible. The whole Difficulty in this Matter is, how
many Parts a certain Portion of Matter can be divided
into.
3: In order to folve this Difficulty, we must remem-
ter is divifiber, that all the Variety that we can conceive to be in
Matter, ariſes from the Forms which diſtinguiſh its Parts
can be affign- from each other; for of its felf it is perfectly homogene-
ous, that is, all alike, being only a Subftance extended
into Length, Breadth, and Thickness; wherefore we can-
not but think, that whatever it is capable of in one
Part, it is alfo capable of in all other Parts. As there-
fore we cannot doubt but that it is diviſible in ſome
Points, fo alfo is it divifible in all the Points that can be
affigned.
4.
4. That the
able in Mat-
Tab. I. Fig. I.
X
Now that the Number of Points which we can con-
Number of ceive in a determinate Quantity of Matter (an Inch for
Points affign- Example) is indefinite; there are many Demonftrations
ter, is indefi- in Geometry to fhow, one of which I fhall give, which
nite, and that feems to me very easy. Let two indefinite Lines AB, CD,
Matter is in- be drawn parallel to each other, and at an Inch diſtance;
definitely di-
vifible. then the Line EF, which is perpendicular to them, and
limited by them, will be alfo an Inch long. Then let
the Point A, in the Line AB, be taken on the left
Hand of the Line EF, and, if you will, at an Inch
diſtance from it, on the Line CD to the right Hand of
EF, let as many Points G, H, D, &c. as you pleaſe be
taken, and at any diſtance from each other; to which
let as many ftreight Lines be drawn from A, as AG,
AH, AD. Then it is evident, that the Line AG will
paſs through the Point I of the Line EF, that the Line
AH will país through the Point L which is higher, and the
Line AD will pass through the Point M which is higher ftill,
and ſo on; and becauſe the Line CD is indefinite, and
an indefinite Number of Points, fuch as G, H,D may be
taken upon it, it will follow, that Lines drawn from A
to all thofe Points, will mark an indefinite Number of
Points on the Line EF different from each other, and
which approach nearer and nearer to the Extremity E,
without any one of them ever paffing through the Point
E, becauſe the Line CD is fuppofed to be parallel to AB.
Wherefore, becauſe the Length of EF was taken at pleaſure,
and the fame Demonſtration holds for any other Length
whatſoever; we must acknowledge, that an indefinite
Number of Points may be affigned in any determinate Por-
tion of Matter, and confequently that Matter is indefinitely
diviſible.
5. This
Chap. 9. of NATURAL PHILOSOPHY.
33
072.
5 This Truth may alſo be demonftrated from this Con- 5. Another
fideration, that there are fome Quantities that are incom- Demonftrati-
menſurable, that is, have no common Meaſure. Thus,
ſuppoſe ABCD to be a Square, it may be geometrically Tab. I.Fig.2.
demonftrated, that the Side AB, is incommenfurable to the
Diagonal AC. Let us then imagine in our Minds the
Line AB, which is an Inch long, fuppofe, to be divided
into a hundred Thouſand equal Parts, and every one of
theſe into a hundred Thousand other Parts that are equal
alfo, and again, every one of theſe into a Hundred Thouſand
other Parts equal to one another ſtill, we may go on in the
Divifion thus, for an Age together, without ever being
able to come at Parts fo fmall, as to fay, that the Line
AC contains a certain determinate Number of them and
no more. Now this could not be fo, if Extenfion were
not indefinitely diviſible; for then after we had divided
the Line AB, for inftance, into as many Parts as it is
poffible for Extenfion to be divided into, the Line AC
would neceffarily contain a certain determinate Num-
ber of thoſe Parts. We must therefore conclude, that
every Thing which is extended, and every Portion of
Matter, is indefinitely divifible.
6. An Ot
6. This Conclufion of Ariftotle's, hath been affented to
by all his Followers, except a very few, and they depart- jection a-
ed from it only, becauſe they thought they contradicted sainf? this.
themſelves: For, fay they, if two Bodies be fuppofed un-
equal, and if they can be divided indefinitely, it will follow,
that the Number of Parts of which the one is compo-
fed, is equal to the Number of Farts of which the other
is compofed, and from thence it will follow, that
they are both equal, which is contrary to the firſt Sup-
pofition.
Jer to this
Ejection.
7. But here is a double Miſtake. Firſt, they did not 7. An An-
confider, that Equality and Inequality are Properties. of
finite Things, which can be comprehended and compared
together by humane Underſtanding, but they cannot be
applied to indefinite Quantities which humane Under-
ftanding cannot comprehend or compare together, any
more than it can a Body with a Superficies, or a Super-
ficies with a Line. But, if it could be faid, that of two
unequal Bodies, divided in the foregoing Manner, as the
1. A certain determinate Num-
ber, &rc.) For if the Line
Tab. I. AB could be divided into
Fig. 2. thofe finalleft Parts, the Line
AC, and all other Lines
could be divided alfo into them; fo
that one of thoſe ſmalleſt Parts would
be the common Meaſure of the Lines
AB, AC, and of all other Lines.
D
Line
34
Part 1
ROHAULT'S SYSTEM
1
8. Another
Objection.
9. Answer.
10. Concern-
Line EF was divided, the Number of the Parts in the
One, was equal to the Number of the Parts in the Other;
we could not conclude from thence, that the two Bo-
dies themſelves were equal, becauſe the Parts of the one,
are bigger in Proportion than the Parts of the other:
There is therefore no Contradiction in this particular, but
the foregoing Demonftration holds in its full force.
8. Others attack the indefinite Divifibility of Matter,
another way; by faying, that it would from thence fol-
low, that a fmall Portion of Matter, fuch as a Cube, a
quarter of an Inch high, might be divided into as many
thin fquare Pieces, as would cover the whole Globe of
the Earth, if it were much bigger than it is; which, they
think, is abfurd.
9. But theſe have no more Reafon of their Side than
the other; for their Objection is founded upon this fingle
Maxim of their own, That every Thing is abfurd, which
our Imagination can't comprehend: This is a very grofs Mi-
ſtake, and unworthy of a Philofopher, who cannot but know,
that there are an infinite Number of Truths, which it is
certain our Comprehenfion cannot attain to. Many Exam-
ples might be given of this, but I fhall content my felf
with Two, both which relate to the Subject we are now
treating of, viz. The Sheets of Gold made by Gold-
beaters, and the Gold Wire made by Wire-drawers.
10. In order to a clear Conception hereof, we muſt firſt
ing the Divi- know, that it appears by Experience, that the Weight of
fion of Goid
made by Gold-
an equal Quantity of Gold and Water is as 19 to 2, fo
that if a Cubick Foot of Water weighs 71 Pounds,
beaters.
1
1. That the two Bodies themselves
are equal, &c.) What is faid of
Quantities decreafing infinitely little,
may alſo be underſtood of Quantities
increasing infinitely great; that is,
Quantities infinitely great, are not
therefore all equal to each other. For
a Line drawn from a Point infinite-
ly, one way, is but half a Line drawn
from a Point infinitely, two ways.
And a Rectangle of an infinite
Heighth, upon a finite Bafe, may be
4, &c of a Rectangle of an
infinite Height alfo, upon a propor-
tionable Bafe. And, in Heterogene-
ous Quantities, an infinite Line, is
not only not equal, but is infinitely
lefs than an infinite Superficies, and
an infinite Superficies, than an inf
pite folid Space. And in a folid Space,
Cylinder infinite in Length, is not
2
I
27 37
only not equal in Quantity, but is
really infinitely lefs, than an infinite
folid Space of two Dimenfions, viz.
Length and Breadth; and an infinite
folid Space of two Dimenfions, is
infinitely lefs than an infinite Space of
all the Dimenſions. Whence, by the
way, it appears, how weakly they
argue, who, becaufe Space (and the
fame is true of Duration) may be di-
vided into in umerable Parts which
are unequal; and in infinite Space
(or Duration, the Number of the
greateft Parts is as much infinite as
abfurd, becauſe they believe all In-
that of the lealt; which they think
conclude from hence, that there can
finites to be equal in every reſpect;
be no fuch Thing at all as Infinite
Space (or Duration.)
¹ a cu-
Chap. 9. of NATURAL PHILOSOPHY.
35
92
84
I a cubick Foot of Gold will weigh 1349 Pounds or 221584
Ounces. 3 Now a cubick Foot contains 2985984 Cu-
bick Lines, and therefore 4 an Ounce of Gold contains
138,722 cubick Lines. Wherefore an Ounce of Gold,
reduced into the Form of a Cube, will be 5 very near
5 Lines high, and its Baſe 6 about 2623 fquare Lines.
This being fo, the next Thing to be known, is, that the
Gold-beaters make out of an Ounce of Gold 2730 whole
Leaves of 34 fquare Lines each, befides what they call
the Wafte, which is the fmall Shreds that are cut off, and
amount to almoſt half: The 'Superficies of 7 every one of
thefe Leaves is 1156 Lines fquare, fo that if they were
all placed regularly by one another, they would make
one Superficies of 3155880 fquare Lines; to which if we
add9 but a third Part, which is the leaft that goes into
Shreds, it will follow, that a Gold-beater makes out of
an Ounce of Gold 4207840 ſquare Lines. Now fince this
Superficies 19 exceeds the Baſe of a Cube of Gold of an
Ounce weight 159092 times, it is certain, that That Cube,
which, as was faid before, did not exceed 5 Lines in
Height, is dvided into 159092 fquare Leaves.
II. The Di-
II. Though this Divifion of Gold be very furprizing,
yet it is very far fhort of what is done by Wire-drawers. vifion of Gold
I have ſeen ſeveral Ingots of Silver in the Figure of Cy- by Vire-
linders, which weighed eight Pounds a piece; one of
them, which feemed to me more regular than the reſt,
was two Foot and eight Inches long, and two Inches and
1. A cubic Foot of Gold, &c.) For
1: 1971: 1349.
For 16
2. Or 21584 Ounce
Ounces make a French Pound. See
is very nearly 5 though is ftill
nearer, For the Cube of sis
T99
137 And the Cube of sis
ह
8
6. About 262
the Square of 5
2623.
Preftet. Nouvel. Elem. Mathemat. 3.136343
Edit. 1. part. lib. 2. pag. 55.
3. Now
4 cubic Foot) The Propor-
tion between a Line and a Foot, is
as 1 to 144 now in this continued
geometrical Proportion, the Number
is 2985984: Therefore becauſe Cubes
are in a triplicate Ratio of their Sides,
a cubic Line is to a cubic Foot, as I
to 2985984, that is, a cubic Foot
contains 2985984 Lines.
4. An Ounce of Gold) A cubir
Foot of Gold, which weighs 21584
Ounces, contains 2985984. cubic
Lines; therefore by the following
Proportion, it is, 21584 Ounces":
2985984 cubic Lines 1 Ounce.
138 7392 cubic Lines.
1 58
::
5. Very near 54 Lines high) Fo
the Cube Root of 138-1321
739
84
Square Lines) For
is pretty nearly
7. Every one of thefe Leaves) For
the Side of a Leaf, was faid before to
be 34 Lines the Square of which is
1156.
8. Make one Superficies) Multiply
1156 the Number of fquare Lines in
one Leaf, by 2730 the Number of
Leaves, and it will make 3155880.
9. But a third Part) To which
Superficies, if we add a third Part of
355880 that is, 1051960 it will
nake 4207840.
10. Exceeds the Baſe) That is, the
Superficies 4207840, contains the
Bale of that Gube, or 2613 159092
times.
D 2
nine
Drawers.
36
Part I.
ROHAULT'S SYSTEM
nine Lines about; fo that the Cylindrical Superficies
was 12672 fquare Lines. After this Superficies was co-
vered over with ſeveral Leaves of Gold, which all together
weighed half an Ounce; the whole Cylinder was drawn
through Holes made in a Plate of Steel, till it became
fuch as the ſmalleſt Wire that is made in this City; I took
25 Fathom or 150 Foot of it, and weighed them in an
exact pair of Scales, and found that they weighed but 36
Grains, wanting about of a Grain. Wherefore 2 the
whole Cylinder ought to have been drawn into a Wire of
307200 Footlong: Whence it follows, 3 that it is 115200
times longer than it was before, and that its Superficies is
become 4 three hundred and forty times as much. To
which if we add, that when this fmall Wire is made into
a thin Plate, to cover Silk with, the Superficies is twice
4
1. The Cylindrical Superficies) For
two Feet and eight Inches (that is
384 Lines) which is the Height of
the Cylinder, multiplied by two
Inches and nine Lines (that is 33
Lines) which is the Circumference
of the Bafe, makes 12672.
2. The whole Cylinder) Firft let
the whole Cylinder (which, as was
faid before, was 8 pounds) be redu-
ced into Grains
8 Pounds by 16,
which makes 128
'Ounces.
128 Ounces by 8,
which makes
1024 Drachms.
1024 Drachms by
by multiplying 3, which makes
3072 Scruples.
3072Scruples by 2,
which makes6144
half Scruples.
6144 half Scruples
by 12, which
makes 73728
Grains.
Then by the following Proporti-
on; 36
Grain : 150 Feet: 73728
Grains: 307200 Feet.
3.
That it is 115200 times longer)
For multiply 2 Feet and eight Inches
(which is the Length of the Cylinder)
or 32 Inches by 115200, and it will
make 3686400 Inches, that is,
307200 Feet (the Length of the
whole Wire.)
4. Three hundred and forty times as
much) Let the whole Cylinder of Sil-
as
ver which is to be drawn into Wire,
be called A, and ſuppoſe another Cy-
linder B of an equal Bafe, but 115200
times higher, and let the Cylinder of
Wire be called C. It is manifeſt that
the Superficies of the Cylinder B, and
the Superficies of the Cylinder A, are
to one another as 115200 to 1, that
is, as the Height of the Cylinder B
to the Height of the Cylinder A,
that is, as the Bafe of the Cylinder
A, to the Baſe of the Cylinder B
(for the Bafes of equal Cylinders are
reciprocally as theirHeights) that is, as
the Bafe of the Cylinder B, to the
Bafe of the Cylinder C. Now if we
fuppofe, according to Cavallerius's
Doctrine Indivifibles, that the Su-
perficies of Cylinders confift of an in-
finite Number of Circumferences of
Circles equal to the Bafes, then the
Superficies of the Cylinder B, will be
to the Superficies of the Cylinder C,
as the Circumferences, or as the Ra-
dius's of their Bafes; now the Radi-
us's are to one another in a fubdupli-
cate Ratio of the Area's of the Ĉir-
cles If therefore the Superficies of
the Cylinder B, be fuppofed 115200,
the Superficies of the Cylinder C will
be a mean Proportional between
115200 and 1 (that is, 340 very
nearly) and the Supecficies of the
Cylinder A will be 1. 2. E. D.
1
5. The Superficies is twice as big)
If the Cylinder be made flat, its
whole Superficies is made into two
Parallelograms, which becauſe they
lie one upon another, form a thin
Parallelepipedon, capable of being
made as thin again, which is done by
the
Chap. 9. of NATURAL PHILOSOPHY.
37
as big, fo that it then is encreaſed to fix hundred and
eighty times as much as it was at firſt, 6 and therefore
contains 8616960 fquare Lines. Now after this Wire is
made into fo thin a Plate, its fuperficies is ftill covered all
over with Gold; fo that only half an Ounce of Gold
with which the Plate is covered, is made fo thin, that its
Superficies is 8616960 Square Lines. 7 Which Super-
ficies exceeds 325795 times the Baſe of a Cube of Gold
of an Ounce weight, and twenty fix fquare, Lines and
in Breadth; from whence it follows, that the Thickness
of the Gold which the Silver Plate is covered with, is not
above
of half the Height or of the
's 79
whole Height of a Cube of Gold of an Ounce weight;
fo that the Quantity of 5 Lines is divided into 65 1590
equal Parts.
3:336
:
Matter, teach
us to form a
12. If we confider further, that Gold is capable of be- 12. The fore-
ing divided ſtill more, if there were any Occaſion for its rations of the
going Confide-
and above all, if we confider that what we have now ex- Divifion of
amined is done by Men, and with Inftruments that are
very grofs and dull, and that there are in Nature many better Judge-
Things, which are vastly more fine and fubtile; we fhall ment of the
clearly fee, that what exceeds our Imagination, is not God
Power of
therefore impoffible; and that it is not for us to prefume,
as many do, to fet Bounds to the Power of God.
13. That
no Divifion
13. Laftly, We are carefully to obferve, that That Di-
vifion which we make in our Minds and Imaginations, there can be
makes no Alteration at all in Matter, but that all real without Mo-
Divifion arifes from Motion; that is, in order for a Por- tion.
tion of Matter to be really divided from that to which it is
united, it muſt neceffarily be feparated from it. And
hence it is, that Motion is fo neceffary, and the Knowledge
of it fo ufeful, that Ariftotle fays, that he who does not
underſtand Motion well, muft neceffarily be ignorant of
all natural Things.
the Workmen, who beat it as thin as
they can, fo that the Superficies of
the Cylinder is thereby doubled.
6. And therefore contains) Multi-
ply 12672, the Superficies of the
Silver before it is beaten, by 680, and
it will make 8616960.
7. Which Superficies exceeds) di-
vide 8616960 by 261 and it will
make 325745.
8. Of half the Height) Becaufe the
Gold with which the Silver Wire is
is, half a Cube of Gold of an Ounce
covered was only half an Ounce, that
Weight.
D 3
CHAP.
ROHAULT'S SYSTEM
Part I.
7
2. What it is
to be moved.
2. What Mo-
Bre.
B
CHAP. X.
Of Motion and Reft.
ECAUSE it is eaſier to underſtand what Motion
is, by Experience, than to give a Definition of it,
or to find out the Caufe, I fhall here make uſe of a fa-
miliar Example, agreed upon by all, which may ſerve to
explain to us the Nature of Motion.
1. Suppofe a Man in a calm Day walking on Foot in
a Park planted with Trees, and that at the Beginning he
is obferved to be between the firft Trees in the Walk,
and then between the Second, and fo to continue on
walking till he comes at the End; no Body doubts but
the Man thus walking moves, and that every Step he takes
is a real Motion. Confider now, that the Motion of
this Man is ſomething new, which was not in him be-
fore; and then if we take an exact Account of what we
conceive to have come to him fince he began to be mo-
ved, and reject every Thing which we certainly know is
not Motion, we are fure that what remains, is, without
doubt, the Thing we enquire after, and that this will ſhow
wherein Motion properly confifts.
2. Now becauſe we do not acknowledge a Vacuum, as De-
tion and Rest mocritus and Epicurus did, therefore we cannot fay with
them, that this Man which we are fpeaking of, applies
himſelf to different Parts of Space, becauſe we do not
diſtinguiſh Space from Matter as they did; wherefore in
the Example now mentioned, there are three Things to
be confidered by us. Firſt, The Defire of Walking in
the Man: Secondly, The Effort he makes to put this
Defire in Execution: And Thirdly, The Correfpondence,
or the fucceffive Application of the external Parts of this
Man, to the different Parts of the Bodies which encom-
Now it is evi-
paſs him, and immediately touch him.
A
dent, that the Defire which this Man has, is not the Mo-
tion of him; for Defire is nothing but Thought, and we
acknowledge many Things to be moved, which we do
not allow to have any Thought. So likewife we ought not
to think, that the Motion of the Man confifts in the Ef-
fort which he makes towards Walking: For though we
may truly fay, that all Bodies which move, have an Ef
fort, (as we know they fometimes have, though they do
not move) yet we are rather to think, that this Effort is
the
Chap. 1o. of NATURAL PHILOSOPHY.
39
the Caufe of the Motion, and not the Motion it felf.
Nothing therefore remains but that Motion confifts in 1 the
fucceffive Application of a Body to the different Parts of
thofe Bodies which are immediately about it; whence it
follows alfo, that the Rest of a Body, is the continual Applica-
tion of that Body to the fame Parts of thofe Bodies which
are about it and immediately touch it.
D 4
1. Succeffive Application of a Bo-
dy, &c.) The Difpute about the Na-
ture and Definition of Motion, a-
mongſt the Writers of Philofophy,
has always been very perplexed. I
fuppofe, becauſe, not fufficiendy at-
tending to the different Senfes of an
ambiguous Word, they endeavoured
to comprehend that in one Defini-
tion, which ought to have been ve-
ry exactly diftinguished into its dit-
ferent Parts. That Motion (or ra-
ther the Effect of Motion) in gene-
ral, is a Tranflation of a Body from
one Place to another, is
is pretty well
agreed amongſt them all. But what
is meant by being tranflated from one
place to another, here the Contro-
verfy lies, and Philofophers differ
widely. They who define Motion by
comparing the Thing which is moved,
not with the Bodies that encompaſs it,
but only with Space which is im-
moveable and infinite, can never
know or underſtand, whether any
Body at all refts, nor what the abſo-
lute Celerity of thoſe Bodies that are
moved is; for befides, that this whole
Globe of the Earth revolves about
the Sun, it can never be known whe-
ther or no the Center of this whole
Syftem, in which all the Bodies re-
lating to us is contained, reſts, or is
moved uniformly in a ftreight Line.
Again, they who define Motion, by
comparing the Thing which is mo-
ved, not with infinite Space, but with
other Bodies, and thofe at a very
great Diſtance, theſe neceffarily make
Tome Body the Mark by which all
Motion is to be meaſured, which,
whether it felf is at reft, or, with ref-
pect to Bodies at a ftill greater di-
tance, is moved, is impoffible to be
known likewiſe. Laftly, They who
define Motion by comparing the
Thing which they fay is moved, not
with diftant Bodies, but only with
that Superficies which immediately
touches it; it is very weak in them
to fay, that thofe Things are truly
at reft, which being connected with
3. It
the Particles of other Bodies, are mo-
ved with the greateſt Swiftnefs; as
the Globe of the Earth which is in-
compaffed with Air, and revolves
about the Sun. And on the contra-
ry, that they only can be faid to be
moved, that with the utmoft Force,
and Refiftance which they can make,
can do no more than barely hinder
themſelves from being carried along
with other Bodies, as Filhes which
ftrive against the Scream.
But if we rightly diftinguish the
different Senfes of the ambiguous
Word, this whole Miſt will immedi-
ately vaniſh. For a Thing in Mo-
tion, may be confidered in three
Refpects, by comparing it with the
Parts of infinite and immoveable
Space, or with Bodies that furround
it at a distance, or with that Super-
ficies which immediately touches it.
If thefe three Confiderations be ex-
aly diftinguiſhed into their feveral
Parts, all future Difputes about Mo-
tion will be very eaſy. First, then,
a Thing in Motion may be compa-
red with the Parts of Space: And,
becaufe the Parts of Space are infinite
and immoveable, and cannot under-
go any Change like Matter, therefore
that Change of Situation, which is
made with reſpect to the Parts of
Space, without any regard had to
the Bodies which encompafs it, may
rightly be called, abfolutely and truly
proper Motion. Secondly, a Thing
in Motion may be compared with
diftant Bodies, and becauſe a Body
may in this manner be transfer-
red along with other Bodies which
immediately furround it, therefore
that Change of Situation which is
made with refpect to thofe Bodies
which are at a diſtance, and not to
thoſe which are near, may properly
be called, relatively common Motion.
Laftly, a Thing in Motion, may be
compared with the Superficies of thofe
Bodies which immediately touch it:
And becauſe, whatſoever is thus mo-
ved, may poffibly have no abfolute
ROHAULT'S SYSTEM Part I
3. In order to
determine
whether a Bo-
3. It is to be obferved here, that when we fpeak of
Motion or Reft, we always mean an immediate Applica-
dy be in Mo- tion, and have no Regard to the Relation a Body ftands
in to Things at a diſtance, any further than to confider
need of com-
fuch fort of Relation as a mere external Denomination
paring it with only, which makes no Alteration in the Thing, and which
tion or no,
there is no
Bodies at a
distance.
-or common Motion at all (as if an
Arrow were ſhot towards the Weſt,
with the fame Swiftnefs, that the
Earth turns towards the Eaft ;) and
on the contrary, that which in this
refpect is at reft, may really be
transferred with both abfolute and
common Motion (as Bodies hid in the
Bowels of the Earth) therefore that
Change of Situation which is made
with refpect to thofe Superficies,
which immediately touch the Thing
moved, may rightly be called Motion
relatively proper.
First, Abfolutely and truly proper
Motion, is the Application of a Body,
to the different parts of infinite and
immoveable Space. And this is in-
deed alone abfolute and proper Mo-
tion, which is always generated and
changed by the Forces impreffed up-
on the Body that is moved, and by
them only; and to which alone are
owing the real Forces of all Bodies to
move other Bodies by their im-
pulfe, and to which they are in pro-
portion (See Newt. Princip. Book I.
Def. 2,-8.) But this only true Mo-
tion cannot be found out or deter-
mined by us, nor can we diſtinguiſh,
when two Bodies any way ftrike
against each other, which the true
Motion, and confequently the true
Force from whence that impulfe
arifes, belongs to; whether to that
which feems to us to move fwifteft,
or to that which moves floweſt, or
perhaps feems to be quite at reſt;
becauſe it cannot be demonftrated
whether the Center of Gravity, as
was faid before, or of the whole
Syſtem (which we may properly e-
nough define to be, One Point in In-
finite Space,) be at reft or no.
Secondly, Motion relatively common
is the Change of Situation" which is
made with respect, not to thoſe Bodies
which are nearest, but to fome that are
at a distance. And this fort of Motion
we mean, when we fay, that Men,
and Trees, and the Globe of the
Earth it felf revolve about the Sun:
is
when we confider the Quantity of
Motion, or the Force of a Body in
Motion to ſtrike against any Thing.
For Example, when a Ball of Wood,
with a piece of Lead in it to make
it heavy, is thrown out of our Hand,
we commonly reckon the Quantity
of Motion, or the Force with which
the Ball ſtrikes, from the Celerity of
the Ball, and the Weight of the in-
cluded Lead together. I fay we com-
monly reckon it fo, and indeed truly,
with refpect to the Force it felf, or
any fenfible Effect of it; but whe-
ther that Force or true Motion be
really in the Ball that ftrikes, or in
the Earth which feems to be ftruck,
this, as was ſaid before, we cannot
certainly determine.
Laftly, Motion relatively proper,
is
the fucceffive Application of a Body to
the different Parts of Bodies which
immediately touch it. And this is
the Motion we generally mean in
Philofophical Difputes, where we en-
quire into the Nature of particular
Things, as when we fay, that Heat
or sound, or Liquidnefs, confift in
Motion. But particular Notice ought
to be taken, that the fucceffive Appli-
cation of a Body is fo to be under
ftood, that it is to be applied fuc-
ceffively to the different Parts of the
Bodies immediately touching it, with
its whole Superficies taken together
(par tout ce qu'il a d'exterieur, as
the French expreſſes it ;) as when a
Ball that is thrown, glides againft
the different Parts of the Air with
its whole Superficies; and when our
Hand is moved up and down, it is
fucceffively applied with its whole
Superficies, to the different Parts of
the Air on the one Side, and of the
Joint by which it is faftned to the
Body on the other Side. It was to
no purpoſe therefore for Mr. Le Clerc
to find fault with this Definition,
in his Phyf. lib. 5. Chap. 5. It will
follow, fays he, that the Banks and
the Channel of the River are as
much moved as the Water, becanfes
And we mean this Motion alfo, they are as far removed from the
1
Wate
Chap. 1o. of NATURAL PHILOSOPHY.
is nothing real in the Subject under Confideration. Thus,
the Man whom we fuppofe walking amongſt the Trees,
may always keep at the fame diftance from the fame Parts
of the Water that runs in a Canal juſt by, and yet we don't
fay that he is at reft; and another Perfon fitting in the
Walk, may be againſt different Parts of the Water, and
yet we don't ſay that he is in Motion. Whence it follows,
that they are very much miſtaken, who, in order to de-
termine whether a Body be at Reft, or in Motion, com-
pare it with immoveable Parts which they imagine to be
beyond
+
Water that runs by, as the Water
is from the other Parts of the Chan-
nel and Banks. But the Cafe of the
Water is very different from that of
the Banks. The whole Superficies
of the Water is fucceffively applied
to different Parts of the Bodies which
furround it, and immediately touch
it, and therefore is transferred from
fome of thoſe ſurrounding Bodies to
others. But the Banks are partly fix-
ed to the Earth, and therefore are not
transferred from thofe Bodies which
immediately furround them. For
when we fay, that a Body is trans-
ferred, we mean that the Whole of
it is transferred. Wherefore an I-
fland ſticking up in the middle of a
River, is not moved (not ſo much as
with this mere relative Motion) tho'
the Water flides by it, becaufe it is
firmly fixed in the Earth, and is not
transferred from that which imme-
diately touches it. So a Body e-
qually poifed in a Liquor whoſe
Parts run upon it with equal Force,
is not moved; becauſe though every
particular Part of the Superficies of
it be every Moment applied to dif-
ferent Parts of the Liquid that fur-
rounds it, yet the whole Superficies
of it is not transferred at once from
the concave Superficies of the Parts
which furround it, confidered as one
whole Superficies.
Further, according to theſe diffe-
rent Definitions of Motion, are we to
understand the Word Place in dif-
ferent Senfes. For when we fpeak
of truly or abfolutely proper Motion
(or Reft) then by Place we mean,
that Part of infinite and immoveable
Space which the Boty poffeffes; when
we fpeak of Motion relatively com-
mon, then by Place is meant, a Part
of fome particular Space or movea-
ble Dimenfion, which Place it felf is
1
truly and properly moved, along with
that which is placed in it: And when
we fpeak of Motion relatively proper
(which indeed is very improper)
then by Place, is meant the Superficies
of the Bodies (or fenfible Spaces) which
immediately furround the Thing moved.
As to the Definition of Řeſt, all
are very well agreed in it: But whe-
ther Reft be a mere privation of Mo-
tion, or any Thing pofitive, this is
fharply difputed. Cartes and fome
others contend, that That which is
at reft, has ſome kind of Force, by
which it continues at Reft, and
whereby it refifts every Thing that
would change its State; and that
Motion may as well be called a
Ceflation of Reft, as Reſt is a Ceffa-
tion of Motion. Malebranch in his
Enquiry after Truth, Book 6. Chap.9.
and others contend on the contrary,
that Reft is a mere privation of
Motion; their Arguments may be
feen briefly explained in Mr. Le
Clerc's Phyf. Book 5. Chap. 5. One
Thing only I would obferve by the
way, relating to this Matter, and that
is, that Malebranch and Mr. Le Clerc,
who follows his Opinion, in the
following Argument, beg the Que-
ftion. Suppofe, fay they, a Ball at
reft; fuppofe that God fhould ceaſe
to will any Thing concerning it:
what would be the Confequence?
It would be at reft ftill. Suppoſe it
be in Motion; and that God ſhould
ceafe to Will that it ſhould be in
Motion, what would follow then?
It would not be in Motion any lon-
ger. Why not? Becauſe the Force,
whereby the Body in Motion continu-
ed in the State it was,is the poſitive Will
of God, but that whereby it is at Reſt
is only privative: This is a manifeſt
begging of the Queſtion. In reality, the
Force or Tendency bywhich Bodies.whe-
ther
*
42
Part I.
ROHAULT's SYSTEM
4. Aremark-
beyond the Heavens, where it is very uncertain, whether
there be any Parts of Matter more immoveable than thoſe
néar us.
4. Having thus explained the Nature of Motion and
able Inftance Reit; when we ſee a Fith in the River keeping it ſelf
of a Body in
motion and of for fome time right againſt the fame Part of the Bank,
another Body and neither the Stream which furrounds it, carrying it
at Reft.
5. That to re-
fift fome fort
of Motion, is
to move to-
downward, nor its own Force, by which it ftrives againſt
the Stream, carrying it upward, we fay that it is really in
Motion, becauſe it really agrees in every particular, with
another in a Pond, which is by all allowed to be in Mo-
tion; for the Effort of the Former, makes it to be fuc-
ceffively applied to the different Parts of the Running
Stream, in the fame manner, as the Effort of the Latter,
makes it to be applied to different Parts of the Water in
the Pond. On the contrary, when we ſee a Stake float-
ing on the Water, and carried along with the Stream, we fay
that it is at Reft, becauſe it is incompaffed with the fame
Parts (which is the general Reaſon why we ſay a Body is
at Reft) though at the fame time, the Stake and the Ri-
ver together, are but one Thing in Motion.
5. When a Fiſh that moves it ſelf in the manner
now deſcribed, is not carried along with the Stream, we
are uſed to ſay, that it reſiſts the Stream; ſo when a Bo-
wards the dy by its Refiſtance, hinders it felf from being carried
contrary part. along with another Body with which it is entirely fur-
rounded, we may as well fay, that it moves the contrary
way.
6. That Mo-
are only
Modes of ex-
ifting, and
are each of
6. Becauſe we cannot conceive any Application to
tion and Reft different Parts, without fuppofing a Body fo applied, fo
that Motion depends neceffarily upon the Thing moved;
therefore we are not to think that Motion is any real Being,
but only a Mode of the Body in Motion; and ſo likewiſe,
that Reft is only a Mode of the Body which is at Reſt.
Whence it follows, that Motion and Reft add nothing
more to the Body in Motion or at Reft, than Figure does
to a figured Body; and fince a Body may either be moved
them but
Accidents of
Matter.
ther in Motion or at Reft, continue
in the State in which they once are;
is the mere Inertia of Matter ; and
therefore if it could be, that God
fhould forbear willing at all; a Body
that is once in Motion, would move
on for ever, as well as a Body at
Reft, continue at Reft for ever. And
the Effect of this Inertia of Matter
is this, that all Bodies refift in pro-
portion to their Denfity, that is, to
the Quantity of Matter contained in
them; and every Body ftriking up-
on another with a given Velocitys
whether that other be greater or
lefs, moves it in proportion to the
Denfity or Quantity of Matter in
the one, to the Denfity or Quantity
of Matter in the other.
or
في
Chap. 10. of NATURAL PHILOSOPHY.
43
determine the
Quantity of
Motion.
or not moved, we conclude, that Motion and Reft are
only accidental to Matter.
7. Motion has always been acknowledged to be a Spe- 7. How to
cies of Quantity, which is meafured partly by the Length
of the Line, which the Body in Motion runs; for Ex-
ample, when a Body of a given Bignefs, fuppofe a Cubic
Foot, moves a given Space, fuppofe fixty Foot, we call this
a determinate Quantity of Motion, and it is twice or thrice
as much, if the fame Body runs 120 or 180 Feet.
I
8. Another
Way to mea-
8. It is alfo partly meaſured by the Quantity of Matter
which moves together: For Example, If a Body of two
Sure the
Cubic Feet runs through a Line fixty Foot long, it has twice Quantity of
as much Motion, as a Body of one Cubic Foot, which runs Motion.
through the fame Line: For it is evident, that we ought
to reckon as much Motion, in each half of the Body of
two Feet, as in the whole Body of one Foot.
9. Whence it follows manifeftly, that in order for une- 9. How tw
qual Bodies to have equal Quantities of Motion, the Lines unequal Bo-
dies may have
which they run through, ought to be in reciprocal Pro- equal Quan-
portion to their Bulk. Thus, if one Body be three times tities of Mo-
as big as the other, the Line which it runs through, ought
to be but a third Part of that of the other.
tion.
at the Ends
10. When two Bodies hung at the Ends of a Ballance 10. How tw
or Leaver, are to one another, in reciprocal Proportion Bodies hung
to their Diſtances from the fixed Point; they muft ne- of a Ballance
ceffarily, when they are moved, defcribe Lines which are may be in
to each other, in reciprocal Proportion to their Bulks. æquilibrio.
For Example; if the Body A be three times as big as the Tab.I.Fig.3.
Body B, and theſe Bodies be ſo faſtned to the Ends of the
Leaver AB, whofe Point C is fixed, that the Diſtance BC
be three times as much as the Diſtance AC, the Leaver
cannot incline either to the one Side or the Other, but
the Space BE along which the leffer Body is moved, will
be three times as much as the Space AD along which
the greater Body is moved; wherefore the Motion of the
1. By the Quantity of Matter) That
is, of the Matter which belongs pro-
perly to the Body in Motion For,
the fubtile Matter, if there be any
fuch Thing, with which the fmall
Pores of terreftrial Bodies are filled, is
not transferred along with them,
with the fame common Motion;
Therefore if a Ball of Iron, and a
Ball of Wood of the fame Bignefs
be moved with the fame Celerity,
there will be more Motion in the
Ball of Iron, than in that of Wood.
So likewiſe, if two equal leaden
$
Balls, the one folid, the other hollow,
and empty, be moved with the fame
Celerity; the folid Ball will have
more Motion than the hallow One,
and will ſtrike a Body againſt which
it is thrown with greater Force. And
the Quantity of Matter which is pro-
perly contained in any Body is to be
determined by its Weight. Where-
fore the Quantity of Motion is not
to be meaſured by the Celerity and
Bignefs, but by the Celerity and
Weight of the Body in Motion;
which is carefully to be obſerved.
onc
44
Part I.
ROHAULT's SYSTEM
11. The Rea-
quors ballance
each other.
one Body, will be exactly equal to the Motion of the
Other. This being fo, there is no Reafon to think, that
the Body A, with four Degrees, fuppofe, of Motion
downwards, fhould lift up the Body B with four Degrees
of Motion, rather than the Body B with four Degrees of
Motion tending downwards alfo, fhould lift up the Body
A with four Degrees of Motion; wherefore we ought to
think that they will be in aquilibrio. And this is the
Foundation of Mechanicks.
I
11. So likewife when any heavy Liquor is contained in
fon why Li- an inverted Siphon, whofe Tubes are wider one than the
other, if we imagine the Height of the Liquor in each
Tube to be divided into very many equally thin Planes;
one of theſe Planes in either Tube, cannot by finking,
raiſe the Liquor in the other Tube, but the Sinking and
the Rifing muſt be in reciprocal Proportion of the Quan-
tity of Parts which fink to thoſe which rife. Thus, if
Tab.I. Fig.4 the Width of the Part AB, the larger Tube of the Si-
phon ABCD, be a hundred times as much as the Width
of the Part C, the traiter Tube; and confequently,
the Quantity of the Parts of the Liquor in the Plane AB,
a hundred times as many as the Quantity of Parts in the
Plane C; then the Riling or Sinking of the Parts on the
Side AB, will be to the Rifing and Sinking of the Parts on
the Side C, in a centuple reciprocal Proportion: Where-
fore the Motion of all the Parts in the Tube AB is ex-
actly equal to the Motion of all the Parts of the Tube C.
So that they in the one, are no more able by finking, to raiſe
thoſe in the other, than theſe Latter are able by finking
to raiſe the Former. Whence it follows, that if each
Tube be divided into an equal Number of Planes, that
is, if the Liquor be of an equal Height in them both,
1 it muſt keep it felf in equilibrio, unleſs diſturbed by
fome external Caufe.
12. Since
1. And this is the Foundation of
Mechanicks) Upon this is built that
famous Problem of Archi-
fes, is fully explained below in the
Notes on the 14th Chap. Artic. 9.
1. It must keep it ſelf in æquilibrio)
Δὸς ποὺ τῶν τὴν
Hence it follows, That´all Li-
Tab. I. mides, Ads wou sŵy Thy
την κινήσω. Το
quors prefs upon Bodies
move 2
that are under them, Tab. XVII,
according to their per- Fig. 1.
pendicular Height, and
Fig. 3.
given Weight, with a si
ven Force: For by increafing the Di-
ftance C B, the Force of the Body B
may be increafed infinitely. For the
manner how this is done by increa-
fing the Number of Leavers, Wheels,
Pulleys, Screws, &c. fee Wilkins's
Mathematical Magick, and others.
The Force of every one of which
Mechanick Powers, and whence it ari-
not according to their Breadth. Which
Paradox may alſo be demonſtrated in
the following Manner. Let ABCDFE
be a Veffel filled with Water: Now
becauſe the Column BF is heavier
than the Column HG, it is manifeft,
that if the Veffel were open at H,
the Column GH would' rife till it
became
Chap. 1o. of NATURAL PHILOSOPHY.
45
Λάουστ.
12. Since it is only the Effential Properties of any Sub- 12. That God
ject, which can be deduced from the Effence of it, after is the firſt
it is known; it is to no Purpoſe for us to endeavour to
find out how Motion could be firſt produced in Bodies,
becauſe this is not an effential Property; we fhall not there-
fore ſtand to argue upon this Subject: But as we own
God to be the Creator of Matter, fo likewife we own
him to be the firſt Mover of it.
became in equilibrio with the Co-
lumn B F. Since therefore the Co-
ver which fhuts up the Veffel at H,
hinders the Column GH from rifing,
it is evident,that theWater at H preffes
the Cover of the Veffel upwards with
a Force equal to the Weight of BL,
and becauſe all Preffure is reciprocal,
it is evident alfo, that the Water at
G preffes the Bottom of the Veffel
downwards with the fame Force;
to which Force the Weight of the
Column GH is to be added, by which
means, the Force of the Water pref-
fing upon G, will be the fame as if
the Column GH were equal in height
to the Column FB, that is, as it it
were filled up to M. The faine may
be demonftrated likewife of all the
other Columns ; whence it is mani-
felt, that the Bottom ED is prefled
in the fame manner, as if the Veflel
every where of equal Thickneſs, were
filled with Water to NO.
But the Truth of this Demonſtra-
tion depends upon this Suppofition,
that the Liquor contained in the Vef-
fel be fuch as cannot be comprefled :
as Water which cannot be comprelled.
What therefore was faid of all Li
quors, is to be underſtood of fuch Li-
quors, viz. that they prefs upon Bodies
that are under them, according to their
perpendicular Height, and not ac-
cording to their Breadth.
Corol. 1. If the Tube AB be ſtop-
ped clofe with a Cover, and the lit-
tle Tube CD be filled with
Tab. I. Water up to D, the Water
Fig. 4.
contained in thisTube,will
prefs upon the Water below
in the great Tube, and this Preflure
will diffufe it felf through all the
Water, and thruft againit the Sides
and Cover of the Vellel thus cloſed ;
and it a Hole be made in the Cover,
for the Water to get out at, it will
fly out thence with as much Force,
as if the little Tube CD were as
broad as the Tube AB.
Tab. I.
Fig. 4.
13. But
Corol. 2. If two Cylinders be exac-
ly fitted to the Tubes AB,
CD, Weights laid upon
them will be in equilibrio,
if they are in proportion to
the Width of the Tubes. For Ex-
ample, if the Tube AB be four times
as wide as the Tube CD, one pound
Weight laid upon the little Cylinder,
will be equal to the Force of four
Pound Weight laid upon the great
Cylinder; which Experiments may
be infinitely diverifyed.
Corol. 3.
Hence it is eafie to ex-
plain that Paradox, which fo much
perplexed the Famous
Dr. Henry Moor, and Tab. XVII.
other learned Men, viz. Fig. 2.
why a flat round Board,
fuch as a Trencher, when it is puc
into Water, fhould rife up immedi-
ately, though the Weight of the in-
cumbent Water be much greater, than
that under it, and yet there be no
fuch Thing in Nature as Lightneſs
to lift it up. Let ABCD be a Vef-
fel full of Water, F a round Board
immerfed in the Water. Now be-
caufe, from what has been already
faid, the Columns of Water Hb, Hb,
communicate all their Weight to the
Column dd, and if the Column d¿
fhould defcend, the Columns Hb, Hb
would aſcend with a Celerity, pro-
portionably greater, as they are lefs
thick; whence it is evident, that
theſe ought to be in equilibrio with
each other (in the fame manner as
in the Siphon Tab. I. Fig. 4.) if the
Column Ad be all Water. But be-
cauſe part of this Column is noɛ
Water, but the Board F, which is
Specifically lefs heavy than Water;
therefore the equilibrium is altered,
and the Column GGdd having lefs
Force (compounded of the Magni
tude and Velocity) than the Columns
Hb, Hb; it muſt riſe ſo far, that
there must be as much of the Wood
above
46
Part I
ROHAULT's SYSTEM
13. That it
is fufficient to
allow, that
God once cre-
13. But becauſe it is not the Part of a Philofopher to
make him working Miracles every Moment, and to have
perpetual Recourfe to his Power, we fhall take it for
ated Motion, granted, that when he created the Matter of this World,
he impreffed a certain Quantity of Motion upon the Parts
of it, and that afterwards, by the common Courſe of his
Providence, he hindred Things from returning into their
original Nothing, and preferved always the fame Quan-
tity of Motion, fo that what remains for us to do, is on-
ly to enquire into other Circumftances of Motion, and
to examine Second or Natural Cauſes.
•
above the Superficies of the Water,
as it exceeds in Bignefs a Quantity
of Water of equal Weight. If the
round frenchef F were fo exactly
fitted to the Width of the Veflel, that
no Water could get between it and
the Sides of the Vellel, fo as to com-
municate its Weight to the Water
below, and by that means force the
Board upwards; or if the Board
touched the Bottom of the Vellel fo
clofe, that no Water could get in
between it and the Bottom, then
the Board would not rife at all. As
I have often tried in Quickfilver,
which does not wet the Board, and
therefore will eafily let it go cloſe to
the Bottom of the Veffel.
1. The fame Quantity of Motion)
Some other Principle (befide the Iner-
tia of Matter) was neceffary for put-
ring Bodies into Motion; and now they
are in Motion, fome other Principle is
neceffary for conferving the Motion.
For if two Globes joined by a fender
Rod, revolve about their common Cen-
ter of Gravity with an uniform Mo-
tion, while that Center moves on uni-
formly in a right Line drawn in the
Plane of the circular Motion; The
Sum of the Mctions of the two Globes,
as often as the Globes are in the
right Line defcribed by their common
Center of Gravity, will be bigger than
the Sum of their Motions, when they
are in a Line perpendicular to that
right Line. By this Inftance, it ap-
pears,that Motion may be got or loft. By
reason of the Tenacity of Fluids, and
Attrition of their Parts, and the
Weakness of Elafticity in Solids, Mo-
tion is much more apt to be loft than
got, and is always upon the Decay.
For Bodies which are either abfolute-
ly hard, or fo foft, as to be void of
Elafticity, will not rebound from one
another. Impenetrability makes them
I
only flop. If two equal Bodies meet
directly in Vacuo, they will by the
Laws of Motion flop where they meet,
and lofe all their Motion, and remain
in Reft, unless they be claftick, and
receive new Motion from their Spring.
If they have so much Elafticity as fuf-
fices to make them rebound, with a
quarter, or half, or three quarters of
the Force with which they come toge-
ther, they will lose three Quarters, or
Hulj, or a quarter of their Motion.
And this may be tried, by letting two
equal Pendulums fall against one ano-
ther from equal Heights. If the Pen-
dulums be of Lead or foft Clay, they
will lofe all, or almoſt all their Motions :
If of claftick Bodies, they will loſe all
but what they recover from their Ela-
ficity. Newton's Opticks the 2d
Edition, in Engliſh, p. 373•
It it be asked how Motion, which
is thus perpetually loft, fhould be per-
petually regained. The Anfwer is;
That it is regained by certain active
Principles, fuch as are the Caufe of
Gravity, by which Planets and Com-
mets keep their Motions in their Orbs,
and Bodies acquire great Motion in
falling. The Cauſe of Fermentation,
by which the Heart and Blood of
Animals are kept in perpetual Mo-
tion and Heat; the inward Parts of
the Earth are conftantly warmed, and
in fome Places grow very hot. Bo-
dies burn and ſhine; Mountains take
Fire, the Caverns of the Earth are
blown up; and the Sun continues vi-
olently hot and lucid, and warms all
Things by his Light; (and the Cauſe
of Elafticity whereby Bodies reſtore
themfelves to their former Figures;
all which Caufes fhall be treated of
in their proper Places) For we meet
with very little Motion in the V Vorld,
befides what is owing to theſe active
Principles. Ibid. p. 375.
СНАР:
Chap. 11. --
47
of NATURAL PHILOSOPHY.
CHAP. XI.
Of the Continuation and Ceſſation of Motion.
HOW
a in Motion, could
it comes to paſs that a Body in Motion, ſhould
1. That a
can never of
it ſelf begin
MOVE.
OW
continue to be moved, is one of the moſt confi- Body at Ref
derable Queſtions relating to Motion, and has very much
perplexed the Skill of Philofophers; but upon our Prin- to move, nar
a Body in
ciples, it is not difficult to account for it: For, as was
For, as was Motion of it
before obferved, nothing tends to the Deſtruction of it self ceaſe t●
felf, and it is one of the Laws of Nature, that all Things
will continue in the State they once are unless any external
Cauſe interpofes; thus that which exifts to Day, will
endeavour, as far as it can, to exist always; and on the
contrary, that which has no Exiſtence, will endeavour, if
I may ſo ſpeak, never to exift; for it never will exiſt of
it felf, if it be not produced by fome external Caufe: So
alfo, that which is now a Square, will, as far as is in its
Power, always continue a Square. And as that which
is at Reſt, will never of it felf begin to move, unleſs
fomething move it; fo that which is once in Motion, will
never of it felf ceafe to move, unleſs it meets with fome-
thing that retards or ftops its Motion. And this is the
true Reaſon why a Stone continues to move after it is
out of the Hand of him that throws it,
2. That it is
a mistake to
think that
Bodies in Mo
themſelves
tend to Ref.
2. We fhall therefore have but little regard to that com-
mon Saying of Ariftotle's, That every Thing in Motion tends
to Reft, becauſe there is no good Reafon for it. For if
this Opinion feems to have fome Foundation from what tion do of
we experience on the one Hand of the Things on the
Earth, where a Stone or any other Body in Motion does
not continue always to move; yet it is overthrown by
what is obferved on the other Hand in the Heavens,
where from the Obfervation of many thouſand Years,
we find no Diminution of Motion.
Hotle's Opin
nion cannot
proved by
3. To which we may add, that this Opinion is not fo 3. That Ari-
eafily fupported, by the Experience of what is done here
upon the Earth, as is imagined: For though indeed it be
be very evident, that we fee the Bodies which were in Mo- Experience.
tion, ceaſe to move, and to be at perfect Reſt yet it is by
no means evident, that they tend to this of themſelves:
For no Body can ever think, that a Cannon-Ball, after it
has entered three or four Foot into a Wall, has an In-
clination after that to be at Reft. On the contrary, when
we
ROHAULT's SYSTÉM
Part I.
dies is the
we perceive that this Ball enters deeper or lefs deep, ac-
cording to the Difference of the Bodies that receive the
Force of it, we afcribe, with more Reaſon, the Ceffa-
tion of its Motion to the greater or leſs Reſiſtance made
by thole Bodies.
4. That the 4. This Opinion was 'peculiar to Ariftotle, and no Body
Air refifts
would have ever come into it, if they had confidered,
Motion, and
that the Re- that Air, though it does not refiſt Motion ſo much as a
fiftance of Bo- Wall, yet it makes fome Reſiſtance, as we experience in
a Fan moved quick; for then when they had ſeen a Can-
non-Ball or a Stone, not always continuing to move in
the Air, they would have thought, that this was cauſed by
the Reſiſtance which the Air makes to the Motion of the
Ball, and that the Ball lofes as much Motion as it commu-
nicates to the Air.
Cause of o-
ther Bodies
ceafing to
move.
5. That a
Body in Mo-
sion, lofes fo
much of its
I
5. Now in order to find out how much of its Motion
a Body loſes when it ſtrikes againſt other Bodies, you muſt
remember, that we ſuppoſed that God created a certain
own Motion Quantity of Motion, and that by the common Courſe of
as it commu- his Providence, he preferves as much Motion in Mat-
sher Bodies. ter, as he impreffed upon it at the Beginning; whence
nicates to o-
it follows, that if a Body in Motion, ftrikes directly up-
on another Body at Reft, and puſhes it before it, it muſt
neceffarily loſe as much of its own Motion, as it communi-
cates to the other, in order for them, to go on together with
the fame Celerity as if the two Bodies were one common
Maſs. Wherefore if a Body in Motion be three times
as big as the Body at Reft, it will lofe a fourth Part of
its Motion; and instead of running, fuppofe, a Line of
four Fathom, in a given time, it will run but a Line of
three Fathom, that is, it will move with a fourth Part
lefs Celerity, than it did before.
6. That a 6. If a Body in Motion, ftrikes upon another Body in
Body in mo- Motion alfo, it will make that move fwifter; but it will
tion lofes less
not loſe, ſo much of its own Motion, as if this latter had
of its Motion,
when it firikes been wholly at reft; becauſe all that it has to do, is only
against ano- to add fome Degrees of Motion to thoſe it has already, in
ready in mo- order to make the Bodies move with the fame Celerity:
One Example will make this clear. Suppofe a Body to
have a certain Quantity of Motion, for inftance, twelve
ther Body al-
tion, than
upon a Body
at Reft.
1. That God created a certain
Quantity of Motion) See above Chap.
X. Art. 15. But though Motion
may be deſtroyed, and hard Bodies
that have no elaftick Force, when
they ſtrike againſt each other, are
not reflected, but lofe their Motion;
yet in other Cafes, Bodies perfectly
hard, communicate their Motion to
each other, according to thofe Laws
which the Author is explaining.
Degrees
{
Chap. 11. of NATURAL PHILOSOPHY
49
Degrees, and that it ftrikes upon another which is at
Reft, according to what was now faid, if the firft Body
be as big again as the other, it ought to communicate
four Degrees of Motion to it, and keep eight to it felf.
But if the Body which has twelve Degrees of Motion, ftrikes
againſt the other moving with three Degrees, it ought to
increaſe its Motion but two Degrees, to make it have as
much as it ought to have; becauſe this being but half as
big as the other, it will by this means have Motion e-
nough to go as fwift as the other: And therefore that Bo-
dy which before kept to it felf only eight Degrees of Mo-
tion, will now keep Ten.
**
1
Ì. If a Body in Motion, be three
times as big as another Body at Reft,
and ftrikes againſt it with thirty two
Degrees of Motion, it will give it
eight Degrees of its Motion, and
keep Twenty four to it felf: But
if the latter Body had four Degrees
of Motion before, it will give it but
five Degrees, and keep Twenty Se-
ven. By the fame way of Reafon-
ing, it is eaſy to find out other Laws
of communicating Motion in Bodies
that are perfectly hard. But becauſe
the hardeſt Bodies of all have alſo an
Elaftick Force, and becaufe the Cafe
of Elaftick Bodies, is different from
this, and more difficult, you may find
the Principal Laws by which their
Motion is communicated, explained
by theſe learned Perfons; Sir Chri-
ftopher VVren, Dr. V Vallis, Mr. Hu-
gens, in his Philofophical Tranfacti-
ons, Numb. 43, and 46, and more
fully by the fame Mr. Hugens in his
Pofthumous Works, and by Mr. Mar-
riot, in a whole Book wrote upon
this Subject, and alfo very fully by
Dr. Keil in his Lectures upon Na-
tura! Philofophy. But this whole
Matter may be comprehended in the
following
PROBLEM.
The Weights and Velocities with
which two Spherical Bodies, perfect-
ly Elaftick, whofe Centers are mo-
ved in the fame ftreight Line, meet
each other, being given; to find their
Velocities after they have met.
In the following Computation, the
Man of Elaftick Bodies after ftri-
king againſt each other, is fuppofed
to arife from two Cauſes.
י
E
7. If
1. From fimple Impulfe. By the
Force of which alone, if the Bodies
had no Elaftick Force, each Body af-
ter they had met, would either whol-
ly reft, viz. if they meet each other
with equal Motion; or they would
go both on together, as if they were
united into one Body, with the fame
Velocity; and the Sum of their Mo-
tions (if they moved both the fame
Way) or the Difference of their Mo-
tions (if they moved contrary Ways)
would contiuue the fame after their
meeting as before:
II. From Elaftick Force. Which
in Bodies perfectly Elaftick, is equal
to the Force with which they are
compreffed; that is, when two fuch
Bodies are ftruck againſt each other's
it is equivalent to that Motion which
one of them would gain or loſe by
fimple Impulfe only. This Force acts
the contrary way, and therefore the
Motion which is produced by it,
must be fubftracted from that Mo-
tion, which is in the Body impelling.
and added to that Motion which is
in the Body impelled, by the Force
of fimple Impulfe only, in order
to find their Velocities after Refle-
Єtion.
This being fuppofed. Let A and
B be two perfectly Elaftick Bodies,
and let A either overtake B, or meet
it Let their Velocities be a and
b; Then the Motion of A will be Ac,
and the Motion of B, will be Bb, and
the Quantity of Motion, in them
both together, if they be moved the
fame or contrary ways will be
AaBb, which (by the 1ft Pofition)
will be the fame after their Impulfe
as before. Now (if they had no E-
laftick Force) their common Velo-
city
I
50
Part I.
ROHAULT's SYSTEM
7. How a
Motion.
7. If a Body which was moved by another, be by any
Body lofes its Means turned out of the Way, fo that That from which
it received its Motion, is left to move freely, it will con-
tinue only to move as it did after it had moved the other,
and
city after they had met, would be
Aa± Bb
A+ B
of A,
>
and therefore the Motion
A2a+ABb
ABa B2b
A+ B
Aзa± ABb
A B
and that of B›
A+B
Now if the Motion
which remains in A
after the Impulfe, be ſubſtracted from
the Motion Aa, which it had at firſt,
there will remain the Motion
ABа ABb
A+ B
,which theBody A has loft
by Simple Impulfe only. Now if this
Motion be fubftracted from the Mo-
tion
A 2a + ABb
which is in A, and
A+ B
ABa Byb
added to the Motion
A+ B
which is in B after their Meeting, from
the firſt Cauſe only, the Remainder
A₂a + 2ABb ABa
A+ B
S
will (by
the 2d Pofition) be the Motion of
2ABaB2bFABb
A, and the Snm
A+ B
will be the Motion of B, from both
Caufes together, after Reflection.
And by dividing ſeparately thefe Mo-
tions by their Bodies, we ſhall have
Aa 2Bb Ba
for the Velocity of
A+ B
2Aa Bb
A+ B
A, and
AB
for the Ve-
locity of B after Reflection. Q. E. J.
(See Newt. Algebra. Pağ. 91.
Probl. 12.)
N. B. It may fo happen, that the
Body A, whether it overtakes B, or
meets it, may lofe all its Motion, or
may be driven back the contrary to
that it moved before they met. Where-
fore in this Cafe the Quantity
Да + 2Bb Ba
by which the Ve-
A fi b
locity after Reflection is expreffled,
will either become Nothing (the Ne-
gative and Poſitive Terms deftroying
one another) or Negative. So like-
wife it may happen, that when the
Body B meets A, it may, after their
Meeting, either reft, or go on to be
moved the contrary way to that A
was moved in, before they met;
and then the Quantity by which the
Velocity is expreffed, will either be
Nothing, or (as at firft) Negative.
But if it be driven back the fame
way that A was moved in at firſt,
the Quantity by which the Velocity
is expreffed, will be pofitive. For
fince the Velocity that way which A
was at first moved in, is expreffed
by the Sign+; 'tis evident, that
the Velocity the contrary way, ought
to be expreffed by the contrary Sign
throughout the whole Computa-
tion.
From thefe general Quantities now
found, by which the Velocities of
the Bodies A and B are expreffed, it
is eaſy to deduce the Laws of Moti-
on which are obferved by any per-
fectly Elaftick Bodies after Reflecti-
on, in any given Cafe whatfover.
For Example.
1. If the Velocities of two Bodies
meeting each other, be reciprocally
as their Weights, in this Cafe it
will be Aa Bb, and therefore the
Quantity by which the Velocity of A
Aa- Ba
is expreffed, =
A + B
Ab Bb
A+ B
and that of B, =
a;
=b.
That is, each Body after their Im-
pulfe, will go back with the fame
Velocity. with which they met each
other.
2. If A ftrikes againſt B, when it
is at reft, the Velocity of A will be
(the Quanuity B, and confequently
its Multiples Bb, &c. vaniſhing) =
Aa
A
be =
Ba
and the Velocity of B wilp
B
2 A a
A+B
That is, as the Sunt
of their Bodies is to their Difference;
fo is the Velocity of the Body A be-
fore Reflection, to its Velocity af-
ter Reflection. And as the Sum of
the Bodies, to double the impelling.
Body, fo is the Velocity of A be-
fore Reflection, to the Velocity of
B after Reflection.
3- IF
Chap. 11. of NATURAL PHILOSOPHY.
and not as it moved before it communicated any of its
Motion; becauſe the Manner in which any Thing ought
to continue to exiſt, and to preſerve it felf, is that which
it has this very Moment, and not that which it had ſome
Time
E 2
3. If A be equal to B, and ftrikes | paring their Products together. There-
againſt it when it is at Reſt, the Ve- fore, &c. Q. E. D.
locity of A will be
o.
a.
And the
Which
Velocity of B will be
ſhows that the Body A after ftriking,
will be at Reft, and the Bod B will
be moved with the fame Celerity
after the Impulfe, that A was mo-
ved with before the Impulfe.
4. If A and B be equal, and meet
each other with unequal Velocity, the
Velocity of A after meeting will be
b; and the Velocity of Ba.
That is, each of them will return
back after meeting, having changed
their Velocity.
--
5. If A and B be equal, and A over-
takes B, the Velocity of A will be
b, and the Velocity of B = a.
That is, they will both move the
fame way they did before, having
changed their Velocity.
LEMMA.
If there be three unequal Quanti-
ties A, B, C ; and A be leſs than B,
and B lefs than C. I ſay, (1.) that
AC
B
B
B+..
AC
B
is lefs than AC (2.) that
is leaſt of all, when B is a
(6.) Let there be three Elaftick
Bodies, as mentioned in the Lemma,
A, B, C, and let A ſtrike againſt B
at reft, and after that, let В ftrike
againſt C at reft alfo; I fay, that by
Cat
this Means, the Body C will acquire
greater Velocity, than if it had been
ftruck immediately by A alone, with-
out the Interpofition of B; and that
it then acquires the greateſt Velocity,
when B is a mean Proportional be-
tween A and C. (And the fame
holds true, if the Motion begins
with the Body C.
For by the Second Law, explained
above, the Velocity of C, if it were
impelled by A only, and the Body B
not between them, will be
a
or 4A1-C
2A +2C
2 dl a
A+C
And by the fame
Law, the Velocity of C, when ſtruck
by the Body B with that Motion
which was given it by A will be
4 Аа
ATBA, which two
B.
Fractions, becauſe they have the fame
common Numerator (4 A a) are to
one another as their Denominators,
mean proportional between A and invertely. Wherefore the Velocity of
C
DEMONST.
The first part is evident from Prop.
25. Book 5. of Euclid. The Second
Part may be demonftrated thus. Let
M be a mean proportional between
A and C: then M² = AC. Now
if M and B be equal, it is B –
2 M or 2 B, But if there be
any difference between M and B, let
that Difference be D; and it will be
M²
A C
M+D
AC
B
M ±D
But M +D+
≈ B +
M*
B
MED
is greater than
1 M as is evident by multiplying
each of them by MD and com-
C in the firſt Cafe, is to its Velocity
in the Second, as A+C+B+
But (by the
A C
B
[0 2 A † 2 C.
A C
Lemma)'B +
B
is lefs than
AC, and leaft of all when A, B,
and C are in continual Proportion.
Therefore A + C + B +
A C
B
is lefs than 2 A + 2 C. That is, the
Velocity of C, in the firſt Cafe, is lefs
than its Velocity in the Second, and
this Inequality is greateft, when
A, B and C, are in continual Propor-
tion. If the Motion begins at the
Body C, then if reprefents its Ce-
lerity, and be fubftitured in the Room
of a, the Demonſtration will be the
fame,
7.The
ROHAULT's SYSTEM
Part I.
8. That
greater Bo-
to move lon-
ger than lef
fer ones.
dies continue
9.
An Ex-
ample.
Time before, but has not now. Wherefore a Body which
has loft fome of its Motion, by meeting Another, may
lofe more of it by a ſecond Meeting, or a Third, and fo
on, 'till at laft it may be quite ftopped, as we often
fee.
I
8. From what has been faid, it follows firft, that if two
like and unequal Bodies, be moved in a ftreight Line with
the fame Celerity, the greater Body ought to move longer
than the leffer, becaufe, the Quantity of Motion in each
of theſe Bodies, is in proportion to their Maffes, but they
communicate and lofe their Motion in proportion to their
Superficies only, with which they ſtrike againſt other Bo-
dies, amongſt which they are moved; now though the
bigger Body has more Superficies than the Leffer, yet it
has not ſo much in proportion to its Bulk, and confequent-
ly it does not loſe every Moment fo much of its Motion
as the leffer one does.
9. One Inftance will make this clear. Suppofe the Bo-
dy A to be a Cube two Foot every Way, and the Body
Tab.I. Fig.5. B, a Cube of one Foot; which being fuppofed, the Su-
perficies of the Body A will be four times as much as the
Superficies of the Body B, but the Maſs of it, will be
eight times as big: And confequently, if theſe Bodies move
with the fame Celerity, the Body A will have eight times
7. The more Bodies there are of
a different Magnitude, between any
two Bodies, fo much the greater will
the Velocity of the Laft be: And it
will be the greateſt of all, if the Bo-
dies be in a continued Proportion.
This eafily follows from the pre-
ceeding Article.
1
:9 +
b
8. Perfectly elaftick Bodies recede
from each other after Reflection, with
the fame relative Velocity, that they
approached each other with before
Reflection; that is, in any given
Time, the Diſtance between the two
Bodies before, and after their meer-
ing, will be the fame, at the End of
that time. For the diſtance of the
Bodies in any given time, before they
meet, may be expreffed by a
viz. the fame Quantities by which
the Difference of their Velocities, if
they be moved the fame way, or the
Sum of their Velocities if they be
moved different Ways, is reprefented:
Alfo the Spaces which they defcribe
feparately, in a given Time, after Re-
flection, may be exprefled by the
fame Quantities, by which their Celeri-
ties are exprefled; wherefore, if from
2 AaBb Ab
the Quantity
A + B
which expreffes the Space run through
by the Body B after meeting, the fame
way that A moved before meeting,
Aa± 2 Bb-Ab
be fubftracted
A B
+
which expreffes the Space run through
by the Body A in the fame time, and
the fame way; the Remainder
AaAb+BaBb
A + B
ab, will
give the Diſtance of the two Bodies
at the End of the given Time after
Reflection.
And by the like Reaſoning other
Laws may be found.
1. The greater Body ought to move
longer) It is to be obferved, that this
is faid of Similar, that is, homogene-
ozs Bodies. Otherwife we are to un-
deftand by it, not the Greateſt, but
the heaviest Body: For the Motion
of Bodies that have the fame Celeri-
ty, is not as the Maſſes of thoſe Bo-
dies, but as the Weights of them.
See the Notes Chap. x. Art. 8.
23
Chap. 11. of NATURAL PHILOSOPHY.
53
as much Motion as the Body B; fo that it ought to loſe
eight times as much every Moment, in order for them
to ceaſe together. But this cannot be, becauſe the Body
*A, having but four times as much Superficies as the other,
can meet with but four times as many Bodies, and not
with eight times as many; wherefore the Body A will
move pretty quick, when the Body B will have no Mo-
tion at all, as is confirmed by Experience; for if a Bullet
and a ſmall Shot come at the fame time out of a Gun,
the Bullet will be carried vaftiy further than the fmall Shot.
10. Secondly, Hence it follows alſo, That a long Body,
fuch as an Arrow, will continue to move longer, when it is
hot lengthwife, than it would do if it went crosswife, for
it meets with fewer Bodies to transfer its Motion to,
and therefore it keeps the more to it ſelf.
"
10. That a
Body will con-
longer, when
it goes one
way than
when it goes
tinue to move
another.
11. That a
Body which
moves almoft
tinue its
Motion long-
eft of all.
12. How A
II. Thirdly, If a Body moves almost wholly within it ſelf, so
as to transfer very little of its Motion to the Bodies that fur-
round it, it ought to continue moving longest of all: Thus
we find by experience, that a finooth well polifhed Brass within it felf,
Ball, of half a Foot Diameter, fupported by two Pivots, ought to con-
will, with a ſmall Stroke continue to run round for three
or four Hours.
12. But becauſe a Body cannot ſo transfer its Motion to
another as not to partake with that Body to which it is
transferred, but will retain fome to it felf, though it be
never fo little; therefore it ſhould ſeem that a Body once Ref.
in Motion, ſhould never afterwards be entirely at reſt,
which is contrary to Experience. But we ought to con-
fider, that two Bodies which have but very little Motion,
may be fo connected and adjuſted to each other, as to
be in a manner at Reſt,
at Reft, which is all that Experience
fhows us.
a
13. Becauſe the World is full, a Body moving in a
ftreight Line, muft of neceffity pufh another, and that
Third, but it ought not to go on thus infinitely; for fome
of thoſe which are thus pufhed, will be forced to turn out
of the Way, in order to take the Place of that which was
firſt moved, that being the only Place where they can
go, and which is free for them: Wherefore when any
Body is moved, 2 a certain Quantity of Matter muſt al-
1. Should never afterwards be en-
tirely at Reft) This is falfe, becauſe
built upon a falfe Foundation, viz.
that Motion cannot be deſtroyed. See
the Notes above, Chap. x. Art. 13.
2. A certain Quantity of Matter)
This is for the moft part true, not
becauſe the World is full, but becauſe
the State of the Air, and other Flu-
ids in which Bodies are moved, is
fuch, that when any Body is moved
out of its Place, thefe, by reafon of
their Fluidity, immediately run into
its Place.
E 3
ways
Body may
Seem to be
wholly at
13. That a
Body in di-
makes other
Bodies turn
rect Motion
in a Circle, in
order to take
its Place.
14
Part I.
ROHAULT'S SYSTEM
?
14. That this
Circle, is the
ways neceffarily be moved in the Form of a Ring or at
Circle, or fome way equivalent thereto.
14. This Truth, though it was known long ago, yet
Motion in a Philofophers, for want of duly attending to it, and well
Caufe of ma- weighing and confidering its Confequences, have thought
my furprizing it impoffible to account for all the Motions we ſee in Na-
Motions.
15, The Ob-
Scurity of the
traction, Sym-,
pathy and
Antipathy.
;
ture by Impulſe alone, which is the only way that we
can conceive clearly, by which one Body moves another
by pushing it; and which fo naturally follows from the
Impenetrability of Matter, which all the World agree in
And this is the Reafon why they introduced into their
Philofophy Things, indeed very fpecious, fuch as Attra-
ction, Sympathy, Antipathy, the Fear of a Vacuum, &c.
but which, at the Bottom, are mere Chimera's, invent-
ed to make them appear to give a Reafon of that which
they did not all underftand, and therefore ought not to
be uſed in the better fort of Natural Philofophy..
}
15. For as to Attraction, Sympathy, and Antipathy's
they ought not to be allowed at all, by reafon of their
1. Attraction) Since nothing acts
at a Diſtance, that is, nothing can
exert any Force in acting where it is
not; it is evident, that Bodies (if
we would ſpeak properly), cannot ar
all move one another, but by Con-
tact and Impulfe. Wherefore At-
traction and Sympathy and all occult
Qualities, which are fuppofed to a-
rife from the Specifick Forms of Things
are justly to be rejected. Yet becauſe,
befides innumerable other Phænome-
na of Nature, that univerfal Gravita-
tion of Matter, which ſhall be more
fully handled afterwards, can by no
means arife from the mutual Impulſe
of Bodies (becaufe all Impulſe muſt
be in proportion to the Superficies,
but Gravity is always in proportion
to the Quantity of folid Matter, and
therefore muft of Neceffity be afcri-
bed to fome Caufe that penetrates the
very inward Subftance it felf of fo-
lid Matter) therefore all fuch Attra-
tion, is by all means to be allowed,
as is not the Action of Matter at a
Diſtance, but the Action of fome
immaterial Caufe which perpetually
moves and governs Matter by certain
Laws. Have not the fmall Particles
of Bodies certain Powers, Virtues or
Forces, by which they act at a diſtance,
not only upon the Rays of Light for
reflecting, refracting and inflecting
phem, but also upon one another for
Qb-
producing a great part of the Pheno-
mena of Nature. For it is well known,
that Bodies act one upon another by
the Attractions of Gravity, Magne-
tifm and Electricity; and thefe In-
ftances fhew the Tenour and Courſe of
Nature, and make it not improbable
but that there may be more Attra-
Etive Powers than theft. How thefe
Attractions may be performed, I do
not here confider. What I call At-
traction may be performed by Impulle
(not Bodily Impulfe) or by fome
ther Means unknown to me. I uſe
that Word here, to fignify only in ge-
neral any Force by which Bodies
tend towards one another, whatsoever
be the Caufe. For we must learn
from the Phenomena of Nature, what
Bodies attract one another, and what
are the Laws and Properties of the
Attraction, before we inquire the
Caufe by which the Attraction is
performed. The Attractions of Gra-
vity, Magnetism and Electricity reach
to very fenfible Difrances, and fo have
been obferved by vulgar Eyes; and there
may be others, which reach to fo Small
Diſtances as hitherto escape Obferva-
tions and perhaps electrical Attracti-
on may reach to fuch Small Diſtan-
ces even without being excited by
Friction. Newt. Opt. p. 350.
It feems to me farther, that these
Particles (of Matter) have not only
A vis
Chap. 11. of NATURAL PHILOSOPHY.
55
Obfcurity. That they are obfcure, is very evident; for
if we take a Loadftone; for Example, It is manifeft to all
the World, that to fay it has an attractive Vertue or a
Sympathy with the Iron, does not at all explain the Na-
ture or the Properties of it. And as to the Fear of a Va-
cuum, I referve the Notion of That to the following
Chapter, where we fhall compare the Reaſoning of the
Antients and our own together.
For
|
a vis Inertiæ, accompanied with fuch
paffive Laws of Moiion, as naturally
refult from that Force; but also that
they are moved by certain active
Principles, fuch as is that (Attra-
&tion which we call the Attraction)
of Gravity, and that which cauſes
Fermentation, and the Cohesion of
Bodies. Thefe Principles I confider
not as occult Qualities ſuppoſed to re-
Jult from the Specifick Forms of
Things, but as general Laws of Na-
ture, by which the Things themselves
are formed: Their Truth appearing
to us by Phenomena though their
Canfes be not yet difcovered.
these are manifeft Qualities, and their
Cauſes only are occult. And the
Ariftotelians gave the Name of oc-
cult Qualities not to manifeft Qualities,
but to fuch Qualities only as they
fuppofed to lic hid in Bodies, and to
be the unknown Cauſes of manifeſt
Effects: Such as would be the Caufes
of Gravity, and of magnetick, and
electrick Attractions, and of Fermen-
tations, if we ſhould fuppofe that these
Forces or Actions arofe from Qua-
lities unknown to us, and uncapable
of being difcovered and made mani-
feft. Such occult Qualities put a
Stop to the Improvement of natural
Philofophy, and therefore of late Tears
have been rejected. To tell us that
every Species of Things is endowed
with an occult Specifick Quality by
which it acts and produces manifeft|
Effects, is to tell us nothing. But
to derive two or three general Prin-
ciples of Motion from Phenomena, and
afterwards to tell us how the Pro-
perties and Actions of all corporcal
Things follow from those manifest
Principles, would be a very great Step
in Philofophy, though the Causes of
thofe Principles were not yet diſcover-
ed: And therefore I fcruple not to
propofe the Principles of Motion a-
bove-mentioned, they being of very
general Extent, and leave their Cau-
Jes to
Jes to be found ost. Id. Ibid.
P. 374-
We have the Authority of the
oldeſt and moſt celebrated Philofophers
of Greece and Phoenicia, who made
a Vacuum and Atoms, and the Gra-
vity of Atoms, the first Principles
of their Philofophy; tacitly attribu-
ting Gravity to fome other Carf than
denſe Matter. Later Philofophers
banish the Confiderations of Such
Cauſe out of natural Philofophy, feign-
ing Hypothefes for explaining all Things
mechanically, referring other Caufes
to Metaphyficks. Whereas the main
Bufinefs of Natural Philofophy is to
argue from Phenomena without feign-
ing Hypothefes, and to deduce Cauſes
from Effects, till we come to the very
First Caufe, which certainly is not
Mechanical, and not only to unfold
the Mechanism of the World, but
chiefly to refolve Theſe and fuch like
Questions, &c. Id. lbid. p. 343.
¦
CHAP.
E 4
56
Part I.
ROHAULT's SYSTEM
x. What was
originally
Fear of a
Vacuum.
CHA P. XII.
•
Of ſuch Motions as are commonly ascribed to the Fear
of a Vacuum.
HERE is no Subject more capable of fhowing us
T
the Difference betwixt true and falfe Philofophy, or
meant by the at least betwixt Reafoning juftly and not juſtly, than this
For we ſee manifeftly, that the one leads us, if not to
the Truth, yet to fo great an Appearance of Truth, that
the Mind acquiefces in it; but the other gives us only
Words, which we can form no Idea's from. For Proof
of This, Let us take for inftance a Syringe, one End of
which being put into the Water, and the Sucker drawn,
let us hear how the Antients reaſoned about it. First, They
obferved, that there could be no Vacuum in Nature; then they
confidered, that there would be one, if the Sucker were
drawn, and no Water followed; whence they concluded,
that the Water ought to enter in proportion to the draw-
ing the Sucker; and hence they faid the Water afcended,
left there fhould be a Vacuum.
2. How the
2. Afterwards, the Manner of the Expreffion was chang-
Senfe of this ed, without altering the Notion, and it was faid, that the
bas been cor- Water afcended, for fear there fhould be a Vacuum in
rupted.
Nature: And this Expreffion being equivocal, it was ta-
ken in a bad Senfe, and as it is cuftomary to carry Things
to Extremity, the Word Fear was changed to Horrour;
fo that it was affirmed, that the Water afcended, out of
the Horrour which Nature had of a Vacuum, as if Nature
(in the Senſe that Philofophers underſtand that Word) was
capable of Horrour.
3. The Fear of a Vacuum in this latter Senfe, is very
ridiculous, wherefore I am apt to think that the Philoſophers
took it in the former Senfe only: But which way ſo ever
it be underſtood, it does by no means anſwer the Que-
tion; any more than it would, if any one ſhould ask, how
Wood came from very remote Parts to Paris, and it
ſhould be anſwered, it came out of the Fear of Cold; this
is no anſwer to the Queſtion, becauſe the final Cauſe is
alledged inſtead of the efficient Caufe, which was the Thing
demanded.
4. How
Chap. 12. of NATURAL PHILOSOPHY.
57
However, if the Reaſoning of the Antients were 4. That the
juft, and built upon a good Foundation, though it could Reasoning
not make us underſtand how the Water afcends, that is, the Fear of a
drawn from
explain to us the efficient Caufe of fuch Afcent; yet Vacuum, does
it fhould prove, at leaſt, that it ought to afcend; and not fufficient-
ly agree with
their Reaſoning fhould agree with Experience. And that Experience.
you may fee that it is defective here alfo, it is to be ob-
ferved, that if the fole Reafon, why any Space is filled,
is for fear there fhould be any Vacuum in Nature, and
this makes the Water afcend; as this Reafon is always
the fame,, it will follow, that the Water ought always
to aſcend, ſo long as the Sucker of the Syringe is draw-
ing. be it never fo long; now Pumps being only long
Syringes, they ought to raiſe up Water to any Height
whatfoever; yet Experience fhews us, that we cannot by
Pumps, raiſe it above One and thirty Foot and a half,
after which, the Water ftops, and will not follow the
Sucker. Whence we ought to conclude, that the fear of
a Vacuum, taken in the moſt favourable Senſe poffible,
is not at all the Caufe of the Waters afcending, fince it
does not agree with Experience.
5. Various
5. Having feen the Defect of the Reaſoning of the
Antients, let us fee if we can fay any Thing better found- Suppofitions
ed. And that I may not be guilty of the fame Fault, I this another
to explain
fhall offer fome Particulars, which are very clear and way.
intelligible to all the World, in order to draw fome cer-
tain and undoubted Confequences from a Foundation
which cannot be contefted.
6. The firft
Suppofition
6. Let us fuppofe firft, That fome Body endeavours.
to draw the Sucker from the Bottom of the Syringe Tab.. Fig.6.
ABC, the Hollow of which it exactly fits, that the whole
Syringe is in the Air, and that the Hole C is open: This
being fuppofed, it is evident, that the Sucker D cannot
be drawn towards E, but it will pufh the Air, which will
puſh that beyond it, 'till, as was faid above, it turns in
the Lines here defcribed, or fome fuch like, in order to
enter into the Place from whence the Sucker was drawn;
whence it follows, that the Air was moved by a real
Impulfe.
7. Let us fuppofe Secondly, That the
were ſtopped, and that there were no Pores
the Syringe or the Sucker; In this Cafe, I fay,
1. It would be impoffible) This
would indeed be true, if the World
were full: But becauſe we have af
firmed it to be otherwife; fo much
Force only is required to draw the
Hole at C, 7. The Second
either in
Suppofition
it would
I
Sucker, as can lift the whole Weight
of the incumbent Air. Nor need
we here trouble our felves with any
Occult Pores or fubtile Matter.
be
ROHÄULT'S SYSTEM
Part I
* The Third
be impoffible to draw the Sucker, the leaſt that can be,
becauſe the World being full, the Air which ought to
puſh the Sucker, would have no Place to go to.
8. On the other hand, Let us fuppofe, that the Syringe
Suppofition. thus ftopped, has Pores, though fo very fmall, as not to
be perceived by our Senfes, and that amongst the Particles
of the Air, there are fome fo fubtle, as to be able to en-
ter thefe Pores. This being fuppofed, there is no Reaſon
why the Sucker may not be drawn, though the Hole at
the Bottom of the Syringe be ſtopped: For then the Suck-
er may make Room for it felf, by preffing the groffer parts
of the Air, and by fqueezing out the fubtle Parts, which
are forced to enter the Syringe.
9.
That the
greatest Part
of terrestrial
Pores, and
zicles.
9. In order to know whether the Sucker of the Syringe
can be drawn when the Hole at the lower End is ftop-
Bodies have ped; we muſt firſt know, whether the Syringe or the
Sucker have any Pores in them or no, and after that, whe-
that the Air
ther there be any Particles in the Air fubtil enough to
confifts of two
Sorts of Par- enter in at thefe Pores: For according to one or other of
theſe Suppofitions, will the Thing be poffible or not pof-
fible. And becauſe neither of them can be determined
by our Senfes or by Reafon, and there being no Contra-
diction in either, it muſt be decided by Experience; now
we find by Experience, that if the Syringe he not too
thick, we can draw the Sucker without much Difficulty;
from whence it is evident, that there are Pores either in
the Syringe, or in the Sucker, or rather in both of them;
and that amongſt the grofs parts of the Air, there are föme
fo fine, as to pass through the Pores of moſt terreſtrial
Bodies.
10. Another
that the Air
is weighty.
10. This Experiment helps us to another very confide-
very confide- rable, which is, that if, after we have drawn the Sucker
rable Experi-
ment; and
a little, we let it go again, it returns of it felf, and that
with fuch a Force, as to ſtrike againſt the Bottom of the
Syringe; the Reaſon of which we fhall fee, if we remem-
ber that a Body never begins to move of it ſelf, if it be
not puſhed by another which immediately touches it;
now, if we obferve, that there is nothing but the Air,
that immediately touches the Sucker, we muſt think that
it is the Air that caufes this fuprizing Motion; for, con-
1. If the Syringe be not too thick)
The Thickneſs of the Syringe figni-
fies nothing (nor the occult Paflages,
nor the fubtle Matter, as was faid on
the Article above;) but the Thickneſs
of the Sucker; which the greater it
is, fo much a Greater, and confe-
quently fo much a heavier Column
of Air muft it fuftain. But the Au-
thor may be excufed, if he means
the Bignefs of the whole Syringe.
fidering
Chap. 12. of Natural PHILOSOPHY,
fidering that the Air always contains in it a great Quanti-
ty of the Particles of Water, and other terreftrial Bodies,
which though they be feparated from each other and dif
perfed, yet do not lofe any of their Weight; though we
do not fully underſtand the particular Nature of the Air,
nor in what its Weight confifts; we fhall make no Dif-
ficulty to affert, that the groffer Air is heavy, and confe-
quently, that by its Weight, the Sucker is forced into the
Syringe, from whence it fqueezes out the fubtil Matter
through thofe Pores which it ſelf entered in at.
11. That the
Air by its
VVeight may
11. But though the Air by its own Weight, preffes
chiefly downwards, yet this does not hinder, but that it
may alſo preſs upwards, and force the Sucker of the in-
press up..
verted Syringe up into the Syringe; for the Column of wards.
Air which anfwers to the Bottom of the Sucker, is for-
ced upwards by the Weight of thofe Columns of Air
which are on the Sides, in the fame manner as the Water
which is at the Bottom of a heavy laden Boat, is preffed up-
wards against the Bottom which refifts it, by the Weight of
the Water which is of confiderable Height round the Sides.
12. When we once underſtand this Force of the Air to
prefs upwards, we fhall not at all wonder, that when we
hold out our Hand flat in the Air, we do not feel the the incum-
Weight of it; that is, we do not perceive our Hand bent
preffed downwards, by the Weight of the Column of Air
which is upon it: For this Column has no more Force tor
prefs it downwards, than the Column which is underneath
has to prefs it upwards.
13. As to the Preffure which is made all over the Body,
when it is immerfed in a heavy Liquid; it is certain, that
we ought not to perceive it, i though the Weight of the
Liquid be very great, any more than we do the Preffure
1. Though the Weight of the Liquid
be very great) The Caufe of this is
excellently well explained by Jo.
Alph. Borellas, de Motibus Nat. a
Gravitate factis. prop. 29. & feq.
After he had fhown, that Sand in a
very ftrong Veffel, cannot any way
be divided, and that a Wedge will
by no means enter into it; and alſo
that Water in a Bladder, equally com-
preffed on all Sides, can neither be
Itreightened nor bent, nor at all mo-
ved
:
So likewife, fays he, in the Bo-
dy of an Animal, there is contained
within the Skin, fome Parts, which
are hard and folid, fuch as the Bones;
ibers that are foft, fuch as the Ten-
}
12. Why we
do not feel
the Weight of
13. Why we
do not feel the
rare of
Preffure
the Air, and
allo why Di-
of vers do not
feel the
Weight of the
į
dons, Nerves, Membranes and Muf- Water.
cles; and others that are Fluid, Wa
try or Oily. Now the Bones in an
Animal cannot be broken or disjoini-
ed, unless the incumbent Weight pref-
fes one way only, as it does on Parters:
But if the Preffure diffuſes it ſelf all
round, fo as to press upwards and
downwards, and fideways, with equal
Force, fo that there be no part of the Skių
but what is preffeä; then it is impoffibles
that any Thing should be ſeparated or
put out of the Way. The fame may be
faid of the Nerves and Mufcles, which
though they be foft, yet because they con-
fift of strong and tough Fibres, they can
all fupport one another, and refift an
miverfalls
ROHAULT's SYSTEM
Part I.
1
+
of the Water, when in diving into the Sea, there are ma-
ny Fathom of it over our Heads. The Reaſon of which
is, that before we can feel the Weight of any Body, there
muſt be fome Alteration made in the Difpofition of our
Organs. But when the Air or Water have made all the
Efforts they are capable of, to prefs or thruft inwards the
external and groffer Parts of our Body, and theſe Forces
are counterballanced and put in æquilibrio, by the Reſiſt-
ance and Effort of the Fluids and moveable Parts within
us, the Action of which we are infenfible of; after this,
I fay, they can do no more, and confequently the State
of our Body will not be changed, nor the Difpofition of
its Organs, to which they are fo uniformly applied, and
with fuch equal Forces, that no one fingle Part can move
outwards, to give way for any other to be thruſt inwards;
and therefore the Effort which they continually make to
prefs us inward, is rendred ineffectual.
niversally diffufed fphærical Compref-
fion; the fame may alſo be ſaid of the
Blood and other Humours of an Ani-
mal, which are of a watry Nature;
for, as it is evident, that Water can-
not be condenſed, ſo likewiſe the H1-
mours of an Animal, contained in the
Cavities of its Veſſels, though they
may be bruised by an Impulse made
from one or a few particular Places,
yet they can never he forced out of
their Veffels, or torn asunder by an
univerfal Compreffion every Way. So
long therefore as the Solid, Tendinous,
or Fleshy, or Liquid Parts, do not
undergo any Separation, Contufion,
nor are disjointed, nor their Situati-
on at all changed; it is impoffible,
that any Pain or Uncafiness should
follow in the Animal, which cannot
arife from any other Canfe, but fepa-
rating that which is one continued
Thing. Wherefore when Divers, &c.
And this is confirmed by what
the famous Mr. Boyle obferved, in
Dis Second Appendix to the Eleventh
Hydrostatick "Paradox, viz. that a
Tadpole, an Animal whofe Fleſh is
very tender and foft, put into a
Veſel half full of Water, ſo cloſed
up, that the Air contained in it,
being condenſed eight times as much
as in its natural State, preffed upon
the Water as much, as if a Column
of Water of Three hundred Feet in
Height laid upon the Animal; mo-
ved it felf 'notwithſtanding, and
ſwam about very quick, and found
no Inconvenience, that could be per-
ceived.
However, becauſe in moſt Ani-
mals there is a great deal of Air,
which may eaſily be compreffed and
condenſed; therefore, though no par-
ticular Member is disjointed, when
an Animal is immerfed very deep
in Water, yet they muſt all of them
neceffarily be freightned and con-
tracted, by the equal Weight and
Preffure of the incumbent Water on
all Sides, as the famous Mr. Boyla
fays, happened to the Tadpole in the
forementioned Experiment.
whofe
Befides, thofe Animals,
Lungs are ſo formed as to contain
a great deal of thin Air and Breath
in them, though the other Parts of
them be not at all hurt, yet their
Breafts muft of neceffity be ftreight-
ned and contracted, in the fame
manner, as the Cork is uſually thṛuft
into an empty Bottle, by the Weight
of the Water, when it is funk very
deep. Therefore Men, whofe Lungs
are very large, when they dive very
deep into the Sea, though they find
no manner of Inconvenience in any
other Part of their Bodies, yet they
labour under a Difficulty of Breath-
ing, and a Pain in their Breaſt,
(though they have Air enough con-
veyed to them to breathe.) And thus
the famous Mr. Boyle tells us of a
certain Diver, that when he walked
at the Bottom of the Sea, the Blood
flew out at his Noſe and Eyes.
14. Let
Chap. 12. of NATURAL PHILOSOPHY.
14. How the
Water is
drawn into
the Syringe.
14. Let us, in the fourth Place, Suppofe the Sucker
which is in the Syringe, as far as it can be thruft, to be
drawn when the Hole C at the Bottom is in the Water;
it ſhould ſeem as if the Air which the Sucker that is
drawn preffes upon, ought to prefs upon the Water, and
make it to rife in the Syringe, becauſe it overtakes it, in
the way which we fuppofed it to go, in order for it ſelf to
enter in, if the End of the Syringe had been in the Air,
and not in the Water, and that it ought to afcend as far as
the Sucker is drawn. But there is no Neceffity that this
fhould always happen; For having made it appear, that
both the Syringe and the Sucker are full of Pores, and
that the Air is full of Matter, fubtle enough to pafs through
them; and alſo that the Water, by reaſon of its Weight
afcends with greater Difficulty; the Sucker may poffibly
be drawn, and the Water not neceffarily afcend, to fill
the Syringe, becauſe it was filled before with that fubtil
Matter, intermixed with the Air. However, Experience.
fhows us, that the Water does afcend, and that the Sy-
ringe is filled with it, and not with the fubtle Matter,
at leaft to the Height of One and thirty Foot and a half,
but no further. The Reafon of which is, that the Air
being heavy, preffes upon the whole Superficies of the
Water in which the End of the Syringe is immerfed;
and when the Sucker is drawn, the Water which anſwers
to the Hole in the End, not being preffed by the incum-
bent, Air, the Weight of that which preffes upon the reft
of the Surface, thrufts it up, and makes it afcend in the Sy-
ringe; in the fame manner, as the Water in a Pail is made
to afcend up a Trunk, fuch as they fhoot with, open at
both Ends, and one End fixed in a Hole in a Trencher
which exactly fits the whole Superficies; upon depreffing
the Trencher, the Water is forced up. In like manner,
the Moving of the Sucker, is the general Caufe of the
Entrance of fome Matter into the Place which it leaves;
but the Weight of the Air determines the particular
Matter.
Syringe ought
15. Since we find by Experience, that the Sucker of to rife but to
a Syringe may be drawn, when the Hole at the End is Height, and
ftopped, this is fufficient to convince us, that the groffer that a Co-
Air is not of an infinite Weight; for if it was, it would weighs as
be impoffible to draw it, which being fo, it is eaſy to forefee, much as a
that the Air by its Weight cannot raife the Water in a Sy- and thirty
ringe above a determinate Height; fo that if, after this Foot and a
Height, we continue to draw the Sucker, the Syringe,inftead half of Wa-
ter of equal
of being filled with Water will be filled with fubtle Matter, as Thickness.
was
15. That the
Water in a
a certain
ROHAULT's SYSTEM
Part I
16. That we
ought not to
perceive the
Weight of
the Air that
is drawn into
but we ought
the Syringes
to perceive
that of the
VVater.
17. When a
Tube filled
with Water
ought to
was before obferved in Pumps: And fince the Water always
riſes to about the Height of Thirty one Foot and a half,
above the Level in which the End of the Pumps is im-
merfed, we ought to conclude, that a Column of Water
of this Height, weighs as much as a Column of Air of
equal Thickneſs, which reaches to the upper Surface where
the groffer Air terminates.
16. If the Sucker of the Syringe flips very eafily a-
gainſt the concave Surface, againſt which it rubs, and if
it had no Weight at all, the Air would very easily be
drawn in, becauſe there is juft as much Force to thruſt
it upwards, as there is Weight upon the Sucker to thruſt
it downwards: But if Water or any other heavy Liquor
is to be raiſed; there muſt then be as much Force uſed,
as is equal to the Weight of the Liquor to be raiſed;
becauſe the Liquor, tending downward, bears upon the
Air, which preffes againſt the Bottom of the Sucker,
and takes off fo much of the Force it had to make it
rife.
17. There may be many Confequences drawn from
what has been faid of the Syringe, which if they be a-
greeable to Experience, are fo many Confirmations of the
empty it ſelf. Truth of our Explication. For Proof hereof, let us fup-
pofe, for Example, that after having filled a Tube with
Water, one End of which is ftopped with the Matter
with which it is made (which they call hermetically feal-
ed) and the other, with the End of one's Finger, we put
the End of the Tube which is ſtopped with our Fin-
ger into a Veſſel of Water, and then take our Finger
away; This being fuppofed, if we confider that the Air,
which preffes upon the Water in the Veffel, refifts the
deſcent of that which is in the Tube, we fhall foreſee,
that if the Tube be not above One and thirty Foot and
a half long, it will not empty it felf at all; but if it be
longer, the Water ought to defcend till there is One and
thirty Foot and a half in the Tube, and then ſtop, be-
caufe the Air has only Force enough to counterpoife fuch
a Quantity: And this is agreeable to Experience.
18. That an
inclined Tube
oright to con-
tain more
Water than
an upright
one.
18. We here fuppofe, that the Tube, which is above
Thirty one Foot and a half long is held upright, and does
not incline one way or the other: For if it inclines any
way, then, becauſe the concave Surface of the Tube
fuftains part of the Weight of the Water, for that Rea-
fon, the Water will not have fo much Force to deſcend
as it has ordinarily, and fo the Air is able to fupport a
greater Quantity than One and thirty Foot and a half in
the
Chap. 12. of NATURAL PHILOSOPHY.
the Tube; that is to fay, according to the Laws of Me-
chanicks, if the Water in the inclined Tube begins to de-
fcend, it will ſtop, when the upper Surface of it, is One
and thirty Foot and a half perpendicularly above the Su-
perficies of the Water in the Veffel, and fo we find it
does.
nefs.
19. And it is remarkable, that if we make uſe of Tubes 19. That the
of different thickneſſes, and Veſſels of different breadth, Vater ought
to be of equal
there is no difference in the Height of Water contained Height in
in the Tubes: For fince the Water which is in each Tube, Tules of dif-
poffeffes the Place of that Quantity of Air, which laid ferent Thick
upon the fame Part of the Superficies of the Water in the
Veffel; it cannot but be in æquilibrio with the Air without,
becauſe, it weighs juſt as much as that whofe Place it pof-
feffes. And thus it is in all Tubes whatſoever, the Wa-
ter riſes to the fame height, which we fee by Experience
in a particular Tube, that it ought to rife to; for as thefe
different Columns of Water are of the fame height; if
that, for Inſtance, which is four times as thick as another,
weighs four times as much as that other; then the Co-
lumn of Air, the Place of which this grofs Column of
Water poffeffes, weighs four times as much alo.
20. Neither ought we to find any difference in the Height
of the Water which is in the Tube, whether the Expe-
riment be made in the open Air, or in a Chamber, pro-
vided there be a Window in it, or at leaſt any Chink
through which the Air can enter; for according to the
Laws of Mechanicks, the Weight of the Air is juſt the
fame, whether it preffes perpendicularly, or winding
oblique.
or
20. That
there will be
no Alteration
made in the
Height of the
Water, if the
Experiment
be made in a
Place that is
Shut up.
21. That the
21. Neither ought there to be any Difference in this Height of the
Water ought
Height, if after the Experiment be made, the Room be
to be the fame,
entirely cloſed up; for though the Column of Air which though the
fupported it before, by preffing upon the Liquor in the Place in which
Veffel, be now intercepted by the Ceiling, yet that part ment is made,
the Experi-
of the Column of Air which is below the Ceiling, preffes be entirely
as much upon this Liquor as it did, when it bore the closed p.
Weight of the Reft of the Column, becauſe the Refift-
ance of the Ceiling does as it were prefs upon it, and 22. That the
hinder it from expanding it felf.
Height of
the Water
ought to be
greater, if the
Place had
been entirely
22. It is true, that if, before the Experiment be made,
the Chamber be fo exactly fhut up, that the Air within
has no Communication with that without, then the Li-
quor contained in the Tube ought not to defcend quite fo clofed up be
far; becauſe as the Tube empties, and the Liquor in the fore the Ex-
Veffel rifes, the Air which is in the Chamber cannot rife in made.
Pro-
periment was
ROHAULT's SYSTEM
Part I.
7
23. That
Proportion Confequently it muſt be condenſed, and
therefore will have force enough to fuftain a little more
Liquor in the Tube; but this cannot be perceived un-
leſs it be a very little Place in which the Experiment
is made.
23. From what has been faid, it is eafy to appre-
Quickfilver hend, that if inſtead of Water, any other Liquor that is
ought not to
remain in the heavier or lighter be uſed, there will remain more or leſs
Tube, above of it in the Tube; fo that Mercury or Quicksilver, which
the Height of is about fourteen times as heavy as Water, ought not to
Twenty Seven
be ſuſtained by the Air, but to about Seven and twenty
Inches and a half, which is very near a Fourteenth Part
of the Height that Water is ſuſtained, and the reſt of the
Tube, how long foever it be, ought to be filled with
fubtle Matter. And this is confirmed by Experience.
Inches and a
kalf.
24. That Ex-
made with
24. But that the Experiments may be more ſenſible,
periments are the Tubes fhould be made of Glaſs, becauſe that is tranf
anore easily
parent And Quickfilver being fo heavy, that we are not
Quickfilver. obliged to have Tubes much longer than Twenty feven
Inches and a half, their Smalnefs makes them more eaſy
to be managed, and to obferve a great many particulars,
which it would be difficult to do in Tubes that are ve-
ry long.
25. That
there is no
Vacuum in
the Top of the
Tube.
26. Another
Proof.
25. First then, This may give Occafion to thoſe who
believe the Poffibility of a Vacuum to obſerve ; That
there is no Vacuum in the Top of the Tube, but the Place
which is left by the Mercury, is filled by fome Matter,
becauſe the viſible Objects behind the Tube, 1 affect our
Eyes ſtill, and are as plainly fenfible as they were before,
which they could not do, if there were a Vacuum; be-
cauſe their Action would be interrupted. And if the Eye
were placed directly againſt the Tube, we ought not to
fee any more than in the Dark, or than if an opake Bo-
dy were between; but we find it otherwiſe.
26. To this we may add, that 2 Nothing or a Vacuum
has no Properties, and that if we put the Top of the
1. Affect our Eyes) It don't at all
follow, that there is no Vacuum in
the Top of the Tube, becaufe the Space
out of which the Quickfilver came,
is tranfparent; For why cannot the
Rays of Light, paſs through an en-
tirely void Space? On the other Hand,
they can't poflibly pass through a
Space that is quite full: See what
is faid of the Nature of Light, in
its Place.
4. Nothing, or a Vacuum has no
Properties) It is very true indeed,
that Nothing has no Properties; But
how does it follow, that Space which
is void of Matter, has therefore
Nothing in it, or is it felf entirely
Nothing. But it may be allowed that
there is fome finer Matter in the
Top of the Tube, or perhaps a little
Air flipt under the Quickfilver which
is rarefy'd by the Heat; but the
Space is very far from being full.
Tube
Chap.12. of NATURAL PHILOSOPHY.
65
"
Tube very near the Fire, we perceive a Rarefaction, in
the fame manner, as in a Thermometer, which makes the
Mercury fall, whence it follows, that there is fome real
Matter in it.
Į
Tube is not
27. However it is eaſy to fee that this Space is not full 27. That the
of common Air, for if the Tube be not quite filled with Top of the
Quickfilver, but an Inch or two be left for Air, and ſtop • full of gross
ping the End of the Tube with our Finger, it be invert- Air.
ed; we obferve that the Quickfilver defcends flowly, and
we have time to fee the Air afcend in the Form of
Drops. Whereas let the Tube be entirely filled with
Quickfilver, and immerſed in the other Quickfilver, that it
may empty it felf in the ordinary way; then if the Tube be
ftopped with the Finger and inverted; the Quickfilver will
not fall flowly, but all at once, as if it were one hard Body,
nor fhall we perceive any Thing to afcend through it.
·
".
28. For a further Confirmation of this Opinion, viz. 28. The Third
That when the Quickfilver defcends from the Top of the Proof..
Tube, it is not filled with common grofs Air, we may obferve;
that if the Top of the Tube be made large, in the Form of a
Veffel, and fome Sort of Animals, as Birds, Rats, and Mice, be
put into it, they will die, in a very fhort time; that others,
fuch as Flies, feem to die, but being preferved afterwards,
two or three Days in a more temperate Place they revive.
and fly away; and others, fuch as Worms and Frogs are prefer-
ved alive, and not hurt, unless they continue very long in it.
29. It may here be demanded, how the fubtil Mat-
ter, which fills the Top of the Tube, gets through: To
which it may be anſwered; that it feems rather to pafs
through the Pores of the Glafs, than thoſe of the Quickfil- Top of the
ver, becauſe the Quickfilver being very heavy, the Pores
of it ſeem to be rather too fmall for it to pass through them:
Though I fhall be of another Opinion, if what I have
heard from England be true, viz. that a Tube of fix
Foot long, will not empty it felf at all, if the Quickfil-
1. That a Tube fix Foot long) This
Experiment is thus related by the
famous Dr. Vallis in his Hydro-
faticks, Prop. 13. If the Quickfil-
ver fufpended in an inverted Tube, be
very exactly cleared of all Air before
it be inverted (which cannot be done
by great Care and Niceness,) and if
the Tube be cautionfly inverted, and
fixed in a firm Place ſo as not to be
in the leaft fhaken the Quicksilver
(though the Orifice at the Bottom be
open) will remain ſuſpended, much |
F
:
ver
beyond the forementioned Height (viz.
to 40.50, or 60 Inches ;) but if the
leaft Air gets into the Quickfilver
thus fufpended, or if the Tube be ha-
ken, the Quicksilver will immediately
rush down to the ufual Height (and
after fome Reciprocations,) will ſtand
fill.
Which Experiment, having been
often repeated by the Lord Brownker,
the famous Mr. Boyl, Mr. Huygens
and others, has fucceeded; fo that
there is no doubt of the certain Truth
29. VVhas
Pores the fub
which is in the
til Matter
Tube may
pass through.
66
ROHAULT'S SYSTEM
Part I.
ver with which it is filled, and that in which it is im-
merfed, have ftood fome time in a Place void of
grofs Air: For in inquiring into the Reafon of this
Phænomenon, we can find no other but this, that
is cleared of fome
the Quickfilver thus prepared
Matter
of the Phænomenon; but upon what
Caufes fo furprizing a Thing depends,
is not fo well agreed.
3 {
The Lord Brounker thought, that
the VVeight of the Air was much
greater than answers to the Height of
about 29 Inches of Quickfilver, but
that the Quickfilver was depreffed to
that Height, by the Air which was in-
vifibly mixed with it (unless it was
cleared of it) And after it was
cleared of it, and there remained no-
thing to refit the VVeight of the ex-
ternal Air, but only the bare weight
of the Quickfilver then it was found
to be otherwife; and the Quicksilver
was supported to a greater Height,
by the Ballance of the Air. This is
indeed very ingenious; but that which
weakens very much this Explication,
is, that
upon the leaſt ſhaking of the
Tube, the Quickfilver immediately
rufhes down which could by no
means be, if it were fupported by
an equal Weight of Air or Æther.
nate and immutable Affection of all
Matter, neither can this Explication
be admitted. And indeed this very
learned Perfon confeffes, that he him-
felf was not fatisfied with it. There
fore he adds. That the Superficies of
the Tube however well poliſhed, can-
not be thought to be so free from all
Ruggedness or Inequality, but that
there must remain fome Roughness,
which must caufe Cohesion, or (if it
be moved) Some Friction of the ad-
jacent Body, whereby the Motion must
be fomething hindred.
And indeed this Opinion comes
nearer the Truth; and that chiefly
becauſe upon the leaft fhaking of the
Tube, the Quickfilver falls down,
whence it is manifeft, that the Suf-
penfion does not depend upon any
permanent Caufe, fuch as the Gra-
vity of the Air or Ether, but upon
fome accidental Thing, fuch as fome
kind of Adhafion. However, `be-
caufe there does not appear to be any
fuch Roughness in the Superficies of
Wherefore the famous Dr. VVallis
the Glafs, as this learned Perfon ima-
attempted the Thing another way.
He imagined, that all real Gravita gines; it feems to be most proba-
ble, that the Quickfilver remains thus
tion, proceeded from the Preffure or
Spring of the Air or Ather, without fufpended from the Contact or Agree-
ment of the Parts, the Force of which
which those inactive Bodies which
we call heavys if once at rest, would is always greateſt in every Effect of
remain ſo, without any real gravita- Nature. Thus, a plain and fmooth
tion, or without defcending, having no Loadfione applied to a Ball of Iron
more Tendency to move downwards than fufpended on a firing from a Nail
fideways. The Quickfilver therefore, will draw it much further from the
when it is cleared of all Air from Perpendicular, than in proportion to
within, and ſuſpended in the aforesaid the Magnetick Force, if it be pulled
back with a gentle and even Hand,
manner, when it is at reft, will con-
tinue ſo, and retain its Pofitions be and be not feparated by any acc
dental Shake. So alfo Water will
yond the common Height necessary to
an æquilibrium, because it is free afcend in a l'acuunt, în ſmall Glaſs
from all Preffure of the Air, and Tubes open at both Ends. And two
is not preffed upon, either by its Gra-fmooth polifhed Marbles will not
vity, or by its Spring: But if it be be feparated, though the grofler Air
put in Motion, either by any shaking be removed. And fo the Parts of all
of the Tube, or by any Motion within, hard Bodies (and in fome meafure
from the Spring of the Air which was alfo of Liquids) cohere together by
Contact, that is, by that attraction
at first left in it, or is fince got in, then
which always arifes from Contact
it will continue that Motion down-
See what is faid below at Chap. 22.
wards (that way being open.)
Artic. 9.
But fince it is now allowed, that
Gravity does not depend upon the
Air or Æther, but is an original con
?
·
All the Author's Pains therefore a-
bout fubtil Matter, and about the
Pores
Chap 2 of NATURAL PHILOSOPHY.
67
Matter, which before kept its Parts at a Diſtance, and
made the Pores fufficiently wide and long, to give free
Paffage to the fubtil Matter; and becauſe it cannot thruſt
the fubtil Matter into the Place which it is difpofed by
its Weight to quit, therefore it does not defcend at all:
However, not having had any Opportunity to fee how
well this Experiment fucceeds, and not venturing to fay
that it is falfe, we remain in fufpenfe, and do not deter-
mine which Body it is, through the Pores of which the
fubtil Matter paffes, to fill the Top of the Tube.
30.
30. VVhat
quence will
go. But to return to our Difcourfe, and to continue
and to continue
to draw the Confequences which we think deducible from the Confe
what has been faid above; Let us fuppofe a Tube filled be, if the
with Quickfilver, and immerfed as ufual in a Veffel, in- Tube be lifted
to which Part of the Liquid runs, 'till it is about the that the low-
up a little, fa
Height of Twenty feven Inches and a half, and then it be er End of it
lifted up a little above the Surface of the Quickfilver, fo that be orst of the
one Drop only of it may run out; then becauſe the Quick- Quickfilver.
filver, that remains in the Tube, does not weigh ſo much
as the Air without, it ought to be impelled with Violence
to the Top of the Tube, and after that, its own Weight
ought to make it defcend on the one hand, as much
as the Air makes it afcend on the other; and fo we find
it does.
Veight of
feel the
the Quick fil-
ver that is in
the Tube.
31. If, after having made the Experiment as ufual, we 31. That we
take the Tube out of the Veffel in which it is immerfed, ought not to
ſtopping the lower Hole with our Finger, but not preffing
very hard upon it, then we ought not to feel, nor do we
indeed feel the Weight of the Quickfilver: For though it
lies upon that part of the Finger, which anfwers to the
Hole of the Tube, yet it is not heavy, becauſe it preffes
neither more nor lefs, than the external furrounding Air,
which is applied to the other Part of the Finger, preffes
upon it, and repels it. And if in this Cafe, the Tube
be opened at the Top, by fuddenly removing that
Pores through which that fictitious,
Matter fhould pafs, is to no Purpofe.
For if there were a Pallage for that
Subtile Matter, either through the
·
nor
uckfilver or the Glafs yet it
would not be able to force the
Quicksilver up into the Tube, n nor
to fultain it there: And if there be
no Paflage for it through either of
them, then it would not fuffer the
Quickfilver to fubfide again, as it
does when the Glafs is thaken. But
indeed the Particles of Quickfilver,
F 2
when it is firft cleared of all Air,
cohære by mutual Contact, both with
one another and with the Glafs, from a
certain Attraction, which ceafes, as
foon as the Tube is fhaken, whereby
the Particles are feparated from each
other, and from the Glafs. And the
fame Experiment has been made in
Water well cleard alfo of Air, by
which means its Parts approached
nearer to Contact. See Newt. Op-
ticks, pag. 337
which
68
ROHAULT'S SYSTEM
Part I
32. FVhat
ought to be
the Confe-
quence of fil-
ling up the
Tube with a-
ny other Li-
quòr.
33. And
led with
Air.
}
which it is ſtopped with, then we ſhould feel the fame as
if the Finger which is applied to the lower Hole received
a hard Blow, becauſe the groffer Air, which defcends
quick, and with great Force into the Tube, adds on a
fudden new Weight to that of the Quickfilver; and this
is confirmed by Experience.
1
་
32. If the Tube be not filled with all Quickfilver, but
fome other Liquor be put in alfo, we may determine
how far each of them ought to defcend, by confidering
how much that other weighs compared with the Quick-
filver. For Inftance: Suppofe the Tube filled with Quick-
filver all but an Inch, and we would fill the reft with
Water; becauſe Water weighs but a fourteenth Part fo
much as Quickfilver, we ought to conclude, that it will
make it defcend below the ordinary Station, the four-
teenth Part of an Inch, and confequently the Water will
be Thirteen of the fourteen Parts above that Station.
1.
33. The like Calculation may be made, whatever hea-
what, if it be vy Liquor be put in inftead of Water: However, it is
to be obferved, that the fame Reafon will not hold good
for grofs Air. For fince we know by Experience, that
it has a Power of expanding it felf very much, and can
eafily be mixed with the fubtil Matter, we conceive that
by mixing it felf with that fine Matter with which the
Top of the Tube is filled, it preffes againſt the Top of
the Tube on the one Part, and upon the Top of the
Quickfilver on the other Part, and fo by this means for-
ces it much lower than it would force it by its own
Weight, which compared with Quickfilver bears no pro-
portion to it.
34. That the
34.
We foreſee alfo, that an Inch of Air will make the
Effects of Air Quickfilver defcend fo much the lower, by how much leis
are different the Tube exceeds Twenty feven Inches and a half in
according to
the different Length, becauſe the Power of dilating it felf, does in a
Lengths of manner reſemble a Spring: For as a Spring, the more
the Tubes.
it is bent, with fo much greater Force does it unbend it
felf; fo the Air, the more it is compreffed, with fo much
the greater Force does it dilate it ſelf, and in all this, our
Reaſoning is confirmed by Experience.
35. A very
35. But to give a plainer Proof how much a little
good Experi- Air, when the Weight of the Column which it fuftains is
Carp's -Blad- removed, is capable of expanding it felf; we need only
der, to show take a Carp's Bladder, and cutting off the leſſer Part at the
ment of a
how much the
Air is capa
ble of cx-
panding it
Lelfor
I.
How much a little Air) See
the Notes on Part III. chap. 2.
1
Art. 3.
below.
Neck
Chap. 12. of NATURAL PHILOSOPHY.
69.
Neck, where it is joined to the Greater, prefs the greater
Part fo cloſe, as to fqueeze out almoſt all the Air that is
contained in it: Then tie it up to keep in that which re-
mains, which is not bigger than a fmall Lentil: After this,
let it be put into the Top of one of the Tubes made
large like a Veffel, and filled as ufual with Quickſilver,
and managed in the fame manner as the formentioned
Experiments, and then we ſhall fee how furprizingly the
Bladder will fwell round almoſt all at once, and appear
to be blown as big as it was before the Air was let
out.
that
36. Now though there be much more fubtil Matter
in the Bladder thus diftended, than groſs Air; yet we are
not to think, that it is that which preffes upon the inter-
nal Parts of the Bladder, and ſwells it thus; this Effect can-
not be produced by it, becauſe it can eafily return through
the Pores by which it entered; it is more likely,
this fine Matter agitates that little grofs Air which remains
in the Bladder with great Violence, which Agitation is the
immediate Cauſe of the Bladder's fwelling: And this is fuf-
ficiently evident; for if the Bladder be entirely emptied of
the grofs Air, it will not fwell at all, and if there be a little
too much, it will break.
36. VVhat
the immedi-
ate Canfe of
the Dilatation
of the Carp's
Bladder is.
A
markable Cir-
37. In order to make this Experiment well, it ſhould 37.4-
be done with a Tube open at both Ends, and the upper mfiance of
End fhould be covered with a Hog's-Bladder, moiftned this Experi-
firſt in Water, that it may ftretch the better, and this ment.
will give us opportunity of obferving another Circum-
ſtance very curious, and that is, that as foon as the Quick-
filver begins to defcend, we fhall fee the Hogs-Bladder
ftretched, and forced into the Tube; the reafon of which
is, that then a very heavy Column of Air preffes upon
it, and there is none under it to fupport it.
38. If the Bladder be pricked with a Needle, and the Nee- 38. Another
Circumſtance.
dle be pulled out a little, to let fome of the grofs Air in,
and then the Hole be ſtopped; the grofs Air which enters
in, will expand it felf round the Carp's-Bladder, and prefs
upon it, and make it appear more or lefs wrinkled, ac-
cording to the Quantity of Air let in.
39. This Experiment may ferve to undeceive thofe, 39. The Ufe-
who upon reading Aristotle have been of Opinion, that fulness of this
Experiment.
Air made ten times rarer than it is, neceffarily changes its
Nature, and is converted into Fire. For the Falfity of this Ima-
1. That this fine Matter) Not no fuch Thing, but only the E
that Matter, for probably there is lafticity of the Air it ſelf.
I
F 3
gination
70
Part I.
ROHAULT'S SYSTEM
40. That the
various.
gination is clearly feen, by fhowing that the Air contamed
in the Carp's-Bladder is rarifyed above a hundred Times,
and yet does not at all alter its Form.
40. When I fpoke of the Height which the Quickfilver
Height of the ftands at in the Tube, I limited it to Twenty feven Inches
Quickfilver is
and a half, which is the common Height obferved at
Paris; but to fpeak exactly, it is fometimes higher, and
fometimes lower; becauſe the Air at different times is
lighter and heavier.
ན་”ཞེ",
41. That the
greatest Cold
ought not to
alter the
Height of the
Quickfilver,
and what the
Caufes are,
that ought to
alter it.
41. One of the beſt Obſervations that I have met with
upon this Subject is this: That though we know by Ex-
perience, that the Air is condenſed by Cold, yet I have
never found that the greateſt Cold, made any Alteration
of the Height of the Quickfilver in the Tube. The Rea-
fon of which, in my Opinion, is, that the Cold being ve-
ry near the fame over a great Part of the Superficies of
the Earth, the Air does not pafs from one Country to a-
nother fo that the Bulk or Quantity of it is increa-
fed, but it being condenfed only from the Top to
the Bottom, it is the fame Quantity of Air, that preffes
upon any particular Place of the Earth; fo that all the
Difference that there can arife in the Air, muft be impu-
ted to more or lefs Vapours and Exhalations, which are con-
tained in it at different Seafons, and to the Winds which
blow fometimes upwards and fometimes downwards.
•
1. Vapours and Exhalations) It
has been long obferved, that in cloſe
and rainy Weather, the Quickfilver
does not riſe ſo high, as when it is
dry and clear; which has been
thought by fome to overthrow the
whole Theory of the Weight of the
Air; and indeed it is very difficult, to
explain particularly the Cauſes of all
the various and minute Changes of
the Heavens; a great deal is owing
to the Winds, which blow fometimes
upwards, fometimes downwards, and
fometimes fideways, a great deal to
Vapours, a great deal to Steams ri-
fing out of the Earth; fomething muft
be afcribed to the Alteration of the
Heavrns in the neighbouring Coun-
tries, and perhaps fomething to that
Flux and Reflux which the Moon
caufes in the Air, which is much
greater than that in the Sea, &c. To
account for all which particularly
and exactly, would be endiefs. How-
ever, to propofe fomething which
may come pretty near the Truth; it
is to be obſerved, that the Air it felf
42. As
is heavier than the Vapours, and fit-
ted to ſupport them, becauſe its
Particles are grofler, and arife from
denfer Bodies, than the Particles of
Vapours.
In the firft Place therefore, this
Weight of the Air, in any particular
Country, may be fo changed by the
VVinds, that the Atmoſphere may
be condenfed and made heavier, by
bringing a greater Quantity of Air,
and heaping it together; viz. when-
ever two Winds blow at the fame
time from contrary Parts of the
Heavens; or fome of the Air may
be carried or blown away by them,
and thereby an Opportunity given to
the Atmoſphere to unfold it felf, the
incumbent Weight being taken off,
viz. as often as two Winds blow
from the fame Country to oppofite
Parts of the Heavens; or whenfoe-
ver any one particular Wind is very
ftrong; for it is found by Experience,
that an artificial ſtrong Wind makes
the Air lighter, and the Quickfilver
in the Tube to fall very much. See
the
Chap. 12. of NATURAL PHILOSOPHY.
71
ther the Heat
42. As to any Alteration in the Height of the Quickfil- 42. That nei
ver, which may be thought to arife from the Dilatation of in the Sum-
the fubtil Matter in the Top of the Tube, by the Heat mer, nor the
of the Summer, or the Contraction of it by the Cold of Cold in the
the Winter, it cannot be at all fenfible: For Experience all fenfibly di-
fhows us, that if this Matter be heated by a Fire, much late or con-
more than it can be by the Heat of the Sun, it will not the Matter in
denfe the fub-
the Philofophical Tranſactions, Numb. I
292.
higher in the Tube, then the Hea-
vens are fair, but a little thicker, and
not quite fo blue, by reafon of the
Vapours which are every way equal-
Secondly, Cold and nitrous Parti-
cles or the Air it felf, condensed by
Cold from the North muft condenfely
the Atmoſphere where-ever it comes,
and make it heavier.
Thirdly, Heavy and dry Exhala-
tions make the Air heavy (in the
fame manner as the Specifick Gra-
vity of any Menftruum is increaſed,
by diffolving Salts and Metals) and
its elaftick Force, as it is called, muft
thereby become ſo much the ftrong-
er.
Fourthly, When the Air by theſe
and fuch like Caufes is become heavy,
then is it more able to fupport the
Vapours; which when they are en-
tirely mixt with it, and fwim about,
and are every way difperfed in it,
make the Sky ferene and clear: But
when the Air from the contrary
Caufes, is made lighter, then is it
unable to fupport the Vapours with
which it is always filled, and fo be-
ing put into fome fort of violent A-
gitation, they gather themſelves into
Clouds and Milts, and being formed
into Drops, fall down.
From thefe Obfervations, it is ve-
ry evident, that the fame Cauſes,
which make the Airheavier,and more
able to fuftain the Quickfilver in the
Tube, make the Heavens alfo clear
and dry; and the fame Caufes by
which the Air is made lighter, and
lefs able to fuftain the Quickfilver,
are Showers and Rain produced
alfo.
Hence it follows. Firft, That when
the Air is lighteſt, and the Quickfil-
ver falls loweft in the Tube, then the
Clouds move very low and quick;
and that clear Air which after Rain,
appears between the thick Clouds,
being difcharged of its Vapours, feems
moft tranfparent and bright, and gives
the beſt and eaſieſt profpect of Things
at a diſtance.
Secondly, When the Air is more
heavy, and the Quickfilver is raifed
difperfed about; and as has been
by many obferved, it does not afford
fo good a Profpect of Things at a
diſtance; and if there do appear any
Clouds, they are very high and move
very flow; and when the Air is
heaviest of all, the Earth is fome-
times covered with very thick Clouds,
which feem to confift of heavier fort
of Exhalations, which the Air at that
time is capable of fuftaining, but
which cannot fwim in lighter
Air.
Thirdly, Hence it is, that in our
own Country, when the Cold is great-
eft, and the North and North-Eaft
Winds blow, the Quickfilver in the
Tube is higheft; becauſe at that time
two Winds blow together upon our
Country from oppofite Parts of the
Heavens; for in the Atlantick Oce-
an, at the fame Latitude with us, the
Wind blows almoſt always from the
Weft. To which we may add, that
the Air which is brought hither by
the North Wind, comes condenfed
by the Cold.
Fourthly, In the moſt Northern
Countries, there is greater Variation
of the Height of the Quickfilver in
the Tube, than in thofe Countries
which are more South, becaufe in
thofe Countries, theWinds are ſtrong-
er and more variable; aud oppoſed
by each other in a lefs Tract of
Land; whence the Air is fometimes
more heaped up and condenfed, and
fometimes carried away and light-
ned.
Laftly, Between the Tropicks,
there is the leaft Variation of all, in
the Height of the Quicksilver in the
Tube, becaufe there the Wind is for
the most part very gentle, and blows
the fame way.
See the Philofophical Tranſactions,
Number 181.
F 4
make
Winter, do at
the Tube.
72
Part I.
ROHAULT'S SYSTEM
43. How
much the
make the Quickfilver defcend at all; and if the Heat of
Summer can do nothing towards fenfibly dilating it, the
Cold of the Winter can much lefs do any Thing towards
the condenfing it.
43. But whatever be the Caufe of the Quickfilver's ri-
greatest Dif- fing and falling in a Tube, where the Experiment is con-
ference in the tinual; the greateſt Height that I have obferved for fif-
Height of the
Quickfilver teen Years, in a Tube which I prepared for that Purpoſe,
was Twenty eight Inches, and a third Part of an Inch;
and the loweſt was Twenty fix Inches and feven twelfth
Parts of an Inch, fo that the greateſt Difference in the
Height of the Quickfilver, was an Inch and three quar-
15.
44. That the
Places of a)
different
Height.
ters.
44. Though all theſe Experiments are fufficient to con-
Height of the vince us, that it is by the Weight of the Air, that the
Quickfilver
ought to be Water or Quickfilver is fupported or made to rife in the
different in Tube; yet it is eafy to conceive how there may be an
Alteration made in the Height of the Quickfilver, and yet
no Change made in the Air it felf: In order to this, we
need only make the Experiment in two different Places,
the one the higheft, and the other the loweſt that we can
come at: For there being a lefs Quantity of heavy Air in
the higheſt Place, the Quickfilver cannot be ſupported by
it to fo great a Height as in the loweſt.
45. The first
45. Now in order to try if Experience would agree with
Experiment. our Reaſoning, I filled a Tube three Foot and a half long,
with Quickfilver, and immerfed it into a deep and ftrait
Veffel, into which it emptied it felf as ufual, after which
I fixed them both in a Wooden Frame, made for that
Purpoſe: And now the Inftrument being fuch as could
conveniently be carried from one Place to another, with-
out any Danger of fpilling: I carried it to the Surface of
the River Seine, which happened then to be frozen, and
obferved exactly the Height of the Mercury: After which,
I went up one of the Towers of the Church of the Virgin
Mary at Paris, which is about Two hundred and fixteen
Foot higher than the Place where the firft Experiment
was made, and here I found the Quickfilver was not fo
high in the Tube as before, by near three Lines, that is,
near a quarter of an Inch.
46. Another
more fenfible
1
46. The fame Experiment was tried in Auvergne, in one
Experiment. of the loweſt Places of the Town of Clermont, and upon
the Top of a neighbouring Mountain, called Puy de Dome,
which is about Three thoufand Foot higher than the Val-
ly, and the Difference in the Height of the Quickfilver was
found to be above three Inches,
47. As
Chap. 12. of NATURAL PHILOSOPHY.
73
47- As this Experiment is more fenfible than mine, if 47. Me-
it was made, as there is Reafon to think it was, with all thod of find-
the Exactnefs one could wifh; it furniſhes us with an of the Air.
ing the Height
eafy Method of finding the Height of the whole Air, fup-
pofing it to be every where of the fame Denſity as it is
near the Earth: For fince upon taking away Three thou-
fand Foot of Air, the Quickfilver finks three Inches, this
is a Proof, that a Column of Quickfilver of three Inches
high, weighs equal to Three thouſand Foot of Air, and
confequently the Height of the whole Air, which coun-
terpoifes Twenty feven Inches and a half of Quickfilver,
is Twenty feven thouſand and five hundred Foot high.
48. As therefore we conclude, that when there is lefs 48. That all
Height of the groffer Air to prefs upon the Quickfilver filver would
the Quick-
in the Veffel; there ought alfo to be lefs Height of that fall out of the
in the Tube; for the fame Reaſon, if we fuppofe that Tube, if there
there were no grofs Air at all to prefs it upwards, we Air to prefs
were no grofs
ought to conclude that all the Quickfilver would fall down, upon the Vef
fo that That in the Tube would be level with that in the fel.
Veffel.
Inftrument to
49. Some have imagined it impoffible to make any 49: A De-
Obfervation by which it ſhould appear, that Reafon and cription of an
Experience agree in this Particular; becauſe there is no make this
Mountain high enough to carry us up to the upper Sur- Experiments
face of the Air; and becauſe, if there were, the Air would
be fo thin, that we could not breathe in it. But I thought
of a Means to remove theſe two Difficulties, and by
which the Thing might eafily be effected; and that was,
to prepare fome fmall Room, with tranfparent Walls,
which one might ftand without and look upon, without
any Danger from what might happen within. I caufed
therefore a Glafs Inftrument to be made, according to the
following Repreſentation. BC is a Tube, upwards of Tab.I. Fig.7.
Twenty feven Inches and a half long, and is open at C:
AB is a large Cavity, which has a Communication with
BC by the Part B L, and is cloſed, and has no Aperture
at A: DE is a ſmall Glaſs Tube ſtopped up at the End
D, and ſticks out of the Cavity AB by the Length FE,
and is open at E: Befides there is a fmall Hole F in this
little Tube, where it is cemented on the outfide to the
Glafs AB in fuch a manner, that the Cavity of the little
Tube has a Communication, with the large Cavity AB
by this little Hole F: Laftly, by means of the Neck BG,
the external Air has a Communication with that in the
whole Tube ABC.
•
50. I
}
>
74
Part I.
ROHAULT'S SYSTEM
1
50. How the
foregoing In-
ftrument is to
"be uſed.
51. Surpri-
zing Effects
Inftrument.
50. I firſt ſtop the Hole G with a Hog's-Bladder, and
turning the whole Inftrument, fo that the End C may be
uppermoft, then I pour in the Quickfilver at the Hole E,
which at firft falls only into the little Tube DFE, but
when it is full up to F, then continuing ftill to pour in,
it runs through the Hole there, and fills the Cavity AB
which furrounds this Tube, which I fill up as high as B;
then I fill the rest of the large Cavity, pouring the Quick-
filver in at C, 'till it rifes as far as the Hole E, which I
ftop then with a Hog's-Bladder; after this, I continue to
pour the Quickfilver in at the Hole C, 'till the Tube BC
is quite full, Having done this, I ftop the Hole C with
my Finger, and invert the whole Inftrument which is full
of Quickfilver as ufual, and immerſe it in a Veffel of the
fame; Then the Cavity AF empties itſelf as far as IL, and
at the fame Time, the little Tube DFE empties itſelf to
the fame Height, and the Tube C empties it felf to H,
which is Twenty feven Inches and a half above the Quick-
filver in the Veffel: And thus we fee that Reaſon and Ex-
perience agree; for as there is no grofs Air to preſs upon
the Surface IL of the Quickfilver which remains in the
Bafon IFL, fo there is nothing to force it to rife in the
little Tube DFE.
51. Now if the Hog's-Bladder which ftops the Hole
from the En- at G, be pricked with a Needle, it is evident, that the
trance of the groller Air which enters into the Cavity ABG ought to
Air into the produce Two very different, and therefore very remark-
able Effects: The firft is, That preffing upon the Quick-
filver which is directly under G, it will cauſe it to de-
fcend; and alſo preffing upon the Surface IL of the Quick-
filver which remains in the Bafon IFL, it will make Part
of it to aſcend in the little Tube DFE, and fill it quite
full, provided it does not exceed Twenty feven Inches
and a half in Length. The Experiment will be more plea-
fant, if after the Hog's-Bladder, with which the Hole G
is ſtopped, be pricked, the Needle be pulled back feve-
ral times a very little, to let a little Air in at a time through
the Hole, and then thruft forward to ftop it again; for
then you will have the Pleaſure to fee the Quickfilver in
the little Tube DFE afcend by little and little at the fe-
veral times, and that in the Tube BC deſcend in the fame
manner. Then if the Needle be pulled out all at once,
you
Chap. 12. of NATURAL PHILOSOPHY.
75
·
you will fee at the fame time it will rife as much on the
one Hand, as it falls on the other.
be drawn into
is no Air to”
Support it.
52. If the Liquor with which the Bafon belonging to 52. That the
the Tube is filled, falls all down, becauſe there is no Air Water cannot
to fupport it, as we fee in the foregoing Experiment, where a Syringe
the little Tube DFE, is entirely emptied of the Quick- where there
filver; the Reafon holds ftronger for its not rifing, if
there be no Air to thruft it up; wherefore there is no
need of making any Experiment, to be affured, that the
Water ought not to rife in a Syringe, when the Sucker is
drawn, if the Veffel in which the End of the Syirnge is
immerfed, be ſo ſtopped, that the external Air cannot en-
ter into it. But if any one be ſtill ſo obftinate, as not to
be content without referring it to Experience, he need
only put the End of the Syringe into the Mouth of a
Glafs Bottle, which is round and ſtrong, and full of
Water; but not begin to draw the Sucker, till the Mouth
of the Bottle be well ftopped with Wax, or fome fuch
Thing, to prevent the external Air entring; and then
he will fee that the Water will not rife at all in the Sy-
ringe.
53. Why the
Air does not
Weight of the
the Water in
a Syphon.
53. That we may go on to explain the moſt confidera-
ble Phenomena of Hydraulick Inftruments; I come now
to give an Account of the Syphon. Let ABCD then be at all raife
fuch a Syphon, the ſhorter Arm of which CD is put in-
to a Veffel of Water; Then, as has been often faid, the Tab. II.
Air which preffes upon the Water which is in the Vef-
fel, ought not to make it rife up in the Syphon, becauſe
the Air which is in the Syphon hinders it.
Fig. 1.
54. What is
Syphon.
1
54. But if the Water in the Veffel be made to rife up the Caufe of
into the Syphon, either by fucking it at the End A, or the Waters
any other way, ſo that it be filled quite full
of Water, rifing in the
and then we take our Mouth away from the Hole A, the
Water will not ceafe to run, but continue running, fo
long as the fhorter Arm CD remains in the Water in the
Veffel: The Reafon of which is this. So long as the
fhorter Arm CD is immerſed in the Water, the Force of
the Air indeed, which preffes upon the Water in the Vef
fel, and which endeavours to make it rife in this Arm, is
not fenfibly greater or lefs, than the Force of the Air
which endeavours to repell it, when it offers to run out
at the Hole in the other Arm: But becauſe the Force of
1. You may find the Defcription
of an Inftrument not much unlike
this in the Experiments of the Aca-
demy del Cimento. But the Air Pump
६
of the famous Mr. Boyle exceeds
them all, and is fo well known, that
I need not deſcribe it.
each
..
*.
76
Part I.
ROHAULT'S SYSTEM
55. How high
the Arms of
the Syphon,
must be for
the Water to
afcend.
56. How the
each of theſe two Arms is diminished, in proportion to
the Weight of the Water which each of them impels;
and the Weight of the Water in the longer Arm being
heavier, than that in the fhorter Arm; it follows, that,
there remains more Force in the Air which acts upon the
Water in the Veffel, to make it rife in the fhorter Arm,
than there does in the other to repel it; fo that it is in-
deed made to rife, and forced to run out through the
longer Arm, notwithflanding the Refiftance of the Air
which oppoſes it.
55. I here fuppofe, that the Arms of the Syphon do
not exceed that Height of the Liquor which the Air would
ſuſtain in a perpendicular Tube; for if they be longer, the
Liquor with which the Syphon is filled, will divide at the
Top, and defcend in each of the Arms; which is con-
firmed by Experience.
י
56. After fo many different Explications as have been
Air is drawn already given, I don't think it neceffary to inlarge much
into a pair of
Bellows. upon explaining how the Air enters, and is received into
a Pair of Bellows; for it is eafy to apprehend, that when
the Sides are ſeparated from each other, they thruſt for-
ward the Air, which not being able to move freely eve-
ry way, I becauſe the World is full, or at least not be-
ing able to enter in at the Nole with Eafe, and quick
enough to fill readily that Space which is left by the Sides
of the Bellows when they are opened; is turned back,
and enters with Eafe and Swiftnefs through the Holes of
the Bellows.
57. How we
draw in the
Air by Ref-
piration.
58. Whence
it is that we
find no Dif-
ficulty in
breathing.
·
57. It is proper here to obſerve, that we receive in the
Air by Reſpiration, much after the fame manner: For it
is certain, that the Muſcles of the Thorax and Abdo-
men, ferve to diftend, and ſwell the Body, by which
Means the Air being thruſt back, gets into the Hollow of
Lungs through the Mouth and Noftrils.
58. The only Difficulty here is, that fince we fuftain a
great many Columns of Air, which are all heavy, and
which prefs upon the external Parts of our Body, and
thruft it inwards; it fhould feem that we ought to feel
fome Difficulty in breathing, in order to overcome this
Reſiſtance: But the Anſwer is eafy; For if there be ſome
1. Becauſe the World is. full)
Whether the World be full or not,
it is the fame Thing; for it cannot
be, but that the Air by its own
Weight (and Spring) muſt ruſh into
the empty Bellows when they are
open. Which I remark here, to
how, that whatever becomes of the
Fulness of the World, the Explica-
tion of thefe and fuch like Motions,
is the fame.
to
Chap. 12. of NATURAL PHILOSOPHY.
77
to thrust it inwards, there are alſo a fufficient Quantity of
others, which enter into the Cavity of the Breaſt to
prefs it outwards; fo that there is an equilibrium between
theſe Forces or Powers; and this is the Reaſon why we
ought not to find any Difficulty in Breathing, or if we do,
it is owing to fome other Caufe.
if our
59. The fucking in of Air through a Quill is done in
the fame manner as Refpiration; for it is the fame as
Mouth were as long as the Quil.
59. How it is
that we fuck
in Air.
60. Why it is
more difficult
to fuck a
heavy Li-
60. If we try to fuck a heavy Liquor through a Quill
dipped into it, we ought to find fo much the greater Dif-
ficulty as the Quantity of Liquor we make to rife is greater;
becauſe this Liquor preffing by its Weight upon the ex- quor.
ternal Air which endeavours to raiſe it in the Quill, 1 hin-
ders it from impelling and affifting the Air which is in the
Lungs, fo much as it ufually does; by which means the
Air in the Lungs is weakned, and has juft fo much lefs
Force to thrust the Parts of the Body outwards, than the
Air which is applied to the external Surface of the Body
has to thrust them inwards, as the Liquor which is caufed.
to rife in the Quill is heavier.
61. I fhall finiſh what I have to fay concerning theſe 61. Concern-
Sort of Motions, with explaining that Swelling which ing the Ufe
of Cupping-
Surgeons make in the Flesh, by the Application of Cup- Glaffes.
ping-Glaffes; the common Method of which, and that to
which all others may be reduced is this; they take a
fmall round Card, upon which they fix four fhort pieces
of Wax-Candle, which they light, and fet like a Candle-
ftick upon the Part of the Body which they intend to cup:
Then they cover all the Candles with the Cupping-Glais,
bnt do not put it cloſe to the Flefh, 'till the Air that is
within it, is fufficiently heated; then as foon as it is
1. Hinders it from impelling) If,
the entire Weight of the Liquor only
be confidered; we muft fay, that the
Difficulty of Sucking, is therefore
greater or lefs, becaufe, in proporsion
to the greater or lefs Height or Thick-
nefs of the Column of Liquor, the
Breaſt is more or lefs diftended by
the Power of the Mufcles; fo that
the Refiftance of the internal Air
(by which it endeavours to hinder
the Liquor from rifing) must be fo
much more or lefs weakned by Ra
refraction, according to the Power of
the external Air, to raife up the hea-
vy Liquor to the Mouth. But be-
saufe the Columns of Liquor are rai-
fed by the external Air with leſs or
greater Difficulty, according as they
are lefs or greater in Height and not
in Thickness; therefore if we fuppofe
two fuch Columns, one of which is
twice as high as the other, and this
other twice as thick as that; though
it be plain, that in both Cafe, there
is the fame Quantity of Air to be
fucked out of the Quill, and the fame
Quantity of Liquor to be fucked
through it; yet it is evident, that a
greater Diftention of the Breaft, and
a greater Force of the Muſcles is
required, that is, it is more difficult
to fuck or raife up the First than
the Second.
put
78
Part I
ROHAULT's SYSTEM
62. VVhy
the Flesh
Swells.
put cloſe, the Candles go out, and we ſee the Fleſh fwell,
and rife up.
I
62. In order to underſtand the Reaſon of this Experi-
ment, it is to be obſerved, that during that fhort time
that the Candles continue light, the Air which is in the
Cupping-Glaſs, though very much agitated and dilated by
the Flame, does however preſs upon the Fleſh, as much
as it did before, becauſe the Cupping-Glafs being not yet
put quite cloſe, does not take off any of the Weight,
which it had before it was dilated; but it is otherwife af-
ter the Candles are extinguiſhed by the immediate Appli-
cation of the Cupping-Glaſs to the Body: For then the
Air which is contaiued in it, is no longer preffed upon by the
Air without, and as it grows cooler, it has not Force ſuf-
ficient to take up fuch a Compafs, as when it was agita-
ted by the Heat: Wherefore fince all the other Parts of
the Body are preffed upon by the external Air, which alſo
preffes the Cupping-Glaſs to the Body, the one muſt of
neceffity enter into the other; that is, the Fleſh muſt be
thrust into the Cupping-Glaſs, and the Air within it
condenſed.
1. Though very much agitated and
dilated by the Flame, does however
press upon the Flesh as much as it
did before, becauſe the Cupping-Glafs
being not yet put quite close, does
not take off any of the VVeight,
which it had before it was dilated.)
This Explication had been fome-
what more plain, if the Author
faid----though dilated by the Flame,
yet fince it is very much agitated, it
does however. Nor was there any
nced of having recourſe to the VVeight
of the external Air here.
1. VVhat is
meant by the
Determinati-
WH
CHA P. XIII.
Of the Determination of Motion.
HEN a Body moves any particular way, the
Diſpoſition that it has to move that
way, rather
on of Motion. than any other, is what we call its Determination.
2. That fuch 2. Determination is a Mode which is diſtinguiſhed
Determinati- from Motion, and which may remain the fame, how
on is fome-
thing diftinct much foever the Motion be increaſed or diminiſh-
from Motion. ed: Thus a Stone that falls freely in the Air, has a cer-
The first tain Quantity of Motion, and at the fame time; has alfo
a certain Quantity of Determination of Motion downwards,
Proof.
and
Chap. 13. of NATURAL PHILOSOPHY.
75
and if it had been thrown oblique from the fame Place,
fo as to have come to the Ground in the fame time, it
would have had the fame Quantity of Determination, but
a greater of Motion.
3. Another Proof that Determination differs from Mo- 3. Another
tion, is, that it depends upon a different Cauſe from that Proof.
of Morion, thus in a Ball ftruck by a Racket, the Moti-
on is owing to the Force with which the Racket is mo-
ved, but the Determination towards any Part, is owing to
the Situation of the Racket,
4. Since every Thing endeavours as much as it can to
continue in the State in which it once is, it is evident,
that a Body which has once begun to move with a cer-
tain Determination, ought always to keep the fame, that is,
it ought always to move in a ſtreight Line, for this is the
only Determination that is natural to a Body in Motion:
Wherefore when it was faid above, that when any Body
was moved in a freight Line, other Bodies muft neceffa-
rily be moved with a circular Motion, we are not to
think that thofe which thus turn out of a ftreight Line,
tend to do fo themfelves, but that they are forced to
do fo, by meeting with, and being impelled by other
Bodies.
4.
That a Bo-
dy does not
tend of it felf
to go out of the
way, but only
to move on in
Line.
a ftreight
5.
That eve-
moves in a
5. Therefore when we fee a Body move in the Sides
of a Square, we conclude, that in the Places where it ry Body which
changes its Determination, it is forced to turn out of the Circle, is for
way, by meeting other Bodies, the Refiftance of which, ced to do so.
it could not overcome. So likewife if a Body moves
through the Sides of an Octagon, we can't but fay, that
1. Natural to a Body in Motion)
Mr. Perrault in his Tentam, Phyf.
Tom. I. p. 8o. 88. contends, that
Motion in a Circle is às natural as in
a ftreight Line; for terreftrial Bo-
dies turned round, endeavour to go
off from the Center of their Motion,
becauſe they are heavy; but ifa Body
that had no Weight at all were turn-
ed round, it would revolve about its
Center freely without any Impulfe,
and would not endeavour to go off
from it: Thus if a Ball of Wax be
fo made hollow, as to equal in
Weight an equal Bulk of Water,`it
will fo comply with the Motion of
the Water turned round in a Veſſel
full of Water, that it will always
defcribe the fame Circle, and never
attempt to go off from the Center
of its Motion. But (befides that there
is no fuch Thing as a Body void of
all heavinefs) this Affertion is con-
trary to all Reafon, and this very
Experiment proves nothing lefs, than
what this eminent Perfon imagined:
For what can be more evident, than
that this Ball endeavours to go off
from the Center of its Motion, but
cannot get off, becauſe all the Parts
of the Water endeavour at the fame
time to go off from the fame Center,
and with the fame Force, becaufe
equally folid; and therefore fince the
Sides off theVeſſel hinder them from
going all of together, there is no
reafon why the Ball of Wax fhould
recede from the Center, and impel
the Parts of the Water to the Center,
any more than there is for the Parts
of the Water to recede from the
fame Center, and drive the Ball
thither.
it
ROHAULT's SYSTEM
Part I
i
6. If that
Tangent of
that Circle
Tab. II.
Fig. 2.
it is eight times forced to turn out of the way; and fince
a Circle is equal to a Figure of an infinite Number of
Sides; it follows, that a Body which moves in a Circle,
is forced to turn out of the way every Moment, either
by the continual Refiftance of Bodies which it every where
meets with, or becauſe it is retained by fomething which
obliges it to keep always at the fame Diſtance, and to
run through the Circle defcribed, otherwiſe it is certain it
would not deſcribe a Curve Line at all.
6. For Example, if the Body A defcribes by its Mo-
Force ceafes, tion part of the Circle BCD, it must be continually turn-
then it ought
to move in the ed out of its Courſe from one of the forementioned Cau-
fes: If, when it comes to the Point D, it ſhould be no
which it de- longer forced; either becauſe the Bodies which it meets
fcribed before. with, fhould make no further Refiftance, or the Thread
which connected it with the Center, and hindred it from
flying off, fhould break, it would not continue to de-
fcribe the Arch DEB, but it would deſcribe a ftreight
Line, which would run the moſt directly that is poffible
from the Arch CD, that is, it would defcribe the Line DF,
which is the Tangent of this Circle, and makes the leaſt
Angle that can be with the Circumference, and which, as you
fee, grows more and more diſtant from the Center :
This is confirmed by an infinite Number of Experi-
7. Bodies
which move in
a Circle, en-
ther Bodies
ments.
7. And fince a Body in Motion, has always a Tenden-
cy to describe that Line, which it would deſcribe if it
deavour to go were at liberty; and what was faid of the Body A, is to
off from the
be underſtood in general of all other Bodies; we muſt
Center of the
Circle which conclude, that Bodies which move in a Circle, have a
they defcribe, perpetual Tendency to recede from the Center of their
and make o- Motion
Motion; and this they ought to do with a Force fo
approach to it. much the greater, as their Motion is quick. Wherefore,
if the greater part of the Space contained in the Circum-
ference BCDE be full of Bodies which move round the
Center G, they will pufh all the other Bodies with which
they are encompaffed, and drive them as far from the Center
as they can: But if thefe Latter can find no Place to retire
to, they will be forced, in order to give Place to the other,
to go nearer the Center; in the fame manner as when we
dip our Hand into a Pail of Water, the Water is forced to
Body in Mo- give way to our Hand, and to remove from the Bottom,
tion meeting which it has a Tendency to by its own Weight.
8. That a
with another
Body which it
8. It is evident, that a Body lofes fo much of its own
Cannot move, Motion as it communicates to other Bodies: Now if it
ought to be
communicates no Motion at all to others, (we do not
reflected.
here
Chap. 13. of NATURAL PHILOSOPHY.
80
here confider what may be occafioned by its Softneſs,
Weight or Figure) we have no Reafon to think that it
fhould at all abate of its Velocity. Wherefore if a Body
in Motion ftrikes upon another, which it cannot move at
all, we ought to conclude, that it will continue to move
on with the fame Celerity as it did before; but becauſe
the Body which it cannot move, hinders its Determination,
it muſt neceffarily alter this Determination, that is, it will
be reflected.
9. This Second Determination, may indeed be contra- 9.That there
ry to the First: but becauſe the Notion we have of re- is not a Mío-
ment of Reſt
flected Motion is not different from the Noticn we have in the Point
of direct Motion, we ought not to think that theſe Mo- of Reflexion.
tions are contrary to each other, but that the one is on-
ly a Continuation of the other, and confequently, that
there is not any Moment of Reft in the point of Reflex-
ion, as fome Philifophers have imagined.
10. Befides, if a Body which was in Motion, comes
to be but one Moment at Reft, it will have wholly chang
ed its manner of exifting into the contrary, in which there
will be as much Reafon for its continuing, as if it had
been at Reſt a whole Age; in the fame manner, as if
Body which was once fquare, was made round but one
Moment, it will have as much reafon as ever it had, to
continue in this Figure.
a
10. That Re-
flexion world
be impoffible,
if there was
Moment of
a
Reſt.
11. That a
Body which
falls perpen-
dicularly upon
11. When a Body falls perpendicularly upon another,
which is hard and immoveable, it is evident, that the Re-
flexion ought to be made in the fame Line, in which
the Body moved before, there being no Reaſon why it another,
ſhould incline one way rather than another: Wherefore ought to be re-
flected per-
there is no Difficulty in this Matter, except when the pendicularly.
Line in which the Body begins to move makes oblique
Angles with the Superficies of the Body against which it
ftrikes. But the Judgement we are to make of this, de-
pends upon what we are going to fay concerning the Com-
pofition of Motion, and of its Determination.
1. The one is only a Continuation of
the other) But it is not fo. For Be-
dies which are either abfolutely hard,
or fo foft, as to be void of Elafticity,
will not rebound from each other, Im-
penetrability only makes them ſtop.
Newt. Optic. pag. 373. See above,
Chap. x. Artic. 13.
Further, there may be a Moment
of Reft, in the Point of Reflexion;
becauſe the reflected Motion, is not
G
a Continuation of the Direct; but a
new Motion imprefled by a new
Force, viz. the Force of Elafticity.
As to what our Author fays; that
if the Body reited but one Moment,
it ought as much to continue in that
new State of Reft, as if it had reſted
a whole Age; it is indeed true, with
regard to the former Motion; but
fince Elafticity is the Cauſe of a new
Motion, the Reaſon is very different.
CHAP.
ROHAULT's SYSTEM
Part I
1. What is
meant by com-
pound Mo-
tion.
Tab. II.
Fig. 3.
2. Two other
Tab. II.
Fig. 3.
CHA P. XIV.
Of the Compofition of Motion, and of its De-
termination.
LL Motion that depends upon two or more Caufes,
we call Compound Motion: Thus, if one Force act-
ing upon the Body A, would cauſe it to move along the
Line AB, and at the fame time another Force acting up-
on the fame Body A, would cauſe it to move along the
Line AC, the Motion which will arife from the Action of
theſe two Forces, or from theſe two Caufes, will be a
compound Motion.
2. In order to find out what Line the Motion, which
Motions be- depends thus upon two Caufes, ought to be made in;
ing given to
find the com- let the two Lines be drawn, which the Body would move
pound Motion. in, if each of theſe Caufes produced their Effect fepa-
rately. For Example, if the firft Cauſe would in a given
Time, make the Body A move from its Place, as far as
B; and if the Second Caufe would in the fame Time,
make it move to C; let the Lines AB, AC, be drawn ;
then having divided the Time in which this Motion was
made, into as many equal Parts as you will, divide the
Line AB into as many, by the Points E, F, G, and the
Line AC into as many alſo, by the Points H, I, L; ſo
that, if the firſt Caufe acted alone, the Body A, would
come to the Point E, in the firſt Part of the Time, to
the Point F, in the fecond Part, to the Point G in the
third Part, and to the Point B in the Fourth; and if the
fecond Caufe, produced its Effect feparately, the Body
A would come to the Point H, in the firft Part of Time,
to the Point I in the Second, to the Point L in the
Third, and to the Point C in the Fourth: After this, draw
the right Lines EM, FN, GO; BD, CD, parallel to the Line
AC; and the Lines HP, IQ, LR, CD, parallel to the Line
AB: This being done, the Points S, T, U, D, where
thefe Lines interfect each other, will determine the Line
in which the Compound Motion is made.
3.
A De-
3. For it is certain, that the firſt Cauſe is anſwered, by
monftration allowing the Body to move to the Line EM in the first
Part of Time, and the Second is anſwered, if we allow it'
to be found in the Line HP in the fame time; where-
fore both theſe Cauſes are anfwered at once, if the Body
Metion.
comes
Chap. 14. of NATURAL PHILOSOPHY.
8.3
comes to both the Lines EM, HP, at the fame Time,
which it cannot do, but at the common Point S. Again, it is
evident, that the firft Cauſe is anſwered, if we allow the
Body to come to the Line EN in the fecond Part of
Time; and the fecond Cauſe is anſwered, if it be allow-
ed to come to the Line IQ in the fame Time, and con-
fequently it is certain, that, in order to anſwer both theſe
Cauſes together, it must be found in theſe two Lines at
the fame time, viz. in the Point T where they interſect
each other. So alfo we may prove, that the Body ought
to be found in the Point V, where the Lines, GO, LR,
interfect each other, to anſwer the fame two Cauſes, and
at laft in the Point D, where the Lines BD and CD in-
terfect one another 1.
Lines com-
pound Motion
4. Where 2 the fimple Motions are equal, as in the firſt, 4. In what
Figure, the compound Motion is in a streight Line: But
where the fimple Motions are unequal, as in the Second may be made,
Figure, the Motion will be made 3 in a Line differently
curved, according to the different Inequalities of the fim-
ple Motions.
5. How to
determine a
5. If more than two Caufes concur to produce a com-
pound Motion, it may be determined in this manner: Motion com-
Firſt draw the Line in which the Body ought to be mo- pounded of
ved, fo as to anſwer two Cauſes; then, taking the Mo- more than two
tion in this Line, as if it aroſe from one Caufe only, draw Simple ones.
the Line which it ought to deſcribe, ſo as to anſwer this
1. Such a kind of Motion as this,
is that of an Arrow, in the famous
Experiment of a Ship under full
Sail; where an Arrow being ſhot
perpendicular, falls down again upon
the fame Place on the Deck, whence
it was ſhot: For the Arrow has a
double Motion impreffed upon it at
the fame time, one by the Bow or
Hand which ſhoots it, and the o-
ther by the Ship moving along.
Something like this was obferved at
Florence, where a Leaden Ball fhot
perpendicularly up out of a Musket
fixed in a Wooden Carriage made to
move very ſwiftly, fell about feven
Foot on this fide the Mouth of the
Musket, which moved Sixty four Pa-
ces. See Expèr. Acad. del Cimento,
p. 145. Perhaps the Musket, was not
erected exactly perpendicular,
or
was moved fomewhat fwifter after
the Ball was ſhot out, than when it
was hot; or if neither of theſe
happened, yet the Reſiſtance of the
G
Air, which could not but retard
the Motion of the Ball, might per-
haps be the fole Cauſe why the Ball
fell fo much on this fide the Mus-
ket.
Fig. 3.
2. The fimple Motions are equal)
It is to be obferved, that thofe fimple
Motions which are here
compared with each o-
ther, and are called equal
or unequal, are not thofe of different
Determinations (fuch as AB, AC,) but
the Parts of the Motion of one and
the fame Determination (viz. AE,
EF, &c. AH, HI, &c.) compared
together.
3. In a Line differently curved)
When one or both the fimple Mo-
tions is altered gradually and every
Moment; the Line which is defcri-
bed, may be conceived to be bent in-
to an infinite Number of ſmall Lines
which end in a Curve. Such is the
Motion of projected Bodies. See the
Notes on Part II. ch. 28. Artic. 16.
2
Caufe
84
Part I
ROHAULT's SYSTEM
6. That the
Cannon's is a
compound
Motion.
Caufe, and a Third, and fo on, if there be a Fourth or
fifth Cauſe, producing its particular Effect.
6. It is eafy to fee, that the Ball of a Cannon which
Motion of a feems to be driven by the Fire level with the Horizon,
Ball out of a does, notwithſtanding move in a Curve like that deſcribed
in the fecond Figure; for there are two Cauſes which con-
cur towards its Motion, the firft of which, viz. that
which cauſes the Ball to move upon the Level, ought
continually to diminiſh, becauſe it communicates, by little
and little, its Motion to the Air which it difplaces; and
the fecond ought to increafe, becaufe we find by Experi-
ence, that the Fall of a heavy Body is flower at the Be-
ginning than afterwards.
7.
That the
the Mark,
7. The Exactneſs of the Cannoneer in levelling the Can-
levelling it at non to the Mark which he looks at, ought not to make
Shows that the us alter our Opinion, and to think immediately that the
Ball defcends. Ball is carried in a ftreight Line: For if we obferve, that
the Cannon is not every where of an equal Thickneſs,
and that the Line AB by which the Mark is aimed at;
is at firſt above, but goes afterwards below the Line of
Direction CD; we fhall conclude, that if the Ball hits
the Mark, it has doubtless fallen a little, or elſe it would
have gone a little above it.
Tab. II.
Fig. 4.
8. V Vhat is
Tab. II.
Fig. 5.
8. As there are Compound Motions, fo alfo are there com-
meant by com- pound Determinations, and, it may be, when the Motions
pound Deter-
mination.
are the moſt ſimple that can be: Thus we fay, a Deter-
mination is compounded of two others, when a Body
moving in a fimple Line to a certain Place, is at the fame
time carried two different Ways; as if the Body A be
moved with a fimple Motion from A to B; becauſe at
the fame time, it continually approaches the Lines BC,
BD, we fay, that the Determination, by which it is carri-
ed from A to B, is compounded of two others, one of
which would make it go towards D, and the other
at the fame time carry it from A to C; and theſe Di-
ſtances are the Meaſure of its Progress towards theſe dif-
ferent Parts.
9. That one
an may be
9. For the fame Reaſon that we confider any one De-
and the fame termination as compounded of two fimple Determinations,
Determinati- we may as well confider it as compounded of innumerable
compounded of others. Thus the Determination from A to B may be
many diffe- confidered as compounded of the Determinations from A
to E, and from A to F; becauſe when the Body A
moves from A to B, it continually approaches BE and BF
rent ones.
alfo,
Chap. 14. of NATURAL PHILOSOPHY.
85
alfo, from which it was diftant by the Length AE and
AF 1.
1. From this Principle, the Me-
thod of explaining the Forces of the
Mechanick Powers (as they are cal-
led,) may excellently well be de-
duced.
For fince a Body with two uni-
ted Forces, always defcribes the Dia-
gonal of a Parallelogram, in the fame
Time, as it would do the Sides, if
the Forces were feparate; it is evi-
dent, that any Force whatſoever, a&t-
ing in a given Direction, may be look-
ed upon as the Effect of two other
Forces acting in Directions, which
at the fame Point, ſhall on each fide,
be any way inclined to the given Di-
rection, provided they make an An-
gle leſs than two right ones: And
this is abundantly confirmed in Me-
chanicks, for by fuch a Refolution
of a given Force into two others,
the known Properties of the Mecha-
nick Powers, fuch as the Ballance,
the inclined Plain, &c. may easily be
deduced.
Of the Ballance or Leaver. Prop. I.
If two Forces, which act upon the
Arms of a Ballance in given Dire-
&tions that are in the fame Plain
with thofe Arms, ballance one ano-
ther; theſe Forces are to each other
reciprocally, as Perpendiculars let
fall From the Center of the Ballance,
to their Directions.
DEM.---(See Newt. Princ. pag. 14.)
Let C be the Center of the Ballance,
Cp, CP the Arms, Ep,
Tab. xx.
PA the Directions of the
Fig. 1. Forces acting upon the
Arms Cp, CP. Let CE
be drawn perpendicular to pE, and
CD to PA, meeting them in E and
D. On the Center C, and with the
Radius CE, viz. the longeſt of the
Perpendiculars, let a Circle be de-
ſcribed which ſhall interfect the Di-
rection of the Force P in A, and ler
the Line CA be drawn. To which let
AG be drawn perpendicular, and GF
parallel, meeting DPA in F.
It is evident, that the Arms of the
Ballance CP, Cp, may be looked up-
on as Lines that will not bend, ly-
ing in the Plain moveable about the
Center C; and the fame may be
underſtood of any other Lines drawn
through the Center C, and lying in
the fame Plain. Now fince it is ma-
nifeft, that there is no difference in
what Points of the Lines, in which
the Forces P and p act, thofe Forces
are placed; fince wherefoever they
are in thoſe Lines, they will have
exactly the fame Power to turn the
Plain CDAPE about its Center: the
Forces P and p may be fuppofed to
be in the Points A and E. Then the
Force P, fuppofed to be in A, may
be refolved (as was before obferved)
into two other Forces: One of which
may act according to the Line CA
produced, and the other, according
to the Line AG; and which may be
to each other as FG to GA, but each
of them fingly to P, as FG and AG
fingly to AF, as will be evident, if
the Triangle AGF be compleated in
the Parallelogram AGFg. It is alſo
manifeft, that the Force, which is as
FG, and which acts according to the
Line CA paffing through the Center
of the Plain, does nothing at all to-
wards turning that Plane about the
Center C; but the Force which is as
AG, and which draws the Line CA
perpendicularly; fince, by the Hypo-
thefis, it ballances the Forcep, which
draws the Line CE, equal to CA (by
Conftruction) perpendicularly alfo, it
muft neceflarily be equal to it.
Wherefore p will be to P as AG to
AF; or as DC (by reason of the fimi-
lar Triangles FGA, ACD) to CA or
CE: That is, the Forces and Pare
to one another reciprocally as Per-
pendiculars let fall from the Center
to the Lines in which they act.
.
Coroll.
If the Arms lie in a ſtreight Line,
and the Determinations of the Forces
be parallel, it is evident, that the
Forces are reciproaclly as the Length
of the Arms.
2. Hence alfo, in the Angular Bal-
lance PCp, which turns
about the immoveable Tab.XX.
Center C; the Situation Fig. 2,
which it will be in, when
any two given Bodies are fixed to
the Ends P and p, may be determi-
ned. For if the Line Pp which joins
the Ends of the Ballance be divided
to the
in reciprocal Proportion
Weights, and the Point of Divifion
G 3
The
86
Part I.
ROHAULT's SYSTEM
10. That it
10. But it is not neceffary to confider all the fimple
is not necef Determinations, of which one may be compofed: It is fuf-
fary to confi-
der all the
Determina-
tions of which
One
may
be
compofed.
ficient
fiſtance of the Plain it felf, acting in
the Line CP perpendicular to the
Plain But theſe three Forces are to
each other (from what was faid he-
fore) as the Sides of the Triangle
VPC; as will be evident, by draw-
T be made in the Line CT drawn
through the Center, parallel to the
Direction of the Weights: I fay it is
done For PD and pĒ being drawn
parallel, and DCE perpendicular to
CT; it is evident that DCE is di-
vided in C, in the fame Proportioning
that PTp is in T, and that the
Weights may be fuppofed to be pla-
ced in the Points D and E. Where-
fore this will be the Situation of
the Points P and p, that is, of the
Ballance it felf when the Weights are
in aquilibrio.
3.
Tab. XX.
Fig. 1.
In the Ballance or Leaver, it
is evident, that two For-
ces, fuch asP and p, which,
when the Ballance librates
to and fro, are recipro-
cally as the Velocities of the Points
D and E, reckoned according to the
Directions of thoſe Forces, will bal-
lance each other.
Of the inclined Plain. Prop. II.
If a Force, with a given Direction,
fupports a Weight upon an inclined
Plain; that Force is to the Weight,
as the Sine of the Inclination of the
Plain, to the Sine of the Angle which
is made by the Line in which the
Force acts, and the Line perpendicu-
lar to the Plain.
DE M.
Let AB be the inclined Plain, P the
Weight fupported, DPV
Tab. XX. the Direction of the Force
Fig. 3.
which fupports theWeight.
Let PC be drawn perpen-
dicular to AB; and from the Point
C, let CB be drawn parallel to the
Horizon, and perpendicular to the
common Section of the Plain and the
Horizon, meeting the Plain in B;
and CA perpendicular to the Horizon
and alfo to CB, meeting the Plain in
A, and the Line in which the Force
acts in V.
a Line through P parallel to VC,
and compleating the Parallelogram
The Force therefore is to theWeight
which it fuftains, as PV to VC; that
is, as the Sign of the Angle VCP, or
ABC, to the Sine of the Angle CPV
or CPD. 2. E. D.
Coroll.
1. If the Points V and A coincide,
that is, if the Force acts according
to the direction BA, the Angle CPD
will be a right Angle; and therefore
in that Cafe, the Force is to the
Weight, as the Sine of the Inclina-
tion of the Plain, to the Radius, or
as the Height of the Plain AC, to its
Length AB. And in this Cafe, the
Force which is required to fupport a
given Weight is leaft of all; becaufe
the Proportion of the Sine of the
Inclination of the Plain, to the Radi-
us is less than its Proportion to any
other Sine whatſoever.
2. If the Point V falls above A;
the greater the Angle APV is, ſo
much the more Force is neceffary to
fupport the given Weight upon the
Plain AB. Infomuch, that by in-
creafing the Angle APV, the Propor-
tion of the Sine of the Angle ABC,
to the Sine of the Angle CPD, is al-
fo increaſed, 'till PV, AV, becom-
ing parallel, and the Angles VCP,
CPD for that Reaſon equal, the Force
and the Weight will alfo become e-
qual.
3. So likewife, if the Point V falls
below A, as at v, the Force requi-
fite to fupport the given Weight, is
again increaſed; the Angle APʊ being
increaſed, till Po, v C become equal,
the Force and the Weight will be-
come equal again. Further, when
Now P may be conceived to be the Lines Pv, PC coincide, and the
held unmoved by three Forces acting Angle PC by that means vaniſhes
together: one of which is the Force the Sine of the Angle ABC will
of the Weight it felf tending down- bear an infinite Proportion to the
wards in a Line parallel to VC; the Sine of that; that is, no finite Force
Second is the Force acting in the whatſoever, acting in a Line per-
Line DPV ; and the 'Third is the Re-pendicular to the Plain, will be able
1
ΤΟ
Chap. 14 of NATURAL PHILOSOPHY.
87
ficient to confider thoſe which we have occafion for in
the explaining any Difficulties; herein imitating Geome-
to fupport the Weight upon the
Plain.
4. If the Line in which the Force
acts be parallel to the Bafe of the
Plain, the Weight is to the Force
which fupports it, as BC to CA, or as
the Bafe of the Plain to the Height
of it.
Tab. XX.
Fig. 4.
5. If from the Point P, PF be
let fall perpendicular to
BC, and from the Point
C, CG perpendicular to
VP; it will eaſily ap-
pear, that PV is to VC (that is, the
Force is to the Weight) as CF to CG.
Wherefore the Force and the Weight
will then ſupport one another upon
an inclined Plain, when they are to
each other reciprocally as Perpendi-
culars drawn from the Point C to the
Lines in which they act; (or, if
GCF be looked upon as an angular
Ballance moveable about the Center
C) reciprocally as the Velocities of
the Points G and F reckoned upon
the Lines in which the Forces act.
Of the Wedge. Prop. 3.
If three Forces acting togther up-
on an Ifofceles Wedge, in Lines per-
pendicular to the three Plains of
the Wedge; two of which Forces,
viz. thofe acting upon the Sides are
equal to each other, and the Direction
of the Third which acts upon the
Baſe of the Wedge, paffes through its'
Vertex; if, I fay, theſe three Forces
fupport each other, the Force acting
upon
the Bafe, will be to the other
Two, as the Bafe of the Wedge, to
the Sum of its Sides.
DE M.
Let ABC repreſent a Wedge; and
let CG be perpendicular
Tab. XX. to AB, and GD, G₫
Fig. 5.
perpendicular to AC,
BC; and thefe will be
the Directions of the three Forces.
In the Lines GD, Gd produced, let
DE and de be taken equal to each
other, which may therefore repre-
fent the two equal Forces, which act
upon the Sides, in the Directions
1/4
ters,
ED, ed. Let EF, ef be drawn
parallel to AB, and DF, df, paral-
lel to GC, fo as to form the Trian-
gles DEF, def. Now each of the
Forces ED, e, may be imagi-
ned to be refolved into two o-
ther Forces, which are to each other
as EF to FD, and ef to fd: And
to act in thoſe Lines: And thoſe
two, which are as EF, ef, becauſe
they are equal, and oppofite, will
deſtroy each other. But the Force
which acts upon the Baſe AB, in the
Line GC; becauſe it fupports the
two other Forces FD, fd, both
which are the fame way, and act in
a contrary Direction to that Force
upon the Bafe; is therefore equal to
the Sum of them. The Force there-
fore acting upon the Bafe of the
Wedge, is to the Sum of the Forces
acting upon its Sides as DF+df
to. DE + de or (by the fimilar Tri-
angles) AG GB that is AB to
AC+ CB.
Coroll.
The Velocities of the Wedge, and
of the Body refifting it, reckoned in
the perpendicular Direction before
explained, are to each other recipro-
cally as the Force acting upon the
Bafe, to the Force acting upon the
Sides of the Wedge, when theſe For-
ces are in equilibrio.
For when the Wedge ABC is dri-
ven up to the Top, or is
in the Situation ab
in the Situation ab c, it
Tab.XX.
Fig. 6.
is evident, that the Parts
of the Body that is cleaved,
have receded from each other, the
Length gd or GD, in the Direction
of the Line perpendicular to AC
or a c; GC therefore is the Veloci-
ty of the Wedge, and GD the Ve-
locity of the refifting Body. But (by
the fimilar Triangles) GC is to GD,
as AC to AC, that is, as AC – CB
to AB. And the Proportion will be
evidently the fame, whatever Situ-
ation the Wedge be in, between the
Parts of the Body to be cleaved
by it.
G4
of
88
Part I.
ROHAULT's SYSTEM
E
ters, who do not draw from one Point all the Lines that
Of the Screw.
-
A Definition.
J
If the Plain of the Triangle ABC
(whofe Hypothenuſe re-
Tab. XX. prefents fuch an inclined
Fig. 7.
Plain, as was explained
above in the 2d Propofi-
tion) be conceived to be fo fitted to
the Concave Superficies of a hollow
Cylinder (the Circumference of whofe
Baſe is equal to the Line BC) that.
the Plane ABC coinciding with the
Superficies of the Cylinder, the Line,
BC may be bent into the Periphery
of a Circle equal and parallel to the
Circumference of the Bafe; the Line
BA will form a kind of Spiral, af-
cending upon the Cylindrical Super-
ficies, and furrounding it once: So
likewife, if feveral Planes, fuch as
A ac, equal and fimilar to the for-
mer, and whofe right Angles are
fubtended by the Line BA produced,
be imagined to be fitted in the fame
manner, to the fame Superficies, di-
ftant from each other, by the Space
AC or ac (their common Height)
there will be many Spirals formed by
the Lines Aa, &c. all continued
from one to another, and each of
them once furrounding the Cylindri-
cal Superficies. Further, if other
Planes fimilar and equal to ABC be
conceived in the fame manner to be
fitted to the gibbous Superficies of
another Cylinder, whofe Bafe is e-
qual to the Bafe of the Concave Su-
perficies of the former Cylinder ;
there will by this means be Spirals
formed in this gibbous Superficies,
exactly like thofe in the Concave
one before. Now if the latter Cy-
linder, which may be turned about
its Axis, by means of a Leaver paf-
fing through the Center of either of
its Bafes, and lying in the Plane of
that Baſe, be imagined to be ſo pla-
ced within the former Cylinder,
which is fixed and immoveable, that,
the Superficies agreeing, the Spirals
formed in each Superficies, may a-
gree with one another allo; and if it
be fo contrived, that they fhall al-
ways thus agree, when the internal
Cylinder is turned about its Axis, and
its Bafe recedes from or approaches to
the Bale of the external Cylinder; it
can
is evident, that two Screws, the
Male and the Female may be con-
ceived to be thus generated.
Prop. 4.
In the Screw, as the Altitude of
one Spiral, is to the Circumference
of the Circle, whofe Radius is the
Leaver by which the internal Cylin-
der is turned round; fo is the Force
perpendicularly applied to the End
of that Leaver, to the Weight lifted
up by the Screw, when the Force
and the Weight are in aquilibrio.
DEM.
Let the Axis of the Screw be per-
pendicular to the Hori-
zon; and the Pofition of
the Leaver, by which the
internal Cylinder is turn-
Tab. XX.
Fig. 8.
ed about its Axis, will be Horizon-
tal. Let the Weight be placed any
where in the Line of the Axis;
and then that Weight, by means of
the internal Cylinder, will prefs with
equal Force (in Directions perpen-
dicular to the Horizon) upon every
individual Point of the Spirals of the
external Cylinder; and the Sum of
the Forces with which all thofe
Points are prefled, will be the fame
as the whole Weight to be lifted
up. But let us firft confider the
Force, or that part of the whole
Weight, which prefies upon any one
particular Point.
Now it is eafy to
fee, that the fame Force, in a hori
zontal Direction, which is able to
fupport the Weight, which prefles
upon any one Point of the Spiral,
upon the inclined Plain of which
that Spiral is formed; that ſame
Force with the fame Direction, is
alfo fufficient, to fupport the fame
Weight upon the Spiral; and that
there is plainly no difference, whe-
ther this Force be immediately ap-
plied to the Point which is preffed;
or be in any other Line touching the
Bafe of the internal Cylinder. Let
BC therefore be the Circumference
of that Bafe; AC the Radius; AG
the Leaver by which the internal Cy-
linder is turned about its Axis; FGH
the Circle defcribed by the Radius AG.
Thefe Things being fuppofed; from
what has been faid, together with
the
Chap. 14. of NATURAL PHILOSOPHY,
89
can be drawn from it, but fuch only as they think may
be of Ufe in their Demonftrations.
·
the Definition of a Screw, and the
4th Coroll. of the 2d Prop. it fol-
lows, that, as the Height of one Spi-
ral, to the Periphery BC, fo is the
Force applied to the Point C, in a
Direction perpendicular to AC, to
that part of the whole Weight, which
that Force fupports upon any one
point of the Spiral. And (by the
Property of the Leaver) as the Cir-
cumference BC, is to the Circumfe-
rence FH; (that is, as AC to AG;)
fo is the Force exercifed in G to the
Force exerciſed in C, becauſe the Di-
rections of theſe Forces being paral-
lel, they have equal Power in the
Leaver ACG, whofe Center is A.
Therefore (equally by Perturbation)
as the Height of one Spiral to the
Periphery FH; fo is the Force which
exerciſed in G, fupports that part of
the whole Weight, by which any
one Point of the Spiral is prefled; to
that part of the Weight it ſelf: And
as the Force which fupports that one
particular Part of the whole Weight,
is to that one particular part of the
Weight; fo is the Force which, act-
ing in the fame Direction, fupports
all the Parts of the Weight, that is,
the whole Weight; to all thofe Parts
together, that is to fupport the whole
Weight. Therefore, &c. Q. E. D.
Coroll.
The Circular Velocity of that
Force by which the Screw is turned
round, and the Velocity of the
Weight which is lifted up by means
of the Screw, are to each other re-
ciprocally as thoſe Forces when they
are in aquilibrio. For it is evident,
that in a whole Revolution of the
Leaver, the Weight is raiſed juſt the
Height of one Spiral, and that in
every Part of the Revolution, the
Weight is raiſed proportionably.
Of the Pulley or Windleſs. Prop. 5.
It is evident, that the Pulley may
accounted for, in the fame man-
•
ner as the Ballance or Leaver, in
which the Forces are imployed ei-
ther on the fame Side of the Center,
or on both Sides: Which, when they
are in aquilibrio, are to each other re-
ciprocally as Perpendiculars, let fall
from the Point which repreſents the
Center of the Leaver, to their Dire-
Єtions. And hence the Forces of
Engines, which confift of many
Pulleys, according as they are diffe-
rently framed, may eally be ex-
plained. If the Compofition of the
Pullies, or the manner of framing the
Windlefs be fuch, that the Ropes
which are fitted to the Pulleys, are
parallel to one another and the
Weight be fo fufpended in the midſt
of the Ropes, as to draw every one
of them with equal Force, it is felf
evident, that the Force, is to the
Weight which it fupports; as One,
to the Number of Ropes. For when
that Force is applied to one of the
Ropes only, it is directly oppofed
to that part only of the whole
Weight, which draws that Rope ;
the Pin to which the Windlels is
fixed, fupporting the other Parts of
the whole Weight.
It is alfo evident, that in this En-
gine, the Force and the Weight,
when they are in equilibrio, are to
each other reciprocally, as their Ve-
locities, when the Force raifes the
Weight. For it is manifeft, that
thefe Velocities are to each other,
as the Decreaſe of the Length of all
the Ropes which fupport the Weight
taken together, to the Increaſe of
the Length of the Rope to which
the Force is applied, in the fame
time; and that juſt ſo much as is
loft in a given time in all the Length's
of the Ropes which fupport the
Weight; the very fame is gained,
in the fame time, in the one Length
of that Rope to which the Force is
applied.
CHAP.
ROHAULT's SYSTEM
Part I.*
}
1. What is
mcant by
Reflexion
and Refra-
Zion.
2. An In-
T
CHAP. XV.
Of Reflexion and Refraction.
HAT we may apply what has been faid to fome Ad-
vantage, we fhall, by the help of it, explain the
Manner of Reflexion and Refraction. But to avoid the
Error of the Antients, who confounded thefe two Things
together, we obferve; that by Reflexion is meant nothing
elfe but the Bending, or Alteration of the Determination,
when a Body in Motion, ftrikes againſt another Body
which it cannot penetrate; and by Refraction is meant the
Bending or Alteration of the Determination, when a Body
in Motion, paffes out of one Medium into another, which
receives it with more or leſs Difficulty.
I
2. Suppoſe, for Example, that the Body A, which is
Stance of Re- perfectly hard, moves with a fimple Motion, in the Line
flexion.
AB, and that it meets with the Body CDEF, which I
Tab. II.
Fig. 6.
ſuppoſe to be perfectly hard likewife, and not to be ſhaken :
Then, from what has been ſaid, it follows, that the Body
A ought to continue in Motion, becauſe it does not
communicate any part of its Motion, and it ought to be
ftruck back, becauſe it cannot go on in a ftreight Line:
But let us fee how, and which way: And that we may
not multiply Difficulties, we do not now confider, what
will arife from its Bignels, Figure or Gravity: Let us
fuppofe likewiſe, that the Air makes no Refiftance to it,
and that it moves with equal Velocity.
3. That the
Angle of Re-
flexion is e-
Angle of In-
gual to the
idence.
3. This being fuppofed, let a Circle be deſcribed on the
Center A, and with the Diſtance BA; and for the fame
Reaſon that the Body A comes from the Circumference
to the Center in a given Time, it ought to go from the
fame Center to fome Point of the Circumference of this
Circle in the fame Time: Now to determine that particu-
lar Point, from the Points A and B, let the Lines AG,
BH be drawn perpendicular to the Superficies CF, and
the Line AHI, parallel to that Superficies: Now we may
obferve, that though the Body A is carried with a fimple
Motion, it is however true, that with reſpect to the Bo-
dy CDEF, its Determination in the Line AB, is com-
pounded of two others, the one of which makes it go
towards the right Hand, by the Length of the Line AH,
■. Ought to continue in Motion) See above, Chap. x. Art. 13.
or
Chap. 15. of NATURAL PHILOSOPHY.
91
or which is equal to it, GB; and the other makes it come
downwards towards GB, by the Length of the Line AG.
Now we may further obferve, I that the Body CDEF
refifts the Determination downwards, but that it does not
at all refift the Determination towards the right Hand,
that is, that part of the Motion which is determined to-
wards the right Hand, which confequently 2 ought to
continue as it began. So that the Body A having in a gi-
ven Time with this Determination, paffed through the
Space contained between the Lines AG, HB, that is, mo-
ved the Length of the Line AH or GB, it ought in the
fame time to pafs through an equal Quantity again, or
which amounts to the fame Thing, it ought at the End
of this Time, to be found in the Line IL, which I fup-
pofe to be perpendicular to the Superficies CF, and the
fame Diſtance from HB, as HB is from AG. So that,
to fatisfy that part of the Motion which is towards the
Right, which does not alter at all, we find that the Body A
at a certain Moment of Time, ought to be ſomewhere in
the Line IL. But to fatisfie the whole Motion, we have
before ſhown, that it ought in the fame Moment to be
fomewhere in the Circumference of the Circle: There-
fore, that theſe two may be both fatisfied together, we
ought to conclude, that it will at the fame Time, be in
the Circumference of the Circle, and in the Line IL to-
gether, which can be no where elſe but in the Point I
which is common to them both. Thus we fee the Body
A which began to move in the Line AB, is reflected in
the Line BI, which makes with the Superficies C the Angle
1. That the Body CDEF refifts the
Determination) If the incident Body
A, and the Body CDEF upon which
it ftrikes, are void of all Elafticity;
the Body CDEF not only refifts this
perpendicular Determiuation, but en-
tirely deftroys all the Motion that
arifes from that Determination (See
the Notes on Chap. x. Art. 13.) fo
that the Body A, is afterwards mo-
ved, with the other part of its Mo-
tion only, along the Superficies BLF.
But if either, or both theſe Bodies
be perfectly elaftick, then a new
Motion will be impreffed upon the
Body A, equal to the Motion which
was loft, and with a contrary De-
termination; fo that, when it comes
to the Superficies GL, with the De-
termination AG, it will then recede
from it with the Determination LI.
This is carefully to be obferved, be-
caufe it is neceflary to
the com-
pleating this Deinonftration, by
which it appears, that the Angles of
Incidence and Reflexion are equal.
For the Nature of this Elaftick Force
being rightly underſtood, the De-
monſtration concerning the reflect-
ing of Elaftick Bodies, will hold in
the fame manner as in perfectly
hard Bodies, according to the Au-
thor's Principles. See further, the
Notes on Chap. xi. Art. 6. Tab. II.
Fig. 6.
2. Ought to continue as it began)
Hence it follows, that the Lines of
Incidence and Repercution are in a
Plane perpendicular to the Superfi-
cies of the reflecting Body. See the
Notes on Chap. xxxiv, Art.2.
IBL,
Tab. II.
Fig. 6.
92
Part I,
ROHAULT's SYSTEM
4. An Ex-
ample of one
Sort of Re-
fraction.
Tab. III.
Fig. 1.
5. Another
I
which
IBL, which is called the Angle of Reflexion,
may eaſily be demonſtrated to be equal to the Angle ABG,
which is called the Angle of Incidence.
4.
Let us now come to Refraction, and that we may ex-
plain the Nature of it fully, I fhall here make uſe of the
Example of a Ball, as was before done in Reflexion.
Suppofe then the Ball A to be moved along the Line AB
in the Air, but ſtriking obliquely upon the Water below
CD, inſtead of going on directly towards E, it tends to-
wards F, this Sort of bending, 2 meaſured by the Angle
EBF is what we call Refraction.
5. If the Body A, after it is arrived at B in the Line
Sort of Re- AB, inſtead of being turned towards F, is turned to-
fraction.
Tab. III.
wards G; this is Refraction alfo, but of a different Sort
Fig. 1.
from the other: Now in order to diſtinguiſh theſe two
Sorts of Refraction, let the Line HB, be drawn through
the Point B, where the Body A paffes out of one Me-
dium into the other, perpendicular to the Superficies CD,
which divides the two Mediums, and the Kind of Refra-
Etion is determined, by the Approach to, or Recefs from
this Perpendicular. For Example, if the Body which
moves along the Line AB, when it is turned out of the
way, afterwards moves along the Line BF, this is called
Refraction from the Perpendicular; but if it afterwards
moves along the Line BG, then it is called Refraction to
the perpendicular.
Body is turn-
ed ont of its
Courſes we
must think,
that it meets
with fome
Obftacle on
that part
6. When a 6. Theſe two Sorts of Refraction have been obſerved a
long time, but the Caufe of them was not at all known.
And we may venture to ſay, that this is one of thoſe
Things which the Antients were ignorant of, and the
Diſcovery of which is owing to one of the principal Men
of this Age; and agreeable to his Opinion, I thus explain
this Matter: Since we are fure, that every Thing, as
from which it much as it can, perfifts in that State in which it is, after
we find by Experience, that a Body quits the ftreight
Line in which it began to move, we muft neceffarily
think, that it has met with fome Obſtacle on that part
from which it removes: Thus, if, when the Body A is
come to the Point B, it is turned out of its Courſe to-
wards the Point F, we ought to conclude, that it meets
TILYNS.
Tab. II.
Fig. 1.
1. Which may eaſily be demonftra-
ted) For BL GB by
and
the Hypothefis ;
LI = GA, becauſe GL
Tab. II.
Fig. 6.
and AI are paratiel, and
the Angles L and G are right An-
gles, by the Hyp. Therefore the Tri-
angles ILB, ACB are equal and fi-
milar.
2. Meaſured by the Angle EBF)
See the Notes upon Art. 11. of this
Chap.
with
Chap. 15. of NATURAL PHILOSOPHY.
with more Reſiſtance on the Side M, than on the Side
N;
and if it is turned toward G, we have Reaſon to
think, on the contrary, that it has met with more
Reſiſtance on the Side N, than on the Side M.
dium which
makes the
7: We may reaſon in the fame manner, in order to 7. That the
Body in Mo-
determine on which Side, a Body moving out of one tion, recedes
Medium into another, will be turned. For fince we be- from the Me-
fore knew, that the unequal Reſiſtance, which a Body
in Motion meets with on different Sides, (according to greatest Re-
the different Mediums through which it paffes) would fiftance to it.
force the Body to turn out of its Courſe, and to remove
from that Side where it finds the moſt Reſiſtance; when
once we come to know, that there is more Reſiſtance on
the one fide than on the other; we conclude, that it will
turn out of the way, by removing from the Medium
where the Reſiſtance is greateft. And thus when we once
come to know that Water refifts the Motion of a Ball
more than Air, we ought to think, that the Ball which
moves in the Air from A to B, in paffing into the Wa-
ter which is below B, will turn towards F, and fo will
recede from the Perpendicular.
8. The Way
how to deter-
mine the par-
Refraction.
8. This may be applied to all forts of Bodies, and to
all forts of Mediums, and therefore we may lay it down
for a general Maxim, that when a Body paffes obliquely ticular fort of
out of one Medium into another, which makes a greater
Reſiſtance to it; it ought fo to turn as to remove from the
Perpendicular, and, on the contrary, when it paffes out
of one Medium into another, where it finds lefs Refift-
ance; it ought to be fo turned, as to approach towards
the Perpendicular.
9. That a
Body which
falls perpen-
dicnlarly up-
on another,
ought not to
be refracted
at all in en-
9. I exprefsly added, that the Body which paffes out
of one Medium into another, muſt fall obliquely upon the
Superficies which feparates the two Mediums, in order to
be refracted; for if it falls perpendicularly upon this Super-
ficies, as there is nothing to refift its Motion more on the
one fide than on the other, fo it ought not to be turned
out of its courſe at all, but to continue to move in the tring into it.
fame Line.
1. To all Sorts of Bodies) For this
Reafon the Rays of Light which
pafs out of Air into Water, are reflect-
ed towards the perpendicular, contra-
ry to what we ſee in a Ball thrown
out of our Hand; becaufe Water
which refifts the Motion of the Ball
more than Air, on the contrary, re-
fills Light lefs. (See Chap. 27. Art.
|
|
10. The
38.) or to fpeak more truly, it acce-
lerates the Motion of Light more by
attracting it; as will be thown at-
terwards.
2. But to continue to move in the
fame Line) Yet fome have thought, as
J. Foffius, Willebrord Snell, that they
have feen a perpendicular Ray of
Light, fome way refracted and con-
tracted
94
Part I
ROHAULT's SYSTEM
10. An Ex-
Motion of a
dy.
Tab. III.
Fig. 1.
10. The exact Quantity of the Refraction of a Body
ample of the paffing obliquely out of one Medium into another, may
refracted Bo- be determined, provided we know how much the one Me-
dium refifts its paffing more than the other. Suppofe, for
Inftance, the Line ČD ſeparates the two Mediums, the
upper one of which is Air, and the under one Water, and
that the Water refifts the Motion of the Ball A twice as
much as the Air; then let us imagine, that this Ball has
run the Length of the Line AB with fuch a Velocity, as
takes up a Minute, and is then ready to enter the Water
obliquely: and that the Thing may be the eaſier apprehend-
ed, we meddle not with what might happen on the ac-
count of the Bignefs or Weight of the Ball. Let us ima-
gine further, that its Motion in the Air has been all along
uniform, and that after having loft half its Velocity by
meeting with the Superficies of the Water, it loſes no more,
though it finks never fo deep; for the Deviation 1 is
made only in the Superficies, and the Water which reſiſts
all its Parts equally, can only make the Ball take up more
or leſs Time in moving through a given Line, and not
cauſe it to move out of it.
11. How Re-
fraction is
made.
Tab. III.
Fig. 2.
11. This being fuppofed, having deſcribed a Circle on
the Center B, and the Diſtance AB, let us confider, that
the Ball having taken up a Minute of Time in moving
from the Circumference of the Circle to the Center,
where it loſes half its Velocity, ought afterwards to take
up two Minutes in moving from the Center to any Point
in the Circumference: Now in order to determine where
this Point ought to be, we obſerve, that though the Mo-
tion of this Ball was ſuppoſed to be a fimple Motion, yet
its Determination in the Line AB, with reſpect to the Su-
perficies of the Water, is really compofed of two Deter-
minations, one of which cauſes it to move from the Left
to the Right, the Length contained between the Lines
AF and BG, which are perpendicular to the Superficies
of the Water, that is, the Length of the Line AG or FB;
the other Determination makes it defcend downwards the
Length contained between the two Parallels AG, CD,
that is, the Length of the Line AF. We muſt further
tracted into it felf; which is, becauſe
when we look upon any Thing in
the Water, it feems to be nearer us
than it really is; fo that herein they
falfely afcribed that to Refraction
(of which there is none in the per-
pendicular) which was to be afcri-
bed to the diverging of oblique Rays
after Refraction, from the Point
|
nearest to us. But for the real and
manifeſt Refraction of perpendicu
lar Rays, which is made in Iſland
Chrystal, See Newt. Opt. pag. 229.
2. Is made only in the Superficies)
It is otherwife in the Reflexion and
Refraction of Light. See below,
Chap. xxvii. Art. 35. 37.
obferve,
Chap. 15. of NATURAL PHILOSOPHY.
·95
obſerve, that the Superficies of the Water refifts the De-
termination downwards, which confequently muſt be al-
tered; 1 but it makes no Reſiſtance at all to the Deter-
mination from Left to Right, wherefore this will not be
at all altered, but the Ball which moved in this manner
the Length FB during the Minute which it took up in
going from the Circumference of the Circle to the Ĉen-
ter, ought to move twice this Length in two Minutes,
in going from the Center to the Circumference: Let
BL therefore be taken equal to twice BF, and the Line
ELM drawn perpendicular to CD, and the Ball ought
to be found fomewhere in this Line, two Minutes after
it has parted from B; but it was before ſaid, that it ought
at the fame time to be in the Circumference of the Cir-
cle alfo; whence we conclude, that the Ball will be at the
fame time in this Line, and alfo in the Circumference of
the Circle; that is, in the Point M, where they interfect
each other. So that inſtead of continuing its Courſe in
the Line AB produced to N, it will be carried along the
Line BM, which is from the Perpendicular, 2 and the
Refraction will be meaſured by the Angle MBN. From
what has been faid, it is plain, that if the lower Medium
had refifted the Ball less than the upper one, the Refraction
ought to have been contrary, that is, to the Perpen-
dicular.
when the Ball
falls very ob-
12. Without altering any Thing before fuppofed as to 12. The dif
the Difference of the Refiftance of the two Mediums, ficulty there is
and the Velocity of the Ball, let us now ſuppoſe, that
the Ball, in order to go to the Point B, comes from ano- lique.
ther Point more diftant from the Point P than was fup-
pofed in the former Example, fo that the Line FB which
is the Meafure of the Determination towards the right
Hand be longer than half the Radius of the Circle, and con-
fequently the Line BL, which is twice as long, be longer
than the whole Radius; it ought to follow, according to
the foregoing Reaſoning, that the Line ELM will fall with-
out the Circle, and not interſect it at all; And ſo our
1. But it makes no Refiftance) | mathematically the Nature of Re-
But it does reſiſt that Determination fraction.
alfo, as it enters; for the Ball in en- 2. And the Refraction will be mea-
tring: ftrikes or rubs against the far-fured) It is a right Obfer-
ther
part of the Hole which it enters
into for which reaſon, and becauſe
the Motion of the Ball is afterwards
perpetually retarded as it palles
through the Water which refilts it,
this Inftance is not fufficiently accom-
modated to explain accurately and
vation of Cartes here, that Tab. III.
Refraction univerfally and Fig. 2.
in all Incidencies is to be
meafured by the Proportion of the
Lines AG and OM, and not by the
Angles ABG, and HBM or NBM.
See Cartes's Dioptr. Chap.2. Art. 7.
Ar
96
Part
ROHAULT'S SYSTEM
"
13. That a
trate it at
all.
Argument feems to conclude, that the Ball ought to be
in two different Places at the fame time, viz. in this Line,
and in the Circumference of the Circle; which is im-
poffible.
13. It muſt be confeffed, that here is fome Miſtake,
Body which whencefoever it ariſes; for every Argument that leads to an
falls too ob-
Impoffibility, is defective either as to the Form or as to
lique upon a-
nother, ought the Matter of it. But let us not imagine that there is
not to pene- any Fault in the Form of this Argument which feems to
conclude in an Impoffibility; let us rather fay, that it be-
ing conclufive, it is a certain Sign, that the Fault was in
fome of the Suppofitions that were made. And ſo indeed
it was, for we fuppofed that the Ball, when it had loſt
half of its Motion by meeting the Superficies of the Wa-
ter, would enter into it, though it fell never fo oblique,
which is not fo. For we fee by Experience in a Sea-
Fight, that Cannon-Balls which are fhot too oblique upon
the Water, are reflected by the Superficies of the Sea, and
kill the Soldiers upon the Decks of the oppofite Ships.
And we obferve the fame Thing in Stones which Chil-
dren make Ducks and Drakes with in the Water.
I. That the
Pofition of
hard Bodies
put into Li-
A
CHA P. XVI.
Of hard Bodies put into Liquors.
LL that can be faid of the Place which a Body ought
to poffefs in any Liquor according as it is more or
leſs heavy, does properly belong to the Doctrine of Mo-
tion. For theſe Bodies are in Motion when they fink in
Motion. the Liquor, and they are in Motion alſo when they rife
from the Bottom, to the Superficies.
is an
quors
Effect of
2. That the
Superficies of
a heavy Li-
quor contain-
ed in a Fef
fel, ought to
be level.
Tab. III.
Fig. 4.
2. That we may not pafs by any Thing therefore which
may be of uſe afterwards, let ABCD be a Tub filled with
Water, and fuppofe first, that this Water is upon the Le-
vel, that is, no one Part of the Surface AD higher than
another; then imagining it to be divided into a great ma-
ny Columns, perpendicular to the Bottom of the Tub,
let us examine one of theſe Columns, as EFGH.
firſt it is obfervable, that though this whole Column en-
deavours to fink down, yet it cannot, becauſe the fmal-
ler Columns, into which this may be fubdivided, muft
bend at the Bottom of the Veffel before they can return
And
up-
Chap. 16. of NATURAL PHILOSOPHY.
*
1
upwards, but that they cannot do, becauſe they meet and
fupport each other, and are alſo fupported by the little Co-
lumns on all Sides of them, which tend downwards like-
wife, and with equal Force. So that the Water in the
Tub ought to continue upon the Level, and to remain in
perfect Reft and Equilibrio, if there be nothing elſe but
its own Weight to move or ſhake it. Whence it is ma-
nifeft, that if we fuppofe the Water in the Tub to be
higher in one Place than in another, that it cannot conti-
nue fo, becauſe thoſe little Columns of Water which are
longer than the other, will have more Power to defcend
than they, and will never leave crouding them up, till the
Surface of the Liquor is come to a Level, when they will
all be in equilibrio with each other. Therefore when a
"
heavy Liquor is contained in any Veffel, we are to think
that its Weight difpofes the Surface of it to be upon the
Level, and that it will continue fo, unless altered by fome
foreign Caufe.
:
3. That a
hard Body
put into a
3. Let us confider further, that if there be put into the
Water in this Tub any hard Body, fuch as I, of equal
Gravity with the Water; as its Weight would have nei- Liquor of e-
ther more nor lefs Effect than the Water whofe Place it qua! Gravity;
poffeffes; there is no Reaſon why any Alteration fhould be ought to reft
made in the Column EFGH, fo that the Body I muft of it."
continue where it was placed.
in
any part
which is hea™
4. But if we imagine this Body to be heavier, by an 4. With what
Ounce, fuppofe, than a Quantity of Water of equal Bulk, Force a Body
it is manifeſt then, that all the Columns, of Water will vier than Wa
not be in æquilibrio, but the Body will go to the Bottom, ter, ought to
not with its ordinary Weight, but only with the Diffe- Sink to the
rence betwixt that and the Weight of a Quantity of
Water of equal Bulk, that is, with the Force of an Ounce
weight.
Battomi
the real
5. But fince Water was here taken only for an Exam- 5. That we
ple, and the Reaſoning holds the fame, when applied to cannot feel
any other heavy Liquor; we may affirm in general, that Weight of a-
in ſupporting a heavy Body, we ought only to feel the ny Body by
Excels of its Weight above that of an equal Bulk of the our Senfess
Liquor in which it is. Hence it is, that we are not fur-
prifed to find by Experience, that a pretty lufty young
Man who weighs a Hundred and thirty eight Pound in
the Air, does not weigh above eight Ounces in the Wa-
ter. But we have before fhown, by many Experiments,
1. Upon the Level) That is, as to
Senfe. But in reality it is part of
the Sphærical Superficies of the
Earth.
H
that
ROHAULT'S SYSTEM
Part I.
1
98
the Liquor,
that the Air itfelf is heavy, wherefore we do not by our
Senfes feel the true Weight of a Body in the Air, but on-
ly the Difference of the Weight of the Body and of the
Air; and conſequently, unleſs we are under any particu-
lar Indifpofition, we ought never to feel our felves lighter,
but only when the Air is heavier.
6. That a 6. It is evident, that if the Body I, juft now mention-
Body which ed, had been fuppofed lighter than that Bulk of Water,
is lighter than whofe Place it poffeffes; the Column EFGH would not
be heavy enough to be in equilibrio with the reſt of the
Water in the Tub; wherefore this Column will be forced
to give way, till the Body I be got up to the Surface
AD, beneath which, fo much of it will remain, as pof-
feffes the Place of a Quantity of Water equal in Weight
to the Body.
ought to rife
up, and that
with fome
Force.
Tab. III.
Fig. 4.
7. How to
find whether
7. From what has been faid, we may draw two very
and uſeful Inferences. Firſt, That if a Body
hard Body important and ufeful Inferences.
weighs more put into any Liquor, finks to the Bottom, it is certain that
or less than Body is heavier, than an equal Bulk of the Liquor, but
if it fwims on the Top, it is an infallible Sign, that it is
lighter.
an equal
Bulk of any
Lignor.
8. The way to
8. Secondly, If a hard Body be put into two Liquors,
find which is and rifes in the one, but finks in the other, the former
the heaviest muft neceffarily be heavier than the latter.*
of two Li-
quors.
}
}
9. This
*It is worth while to explain, ater. Becauſe then the Column EFGH
is heavier than the Columns which
little more fully, and in better Or-
der, the Hydroftatick Propofitions, furround it. See Art. 4. of this
Chap
which are urged too briefly and con-
fufedly in this Chapter..
1. Therefore. All Water gravitates
in every Place, even in Water it felf
(and the fame is to be underſtood of
any other Liquor) and by reason of the
equal Preffure of its Parts on all
Sides, its Superficies ought to be plain
and level. This is demonftrated in
the Second Article of this Chapter,
and by the famous Mr. Boyle in his
Hydrostaticks. Paradox 1.
2. A hard Body, fuch as I, equal
in Weight to a Quantity
Tab. III. of Water of the fame
Fig. 4.
Bulk, put into Water,
ought neither to fink nor
rife, but to rest in any Place. For
the Column EFGH gravitates nei-
ther more nor leſs than the Columns
which furround it, and therefore i
ought to keep in equilibrio. See
'Art. 3.of this Chap.
3. A Body, fuch as Is heavier than
Vater, ought to fink_in_the_ V¥a-
§
4. A Body, fuch as I, heavier than
Vater, ought to have just so much
VVeight in VVater, as it exceeds in
Veight an equal Bulk of VVater.
For fince the Body A poffeffes the
Place of an equal Bulk of Water in
the Column EFGH; it is manifeſt
that by how much that Body exceeds
that equal Bulk of Water in Weight,
by juff fo much is that Column hea-
vier than it was before. See Art. 4.
of this Chapter, and Archimedes of
Bodies put into Fluids. Prop. 7.
Hence, fince the Proportion of
Weight betwixt Gold and Water is
known, Gold may be proved and
valued, by weighing it in Water. See
Boyle's Hydrostatick Medicine.
5. Any Body fuch as I, put into
Vater, is not only preffed downwards
by the incumbent Water, but is alfo
preffed upwards by the Water that
is under it. This is evident from the
firſt Propofition. See alfo Boyle's Hy-
6. The
drostaticks, Paradox 3•
Chap. 18. of NATURAL PHILOSOPHY.
99.
9. This being fo, if we examine the Opinion of fome 9. A Miſtake
Philofophers, viz. that there are certain Places natural to in fome Phi-
lofopkers.
all Bodies where they of themſelves continue at reſt, and
have no Tendency to go out of them, and that this is the
6. The heaviest Body of all, fuch
as I, a Cube of Gold, if it be put
So deep into the VVater, that the
Depth of the Vater from EH to the
lower part of that Cube be twenty
times as much as the Thickness of I
is, that Cube will be ſo preſſed up-
wards by the VVater that is under
it, that, if the incumbent VVater
EIH were removed, it would not fink.
For fince the Cube I is juft of the fame
Weight as the Water which reaches
from EH to the Bottom of the Cube;
all which Water we now fuppofe
to be removed; it is evident, that
the Column FIG in this Cafe, is
in equilibrio with the Columns which
furround it, and therefore the Cube I
cannot fink. See Hydrostatick Pa-
rad. II.
7. A Body, fuch as I, lighter than
VVater, let it be preffed never so
much by the incumbent VVater, ought
to rife notwithstanding. For in this
Cafe, the Column EFGH is lighter
than the Columns of Water which
furround it. See Art 6. of this
Chap.
8. VVhen a light Body is rifen to the
Top of the VVater, ſo much of it ought
to remain under the VVater, as is
equal to a Bulk of VVater weighing
as much as the whole Body. This is
the Fifth Propofition of Archimedes
concerning Bodies put into
Tab. III. Fluids, and is eaſily de-
Fig. 4.
monftrated from what has
been already faid. For it
is manifeft, that when the lower
Part of the Body fwimming in the
Water, is funk in this Proportion,
the whole Column EFGH is in 2-
quilibrio with the Columns that fur-
round it and if the fame Body be
funk deeper, this Column will be
lighter than the reſt of the Columns;
if not fo deep, it will be heavier.
9. In every Body that is lighter
than V Vater, the Proportion of its
VVeight to the VVeight of VVater,
is as that part of it under the VVa-
ter to the whole Body. This Propofi-
tion follows from the preceeding one,
and is more at large demonftrated by
Archimedes, Book II. Prop. of Bodies
put into Fimids.
Rea-
10. All VVater preffes upon the
Bodies under it, in proportion to its
perpendicular Height, and not in pro-
portion to its Breadth, This noble
Propofition is at large demonftrated
in my Notes upon Chap. 10. Art. 1 1.
I.
11. This Preffure acts upon Bodies
immersed in the VVater, not only on
the Top, but on the Bottom and the
Sides, every way equally. This Pro-
pofition follows from the foregoing
one, and is demonftrated from the
Nature of Water, whereby every
Preffure is propagated equally and
entire every way. See alfo Boyl's
Hydroft. Paradox 7.
12. Hence, a wooden Trencher put
under VVater, immediately rifes up;
though there be a much greater Quan-
tity of VVater lying above it, than
is under it; neither is there any fuch
Thing in Nature as Levity, to lift
it up.
This Propofition you have
demonftrated in my Notes on Chap.
X. Art. 11. Coroll. 3.
wooden
13. However, If the
Trencher be exactly fitted to the VVidth
of the Veffel, fo that no Water can
get in between it and the Sides of the
Veſſel, which by communicating its
Weight to the Water beneath, might
force the Trencher up ; or if the
Trencher goes fo close to the Bottom of
the Veffel, that no VVater can get in
betwixt it and the Bottom, then the
Trencher will not riſe at all. Which
is a manifeft Proof, that there is no
fuch Thing as Levity in Nature.
See the fame Place.
It is very hard to prove this Pro-
pofition by Experiments, becauſe
Water is fo apt to wet and run all
about. But I have tried it with
Quickfilver, which will not wet inoft
Bodies; for after I had gently put
a Piece of Money on the Bottom of
a Veflel full of Quickfilver; the Mo-
ney did not rife up; but if I thaked
the Vaffel, or lifted up the Money
ever fo little with a Needle, that fome
of the Quickfilver might get be-
twixt the Money and the Bottom
of the Veflel, the Money was
im-
mediately raiſed up,
H 2
14,7
'
INÒ
ROHAULT'S SYSTEM n. Párt I.
-
t
Reaſon why Water has no Weight in Water; we ſhall not
fcruple to affirm, that this is as grofs an Errour, as, it
would be in a Man, who, feeing a large Cannon in one
Scale, and Seven or eight thouſand Pound Weight in the
other, ſhould affirm, that the Cannon did not weigh any
Thing in this Place, becauſe he can eaſily lift it up or
down: For this Opinion of theſe Philofophers is founded
upon this Experiment, that in drawing Water out of aWell,
we do not begin to feel the Weight of that with which
the Bucket is filled, till it comes into the Air; whereas
they ought to think, that as the Cannon is always hea-
vy, and we could not eafily lift it, but for the Weight
which keeps it in equilibrio; fo alfo the Water weighs al-
ways the fame; and the Reaſon why we don't perceive
its Weight when the Bucket is under Water in the
Well, is, becauſe we are affifted by the reft of the Wa-
ter in the Well, which is in equilibrio with that in the
Bucket.
Tab. I.
Fig.4.
15. So likewiſe, it may be, that
Oyl having Water on each Side of it
may not rise up, viz. thus, if, when
the Syphon is filled with Water up
to ABC, Oyl be poured upon the
Water in each Arm, and Water be
14. It is poffible for Water to de- by the Oyl's mixing with the Wa-
prefs and fink a Body light-ter, it is more proper to uſe a Syphon
er than it felf. This may be with fmaller Arms. See Boyle's 8th
done by gently putting the Hydroftatick Paradox.
Syphon ABCD, filled with
Öyl as high as ABC, into theWater till
the ſhorter Arm AB be under Water;
for then the Water preffing upon the
Superficies AB, will lift up the Oyl
the Oyl
fo much the higher towards D, as the
Syphon is let down deeper into it.
And from hence alfo it is as clear as
the Sun at Noon-Day, that there is
no fuch Thing as Levity in Nature.
But left the Experiment ſhould fail
1
again poured upon that Oil to bal-
lance the Preffure of the lower Wa-
ter upwards. See Boyle's Hydroſta-
ticks, Paradox 9.
CHAP.
Chap. 17. of NATURAL PHILOSOPHY.
TOI
CHAP. XVII.
S
A
Of Accretion, Diminution, and Alteration.
•
meant by Ac-
cretion and
Diminution.
S Aristotle in treating of local Motion confiders alfo 1. What is
the other Changes that happen to natural Bodies,
fuch as Accretion, Diminution and Alteration, which he
calls Motion likewife; fo we after his Example, fhall not
wholly neglect theſe, but ſhow that it was not without
Reaſon, that he brought them under this Head, fince
they are indeed the Effects of local Motion. All the
World agree, that by Accretion and Diminution is meant
the fenfible Increaſe or Decreaſe of the proper Subſtance
of a Body; Thus we are fure, that the Trunk of a
Tree is increaſed when we fee it bigger than it was be-
fore.
creaſed and
2. Since we obferve, that Trees, and in general all 2. How Bo-
Bodies ftand in need of Nouriſhment, to make them dies are in-
increaſe, and that it is impoffible to conceive how a diminished.
Body fhould increaſe and become bigger without fome
Parts being added to its former Bignefs; this is a con-
vincing Proof, that every Body which increaſes, receives
fome Augmentation of Matter. And as this is true of a
Body which increaſes, fo may we alſo affirm, that eve-
ry Body which decreafes, lofes fome of the Matter which
it had before.
3. However this does not hinder us from making a 3. That In-
difference betwixt Increase and Rarefaction; and betwixt creafe is dif
Decreaſe and Condenfation: For the Matter which is ad- ferent from
Rarefaction.
ded to a Body increafing, and that which is taken from
a Body decreafing, is looked upon as belonging to it,
and as part of its proper Subftance; but, as was before
obferved, the Matter which enters into the Pores of a Bo-
dy to rarify it, or that which gets out of its Pores, that
it may be condenſed, is looked upon as Matter that does
not belong to it.
4. The Idea we have of the Accretion of Tree, be-
Tree, be- 4.That there
ing different from the Idea we have of its being tranf is a great
deal of dif-
planted, it muſt be owned, that Ariftotle had Reafon to ference be-
make a difference betwixt Accretion and local Motion, "twixt Accré-
Body, and the
However, as a Tree cannot be tranfplanted, but by the tion in a
local Motion of its whole Body, fo we cannot conceive local Motion
how it fhould increafe but by the local Motion and of it.
H 3
Union
}
1Q2
Part I.
ROHAULT'S SYSTEM
5. How Bo-
tered.
Union of the fmal Particles which contribute to the in-
creaſing it.
5. When a Body neither increaſes nor decreaſes, but
dies are al- is fomewhat changed, if this Change be not fo great that
we do not at all know it, we call it, as was faid before,
Alteration; hence it is eafy to fee, that there can be no
Alteration without local Motion: For how can there be
any Change in a Body, if none of the Parts which com-
pofe it, and upon the particular Order of which its Nature
depends, have changed their Situation? This being fo, it
is very evident, that there must be an Alteration in a Bo-
atton
dy, when the fenfible or infenfible Particles of which it
is compofed, are put out of their Order, or any great
Change made in their Figure: Or it may alfo fuffer an
Alteration, by the Acquifition of fome new Particles, or
by the Lofs of fome of its old ones; all which cannot be
without local Motion: Thus, when there is an Alteration
in a bruised Apple, we can eafily imagine that many of
its Particles have been forced to change their Situation,
and perhaps fome of them have alfo changed their Fi
gure. If after this, any one ftill doubts whether there
may not be fome kind of Alteration in which there is
fomething elfe befides what proceeds from local Motion, I
think he cannot be fatisfied better, than by what we are
now going to ſay of Forms.
1.That Forms
aught 10 be
treated of by
themſelves.
CHA P. XVIII,
Of FORMS,
FORMS are the matter For funce Matter
ORMS are a Subject that we cannot hope to treat
a Subject we c
of, as we have done of Matter
is a common Subftratum, which, when once we underſtand
what it is in Wood, we cannot at the fame time but un-
derftand what it is in Fire, and in every Thing elſe; one
fingle Reflection is of it felf fufficient to gain the Know-
ledge of it. But becauſe the Form of any Thing, is that
which makes it to be that particular Thing, and diftin
guifhes it from every Thing elſe; it does not follow, that
if we know the Form of Wood, we therefore know the
Form of Fire, or any Thing elfe. Wherefore if we
would fucceed herein, and fay fomething more than ordi-
nary, we muſt deſcend to Particulars, notwithſtanding the
Cu
"
Chap. 18. of NATURAL PHILOSOPHY,
103
;
Cuſtom of Philofophers, who feldom do fo, but for the
moft part content themfelves with propofing abundance
of looſe Queſtions, which we may look upon as fuperflu-
ous, and from which we can gain no Advantage.
ftantial
rationa! Soul,
there are any
others,
2. Of Sub-
2. However, I do not affirm, that it is an uſeleſs En-
quiry, if it ſhould be asked here, as ufually it is, whether Forms, and
there be any fuch Things as Subſtantial Forms, that is, that the In-
Forms which are real Substances; and confequently have a france of the
diſtinct Exiſtence from that of Matter. But thus much does not
at leaſt, I may venture to affirm, that the Solution of this prove that
Difficulty, depends upon the particular Knowledge of
the Things. The Inftance of the rational Soul proves no-
thing here; for though we know that this is a Subítance
really diftinct from the Body, to which it is united, and
that it does not at all depend upon it for its Exiſtence,
yet we can conclude nothing from hence as to the Forms
of other Beings which are purely material.
3. That the
rational Sou!
is not the
as a Body.
3. But if we confider this Matter more cloſely; though
I acknowledge, as all the World do, that the Soul is that
which particularly makes a Man to be a Man; and con- Form of the
fequently that it is truly the Form of a humane Body as humane Fody
humane; yet I can't agree, that it is, properly ſpeaking, the
Form of all that which is fenfible, and is called the Body
and confidered fimply as a Body, any more than it is the
Form of any of its Parts, confidered as different from
each other: For in this Senfe, every one of them has its
particular Form fo clofely connected with the Matter of
it, that it continues as long as the Part fubfifts, even af-
ter the Soul is ſeparated from the Body. And indeed af-
ter fuch Separation, every part appears the fame, as it did
immediately before. For, that which was Fleſh, for In-
ftance, is Flefh ftill, and that which was Bone, is Bone
ftill, and fo of the reſt.
4. The Cauſe of many People's Miftake, who con-
found the Properties of the Body with thofe of the Soul,
is this; that a dead Body, when the Soul is feparated from
it, is uncapable of many Functions which we obferved in
it before, fuch as moving it felf, Refpiration, Nouriſh-
ment, &c. fo that they perfwade themfelves that all thefe
Things depend upon the Soul, and would not have ceaſed
in the Body, if the Soul had not departed from it:
Whereas we ought rather to think, that the continuing
of the Soul in the Body, depends in fome meafure upon
the Difpofition of the Body to perform thefe Functions,
and that the Separation is a Confequence of thefe Functi-
ons not being able to be performed. For every Day's
H 4
Ex-
4. An Error
among ft Phi-
lofophers.
104
Part I
ROHAULT's SYSTEM
are effential
Forms.
Experience fhows us, that Death never comes, nor is the
Soul ever feparated from the Body, till it is fome way
hurt, or by fome Means fpoiled and corrupted. And we
have no Example of the Soul's being feparated from a
found and perfect Body, and that this Body did not be-
gin to be corrupted, till after, and becaufe the Soul was
ferated from it.
5. That there It would therefore be unreasonable, upon the fingle
Inftance of the Rational Soul, which is very different from
the common Forms of Bodies, and without firſt knowing
the particular Form of all Kinds of Bodies, to affirm here
rafhly, that there are fubftantial Forms in Things merely
corporeal; however we may venture fafely and confi-
dently to affert, that there are fome Forms which are ef
fential, that is, fuch as belong neceffarily to their Subjects:
Thus to be liquid is effential to Water, becauſe there is no
Water which is not Liquid; we may alfo affirm, that
there are other Forms which are only accidental, becauſe
they fo belong to the Subject, that it can exift without
them, and not ceafe to be what it was. Thus Coldness
is an accidental Form of Water, becaufe Water would
ftill be Water, if it was made hot.
not certain
6. That it is 6. It might very eafily be, that Ariftotle might ac-
that Ariftotle knowledge effential Forms and not fubftantial Forms; for
did allow of it is certain, that the Greek Word which he ufes, may
fubftantial
as well or better fignify the one than the other.
'Forms.
7. That
Artificial
So natural.
7. Forms are commonly diftinguiſhed into Natural and
Artificial: They call thofe Natural, which belong to the
Forms are al- Subject without the Affiftance of Men, Thus a Portion
of Matter receives the Form of Marble in the
Bowels of the Earth. Artificial Forms are thofe that
proceed from Art; thus the Form of a Clock is
&
called Artificial, becauſe it is owing to the Labour of
the Clock-maker. I agree, that if the Name had been
given with regard only to the Caufes by which they were
produced, it would have been reaſonable to call the one
Natural, and the other Artificial; but fince it is inferred
from thence, that the Natural Forms are different from the
Artificial Forms, and that they act from internal Princi-
ples, which are very different from thofe of Artificial
Forms; there lies the Miftake. For Artificial Forms are
as natural as the Natural Forms themſelves, becauſe e they
proceed from Caufes purely natural; and Art, as was
faid before, does nothing else but apply active Things to
paffive Ones.
8. If
Chap. 18. of NATURAL PHILOSOPHY.
105
8. It is much more reaſonable to divide Forms into 8. The Divi-
Simple and Compound. Simple Forms are thofe of fimple fion of Forms
into fimple
Beings, that is, of Beings that are capable of but a and com.
few Properties; and compound Forms are thofe of com- pounded.
pound Beings, that is, Beings that are capable of a great
many Properties. For Inftance, the Form of a hard Bo-
dy, whatever that Form may be, is a fimple Form compared
with the Form of Wood, which, with refpect to the for-
mer, may be faid to be compounded; becauſe a hard Bo-'
dy, as hard, is not capable of fo many Properties as
Wood.
ought to be
9. This Obfervation is more remarkable than one 9. That fim-
would imagine. For it is evident, that fimple Things may ple Forms
be known, when we don't at all know thofe that are com- underfood
pounded of them: Whereas we cannot know thoſe that first.
are compounded, but we muſt have a dinſtinct Knowledge
of thofe Things which go towards their Compofition.
Wherefore in order to underſtand particularly the Forms
of Bodies, it is neceffary that we firſt begin with thoſe that
are fimple, and afterwards come to thofe that are`com-
pounded.
CHAP. XIX.
Of Elements according to the Opinion of the
IF
Antients.
I.
mean by E-
lements.
F we once have a clear Notion of what Philofophers 1. What Phi-
mean by the Word Element, we cannot doubt, but lofophers
that the Forms of Elements are the moft fimple of all.
It is to be obſerved therefore, that the principal Deſign
of Philofophers is to explain how every Thing is gene-
rated, in fuch a manner as to let us know the different
States through which fuch Things pafs from their firſt
Principles till they are entirely compleat, and in that perfect
State in which we fee them. And in order to this, fince
they find by Experience, that every Thing is not made
indifferently out of another, and that Stones, for Inſtance,
and Marble are not proper to be converted into Fleſh,
neither will they ferve to nourifh it and make it grow; fo
they judge by proportion, that all forts of Bodies are not
compounded of Principles alone, connected together in
the moft fimple manner poffible; but fome very fimple
Things
106
Part I
ROHAULT'S SYSTEM
2. That there
ought to be
more Ele-
ments than
Things only, of the Mixture of which all other Things
are afterwards compofed. Theſe very fimple Things,
whatever they be, which thus arife from the firſt De-
termination and Connexion of Principles, are what Phi-
lofophers call Elements: So that Elements differ from
Principles in this, that a Principle, fuch as Matter, for Ex-
ample, is, as it were, an incompleat and undetermined
Thing, whereas an Element, is a compleat aud determined
Thing.
2. This being explained, there muft, without doubt, be
more than one Element, otherwiſe there would be but one
uniform Simplicity in Nature, and no compounded Things.
one, and what But Philofophers have not agreed what is meant by Ele-
the Opinion of ment, the Reafon of which, is, becauſe they have not ſo
the Antients much inquired into the Nature of Things themselves as into
ing Elenicuts. the Senfations which they are apt to raise in us.
Was concern-
3. How A-
Forer Ele-
ments.
Thus fome
Philofophers who confidered the Senfe of Seeing only, have
afferted that Light and Dark, Transparent and Opacous were
the Elements of Things. And others, who referred eve-
ry Thing to Feeling, have pretended that Hard and Li-
quid, or Hot and Cold were the Elements.
3. Ariftotle may be placed amongſt the Number of theſe
riftotle made laft, though he went in a Way fomewhat different from
theirs. He confidered firft, the principal Qualities that
come under the Senfe of Feeling, fuch as Heat, Cold, Dry-
ness or Hardness, and Moiftness or Liquidness: And after
he had obferved that two of thefe Qualities might meet
in the fame Subject, and that the Four might be coupled
four different Ways, he compofed four Elements; of
which the Firſt is Cold and Dry, the Second is Cold
and Moift, the Third, Hot and Moift, and the Fourth, Hot
and Dry.
4.
What
Names he
gave to them.
4. Then, in order to give Names to them, he examined
what thoſe Things in Nature were, in which one Ele-
ment feemed to prevail, or in which its Qualities were
moſt ſenſible. Thus, imagining the Earth to be both
the coldeſt and drieft Thing in the World, he called his
First Element, Earth. So likewife, becauſe he thought
that Water was the coldeft and moiſteſt Thing, he cal-
led his Second Element, Water. Further, imagining alfo,
that there is nothing more moiſt and hot than Air, he
called his Third Element, Air; And lastly, not doubting,
but that Fire is the hotteſt and dryeft Thing in the World,
he called his Fourth Element, Fire.
5. Ari-
Chap. 19. of NATURAL PHILOSOPHY.
107
have been
5. Ariftotle's making uſe of Names which were before 5. That theſe
ufed to fignify other Things, hath given occafion to many, misunderstood
who did not rightly apprehend his Meaning, weakly to by fome.
believe, that This Earth which we inhabit, This Water
which we drink, This Air which we breathe, and This Fire
which we kindle, are the Four Elements. But this will appear a
very grofs Miſtake, to any one who confiders, that the
Name Element is given only to the most fimple Body,
whereas the four now mentioned are the moſt compound-
ed of any we know.
6. That the
ceived.
6. But if we ſuppoſe the Elements of Ariftotle to be as
fimple as he makes them, and if we compare them with Elements e-
ſtabliſhed by
thole which other Philofophers have attempted to intro- Ariftotle and
duce; we do not find any Advantage they have, why we others, ought
fhould prefer them above others; becauſe in this Matter not to be re-
we have no more reaſon to confider the Qualities of Feel-
ing, than thoſe of Seeing, or any other Senfe. But nei-
ther the one nor the other ought to be allowed, and that
for theſe two Reafons, which feem to me very ftrong.
The Firſt is, That in order to eſtabliſh Elements through-
ly, it ought to be upon the Determinations which may
happen to Matter abfolutely and in it felf, and not upon
the Relations which the different Forms of which it is
capable may have to our Faculties to raiſe Senſation. The
Second is, that all theſe pretended Elements being deter-
mined by fenfible Qualities, of which we have no clear
Notion; it is impoffible, but that there muſt remain fome
Obfcurity, into which no Philofopher can fo far pene-
trate as to be able to fee what will arife from their Mix-
ture; in the fame manner as a Phyſician cannot tell what
is the Vertue of a Medicine compofed of many fim-
ple ones, of which he has only a confufed Know-
Ledge.
CHAP.
T
108
Part I.
ROHAULT'S SYSTEM
1. The Me-
thod of the
Chymifts, in
finding out of
Elements.
z. What the
the Chymifis
is.
I
CHAP. XX.
Of the Elements of the Chymifts.
Cannot tell whether thefe or fuch like Reaſons, in-
duced the Chymifts to reject thoſe Elements which the
Antients would have introduced; thus much is certain,
that they have propofed others very different. And in
order to eſtabliſh them, as they profefs an Art which
confifts principally in ufing Fire after different manners,
to feparate as much as is poffible, the different Parts of
which different Bodies are compofed, they have pretend-
ed, that this Refolution is the only Way to find out what
are the true Elements which Nature makes uſe of in the
Compoſition of Bodies; as the taking a Machine to
Pieces, is the only way to find out what it is compo-
fed of.
2. Thus, in working upon certain Bodies, upon Wine,
Mercury of fuppofe, they put a large Quantity of it into an Alembick,
and by means of Fire, make fome of its Parts exhale,
which being then condenſed by the Cold, fall down into
another Veſſel in the Form of a ſtrong, fubtil, and pene-
trating Liquor, to which they are pleaſed to give the Name
of Mercury, Spirit, or Aqua-vita.
3. What it 3. After this, continuing the Alembick upon the Fire,
is that they they make it diſtill a Liquor which has no Tafte, and
call Phlegm
and Sulphur. this they call Phlegm; and fo they go on till there re-
mains nothing in the Alembick, but a glutinous Subſtance
like Honey. Then they put this glutinous Subftance in-
to a Retort, and with Fire they make it again diftill a
Phlegm like the former, and then an acid Liquor which
they call Mercury alfo; and after that, another Liquor not
quite fo fluid, fomewhat like Oil, and which is inflammable
like it, to which they give the Name, Sulphur.
4. What it is
Caput Mor-
tuum, and
Salt.
4. Laftly, They take that which remains in the Retort,
that they call and which prefently grows dry, and burn it, and
the
put
Aſhes into an Earthern Pot or Pan, with a certain Quan-
tity of Water, which in a fhort time becomes Salt, then
ftraining it off clear into another Veffel, there remains.
in the Pot a kind of duity infipid Earth, which they call
Caput mortuum or Terra damnata, then with a gentle Fire,
they make the clear Water which is in the other Veffel
to evaporate intirely, and after that, there remains at the
Bot-
Chap. 20. of NATURAL PHILOSOPHY.
109
Bottom of the Veffel, a hard brittle Body which is very like
Salt, and therefore they call it Salt.
5.
That Mer-
cury, Phlegm,
Sulphur, Salt,
are the Ele-
5. Hence they conclude, that theſe five Subſtances, viz.
Mercury, Phlegm, Sulphur, Salt and Caput mortuum, are
the Elements of Wine: And becauſe whatever they can and Caput
extract out of any other Subject, reſembles one or other mortuum,
of theſe, therefore they conclude in general, that theſe ments of the
Things, are the only and the true Elements of all the Chymifts.
mixed Bodies which are in the World, and that all the
Variety that we fee is owing to the different Mixture of
theſe.
6. I fhould think it a great Piece of Injuftice not to
give the Chymifts that Commendation which is due to
their Induſtry and laborious Application. Without doubt
the whole World, and the Philofophers particularly, are
very much obliged to them for the Pains they have taken,
and which they continue to take, to make a great Num-
ber of Experiments, whereby they come to the Know-
ledge of diverfe Properties of many different Things. This
gives them opportunity to find out and diſcover the Na-
ture of Things, and at the fame time, ferves for a Rule
to try the Truth of their Principles by, and to juſtify their
Reaſoning and the Confequences which they draw from
thence. However I think their manner of treating of Phi-
loſophy is not fatisfactory, nor their Elements ſuch as ought
to be allowed.
6.
How Chy-
feful to Phi-
lofophers.
mistry may be
7. Though the exceffive Commendations which they 7. The Er-
give themſelves, and with which their Books are filled, as rour of the
if they were the only Philofophers, and the Secrets of Chymifts.
Nature depofited in their Hands alone; and though the
large Promiſes they make, which for the moſt are falſe
and vain, have rendred them almoft univerfally contempti-
ble to the World; and the obfcure Terms, and almoſt
perpetual Equivocations which they ufe, have made them
ridiculous alfo to a great many: Yet I do not depart from
their Opinions upon this Account. For as to theſe ex-
ceffive Commendations, and vain Promiſes, they are only
perfonal Faults which any one may eafily lay afide, and
which fome Chymifts of my Acquaintance are entirely
free from; who far from being vain and proud like others,
are on the contrary, fo modeft, that if they had nothing
elſe to recommend them, they ought upon this Account
to be placed in the Rank of Gentlemen. And as to the
Obfcurity of their Terms, fome of which are authorized
by Cuftom, that is eafily difperfed, if we give but our
felves the Trouble to explain them.
8. That
110
Part I.
ROHAULT'S SYSTEM
}
and thoſe
which they
do get toge-
ther are al-
tered. .
8. That they 8. That which makes me not to approve of the Me-
cannot get to thod of the Chymifts, is, firft, becauſe it is defective;
gether all the
Parts of a for it is certain, that let them take never ſo much Pains,
mixed Body; they can only get together the fenfible Parts of which
a Body is compofed: For as to thoſe which reſemble that
fubtil Matter, the Exiſtence of which, we demonſtrated
above, and which go to the Compofition of a great ma-
ny Things, theſe eſcape all their Pains. But further, thạt
which they give the Name of Principle to, cannot but
be very much altered, and very different from what it
was in the Mixture: For it is impoffible, but that the
different Parts which they extract, when they are put in
Agitation by the Fire, and dafhed one against another,
muſt be changed both in their Figure and in their Na-
ture. And this is confirmed by Experience, for if all the
Parts into which the Mixture is refolved, be mixed toge-
ther again, the Reſult will not be at all like the former
Mixture.
9. That, al-
9. To this may be added, that the Chymifts deceive
lowing of
themſelves, in ſaying, that there are but five Elements: For
their Opinion,
there ought to allowing of their Method, and the Manner upon which it
be more than is founded, we muſt ſay, that there is a great Number, yea
five Elements. ſo great, that it is impoffible to know them all. Thus
there are a great many Sorts of Mercury, Sulphur, Salt, &c.
But to mention Salt only; we find almoſt as many dif-
ferent Salts, as there are different Mixtures. For Exam-
ple, That which is extracted out of an Afh-tree, is
Cauftick, that is, will corrode and burn the Fleſh, if ap-
plied to it; but that which is extracted from an Oak will
not do fo.
10. That they
10. But that which fhocks me moft in the Reafoning
have but a of the Chymifts, is the Confufion that they are unwilling
confufed No-
tion of their to get out of, and the Averfion they have to clear and
●wn Elements. diftin&t Knowledge, which it is fo natural to defire. For
Inſtance, if we ask them what they mean by Sulphur,
they will anfwer indeed, that it is a fat inflammable Sub-
ftance; but if we go on to ask what this fat inflammable
Subſtance is, which they call Sulphur, and in what this
Property of being Inflammable confifts, they will not only
not give us any further Anfwer, which indeed is no great
Matter, becauſe they have none to give; but they will be
offended at our Curiofity, and that we ſhould have any
Defire to be fatisfied herein; So that their Science extends
no further than to give Names to Things whofe Natures
they underſtand not, and confequently from the Mixture
of which, it is impoffible to forefee what will arife, which
is
Chap. 20. of NATURAL PHILOSOPHY.
III
}
is one of the principal Conditions which we require in
Elements.
•
11.The pre-
Uſe
tended Ufe of
the Elements
the Antients,
11. Perhaps it will be faid here in favour of the
Elements of the Chymifts, and in favour of thoſe of the
Ariftotelians, that though we do not know diftinctly what of the Chy-
they are in themſelves, yet we know at leaſt what they mifs and of
are capable of, that is, the Senfations they raiſe in us, or
the Convenience or Inconvenience we receive from
them, which they think fufficient to determine what the
Effect of their Mixture will be. For, fay they, we may
lay down two general Rules hereupon; Firft, That if
two Things Separately, are capable of producing the fame
Effect, they will alſo be capable of producing it when they
are mixed together. Secondly, That if Two Things fepa-
rately, are capable of producing two contrary Effects, when
they are compounded together, they will produce fome middle
Thing between these two Effects. And theſe cannot be de-
nied to be of good Ufe.
1
>
may be the
12. Though thefe Rules are for the moft part found 12. This pre-
to be true, yet it will be very wrong to truſt too much tended Use,
to them, and I doubt not but the Chymifts themſelves Occasion of
will difown them; for they know very well, that he our making
who exactly follows them, will many times form a Judge- many falje
ment contrary to Experience.
Judgements.
Inftance.
13. For Inftance, if we follow theſe two Rules ftrict- 13. The firft
ly, we muft affirm, that two Bodies which ſeparately are
cold, ought together to make one cold Body:
14. We muſt affirm, that two liquid Bodies will com- 14. II In-
pofe one liquid Body.
15. That two transparent Liquors will compofe
transparent Liquor.
one
fiance.
15. III In-
france.
16. That two red Liquors mixed together, will make 16. IV In-
one red Liquor.
fiance.
17. That a Body of a Yellowish Colour, mixed with a 17. V In-
Body of a Green Colour, ought to compofe a Yellowiſh ſtance.
Green.
18. That two Things which may be feparately taken 18. VI In-
without any danger, may alfo be taken together without fance.
any.
19. However, we know that every one of theſe are con- 19. The firſt
Experiment
tradicted by the following Experiments. For Examples of the contra-
cold Lime, having cold Water fprinkled upon it, grows.
fo hot, as to be ready to burn. Further, If Oil of Vi-
triol and Oil of Tartar, each of which are cold, be mix-
ed together, we fhall perceive a fudden. Ebullition, and
at the fame time a very fenfible Heat.
20. If
112
Part I.
ROHAULT'S SYSTEM
),
20.II. Expe-
riment.
21. III. Ex-
periment.
22. Of Sym-
20. If Spirit of Wine and Spirit of Urine, each of which
are very fluid, be mixed together, they will, in a Mo-
ment almoſt, unite into a Body not at all fluid, but pret-
ty hard.
21. If about an Ounce of Litharge of Silver be put in-
to a Pint of diftilled Vinegar, and boiled half a Quarter
of an Hour, and if a Piece of unflacked Lime be steeped
Four and twenty Hours in a fufficient Quantity of Wa-
ter (it muſt be in an Earthen Pot varnished, new and
clean;) and afterwards each of thefe Liquors be ftrained,
they will be very tranfparent; but when they are mix-
ed, they will become opacous and of a very brown
Colour.
22. In the Uſe of theſe two Liquors confifts the whole
pathetick Ink. Secret of the Ink, which they call Sympathetick Ink.
They write that which they would not have feen, with
the firſt Water, and the Writing diſappears the Moment
that it is dry: Then, he who receives the Letter, wipes
over the Paper with a Sponge ever fo little moiftned
with the other Water, and the Writing begins to
appear of a reddiſh Colour, tending to a Black. If
theſe Waters are freth made, and Care be taken to co-
ver the Pot in which the unflacked Lime is infuſed, the
Sponge that is moistned need not touch the Writing, in
order to make it appear, it is fufficient, if it paſs by it
at a little diſtance: Nay I have often feen the Lime-
Water ſo ſtrong, that when the Letter written with
the firſt Water was laid upon a Table, and covered
with a Quire of Paper, the upper Leaf of which only
was moiſtned with the Second Water, the Writing grew
black.
23. IV. Ex-
periment.
24. V. Ex-
periment.
23. If a Piece of Brafil Wood be boiled in Water over
the Fire, we ſhall preſently have a Liquor pretty red;
which if it be afterwards poured into a Glafs in which
there is ever fo little Vinegar, this Colour will be chang-
ed into an Amber-Colour, and that fo quick, that the
firſt Colour will diſappear entirely, as foon as the Water
touches the Bottom of the Glafs.
24. It is certain, that Nut-Galls are of a Yellowiſh Co-
lour, and that when they are reduced to Powder, there
is no more Blacknefs in them, than in the Copperas which
is green; and yet if theſe two be infuſed in common
Water for a few Days, or if you would have it quicker,
if the Water be boiled an Hour or two over the Fire,
they will be of one black Colour, and not differ from
Ink but only in this, that they want the Gum Arabick.
25. Phy-
Chap.2o. of NATURAL PHILOSOPHY.
113
25. Phyfitians order fometimes a few Drops of Spirit 25. VI. Ex-
of Nitre or of Oil of Vitriol to be taken in Broth or fome periment.
other Liquor, and theſe two Things taken feparately and
in proper Cafes, are good Remedies, but if they be ta-
ken together, they are Poifon. Now this Experiment,
together with the foregoing ones, and many others that
might have been added, do fo evidentlý fhow the Un-
certainty of the two forementioned Rules, and confe-
quently the little Ufe of the Elements of the Antients and
of the Chymifts, that there is no need of adding any
Thing more: That which now remains to be done, is
to endeavour to diſcover what are the true Elements of
natural Things.
CHA P. XXI.
Of the Elements of natural Things.
1. That we
upon the annot be
mifiaken in
afcribing Fi
Parts of
Matter.
T
HAT we may act here with all poffible Caution,
and eſtabliſh the Number of Elements,
Confideration of Things as they are in themfelves, wich-
out any regard to the Manner of their affecting us; we gres to the
obferve, that the first Thing that we can conceive to hap-
pen to Matter, is, that it may be divided into a great
Number of Parts, all which are of a certain Figure. This
Confideration is of great Importance; for if we attend e-
ver fo little to it, we fhall be furprized at fome Perſons,
who are ready to laugh, when we obferve to them, that
the Parts of Matter are of a certain Figure, and yet can
ſeriouſly hearken to thoſe who tell them of occult Qua-
lities, which they cannot at all comprehend.
2. We obferve further, that befides thofe grofs Bodies, 2. That there
fuch as we can take notice of, with which we are fur- are a Multi-
tude of very
rounded; there are an infinite Number of others very ſmall, fmall Bodies.
which eſcape our Sight, and which were not at all known
to the Antients. Though even amongſt thefe, if we
ftrictly examine them, fome may be made appear to us,
fuch as the little Eels, which fpring up almoft in a Mo-
ment, in the best fort of Vinegar fet in the warm Sun; but
it is certain, we had not known of theſe fmall Creatures
to this very Day, were it not for the happy Invention
of the Microſcope, in this Age. Thus, for Example, Specks
of Mould upon the Covers of Books, have been long ob-
ferved,
I
+
114
Part I
ROHAULT'S SYSTEM
1
*
3. That theſe
fift of Parts
fill Smaller.
4. That Ele-
san be of
Matter.
ferved, and alfo, that a Mite, which is much lefs than a
Grain of Sand, is an Animal, becaufe we can fee it move
along; but it is fince the Invention of Microſcopes that
we can with pleaſure fee not only that they are fo, but
that every Speck of Mould is a little Garden covered with
Plants, every one of which has its Stalk, Leaves, Buds
and Flowers; and that a Mite has its Back covered with
Scales, that it has three Feet on each Side, and two black
Spots in the Head, which we fuppofe to be Eyes, becaufe if
the Point of a Needle be put in its way, it will turn afide.
3. Since fuch fmall Bodies are diſcovered and ſeen by
the Microſcope, we may reaſonably judge that there are
Parts incomparably lefs yet, which efcape all our Senfes,
all the Induſtry of Man, and exceed even our Imagina-
tion it ſelf. And that this may be clear by one Exam-
ple; Since a Mite walks along, it must have Legs, and
thefe Legs muſt neceffarily have Joints. In order to move
the Joints, there must be Mufcles, Nerves and Tendons,
and in theſe Nerves Fibres, fuch as we fee in thoſe of lar-
ger Animals, or at leaft, fomething equivalent to them:
And if we would carry this Confideration yet further, and
fpeak of the Heart, Blood, Brain, and Animal Spirits, we
fhall be quite at a Lofs, and forced to confefs, that our
Imagination is unable to comprehend or reprefent the
extreme Smallneſs of the leaft Parts of which a Mite is
compofed. I defire that thefe Things may be well confi-
dered, and I have purpofely urged them, to avoid the
Impertinence of thofe Perfons, who ridicule every Thing
propoſed to them, which does not agree with their grofs
Notions; and who make a Jeft of it, when we men-
tion that fubtle Matter to them, whofe quick Motion and
Smallneſs makes a Paffage for it, and finds it a Place
every where.
-
4. Having laid down theſe Obfervations, fince we are
ments arife affured, that the fmalleſt Bodies in the World, as well as
Divifion that the Larger, arife from the Mixture of Elements; and
fince it is certain, that a fufficient Number of the fmal-
left Parts, may compofe as great a Body as we will; we
muft conclude, that there ought to be as many Elements,
as there can be remarkable Differences in the infenfible
Parts of Matter upon their firft Diviſion,
5. That we
5. Now that my Mind may be the clearer underſtood,
do not here
I muſt repeat the Advice which I before gave, viz. That
Speak of the
"Divifien that I confider Things in their mere natural State. And though
was made at I am very well aware, that the firſt Divifion of Matter
the Creation
of the World. Was made by God, and as he pleaſed, when he created
the
Chap. 21. of NATURAL PHILOSOPHY.
IIS
the World; yet that is not the Divifion I am here ſpeak-
ing of, becauſe I believe the Creation to be a Myſtery.
which I cannot fearch to the Bottom of. So that I fpeak
of another Divifion, which may be made agreeably to the
Notions we have, and of which all the Things in the
World are the Confequences.
6. Thus, confidering as far as I am able all Matter, 6.What that
I first divide it in my Mind into an infinite Number of Diviſion is
which I Sup-
Parts very near equal, not troubling my felf what Figure pofe Elements
they are of, becauſe, there may be a great many other Figures, to arife from.
belides Cubick which comes firft into every one's Thought,
that may produce the fame Effect. After this, I fuppofe
that God turns every one of thefe little Particles, in ma-
ny different Manners, about their feveral Centers, in or-
der that a true Diviſion of them from each other may
begin to be made.
í
7. This being fuppofed, it cannot be but that all theſe 7. That there
Particles of Matter muſt be broken where-ever they are
must neceffa
rily be three
angular, or are intangled with thoſe that join to them; fo Elements.
that thoſe which were fuppofed before to be very fmall,
muft become ſtill fmaller and fmaller, till they are got
into a Spherical Figure. Thus we have two Sorts of
Matter determined, which we ought to account the two
firſt Elements. And of theſe two we here call that which
confifts of the very fine Duft which comes off from thoſe
Particles, which are not quite fo fmall, when they are turn-
ed round, the first Element. And theſe Particles thus made.
round, we call the Second Element. And becauſe it may
be, that fome of the fmall Parts of Matter, either fingly
or united together, may continue in irregular and confufed
Figures, not fo proper for Motion, we take them for the
third Element, and join them to the other two.
lements.
8. As to the chief Properties of these three Elements, 8. The Pro-
it is to be obſerved, that it is no Contradiction to ſuppoſe perties of E-
them to be changed from one Sort to another: Thus the
Particles of the Third Element may fometimes be made
round, and acquire the Form of the Second. And thoſe
of the Second and Third may be broken, and ſo converted
into the Firft. But none of theſe three Elements will bet-
ter preſerve their Form than the Second, becauſe it is more
folid, and the Spherical Figure, which it is of, will allow
it to move about it felf, without being intangled with the
Particles about it. On the contrary, none are fo eafily
changed as the First, becauſe its Particles moving very
quick and being very fubtle, they cannot refift the Shock
of the Particles belonging to the other Elements, when they
I 2
meet
116
Part I.
ROHAULT'S SYSTEM
9. The Pro-
meet with them, but are forced at all times to fuit
their Figures to thofe of the Places through which they
pafs, and where their Motion carries them.
9. The First Element ought alfo to have more Motion
perties of the than either of the other Two, for though all the three
First.
Elements, were at the Beginning equally moved by the
Firſt Mover, yet it muſt afterwards happen, that the first
Element having oftentimes met with other Bodies which
refifted it, and which it could not move, will be reflected
back, without lofing any of its own Motion; whereas
the other Elements cannot meet this, but they will move
it, and fo increafe its Motion by diminishing their
10. How the
Firft Ele-
ment acquires
other Two.
own.
10. And fince the First Element is often forced to run
into thofe little Intervals which are between the ſmall
greater Velo- Globes of the Second Element, it must neceffarily be, that
city than the many of its Parts being compreffed, will leave the Place
where they are, and get forward; and fo having a Moti-
on compounded of their own Motion, and of that of
the Parts which follow them and prefs upon them, they
will acquire a greater Velocity than the Parts of the Se-
cond Element which force them on. In the fame man-
ner as the Air contained in a Pair of Bellows goes out
with much greater Velocity, than the Sides of the Bel-
lows approach each other, and which by their approach-
ing, puſh it, and make it to go out.
11. Why we
proper Names
to thefe Ele-
ments.
II. I would have it obferved by the way, that I might,
do not give after the Example of Ariftotle, give Names to the three
forementioned Elements, from the Things which partake
moft of them: Thus, I might give the Name Fire to the
First Element, Air to the Second, and Earth to the Third
But befides that this would be to act contrary to Order,
becauſe I have not yet proved, that Fire is for the moſt
part compofed of the First Element, Air of the Second,
and Earth of the Third; there is yet another Reaſon that
ought to hinder me from doing it, and that is, that I
fhould give Occafion for abufing them, and for having
them underſtood in another Senfe than what I intend they
fhould be.
12 That these
Imaginary.
12. Perhaps it will be here faid, that Matter was not di-
three Ele-
vided in the Beginning as I have fuppofed; But tho' I agree
ments are not it may be fo, this makes nothing againft me; for it fig-
nifies very little how Matter was divided at the Beginning;
and in what manner foever it was divided, there is no
doubt but it is now divided into thoſe three Sorts of Matter
which I have deſcribed; it being certain, that they necef-
farity
Chap. 21. of NATURAL PHILOSOPHY.
F17
farily follow from the Motion and the Divifion of the
Pasts of Matter which Experience obliges us to acknow-
ledge in the Univerſe. So that the Three Elements which
I have eſtabliſhed, ought not to be looked upon as ima-
ginary Things, but on the contrary, as they are very ea-
fy to conceive, and we ſee a neceffity of their Existence,
we cannot reſonably lay afide the Uſe of them, in ex-
plaining Effects purely Material.
1. We cannot reaſonably lay afide)
Theſe three Elements are to be look-
ed upon as fictitious and imaginary,
becauſe they depend upon a Plenum
every where, which we have before
rejected. But concerning the true
Elements of Nature, the illuftrious
Newton thus explains himſelf.
even
It ſeems probable to me, that God
in the Beginning formed Matter in fo-
lid, maffy, hard, impenetrable, mo-
veable Particles, of ſuch Sizes´ and
Figures, and wioh fuch other Proper-
ties, and in fuch Proportion to Space,
as moft conduced to the End for which
he formed them; and that these Pri-
mitive Particles being Solids, are in-
comparably harder than any porous
Bodies compounded of them';
fo very hard, as never to wear or
break in Pieces: No ordinary Power
being able to divide what God him-
Self made one in the firf Creation.
While the Particles continue entire,
they may compofe Bodies of one and
the fame Nature and Texture in all
Ages But fhould they wear away or
break in Pieces, the Nature of Things
depending on them, would be changed.
Water and Earth compofed of old worn
Particles and Fragments of Particles,
would not be of the fame Nature and
Texture now, with Water and Earth
compofed of entire Particles in the
Beginning. And therefore that Na-
ture may be lafting, the Changes of
corporeal Things are to be placed only
in the varions Separations and new
Affociations and Motions of thefe per-
manent Particles compound Bodies
being apt to break, not in the midft
of folid Particles, but where thofe
Particles are laid together, and only
touch in a few Points. Opticks pag.
375
Further, nothing can be more ab-
furd than to imagine, that all thefe
furprizing Things in the Univerſe, a-
rife and were formed out of thoſe
three Elements of Cartes, and by
I 3
CHAP.
the Motion impreſſed upon them in
the Beginning, without any Inter-
pofition afterwards, either of God
himfelf, or any other intelligent
Caufe. For according to that Hypo-
thefis, the Followers of Cartes have
not ſo much as dared to attempt ex-
plaining how all Kind of Plants and
Animal Bodies (which are the prin-
cipal and moft excellent Part of this
Uuiverfe) were at firſt made, and by
what Laws of Motion they were
framed. How much better does the
forementioned admirable Perfon ex-
prefs himſelf.
Now all material Things feem to
have been composed of the hard and.
folid Particles abovementioned, vari-
onfly affuciated in the first Creation by
the Counsel of an intelligent Agent.
For it became him who created them
to fet them in order. And if he did
fo. 'tis unphilofophical to feek for any
other Drigin of the World, or to pre-
tend that it might arife out of a
Chaos by the mere Laws of Nature ;
though being once formed, it may
continue by thofe Laws for many
ges. For while Comets move in very
excentrick Orbs in all manner of Po-
fitions, blind Fate could never make
all the Planets move one and the fame
way in Orbs concentrick, fome incon-
fiderable Irregularities excepted, which
may have risen from the mutual Adli-
ons of Comets and Planets upon ane
another, and will be apt to increaft,
'till this Syftem wants a Reformation.
Such a wonderful Uniformity in the
Planetary Syftem must be allowed the
Effect of Choice. And ſo must the
Uniformity in the Bodies of Animals,
they having generally a right and a left
Side ſhaped alike, and on either -Side
of their Bodies, two Legs behind,
and either two Arms, or two Legs,
or two Wings before upon their Show!-
ders, and between their Shoulders a
Neck running down into a Back-bone,
and a Head upon it; And in the
Head
{
118
Part I.
ROHAULT's SYSTEM
F
1. VFhat is
meant by
hard and li-
!
CHA P. XXII.
Of the Form of a Hard and of a Liquid Body, or
of Hardness and Liquidity.
BE
1
ECAUSE it is by means of our Senfes, that we
find out the principal Differences obferved in
quid Bodies. Things; I think we cannot do better, than to confult
them one after another, to. find out in what Order the
Forms of natural Bodies ought to be treated, beginning
with thoſe which diſcover to us the feweſt Properties of
their Objects. And fince the Senfe of Feeling is the grof
ſeft of all, and that which takes up the leaft Compals of
our Views, I will begin my Inquiry with that. Now when
we make uſe of the Senſe of Feeling, to difcover what
Sort of Bodies they are which furround us, we obſerve
that there are fome which refift the Motion of our Hands,
and will not be divided without great Difficulty; on the
contrary, there are others which do not refift them at all,
but are very eafily divided all ways; the first of theſe we
call bard Bodies, and the other liquid Bodies; and we fay,
that a Body is fo much the harder, as there is greater Diffi-
culty in dividing it, and another fo much the fofter, as
it refifts lefs, and is divided with greater eafe. And thoſe
Bodies which are of a middle Sort, betwixt hard and li-
quid, and which refift our Feeling, or the Motion of our
Hand but a little, thefe we call foft.
Head two Ears, two Eyes, a Nofe, a
Aouth, and a Tongue, alike fituated.
Alfo the first Contrivance of those
very artificial Parts of Animals, the
Eyes, Ears, Brain, Mufcles, Heart,
Lungs, Midriff, Glands, Larynx,
Hands, VVings, fwimming Bladders,
natural Spectacles, and other Organs
of Senfe and Motion; and the Inftinct
of Brutes and Infects can be the Ef-
fect of nothing else than the Wisdom
and Skill of a powerful everlasting
Agent, who being in all Places, is
Tore able by his VVill to move the
Bodies within his boundless uniform
Senforium, and thereby to form and
reform the Parts of the Universe, than
we are by our VVill to move the Parts
of our own Bodies. And yet we are not
20 confider the FVorld as the Body of
|
God, or the feveral Parts thereof as
Parts of God. He is an uniform Be-
ing, void of Organs, Members or Parts,
and they are his Creatures fubordinate
to him, and fubfervient to his VVill.
And he is no more the Soul of them,
than the Soul of a Man is the Soul
of the Species of Things carried through
the Organs of Senfe, into the place of
his Senfation, where it perceives them
by means of its immediate prefence
without the Intervention of any Third
Thing. The Organs of Senfe are not
for enabling the Soul to perceive the
Species of Things in its Senforium,
bus only for conveying them thither` ;
and God has no need of fuch Organs,
he being every where prefent to the
Things themſelves,
Things themselves. Ibid. p. 378.
2. We
Chap. 22. of NATURAL PHILOSOPHY.
119
2. We obferve alfo that a Body, which refifts the Touch 2. That hard
and liquid
and is with Difficulty divided, keeps it felf alfo within its Bodies are the
proper Limits, and preferves its Figure, without wanting fame kind of
a Veffel to contain it, and on the other hand, that a Bo- Bodies as the
dry and moist
dy which does not refift the Touch, does not contain it Bodies of the
felf within its Limits, but runs and fpreads about, if it be Ancients.
not put into ſome Veffel. Wherefore Ariftotle having
given the Name of Dry to a Body. which is contained
within its proper Limits, and that of Moift, to a Body
which does not do fo, but wants to be contained within
the Limits of another; it follows, that the hard Body we
are fpeaking of, is the fame as what Ariftotle called
Dry, or at leaſt a Species of it; and alfo that the Li-
quid is the fame with the Moift, or at leaft a Species
of it.
of Ariftotle,
3. As Ariftotle has not explained what Dryness and 3. In wah
Moiftnefs confift in, fo neither has he explained the the Followers
Nature of a hard and a liquid Body. But most of his make Hard-
Followers contend, that a Body is hard, becauſe it com- nefs and Li-
prehends a great deal of Matter in a little Compafs, and quidness to
that a Body is liquid, becaufe it contains but a little
Matter in a great Compafs; fo that they make Hardness
to confift in Condenfation, and Liquidness in Rare-
faction.
•
i
4. It is to be obferved, that they would be underſtood 4. That their
to fpeak here of a Rarefaction, without the Addition of Opinion goes
upon a falle
any Matter at all, not fo much as of foreign Matter; and Suppofition.
of a Condenfation which does not in the leaft fuppoſe any
Sort of Matter to come out of the Pores of the conden-
fed Body; which Things are directly oppofite to what has
been before eſtabliſhed; wherefore it cannot be thought
ftrange, if we do not agree together as to the Nature of
hard and liquid Bodies.
But if Rarefaction and Coudenfation were made as 5. A Confu-
they pretend, yet it were eafy to prove that they are mif- tation of the
Opinion of the
taken in their Notion of Hardneſs and Liquidnefs: For as Ariftotelians,
the producing one Piece of white Marble, is fufficient to and the Rea-
fhow, that the Nature of Marble does not confist in Son why Vef-
fels full of
Blacknefs, fo it fhall fuffice to bring one Inftance of a Water are
Body which dilates it felf when it grows hard, in order to broken by the
fhow that Hardnefs does not confift in Condenſation :
Thus we fee that Water is dilated, when it is turned in-
to Ice, for the Veffels which contained it, and juft
14
held
Frof
120
Part I
ROHAULT'S SYSTEM
telians, as to
the Froft.
held it, cannot then contain it,
broken,
but are many times
6. A Miſtake 6. I know very well, that it will here be anſwered as
of the Arifto- ufual, that the Veffels would not be broken, but for fear
the Reason
of a Vacuum: That is, becauſe their Sides approach one
zoby Veffels another, that there may not be any Space left between
are broken by their Concave Superficies and the Convex Superficies of
the Water which is condenfed. But if this were true, it
would follow, that all the Glafs Tubes which we ufed
in the forementioned Experiments, ought alfo to be bro-
ken, when no Air got into the Place out of which the
Quickfilver came, which did not come to pafs, as I have
oftentimes tried.
7. Another
Proof that tee
is not conden-
fed VVater,
and why it
Swims upon
the VVater.
8. An ocular
of the Water,
7. Add to this, that if Ice were only condenfed Wa-
ter; to make for Inftance, a Cubick Foot of Ice, there
must be more than a Cubick Foot of Water, and con-
fequently a Piece of Ice would weigh more than a Quan-
tity of Water of the fame Dimenfions. From whence it
follows, according to what has been before demonſtra-
ted, that Ice ought to fink to the Bottom
and not ſwim at the Top, as we find it does.
8. But for the full Conviction of thoſe who ſeem to
Demonftrati- defy all Arguments, and truft only to what they fee, let
on of the fame ·
Thing. them but take a Glafs of the Shape of an inverted Cone
or Pyramid, and after having filled it quite full of Water,
expofe it to a great Froft, that the Water may become
Ice, then if the Glafs holds but half a Pint, we thall fee
the Ice riſe up about the fixth Part of an Inch above
the Mouth of the Glafs, which is a Dilatation fenfible e-
nough not to doubt of the Fact.
9. VVhat
9. This then is a certain Truth, that every Body which
the Nature of becomes hard, is not condenſed; and therefore Hardneſs
bard Body
confifts in.
does not confift in Condenfation, nor confequently does
Liquidnefs confift in Rarefaction; for as Water is dilated
by freexing, fo is Ice condenfed by thawing. Having
thus fufficiently confuted an Opinion which has been fo
long received, and not thinking it worth while,
to fhow
how little Foundation there is for other Opinions which
have been received only by a few, I come now to eſta-
bliſh my own. And firſt I examine the Appearances of
{
1. So great is the Force of free-
Zing Water, that not only Bowls
and Glafs Cups, but alfo large Vef-
fels of Brafs and Silver are broken
by it. See Experim. Acad. deļ Cim.
P: 72
2. A Dilatation fenfible enough)
Yet it must not be diffembled, that
fomething may poffibly be here af
cribed to the Contraction of the
Glafs. See the Notes on
Pr 23,
Art. 36.
a hard
Chap. 22. of NATURAL PHILOSOPHY.
a hard and of a Liquid Body, and find, that the one con-
tains it felf within its proper Bounds, and the other does
not: And becauſe to be contained within its proper
Bounds, is the fame Thing as not to be moved; Icon-
clude, that to be hard, is to be compofed of Particles which
are fo at rest among themselves, that their Connexion and
Order, is not disturbed by any Matter that moves between
I
1. So at rest among themſelves)
Though all hard Bodies have Parts in
fome meaſure at reft, and many
liquid Bodies (viz. fuch as are made
liquid by Heat) are manifeftly very
much agitated; yet becaufe fome-
thing more than the bare Reſt of the
Parts feems requifite to conftitute
Hardneſs; (for a Heap of very fmall
Sand, whofe Particles are all at reft,
is not a hard Body;) and becauſe
Motion does not feem always ne-
ceflary to conftitute a liquid Body,
(for fome liquid Bodies are very
cold;) I think it therefore worth
while to add fomething here, to ex-
plain this Matter more fully.
First then, Let us hear what the
famous Newton fays, concerning that
Force by which the primary and na-
turally indivifible Corpufcles of which
the Particles of all Bodies are com-
poled, are connected and cohere to-
gether.
The Parts of all homogeneal hard
Bodies which fully touch one another,
ftick together very firongly. And for
explaining how this may be, fome have
invented hooked Atoms, which is
begging the Question; and others tell
and others tell
us, that Bodies are glued together by
Reft, that is, by an occult Quality or
rather by Nothing; and others that
they stick together by confpiring Mo-
tions: I had rather infer from their
Cohafion, that their Particles attract
one another by fome Force which in
immediate Contact is exceeding frong,
at ſmall diſtances performs the Chymi-
cal Operations abovementioned, and
reaches not far from the Particles with
any ſenſible Effect---Now if compound
Bodies are ſo very hard, as we find
Some of them to be, and yet are very
porous, and confift of Parts which are
only laid together, the fimple Parti-
cles which are void of Pores, and were
never yet divided, must be much har-
der. For fuch hard Particles being
heaped up together, can fearce tonuch
one another in more than a few |
Foints, and therefore must be fepa-
them.
rable by much lefs Force than is re-
quifite to break a folid Particle, whofe
Parts touch in all the Space between
them, without any Pores or Interfti-
ces to weaken their Cohafion. And
how fuch very hard Particles which
are only laid together, and touch only
in a few Points, can fick together,
and that fo firmly as they do, with-
out the Affiftance of fomething which
causes them to be attracted or preſſed
towards one another, is very difficult
to conceive.----Now the smallest Par-
ticles of Matter, may cohere by the
frongeſt Attractions, and compofe
bigger Particles of weaker Virtue ; and
many of these may cohere, and com-
pofe bigger Particles whofe Virtue is
ftill weaker: And so on, &c.
Op-
ticks Ibid. p. 364.370.
It is evident therefore, that the
Particles of which the original and
ſmalleſt Parts of Matter are compo-
fed, ftick together and are united,
not by Reft (which is really nothing
at all) but by mutual Attraction.
(See the Notes above on Chap. xi.
Art. 15.) And it is manifeft, that
all Bodies, fluid and folid are equally
compounded of fuch fort of Particles
entirely folid and perfectly hard.
But that which is next to be enqui-
red into, is, what the Figure and
Compofition of the larger Particles
muſt be, in order that the Bodies
compoſed of them, may be hard or
liquid.
Secondly therefore. That Body,
whofe Particles are fo fitted to each
other, as to touch one another in
Large Superficies's, will, by the very
trong mutual Attraction of its Parts,
be a very hard Body; and according
as thofe Parts afterwards either tonch
one another only, or are moreover
intangled with each other, will the
Body be more or leſs brittle, and ca-
pable of being made liquid by Heat,
with more or lefs difficulty:
Ice, Wax, Glafs,
Wood, &c.
As
Metals, Bones,
Thirdly,
122
Part I.
ROHAULT'S SYSTEM
}
}
10. What the
Nature of a
liquid Body
confifts in.
them. Whence it follows, that a Body is fo much the
harder, as it has more Parts which immediately touch each
other without moving.
10. On the other Hand; becauſe, not to contain it
felf within its proper Bounds, is the fame Thing as to
move it felf; and becauſe we cannot conceive any more
effectual Cauſe of that Motion which we fee in a liquid
Body, than the Motion of its infenfible Parts; I therefore
conclude, that Liquidness confifts in the perpetual Agi-
tation of the infenfible Parts of the liquid Body. Thus for
Example, when a Glafs full of Water fet upon a Table
is at reft, though we cannot perceive any fenfible Agi-
Thirdly, That Body whofe Parti-
cles touch one another in lefs Super-
ficies, and therefore are not fo hard,
may yet be more folid; and therefore
Gold is heavier than a Diamond
though not ſo hard.
Fourthly, That Body, whofe Par-
ticles, when they are compreffed, ap-
proach towards each other, but do not
flip under one another, is an elastick
Body, and returns to its Figure, by
that Force which arifes from the
mutual Attraction of its Parts.
Fifthly, That Body, whofe Particles.
flip under each other, is a foft Bo-
dy, which yields to the Stroke of a
Hammer.
Water will rife in ſmall Tubes open
at both Ends in a Vacuum, And
Drops of Liquors hanging upon a
hard Body, and juſt ready to fall,
will gather themſelves into round Fi-
gures in a Vacuum: viz. by fuch a
mutual Attraction of their Particles,
as that by which the polished Mar-
bles ftick together. Further, Thefe
fluid Bodies, if they have Particles
which can eaſily be intangled with
one another, as Oil, or ſuch as may
be made ſtiff by Cold, and faſtned
together, as if they had Wedges put
between them, as Water, fuch Bodies
eafily grow hard. But if they have
fuch fort of Particles, as can neither
be intangled with each other, as Air,
nor made ſtiff by Cold, as Quick-
filver, then they cannot by any means
be made to congeal,
Sixthly, That Body, whofe Par-
ticles touch one another in very ſmall
Superficies, is a crumbling Body, as
Snow, or fuch whofe Parts may very
eafily be feparated; as two well po-
lifhed Marbles, which ſtick together
in a Vacuum, but are pulled afundering
by the leaſt Shake.
Seventhly, If the Parts of a Body,
either do not touch one another at all,
or at leaſt will very easily flip, and are
of fuch a Bignefs, as to be eafily agita-
ted by Heat; and the Heat be fufficient
to agitate them, though perhaps it
be much leſs than is required to
keep Water from freezing; or if
they be not agitated by Motion, but
are only fmall, round, flippery, of
fuch a Figure, and Bignefs, as make
them very easily agitated and give
way; that is, a fluid Body. And
yet the Particles of fuch fort of Bo-
dies which are most fluid, do in
fome meaſure cohere together; as is
evident from hence, that Quicksilver
very well cleared of all Air, will
fland 60 or 70 Inches high in the
Barometer (as was faid before). And
Eighthly. If the Parts of a Body
be very fmall, fpherical, and exceed-
denfe, fuch a Body may alſo be
fluid, and yet be much heavier, than
harder Bodies, whofe Particles are
not fo folid, but which touch one
another in larger Superficies.
Ninthly, Thofe Bodies, whoſe Par-
ticles are agitated with a very quick
Motion all ways, whatever the Fi-
gure of them be, will be liquid, as
Metals that are melted, &c. But
fuch Bodies grow hard, as foon as
that violent Motion ceaſes.
Laftly. Thofe Bodies, fome of
whofe Particles are intangled with
each other, fome of them touch one
another in large Superficies, and fome
are loofe, and will eafily flip under
each other, thefe are flexile as Lea-
ther, or very pliant as Twigs, Glue,
Pitch, &c.
2. That Liquidness confifts) Sce
the Notes on the foregoing Artic.
tation
Chap. zz. of NATURAL PHILOSOPHY
123
tation in it, yet notwithftanding, fome of its Parts are in
Motion downwards, and at the fame time others of
them are in Motion upwards, fome of them move from
the Right to the Left, and others from the Left to the
Right, in a word, there are fome parts of the Water
which move in all manner of Determinations; whence
it follows, that That Body is the moft liquid, whofe
infenfible Parts are the fmalleft, and the moſt agi
tated.
fifts in.
11. If what I have now faid of Liquidneſs be joined 11. What the
to what was before faid concerning Hardnefs, we fhall Nature of a
Soft Body con-
eafily conceive that a foft Body, which feems to be of a
middle Nature betwixt a hard and a liquid Body, and to
partake of them both, is therefore foft, becauſe it is com-
pofed of two Sorts of Parts, the one in fome meaſure at
reft, and connected with each other, while the other are
in Motion, and thereby cauſe ſome ſmall Agitation in the
former.
1
12. Now that which confirms me in my Opinion con- 12. Why a
cerning the Nature of hard and liquid Bodies, is, that the hard Body
refifts the
chief Properties of them are neceffarily deduced from Touch.
thence. And Firft, Suppofe the Nature of a hard Body
to confift in what I have faid, it follows from thence,
that it must be with Difficulty divided: For, for Inftance,
if I put my Finger to any of its Parts, I ought to
feel the Refiftance, not only of thofe Parts which
I touch, but alſo of all thofe Parts which are behind
them, and many times it is much eaſier to move the
whole hard Body, than to feparate oné Part from it, be-
caufe the reft of the Body has a ſtronger Connection
with, and is more at reft, with refpect to this Part, than
the neighbouring Bodies have with the whole Body.
Į
་
2
13. On the contrary, fuppofe the Nature of a liquid 13. Why
Body to confift in what I have faid, it follows from thence, liquid Body is
cafily divi-
that a Liquid muſt be very easily divided. And indeed if dea.
put my Finger to it any way, it meets with no Re-
fiftance, for thofe few infenfible Parts which my Finger
touch, being in Motion already, are very ready to quit
their Place; neither are they fupported nor hindred
by the Refiftance of thoſe which are beyond, which are
alfo in continual Motion, and therefore eafily yield to
them, and open a Paffage for them all ways.
14. What I have advanced concerning the Nature of 14. Why ma-
a hard and of a liquid Body, is ftill further confirmed from "y Bodies are
preferved un-
hence, that all the Confequences that can be drawn from corrupted,
it, help to explain fome Experiment, which perhaps it
within the
parts of a
Would hard Body's
1
}
124
15. Ofthe
Bodies.
A
ROHAULT'S SYSTEM
Part I.
would be impoffible to explain without it. And firſt, if
we confider that fome Bodies are eafily altered, only by
diſturbing the Order of their Parts, and that every Thing
endeavours as much as it can to continue in that State in
which it is, and confequently that which is once at reft,
will never begin of it felf to move; it will not be diffi-
cult to find out a very eafy way to preserve a hard Body
a very long time, viz. by inclofing it in another hard Body;
whofe Parts being at reft among themſelves, can make
no Impreffion upon it, and are moreover a Guard upon
it, againſt the Affault of any external Cauſes which might
tend to corrupt it. And thus we fee that Salt, Sugar, and
Metals, are preferved by being thus incloſed in hard
Bodies.
I
15. On the other hand, it is eaſy to foreſee, that the
Vertue of Li- contrary ought to happen, if hard Bodies be put into Li-
Folve certain quids: For the Parts of Liquors being in continual Agi-
tation, they may eafily fo shake and move the Parts of hard
Bodies, as to force them out of their Places, and carry
them along with them. And thus we find it by Experience,
in all hard Bodies that can be altered, as in Sugar and
Salts, which are diffipated and fink to the Bottom of the
Water almoſt in a Moment; infomuch, that if we throw
a Pound of Sugar into a great Tub of Water, it will in-
tirely diſappear in a fhort time; and the Parts of it, 2 will
alfo be fo diffipated, and fpread amongst all the Drops of
Water, that there will not be one of them but what is
impregnated with it.
16. VVhy a
Liquor does
not entirely
diffolve cer-
zain Bodics.
16. And fince hard Bodies may be compofed of Parts
of different Bigneffes, as well as liquid Bodies, it is eaſy to
conceive. that there may be fuch a Liquor as will car-
1. In continual Agitation) See
the Notes upon Art. 9.
afunder, as the Quantity of VVater
in which they float, will allow. And
does not this Endeavour imply, that
they have a repulfive Force by which
they fly from one another, or at leaſt,
that they attract the VVater (See
the Notes on Chap. xi) more ſtrongly
than they do one another. For all
Things afcend in VVater, which are
less attracted than V Vater by the
gravitating Power of the Earth;
2. VVill also be diffipated) The il-
luftrious Newton thus expreffes him-
felf upon this Subject in his Opticks,
P. 362. If a very small Quantity of
any Salt or Vitriol be diffolved in a
great Quantity of VVater; the Par-
ticles of the Salt or Vitriol will not
fink to the Bottom, though they be
heavier in Specie than the VVater,
but will evenly diffuſe themselves in-fo
to all the VVater, jo as to make it
as faline at the Top as at the Bot-
tom. _ And does not this imply, that
the Parts of the Salt or Vitriol re-
rede from one another, and endeavour
to expand themſelves, and get as far
all the Particles of Salt which float
in VVater, and are less attracted
than Vater by any one Particle of
Salts muſt recede from that Particle,
and give way to the more attracted
VVater.
чу
Chap. 22. of NATURAL PHILOSOPHY.
125
ry away with it only fome certain Parts of a hard Body,
and that others will not be difplaced by it. Thus Water will
only waſh off the fineſt Parts of Liquorifh, and leave the
groffer ones at reft with each other.
17. Of the
17. It may
may alſo ſo happen in hard Bodies, that the Parts
diffolving
of them which are pretty near equal, may yet be fo folid; Power of A-
and on the contrary, all the Parts of a certain Liquor qua fortis.
may be fo fmall, that the Parts of the hard Body will
not be at all moved by them, as they would be by the
groffer Parts of another Liquor; which doubtlefs is the
Reaſon why common Water, will not diffolve Silver, and
why Aqua Fortis, which the Chymifts call Spirit of Ni-
tre, will eafily diffolve it, but is too weak to diffolve
Gold.
18. However, it is not only the Grofness of the Parts 18.VVhy
of any Liquid, which renders it capable of feparating the does not dif-
Aqua regia
Parts of a hard Body; the Pores which are between the folve Silver.
Parts of a hard Body, do alfo contribute towards it: For
they may be of ſuch a Figure, and alſo ſo ſmall, that the
Parts of the Liquid cannot penetrate them; from whence
we may conclude, that the Parts of the Salts of which
Aqua regia is made, are put together in ſuch a manner,
as to compofe Bodies 2 too grofs to enter the Pores of
Silver, and fo only fliding by them, they can neither go
in, nor divide the Parts: Wherefore it is not to be won-
dred at, if this Water will not diffolve Gold.
1. VVill easily diffolve it) Con- |
cerning the diffolving of Metals the
fame celebrated Perfon fays thus.
VVhen Aqua Fortis, or Spirit of
Vitriol poured upon Filings of Iron,
diffolves the Filings with a great
Heat and Ebullition, is not this Heat
and Ebullition effected by a violent
Motion of the Parts, and does not
that Motion argue, that the acid
Parts of the Liquor ruſh towards the
Parts of the Metal with Violence, and
run forcibly into its Pores, till they
get between its outmoft Particles, and
the main Mafs of the Metal, and
furrounding thofe Particles, loofen
them from the main Mafs and fet
them at liberty to float off into the
VVater? And when the acid Par-
ticles, which alone would diftil with
an ealy Heat, will not feparate from
the Particles of the Metals, without
a very violent Heat, does not this con-
firm the Attraction between them.
Opticks, p. 352. Now this fame
A
19. It
Aqua fortis which eafily diffolves
Iron or Silver, will not diffolve Gold
at all, the Reaſon of which is, be-
caufe its Particles, which are more
ftrongly attracted by the Particles of
Iron or Silver than by one another,
are on the other hand more ſtrongly
attracted by one another than by the
Particles of Gold. The contrary to
which we are to underſtand of that
Force by which Gold is diffolved in
Aqua regia.
2. Too grofs to enter) Mr. Clerc
in his Phyficks, Book II. Chap. iv.
Sect. 24. contends on the contrary,
that the Parts of Aqua regia, are
Sharper and smaller than those of
Aqua fortis, and therefore can enter
the very small Pores of Gold only, and
Separate its Parts, which like VVedges,
they drive from one another, whilst
the groffer ones move about the Su-
perficies of the Gold to no purpoſe,
they not being able to diffolve the con-
tinnity of it, becauſe they cannot en-
ter
126
Part I.
ROHAULT'S SYSTEM
19. The Me-
thod of Sepa-
rating Gold
from Silver.
19. It is from the Confideration of the different Pro-
Perties of the feveral Sorts of Aqua fortis, that the Refi-
ners of Gold have lately found out a way of ſeparating
Gold from Silver mixed with it: The whole Secret of which
confifts in putting the Mafs compofed of Gold and Sil-
ver into Aqua fortis, which will diffolve the Silver only;
for then its Parts will be brought out by thoſe of the
Liquid, till the pure Gold will remain like Sand or Dregs
at the Bottom of the Veffel; fo that by inclining it gent-
ly, and pouring the Aqua fortis into another Veffel, it
will carry the Silver along with it, and leave the Gold
at the Bottom: After this, they feparate the Silver from
the Aqua fortis in the following manner; they put a
Quantity of common Water to the Aqua fortis, to make
it lefs corrofive, and then put in a Piece of Copper, againſt
which the Particles of Silver brought out by the Liquid
ftriking, they are ſtopped by it; in the fame manner as
Duft flying about a Room is ftopped by the Hangings
or any other Furniture which is foft, or as a Stone ſticks,
when it is caft into Mortar. The Gold and the Silver
being thus feparated from one another in Duft, may each
of them be melted in a Crucible, and then made diftinct
Maffes of.
1
20. VVhy the 20. It may here be asked, why the fmall Particles of
Parts of ma- Salts and Metals, fwim thus in all the Parts of common
ny Bodies
Water or Aqua fortis indifferently, and whence it is, that
heavier than they do not fink to the Bottom of the Veffels? For this
which are
VVater, do
not fink in it.
ter its Pores. And again, 'Sect. 28.
He fays, That from the Mixture of
many Salts, the Parts of the Aqua
regia become ſmaller, and more fitted
to enter the smallest Pores, and Sepa-
rate the ſmalleſt Parts; between which
they are driven like VVedges, by the
Motion of the Liquid in which they
Swim; but when they enter into wider
Pores, they have no Effect; in the
fame manner as the Force of VVedges
to feparate Things joined together, is
nothing unless they be driven into
freight Fiffures. Since therefore the
Pores of Gold are the smallest of a-
ny Metal, they will admit the Par-
ticles of Aqua regia only, and the grof-
fer Parts of Aqua fortis cannot enter
into them. Now the fame Parts of
Aqua regia are too fubtle to have
Strength enough to remove the Sides
of the Pores of other Metals; for
they want the groffer Parts of Aqua
fortis which fill and divide the larger
Peres. Thus far he; but what he
ነ
fays, he does not confirm by any
Arguments or Reaſons, unleſs it be
this, that Silver feems to have larger
Pores than Gold, becauſe it is lighter;
but from the known Properties of
Silver, irs hardneſs, ſmoothneſs, &i.
we may with much greater Proba-
bility collect, that it confifts of ſmal-
ler Particles, and therefore has fmal-
ler Pores, though more of them;
But that Gold on the con-
trary, confifts of lar- * See Part
ger Particles or Lumps, III. Chap.
and fo has larger Pores, vi.Art. 13.
'but much fewer. And as
*
to the Nature of the Liquids, I
fhould think, that the Parts of the
Aqua regia, would become not
fmaller, but larger by the Mixture
of many Salts. But all this depends,
as was faid before, not fo much up-
on the Bignefs and Figure of the Pores,
as upon the different Attraction of the
Paris.
fhould
Chap. 22. of NATURAL PHILOSOPHY.
127
fhould feem to follow from what was before demonftra-
ted concerning hard Bodies fwimming in Liquids, becauſe
every Particle of Salt or Metal is heavier than an equal
Mafs of the Liquid in which it ſwims. However, it is
to be obſerved, that when we reaſoned in that manner,
we confidered only the Gravity of the hard Body and the
eafinefs of the Liquid to be divided; we did not then know
of the Motion of the Particles of the Liquid, by which
they carry up with them as many Particles of Salt or Me-
tal, as would deſcend by their own Weight; in the fame
manner as the Babbling up of new Wine, makes other
Bodies which are heavier, fwim, and not fink to the Bot-
of the Tub, where we fee that they do at laſt ſub-
fide and compofe the Lees, when this Motion, which is
greater than the ordinary Motion of the Liquid, ceafes.
To which may be added, that the Particles of the dif
folved Body are in fome meaſure intangled with thoſe of
the Liquid, which they go along with; which fhows us
more particularly that this hinders them from being able
tom
tó fink.
21.That
a certain
21. And that which is remarkable here, is, that as the
Particles of the Liquid are finite, and the Force by a certain:
Quantity of
which they are agitated is limited; it muft neceffarily fol- Water will
low, that when they have once laid hold of as many Par- diffolve only
ticles as they can contain, they cannot after that ſeparate Quantity of a
any more, nor overcome the Refiſtance of the remaining hard Body
Particles which are at reft, wherefore the hard Body will
be no farther diffolved. And thus we find by Experi-
ence, in common Water and Aqua fortis, that they will
diffolve but a certain determinate Quantity of Salts or
Metals. Thus, for Example, if, after a Pint of common
Water has diffolved a certain Quantity of Salt, one Grain
only be put in, it will continue whole in the Water, as
it would do in a dry Place.
22. How the
Chryftalizati-
mifts is made.
22. And from hence it follows, that if after a Liquid
has feparated all that it can from a hard Body, it be eva- of the Chy-
porated to a certain Quantity, that which remains will
not be able to contain all the Particles of the diffolved
Body, wherefore many of them will be forced to unite
together, and to compofe fomething fenfible; and thus
it is, that if Water be boiled, having firft been ftrain-
ed like Lye, through Earth charged with Nitre as
much as it can be, and then taken off from the Fire,
1.The Motion of Particles) Not by ↑ See above on Art. 15.
their Motion, but by their Attraction.
-->
and
728
Part I.
ROHAULT'S SYSTEM
23. That the
Water
which will not
diffolve one
certain Body
will yet dif-
Solve a Body
of another
Sort.
any longer:
24. How the
and permitted to fettle a little, a great many Particles of
the Salt-peter which are difingaged from the Particles of
the Water, will ceafe to move, and ftriking many of
them together againſt the Concave Sides of the Veffel,
will at laſt compoſe thoſe curious Bodies in the Form
of Hexagons, which we ſee ſtick there. And in the fame
manner we may apprehend, how all the other Chryftali-
zations of the Chymifts are made.
23. Though a certain Quantity of any Liquid, will dif-
folve but a determinate Quantity of a certain hard Body,
yet this does not hinder, but that other hard Bodies may
be diffolved by the fame Liquid; becauſe their Particles
may be of fuch a Figure, as to fuit with the Particles of
the Body already diffolved, in ſuch a manner, as may oc-
cafion more diffimilar Particles, to move with greater
Eafe, than the fimilar Ones could move. And thus Ex-
perience ſhows us, that after Water has diffolved as much
Salt as it can, it will yet diffolve a ſmall Quantity of Vi-
triol or Alum.
24. If a Body be put into a Liquor, to whofe Parti-
Precipitation cles it will more eafily unite it felf, than to thofe of ano-
of the Chy-
mifts is made. ther Body which it had before diffolved; and fuppofing alfo
that it cannot comprehend theſe two Sorts of Particles
together, 2 it muſt be forced to let go the Particles which
it had before embraced, which will confequently fubfide
to the Bottom of the Veffel. Thus if a little of that dif
folved Salt, which Chymifts call Oil of Tartar, be poured
upon Aqua fortis which before had diffolved Silver, the
Metal will be forced to fubfide to the Bottom of the
Veffel. And this Inftance fhows us the Reafon of all the
Precipitates of the Chymifts.
1. Thofe curious Bodies) Concern
ing which the admirable Perfon be
fore cited, fays thus. VV'hen any ſaline
Liquor is evaporated to a Cuticle, and
let cool, the Salt concretes into regular
Figures; which argues, that the Par-
ticles of the Salt before they concreted,
floated in the Liquor at equal Diſtan-
ces in Rank and File, and by con-
Sequence, that they acted upon one a-
nother by fome Pomer, which at equal
Distances is equal, at unequal Diffan-
ces, unequal. For by fuch a Pomer
they will range themſelves uniformly›
and without it, they will float irre-
gularly, and come together irregular-
ly. Opticks, p. 363.
2. It must be forced to let go) If
fuch a Body be put into fuch a Sort
of Liquor, that the Particles of the
Liquor will be more ftrongly at-
tracted by the Particles of this Bo-
dy, than by the Particles of that Bo-
dy which was diffolved in it before,
the Particles of the Liquor being by
this ftronger Attraction removed
from the firſt Body to this Other,-
will fuffer the Particles of the firſt
Body to fink to the Bottom, in the
fame manner as Iron is feparated
from a Loadſtone, by putting a
ftronger Loadſtone to it.
A
25. We
Chap.32. of NATURAL PHILOSOPHY.
119
*
25. We must not here omit another Circumftance ve- 25. How two
ry confiderable, and that is, that the Particles of two Li- Liquors mix-
ed together,
quors may be of fuch a Bignefs and Figure, as to intangle may compofe
one another when they meet together, and fo move with one hard Bo-
dy.
more difficulty; whence it follows, that they will com-
pofe One Body which is not fo liquid: So likewife, if the
Particles of the two Liquors adjuſt themſelves to each
other, ſo that the greateſt Part of them are hindred from
moving, then all the Particles together will form a Body
pretty hard. Thus we fee, that if an equal Quantity of
Spirits of Wine and Spirits of Urine, each of which Li-
quors are very fluid, be mixed together, they will unite
into a pretty hard Body.
26. How a
hard Body
may ariſe out
26. We may add to what has been faid about the Mix-
ture of different Liquors, that there may be found one, which
is compofed of fuch fort of Particles, that fome of them of a liquid
being much larger than others, they cannot continue their one only.
Motion, but by means of the fmaller ones; fo that if
theſe be any way difingaged, the Weight of the other
alone, or the Irregularity of their Figure, will make them
continue at reft with each other, and according as they
are more or less cloſely united together, they will compofe a
Body more or lefs hard: And this is the Reaſon why fome
of the Particles of Milk or Blood curdle, while others
which are more proper to continue their Motion, being
difingaged from thefe, compofe a Serum, which remains
liquid. And this is alſo the Reaſon why, in fubterraneous
Caves, which they call dropping Caves, certain liquid Drops
which diftill from the Roofs harden into Stone, after they
have been a little while in the open Air.
27. Of the
Cafes of
Liquidness.
27. Having fufficiently fhown by theſe Experiments,
that the Particles of liquid Bodies are in continual Agita
tion, we are to enquire next, what the efficient Caufe of
this Motion is, firft, in Water and other fuch like Li-
quids, which feldom grow hard, but more particularly.
in Air, which never hardens, but always remains li
quid. Wherefore in the firſt Place it is reaſonable to
think, that the † Figures of the Particles of Liquids are
not altered, fo long as we cannot perceive any kind of their Figures
Alteration in them: But further, becauſe they cannot move were conti-
nually alter-
with regard to each other, as they ought to do, to com- ed, there
pofe a Liquid, without leaving a great many Interftices would be no
round them; which there being I no Reafon to think need of fubtil
empty, they muft neceffarily be furrounded by fome Mat- up their in-
1. No reason to think empty) See the Notes on Chap. viii. Art. 2.
K
1
ter
1. For if
Matter to till
terfices
i
13·0
Part I
ROHAULT'S SYSTEM
28. How Li-
quers are e-
vaporated.
ter which is very ſubtle, fuch as that which we before
called the Firſt or Second Element. And as the Particles
of hard Bodies diffolved in any Liquid, are kept in Mo-
tion by the Particles of this Liquid; fo we ought to think,
that the Particles of Water, and of all Bodies which do
not congeal, but always remain liquid, are in perpetual
Agitation, becauſe they fwim in the Matter of the First and
Second Element.
28. If this Matter be very much agitated, it is eafy to
conceive, that it may move the Particles of the Liquid
in ſuch a manner, as to diffipate them from each other,
and make them fly into the Air, and this is called Eva-
poration.
29. How they
29. On the other hand, if its Motion be very faint, or
are congealed, if it be more than ordinarily fubtil, it will follow, that
it will not be capable of preſerving the Liquidneſs of ſome
groffer Bodies; in the fame manner as we fee the Water
running amongſt Bulrufhes, keeps them in Motion, and
diſtinct from each other, whereas in the Air, they are
confufed and mixed together, without any Motion; and
thus the Water is frozen in Winter, and turned into Ice.
But we cannot ſhow a Reaſon why this happens at one
Time of the Year, rather than at another, till we come
to know ſomething more of the Syftem of the World.
30. Why Some
Bodies grom
Soft before
they become
liquid.
31. Why o-
without grow-
I
30. If the Difpofition of the Particles of a Body be.
fuch, as to leave Pores between them large enough to re-
ceive the groffer Matter of the Firſt and Second Elements,
this Matter may ſhake the Particles a little, before it quite
feparates them, and moves them from each other, and
confequently the Body ought to grow foft, before it becomes
liquid, as we fee Wax does.
31. But if the Pores of a hard Body are fo fmall, that
ther Bodies, only the moſt ſubtil Matter of all can pass through them,
become liquid in this Cafe, that which is more grofs, and which is alone
ing foft.. able to ſhake thoſe Particles which make the leaft Re-
fiſtance to it, can only apply it felf to the Superficies of
the Body; whence it follows, that it will have diffolved
1. And thus the Water) Since nei-
ther the Force it freezes with, is al-
ways proportioned to the Cold, but
feems to have fome Dependance up-
on other Changes in the Heavens;
nor is the Cold, unleſs fo far as it is
merely comparative (See the Notes
on Chap. xxiii. Art. 54.) owing to
the Particles being at Reft; nor
can Hardneſs it felt (See the Notes on
Art. 9. of this Chap.) arife from the
mere Reft of the Particles. Con-
gealing must neceffarily be afcribed
either to nitrous Particles; or to the
Particles of fome other Salts, which
like Wedges fixed between the Par-
ticles of Water, join them together
and make them cohere: However
there is hitherto nothing certain found
out concerning theſe Particles.
Chap. 22. of NATURAL PHILOSOPHY.
ІІ
all the external Parts of the Body, before it makes any
Alteration within it. And fo fuch a Body will be entire-
ly diffolved without being made foft, as we find Ice
does. *
32. How Wa-
Plaister of
Paris.
ter hardens
32. It is not at all furprizing, that Water, which is li-
quid, fhould ſoften a great many hard Bodies which it pe-
netrates and diffolves, and that, when it is mixed with
Plaiſter of Paris, for Example, there fhould arife a Com-
pofition pretty liquid: But it is very furprizing, that af
terwards it fhould acquire a Hardness which it would ne-
ver have had without mixing Water with it, which one
would think, ſhould rather help to foften, than to harden it.
Nor can we think, that this arifes from a fudden Evapo-
ration of the Parts of the Water; for if it be weighed
when it is liquid, and weighed again when it is grown hard,
we canot perceive that it has loft any of its Weight. My.
Opinion concerning the Matter is this, that the Fire has
formed a great many Pores in the Plaiſter, of ſuch a Big-
nefs, as the groffer Particles of the Air cannot penetrate,
becauſe they are not folid enough to remove the Obfta-
cles they meet with, which the Particles of the Water,
which are more folid and penetrating, are able to do.
Wherefore, when the Plaifter is moistned with, or put
into fuch a Quantity of Water only,, as is fufficient to
furround every Grain or Lump of it; and after that they
come to be ſtirred up together, then the Particles of the
Water which force themſelves into the Pores, like fo
many ſmall Wedges opening and ſplitting them, divide
thefe Grains into ftill fmaller Parcels. And becauſe theſe
Parcels have a larger Surface than the Grains had before,
of which they are but the Duft, it is more than the Wa-
ter is able to furround. Infomuch, that the greateſt part
of them touching one another clofe, and continuing at
reft, it is no wonder if they compofe a hard Body.
33. From
*The true Caufe why fome Bo-
dies grow foft before they melt, and
others not, ſeems to be this; that
thofe Bodies which grow foft, are
compofed of diffimilar Parts, fome
of which melt fooner than thoſe they
are mixed with.
1. If they compofe a hard Body)
Mr. Le Clerc attacks our Author here
with three Arguments in his Phyficks,
Book V. Chap. xiv. Sect. 25. Firß,
fays he, This Answer does not agree
with a Mass made up of Meal and
Water kneaded together, and baked ;
K 2
and other fuch like Things that might
be inftanced in. But can any Thing
be more evident, than that the Eva-
poration of the Water produces the
fame Effect in Bread, as the Diffolu-
tion of the Lumps in Plaifter of
Paris? For though not all, yet cer-
tainly fome of the Water is diflol-
ved into Vapours, in proportion to
the Heat, wherefore the external
Part of the Bread is much harder than
the Internal. Secondly, He fays, He
does not show why the Particles of Wa-
ter fo divided touch one another cloſe.
bur
1
ROHAULT SYSTEM
Part I
132
33. That too
Quantity of
Water hin-
33. From hence we draw this Confequence, that if
great a the Plaifter be put into fuch a Quantity of Water as is
Lufficient to furfound all the fmall Parcels which the
ders the Plai- Lumps are divided into, they will be hindred from reft-
fter from
growing hard. ing, and fo the Plaifter will not grow hard at all; and
thus the Mafons find it by Experience, and this is
what they
they mean, when they ſay their Plaiſter is
34. Why
Water does
not harden
Lime.
drowned.
34. Notwithſtanding this, it is not to be wondered at, if
there be fome Bodies which the Water will divide, and yet
not at all help to unite and harden their Parts into one
Mais, as it does thofe of Plaiſter of Paris; for the Par-
ticles of thefe Bodies may be of fuch a Figure, as ſcarce
to touch one another at all, and fo cannot unite together
to compoſe one Whole: To which it may be further ad-
ded, that the Water has fo quick a Motion within ſome
Bodies, that it further feparates the Particles already difu-
nited; and by this means the Pores or Intervals, which
are between them, become fo large, that the Air has Pow-
er to get in, and hinder fuch Particles from touching one
another. And this is the Reafon why Lime, which is di
vided by Water, does not yet become hard like Plaiſter
of Paris: For if a Piece of Lime, which has been wet-
ted with a little Water, be divided without meddling with
..
if the Parts of the Plaifter muſt be
together by Wedges, it feems
much more probable, that the burnt
Parts (for the Plaifter is made of
Stone half burnt) growing a little
hot, by the Water being poured on
it, draw the volatile Salts out of the
Parts which are not burnt, whichi
Particles of the Salts, being fixed in
the Pores of the Plaifter, keep its
Parts together. For the ſtiff Particles
of Salt, feem much more proper to
perform the Office of Wedges, than
the limber and flexible Parts of Wa-
But indeed, Plaiſter of Paris,
ter.
Clay, and fuch kind of Bodies, do
therefore grow hard in this manner,
becaufe the Water in exaporating, fo
attracts their Parts to each other,
which before did not touch one ano-
ther, that afterwards touching one
another in larger Superficies, they
cohere together by that mutual Attra-
ction, which depends upon imme-
diate Conta&. See the Notes on Art.
the 9th of this Chap.
But he does exprefsly fhow this in
thefe Words. And becauſe theſe Par-kept
ticles have a larger-Surface than the
Grains bad before, of which they are
but the Duft, it is more than the
Water is able to furrounds Info
much, that the greatest Part of them
touching one another clofe, &c. What
could have been faid more exprefs.
But (I fuppofe) this learned Gentle-
man, when he tranflated this Place
into Latin, being nor very attentive,
overlook'd the connective Particle,
aellement que. Thirdly, He fays,
That he fuppofes Hardness to arife
from immediate Contact and Reft,
which we have before confuted. Coń-
cerning this, See the Notes on Art.
the 9th of this Chap. Having thus
confuted the Opinion of our Author,
the learned Gentleman conjectures,
«That the Particles of Water which
diffolve the groffer Lumps of the
Plaifter, are fo fixed into the lef-
"fer Particles, as, like Wedges, to
join many of hem together, and fo
compofe & more folid Mass. But,
:
::
?
:
*
Chap. 22. of NATURAL PHILOSOPHY.
133
it, the Duft into which it diffolves it felf, is of two or
three times as much Bulk as it was before.
Matter of the
cond Element
does not flop
First and Se-
in the Pores
of hard Bo-
35. When the Water penetrates the Pores of certain 35. That the
Bodies which it cannot entirely divide; it is evident, that
it will ftop for fome time; becauſe it muſt lofe its Mo-
tion, by ſtriking againſt the Particles which it touches:
But it is otherwiſe with the Matter of the First and Second
Element, when it paffes through the Pores of hard Bo-
dies: For as theſe Pores, as ſmall as they are, are formed
by its continual paffing through them, fo it leaves them
big enough always to find a Paffage through, without ever
being ſtopped.
dies.
the Matter of
36. However, it is to be obſerved, that by bending a 36. What the
hard Body, fuch, for Example, as the Blade of a Sword, Confequence of
the Particles will be made to expand themſelves on the the Second E-
Convex Side, and to contract themſelves on the Concave lement paſſing
through very,
Side. So that its Pores will become ſmaller and ftreight-mall Pores
er on this Side, but this ought not to hinder the Matter ought to be.
of the Firſt or Second Element from entring in, becauſe
being very fine, and moving very quick, it ought rather
to alter its own Figure and become longer, or to wear in
pieces the Matter which ftreightens it, than to be hin-
dred in its Paffage; and fo the Ports will not be ſtop-
ped up by it.
37. But becauſe the fubtil Matter which paffes through
the Pores which are ſo very ſmall, cannot endeavour to
wear the Particles of the hard Body through which it
paffes, but it muſt at the fame time endeavour to reſtore
the fame Particles to the State they were in before the
Body was bent; it follows, that this ought to make the
Body grow ftreight again. And thus we experience the
Property which is called Stiffness, and which Workmen
call the Power of Springing.
38. However, this Property ought not to be found in
all Sorts of hard Bodies indifferently, becauſe there are
fome, whofe Pores are fo large, that though they be
ftreightened by bending the Bodes, yet they will be ſtill
1. The Power of Springing) Since
this fubtil Matter, as was before pro-
ved, is only fictitious, it is much more
probable,that if a Body be compound-
ed of fuch Șort of Particles, that it be
compact, and bends or yields inward
to Preffion without any fliding of its
Parts, it is hard and elaſtick, return-
ing to its Figure with a Force ari-
fing from the mutual Attraction of
its Parts. Newt. Opt. pag. 370
But if the Parts of the Body dip
under one another, then the Body is
of that Sort, which will yield to the
Stroke of a Hammer; But con-
cerning the Laws of the Communi-
cation of Motion, in fuch Bodies as
have a Power of ſpringing back, or
are Elaftick, as they call it, when
they meet other with certain Forces.
See the Notes on Chap. xi. Art. 6.
K 3
wide
+
37. What the
Force of
Springing back
confifts in.
3
38. Why it is
not fo
found in
all bard Bo-
dies.
}
134
Part I.
ROHAULT's SYSTEM
becomes Ela-
wide enough to give a free Paffage to the fubtle Matter.
Thus we can perceive by our Senfes, that the Parts of Steel
which is not tempered, are larger, and confequently the
Pores wider, than thofe of tempered Steel; whence it is
eafy to apprehend that the Pores may be ftreightned, with-
out hindring a free Paffage of the fubtle Matter through
them; whence it follows, that when it is bent, it will not
fpring back again.
39. Why a
39. Now to fhow, that the Power of Springing confifts
Plate of Iron intirely in the Smallneſs of the Pores of a hard Body, let
Stick, by being us confider, that if a Plate of untempered Steel, be beat-
beaten, when en upon an Anvil when it is cold, it will acquire a Pow-
it is cold.
er of Springing which it had not before. But it is mani-
feſt, that this Beating does nothing elle but make the Parts
approach nearer one another, and by this Means ftreightens
the Pores; whence it follows, that herein confifts this
Power.
40. How this
40. It may be further obſerved, that if a Spring be held
Power may be bent a long time, without being allowed to recover it
lofi.
felf, the fubtil Matter will be forced to alter its Figure
by growing longer, if it be not able to wear in pieces the
Matter of the hard Body; or if it be, the Pores will grow
bigger and bigger, fo as that the Matter of the First and
Second Element may paſs freely through them; and this
is the Reaſon why the Body ought to lofe the Power of
recovering it felf, in proportion as it is capable of being
worn, which agrees with Experience.
i
41. Whence
which a
Spring un-
bends it felf,
arifes.
41. The Force with which a Body unbends it ſelf, de-
the Force with pends partly upon the Swiftness of the Motion of the fubtil
Matter, and partly upon the great Number of Pores through
which it paffes at a Time: But it depends chiefly upon
the Difpofition of thefe Pores as they become infenfibly streighter
and freighter. For by this means, that which gets into
them ought to have the fame Force, and to produce the
fame Effect, as a Body which paffes between two others,
whofe Superficies are almoft parallel. Now according to
the Laws of Mechanicks, though the Body which thus
paffes between two others be very fmall, and moves but
flowly, it will notwithſtanding, have an incredible Force
to ſeparate thoſe two from cach other.
break in re-
42. VVhy
42. When the fubtil Matter begins to remove the
Some Bodies Parts of the Body which are in its way, it has their whole
Storing them- Refiftance to overcome, and alfo fome of the Refiſtance
felves. of the furrounding Bodies: Now becauſe every Thing
endeavours of itſelf to continue in that State in which it
once is, and therefore the Bodies which have received a
certain
เ
Chap. 22. of NATURAL PHILOSOPHY.
135
certain Motion, continue of themſelves in that Motion;
this fubtil Matter cannot continue to impell them, but it
muft increaſe their Motion; and it may fo happen, that
by its impelling and moving them in this manner, it may
fo far divide the Particles of the Body, through which it
paffes, from each other, as intirely to feparate and. break
them; eſpecially if the Body be brittle.
in.
43. Now in order to underſtand how it is, that ſome
43. That
the Limber-
Bodies will bend without breaking, and that on the con-
ness or Brit-
trary, others will very eaſily break; it is to be obſerved, tieness of a
that the Texture of fome may be fuch, that their Par- Body confifts
ticles may be intermixed with each other, like the Rings
of a Chain, or the Threads of which a Cord is compo-
fed. Now it is eafy to conceive, that thefe Bodies may
be wound ſeveral times round without breaking, becauſe
their Particles are fo hooked together, that they may be
bent any way. On the other hand, there may be Bodies
which are not of fuch a complicated Texture, which are
hard only, becauſe their Particles touch one another in a
few Places: Whence it follows, that one cannot fe-
parate them ever fo little, but their whole Continuity
will be deſtroyed; and thefe are what we call brittle
Bodies.
Place in
very unequal;
44. Leather may ferve for an Inftance of a limber Bo- 44. VVhy the
dy, that is, of a Body that will bend without breaking; which a lim-
and Glaſs, on the other hand, for an Inftance of a brittle ber Body
Body; that is, one that will break before it will bend; breaks, is
And there will be no doubt, but that the Limberness of and that of a
the one, and the Brittleness of the other, confifts in what brittle Body
I have faid; if we confider the Place where a Piece of very mouth.
dry Leather is pulled aſunder, and the Place where a Piece
of Glafs is broken: For the Leather appears unequal, and
as it were untwiſted, which is an evident Sign, that the
Particles which are at the End of one Part, entered in
between the Particles which are at the End of the other
Part; and on the contrary, the Breach of the Glafs appears
very well poliſhed, which is a Sign, that the Particles of one
of its Pieces, touched the Particles of the other Piece on-
ly, without entring in between them.
ly
45. If Glafs, which is very brittle, have very large 45. Vy
Pores on one side of its Superficies, and which grow leſs Glaſſes new-
and lefs towards the other Side, there cannot enter into made, are
apt to break,
theſe large Pores, fubtil Matter enough to fill them, but without being
that by continuing its Motion very quick towards the meddled with.
freighter Parts of the Pores, it muſt wholly difunite the
Parts. Now when a Drinking-glafs, which is juft made,
K 4
grows
136
Part I.
ROHAULT's SYSTEM
46, To hinder
thus break-
ing.
grows cold on a fudden; it is impoffible but that the
Pores must be larger where the Glafs is thickest, becaufe
the Heat, which dilates Bodies, continues longer here than
in the other Parts: Wherefore the fubtil Matter which
enters into theſe large Pores, going on ſwiftly, and with
great Force, muft break the Glafs in the Places where
the Pores are fenfibly lefs. And this fo commonly hap-
pens, that it is fomething ftrange, if a hundred Glaffes
be expofed to the Air as foon as they are made, if one
of them eſcape without breaking.
i
46. The Glafs-makers have a Way to prevent this In-
Glaffes from convenience, by putting the new-made Glaffes into the
Arch of the Furnace, where they are removed by little
and little out of the Flame, fo as not to get above the
Space of nine or ten Foot, in fix Hours time, and then
they are expofed to the open Air; and fo all the Parts
growing infenfibly Cold, the one as well as the other,
the Pores are equally ftreight every where, and the fub-
til Matter which can enter into one of them, can run
from thence freely, through all other Parts of the Glafs
where the Paffages are equally open.
47. A ſurpri-
Drop.
47. What we have now faid concerning the Cauſe of
ing Property Glaffes being broken as it were of themfelves, opens a
of a Glass
Way for us to explain a kind of a Miracle in Nature,
which was lately diſcovered and brought hither from
Tab. III. Holland, and, which has travelled through all the Uni-
Fig. 5.
verfities of Europe, where it has raifed the Curiofity, and
confounded the Reafon of the greateſt Part of the Phi-
lofophers. It is a kind of a Drop of thick Glafs, and ſuch
as the Glaſs-Windows are made of, near the fame Shape
and Bigneſs as deſcribed in the Figure. It is entirely So-
lid, except perhaps we may fometimes fee a few mall
Bubbles of Air in the thickeft Part of it, as at D, where
it will bear pretty hard Blows of a Hammer without
breaking. And yet, if the little End of it be broken off
any where near B, the whole Body will burst in Pieces
with a Noife; and we fhall fee it ſcatter it felf all round,
1. Muft break the Glass) But it
may be (and it is more likely) that
the Cold, by ftopping the Motion
of fome of the Parts on a fudden,
whilft the reft are in great Motion,
breaks Vefleis made of Glafs. For
thus almoſt all Bodies are broken by
the unequal Motion of their Parts:
Hence a 1 ile by one Blow burfts a-
funder many times into fix hundred
Pieces. Hence the Chymifts Vef
fels are often broke. Hence they
who cut Drinking-Glaffes into Spi-
rals, firft put a red hot Iron near them,
and then pour cold Water on the
Part of the Glaſs which is heated.
And hence Drinking-Glaffes are re-
ported to be broken only by the
Voice bending them.
and
Chap. 22. of NATURAL PHILOSOPHY.
137
**
and to a good diſtance, in a Powder, which though ve-
ry fmall, has its Parts cracked in fo many Places, that
it is eafy to divide them by preffing them between ones
Fingers, which may be done without any Danger of
pricking them, as there is, if we fhould handle a piece
of Glafs fo, after it is powdered in a Mortar.
48. To fay the Truth, this Phenomenon is fo fingular,
that it is no wonder it fhould at firft Sight furprize us.
But if we confider it more cloſely, it is eafy to obferve,
that there is nothing elſe appears, but only the local Mo-
tion of the Parts of the Body, which are carried from
the Center to the Circumference: Now as we cannot
conceive how a Body fhould begin to move of it felf,
without being put in Motion by another Body which
was in Motion before; fo it is eafy to imagine, that the
ſcattering about of the Particles of the Glass-drop, is
owing to fome Matter which getting into its Pores, preffes
upon them and divides them, in the fame manner as we
fee a Wedge when it is driven into a Body with great
Force and Velocity, ſplits it, and feparates the Parts from
each other. And there is no Doubt at all, but that this
is the fame Matter which breaks the Glaffes in the Glaſs-
Houſe, when they are fuffered to cool too foon.
48. Of the
externalCaufe
of the Motion
of the Parts
of the Drop,
ought to be.
49. Now in order to underſtand how this Drop could 49. What
the particular
acquire a Difpofition proper to produce this Effect, there Difpofition of
is Reaſon to guefs, that the Workman, who makes a Se- the Parts of
cret of it, has a Way of cooling it all at once, by dipping the Drop
it when it is very hot into fome Sort of Liquor, which hin-
ders it from breaking in pieces: For we fee by Experience,
that Glafs which is fo cooled in Water, breaks into ſmall
Pieces. But be this Liquor what it will, it is certain, that
the Parts of the Drop, which are neareſt the Surface, cool
firft, and by communicating their Motion to this Liquor,
loſe what they had before, which kept them at a little di-
ſtance from each other; and fo they are condenſed, and
contract their Pores, and fit them to the fineſt Parts of
the fubtil Matter, which preferves its Paffage through
them. But this is not the Cafe of the internal Parts of
the Drop, which not being cooled till after the other,
cannot contract themſelves fo, becauſe thoſe other being
grown hard, and difpofed like an Arch, do not at all
prefs upon them; fo that the Pores which are amongſt
the Parts neareft the Middle, are large, and grow lefs and
lefs as they come towards the Superficies. And this be-
ing allowed, there is a plain Reaſon for what cauſes fo
great Admiration.
ས
50. It
1.
138
+
.*
Part I.
ROHAULT's SYSTEM
50. That it
ought to bear
the Blows of
50. It is no wonder that the Drop will bear the Blows
of a Hammer, becauſe it is thick enough for that:
a Hammer. For other Pieces of Glafs of the fame Bignefs will do
the like.
51. That they
ought not to
break of
themſelves.
52. How it
5r. It is alſo manifeft, that they ought not to break
of themſelves, as the forementioned Glaffes do, be-
cauſe the ſubtil Matter which paffes through them, finds
as free a Paffage to come out, as to enter in.
52. But when the little End is broken off near the place
flies in picces, marked B, we can there fee very large Pores into which
the larger Particles of the fubtil Matter entring in a great
Quantity, and continuing to move from thence very fwift-
ly, towards every part of the Superficies, where the Pores
grow ftreighter, they cannot but ſeparate every way the
Parts of the Glafs, and fo divide them into that Powder
which we fee.
53. VVhy it
the
very End
ken off.
Tab. III.
53. This Truth is confirmed by obferving, Firft, That
does not break the Extremity of all, which is at A, is fo fmall, that
in pieces when
there could be no fenfible difference in cooling between
of all is bro- the infide and the outfide, fo that the Pores there are of
an equal Bignefs throughout. Wherefore if the End be
broken off thereabouts, this will not give leave to the
fubtil Matter to let in its groffer Particles, any more than
if it were not broken at all, and confequently the Drop
ought not to burft in Pieces; as by Experience we find
it does not.
Fig. 5.
54. That the
Drop, when
beated again,
paght to lofe
its Vertue of
Eurfling a-
kunder.
54 Further, if one of theſe Glaſs Drops be made red
hot in the Fire, and then fuffered to cool flowly, its
Pores will then become very near equal, in like manner,
as Workmen neal Steel. After which, if the End of the
Drop be broken off any where, becauſe there can no fub-
til Matter enter in, but fuch as can go out on all Sides
with as great Eafe as it entred in, therefore the Drop 2 ought
not to burſt in Pieces at all, which alfo we find true by
Experience.
1- Separate every way) Becauſe
Glafs is a Body which has a Power
of Springing, it is probable, that this
Glafs Drop is broke in the fame
manner, as a Steel Bow burfts in
pieces fometimes, when it is loofned
on a ſudden; viz. by the too great
Celerity and Force of that Motion
which arifes from the mutual Attra-
tion of its Parts. For its Parts
from the Center to the Circumfe-
rence, feem to be like fo many Bows
bent. And hence perhaps it is, that
after it is burft in Pieces, its Fiffures
are difpofed like fo many Radii
drawn from the Axis to the Superfi-
cies, as Mr. Hook obferved in a
Glafs Drop covered over with Glue.
See Hook's Micrography Obſerv. 7th.
2. Ought not to burſt in pieces) For
the fame Reaſon, that there is no
danger of breaking a Bow when it is
gradually loofned.
$5. Laftly
Chap. 22. of NATURAL PHILOSOPHY.
139
ments of La-
Tab. III.
Fig. 5.
55. Laftly, To confirm what has been faid of the In- 55. Some cu-
rious Experi-
equality of the Pores which are in the Middle, and thoſe
near the Superficies in theſe Sort of Drops, I carried three pidaries.
of them to three different Lapidaries; The firft of them
I ordered to cut the Drop which I gave him, with Pow-
der of Diamond about the Place C. I ordered the Se-
cond to drill a Hole in his, with the fame Powder about
D, and I ordered the Third to put his upon the Wheel,
and grind it plain at E, with Powder of Emery: Now
after thefe three Workmen began feparately to work up-
on them with as much Caution as they do upon Pearls or
Stones of a great Value, and had ground with thele Pow-
ders as much off from the Drops as amounted to the Thick-
nefs of a French Two-pence, which I reckon is as far as the
fmall Pores reach, I faw each of them burſt in pieces as
uſual, to the great furprize of the Workmen, who did not
at all expect any ſuch Thing.
ferences in
56. But to return now to the Confideration of Liquids. 56. Of two
I obferve firft, That if they be all reduced to two Species, principal dif-
the one comprehending all thoſe which we call thin, and Liquors.
the other, all thoſe which we call fat, it will not be dif-
ficult to determine what their principal Difference con-
fifts in. For fince the Former is very eafy to evaporate,
but the Latter evaporates with great Difficulty, we can-
not but think, that the Particles of the one, muſt be of
very fimple Figures to be able to difingage themſelves
from each other, and the Particles of the other, of more
entangling Figures, fomething like Branches of Trees, by
which they hold each other together.
57. VVhy
Pater, when
poured down,
57. And this is confirmed from hence, that if a Vef-
fel full of thin Liquor be fo inclined, as to pour it out
flowly, the Liquor will run about and divide itſelf into a is difperfed in
great many diſtinct Drops; whereas if it be a fat Li-
quor, it will go on in a long Thread, whofe Parts are un-
interrupted.
58. This being fuppofed, we fhall not think it at all
ftrange; that Oil or Air is fo hard to mix with Water:
the Reaſon of which is, becauſe the Particles of theſe
Liquors unite together much eaſier than they do with the
Particles of the other: Whence it is, that if Water and
Oil put into the fame Veffel, be fo fhaked up together,
that they ſeem to compoſe but one Liquor, they cannot
continue fo long, before the Particles of the Oil which
meet each other, will entangle themſelves fo as to com-
poſe ſeveral Drops, which becauſe of their Lightneſs, riſe
up, at the fame time that the Particles of the Water, whoſe
Motion
Drops.
58. VVhy
Some Liquors
will not miz
together.
140
Part I.
ROHAULT''s SYSTEM
59. That the
Drops of one
Liquor which
Swim in ano-
ther Liquors
Te round.
60. The Opi-
Ariftotelians
Motion caufes them alfo to meet, join together likewiſe,
and compofe other Drops which fink downwards: And
this is the Reaſon why thefe two Liquors entirely clear
themſelves of each other, and become diftinct, the one
at the Top, and the other at the Bottom.
59. It is worth obferving, that the Drops of Liquors,
which fwim in a large Quantity of other Liquors which
they will not mix with, are all round like Balls. This
cannot be perceived in Drops of Rain as they fall in the
Air, by reafon of the Swiftnefs of their Fall; on the con-
trary, they ought rather to appear long, fo, as we ſhould
take them for fmall Columns; for the fame Reaſon that
a lighted Torch moved quick, appears like a long Train
of Fire. A better Way then for us to take, in order to
fee if the Drops of Water which ſwim in the Air be
round, is to put a little Water into the Hollow of one's
Hand, and to throw it up into the Air, about the Height
of our Eyes; for then it will divide it felf into a great
many fmall Drops, which beginning to defcend very
flowly, give the Spectator an Opportunity of obferving their
Figure.
60. This Phamenon has always been obferved, and a
nion of the Reafen for it attempted to be given, by faying, that the
concerning the Parts of the fame Liquor have a mutual Aff.Etion for each
Roundness of other; whence follows a Defire of uniting together, which
thefe Drops. cannot be done perfectly, but by compofing a Ball, for
61. A Con-
if they compofed any other Figure, thofe Parts which
were moſt diſtant from the Center, would tend towards
it with a greater Force than thoſe which are nearer it, and
confequently make them give way, and remove back till
they are all equally placed about the Center, and fo be-
come round.
61. But becauſe theſe Words, Affection and Defire have
futation of no Meaning, that we can apprehend, unleſs they be afcri-
the Opinion of bed to Subjects which are capable of Knowledge, there-
hans. fore we cannot apply them to the Parts of Water, with-
the Ariftote-
out fpeaking very improperly and obfcurely. Wherefore,
thefe are fo far from explaining a Thing which ought to
be very eafy, (for we are only inquiring into the Figure of
a Body;) that they perplex it with Terms which have no
clear and diftinct Signification when applied to fuch Sub-
jects. Further, let this Defire of uniting be explained how
it will, it is very abfurd to afcribe it to Subjects which
feem naturally to be fitted to difunite from each other, be-
caufe Nature has made them fo capable of diſuniting.
62. In
Chap. 22. of NATURAL PHILOSOPHY.
141
dies which ere
compelled out
than a
cle rather
Tab. III.
Fig. 6.
62. In order then to find out the Cauſe why the Drops 62. That Bo-
of Liquors which ſwim in others, are round, we muſt
keep this Truth in our Minds: That every Thing endea of the way,
vours, as much as it can, to continue in that State, in tend rather to
defcribe the
which it once is, and confequently, that which is in Moti- Circumference
on, would continue to move with the fame Determination of a Circle
with which it began, that is, according to what was before
Streight Line,
faid, in the ſame ftreight Line. Thus, if the Body A, for and the Cir-
Example, is moved along the Line AB, it is determined cumference of
at the Beginning of this Motion to go towards C, and it a larger Cir-
will never of its felf tend to go towards E or towards D. than of a
However, if when the Body is come to B, it meets with Smaller.
any Obſtacle there, it may turn out of the Line BC and
go in fome other Line. But becauſe it is forced out, it
follows, that it will go as little out as it can, that is, when
it quits the Line AB at the Point B, it will tend to move
in à Line which will make the leaft Angle that can be
conceived with the Line BC. And becauſe the Line BD
does not make ſo fmall an Angle with the Line BC as
BE does, we cannot but think, that the Body A tends ra-
ther to move in the Line BE than in the Line BD. And
becauſe the Circumference of a Circle, of which BC is
the Tangent, makes a lefs Angle with BC than any An-
gle comprehended betwixt two ftreight Lines. We muſt
conclude, that the Body A, when it is arrived at the
Point B, will refift turning into the Circumference of a
Circle leſs than into any ftreight Line. Laſtly, Becauſe
it is certain, that the Circumference of a great Circle makes
a lefs Angle with its Tangent, than the Circumference of
a fmall Circle does with its Tangent, we muſt alſo con-
clude, that the Body A, when it is arrived at the Point
B, where it is forced to turn out of its Way, will refift
ftill lefs, the defcribing the larger Circumference BG, than
the ſmaller one BF.
round.
63. This being fo, if the Particles which compofe a 63. Why the
Drop of Liquor, and which are hindred from going on Drops of Li-
in their Motion, by the Liquor which furrounds them, be gors are
compared to the Body A; and all that has been faid of
the Body which made Reſiſtance to it at B, be applied to
the Particles of the furrounding Liquor, which do not.
make fo great Refiftance, but that they can retire back
a little; we conclude, that the Particles of the Drop, do
gradually remove thofe furrounding Particles which get
within the Sphærical Superficies which the Drop may be
com-
142
Part I
ROHAULT's SYSTEM
6. That
Drops, any
way Support-
ed, ought to
be a little
flat.
comprehended under. And becauſe the World is full,
and the Particles which are removed out of their Place,
have no where to go, without removing as many others,
they muſt neceffarily be driven to thofe angular Parts of
the Drop which are without that fphærical Superficies;
and fo the Drop will of it felf become of a round Figure,
though the ſurrounding Liquor contributed nothing elſe
to it, but only not refifting it at all: But becauſe the Par-
ticles of this Liquor, are more hindred from continuing
their Motion in a ftreight Line, by the angular Parts of
the Drop, than by the others which are nearer the Center,
it is evident, that they muſt force them towards the Cen-
ter, and at the fame time make theſe other remove fur-
ther off from it. 2 And in this manner the furrounding
Liquor contributes as an efficient Caufe, towards making
the Drop round. Nay, we may affirm, that it does the
greateſt Part towards it, if, all other Things being alike,
this bemoved with the greateſt Celerity.
64. But it is to be obſerved, that there are two Things
required in order to make Experience agree with this De-
monftration: The Firft is, That the furrounding Liquor
be not more than ufually agitated by any external Force; and
Secondly, That the Drops be not any way ſupported, at
leaſt, when they are of any confiderable Bignefs, for then
their Weight, which is fuperior to the Cauſe which makes
them round, will make themi a little flat, fo that they
will be round only in that Part which is parallel to the
Horizon. As we fee by Experience in Drops of Water
1. The VVorld is full) See the
Notes on Chap. vili.
2. And in this manner) A Portion
of any Liquor, incloſed in another
Liquor, which it does not mix with,
will preferve its Figure, whatever it
be, without any Alteration, if the
Parts of the furrounding Liquor be
at reft, with refpect to each other.
See Nemt. Princip. Book II. Prop. 20.
Cor. 9th.
But if the Parts of the
furrounding Liquor be agitated, the
inclofed Drop muft neceffarily be
compreffed into a globular Figure.
For fince the Superficies of any other
Figure is greater than that of a
Globe, and therefore expoſed to
more Attacks from the Parts with
which it does not mix coming up-
on it on all Sides; and becaufe
whatever is preffed upon on all
Sides, retires thither where it
may be leaft prefled upon; it is e-
vident, that the Parts of the incloſed
Drop, muft gather themſelves into
the Form of a Globe, when they
will be leaft preffed upon. And this
they will do, if there were no fuch
Thing as Attraction. But fince the
Drops of Water and of other Li-
quors, gather themſelves into a round
Figure, in a Vacuum, as well as when
incloſed in any Liquor, the Caufe
of this ought by all means to be af-
cribed to the mutual Attraction which
there is betwixt the Parts of one
and the fame Liquor. (See the Notes
on Chap. xi. Art. 15.) For the Drops
of every Fluid affect a round Figure,
by the mutual Attraction of their
Parts: In the fame manner as the
Globe of the Earth and the Sea af-
fects a round Figure, by the mutual
Attraction of its Parts by Gravity.
Newt. Opticks, pag. 370.
which
Chap. 22. of NATURAL PHILOSOPHY.
143
which reſt upon fuch Leaves of Herbs as they will not
wet, and in thoſe put upon a dufty Table, as alfo in Drops
of Oil or melted Greafe fwimming on Water, which in-
deed are not round, but only on that Part which is level
with the Horizon, for on the other Sides, they are flatter
in proportion to their Bignefs and Weight.
Drops of
65. This laft Obfervation ought to be underſtood only 65. Wh
upon Suppofition, that all Things elfe are alike. For it Quicksilver
is not at all impoffible, but that of two Drops of different are more
Liquors, that which is the moſt heavy, may be the round-round that
Drops of
eft, provided it be alfo the ſmalleſt: The Reaſon of which Water.
is, that all the Particles of the Liquor which furrounds
the Drop, do not help to make it round, but thoſe only
which are applied to the Surface of it; the reft, which
enter into the Pores, ferve rather to diffipate it. Where-
fore a Drop, which is ſmaller and heavier, having its Pores
lefs, and perhaps a lefs Quantity of them than the other,
which is larger and lighter, has alfo its Surface more con-
tinued, and confequently gives more Opportunity to the
Cauſe, which makes it round, to work upon it, and leſs
to that which would diffipate it. Thus we fee, that a
Drop of Quickfilver is always more round than a Drop of
Water a little lighter..
66. On the contrary, Spirits of Wine, being very light, 66.ence is
muſt have ſo many Pores, and the Superficies of it muft is that Drops
of Spirit of
be fo interrupted, that there can be but a very few Vine don'
Particles of the Air applied to it to make it round, the make them-
greateſt part of them pafs through it, and tend to diffi- Selves round.
pate it; alfo this is a Liquor, which it is very difficult to di-
ftinguish into Drops, as may be tried, by putting a little
of it into our Hand and throwing it up into the Air;
for if it be well rectified, it will not fall down in Drops,
as Water does, but it will be fo diffipated by the Air,
that none of it will appear fenfibly on the Ground. So
alfo if it be thrown upon a dufty Table, it will not gather
into round Drops, but fpread it felf about, and mix
with the other Bodies which it meets with, nay even
with Soot it ſelf, which Water will not moiſten.
Bodies and
67. Having thus fhown what kind of Superficies that 67. Vhy a
which is common to two Liquors, the one incloſed in the Liquor will
other, is; it may not be amifs to ſtop a little, and examine moisten fome
what fort of Superficies that ought to be, which is between not others.
two Liquors, the one contained in a Veffel, and the other not :
But becauſe there may be fome Difference in this, according
as the Veffel will be wetted or not wetted by the Liquor
contained in it; it is to be obferved, that a Liquor there-
fore.
144
Part I.
ROHAULT's SYSTEM
68. That the
fore wets a hard Body, becauſe it immediately touches its
Superficies, and that another Liquor does not wet it, be-
caufe it does not immediately touch its Superficies; but
there is room left for the fubtil Matter to paſs between the
concave Superficies of the one, and the convex Super-
ficies of the other.
68. This being fuppofed; we conclude firſt, that if a
the Water in Very clean Glafs, whofe upper Edge is of an equal Height
Superficies of
a clean Glafs all round, be exactly filled with Water, the Surface of the
exactly full, is Water will be perfectly level and plain, becauſe the Air
quite flat.
which touches it, does not prefs more upon one Part than
upon another.
69. That the
which will
wet a Glass,
ought to be
Concave, if
the Glass is
not frill.
Tab. III.
Fig. 7.
69. But if the Glafs be not full of Water, the Superfi-
Superficies of cies ought to be Concave, I becauſe the Air which comes
a Liguor
in at the Mouth of the Glafs, and circulates about the
Glafs and the Water, as if they were one continued
Thing, cannot fo eafily turn to move along the internal
Superficies of the Glafs, as continue its Motion in the
Middle: From whence, being to go out again at the
Mouth of the Glafs, it deſcribes a Curve in a contrary
Pofition, to what it did when it entered in, much the
fame as is deſcribed in the Figure; fo that the Water
is preffed more in the Middle than on the Sides, and confe-
quently muſt riſe towards the Sides.
70. Why the
70. Experience would perfectly agree with this Reafon-
Concave SH-ing, were it not that as the moft convenient Motion for
perficies is not the Air is in a Circle, it fhould feem, that it ought to
Spherical.
71. That the
bend the Surface of the Water into the Form of a Con-
cave Sphere, which yet it does not do ; For the Sur-
face of the Water is curved only towards the Sides, and
is perfectly level in the Middle. But the Reafon is plain;
for if the Glafs be large, a great Quantity of Water muſt
be raiſed up to make the Curvature fo convenient, as
the Water requires, which it is certain is refifted by its
Weight.
71. And for Proof of this; If into a ſmaller Tube of
hollow Sur- Glafs, in which a ſmall Quantity of Water riling at the
Sides makes its Surface fpherical, fome Water be poured,
fo as not to fill it, we may obferve, that it will con-
tinue in the fame manner Spherical, though the Tube be
Tab. III. inclined as you fee in the Figure; where the Curvature
face of the
Water in a
Smaller Tube
not full, is
Spherica!.
Fig. 8.
1. Because the Air) Since all thefe
Phænomena are the fame in a Pacn-
um as in the open Air; we muſt
affert, that the Superficies of any
Liquor contained in any Veſſel is
Gibbous or Concave, according as the
Particles of the Liquor are more or
lefs mutually attracted by each other,
than they are by the Matter of which
the Veflel is made.
ABC
Chap 22 of NATURAL PHILOSOPHY.
145
13
ABC repreſents the Surface of the Water, which is there-
fore above the Level, and manifeftly higher at A than at
C, becauſe that Pofition of the Water agrees better with
the Motion of the Air, which would be more turned
back, and with greater Force in the Place D, if the Wa-
ter were more upon the Level DBE.
alfo
and turned
Fig. 9.
72. The fame Caufe, which hinders the Water from 72. Why a
growing level in an inclined Tube, hinders a Bottle Bottle with a
which has a very ftreight Neck from emptying it felf, when filled
fmall Necks
when it is near inverted,, and the unequal Height of the full of Water,.
two Parts of the Water which endeavour to come out at with the Bot-
the fame time, fhould feem to deftroy the equilibrium of tom upwards,
the Air's Preffure, which repels and fupports it by its will not empty
Weight. For Example; Though in the Bottle here de-Tab. III.
itself.
fcribed, the Height of the Water which endeavours to
come out of the Bottle at C, is greater than of that at A,
and therefore ſhould ſeem to be able to force the Air to
defcend at C, and to rife again by A, and get into its
Place; yet this does not happen, becauſe the Parts of
the Air now defcribe the Curve ABC; and the Dif
ference of the Weight of the Water at A, above that
at C, is fo very ſmall, that it is not able to make the
Aip to defcribe a Line that is more curved, as it muſt
do, if the Water which defcends by C, took up part of
the Width of the Neck.
73. If a little more Water be poured into a Glafs of the 73. That the
common Shape, than will fill it exactly full; as that which Superficies of
would run over the Sides, is more expoſed to the Power when the
a Liquors
of the Air than any other Part is, it follows, that the Glass is heap-
Air ought to push it back towards the Middle, where it ng fulls ought
す
​ought to be higher, in order to its more convenient Mo-
tion. And thus we fee that a Glafs may be filled heaping
full, and that the lefs the Glafs is, the nearer does the Su-
perficies of the Liquors it contains approach to a Sphere;
becauſe it does not faſtain the Weight of fo great a Quan-
tity of Water, and the Force of the Air is fufficient to
bend it in this.
to be convex.
#
74. If the Glafs be greafy, or for any other Reafon will 74. That the
not be made wet, whatever Quantity of Water be put in-
to it, the Superficies ought always to be convex, be-
caufe its Figure does not fo much depend upon the ex-
ternal Air, as upon the Air that flows between the inter-
1. The Superficies ought always)
Thus the Superficies of Quickfilver in
Glafs Tubes, is always gibbous, be-
cauſe it does not wet the Glafs, but
in Vellels of Gold that are not full,
its Superficies is concave, as that of
Water is in Glafs. See the Notes on
Art. 69. above.
Water, in a
Superficies of
Glafs not full,
and which
wetted, ought
will not be
alſo to be con.
vex.
L
nal
146
Part I.
❤
ROHAULT's SYSTEM
are carried
from the Mid-
dle to the
Sides.
nal Parts of the Glafs, and the external Parts of the Li-
quor which it contains; which by its continual moving
round, blunts the external angular Parts which refift its
Motion, and forces them towards the Middle, or elſe
forces them inwards, and fo caufes the Water to raiſe it
felf up towards the Middle, where the Air oppofes its Paf-
fage lefs, becauſe it cannot get thither, but by altering and
bending its Courſe.
75. Why Some
75. From what has been faid in the two foregoing Ar-
Bodies float- ticles, we infer, that the Air which depreffes the Middle
ing on the Top
of the Water, of the Superficies of the Water in a Glaſs not full, ought
from the fame Caule, to drive light Bodies which fwim
upon it, and touch it immediately, towards the Sides:
This I have experienced in fmall Globules of Glafs full of
Air and cloſed up, which an Enameller made as light for
me as he could; for thefe being put towards the Middle
of the concave Superficies of the Water in a ſtrait
Glafs not very full, it was very pleafant to fee them dri-
ven from thence to that Side of the Glafs which was near-
eft to them.
76. That this
cauſed by At-
traction.
76. Becauſe I made ufe of a ſmall Globule of Glafs, and
Motion is not a Veffel of the fame, in this Experiment; fome Perfons
perhaps, may imagine, that this Globule moved towards
the Side, becauſe it was attracted by the Glafs: But it is
very eaſy to confute this Imagination; for not to men-
tion the Obſcurity of that Word, the fame Thing will
happen in a Veffel of Wood, or of any other Matter what-
foever, I which we cannot ſuppoſe to have any Sympathy
with the Globule.
77. That the
Middle in a
Glaſs heaping
full.
77. But that which evidently overthrows this Opinion,
fame Bodies and confirms that which I have advanced, is, that if At-
ought to go
from the Sides traction had any Thing to do here, the Globule ought to
towards the move ſwiftly from the Middle to the Side of a convex
Superficies of the Water in a Glaſs heaping full; for
befides the Attraction, the Declivity ought to help its Mo-
tion. Which yet is not fo; but on the contrary, it moves
from the Side towards the Middle, as it ought to do, if
what I have affirmed be true; becauſe, as was faid be-
fore, it is the Sides which are moſt expoſed to the Force
of the Air, and the fame Cauſe which drives the Water
from the Sides to the Middle, ought alfo to drive the ſmall-
Globule.
1. F'Vhich we cannot ſuppoſe) See the Notes on Chap. xi. Art. 15.
78. But
Chap. 22. of NATURAL PHILOSOPHY.
147
1
muft Body that is
muft Water, when
move Swimming on
Body. Water, does
heavier than
the Top of the
to a [mall
Globule of
78. But it is to be obſerved in theſe Experiments, that 78.Vya
the Body which floats on the Top of the Water,
immediately touch it, or which is the fame Thing,
be wetted by it, that the Air may be forced to
round them both, as if they were one continued
But if the Body which floats on the Water does not im- the contrary
mediately touch it, or is not wetted by it, we experience
the contrary; that is, the Body will defcend from the Sides Glass.
towards the Middle, when the Superficies of the Water is
concave, and from the Middle towards the Sides when the
Superficies is convex, becauſe the Parts of the Air which
paſs under the Body deprefs the Liquor all round, which
produces the fame Effect, as if, when a large heavy
fpherical Body was fixed upon the Declivity of a Moun-
tain, we ſhould take away the Earth equally all round
it, and put Leavers under it to fupport it; for it is evi-
dent, it would by that means be difpofed to defcend to the
Bottom of the Mountain.
79. It is to be obſerved further, that when a Body 79. How fuch
which weighs more than an equal Bulk of Water fwims Bodies as
theſe, may
upon the Water, as a Needle made of Steel will do, the float upon the
Reaſon of it is this; that the Air which preſerves it felf Water.
a Paſſage between the Body and the Water, fupports it
and hinders it from finking: For we ought not to think
that it proceeds from hence, that the Parts of the Water
are harder to be ſeparated near the Superficies, than deeper
in, as we may be apt to imagine; for having cauſed fome
fmall Needles to be made of Glaſs, which were lighter
than the Steel Needles of equal Bignefs, and laid them
gently upon the Water, they always funk down to the
Bottom.
1
80.V Fhy Li-
quors fome-
times rife up
on the Sides
80. From hence, viz. that the Body dipped in the Wa-
ter will be moiſtned, or not moistned, it follows, that the
Water will rife up on the Sides of fome Bodies higher
than it is any where elfe, or that it will be depreffed low- of fome Bo-
er; The Reaſon of the Firſt is, becauſe the Air which dies that are
dipped in
moves from one Side of the Veffel to the other, and paf- them a little
fes over the Body, permits the Liquor to rife in that way.
Hollow which the Air cannot without great Difficulty turn
into: whereas when it paffes under, as in the Second
Cafe, it depreffes the Liquor all round. And of this a
Multitude of Experiments may be made, and an infinite
Number of them are made without any Notice being ta-
ken of them; for every time we dip our Pen into the
Ink, we may obferve, that if it be moiftned, the Ink will
rife;
In 2
148
Part I
ROHAULT's SYSTEM
81. Why the
Water will
rife confide-
rably in the
Part where
two Pieces of
Glafs are fit
ted to each o-
ther, when
they are dip-
ped a little
into it.
?
rife; and on the contrary, that the Ink is depreffed about
the Pen if it is not moilt.
#
If two plain Bodies which the Water will wet, fuch
as two Pieces of clean Glaſs, be put very near one ano-
ther, and dipped a little way into a Veffel of Water; the
Air which moves from one Side of the Veffel to the o
ther, in order to get over the Obftacle that lies in its
way, ought rather to pass over the Top of the two Glaf
fes, than to defcend into that ftreight Place which is be-
tween them: So that the Water is not fo much preffed
here as it is in other Places, where the Air can go with-
out bending its Courfe fo much, and fo it ought to riſe
to a confiderable Height above the Level of the Water
contained in the Veffel; and thus we fee by Experience
that it does.
82. Why the
82. And there is no Doubt but that the Water would
Water is feen rife ftill higher, if the two Pieces of Glafs were clofed
to rife of it
on both Sides, for by that means almoſt all the Air which
felf in small
Glafs Tubes. moves crofs, without bending its Courfe, would be hin-
dred from entring in. Or, which is the fame Thing, we
may take a very fmall Glafs Tube open at both Ends, and
dip it in the Water, for then the Air cannot enter in by
the Sides; fo that the Water muft rife very high in fuch
fort of Tubes, if they be very flender: And indeed I have
made the Water rife a Foot high in a Glafs Tube fo fmall,
that one could fcace get a Horſe-hair into it.
83. Why it
on without
End.
83. However, we must not conclude from hence,
does not rife that it ought to rife on without End in theſe ſmall Tubes;
for it is eaſy to fee, that the Water muſt ſtop, when the
Weight of that which is rifen, tends downwards with
greater Force than the Preffure of the external Air has to
thruft it up.
84. If the Tube be inclined, a greater Quantity of Wa-
84. That a
greater Quan- ter will get in, becaufe, being fome way fupported by
tity of Water the Glafs, it does not tend downwards with fo great
ought to rife
in an inclined Force. Which is confirmed by Experience, according to
the moſt exact Laws of Mechanicks.
1
Tube.
85. Having now explained the Force of the Air as a
85. Why the
Water rifes Liquid to impel Bodies which are cloſe to it, we may
Sometimes
higher in the fay with more Affurance and Certainty than we could
Smaller, than before, what the Situation of a Liquor in an inverted
in the larger Syphon, whofe Branches are of an unequal Thickness,
Arm of one as is here repreſented, will be. For Example, if we con-
phon. fider only its Weight, we may confidently affirm, that
inverted Sy
Tab. I.
Fig. 4.
1. The Air which moves) See-the Notes on Art. 85, of this Chap.
Chap, 22. of NATURAL PHILOSOPHY.
149
if the Water in the larger Branch, reaches up to the
Height AB, it ought to rife to the Height C in the little
Tube, to be upon the Level with the other: But we may
add, that if this Branch be ſo ſmall, that the Parts of
the Air cannot turn in it but with Difficulty, the Water
will rife confiderably higher than in the larger Branch,
fo as to reach to D, according to what was now
proved.
tual Motion.
86. There are few of thofe who enquire after a per- 86. An ima-
petual Motion, but when they fee this Experiment, for ginary perpe
want of rightly underſtanding the Caufe of it, think
they have found out fuch a Motion. And indeed it
looks at firft Sight very probable, that if we take one
of theſe Syphons, in the fmaller Branch of which the
Water rifes very high, and bend this Branch a little lower
than the Height which the Water rifes to, it might be
fo ordered, that the Liquor with which it is filled might
run out into the larger Branch, in order to rife up a-
gain in the ſmaller one, and fo produce a perpetual Mo-
tion: But it is certain, that 2 they are deceived who
make this Conjecture; for beſides that, the Branch of the
Syphon, out of which the Water is to run, ought to be
longer than the other, (which is not fo here, where the
bent Branch is in the Room of a whole Syphon) it is
easy to fee, that the Water, the Moment it endeavours
to come out at the End of this ſmall crooked Branch,
is more expoſed to the Force of the Air, than that which
is contained in the larger Branch; whence it follows, that
its Paffage out muſt be ſtopped.
87. This will appear more evident, if we confider, that 87. That in a
when the End of the fmall Tube of a bent Syphon, Syphon the
Tube of which
whofe Height does not exceed that, to which the Wa- is very small,
ter will commonly rife, be dipped into the Water, it will the Vater
immediately be filled; but if the End of the longer Branch
1. The Parts of the Air) It
looks very probable, at firft Sight,
as if the tiff Particles of the Air,
either paffed over the Mouth of
the little Tube CD; or elſe flick-
ing in it, like little Pieces of Wood
a-crofs it, fupported the Column
of incumbent Air, fo as it ſhould
not prefs upon the Water under
it, with its ufual Weight: But by
often repeated Experiments, it is
found, that the Water will rife as
high in fmall Tubes, though the
grofs Air be exhauited. See The
will not al-
ways ran ont
through the
longer
Exper. of the Academ. del Cimento, Brauch.
p. 55. It is evident therefore, that
all thefe Phænomena's are to be af-
cribed to Attraction. See the Notes
above on Art. 69.
2. They are deceived) It is ma-
nifeft, from Calculation upon Me-
chanick Principles, That all Que-
ftions about a perpetual Motion end
in this. To find out a Weight bea-
vier than it felt, or an elaftick Force
ftronger than it felf; Which is
abfurd.
L 3
be
150
Part I
ROHAULT'S SYSTEM
88. A curious
Air.
be not depreffed lower than ufual beneath the Level of
the Water in the Veffel, it will not run out into the
Air, as it ordinarily does; whence we fee, that the Air
puſhes it back with greater Force than it has to come
out.
88. For a further Confirmation of a Thing which has
Experiment been fufficiently proved, I may add, that fo far is the Wa-
of the Pref- ter from coming eafily out at the End of a ſmall Tube,
fure of the
that fometimes it will be forced to enter and afcend into
it, when it was entirely without before: Which may be
tried, by holding a very clean fmall Tube open at both
Ends perpendicular, and putting a Drop of Water upon
the external Superficies, which may entirely ftop the Hole
at the lower End, when it is got down thither, for then
you will with pleafure fee the Tube filled in the fame
manner as if the End of it was dipped in a Veffel of
Water.
$9. VVhat
Filtration is.
89. After what has been faid in the foregoing Articles,
the Cause of it is eafy to underſtand what is the Caufe of the Filtra-
tion of the Chymifts: For the Piece of Woollen Cloth
which they put upon the Side of the Veffel, in fuch a
manner, as that one End of it is dipped into the Liquor,
and the other End hangs down on the Outfide lower in
the Air, reſembles a bent Tube, in which the Water runs
as in a Glafs-Tube: And it matters not, if this Cloth or
Woollen Tube be full of Holes on all Sides, for the Air
which moves round it, preffes in the Water which en-
- deavours to come out at them, fo that it is like one con-
tinued Covering.
90. That the
Forms of hard
and liquid
Bodies as
fach, are not
fubftantial
Forms.
91. VVhat
Dryness and
Moiftness is.
any
90. Since our Thoughts, or, if you will, our Conjectures
concerning hard and liquid Bodies are confirmed by fo
many Experiments, I think it fuperfluous to add
Thing more. Wherefore I fhall finish this Chapter, in on-
ly remarking two Things: The Firſt is, That if Hardness
and Liquidness confift in Reft and Motion, which have
their Dependence upon fomething elfe; then theſe
Forms are not Subftantial, but only Qualities or Modes
of Exiſtence in the Bodies to which they belong.
91. Secondly, That having explained the Nature of
Hardness and Softness, I have at the fame time explained
wherein Dryness and Moiftness confift. This is evident,
if we underſtand the Word Dry and Moist in the Senfe
of the Antients, who did not diſtinguiſh them from bard
and liquid: As we may fee from hence, that fpeaking of
Moift, they uſe the fame Greek Word as all Interpreters
render bumid or liquid indifferently. It appears further,
thats
Chap. 23. of NATURAL PHILOSOPHY.
ISI
that I have explained what the Nature of Dryness and
Moiftnefs is, according to that Senſe which we now uſe
thofe Words in; becauſe by Dry, we underſtand that which
will not wet any Thing; and by Moift, that which will
wet a Thing, which are two Properties which have been
fully and expressly handled above.
THE
CHA P. XXIII.
Of Heat and Cold.
1. That the
Vlords, Heat›
and Cold,have
HESE Two Words have each of them two dif
ferent Meanings For Firſt, by Heat and Cold, we
underſtand two particular Senfations in us, which in fome two different
Meaſure reſemble thofe, which we call Pain and Plea- Meanings.
fure, fuch as we feel, when we touch Ice, or when
we go near a Fire. Secondly, by Heat and Cold, we un-
derſtand alſo the Power which Bodies have to raife the
forementioned Senſation in us.
2. In what
Senfe it is,
2. I think we cannot underſtand what Heat and Cold,
in the former Senſe of the Words, is, but only by Expe- that we pro-
rience; wherefore our Curiofity will be fatisfied, and our pofe to treat
Pains imployed only in enquiring what that Power confifts of Heat and
in, which certain Bodies have to warm us, and alfo what
that Power confifts in, which we obſerve other Bodies have
to cool us.
3. Ariftotle fays, that Heat is that which collects to-
gether homogeneous Things, or Things of the fame Na-
ture, and diffipates heterogeneous Things, or Things of a
different Nature; and Cold, he ſays, is that which collects
together, Things homogeneous and heterogeneous indif-
ferently. The common Inftances made ufe of to prove
this, are Fire, by the Heat of which, a great many Parts
of Gold may be collected into one Maſs, or two or more
Metals which are mixed together, may be feparated:
And Ice, which by its Coldness, unites together, Water,
Stones, Wood, Straw, fo as to compofe one Body of all
theſe together.
Cold.
3. How Ari-
ftotle de-
and Cold.
fcribes Heat
4. But it is to be obſerved, that the Inftance here gi- 4. That Heat
ven, is fometimes faulty; for if a Maſs, compofed of Gold, colles toge-
ther Things of
Silver, and Copper, be put upon the Fire in a Crucible, a different
it is not true, that theſe Metals will always clear them-
ſelves of each other, fo as to be feparated and placed in
L 4
Nature, as
well as those
of the fame
their Nature.
1152
Part I.
ROHAULT''s SYSTEM
5. That the
Property of
Fire, is ra-
ther to diffi-
pate than to
aollect toge-
ther.
6. That Ari-
Heat and Cold
their proper Order, one upon another, according to
their different Weight. On the contrary, if feveral
diftinct Pieces of Gold, Silver, and Copper be put toge-
ther into a Crucible, the Fire will not fail to mix them
all together.
5. It is true, that if the Fire acts a very long time up-
on a Maſs, compofed of Gold, Silver and Copper; the
Silver and Copper will go all away in Smoak, and fo leave
the Gold alone in the Crucible. But we ought not for
this Reaſon to ſay, that the Fire has a Property of col-
lecting Things together, becauſe this perhaps is only ac-
cidental, that is to fay, by diffipating the Firſt, which
refifts its Force lefs, the Gold remains alone, or laſt, be-
cauſe it refifts its Force more. In the fame manner, as
if Saw-Duft, and the Filings of Lead were mixed toge-
ther in a Plate, we can with our Mouths blow away the
Saw-Duft, and leave the Lead-Filings alone in the Plate.
For it is evident, that it is only the Refiftance of the
Pieces of Gold, which is the Cauſe of that Metal's being
thus feparated from the Silver or Copper. For if it be
left after this upon the Fire, it will continually diminiſh
by little and little, till it intirely vaniſhes, as Refiners have
tried; and this is what they mean when they fay, there is
no Gold of 24 Carats, that is, none that can be refined
fo pure.
6. But if it was true, that Heat always collected toge-
totle has on- ther homogeneous Things, and diffipated heterogeneous
ly faid what
ones, and that Cold collected together all fort of Bodies
do, but not indifferently, this would indeed teach us what Heat and
what they are. Cold do, but not at all tell us what they are: But Ari-
ftotle has been excufed in this, by faying, that in defining
Heat and Cold as he has done, he did not fo much follow
his own Opinion, as that of others.
7. What the
Cold is.
7. I don't know whether his Interpreters have hit right,
Opinion of his when they pretend, that his Opinion was; that Heat, in
Interpreters the Fire, for Inftance, is fomething in the Fire like that
concerning
Heat and
Senfation which is raiſed in us, when we approach the
Fire. And fo likewife, that Cold in Ice, is ſomething in
Ice very
like that Senfation in us, which arifes from touch-
ing it. i
Becauſe in his II. Book of the Soul, Chap. xii.
after he had shown that Senfation is a Paffion, he ſays,
that the Moment any Senfation is rais'd in us, we be-
come like the Object that raifes it.
1. Because in his II. Book) This
Place is not in that Chapter, but in
the vth Coap. of the fame Book, he
fays, πάχει μέν γὰρ τὸ ἀνάμοιον
ὄχι πεπονθὸς ἢ ὁμοιὸν ἐστιν.
8. But
Chap. 23 of NATURAL PHILOSOPHY.
753'
have no Foun-
8. But whether Ariftotle were of this Opinion or no, 8. That they
thus much is certain, that they have no Proof of what dation for
they affirm; for it is no Proof to ſay as they do, that their Opinion.
the Fire cannot give that which it has not; becauſe
taking the Word give, in the Senſe here uſed, there is no
doubt but that the Needle, when it pricks us, gives uș
Pain, and yet there is no reaſon to believe from hence,
that the Needle has in it any Pain like that which it
cauſes in us.
falfe.
9. Further, the Heat of the Fire, and the Cold of 9. That it is
Ice being Properties or Qualities belonging to Bodies abfolutely
which every one acknowledges to be inanimate, they
cannot be like the Senſations which we feel by their Means,
becauſe theſe Senſations belong to us as animate Creatures,
And becauſe the fame Thing may fometimes happen to
raife in us two different Senſations at the fame time, it
will follow from their Opinion, that the fame Thing may
be hot and cold at the fame time, which is impoffible;
yet the Air which we breathe out of our Mouths, may
at the fame time feel hot or cold according as it is dif-
ferently applied to our Hands in blowing upon them.
bot Bodies
10. By reflecting upon this Experiment, which fhows 10. In what
us, that the fame Air feels hot or cold, not only from its the Heat of
being applied in a different manner to our Hands, but confifts.
alfo from the different manner of making it come out
of our Mouths; it is eafy to conjecture, that the Heat of
a Body confifts in a peculiar Motion of its Particles. And
becauſe the nearer we put our Lips together, and make
the Air come out quicker and ſtronger, the lefs we feel
the Heat, hence we conclude, that the Heat of a Body
does not confift in the direct Motion of its Parts. Now
whatever is in Motion, either moves on directly, or elfe
has an unequal and different Motion, as it were about
its own Center; from whence we may infer, that the
Air which comes out of our Mouth, befides that direct
Motion, by which the Whole of it is removed from one
Place to another, it has alſo a great many of its Particles
moved round with a circular Motion about their own
Centers: By which means thoſe which are applied to our
Hands, with this fort of Motion, excite in us a kind of
Tickling. And becaule it is this kind of Motion which
raiſes in us the Senſation of Heat, we ought alfo to con-
clude, that the Heat of Bodies confifts in this Sort of Motion
of their Small Parts.
II. SQ
£14
Part I.
ROHAULT'S SYSTEM
11. The Re-
femblance
there is be-
twixt Heat
end Pain.
}
12. That Bo-
come hot, to
which it is
11. So that what is in the Object is very different from
the Senſation which it raiſes. And this ought not to be
thought more ftrange, than the Difference there is be-
twixt the Figure and Motion of a Needle, which pricks
us, and the Pain which it caufes. For as it is evident
from the Inſtance of Pain, that the Soul being united to
the Body, it is the Appointment of Nature, that certain
Perceptions of the Soul fhould follow from certain Mo-
tions or Diviſions which the Needle caufes in the Body:
So alſo we ought to think, that Nature has appointed
that from that particular Manner in which our Body is
moved by the Fire, there fhould arife a particular Percep-
tion, and this is what we call Heat, taking it in the former
Senſe of the Word.
12. This is confirmed by Experience, which teaches
dies may be- us, that many Bodies are made capable of warming us,
to which we cannot fufpect any Thing has happened but
only Motion. It is to no purpoſe to inftance in them
I fhall content my felf with the following Ex-
certain, no-
thing has
happened but
Motion.
13. The I.
Example.
14. The II.
Example.
*5. The III.
Example.
6. The IV.
Example.
17. The V.
Example.
all :
ample.
:
13. And, Firſt, It is certain that when our Hands are
very cold, we find by Experience, that if they be rubbed
a little while together, we shall feel a confiderable
Heat.
14. Secondly, As was before obferved, Lime having cold
Water poured upon it, though it was before cold, will ac-
quire fuch a Motion of its Parts, that they will be all
difunited in a fhort time, and by that Means will be-
come capable of heating us in fuch a manner, that it will
be very painful to hold it in one's Hand.
15. Rotten Dung, that is, fuch as diffipates it felf by
little and little, becomes fo hot, as to ferve inftead of a
moderate Fire in many Chymical Operations. And Chy-
miſtry furniſhes us with many other Examples not fo com-
mon, which ought to be more known to the World than
they are.
16. For Inftance, if a few Filings of Brafs be thrown
into a large Veffel in which is a little Aqua-fortis, it will
immediately raiſe fuch a Fermentation, that the Bottle
will feem quite full, and at the fame time will be ſo hot,
that we cannot touch it without being burnt.
17. Further, If, as was before faid, Oil of Vitriol and
Oil of Tartar be mixed together, though feparately nei-
ther of them are combuftible, they will immediately ac-
quire an incredible Fermentation on a ſudden, and at the
fame time a very fenfible degree of Heat.
18.It
Chap. 23. of NATURAL PHILOSOPHY,
155
18. It is true, that in theſe Sort of Examples, it may 18. The VI.
with fome Reaſon be faid, there is fomething that we do Example.
not throughly understand, wherefore I fhall ſtay a little,
before I fay what the Cauſe of theſe furprizing Motions
may be: To come therefore to fome more familiar In-
ſtances, we obferve, that two hard Bodies rubbed against
one another, do ſo agitate the Parts of each other, as not only
to burn us when we touch them, but their Motion
will increaſe to fuch a Degree, as to fet each other on
Fire. Thus in very dry Weather, the Wheel and the
Axle-Tree of a Chariot, when it goes very quick, and in
general, all Sorts of Engines which are made of Matter
that will burn, and which move very quick, are apt to
take Fire. Nothing is more common, than to ſee a
Wimble grow hot in boring a Hole in a hard thick Piece
of Wood. So likewiſe, if we file or harp a Piece of Iron
or Steel it will grow fo hot fometimes as to loſe its Tem-
per. And a Saw, which the Wood will not eafily yield
to, acquires a very notable Heat. But nothing fooner
takes Fire than a ſmall Piece of Flint or of Steel, which
is ftruck off, and put into a violent Motion by ſtriking
theſe two against each other. Now in all thefe In-
ſtances, there is nothing added to thefe Bodies but Mo-
tion.
Heat.
19. All the Antients who have conſidered the greateſt 19. An E
Part of theſe Experiments, have afferted that Motion is plication of
the Opinion of
the Principle of Heat; which I acknowledge with them the Antients
to be true; if by Motion they mean the Motion of the concerning
whole Bodies, which is the Cauſe of the two Bodies rub-
bing against each other; but if by Motion they mean the
Motion of their infenfible Parts, I think they have not faid
enough: For the Motion of theſe Parts, is the very Heat it
felf of thofe Bodies.
20. I fee no Objection that can be made againſt this:
For when they object, in order to fhow, that Motion is
not the Principle or Cauſe of Heat, that a Ball out of
a Cannon which moves very quick, does not burn the
Wood which it enters into; or that a Musket Bullet does
not burn the Wood which it penetrates, though it be very
dry; this contradicts the Opinion of thofe only who pre-
tend that Heat confifts in the Swiftneſs of the Motion of
all forts of Bodies how grofs foever. But this Objection
makes nothing againſt us, who affirm, that Heat conſiſts
in the different and violent Agitation of the infenfible
1: The Frinciple er Caufe of Heat) Is not the Heat it felt.
Parts
20. Why a
Cannon Ball
which moves
very quick,
does not grew
bot nor burn
LSS
Part L
ROHAULT'S SYSTEM
21. Why the
Nave of a
Wheel grows
'hot', and not
the Fellows
72. VV by a
Piece of Iron
when filed,
Parts of Bodies. But when a great Bullet moves very
quick, its Parts may be at reft with refpect to each other,
and therefore it is no wonder that they don't burn the
Bodies which they touch.
21. If we reflect upon what has been faid, we fhall
not at all wonder, that the Bands of Iron which are about
for
a Wheel do not grow hot as it does in the Middle,
though they defcribe larger Spaces by their Motion, yer
notwithſtanding this, their Parts are not agitated with re-
ſpect to each other, as thofe in the Middle are, which con-
tinually rub against the Axle-Tree.
J
22. We may very easily answer a great many Questions
which may be put to us by thoſe who will not allow, that
grows hot, but the Form of a hot Body confifts only in the Motion of its
ot the File. fmalleft Parts: Thus when they ask, how it is poffible,
that when a Piece of Iron fixed in a Vice, is filed, the Iron
grows confiderably hot, but the File which moves upon it
is fcarce warm at all: It is eafy to anſwer, that the Parts
of the File moving upon the Iron, and continually grating
it, not only with its own Parts, but alſo with fome of
the Parts of the Iron which it has rubbed off, and which
remain fometime between its Teeth, muft neceffarily ex-
cite a very great Agitation of the Parts of the Iron
which is filed, and confequently heat it very fenfibly. But
this is not the Cafe of the File; for though its Parts are
grated as much as thofe of the Iron, yet becauſe it is
longer, the fame Teeth do not twice together touch the
Body which it grates, but there is always fome fmall
diftance of Time, between the two Rubs of the Parts
of the File, during which Time, that Place which may
have begun to acquire fome fmall Heat, may loſe it
again.
23.VVhy Iron
when it is
filed grows
Botter than
23. There are fo many Things to be confidered in this
Experiment, that a fmall Difference alters all the Circum-
ftances. Whence it is, that a Piece of Copper or Lead,
șther Metals. When it is filed, ought not to grow fo hot as Iron, both
becauſe Copper and Lead are not fo ftiff, and becauſe it
is eaſier to ſeparate their Parts than the Parts of Iron,
fo that the File being never applied twice together to
the fame Part of the Body which it grates, it cannot
fhake its Particles fo much: And this is fo true, that if
we try to file a Piece of Copper, with an old worn File,
which will fhave off but a little at a time, the Heat will
be as great as that produced in the Iron.
1
24. Now
]
Chap 23 of NATURAL PHILOSOPHY
257
Saw grows
24. Now if any one asks, why, in fawing a Plank of 24 VVhy a
Wood, the Saw grows hot and not the Wood: I anfwer, hot, and not
that the Plate of the Saw, fticking in the Slit of the the Vood
Wood, and being rubbed againſt each Side, the Parts of
it muſt be fenfibly fhaken: Whereas it is evident, that
the Plank ought not to grow hot in the Place againft which
the Teeth of the Saw go, for the Reafon juft now given,
viz. becauſe it cuts the Parts off; neither ought it to
grow hot on the Sides, efpecially if the Wood be eafy to
faw, becauſe the Saw advances further and further into the
Slit, and fo does fcarce twice together touch the fame Part
of the Wood.
Flood when
Sawn may
25. It is true, that if the Wood be very hard, and dif- 25. How the
ficult to faw, and if the Saw fticks in the Slit which it
makes, the Plank will then become pretty hot; but we grow hot
fhall not be able to perceive it by our Touch, becauſe
the Parts of the Wood being large, lofe their Motion in
a Moment, and it will take fome time to pull out the
Saw, and to open the Slit fo wide as to put our Hand in
to feel. But though we cannot perceive it by our Touch,
we may fee it with our Eyes; for the Places againſt
which the Saw for fome time grated look burnt, as if
they had been in the Fire. And it happened fome
time ago, that defignedly fawing a Piece of hard Wood,
fixed in a Vice, in a Smith's Shop, with a Saw which
ftuck in the Slit it made, I at firft perceived a Smell
like burnt Wood, and continuing to faw the Wood with
greater Force, feveral Sparks came out of it.
26. VVhy a
Nail driven
into a Piece
does not grow.
26. The Experiment which feems to be the moſt con-
trary to the Principle we have laid down, is, that if we
drive with a Hammer a large Nail into a piece of hard ofood with
Wood, we fhall not find it grow warm while it is dri- 4 Hammer
ving in, but after it is in, and the Hammer does nothing hot.
elfe but beat the Head flat, then it will begin to acquire
fome Heat: Yet is there nothing in this, but what per-
fectly agrees with our Notion of Heat. For as we make
it to confift wholly in the Agitation of the fmall Parts
of the Body; it is certain, that the Nail ought not to
grow hot, while it is moved all together in entring into
the Piece of Wood; but that it ought then to begin to
grow hot, when it ceaſes to move fo, and its Head begins
to be made flat; for it is then only that the fmall Parts
begin to be in Motion, and acquire an Agitation fuffici-
ent to Heat. And indeed, when the Head of a Nail is
made flat, all that is done, is, that there are by that Means
fewer Parts placed one upon another, and more by each
other's
158
Part T
ROHAULT'S SYSTEM
27. That
other's Sides, which cannot be, but by the Motion
and Agitation of thefe Parts, which by their beating
againſt each other, cauſe that trembling in which Heat
confifts."
27. Having thus endeavoured to anſwer the Objections
Flame ought
to be very hot. that might be made againſt us; we come now to draw
fome Confequences from what we have laid down; be-
caufe if thefe agree with Experience, they will help to
confirm us in this, that we are not far from the Truth. In
the firſt Place then, let us confider, that feeing Heat con-
fifts in a certain Motion, or a certain Agitation of the
fmall Parts of a Body, it is certain, that the more the
Parts of the Body are thus moved or agitated, the greater
will the Heat be. Now it is evident, that Flame is
more agitated than any other Body which comes under
our Senfes. For, for Example, it is this violent Agitation
of the Parts of the Wood which nourish the Flame,
that makes the greateſt Part of them fly away, and that of
all the Wood that can be burnt in a Day, ſo very little re-
mains in Aſhes ; which we do not find in the foremen-
tioned Inftances, where there is only a moderate trem-
bling of the Parts of the Bodies which is not ſufficient
to dilunite them entirely. And this is the Reaſon why
Flame ought to be the hotteft' Thing in the World, as e-
very Body knows it is.
28. How a
agitated as
Flame may
yet be hotter.
28. However, this muſt be underſtood with fome Re-
Body that is ftriction, that is, if they agree in all other Particulars;
not so much
for it is not inconfiftent herewith, that there fhould be
fome Bodies hotter, and more capable of heating than
Flame, if they confift of more folid Particles, and
confequently fuch as are more capable of Agitation ;
wherefore Iron, tho' it be not red hot, will burn more,
if we touch it, than the Flame of Straw, or Spirit of Wine.
will do.
burn more
than
ther.
any o-
29. VVhy
29. The Difference that there is betwixt the Groffneſs
Sea-Coal will of the Particles into which the Bodies that are burnt are
refolved, is the Cauſe of ſo much Difference in the Flames.
Thus, Oak being more folid than Straw, but not ſo ſo-
lid as Sea-Coal; their Flames are alfo proportionably more
or lefs burning or ſtrong one than another: And the Ufe
that Smiths make of them, according as they have oc-
cafion, fhows plainly, that Sea-Coal, acts more ftrong-
ly than all other, becauſe when they would heat a
1. Flame is more agitated) Con- |
cerning the Nature of Flame and
Fire, See Part the IIId. and the
whole ix Chap, with the Notes.
Piece
Chap. 23. of NATURAL PHILOSOPHY.
159
Piece of Iron very much, they prefer this Coal to all
others.
30. When a Body melts, and liquifies, as I may
may call
it, by little and little into Flame, it is impoffible but that
the Particles which flip and rub one againſt another, muſt
be diminished and broken into a thouſand Pieces, and fo
make a very fine Duft, which, that it may continue to
move with that violent Agitation which it has acquired,
gets off from that Mafs of which it was before à Part,
and flies into the Air; which is what we call exha-
ling or evaporating: And hence it is, that the Fire
has the Property of diminishing all Bodies which it acts
upon.
ry
a
31. This being allowed, there is no Difficulty in re-
folving that Queſtion commonly asked, viz. How it is
poffible that Heat fhould produce at the fame time two
feemingly contrary Effects: Such as hardning of Clay,
and foftning of Wax. In order to this, we need only ob-
ferve, that Clay is compofed of two Things that are ve-
different from each other, viz. Earth and Water; the
Latter of which may very easily be evaporated, before
the Particles of the Former are confiderably fhaken; and
fince the Clay is ſoft for no other Reafon, but becauſe the
Particles of the Water are in fome fort of Agitation, a-
mongſt the Particles of the Earth, to which they be-
long; it muſt needs be, that when the Water is all eva-
porated, and the Particles of the Earth remain alone,
they will reſt againſt each other, by their own Weight,
and ſo by that means compofe a hard Body. On the con-
trary, the Parts of Wax are pretty near equal; fo that
the groffer Particles are agitated before any confiderable
Quantity of the ſmaller ones can fly away. And therefore
all the Particles of a Piece of Wax being a little in
Motion at the fame time, compofe together a foft
Body.
30. How
Heat deals
and dimi-
wishes the
Bulk of them.
with Bodies,
31. VVhy
Heat hardens
Clay and fof-
tens V Fax.
32. That the
Heat needs
not be very
32. It may be obſerved alfo, that the Heat must be
but moderate, to harden Bodies: For if it be very vio-
lent, it will make them liquid. And thus we fee, that great, to har-
Flame melts not only Metals, but alſo Aſhes, Sand, Stones, den Bodies.
and Flints, of a Compofition of which all Sorts of Glafs
are made.
33. From the different Degrees of Heat, and the va-
rious Texture of the Parts of which a Body is compo-
fed, we may conclude, that very different Effects will be
produced: For firft; If a Body, whofe Particles are very
clofe to one another, be confiderably hot, whatever the
Figure
33. How Heat
Bodies!
rarifies fome
100
.: Part I
"i
ROHAULT's SYSTEM
}
34. How it
condenfes o-
thers.
1
35. VVhy
VVater, when
it is very near
rarer than
when it is not
fo cold.
Figure of thefe Particles be, fo they be not exactly round,
when they are agitated or turned round their Centers, their
angular Points, or the Parts which are moft diftant from
the Center, muft neceffarily meet one another, and turn
one another out of the Way; whence it follows, that
the Heat will cauſe a Rarefaction in this Body, as we ſee
in Milk, and all other Lipuors; and alfo in moſt hard
Bodies, in which few or none of their Particles fly off
when they are hot: Thus red-hot Iron is fomething big-
ger than when it is cold.
34. But if the Particles of a Body be very smooth, and
eafy to be put in Motion, and yet are fo placed with respect
to each other, as scarce to touch one another, fo that the
Compofition is very rare; a very little Heat coming upon
it, and fhaking the Particles, may cauſe them to approach
nearer one another, and the whole Body may be by this
means condenfed. And thus we experience, that Heat
when it melts Snow, reduces it into a lefs compaſs.
35. And becauſe the Particles of almoſt all liquid Bo-
dies muſt every Moment bend themſelves, or fome way
freezing, is alter their Figure, in order whereunto, they muſt be mo-
ved with fufficient Force; therefore if the Heat, or that
which forces them to move, or fo agitates them as to
make them Liquid as ufual, does almoft wholly ceafe,
all that the Particles can do, with that little which they have
remaining, will be to move themſelves without bending
fo much, as to join as near as they can together: And
then the Liquor will be rarified a little, and after it is fo
rarified, the Addition of the leaft degree of Heat, will
cauſe its Parts to approach nearer one another again. Thus
Water is a little rarified before it freezes, and is conden-
fed again by the leaft Heat that can be. But becauſe
fome Skill and Pains is requifite to prove this by Experi-
ence; I will fet down the Means I made uſe of to make
it appcar ſenſibly.
36. An Ex-
periment to
Show that
VVater,when
36. I caufed a Glaſs Veffel to be made like that in the
Figure, the largeſt Mouth of which is at A, and the other
at B, the End of the ſmall Tube BC, which is very flen-
It is extreme- der I poured Water into the Hole A, 'till the Veffel
by cold, is ra- was full, and confequently 'till it arofe up to D, in the
Tab. III. fmall Tube, then I ftopped up the Mouth A cloſe with
Fig. 10. foft Wax, and a Hog's-Bladder tied on: Having thus
prepared it, if the Heat of the Air be fo diminiſhed,
rified.
that
Chap 3 of NATURAL PHILOSOPHY.
161
that the Water be very near freezing,
it will fwell,
and rife up to the Mouth B, where it will fometimes run
over a little: Then, if we put our Hands or any other
Thing that is warm, to the Veffel, we fhall fee the Wa-
ter condenſe it ſelf, and fink in the fmall Tube almoft
down to the Bottom C. It is true indeed, that if we
continue to heat the Veffel, the Water contained in it,
will begin to dilate itſelf again, the Reafon of which, is
that which I have now given.
37. Becauſe we can move our felves with greater Eaſe 37. That the
in the Air than in the Water, this is a Proof that the Quantity of
Parts of the Air are much finer than thofe of Water. determined by
Heat, may be
Wherefore the leaft Heat that can be, muft: dilate the the Rarefa-
Air; and confequently, The Quantity of the Rarefaction of
Etion of the
the Air, will very exactly how the Quantity of Heat here
on the Earth; that is to fay, we can judge that it is hotter,
one Day of the Year than another, by obferving in which
of theſe two Days, the Rarefaction of the Air is greateſt.
38. Now in order to make this Rarefaction fenfible,
there has been invented in our Days an Inftrument called
a Thermometer, pretty like that in the Figure: DE is a
very ſmall Tube of Glafs about two Foot long, like a
Neck belonging to the Bottle A, which is Glafs alfo,
and about as big as a Tennis-Ball. The lower End is bent
and made large, fo as to form another Bottle marked F,
which needs not be ſo big as the Bottle A, and has a ſmall
Hole made in it at B.
Air:
38. A De
ſcription of a
Thermometer
-
Tab. IV.
Fig. 1.
39. The
Manner of
39. The Thermometer is at first entirely empty, that is,
full of Air only, part of which is forced out, by heating preparing and
the Bottle A, at the fame Time that the other Bottle F is the Ufe of the
dipped into a Veffel of Aqua-fortis, tinctured of a Green Thermometer.
Colour, by diffolving a Piece of Copper in it. We
chooſe Aqua fortis rather than common Water, becauſe
it is not fo fubject to freeze, and does not fo eafily eva-
porate. As the Air remaining in the Thermometer grows
cool, it has not Force enough to preſerve that Bulk which
it had before, and fo is obliged to retire up into the Glaſs,
and leave Room for the Aqua-fortis, which by its own
* It will well) Becauſe its Parts
are made ftiff, by the Mixture of
Nitrons Particles, and of other Salts.
(See the Notes on Art. 54.) Howe
ver it muſt be acknowledged, that
fomething ought to be allowed for the
Contraction of the Glaís. For as
Heat, by encreafing the Motion of
the Parts, dilates and extends Glafs
and other Bodies, fo cold by stopping
|
the Motion of their Parts contracts
and condenſes Glaſs and other Bodies.
See the Experiments of the Acad. del
Cimento, p. 109, & The Water
therefore a little before it freezes, ri-
fes in the Tube CB, partly becauſe it
is a little rarified, and partly becauſe
the Glaſs AC is a little condenfed by
the Cold.
M
Weigh,
Tab. IIIe
Fig. 10.
1
162
Part I.
ROHAULT'S SYSTEM
Tab. IV.
Fig. 1.
40. The Rea-
fon of this
Use.
41. That this
does not ex-
Weight, affifted by that of the external Air, gets into the
Bottle F, and from thence rifes up in the Tube towards
C. After this, the Inftrument is taken out of the Veſſel
in which it was dipped, and without doing any Thing more
than fixing it in a Wooden Frame, marked with feveral
Divifions, it fhows how much hotter it is at one time than
another.
40. For the more the Green Liquor is forced to de-
fcend by the Rarefaction of the Air in the upper Part of
the Tube, the hotter it is in the Place where the Ther-
mometer is fixed: And on the contrary, it is a Sign of
greater Cold, when this Liquor rifes higher, becauſe this
fhows that the fame Air has not Force fufficient to pre-
ferve its Bulk, but is obliged to give way to the Aqua-for-
tis, which the Weight of the external Air that preffes
upon the Hole B, continually forces to rife up as high as
it can in the Tube DF.
41. However, we must take care not to be deceived
Thermometer in the Judgement we make of the Heat, by barely look-
actly diftin- ing on the Thermometer; becauſe the Weight of the Air
gnifh all the being not always equal, it may be, that the Air will prefs
Differences of more upon the Liquor contained in the Bottle F, at fome
the Heat.
42. A De-
fcription of
another Ther
mometer.
Tab. IV.
Fig. 2.
Tab. IV.
Fig. 2.
Times than at others, and confequently force it to riſe
higher in the Tube FD, and may occafion us to think
that it is colder than it was before: when perhaps the
Heat of the Air was neither greater nor leſs.
42. This occafioned the making another Sort of Ther-
mometer not long fince, which has but one Bottle of Glaſs
only, and has a long flender Neck as is here reprefented.
At the Hole A is put in as much Spirits of Wine as
will fill the Bottle quite full, and the Neck alfo as high as
the Place marked B, and then putting the End A into the
Flame of a common Lamp fuch as Workmen uſe, ſtop up
the Mouth there, and then the Thermometer is finiſhed.
43. VVhy
43. When the Heat of the Air increaſes, the Spirits of
the Heat con- Wine dilate and riſe above B, and fo force the Air in
denfes the
Air in this the Part of the Neck BA to condenſe. Which it
Thermometer. may eafily do, becauſe when it was inclofed here, it
was very much dilated by the Flame which melted the
Glaſs, in order to ftop the Hole A. On the contrary,
when the Weather grows cold, the Spirits of Wine
contract into a lefs compafs, and defcend below the
Place marked B, and permit the Air to extend it ſelf be-
yond its Limits. By this Thermometer therefore we judge
whether it be more or less hot, by the rifing and
falling of the Spirirs of Wine; and we need not fear
the
Chap. 23. of NATURAL PHILOSOPHY,
163
the Inequality of the Weight of the Air, becauſe it
cannot get in, to make any Alteration in our Obferva-
tions.
Thermometer.
44. Though the Fault in the foregoing Thermometer is 44.1 De-
remedied in this, yet has this another of as ill Confe- felt in this
quence, viz. that becauſe the Spirits of Wine dilate
and condenſe but very flowly, we cannot foon enough per-
ceive the Alteration that is made in the Heat or Coldneſs of
the Air. And there is another Fault ftill, (if it be not
made larger than they ufually are) which is, that the Spi-
rits of Wine, being not capable of a very great Rarefaction,
its Rifing and Falling in the Neck of the Bottle will not be
of fo great Length, as to diftinguiſh the fmall Changes
that happen in the Heat of the Air. But one Remedy
of this, is, as I faid, I to make the Thermometer very large.
I have one in which the Difference betwixt the greatest
and leaft Height of the Spirits of Wine is above three
Foot.
45. After what has been faid concerning Heat, there
remains nothing more to be explained, but that which
we experience in Lime, when either Water is poured upon
it, or it is put into Water: And this may ſerve to explain
why other hard Bodies grow hot as foon as certain Li-
quors enter into their Pores. In order to our Satisfacti-
on in this Matter, we need only confider, that the Stone
of which Lime is made, has fo very fmall Pores that the
Water can ſcarce enter into them; but after it is put
into the Kiln, the Fire which penetrates it, carries away
fome of the internal Particles, and by that Means enlar-
ges the Pores fo much, that afterwards the Particles of
the Water can eaſily enter, being only furrounded by
the 2 Matter of the firſt Element: Wherefore being freed
from the Matter of the Second Element, when they en-
ter into the Pores, they can eaſily acquire all the Force
of the Firſt Element in which they fwim; fo that mos
ving them very quick, and being alfo pretty grofs, they
have Force fufficient to difunite the Parts of the Lime,
and to carry the ſmall Duft of it along with them: And
it is principally in the Agitation of this Duft that the Heat
of the Lime confifts.
1. To make the Thermometer) This
Inconvenience may be remedied by
bending the Neck of the Thermome-
ter into a Spiral; for by that means
the Spirits of Wine will rife eafier
and quicker, and the Difference of
the Degrees of Heat may be more ea-
fily obferved.
2. Matter of the First Element) See
the Notes below on Art. 48.
M 2
46. There
45. I'VIy
Lime grows
hot by having
Vater pour
upon it.
ea
i
164
Part I
ROHAULT's SYSTEM
46. How a
Cock of moist
Hay grows.
hot.
47.VV by Hay
when it is
fcattered does
not heat.
48. How
46. There is no need of wetting Hay in order to have
it grow hot of it felf, it is fufficient, if it be heaped up
whilſt it is green; for every Spire of Grafs contains in
it felf enough of the Moiſture which it fucks out of
the Earth; the Particles of which go and come out of
one Spire into another, and fwim at firſt in the Mat-
ter of the Firſt and Second Element, where confequent-
ly they have only the Velocity of the Second Element.
But afterwards when the Grafs grows dry, their Fibres
fhrink, and their Pores grow fo fmall, that the earthy
Juice which runs out of one into another, ſwims in the
Matter of the First Element only, whofe Velocity it
then obeys, and fo has a Force fufficient to move
the groffer Parts of the Hay, and to heat them by that
Means.
47. I faid exprefsly, that the Hay muſt be heaped;
that the Particles of the earthy Juice which come out
of one Spire of Grafs may enter into another with all
their Motion, becauſe if the Hay be fcattered in the
Meadow, the Juice which comes out of the Spires of
Grafs, is diffipated in the Air, and does not enter again into
others, to cauſe that Agitation which is neceffary to pro-
duce Heat.
I
48. As to the Heat which arifes from the Mixture of
two Liquors, two different Liquors, we need only imagine that their
grow hot when Particles are of fuch a Figure, that they can more cloſely
mixed toge unite when they are mixed together, than when they are
that are cold,
ther.
1. That their Particles) Since there rine with diftilled Vinegar or Spi-
is no fuch thing as this First Element, rit of Vitriol; alfo Sal Armoniac and
by all theſe Experiments, it appears, Corrofive Sublimate reduced to 1
that in Fermentations, the Particles Powder feparately, and then mixed
of Bodies, which almoſt reſt, are put together; if distilled Vinegar be pour-
into new Motions by a very potent ed upon them, they will be very
Principle (namely Attraction) which cold during the Fermentation. (See
acts upon them only when they ap- the Philofoph. Tranſactions N°.274)
proach one another; and caufes them Alfo Sal Armoniac mixed with a
to meet and clash with great Fio double Quantity of Oil of Vitriol
lence, and to grow hot with the Moti- will bubble up and fwell very much,
on. Newt. Opt. pag. 355.
bur and yet the Liquor at the fame time
becauſe Heat does not confift in eve- feel very cold. See the Exper, of
ry Motion, but in a peculiar Mo- the Acad. del Cimento, p. 153. Nay
tion (and of certain Particles per- further, from the Morion of fome
haps) of the fmall Particles of all Bo- Salts which are naturally in all Wa-
dies; if the Fermentagon or E- it is, that Water it ſelf inclo-
bullition arifes from the Mixture of fed in a Glafs, and put into a larger
fuch Sort of Salts as produce Cold. Vefel full of Water, if red-hot
(See the Notes on Ärt. 54 below, Coals be thrown into the Water in
the Fermentation may not only be this larger Veffel, will firſt grow
attended with no Heat, but with a cold (as appears by applying a
fenfible Cold. Thus Salt-petre mix- Thermometer to it) before it re
ed with Spirit of Vitrici or other ceives the Heat communicated by
acid Spirits allò volatile Sult of U-the Water which furrounds it.
ter,
ſeparate,
Chap. 23. of NATURAL PHILOSOPHY.
165
ſeparate; and when they are fo mixed, they fwim in the
Matter of the first Element only, at leaft, during that
little Time we ſee them ferment: Which is confirmed
from hence, that after the Fermentation ceaſes, we find
many Particles united together, and that they compofe
a great many ſmall hard Bodies.
Natue of
49. Having thus explained the Form of a hot Body, it 49. How to
will be eafy to determine that of a cold Body, which is find out the
the direct contrary: For if we conſider, that Cold extin- Cold.
guiſhes, or rather diminishes Heat, there will be no
Doubt, but that thofe are cold Bodies, which cauſe that
particular Motion in which Heat confifts to ceaſe:
Now we know that this Property belongs to three
Sorts of Bodies: Firft, to fuch as have their Particles
at Reſt with refpect to each other. Secondly, to fuch
whoſe Particles may be in fome Agitation, but less than
thoſe of the hot Body to which they are applied; and
Laſtly, Such whofe Particles may be fufficiently agita-
ted with a Motion proper to excite in us the Senfati-
on of Heat, but is attended with a different Determina-
tion which changes and ftops the Morion which the
Parts of our Body are in, and therefore cool it. The
whole Difficulty therefore is, whether Cold confifts in
one of theſe Modes only, or in each of the Three.
50. Now fince there are Three Sorts of cold Bodies,
we may affirm, that Cold confifts in each of these three
Modes. For, Firft, The Cold which is common to all
hard Bodies, cannot confift in any Thing but what is
common to them all, viz. in the Reft of their Particles:
Further, the Cold which we feel in Summer-time, when
we go into the Water, eſpecially when we are up to
the Middle, arifes from hence, that the Particles of the
Water having leſs Motion, than our Bodies have in all
thoſe Parts which are near the Heart, they receive ſome
Motion from us, and at the fame time we loſe it. And
of this we have a very convincing Proof, becauſe the
fame Water feels many times warm when we dip our
Hands into it, becauſe they are not fo hot as our Breaſt.
Laſtly, It is evident, that the Breath which comes out
of our Mouths, when we contract our Lips, or the
Air which we put into Motion with a Fan, in the
Heat of Summer, ought to cool us; if we confider that
the direct Motion of them diminiſhes or alters a little
the Determination and Agitation of that Motion which
is in the Part of the Body where we feel it cool.
M 3
SI. For
so. That
there are
three Sorit of
cold Bodies.
166
Part I.
ROHAULT'S SYSTEM
t
51. VVhy a
cold Body,
when it cools
51. For a Confirmation of this, we may obſerve, that
cold Bodies cannot make any Alteration in the Motion of
another, hot Bodies, without as much altering that Mode in which
warms it ſelf. their own Coldneſs confifts; that is, a cold Body cannot cool
another, without growing warm it ſelf, and fo we find by
Experience.
52.VVhy
fome Bodies
are colder
than others.
53. VVhy the
Air near a
52. We may obferve further, that the more Particles
a cold Body has at Reft, the more thofe of a hot Body
to which they are applied, ought to lofe of their Moti-
on, in order to communicate of their Heat to the other,
Thus Marble having more Particles at reft then Wood
which has more Pores, and is full of a Liquid Matter
which is in continual Motion, ought to feel colder than
Wood.
53. This alfo may ſerve to explain to us, why the Air
which is near Marble, or other Bodies, which have very
cold Body is
colder than in Small Pores, ought not to be quite ſo warm, or ought to be
other Places. a little cooler, than that which is in Places where fuch Bodies
54. Why
Snow feels
colder than
Marble.
are not. For the groffer Parts of the Firſt and Second E-
lement, which cannot enter into the fmall Pores of thefe
Bodies, muft neceffarily be reflected back from them,
and for the moſt part there is only the moft fubtil Mat-
ter about them, which is ready to enter in to them, or
which cannot but come out of them, and confequently
this is not able to agitate the grofs Particles of the
Air, which are proper to raiſe in us the Senſation of
Heat.
54. When I fay that Bodies which have more Parti-
cles at reſt, ought to feel colder than others which have
fewer, I fuppoſe that the Particles of each of theſe Bo-
dies are equally fufceptible of Motion; for if we fup-
pofe that the Particles of a Body are very eafily to be
put in Motion, and to loſe their Reft, this Body, though
very porous, ought much rather to receive within it felf
the Agitation of a hot Body, and by that means cool it,
than another Body which has fewer Pores and more Parts
at reft, but fuch as are not fo eafie to be moved. And
hence it is, that when we touch Snow, which is very
rare, it cools us much more than when we touch Mar-
ble, whofe Particles are much lefs capable of being put
into Motion. I
1. It is much more probable
that Cold (which is not merely
Comparative, as that of fimply Hard
or Liquid Bodies is; but pro-
55. The
duces real Effects, fuch as Free-
zing, Breaking in Pieces, Rarefa-
Etion, &c.) is owing to fome Par-
ticles of Nitre and other Salts
which
"
167
Chap. 23. of NATURAL PHILOSOPHY.
55. The Nature of Heat and Cold being fuch as I 55. How both
have now deſcribed, if you call to mind what was Heat and Cold
are drying.
before ſaid concerning the Form of moift or liquid Bo-
dies; it will be eafy to underſtand how Heat and Cold,
which are direct contrary Qualities, may yet, though by
quite different and oppofite Ways, produce one and the
fame Effect, viz. Drying or Hardning: As we experience in
this, that the fame Things, as Clay, for Inſtance, are made
as dry by the Cold in the Winter, as they are by the great-
eft Heat in the Summer: In order to underſtand the Rea-
fon hereof, we need only confider, that the Parts of moiſt
or liquid Bodies, fuch as Water, lofe all their Motion when
it is very cold; wherefore fince fuch Bodies by this
Means acquire the Form of hard or dry Bodies, it is not
at all furprizing, that Clay which is compofed of Water
and Earth, fhould grow hard and dry, when the Wea-
ther is very cold, feeing the Water alone, to which all
the Softness of the Clay is owing, freezes and grows
hard. On the contrary, Heat caufing the Parts of the
Water, by whofe Means the Matter of the Firſt and
Second Element kept the terreftrial Parts of the Clay
in fome fort of Motion, to evaporate; theſe terreſtrial
Particles, by their own Gravity, will be at reſt with reſpect
to each other, and by that Means compoſe a dry or hard
Body.
56. Why
Heat and
That Moifiure are
For a Principles of
56. Hence we may alfo fee the Reaſon of a Maxim
founded upon a Multitude of Experiments, viz.
Heat and Moiſture are Principles of Corruption.
Body is corrupted when there is a very remarkable Change Corruption.
in it, which doubtlefs may be effected by fuch a Mo-
tion as this. Now thefe two Qualities confift in this
Motion.
57. On the contrary, by Reft, the Parts of Bodies are
$7. Why Cold
kept in the fame Situation, and Cold caufes them to be at hinders Cor-
Reft; wherefore we may lay this down for a Maxim, That ruption.
Cold hinders Corruption.
58. However we muſt not affirm this to be a general 58. Why a
Maxim. For if a Body has Pores large enough to contain great Cold
a good deal of Liquor, and thefe Pores be filled with crumbles
Water ; becauſe Water cannnot freeze without dilating
it felf, it may ſo happen, that in freezing it may break
which are of certain Figures pro-
per to excite that Senfation, and
to produce thoſe Effects. And hence
it is, that Sal Armoniac or Salt-
Petre, or Sult of Urine, and ma-
ny other Volatile Alkalizate Salts,
make the Water with which they
are mixed very cold. See above on
Art. 48.
M 4
the
Stones.
168
Part I.
·ROHAULT's SYSTEM
the Body, which contains it, in Pieces. And thus we fee
that foft Stones, which are expofed to the Froft, crumble
and are reduced almoft to Powder, before the Water which
they have fucked in, can get out.
59. Why 59 This perhaps is the Reaſon of what is faid by ſome
Froft is hurt- of the Antients, That a hard and penetrating Froft has a
ful to Plants, Power of Burning, However, it very often happens, that
we afcribe that Effect to Froft, of which it is only a
very diftant Caufe, and which is immediately produ-
ced by Heat. For Example, when we fay, that Froft
corrupts Fruits and the Buds of Plants, we ought rather
to fay, that the Heat corrupts them whilſt the Froft is
diffolving, because it cannot get into the Pores of the
frozen Fruits, nor make the internal Parts fo foft as
they were before they were frozen, without having firſt
intirely deſtroyed the Connexion and Order of the other
Parts, nor confequently without having altered the whole
Compofition of the Parts.
60. Why Cold
60. For Proof of this we may obferve, that it is the ex-
does not hurt treme Parts of the Plants, which always contain in them
Some Parts of
more Moiſture than the other Parts, that are almoſt the
the Plants.
only ones corrupted by the Cold, and alfo that the Cold
does not hurt them till after they are budded, for before
they bud, the Cold does not hurt them; for which we
can give 2 no other Reafon but this, that Plants before
they put forth their Buds, are not ſo full of Watry Juices,
and their Pores are large enough to fuffer the fubtil Mat-
ter, to put thofe Parts which may have loft their Mo-
tion into Motion again, without neceffarily deftroying the
Connexion of thoſe it first acts upon, and which are
more external, before it comes to apply it felf to the o-
ther which are more internal.
E1. A Con-
¿his.
61. For a Confirmation of the Truth of this foregoing
firmation of Art. we may add, that in Northern Countries, where the
Cold is fo great, that a Man cannot go into the Air with-
out running the hazard of having the extreme Parts of
his Body frozen; if their Nofes or Fingers be frozen, they
do not lofe them, if they keep from the Fire, and rub them
with handfuls of Snow.
1. That the Heat corrupts them)
However for the most part, the Par-
ricles of the Juice being dilated and
made ftiff by the Cold, break in
Pieces, and fpoil the tender Parts
of the Buds, as is obferved by Mr. Le
Clere in his Phyficks, Book V. Chap.
xiii. Sect. 65. Though this Defect
does not appear till the following
Heat ſhows it.
2. No other Reafon) See the Notes
on the foregoing Art:
62. Haz
H
:
Chap. 23. of NATURAL PHILOSOPHY.
169
62. Having thus explained the Four principal Qualities 62. That the
that come under the Senfe of Feeling, viz. Hardness, Roughness
Qualities of
Liquidness, Heat, and Cold; there is no Difficulty in any and Smooth-
other which may come under the fame Senfe, fuch as nefs have no
For all theſe Qualities do fo clearly them.
Difficulty in
Rough and Polished.
follow from the Difpofition of the Parts of Matter only,
that there is no need of any Explication of them; where-
fore I fhall pass on to enquire into the Nature of Taftes.
{
CHAP. XXIV.
Of TASTES.
•
THE Word Tafe is ufed in Two Senfes. For First, 1. The Mean-
it fignifies that Senfation which we commonly have ing of the
Word Tafte.
when we drink or eat. Secondly, we underſtand by this
Word fomething, I know not what, in the Meat and
Drink in which the Power of raiſing this Senſation of Taſte
in us, conſiſts.
2. That all
Meat.
2. Though Taſte in the former Senſe of the Word,
cannot be exactly defcribed, nor particularly known but Men do not
perceive the
by Experience, yet we may make this Obfervation, that fame Tafic in
all Men have not the fame Tafte when they eat the fame the fame
Meat, as appears from hence, that fome Men can eat
with Pleaſure thofe Things which others have an Aver-
fion to: Whence we may conclude: that it is the fame
with Tafting as with Feeling: For if we touch in the fame
Part, two Perfons, the one in perfect Health, the other
juſt recovered of a Distemper, they will be very diffe-
rently affected, viz. the one with an agreeable Tickling,
and the other with an intollerable Pain; in like manner
the fame Meat may cauſe different Senfations in different
Perfons.
-
3. As to Tafte in the other Senfe of the Word, which 3. Ariftotle's
Opinion con-
we are principally to infift upon, Ariftotle's Opinion is,
cerning
That it is a Quality or Property of a moift Body arifing Taftes.
from an earthy Dryness, and a Heat on being fresh boiled.
This Definition contains Three Things, every one of which
have fome Refemblance of Truth. And first, I think
Ariftotle had Reafon to fay, that Tafte is a Property of a
moift or liquid Body, becauſe thoſe that are perfectly dry
or hard, have no Taſte 'till they are mixed with our
Spittle. Further, if we confider that Water has fcarce a-
ny
170
ROHAULT's SYSTEM · Part I.
4- That Ari-
ny Tafte, and Air none at all, though they be both moiſt
Bodies, we muſt confefs, that he had Reaſon to add
fomething more grofs, and of an earthy Nature.
Laft-
ly, he ought to bring in Heat, becauſe we find by Ex-
perience, that in many Fruits it caufes certain Taſtes
which we did not perceive in them before they were
prepared.
4. The Followers of Ariſtotle will readily agree to that
ftotle has not Explication which I have given of his Definition of Tafte;
explained
what Taste is. but it muſt be owned, that though he has faid nothing
but what is true, yet has he given us no Information at
all; becauſe he has not explained what that Affection or
Property of Body is which caufes Tafte, nor wherein it
conlifts.
riftotle.
5. A Mif-
5. Some have attempted to fupply this Defect, by fay-
take in the
Commenta ing, that it is a Quality very like that Senſation which it
ZOTS spon A- raiſes in us; but they are not at all aware what Inconve-
nience this brings us into: For befides that this gives to
inanimate Bodies a Mode of Exiſtence, which does by no
Means belong to them; it would follow from this Opinion,
that two Men could never have different Taſtes of the
fame Meat or of the fame Drink, contrary to what we
have before proved.
6. That Tafte
confifts in the
Grafness, Fi-
Bare and
Parts of the
Bady which
pe bade «
6. On the contrary, fince we are already affured, that
when the fame Meat caufes different Senſations in two
different Perfons, one of them muft neceffarily have a
"Motion of the Senfation different from that in the Thing which raifes
the Senſation, we have Reaſon to think the fame of the
other likewife. It is probable therefore, that the Faculty
of Tafting in us, is very like the Faculty of feeling Pain;
that is to fay, in order to bring this Power into Act, no-
thing more is required on the Part of thoſe Bodies which
cauſe Tafte, but that they move the ſmall Fibres of the
Nerves of the Tongue in fuch a manner as they ought
to be moved, and as Nature has appointed, in order to
the Perception of Tafte; the fame as in order to feel
Pain, nothing more is requifite but to move in a certain
manner the Nerves which are the Inſtruments of Feeling:
And becauſe nothing can move another, unleſs it be in
Motion it felf, and nothing can be applied to the Nerves
of the Tongue, fo as to have any Effect upon them, un-
leſs it be of a certain Bignefs, and of a certain Flgure:
1. The Small Fibres) Concerning
the Organ of Tafte, and its Defcrip-
tion. See Regis's Phyf. Book VIII.
Part II. Chap. iv. and the famous
Lewenhook's Epift.
I there-
Chap. 24. of NATURAL PHILOSOPHY,
I therefore think, that the Form of a Body which cauſes
Tafte, confiſts in the Bigness, Figure and Motion of its
Particles, and that from the Difference which there
may be in theſe Three Things, there may arife different
Taftes.
7. And this is confirmed by a Truth, which follows 7. Why fome
from what I have fuppofed, namely, that if the Particles Bodies have
no Tafte.
of a Body be fo fubtil, that they will fcarcely or not at
all move the Organ of Tafte, that Body will have no
Taſte. And thus we find by Experience, that Water has
ſcarce any Tafte, and Air none at all.
why Air has
8. We may alſo give a particular Reaſon why Air has 8. A parti-
no Taſte, viz. becauſe it ſwims upon our Spittle without cular Reaſon
mixing with it, fo as to make any Impreffion upon the no Tafie.
Nerves of the Tongue; by which we may alſo under-
ftand why fat Liquors have not fo fharp a Taſte as thin
Liquors have.
9. Further, if a Body be of fuch a Nature, as that none 9. Why hard
of thofe Parts are feparated from it, which are capable most part
Bodies for the
of penetrating the Pores of the Tongue, in order to move have no Tafte.
the Fibres of the Nerves, that Body ought to have no
Tafte. And fo we find, in moſt Metals, and alſo in Glaſs
and Flint Stones.
Io. Nor are we to think that there is any Thing in theſe
Bodies, that cauſes them to have no Tafte, but only, the
not being divided; for the Salts which belong to the
Compofition of Glaſs, tafted very ſtrongly before they were
concreted; and Metals which are reduced to a very
fine Powder by the Chymifts, are of fo ftrong a Tafte as
not to be born.
10. How Me-
tals may ac-.
trong Tafte.
quire a very
11. Why
warm Meats
11. Since Heat always increaſes the Motion of a Body;
and fince it is alſo very certain, that the more a Body have a strong-
is in Motion, the more capable it is of moving others er Taffe than
to which it is applied; it follows, that when Meat is thofe that are
hot, it muſt neceffarily have a ſtronger Tafte, than when
it is cold; as every Day's Experience fhows us.
cold.
Meat, when
12. It is alſo very eaſy to fee, that the Heat, in ma- 12. Why
king Meat ready, caufes the Particles to ſtrike one againſt it is made
another, fo that the Corners of many of them must be
1. The Bignefs, Figure and Motion)
Others contend, that not all the Par-
ticles, but the Salts mixed with the
Particles of all Bodies, are the Cauſes
of all Taftes; which is handled at
large by Mr. Le Clerc in his Phyf.
Book V. Chap. xii. And indeed this
is a very probable Opinion; but
whether the Particles of the Salt on-
ly, or any other Particles, be the
Caufe of Taftes, it comes to the
fame Thing; for we muft neceffarily
at laft have recourfe to the Bignefs,
Motion and Figure of thofe Parti-
cles. See the Notes on Art. 38.
broken
ready, has a
different
Tafie from
what it had
when raw.
192
Part I.
-ROHAULT's SYSTEM
13. That
there ought
+
broken off, and they by this Means divided into ſmaller
Particles than they were before, and alſo of a different
Figure; and this is the Reafon, why Meat, when it is
made ready, has a different Tafte from what it had when
raw.
13. As to the Difference that there is in Taftes; fince
we have made them to confift chiefly in the Difference
to be a great of the Figures of the Bodies fo tafting; of which Fi-
gures there may be infinite Variety; this agrees with
Experience, which difcovers to us new Taftes every
many very
different
Tafies.
14. A Mif-
sake of those
who think
that all
Day.
14. This being fo, I cannot approve of their Opinion,
who contend for two extreme Täftes, from a Compofi-
tion of which they imagine all others to arife. Belides,
Taftes arife that it would follow from thence, that all Taſtes would
from a Mix- differ only in degree; which is contrary to Experience,
which shows us, that there is a greater Difference
than fo.
ture of two
Extremes.
15. That
Sweet ought
mot to be op
pofed to Bit-
ter.
16. What
lifts in.
I do not fay that there can be no Inſtances given of
15.
fuch extreme Taftes, which raife in us the moſt diffe-
rent Senſations; but if any fuch are to be allowed, I
fhould rather oppofe a sharp or acid Tafte, to a bitter
Tafte, and not a Sweet to a Bitter, as is commonly done;
becauſe we do not find that a harp Tafte arifes from the
Mixture of Sweet and Bitter; but on the contrary, Sweet
feems rather to arife from the Mixture of the other Two,
as we experience in Fruits, the Sweetness of which feems
to be a Medium, betwixt an Acid and a Bitter.
16. To attempt to treat of every particular Tafte,
Acidness con- would be to undertake a Thing impoffible, and there are
many Things wanting in order to ſpeak with Certainty of the
principal and moft common ones. However amongſt theſe,
fome feem more eaſy to be underſtood than others, fuch
as Acid or Soure like Citron-Juice. For as this Tafte pricks
the Tongue, we may from thence conclude, that Bodies
which affect us in that manner, conſiſt of a great Num-
ber of long and ſtiff Particles, which in fome meaſure resemble
Small Needles.
17. Why all
they are ripe
are foure.
17. This will feem the more probable, if we confider,
Fruits before that this foure Tafte is common to all Fruits before they
are ripe; for this is a Sign that Sourenefs confifts in fome-
thing which is common to them all; but we cannot con-
ceive any Thing elfe common to them all, but this
Difpofition of their Parts, for they are all compoſed of
the Juice of the Earth, which ftops in the long ftreight
Pores of the Stock and Branches which bear the Fruits.
18. That
Chap. 24. of NATURAL PHILOSOPHY.
173
18. That we may underſtand ſomething of other Taftes, 18. What the
Sweet acid
we may confider the Progrefs of Fruits 'till they come Tafie of
to Maturity; for if we can but once know what Figure Fruits confifts
the Particles are of, when we experience a certain Taſte, in.
it will be eaſy to conclude, that this Tafte confifts in
this Sort of Figure. First then, fince all Fruits are ripen-
ed by the Heat of the Earth and Air: (whether this Heat
be cauſed by the Rays of the Sun, as commonly happens
in Fruits that grow in Gardens, or whether it be produ-
ced by Fires kindled under or upon the Earth, as when
Fruits are made to grow in Houſes, in the midſt of Win-
ter;) we cannot help thinking, that a great many Parti-
cles of theſe Fruits are put into fo great Motion, as to
ftrike againſt each other in different Manners, fo that ſome
of the longeſt of them are broken into ſhort ones, others
have their Points only beaten off, and others are made
entirely round. And then
And then it is, that the Fruits have a
fweet acid Tafte. Whence it is reaſonable to conclude,
that the fweet acid Taste of Fruits confifts in this, that fome
of their Particles are long and stiff, and prick the Tongue,
at the fame time that a great many other of them are lefs
penetrating, and fo flip over the Fibres of the Nerves,
without producing any thing more than a kind of Tickling.
19. We may obferve further, that the riper Fruits 19. How they
grow, the more their Particles are broken, blunted and become entire
ly sweet.
made fmall; wherefore fince the Fruits are then fweet-
er, we ought to conclude, that the great Sweetness of
Fruits arifes from hence, that they have a far greater Num-
ber of thoſe Particles which can only tickle, than of thoſe
which prick.
20. But if Fruit continues ripening too long; there is
no doubt, but that all its Particles will be fo bruiſed, that
none of them will be able to prick the Tongue agreeably,
but they will only tickle it in a difagreeable manner:
Now Fruits when they are too ripe, become bitter; whence
it is reaſonable to prefume, that Bitterness confifts in
this, that all the Particles are fo broken, blunted, and made
fmall to that Degree, that there remains no long and stiff ones
amongst them.
20. What
Bitterness
confifts in.
21,ᏤᏤᏤ
they are over-
21. And this is confirmed from hence, that in thoſe
Things which are made ready by Art, the Parts of them Meats when
which are burnt, and whofe Particles are beaten one againſt made ready.
another, and have their Corners broken off, are always become bitter.
bitter, as we experience in Crufts of Breath and in Roaft-
meat when laid too near the Fire.
22. The
174
ROHAULT'S SYSTEM
Part I.
22. Why 22. The Nature of Soure, Sweet and Bitter being
Sweet Things thus explained, we ſhall no longer be furprized, that ſweet
may be refol-
ved into two Things, fuch as Wine, fuppofe, may be refolved into
other, the one two other, the one of which is foure, or acid, the other
bitter; for that which makes any Thing fweet, (with
fuch a Sweetneſs as is agreeable to the Tafte) is compo-
fed of two Sorts of Particles, in the one of which Acidness
confiſts, and in the other, Bitterness.
acid, and the
other bitter.
23. Why bit-
ter Things are
heating, and
acid Things
cooling.
24. How a
bitter Thing
may be cool-
ing.
25. That the
the Figure of
the Particles
of the Body
23. Neither ſhall we any longer be furprized, that
Orange-Peal, Treacle, and many purging Medicines have a
heating Quality, and that acid Things, fuch as the Juice of
Orange and Verjuice, are commonly cooling; fince we are
affured, that Heat conſiſts in fuch a Sort of Motion, as
the fubtil, round and blunt Particles of bitter Things, are
capable of exciting and continuing; and that on the con-
trary, the long Particles, of which acid Things are com
pofed, being ſomething of the Nature of Water, are more
proper to hinder Motion, that is, to quench Fire, than
to kindle it; wherefore they ought to be reckoned a-
mongſt cold Things.
24. Neither is it inconfiftent with what has been faid,
that we fometimes find our felves cooler than we were
before, upon eating bitter Things; for there are ſome of
them ſo eaſy to be corrupted, that they can produce but
a very ſmall Heat, fuch as is fcarce to be perceived;
but yet this Heat may be enough to caufe fuch an Agi-
tation in the Particles of our Blood, as to carry off fome
noxious Matter which made it move too quick before,
and by this Means it will be put into a more quiet State;
and thus we may feel the Heat abated, and our felves cool-
er than we were before.
25. I fhall not infift any longer upon the Explication
Alteration of of particular Taftes. It would be very tedious to go
Taftes arifes
from the Al- through them all, and require a great Number of very
teration of
exact Experiments, which I have not made, nor perhaps
ever fhall. But to confirm my own Opinion as much as I
can, that their Difference confifts in the different Figures
of the Particles of the Body which we taste; I will exa-
mine one particularly, and make it appear, that as often
as our Reaſon fhews us, that there is any Alteration in the
Figure of the Particles, Experience fhews us alfo that there
26. An In- is fome Alteration in the Taſte.
ftance in
Wine, and
rohich we
zafte.
26. Let us take Wine for an Example, and confider it
that the Wood from the very Beginning, 'till it degenerates into fome-
thing that is not at all like Wine. I obſerve in the firſt
ought not to place, that the Moiſture of the Earth, becauſe it is com-
of the Vine
have any
Tafie.
pofed
Chap 24. of NATURAL PHILOSOPHY.
$75
:
pofed of the moft minute Particles of it, has fcarce any
Tafte, and though in the Pores of the Wood of the
Vine it grows in groffer Particles, and fuch as are able to
move the Nerves of the Tongue; yet becauſe it ſticks
among the Parts of the Wood, and is not eafily difinga-
ged from it; therefore it excites but a very fmall Sen-
fation in thoſe who chew the Wood.
27-That a
Bunch of
Grapes, when
27. Further, fince the Particles of the Juice which
get into the Air and diftill through the Stalk of the
Bunch, in order to form the Grapes, ftick together, and it is firft
cannot eaſily be feparated; it follows, that they can ap-
formed, ought
ply themſelves to the Superficies of the Tongue only,
to have ser
and confequently that they can raife but a fmall Senfa-
tion ſcarce to be perceived. And fo we find by Expe-
rience.
little Tafte
28. But fome time after, when the Particles, of which 28. Whence
the ſmall Grapes are compofed,, are ſeparated from each arifes the e-
other, either by the Heat of the Air which agitates them of Verjsce.
ry Sharp Tafe
gently, or by the Acceffion of more fimilar Particles which
thruft themſelves in to increaſe the Bulk of them; it is
manifeft, that they ought then to act feparately, and to
raife the Senſation of a very ſharp Tafte, fuch as we ex-
perience in Verjuice.
29. And the Heat of the Air, which increaſes as the
Fruit ripens, continuing to move the Particles of the
Grapes, it is evident, that they muſt be more and more
blunted thereby, and ſome of them made fo very fmall,
as only to tickle the Tongue agreeably, and to excite that
Senfation of Sweetness which we feel in chewing the ripe
Grapes.
29. How
Grapes gra
Sweet.
it is that the
30. We fee alſo, and it is an Obſervation worth taking 30. Whence
Notice of, that if it be wet Weather about the Time of Wine is sharp
gathering the Grapes, the Water which finks into the if it rains da
Earth, will afford too much Nouriſhment to the Grapes: ring the Vi
Wherefore as there are too great a Number of long Par- tage.
ticles, which there is not time for breaking or blunting,
it follows, that the Grapes will not be fo fweet as they
would otherwife have been. And this is often found by
Experience: For if it rains a little before the Vintage, the
Wine is harper, or, as they call it, harfher. This the
People of Languedoc feem to be aware of, who are at the
Trouble, a little before the Seaſon of gathering the mufta-
dine Grapes, to twift the Stalks of all the Bunches, that fo
they may ripen, and not receive any more new Nou-
riſhment.
A
31. For
176
Part N
ROHAULT''s SYSTEM
31. The Rea-
Jon why new
VVine is
Sweet.
31. For a further Confirmation of what I have faid,
it is worth obferving, that if we taſte of the Juice of the
Grapes just after they are preffed, there ought to be very
little Difference from the Tafte of the Grapes themſelves;
and it ought alfo to continue its Sweetneſs for fome time
after it is put into the Veffel, provided the Veffel be well
ftopped. For though, while it is working, many of the
long Particles which are intangled in one another, have
an Opportunity of getting clear, and fo are capable of
pricking; yet however they cannot caufe any fharp Sen-
fation, becauſe they act in Company with a great many
others which have had fufficient time to be broken and
made fmall, having been preferved in the Veffel care-
fully ſtopped up: And this agrees very well with the
fweet Tafte which we find in New Wine before it is
fined.
32. VVhy 32. If while the Wine is working in the Prefs or
VVine grows Vat, and while it continued to work in the Veffel, the
Sharper by
working. moſt ſubtil Particles, which have moſt Motion, and which
by reaſon of their Smallneſs were lefs ingaged with the
other, be permitted to fly away, and evaporate into the
Air through the Bung-hole, which is left open for that
Purpoſe, there must neceffarily remain fewer of thofe Par-
ticles which tickle the Tongue, and more of thoſe which
prick it. And this is the Reaſon why we ought then to
find the Taſte ſharper, that is, fuch as we experience in
Wine not quite fit to drink.
33. How it
lofes this
Sharpness.
34. How it
may become
very sweet.
33. After this, we may confider the Wine in two Con-
ditions: Firſt, let us fuppofe it ſtopped up in the Veſſel
ſo cloſe, that it has not the leaſt Communication with the
external Air; in which Cafe fome of its Particles will be
broken and blunted, and a great many of thoſe which re-
main whole, will lofe their Stiffneſs, and become plyable,
by rubbing againſt one another, and bending in that ſtrait
Place in which they are inclofed; and by this Means they
will be leſs capable of fhaking the Nerves of the Tongue:
Wherefore the Wine will no longer tafte Sharp, but at
tain that Sweetneſs which we experience in it when it is
fit to drink.
34. And without doubt the Sweetnefs would increaſe con-
tinually, if the Wood of the Veffel did not change the
Liquor a little, and permit the more fubtil Parts of it to
evaporate through its Pores. For a Proof of which, we
may remember, that Wine kept many Years in earthen
Bottles, well ſtopped, and put into Sand in the Bottom of
the
Chap. 24. of NATURAL PHILOSOPHY.
177
!
the Cellar, will in length of Time become as fweet as
Honey.
may grow
Sharp.
35. Suppoſe now, that the Veffel be not ſtopped; the 35. How it
long Particles which flip by one another, may be ſo worn
as to be a little diminiſhed, but there is no Neceffity that
they fhould become limber and pliable: For thoſe of
them that are moſt limber, are at liberty to evaporate
through the Hole of the Veffel, and thoſe which remain
have the more room to move in without being forced
to bend themſelves. So that all the Alteration that will
happen to the long Particles which remain, is, that they
will become more ſharp, and the Wine will be convert-
ed into a Liquor which will prick the Tongue more ſharp-
ly, that is, it will be turned into Vinegar.
36. If the Particles ftill continue to be thus moved for
a confiderable time, they will at laft be fo worn, and be
come fo very flender, as to be extremely pliable, info-
much, that they will have no Power at all to move the
Nerves of the Tongue; and then the Liquor compoſed
of them can have no Tafte, and be very little different
from Water; as we find by Experience.
36. How V7-
negar may
turned into
be
a Liquor that
may have no
Tafic.
37. A re-
37. For a final Confirmation of what I have faid con-
cerning Taftes, I will relate an Experiment which I made markableEx-
periment.
my felf: I took a Pewter Pot, and having made a
Hole in the Bottom of it, I ftopped it with a Piece of
Cloth, and then filled it about half full of very fine
Sand, fo well washed, as not in the leaft to tincture the
Water which drain'd through, and afterwards well dried :
After this, I put in a Quart of full-bodied Red-Wine,
which diftilling through the Hole below, there came out
about a Pint of clear Liquor like Water, which had no
Tafte: Then perceiving that the Drops began to be tin-
ged with Red, I took away the Veffel which I had fet un-
der, and put another in its Room, into which there ran
pretty near the other Pint; and this laft was much leſs
red, and had a much fainter Taſte than the Wine it felf
before it paffed through the Sand. Lastly, mixing this
Liquor with the other, which was very clear, the Refult
was a Liquor of a very faint Colour, and fcarce any
Taſte.
2
38. I think no Body that knows what Sand is, can find 38. The Con-
out any other Reafon for the Alteration of the Tafte of non of this
Chapter,
the Wine by paffing through it but this, that the Parti-
cles of the Wine being forced to go through very nar-
row winding Paffages, are bent a great many times all
N
Ways,
178
Part I.
-1
ROHAULT's SYSTEM
Ways, and have the Figure and Condition of them
changed: From whence we may conclude, that 2 the Form
of all Bodies that have any Tafte confifts in the Difpofition
and Figure of their Particles.
1. Have the Figure, &c.) The Fi-
gure of them is not altered, but only
the Parts which have no Colour or
Tafte, are ſeparated from the red
Parts which have a Tafte.
2. The Form of all Bodies, &c.)
That Taſte confiſts wholly in the Fi-
gure and Compofition of the Parts is
clearly demonftrated by the famous
Mr. Boyle, from the furprizing Al-
teration of Taftes, by varioufly
compounding of Bodies. I think
it worth while briefly to propofe
the Experiments made by that ex-
cellent Perfon, becauſe they ought
to be kept in Memory.
Firft, From two Bodies, one of
which is very acid and corrofive, the
other alkalious aud fiery, may ariſe a
Body without almost any Tafte. This
is done by a certain Compofition of
Spirit of Nitre and Nitre fixed per
deliquium.
Secondly, A Body that has fearce
any Tafie may be feparated into two
Bodies of a very sharp Tafic, yet
very different from each other. This
is done by diftilling the moſt refi-
ned Salt of Nitre by Inflammation,
or with a Mixture of Clay which has
it felf no Tafte.
Thirdly, From two Bodies, one of
which is very bitter, and the other
very falt, may arise a Body which
has no Tafe. This is done by fprink-
ling Crystals of Silver diffolved in
Aqua-fortis with Brine or Salt
Water, and then melting and prepa-
ring them on the Fire till they come
to a Luna cornea as the Chymifts
call it.
Fourthly, From two Bodies mixed
together, one of which is very ſweet,
and the other very falt, may arise
also a Body which has no Tafte. This
is done by pouring a certain
Quantity of Spirit of Sal Ammoni-
ac or Salt of Urine upon red Leat
diflolved in Vinegar, or Sugar of
Lead diffolved in a proper Men-
ftruum.
Fifthly, From two Bodies, one of
which is acid, and the other has no
Tafic, may arise a Body very bitter.
This is done by ftraining Aqua-for-
tis faturated with diffolved Silver:
For it will afford very bitter Cry-
fals.
Sixthly, From two Bodies mixed
together, one of which is infipid, and
the other very corrofive, may ariſe a
Body sweeter than Sugar. This is
done by pouring the best Aqua-for-
tis upon red Lead, and then pur-
ting it over a moderate Fire till it
is faturated.
Seventhly, From the sweetest Bo-
dies of all, without mixing any other
Bodies with them, may be cxtracted
very corrofive Liquors, fuch as will
diffolve certain Bodies. Thus a Spi-
rit that will diffolve Copper may be
extracted from Sugar or Honey.
Eighthly, A Body as bitter as can
be. may be feparated into two Bodies,
one of which is very acid, and the
other without any Tafie. Thus a ve-
ry acid Spirit may be extracted from
Crystals of Silver diftilled over a very
hot Fire, and a Body without any Tafte
will remain at the Bottom.
Laftly, The fame Body_diffolved in
different Liquors, as Aqua fortis, Aqua
regia, Spirit of Salt, diſtilled Vine-
gar, Spirit of Urine, &c. will have
a different Tafte in each of them. So
alfo, the fame Liquor as Aqua-fortis,
mixed with different Bodies, will have
different Taffes, thus with Silver it
will be bitter, with Lead it will be
Sweet, with Copper it will be intol-
lerable. See Boyle of the Production
of Tafies.
;
CHAP.
Chap. 25. of NATURAL PHILOSOPHY
179
BY
CHAP. XXV.
Of SMELLS.
I
Word Smeli,
Y the Word Smell, we may firft underſtand that par- 1. What is
ticular Sort of Senfation which is raiſed in us by the meant by the
Impreffion of certain Bodies upon the Nerves of the in-
ternal Parts of the Nofe: And we may alſo underſtand by
it, that in the Body which ſmells, in which the Power of
exciting the Senfation of Smell in us, conſiſts.
2. That the
Senfation of
alike in all
Smell is not
2. Every Body knows by their own Experience what
Smell is in the former Senfe of the Word, but it is im-
poffible to defcribe and make fuch Perception known to
others. All that we can fay, is, that the fame Object does Perfons:
not raife the fame Senfation in all Perfons, a great many
finding certain Perfumes agreeable to them, which others
cannot bear.
3. This being fo, we fhall only endeavour to find out 3. That Ari-
what Smell is with refpect to the Body fmelling. Ariftotle totle has not
defined what
has not defined it at all in that Chapter where he treats Smell is.
exprefsly of Smells, and 2 where he makes this Excufe,
that Men have not their Smell fo perfect as other Creatures.
ftorelians.
4. Some of his Followers think they underſtand what 4. The Opini
he means 3 from that Place where he fays, that the In- oh of the Ar
ftant we perceive any Thing, we become like the Ob-
ject which acts upon us to caufe that Senfation: And
upon this Foundation it is, that they contend that Smell-
in the Object is fomething very like that Senfation which
it raifes in us. To which they add, that Smell ariſes from
the Mixture of hot and cold, dry and moift, but fo that
the hot and the dry prevail moft.
5. But befides, that this Opinion afcribes to inanimates. A Confu-
Bodies, a manner of Exiſtence which agrees to those tation of this
only that are animated, which cannot be; it would fol- Opinton:
low, that the fame Smells muft be equally agreeable to
all Perfons, contrary to what was obferved before. To
which we may add, that it is wholly inconceivable, (fup-
1. The Nerves of the internal Parts)
For the Organ of Smelling, and the
Defcription of it.
B.8. Part 11.Chap. v.
Colour are. The Reaſon is, becanfe
we have not this Senfation very per-
See Regis Phyffect, but worse than many other Axi-
mals; For Man's Smell is very bad.
Arift. de Anima. lib. 2. cap. 9.
2. VVhere he makes this Excuſe)
It is not fo evident what Smell is,
as what Darkness or Light or
3. From that Place) See the Notes
on Chap. xxii. Art. 7.
Na
pofing
180
ROHAULT'S SYSTEM Part I.
}
6. What the
Nature of
Smells con-
fifts in.
7. Why
Smells are
more percei-
ved when it
is hot, than
when it is
cold.
8. Why cer-
tain Bodies
ceafe tofmell.
9. How Bo-
dies which
Seem to have
no Smell, may
pofing the Idea's which the Ariftotelians give us of the
four principal Qualities that come under the Senfe of
Touching to be true) that the Mixture of them fhould
produce any Thing elſe but Warmneſs, which will be
more or lefs dry or moiſt, according as it has more or
lefs of thofe Qualities mixed with it, which has no Simi-
litude at all to that Idea which they give us of Smell.
Laftly, If this Mixture were Smell, as we perceive it by
Touch, it ought to raiſe a Senfation like to it felf in all
Places where the Organ of Touch is; and then we ought
to ſmell with our Hands as well as with our Noſes; which
is contrary to Experience.
6. If to this it be anſwered; that That which caufes
the Senſation of Warmneſs, when it acts upon the Hand,
may alſo excite the Senfation of Smell, when it acts up-
on the Nofe, Nature having fo ordered it: I agree with
them. But becauſe I know nothing elfe in Bodies but
Magnitude, Figure and Motion, I cannot think there is need
of fuppofing any Thing elfe to make them capable of
impreffing Smell upon the Organ of Smelling: Wherefore
I am of Opinion, that the fame Particles which raiſe the
Senſation of Tafte, when applied to the Tongue, may
alfo raife the Senfation of Smell, when being fo very
fmall to fly about like Vapours or Exhalations, they
come to tickle thofe two extended Parts of the Brain
which anſwer to the moſt inward Recefs of the Noſe.
i
7. This may be proved from hence: Firſt, That we
experience, that the greater the Heat is, and confequently
the more capable of making a greater Number of fuch
Particles as caufe Smell, to fly off; the further do Bo-
dies extend their Smell: And on the contrary, as the
Cold keeps their Particles at reft, and hinders them from
exhaling, fo it is the Caufe of their Smell's being lefs per-
ceived.
*
8. Further, we obſerve, that a great many Bodies ſmell
no longer than whilst they are moift, that is, fo long as
fome of their Particles are in Motion; and that they
ceaſe to ſmell when they are quite dry, or have all their
Particles at reſt.
}
9. Laftly, One of the most evident Proofs that we have
to fhow that Smells confift in the Evaporation of certain
Particles, is this; that moft hard Bodies, which do not of
themſelves, as we fay, raife the Senfation of Smell, when
Some Smell. they come to be burned, or only to be rubbed one a-
gainst another, appear to have a Smell; becauſe by theſe
Means fome of their Particles are made to evaporate.
Send forth
Thus
1
Chap. 25. of NATURAL PHILOSOPHY.
181
Thus Sealing-Wax, when it is lighted, raifes a Smell,
which was not perceived before. And thus Iron rub-
bed againſt Iron, and one Flint against another, raile a
Smell alſo which was not perceived before.
have never
any Smell.
10. I do not however pretend to affirm, that all Sorts
10. Why
of Particles which are carried off from all Sorts of Bo- Some Bodies
dies, ought indifferently to raiſe the Senfation of Smell;
For in order thereto, there ought to be a certain Motion
of the Organ of Smell, and a certain Force to ſhake it;
and there may be alfo Particles fo very fmall as not to
be able to fhake it the leaſt that is poffible: Thus, the
Air which we breathe, and the Vapours which rife out of
Water, have no Smell at all; and, on the contrary, there
may be others fo large as that they may not come to the
Organ at all, or if they do come to it, are rather capa-
ble of quite ruining it, than of fhaking it in ſuch a man-
ner as may raiſe the Senfation of Smell.
11. The Difference of Smells depends upon the fame 11. Wherein
Caufe as the Difference of Taftes does, that is, I upon of Smells con-
the Difference
the Difference there is in the Bignefs and Figure of the Par- fifts.
N 3
1. Upon the Difference) That
Smell, in the fame manner as Taftes,
confifts entirely in the Compofition
and Figure of the Parts, is very evi-
dent from the following Experiments
made by the famous Mr. Boyle.
First, From two Bodies mixt
together, each of which is with-
out any Smell, may be raiſed a very
fromg urinous Smell. This will be, if
unflacked Lime and Sal Ammoniac
be beaten together.
Secondly, By a Mixture of common
Water, which has no Smell, a Body
which has alfo no Smell, may be made
to fend forth prefently a ſtrong Smell.
Thus Camphire diffolved in Oil of
Vitriol has no Smell, but mixed with
Water, it immediately fends forth a
ftrong Sinell.
Thirdly, Compound Bodies may fend
forth Smells which are not at all like
the Smells of the Bodies feparate.
Thus Oil of Turpentine mixed with
double the Quantity of Oil of Vitriol,
after it is diftilled, will not ſmell of
Turpentine but of Brimftone; and
that which remains in the Retort,
if it be forced with a ftronger Fire,
will refemble the Smell of diftilled
Oil of Wax.
Fourthly, A great many Smells
may be raised only by Motion and
ticles
Agitation. Thus a Multitude of
Bodies, as Glass, Stones, &c. which,
though heated, fend forth no Smell,
yet if agitated and bruifed with a
particular Motion, fend forth a very
ftrong Smell and there comes a
Smell like that of a Rofe, out of
Beech-wood while it is turning.
Fifthly, A Body that has a firong
Snell, mixed with another Body that
has no Smell, may loſe all its own
Smell. Thus if Aqua fortis not too.
well dephlegmated be poured upon
Salt of Tartar, till it ceafes ferment-
ing; that Liquor, after evaporation,
will afford Cryſtals without any
Smell, like Salt of Nitre; but if they
be burnt, they ſmell as bad as can
be.
Sixthly, Out of two Bodies mixed
together, one of which has the worst
of Smells, and the other not a very
good one, may arife a pleasant_aro-
matick Smell. This is done by a
certain Mixture of Aqua fortis or
Spirit of Nitre with inflammable
Spirit of Wine.
Seventhly, Spirit of Wine mixed
with a Body that has farce any
Smell, may produce a pleaſant aro-
matick Smell. Thus an equal Quan-
tity of inflammable Spirit of Wine
and Oyl of Dantzick Fitriol, mixed
together,
182
Part I
ROHAULT's SYSTEM
ter another.
ticles which are exhaled from the Body that fmells. As will
be evident to any one who confiders that thofe Things
which have the fame Tafte, have alfo the fame Smell:
Thus all fharp Bodies have a fharp Smell, and all bit-
ter Bodies have a Smell that has fomething of Bitterneſs
in it.
12. How the
12. And this is fo true, that when we are once affured'
Same Body
that the Particles of certain Bodies have changed their
may send
forth different Figures, we always" find by Experience, that they have
Smells one af- changed their Smell alfo. Thus, the Matter gendred in
the Abfcefs of a Land Beaver, expofed for fome Days.
together in the Sun, in a hot Country (which without
doubt daſhes the Parts one againit another, and alters
their Figure) fenfibly alters its Smell, and as ftrong
as it was, it becomes firft tolerable, and at laft is
turned into that valuable Perfume, which we call
Musk.
13- How the
Bulk of
Smelling Bo-
dies dimi-
nishes by lit
tle and little.
T
13. From what we have faid concerning the Nature of
fmelling Bodies, we may conclude, that both their Bulk
and their Weight diminish by little and little. Thus we
find by Experience thofe Smells to be quickly over which
are raiſed by burning: But as to thofe which we perceive
without heating the Bodies, ſuch as thofe of Musk and
Civet, it is a long time before they are fenfibly dimi-
nifhed, becauſe the Motion of their Particles is very flow,
and but a few of them are exhaled at a time. And as but
I
a few of them are exhaled at once, they could not move the
Senfe, without meeting and mixing with a great many
others, which were fometime before evaporated, and flew
about the ſmelling Body.
together, and digeſted, and then di-
filled, will afford a penetrating
Spirit of a very pleafant Smell.
Eighthly, A Body of the most plea-
fant Smell, without mixing any other
Body with it, may degenerate into the
worf ink. Thus the Spirit menti-
oned in the foregoing Experiment, if
it be kept flopped up in a Bottle,
will in a fhort time degenerate into
the ftrong Smell of Garlick.
Lafly, Out of two Bodies, one of
which has no Smell, the other a bad
Smell, may arise a pleasant Smell
like that of Musk. This is done by
putting Pearls into Spirit of Vitriol.
For while they are diffolving, they
See
fend forth a pleafant Smell.
Boyle of the Production of Smells.
1. It is a long time, &c.) Who-
ever confiders the infinite Divifibili-
ty
of Matter, and the inconceivable
Smallneſs of the Parts of Light which
always find an eafy and open Paflage
through Glafs and Diamonds on all
Sides, and every Way, will, I believe,
have no doubt, but that it is whol-
ly owing to the Smallness only of
the Particles emitted, though they
may be very much larger than the
l'articles of Light, that Bodies which
have a Smell, are yet a very long time
before they are fenfibly diminish'd.
CHAR
Chap. 26. of NATURAL PHILOSOPHY.
183
We
3
T
CHA P. XXVI.
Of SOUND.
two Mean-
HE Word Sound was intended to fignify in the 1. The Word
firft Place, that particular Senſation which is raiſed Sound has
in us, by the Impreffion made upon the Ears by what is.
we call founding Bodies. And the fame Word is alfo
uſed to fignify That in the founding Bodies, as in a Bell
or in the Air which furrounds it, which caufes in us the
Senfation of Sound.
here to under-
ftand the
2. After what has been obferved when we spoke of 2. In what
Taſtes and Smells, it is needleſs to fay, that Sound, taken Sense we are
in the former Senſe of the Word, cannot be deſcribed,
nor known any other Way but by Experience. Where- Word.
fore we fhall treat of it here only as That in the founding
Bodies or in the Air, which we call Sound.
Notion of
Sound.
3. Ariftotle has a Chapter particularly upon this Sub- 3. Ariftotle's
ject, wherein he afferts, that Sound is nothing else but the
local Motion of certain Bodies, and of the Medium applied to
the Ear; and that we may be fure that this is his No-
tion, he repeats it above twenty times.
4. I take particular Notice of that extraordinary Care 4. The Notion
which Ariftotle took, to make us underſtand the Notion of Some of his
he had of the Nature of Sound For though he repeat-
ed it ſo often, that it may feem troubleſome to fome Rea-
ders; yet
I find, he has not faid it often enough for ſome
others, who profeffing to follow his Opinions in other
Things, do notwithstanding believe that Sound is a Quality
different from local Motion.
Followers
5. There are ſome, who, to maintain this Opinion, and What Rea-
confute that of Ariftotle, fay, that if Sound be nothing elfe fen they give
for it.
but local Motion, it would follow; that in moving our
Hand, for Inftance, we ought to perceive fome Sound; and
there are others who affert, that according to this Noti-
on, it muſt follow, that a Bell which is heard two Leagues
every Way, muft move the Air fo far all round, which
they think abfurd.
6. However, thefe Objections are of no Weight; for
as. to the firft, it proves no more than this; that Sound
1. A Chapter particularly upon this
Subject) Chap. viii. Book, 2. Con-
cerning the Soul.
6. That they
are mistaken
in differing
from Arifto-
tle.
N 4
does
ROHAULT's SYSTEM
Part I.
7. That the
founding Bo-
dy does not
is requifite to
produce
Sound,
does not confuft in all Sorts of Motion, and eſpecially not
in fuch a Motion as is given to the Hand when it is mo-
ved; which indeed is very true. And as to thoſe who
think it abfurd, that a Bell. fhould move the Air for two
Leagues round, they judge of Nature only by their own
Prejudices, which are no Proofs.
7. I confefs indeed, that fome Force is required to-
two
put a Mafs of Matter, which is extended Leagues
cauſe all that round in Motion; But the Effect produced by the Bell is
Motion which not fo great as we may imagine: For when it moves the
Air in this manner, it only acts upon a Body, which
was in Motion before as it is a liquid Body. So that
it does not fo much act upon it to give it Motion, as
to determine that Motion which it had before, in fuch
a Manner as is proper to produce in us the Senfation of
Sound.
be moved.
8. That it is 8. I fay further, that it is not fo difficult as is imagi
not at all dif- ned, to caufe fuch Sort of Trembling in a Body which
ficalt to put
Some Bodies in is every way furrounded with a Liquid: Experience fhows
Motion, which us this in a large Anvil, (which doubtlels is one of thoſe
feem hard to Bodies which are not apt to be put in Motion;) for we
fee it trembles upon the leaft Blow given it by the Ham-
mer; and we may obferve, that if a few Grains of Mil-
let be put upon it, and it be ftruck on the Side with a
moderate Key; according as the Sound is more or leſs,
the Grains of Millet will jump higher or lower, and
change their Place on the Anvil. Now it could not
caufe this Motion in the Grains,
.it felf.
9. That
in a certain
op only.
?
if it was not moved
9. And to fhow that Sound confifts in a particular Sort
Sound confifts of Motion, we need only confider, that it is always pro-
fort of Moti- duced when we ftrike our Fingers over the Strings of a
Lute, or when we ftrike against any hard Body. Now
to ftrike the String of a Lute, or to ftrike any hard Bo-
dy, is nothing elfe but to move the String out of its
Place, or to put the Body in Motion.
And it is very
abfurd, to think, as the Ariftotelians do, that the Confti.
tution of them is altered, and that we make them to ac-
quire fome Heat or Cold, fome Drynefs or Moiſture which
they had not before.
1. It only acts upon 4 Body.) The
Motion which is in the Air before
It is moved by the Sorinding Body,
contributes nothing at all towards
producing Sound. For as many Par-
ticles of Air as there are, tending the
there are
fame way as they are impelled by the
founding Body, and therefore more
eafily yielding to it than if they
were at reft; juft fo many ought
we always to think are moved the
contrary way, and for that Reaſon
refift the Body in Motion more than
Motion
they would do if they were at reft.
10. And
Chap. 26. of NATURAL PHILOSOPHY.
185
10. And this is confirmed from hence, that if the Ear 10. A Proof
be tickled in the infide ſo as to make any Impreffion up- of this.
of the Truth
on what the Phyficians call the auditory Nerves, we find
a certain Tingling. Whence it is evident, that it is the
fame with the Senfation of Sound as with that of Pain;
and both the one and the other fhows, that by the Ap-
pointment of the Author of Nature we are made fo,
that when certain Nerves are moved, after a particular
Manner, we ſhould have a particular Senſation.
Proof.
11. I can't omit here an Experiment which is often 11. Another
made uſe of to divert Children, and which wonderfully
confirms this Opinion. They put a long Thread through
a pair of Tongs, and wind each End of the Thread
about their Fore-fingers, and then ſtop both Ears with
thofe Fingers; then moving their Bodies backward and
forward, they tofs the Tongs in the Air, and hit them
againſt the Andirons, or any other hard Body. Now
though thoſe that ſtand by, hear but a moderate Sound,
yet the other hear a Sound as loud as that of a large
Church-Bell. It is impoffible to folve this any other way,
but by faying, that the Motion of the Tongs fhakes the
String, which gives its Impreffion to the Fingers, and theſe
move the Parts of the Ear, to which they are applied, and
by this Means the Nerves of the Organ of the Ear are al-
fo moved.
12. Being affured that Sound conſiſts only in fome Sort 12. A Mif-
takę of Ari-
of Motion, all that remains, is to determine what Sort of ftotle's spon
Motion that is: And here I cannot agree with Ariftotle, this Subject
who would have Sound to be the Motion of a Body that of Scand.
is hard, poliſhed and concave; for it is certain, that there
are a great many founding Bodies which thefe Qualities
do not belong to, and alfo, that there are none of them
in Gunpowder when it takes Fire in a Cannon, which
yet makes fuch a prodigious Noiſe.
13. The
of ſome of his
rum Fulmi-
13. Some perhaps, out of Zeal to this Philofopher, may
attempt to defend his Opinion, by faying, that if thofe weak Defence
Qualities required by him in a founding Body, are not Followers.
to be found in the kindled Powder, nor in the Air And of Au
which is ſhaken; yet they are in the Cannon, upon nans.
which he would make the Whole of the Sound to depend.
But without amufing one's felf to find out Reaſons
to confute this Opinion; it fhall fuffice to alledge the Ex-
periment of what the Chymifts call Aurum Fulminans.
What they call fo, is only a Compoſition of three Parts
Hearing and its Deſcription, See Re-
gis Phyf. Book VIII. Part II.Chap.vi.
1. The Phyſicians call the auditory
Nerves) Concerning the Organ of
1
of
186
Part I
ROHAULT'S SYSTEM
14 That
豆
​Rauad con-
Fits in a
in a par-
of Mation
}
of Salt-Petres two of Flowers of Sulphur, and one of
Salt of Tartar, beaten feparately in a Mortar, and then
mixed together. We must take about as much of this
Mixture as we do of Gun-powder to prime a Musket,
and lay it upon an Iron-Plate, or a flat Tile, and put it
upon a Chafingdifh of Fire; then the Powder will grow
hot gradually, and be at once turned into a Flame,
which dilating it felf every way, caufes a Sound almoſt
as loud as the Report of a Musket well charged. In
this Experiment, the Iron Plate or the Tile, ferves only
to hinder the Powder from taking Fire, 'till it is equally
heated all over, and fince the Sound depends upon the
Flame and the Air, which are neither hard, nor polifhed,
nor concave, without doubt this Opinion of Ariftotle's
cannot be fupported.
*
14. We choofe rather to fay, that Sound confifts in
a particular. Sort of Motion of Bodies, than to fay with
dar fort ristotle, that it confifts in the. Motion of a particular Sort of
Bodies. For a more diftinct Explication whereof, we may
obferve, that the Bodies which we call founding Bodies,
are not applied immediately to our Ears, in order to ex-
cite the Senfation of Sound, but for the most part act by
the Interpofition of the Air which they put in Motion
wherefore we ought to find out what the Motions of
each of theſe are, when they produce this Senfation in
I5. That this
15. There are fome Inftances in which it is eafier to
Motion may find out the Manner in which the founding Body is moved,
confidered and there are others in which it is eaſier to find out the
in the found
Body, and Motion of the Air. The former of theſe we will f
the Medi- explain as far as we are able, viz. the Manner in which
founding Bodies are moved.
PALEN
16. What the
firft
16. And to begin with the Lute, or any fuch kind of
Bend of the Inftrument that is plaid upon with the Fingers, it is to
Bring of a be obferved, that the Strings being ftretched, are as ftreight
Inte confifts
as is poffible, and that in playing upon them they are put
out of their Pofition, and bent a little by the Fingers,
but as foon as they are let go, they return again to the
Place out of which they are moved, and the Velocity
which they acquire in returning, makes them go a little.
beyond it; then they come back, and go a little beyond the
Place of Reft again; and thus they go and come feveral
times, or have feveral Vibrations, and in this trembling
Motion confifts their Sound.
1. Turned into a Flame) See this on Part III. Chap.ix. Art.
Phænomenon explained in the Notes
17. The
Chap. 26. of NATURAL PHILOSOPHY.
187
17. The Sound of the Strings of a Violin confifts in 17. What the
the Agitation they are put into by the moving of the Sound of the
String of a
Hair of the Bow over them which is made rough and Violin con-
jagged, almoſt like a Saw, by being rubbed with Rolin. fifts in.
Which is fo true, that if the Hair of the Bow be rub-
bed with Tallow or Oil, the Strings will have no
Sound, becauſe they flip under it, and are not ſhaked by
it.
18. The Sound which a Drinking-Glaſs makes when 18. What the
the Finger preffing hard upon it moves round the upper Sound of a
Edge of it, confifts in the Vibrations like thofe of the drinking-
Strings of a Violin, it being evident, that the Finger here in.
fupplies the Place of a Bow.
Glass confifts
19. V What
a Bell con-
19. The Sound of a Bell conſiſts in a Trembling, pretty
much like that of the String of a Lute: For it is certain, the Sound of
that the Blow given it by the Clapper alters its Figure a ffs in.
little, fo that from being round, it becomes oval: And
becauſe it is made of Metal very ſtiff and ſpringy, that
Part which is moſt diftant from the Center, returns to-
wards it, and fomewhat nearer than it was at firft, fo
that the Places which were at the Extremities of the
longer Diameter, are at the Extremities of the fhorter one;
and thus the Circumference of the Bell changes its Figure
by Turns, all the time it is founding.
20. It will be very eafy for any one to believe what is 20. Proof
now faid, if he obferves, that in laying his Hand upon a of fuch Trem-
large Bell juſt when the Clapper ftrikes against it, he will bling.
feel a manifeft Numneſs.
21. If the Bell be very ſmall, as the Trembling is eafi-
ly ftopt by putting our Hand to it, fo ought the Sound a
to ceafe alfo. And indeed there are very ſmall Bells, which
if they be but very lightly ftruck, will found for a long time;
but if we lay our Hand upon them as foon as they are
ftruck, their Sound will immediately ceaſe.
21. VVhy
Small Bell,
when touched,
ceafes to
found.
22. VVhy the
great Bellis
22. But the Sound of a great Bell is not fo eaſily ſtopped.
by laying our Hand upon it, becauſe it has more Motion, Sound of a
and becauſe it can transfer fuch a fmall Part of its not so easily
Motion to the Hand, and referve enough to make it be topped.
heard.
when it is
23. The Sound raiſed by ſtriking a Piece of Wood, or 23.VVhy a
in general, any hard founding Body, confifts in a Trem- Body founds
bling, like that of a Bell, which is owing to its Spring- ftruck.
ginels.
24. Wherefore Bodies which have not this Property of
Springiness have only a very low and imperfect Sound:
24. Phy
Some Bedies
And Sound.
have but little
188
Part I.
ROHAULT'S SYSTEM
25. VVhat
on of the Air
Sound con
Fifts
And this is the Reafon why Lead and Clay, when they are
ftruck against, have ſcarce any Sound.
25. After what has been faid, it will not be very dif-
fort of Moti- ficult to determine what fort of Motion it is in the Air
it is in which which produces in us the Senfation of Sound; for it is
evident, that this Motion of the Air muft neceffarily
be fuch, as the Trembling of the Sounding Bodies is ca-
pable of producing in it, that is, the Air ought to trem-
ble, and bubble, and alfo by rifing and falling, to divide
it felf into an infinite number of very fmall Particles,
which by trembling and ftriking againſt one another,
muſt have a very quick Motion; fo that the Air muft
be fomething like a Liquor that fimpers and does not
quite boil. This is confirmed by what we fee of a Mo-
tion very like this in a large Tub of Water, by moving
a Stick backwards and forwards in it very quick; for
this Motion of the Stick is very like that of the Strings
of a Lute, only thefe are much larger and the other
flower.
on of this
Motion.
26. A visible
26. We may be certain of this Motion or Trembling of
Demonftrati- the Air, if we confider that the founding Body ought to
imprefs the fame fort of Motion upon it, that it does
upon other Liquors. Thus, if a Glafs be half full of
Water, and we make it found in the Manner before-
mentioned, by moving our Finger along the upper Edge
of it; it muft without doubt thake the Air as it does
the Water; 2 now we fee the Water tremble and boil,
and alfo by jumping out batter and break it felf in
fuch a manner, that a great many fmall Drops fly a
good way out of the Glafs. Whence we must con-
clude, that the Air has the fame Sort of Trembling or
Boiling.
V
1. This Motion of the Air), For
the Parts of the founding Body go-
ing and coming by Turns, thruft and
drive forward as they go thoſe
Parts of the Air which are next
them, and by preffing upon them,
condenfe them; then by returning,
they permit the Parts thus compref-
fed, to fpread and dilate themfelves
again. Thofe Parts of the Air there-
fore which are next to the founding
Body, go and come by Turns agree-
ably to the tremulous Agitation of
the Parts of the founding Body ;
and in the fame manner
as the
Parts of that Body agitate theſe Parts
of the Air, do thefe, being agitated
with the fame Sort of Tremblings,
agitate thofe Parts that are next
them; and theſe in like manner a-
gitate thofe beyond them, &c. This
being allowed, the manner how the
Pulfes are propagated along, and all.
the other Phænomena of Sounds, are
very advantageoufly explained. See
Newt. Philofoph. Princip. Mathemat.
Book II. Prop. 43, &.c.
2. Now we fee the VVater tremble)
You may fee a Cafe of this Experi-
ment very well worth obferving in
the Notes on the 45th Art. of this
Chapter.
27. After
2
Chap. 26. of NATURAL PHILOSOPHY.
189
I
27. After having fufficiently fhown the Motion of the 27. Vence
Air, which is neceffary to make us hear any Sound: It that VVig
ling arifes,
is eaſy to conceive that the Air in paffing by fome hard which is
and immoveable Bodies, may move it felf fometimes in made by blow
fuch a manner. Thus, when we whiſtle, by blowing Hole of a
ing into the
into the Hollow of a Key, it happens, that the Air which Key
enters in, fills one half of the Hole, and the Air which
comes out fills the other half; and theſe two Parcels of
Air fliding by one another with contrary Motions; a
great many of their Parts muſt neceffarily be made to
turn round and to tremble, and the whole Air which
is betwixt him that whiſtles and him that hears muſt al-
fo be made to turn round and to tremble.
28. We may obferve here, that there are Bodies, which 28. How the
are opened by Fits to let the Air through, and which Sound of an
by this means caufe us to hear a particular Sound, which Bagpipe is
Organ Piper
is alſo a very confiderable one. Of this Sort are the made.
Rows of Pipes which compofe an Organ, or the fingle
Pipe of a Bag-pipe. Theſe Bodies themfelves are not mo-
ved in order to produce Sound; but the Air being firſt
put into Motion, endeavours to pass through them, but is
forced to go out trembling, and fo impreffes on the reſt
of the Air the fame Sort of Tremblings as the Strings of a
Violin do, and fo caufes us to hear a Harmony, the Mo-
tions of which are Trembling.
29. And in the fame manner is the Voice of Animals 29. How the
formed: For there is a finall Valve at the End of the Voice of Ani-
Trachea, which performs the Office of the Valves of the med.
mals is for-
Tubes which compoſe an Organ; which Valve we can
contract as we pleaſe, and let the Air out of the Lungs
by Fits. And becauſe this Valve for the moſt part conti-
nues open, therefore the Air in Refpiration comes out com-
monly without any trembling, and confequently without
making any Noife. ..
30. It would be too tedious to explain particularly all 30. a
the different Manners in which Sound is produced. But Cannon makes
becauſe there is fomething fingular in the Sound of a Can- it is dif
a Noife when
non when it is diſcharged, becauſe the Flame feems to charged.,
give but one and not a repeated Shake to the Air, there-
fore it may be worth while to explain how fuch a prodi-
gious Noife is made. It is to be obferved then, that the
Gun-powder, when it takes Fire, is fo extraordinarily
dilated, as to take up above a Thouſand times the Space
1. Is fo extraordinarily dilated)
For the true Reafon of this Dilatati-
on, Sce the Notes on Part. III.
Chap. 9. Art. 13.
that
"
Part I.
190
ROHAULT's SYSTEM
t
31. That the
the Sound
that it did before; fo that it drives before it every Way
all the Parts of the groffer Air which was in this Space,
and theſe Parts can find no where to go, but by preffing
upon other Parts, and driving them on likewife; and at
the fame time they ſqueeze out the fubtile Matter which
mixing with the Powder, compoſe that fenfible Maſs
which we call Flame. Hence it follows, that there is in
the Air two contrary Motions; the one of which gathers
together and unites the moſt fubtile Parts, and the other
difperfes the groffer ones. And this would be done in a
Moment, but that the groffer Air which is condenſed all
round, has a Tendency to return into that Place out
of which it was driven, and towards which, after the Vio-
lence of the Flame is over, its own Weight forces it, and
that with ſuch an Impetus, that it becomes more denſe
than it ordinarily is; whence it will be reflected again all
round, or condenſed anew; becauſe being rarefyed again,
it returns to the Place which it had quitted; and thus it
quits and takes again the fame Place feveral times fuc-
ceffively; and this is the Reafon of that fhort Continuance
of the Noife of a Cannon when it is diſcharged.
31. However it is to be obferved, that the Ear may
Senfation of fometimes be ſo ſtrongly moved, that it may continue to
continues lon- tremble fome fhort time after the Air has done trem-
ger fometimes bling; and for this Reaſon, the Senfation of Sound
may fometimes continue after the Agitation without is
ceafed.
than the
Sound it felf.
32. VVby the
Flash of a
Cannon is
Sound is
32. Becauſe the trembling Motion of the Air in which
Sound confifts, is communicated gradually, fo that it
Seen before the affects thofe Parts which are near the founding Body
ſooner than thoſe that are further off, the Sound muft
neceffarily take up fome time in going along: And ſo we
find by Experience, that if a Cannon be diſcharged at two
or three Miles diftance from us, we fee the Flaſh fome time
before we hear the Noife.
heard.
33.VVhy the
weaker, the
33. And becauſe the Motion which is impreffed by
Sound grows the founding Body upon the Air clofe by it, is tranf
further we are ferred from one Part of the Air to another fucceffively,
diftant from and always paffes from a lefs Quantity to a greater, in
the founding proportion to its Distance from the founding Body;
Body.
therefore near the founding Body, there must always
be more Motion in a given Quantity of Air, than there
is at a greater Diſtance; fo that the Sound ought to grow
weaker as it is further from the founding Body.
34. The
Chap. 26. of NATURAL PHILOSOPHY.
Sound going
34. The Propagation of Sound may very well be com- 34. That
pared with Circles made in the Water, by throwing a
along with the
Stone into it. And as thofe which are made in a running VVind, og
Stream, extend themfelves further towards the lower than to be heard
fooner than
towards the upper Part of the River,
Part of the River, becauſe the when
whole Water in which they are formed carries them in- it.
tire that Way So likewife may we conceive, that if the
Wind carries the Air towards one certain Place, the trem-
bling Motion in which Sound confifts, will fooner go this
Way than the contrary. Thus we find by Experience,
that we hear the Sound of a Cannon, and in general all
other Sounds, 2 fooner with the Wind than against it. And
it may happen, that the Air may be moved to quick, that
its Parts may flee from us as faft as the Sound goes, and
fo we may not hear it at all.
7
as
35. Becaufe Sound is propagated every Way, as it were 35. How
from the Center to the Superficies of a Sphere, it may Echo is ado
fo happen, that the Parts of the Air which would com-
municate their Motion to fuch as are at a greater Diſtance,
may meet fome hard Body which they cannot fhake;
and this may cauſe them fome Way to be reflected back
again, and make them communicate their Motion again to
thofe Parts from which they received it, and thefe to others;
ſo that there will be a new Trembling of the Air inftead
of that which began firft, and hath already ceaſed for
fome Time: Confequently we may hear again the ſame
Sound which we heard at firft; aud this redoubled Sound
is what we call an Echo.
Echo may re-
36. If the Sound meets with ſeveral Bodies at different 36. How
Diſtances, which are capable of reflecting it back again; peat VVerds
if that which returns from the moft diftant Place ftrikes Spoken feveral
upon the Ear, after the Impreffion of the former is times.
1
1. Compared with Circles made in
the VVater) If the Water be put in
Motion, by throwing in a Stone,
or by moving our Finger er a Stick
backward and forward in it, the,
Waves will immediately furround
our Finger and if during the Agi-
tation it be carried ftreight forward
towards any Part without bending,
yet thefe Waves, as if they were
concentrick Circles, will be equally
propagated every Way; which Com-
parifon does very properly fhow us,
that the tremulous Motion of the
Air ought to be propagated not on-
ly the fame way that every one of
the Particles of the founding Body,
1
fuch as the Strings of a Violin, are
agitated; but alfo to be propagated
in a Circle all Ways from the found-
ing Body as the common Center.
2. Sooner with the Vind than a-
gainst it) The Gentlemen at Florence
thought they had found by cerrain
Experiments, that Sound is propa-
gated with the fame Celerity againf
the Wind, as with it, though much
more faint. Exper. Acad. del Ci-
mento, p. 140. But the induſtrious
Mr. Derham found it otherwife in
Experiments made at a much greater
diſtance. · See the Philoſophical Tranf-›.
actions, Numb. 313.
quite
}
192
Part I
ROHAULT's SYSTEM
}
does not al-
quite gone off, it muſt in its Turn produce a new
Senfation of Sound. Whence it is evident, that we may
meet with Echo's which repeat the fame Word feveral
times over.
37.VVhy he 37. According to the Inclination with which the Air
which speaks, ftrikes upon the Bodies which reflect the Sound, ought the
Reflection to be on the one Side or on the other, which
ways hear the
Sound of the is the Reaſon why there are fome Echo's where he who
ſpeaks does not hear the Words that are repeated, when
others who are at fome Diſtance from him can hear them
repeated diſtinctly.
Echo,
in.
38. VVhat
38. As to the Difference of Sounds that we meet with,
the different which conftitutes the different Species of them, as Flats
Species of
Sound confist and Sharps; the mufical Inftruments fufficiently ſhow us,
that they conſiſt in the different Motion both of the
founding Body, and of the Air which is agitated by it.
For the more the Strings of a Lute are trained, the
harper the Sound is; and on the contrary, the loofer the
Strings are, the more flat is the Sound. Now it is cer-
tain, that the more a String is ftretched, the fwifter and
more frequent is the Motion which it impreſſes on the Air:
whence it follows, that a harp Sound confifts in the
Quickness and in the fudden Reiteration of the Motion upon
which the Sound depends, and a flat Sound confifts in the
Slowness.
39. How ſe-
may be heard
together.
39. When two founding Bodies ftrike upon the Air at
veral Sounds the fame time, they muſt impreſs fuch a Motion upon it,
as is compounded of the two Motions which would be
cauſed, if they acted upon it ſeparately; and confequently
the Air ought to put the Organ of Hearing into fuch a
Sort of trembling Motion, as may raiſe a Senfation com-
poſed of each of the Senfations which the Bodies would
raife feparately.
40. VVhat
fift in.
40. And if the Motions of theſe two founding Bodies
Concords con- do fo exactly agree, that the Tremblings which they caufe
in the Air in a given Time are commenfurable, that is,
at the fame time that the one ftrikes the Air, the other
ftrikes it alſo, or at leaft, that they ftrike together every
ſecond or third Stroke; then the Ear will be fo uniform-
ly ftruck upon, and in fuch meafure, that it will perceive
the Diſtance, and be pleaſed with the Cadence; and in
the Strokes being thus commenfurable very probably con-
fifts thofe Concords which Muficians call an Unifon and Octave,
a Fifth and a Third.
41. On
Chap. 26. of NATURAL PHILOSOPHY.
193
41. On the contrary, if the Tremblings impreffed on 41. Why
the Air by the Sounding Bodies be incommenfurable, that Some Sounds
is, if they do not agree in Time nor ftrike together; we
are Difcords.
muft perceive the Inequality of the Sound; and becaufe
they do not move the Ear uniformly, they cannot pro-
duce any Harmony; and in the Strokes being thus in-
commenfurable, confifts very probably the Tones which
Muficians call Difcords.
ons of the
43.Of the
Motion of
Pendulums.
42. From what has been faid concerning the Motion 42. That the
impreffed on the Air by founding Bodies, fome Perfons laf Vibrati
perhaps may be apt to think that thoſe impreffed by the String of a
Strings of a Lute are not equal, but quicker at firſt, and flow- Lute do not
er as the Motion ceaſes; but it is not very difficult to fhow Time than
take up more
that the contrary is true, if we obferve, that the Motion of the the firft.
String when it almoſt ceaſes to be agitated, may be made up
by the Shortnefs of the Way that it has to go: So that it
takes up neither more nor lefs Time in making its firſt and
longeſt Vibrations, than it does in making its laft and ſhorteſt.
43. There must indeed be ſome Pains requifite to prove
the Truth of this by Experiments: For it is impoffible to
do it by the Strings of a Lute, becauſe of the fmall Time
that they take to make ſeveral hundred Vibrations in. But
becauſe the Motion we are ſpeaking of is very like that
of a Weight hanging in the Air at the End of a String, we
may imagine, that what we obferve of the Motion of the
one, may be equally applied to the other: Now we find
by Experience, that if this Weight be drawn from the
Perpendicular, and then let go, fo as it may fwing freely,
all the Vibrations till it ceaſes to move at all, will be made
in the fame Time. For if we will be at the Trouble to
count how many Pulfes of the Artery there are in the
firſt twenty Vibrations, fuppofe, we fhall find as many in the
twenty following Ones, or in any other Twenty, which
you will: Now from this fingle Experiment we may con-
clude that every Vibration of the String of an Inftrument
is made in the fame Time, and that the Laft take up no more
than the Firft. And becauſe this Experiment is very eaſy
to make, and is a curious one, and may ferve as a Prin
ciple from whence many important Conclufions in Mu-
fick may be drawn; it is worth any one's while to be at
the Pains to obferve the Motions of thefe Pendulums, and
to put feveral of them in Motion together. For we fhall
then fee, that thofe which are of an equal Length, and
alike in every other refpect, will perform their Vibrati-
ons in the fame Time; and that thoſe which are of dif
ferent Lengths, require different Times, viz. the Shorter,
O
the
194
Part I.'
ROHAULT's SYSTEM
44. VVhence
the lefs Time, fo that their. Vibrations will be to each
other in a reciprocal Proportion of the Square Root of
their Lengths, and thus what we have faid of the com-
menfurability of Sounds, and the Concords of Mufick, is
confirmed.
44. From hence we may alfo clearly apprehend how
different different Sorts of Voices are made, and why the fame
Sorts of Voices
arife. and Mouth may cauſe by turns a fharp and a flat Sound. The
why the Voices Reafon of which is, that the Epiglottis which is placed at
of Children the End of the Pipe through which we breathe, and which
are generally
Sharper than opens to give a Paffage for the Air in order to form the
thofe of grown Voice, may be lifted up and let down at pleaſure, that
People.
is, fo as to be fometimes to be altogether and from its
Roots open, or fhut, and fometimes in Part only. Now
that which can be lifted up in fuch a manner as this, by
Turns, and as it were with a trembling Motion, to let the
Air out with the fame fort of Motion, reſembles a Pen-
dulum; whence it follows, that the Tremblings of the
Voice muſt be fo much the quicker, the lefs the Epi-
glottis which regulates the Motion, is lifted up, and on
the contrary, they are the floweſt that can be, when the
Epiglottis is at liberty to lift it felf quite up. Upon this
Flexileneſs of the Epiglottis depends all the Variety of
Tones of the Voice; for the Air which comes out of
the Lungs being differently agitated according to the dif-
ferent Pofition of the Epiglottis, impreffes the Motion
it received as it came out, upon the external Air, which
ftriking the Ear differently is the Caufe of all that Diver-
fity which we obferve in Sounds. And becauſe Children
have generally all the Parts of their Bodies proportioned
|
Polygons, and that they are in the
fame Pofition with refpect to the
Earth; then it is evident, that the
Square Roots of the Arches, or of
the Spaces run through, and for the
fame Reafon, their Radius's or the
Length of the Strings, will repre-
fent the Times of the Defcent of
Pendulums, and becauſe the impetus
or Velocity in afcending, is evident-
1. In a reciprocal Proportion) Here
the Number of Vibrations in a given
Time are compared with each other.
But if the Times of the Vibrations
be compared together (which is the
better Way) then we muſt ſay, that
the Vibrations are to each other, as
the fquare Roots of their Lengths
directly. As may be thus demon-
ftrated. We fuppofe that the Ac-
celeration of heavy Bodies in fallingly
is fuch, that the Spaces they run
through, are as the Squares of their
Times (which fhall be demonftra-
ted in its proper Place. See the
Notes on Part II. Chap. xxviii. Art.
16.) then if we imagine fimilar Arcs
of unequal Circles to confift of an
infinite Number of Sides of fimilar
deftroyed equally in the fame
manner, and in the fame time as it
was acquired in defcending; there-
fore the whole Vibrations of theſe
Bodies muft neceffarily have the fame
Proportion to each other, as the
Square Roots of the Lengths of the
Strings. See the Notes on Part II.
Chap. xxviii. Art. 16.
to
Chap. 26. of NATURAL PHILOSOPHY,
195
to their Bignefs, and confequently their Epiglottis, leis
than in grown Perfons, therefore the Voice is generally
fharper.
45. And altogether as eaſy is it to account for an Expe- 45. The Rea-
riment which at first Sight has furprized a great many
Son of the
Perfons; which is, that if two Strings of the fame Lute, Strings that
Sympathy of.
or of different Lutes that are near one another, be Uni- are Concords:
fons, we cannot move the one, I but the other will found
alfo, at leaſt it will tremble; whereas it will not tremble
at all, if we move any other String near it, which is not
a Concord. Now the Reafon of this Experiment is, that
the Strings which are Concords, are capable of the fame
Vibrations; fo that the Air which is put in Motion by
the one, can very conveniently communicate its Vibra-
tions to the other; which cannot be in two Strings that
are not Unifons; for there is no Agreement in them, be
cauſe the Air which is pùt in Motion by the one;
does not find the other at all difpofed to receive its
Motion, and every Stroke except the Firft, is out of
Time, fo that by not agreeing they deſtroy each other's
Motion.
46. This Experiment has raiſed the Admiration of ma- 46. The fame
ny Perfons for a long time, and ſome have undertaken to Sympathy is
to be found in
account for it, by faying, that there is a Sympathy be- other Bodies:
tween the two Strings; but, befide that this is only a Way
of ſpeaking, we may obferve, that the Difpofition which
a Body has to move, when the Air is fhaken by another
Body, 2 is to be found in other Things as well as in the
Strings of a Lute, or other Mufical Inftrument: This I
have experienced in the late Wars, when I have obfer-
ved the Glafs-Windows to tremble very fenfibly upon
the beating of a certain Drum, and at the fame time would
not tremble at all upon the beating of others which were
much louder.
1. But the other will_found alfo) | fick Man that had his Left Hand
So likewife if two Glaffes, by put- cut off; upon the diſcharging of
ting in a proper Quantity of Wa- Cannons, he thought himſelf almoſt
ter, be made Unifons; the preffing thattered and torn to pieces; and
Our Finger hard upon the Edge and of another, that upon fcraping a
moving it round either of them, piece of Iron with a Knite, he
will make the Water in the other could not hold his Water; and of
curl, and dance about.
a Third, that upon tearing thick
Paper his Gums would bleed. See
his Effect of languid Motion.
2. Is to be found in other Things)
Thus Mr. Boyle relates concerning a
2
47 Ta
196
ROHAULT'S SYSTEM Part I
47. What is
the Caufe of
that fhiver-
ing which we
feel upon
hearing a
Trumpet.
48. How we
do render
our felves at-
tentive, fo as
to hear
Sounds di-
ftinctly.
47. To thefe Sort of Motions, I conceive we may af
cribe the Cauſe of a certain Shivering, which we fome-
times feel all over our Body, and which reaches even to
our very Heart, when we hear the Sound of a Trum-
pet, or fuch kind of Inftrument; For it may be that the
Blood is fo difpofed, as to yield eaſily to the trembling
of the Air.
48. And becauſe the Membrane of the Ear, which is
moved by the Agitation of the external Air, and the dif-
ferent fhaking of which caufes different Motions in the
Capillaments of the Nerves of the Ear, is fomething
like the Parchment of a Drum (and is therefore by ſome
called the Drum of the Ear) I am of Opinion, that it is
capable of being more or lefs fhaked, according as it is
more or leſs ſtretched. Wherefore I can eaſily perfuade
my felf, that we fometimes ftretch or loofen it, in or-
der to receive the Impreffion of the Sound more fenfi-
bly, and to make it the better agree with the Motion of
the external Air: fo that Attention confifts in nothing elſe
but in a due ſtretching or loofening this Membrane; and
keeping it in that Pofition in which it will beft receive
the Impreffion and Motion which the Sound gives to the
external Air.
CHAP. XXVII.
Of Light and Colours, and of Transparency, and
Opakeness.
IF
1. The first F in any Thing Exactnefs be required in the Meaning
Sanfe of the of Words, in order not to be furprized by any Equi-
Words Light
Colours. Vocation, it is principally in this of Light and Colours,
which are commonly uſed to fignify very different Things,
and generally confounded by moft Men. Firſt then it is
to be obſerved; that as we have given the Name Pain to
the Senſation, which is raiſed in us by a Needle when it
pricks us; fo likewife have we given the Name Light to
that Senfation which we have, upon looking on the Sur
or a Flame, and that of Colour to the Senfation raiſed in
us by diverſe Objects which we call coloured; thus in
particular, we give the Names of a White Colour and a
Green Colour to the Senfations which Snow or Grafs ufu
fually produce in us,
2. SB-
Chap. 27. of NATURAL PHILOSOPHY.
197
2. Secondly, By theſe Words Light and Colour, we 2. Another
alfo underſtand, that on the Part of the external Ob- Senfe of the
Words Light
jects which is the Caufe of exciting in us the forementi- and Colour.
oned Senfations: Thus by the Light of the Flame, we
mean fomething, I know not what, which occafions the
Senfation of Light to be excited in us; and by the White-
nefs of the Snow, we underſtand fome other Thing, I
know not what, that is the Occafion of our having the
Senfation of Whiteness.
3. And becauſe the Objects which we call luminous,
fuch as the Sun or a Flame, do not affect our Eyes im-
mediately, but act by the Interpofition of fome interveen-
ing Bodies, ſuch as Air or Water or Glafs; yet that which
is impreffed on theſe Mediums, whatever it be, is called
Light alfo, but Secondary or Derivative, to diftinguish it
from that which is in the luminous Objects which is cal-
led original or innate.
3. A third
Senfe of the
Word Light.
parent and
4. We call thoſe Bodies Transparent, through which 4. The Mean-
luminous Bodies act upon our Eyes to raiſe the Senfa- ing of the
Words Tranf
tion of Light, and through which we can alfo fee Co-
lours. And we call thofe Bodies Opake which interrupt Opake.
the Action of luminous or coloured Bodies, or through
which we cannot ſee either Light or Colours.
5.That the
Senfation of
Light or Co-
lour cannot be
5. I do not pretend to declare what Light and Colours
are in the firſt Senſe of the Words, but leave it to eve-
ry one to make them clear to himſelf by his own Expe-
rience; for I think it as impoffible to give another Per- defcribed.
fon a true Notion of that particular Senfation that we
have of Colours, as it is to give it to one that is born
blind.
6. That one
and the fame
vifible Object
ceffarily raife
the fame Sen-
6. However, I may venture to affirm, that as it of
ten happens that the fame Food may at the fame time
raiſe different Taftes in two different Perfons, fo it may does not ne
alſo happen, that two Perfons looking in the fame man-
ner upon the fame Object, may have very different Sen- fation in twe
ſations; and I am the more perfwaded of this, becauſe different Per-
I have experienced it in a particular manner my felf. Sons.
For when I had once quite tired and weakned my right
Eye by looking intently for above twelve Hours toge-
ther through a perfpective Glafs on a Battle betwixt two
Armies, within a League of me; I found my Sight fo
affected afterwards, that when I looked upon Yellow Ob-
jects with my right Eye, they did not appear to me as
they uſed to do, nor as they now do to my left Eye:
And, which is very remarkable, I do not find the fame
Difference in all Colours but only in fome; as for in-
0 3
ftance
198
Part I.
ROHAULT's SYSTEM
7. Ariftotle's
Opinion a-
busit Light.
2. His Opi-
nion about
Celours.
9. That he
Light and
Colours are.
ftance in Green, which appears to me to come near to
a Blue, when I look on it with my right Eye. This Ex-
perience makes me believe, that there may be fome Men
born with that Difpofition, which I at preſent have in
one of my Eyes, and which may continue all their
Lives, and perhaps there are others whofe Eyes are of
the fame Difpofition with my other Eye. However, it
is impoffible, either for themſelves or any other Perfons
to perceive it, becauſe every Body accuftom themſelves
to call the Senfation which a certain Object produces
in him, by that Name which it ufually goes by; which
yet being common to the different Senfations that every
one may poffibly have, is not the leſs ambiguous.
7. Before I come to that Enquiry which I defign, viz.
what Light is, and what the Colour of Objects is, which
is the principal Deſign of this Diſcourſe; I obſerve, that
Ariftotle has treated of the fame Subject, in the 7th
Chapter of his Second Book Concerning the Soul; where,
after having faid, that Colours depend upon Light in order
to their being feen, he concludes, that theſe two Qua-
lities ought to be explained together. And in order to
determine what Light is, he fuppofes that fome Bodies are
transparent, fuch as Air, Water, Ice, Glafs, and fuch like.
And becauſe we cannot fee through any of theſe Bodies in
the Night, he fays, that then they are in Power only tranf-
parent, and that in the Day-time they become actually
transparent: And becauſe it is Light alone that can bring
this Power into Act, he concludes, that Light is the Act
of a transparent Body as tranfparent.
8. As to Colour, he obferves, that fince the Ob-
ject in which it is, does not apply it felf immediately to
our Eyes, in order to raiſe any Senſation in us, it muſt
firft move the Medium which is betwixt that and us; and
becauſe it cannot be perceived through Opake Bodies,
nor can it be feen through thofe that are only tranſpa-
rent in Power, he concludes, that Colour is that which
moves Bodies which are actually transparent.
9. Though Ariftotle in the forecited Chapter, has not
has not fuffi- fearched this Matter to the Bottom, yet he affirms, that
ciently ex-
plained what he has fufficiently explained what Light, and Colour, and
Tranfparency are, and imploys almoſt all the remaining
Part of his Difcourfe, in refuting the Opinions of fome
Philofophers that were before him. However he adds,
that Light is not Fire, nor a Body proceeding from a
Luminous Body, and paffing through a tranſparent one;
but only the Prefence of Fire, or any other luminous Body
X.
with
Chap. 27. of NATURAL PHILOSOPHY.
199
with the transparent Body. But upon confidering this O-
pinion, I fee no reafon to be fully fatisfied with it, as if
it could not be carried any further than Aristotle has
done, or at leaſt, that it cannot be more diſtinctly ex-
plained. For it is certain, we are ſtill at a lofs to find
out more particularly what the Nature of tranſparent
Bodies, and alfo what the Nature of luminous Bodies is,
and further how the Prefence of the Latter operates on
the other, to bring its Power into Act, and last of all,
what that is which moves a Body that is actually trans-
parent.
his Followers
Colours.
10. This fome of the Commentators upon Ariftotle 10. What the
have acknowledged; and though they might have had Opinion of
fome Light from what he has faid in his Problems, and is concerning
particularly from the 61ft of the Eleventh Section, yet Light and
they have either overlook'd what he has faid in this
Place, or at least not rightly underſtanding him, they
have advanced fomething which it does not appear that
Ariftotle ever thought of, viz. that Light and Colours in
the Objects which we call luminous or coloured, are
Qualities exactly like thofe Senfations which they occa-
fion in us, and (as fome of them contend), they arife
alfo from a Mixture of Hot and Cold, of Dry and
Moift. And for Proof of this (befides their thinking,
that they have Arftotle on their Side) they affirm, that
it would be impoffible for luminous or coloured Bodies
to cauſe thoſe Senfations in us which we feel, if there
were not in them fomething very like what they cauſe
us to feel, for, fay they, nothing can give what it has
not.
II.
11. But, befides that Ariftotle has faid nothing pof-
tively concerning what they have advanced, Authority
ſtands for nothing, when we are inquiring after Reaſons
only. And as to what they alledge, it will appear to be
only a mere Sophifm, if we reflect ever fo little up-
on the Pain which we feel when we are pricked by a
Needle; for this fhows us, that it is not at all impoffi-
ble for an Object to be able to excite in us a Senfati-
on which it felf has nothing of. And this is ftill fur-
ther confirmed from hence, that two Men may ſee the
fame Object differently, as was before obferved, I my
felf feeing Yellow differently with my two Eyes.
1. The 61ft of the Eleventh Section)
Where, after having propofed this
Queftion. Why we cannot fee through
an Opake Body. He argues very
04
much for the Propagation of Light in
ftreight Lines. See the Notes on the
latter Part of the 15 Art. of this
Chapter.
12. But
11. That they
have not pro-
ved what they
affert.
200
Part I.
ROHAULT's SYSTEM
12. That it
is not true.
12. But that which most evidently fhows, that it is not
at all neceffary there fhould be any Reſemblance between
the Quality of the Object, and the Senfation it excites,
is this; that we certainly have very ftrong Senfations of
Red, and Yellow, and Blue, and all other Sorts of Co-
lours, upon looking through a Triangular Glaſs Prifm, in
which no one ever fufpected that there was any Thing like
the Senfation which it raiſes in us.
13. The Ab-
13. That which others of them fay concerning the O-
fardity of the
Opinion of riginal of Colours is still more abfurd. For what Con-
Some of the nexion is there betwixt the Idea's we have of Hot and
Ariftotelians. Cold, Dry and Moiſt, and thoſe which they ſuppoſe us
to have of Colours: If what they fay were true, it would
from hence follow, that the fame Object ought to have
as much Variety of Appearances to the Eyes, as it raiſes
different Senfations to the Touch; which does not agree
with Experience: On the contrary, there are fome Bo-
dies, fuch as polifhed Steel, and Lobfters, which when
heated by the Fire, acquire a certain Colour; but when
made cold by dipping them in Water, they do not alter
their Colour.
14. A Com-
14. Leaving therefore the Opinion of Aristotle and
pariſon of the his Followers, concerning Light and Colours, let us now
Senfation of confider what Part we are to take upon this Subject. And
Light with
that of Pain. Firft, Since we have no Reafon to fay, that the Light of
luminous Bodies is any Thing elfe but the Power which
they have to produce in us that very clear and bright Senfa-
tion which we have when they are before us; Why may
we not compare this Power with that which a Needle
has to cauſe Pain in us? Since then the Senfation which
a Needle raifes in us fuppofes only that we are ſenſitive
Creatures, and nothing more is required in the Needle but
its Figure and Hardness, which are alone fufficient to
cauſe a Divifion in the Part to which it is applied: So like-
wife it is reaſonable to think, that the Senfation of Light de-
pends upon this, that we are by Nature made capable of
this Sort of Senfation; and that there is in the Pores of
tranſparent Bodies, a Matter fine enough to penetrate even
Glafs, and yet at the fame time ftrong enough to ſhake the
fmall Capillaments of the Nerves which are at the Bottom
of the Eye. Further, as there must be fome Agent to
pufh the Needle into us, fo likewiſe muſt we think, that
this Matter is pufhed by the luminous Bodies, before it
ean make any Impreffion on the Organ of Light.
75. Thus
istk
Chap. 26. of NATURAL PHILOSOPHY.
201
15. Thus ¹ Original Light conſiſts in a certain Motion 15. What
of the Parts of luminous Bodies whereby they are capable of Light and
our Opinion
of pufhing every Way the fubtil Matter which fills the Transparency
Pores of tranſparent Bodies; and the Effence of fecondary and Opake-
or derived Light confifts in the Difpofition or Tendency of nes is,
this Matter to recede from the Center of the luminous Body
in a ſtreight Line. Whence it is eaſy to infer, that the
Form
}
Shadow. Newt. Opticks pag. 337-
Rays of Light therefore must be
fmall Corpufcles fent forth from lu-
minous Bodies with a very great ce-
lerity. For fuch fort of Corpufcles
(contrary to the Pretion of Motion
propagated in a Fluid) ought to be
tranfmitted through uniform Medi-
ums or void Spaces in ftreight Lines,
without bending into the Shadow 5
as we fee the Rays of Light are
tranfmitted.
Concerning that Force by which
thefe Corpufcles are fent forth with
fuch incredible celerity, that they
are
1. Original Light----Secondary or
derived Light) Original Light con-
fifts intirely in a particular Motion
of the Particles of the luminous Bo-
dy; not whereby they puth forward
that fictitious Matter which Cartes
imagined the Pores of tranfparent
Bodies to be filled with ; but where-
by they ſhake off fome very ſmall
Particles from the luminous Body,
which are fent forth all Ways with
a very great Force: And Secondary
or Derivative Light confiſts, not in
the Difpofition, but in the real Mo-
tion of thofe Particles receding eve-
ry way from the luminous Body in
carried above 7000000 of
freight Lines with incredible Swift-
Miles in a Minute (See the Notes on
nefs. For if Light confifted only Art. 30. below.) the admirable Per-
in Preffare, it ought to be propaga- fon before-cited fpeaks thus. Thoſe
ted to all Diſtances in a Moment of Bodies which are of the fame kind and
Time; which it certainly is not (See have the fame Vertue, the ſmaller they
the Notes on Art. 30. below.) And it are, the stronger is their attractive
would not be propagated in ftreight Force in Proportion to their Bignefs,
Lines, but it would perpetually run See the Notes on Chap. xi. Art. 15.)
in upon the Shadow. For Preffion VVe find this Force ftronger in pro-
or Motion cannot be propagated in a portion to their VVeight in Small
Fluid in right Lines beyond an Obfta- Magnets than in larger ones; for the
cle, which ſtops part of the Motion, Particles of ſmall Magnets, becauſe
but will bend and ſpread every Way they are nearer one another, can the
into the quiefcent Medium, which lies more easily unite their Forces together.
beyond the Obftacle. Gravity tends VVherefore it is reasonable to expect,
downwards, but the Preſſure of Wa- that the Rays of Light, fince they
ter arising from Gravity, tends every are the ſmalleſt of all Bodies (that we
way with equal Focre, and is propa- know of) Should be found to have the
gated as readily, and with as much strongest attractive Force of all. How
Force fideways as downwards, and ftrong this Force is, may be gathered
through crooked Paffages as through from the following Rule. The At-
Streight ones. The Waves on the Sur- traction of a Ray of Light, in pro-
face of ftagnating Water, paffing by portion to the Quantity of Matter it
the fides of a broad Obstacle which contains, is to the Gravity which any
Stops part of them, bend afterwards, projected Body has, in proportion te
and dilate themſelves gradually into the Quantity of Matter contained in
the quiet Water behind the Obſtacle. it, in a Ratio compounded of the
The Waves, Pulfes or Vibrations of Velocity of the Ray of Light, to the
the Air, wherein Sounds confift, bend Velocity of the projected Body, and of
manifeftly, though not ſo much as the the Bending or Curvature of the Line
Waves of VVater---And Sounds are which the Ray defcribes in the Place
propagated as readily through crooked of Refraction, to the Bending or Car-
Pipes as through freight ones. But vature of the Line which the project-
Light is never known to follow crook-ed Body deſcribes ; viz. if the Incli-
ed Pallages, nor to bend into the nation of the Ray to the refracting Su-
|
+
perficies
202
Part I.
ROHAULT'S SYSTEM
Form of a Transparent Body confifts in the Streightness
of its Pores, or rather, that they cross each other all ways
without any Interruption, and on the other hand, a Body
is opake, becauſe none of its Pores are ſtreight, or if they
be, they are not penetrable quite through, and all ways.
16. I
0000000000000001
moved ſhould have its Pores ftreight,
and eafy to pass through from all
Sides, and all Ways, and yet at the
fame time, the thinneft Paper or even
Leaf-Gold, for want of fuch Pores,
fhould exclude the Rays of Light;
is not eafy to be conceived. Where-
fore we muſt feek for another Cauſe
of Opakeness.
perficies, be the fame as that of the pro-
jected Body to the Horizon. And from
this Proportion I collect, that the At-
traction of the Rays of Light is more
than
times
greater than the Gravity of Bodies on
the Superficies of the Earth, in pro-
portion to the quantity of Matter con-
tained in them; viz. if Light takes
up about ſeven or eight Minutes in
coming from the Sun to the Earth----
Now, as in Algebra, where affirma
tive Quantities vanish and ceafe, there
Negative ones begin; fo in Mecha-
nicks, where Attraction ceases, there
a repulfive Vertue ought to fucceed.fage
Therefore a Ray, as foon as it is fhaken
off from a fhining Body, by the vibrating
Motion of the Parts of the Body, and
gets beyond the Reach of Attraction,
is driven away with exceeding great
with exceeding great
Velocity. Opticks pag. 370.
1. In the ftreightness of its Pores)
Thus Ariftotle clearly expreffes him-
felf. The Sight will not penetrate
folid Bodies, because it can go only
through a ftreight Paffage (this the
Rays of Sun are an Evidence of, and
alfo our not feeing any Objects but
what are right before us) when there-
fore the direct Progress of the Sight is
hindred by the Pores not being_all
freight, it cannot pass through. But
the Sight will pass through fluid Bo-
dies, becauſe the Pores are small and
Streight; fo that it is not hindred from
going through them. Wherefore Glass
is tranfparent though it be very thick;
but a piece of Wood is not tranfpa-
rent, though it be very thin, because
the Pores of the former are regular,
and thofe of the latter irregular. Nor
does their being large fignify any thing
if they be not streight; neither are
rarer Bodies the more transparent,
unless their Pores are ſo diſpoſed as
to admit of a Paſſage. Prob. 61. Sect.
11. And indeed that ftreight Pores,
or rather fuch as croſs one another e-
very way from all Sides, are neceffary
to a Body's being tranſparent cannot be
doubted: But how it can be, that not
only Glafs and Diamonds, but alſo
Water, whofe Parts are fo eafy to be
We muſt know then, that all Bo-
dies whatſoever, have in them much
fewer Parts, and much more Pores
or void Spaces, than is requifite for
the greatest Number of Rays of
Light to find a free and open Paf-
For
in ftreight Lines all ways with-
out running upon the Parts.
fince Water is nineteen times lighter,
that is rarer than Gold; and Gold
it felf is fo rare, that it will very
eafily, without making any Refift-
ance, fuffer the Magnetick Effluviato
pafs through it, and will eafily ad-
mit Quickfilver into its Pores, and
will alfo let Water go through it, that
is, it has more Pores than folid Parts;
confequently Water will have above
forty times as many Pores as folid
Parts. And indeed you may think,Gold
and Water, and all other Bodies (with
great Probability) as much rarer ftill
as you pleaſe. For if we conceive the
Particles of Bodies to be fo difpofed
amongst themselves, that the Inter-
vals, or empty Spaces between them,
may be equal in Magnitude to them
alls and that theſe Particles may be
compofed of other Particles much fmal-
ler, which have as much empty Space
between them, as equals all the Mag-
nitudes of thefe ſmaller Particles :
And that in like manner, thefe fmal-
ler Particles are again composed of
others much ſmaller; all which toge-
ther, are equal to all the Pores or
empty Spaces between them, and fo
on perpetually, till you come to folid
Particles, fuch as have no Pores or
empty Spaces within them. And if
in any grofs Body there be, for in-
Stance, three fuch degrees of Particles,
the least of which are folid; this Bo-
dy will have feven times more Pores
than folid Parts. But if there be
four
Chap. 27. of NATURAL PHILOSOPHY.
203
16. I doubt not but that this Opinion will be eſteem- 16. A Con-
ed a Conjecture only. But if it fhall afterwards be made firmation of
this Conje-
appear to have in it all the Marks of Truth, and that dure.
all the Properties of Light can be deduced from it: I
hope that That which at firft looks like a Conjecture
will be then received for a very certain and manifeſt
Truth.
17. And firſt, that we are fitted by Nature to per-
ceive what we call Light, though there were nothing
that bore any Refemblance to it without us, we have
a very convincing Experience: For if, when it is the
Darkeſt that can be, we rub our Eyes in one particu-
lar manner, or if by chance we receive a very hard
Blow upon them, fo that the internal Parts of the Eyes
are very much ſhaken by the Blow, we fee Light, and
very bright Sparks, which ceafe as ſoon as the Motion
ceaſes.
four fuch degrees of Particles, the
leaft of which are folid, the Body
will have fifteen times more Pores
than folid Parts, If there be five
Degrees the Body will have one and
thirty times more Pores than Parts.
If fix Degrees, the Budy will have
Sixty and three times more Pores than
Solid Parts, and so on perpertually.
Newt. Opt. p. 243•
The Reafon therefore why fome
Bodies are Opake, is not the want of
Pores which are paffable on every
Side in ftreight Lines; but either
the unequal Density of the Parts, or
the Largenefs of the Pores, either
filled with other fort of Matter, or
elfe empty; by which means the
Rays of Light in palling through,
are perpetually bent backward and
forward by innumerable Reflections
and Refractions, till at laft they hit
upon the Parts themſelves of the Bo-
dy (See the Notes below on Art. 35.)
and fo are wholly extinguiſhed and
loft. Hence it is, that Čork, Paper,
Wood, &c. are Opake; and Claſs
Diamonds, &c. tranfparent.
For
in the Confines of Parts that are
alike, and of equal Denfity, as the
Parts of Glafs, Water, and Diamonds
are, by reafon of the equal Attracti-
on on all Sides, there is no Reflexi-
on or Refraction; and therefore the
Rays of Light which enter the firft
Superficies of thefe Bodies easily go
on (except fuch as chance to fall
upon the folid Parts, and åre extin-
guilhed. See the Notes on Art. 35.
below) in a right Line through the
whole Body. But in the Confines of
Parts which are very unequal in Den-
fity, fuch as the Parts of Wood or
Paper, compared with each other, or
with the Air, or empty Space in
the larger Pores of them, the great-
eft Reflexions or Refractions are
made, becauſe of the unequal At-
traction; therefore the Rays can by
no means paſs through fuch Bodies's
but are perpetually bent backward
and forward, and at laſt loft. That
this Difcontinuity of Parts is the prin-
cipal Caufe of the Opacity of Bodies,
will appear by confidering that O-
pake Substances become transparent,
by filling up their Pores with any Sub-
Stance of equal, or almoſt equal Den-
fity with their Parts. Thus Paper
dipp'd in Water or Oil, the Oculus
Mundi Stone Steeped in Water, Lin-
nen-Cloth oiled or varniſhed, and
many other Subſtances ſoaked in fuch
Liquors as will intimately pervade
their little Pores, become by that means
more transparent than otherwife; ſo,
on the contrary, the most transparent
Substances may by evacuating their
Pores, or feparating their Parts be
rendered fufficiently opake, as Salts
or wet Paper, or the Oculus Mundi
Stone, by being dried, Horn by being
fcraped, Glass by being reduced to Pow-
der, or otherwife flawed ;--and Water
by being formed into many fmall Bub-
bles-- become Opake. Newt.Opt.p.224.
18. Fur-
17. That me
are fitted to
perceive
Light.
204
ROHAULT's SÝSTEM Part I.
18. That
there is ſuch a
til Matter,
was proved
before.
18. Further, That there is fuch a Thing as fubtil Mat-
Thing as fub- ter which penetrates the Pores of tranfparent Bodies, the
Difpofition of which to recede from the Center of the
luminous Body in ftreight Lines, may here be called
fecundary or derived Light, has been fufficiently proved
before, when we fhewed the Neceffity of the fecond Ele-
ment; and we may venture to affirm, that none of thoſe
Things would come to pafs without it, which we have
before obſerved to come to paſs, when we explained thoſe
Motions which are ufually afcribed to the Fear of a
Vacuum.
19.
That lu-
minons Bo-
Matter all
Ways; and
what it is
that Flame
confifts in.
19. Nothing further remains, but to fhow that lumi-
ases push this nous Bodies do actually pufh this Matter every Way;
which they will be found to do, if it be true, that the Parts
are very ſmall, and very much agitated. Let us then ex-
amine all the luminous Bodies that we know, and fee if
the Parts of which they are compoſed, be not as fmall, and
as much agitated as we fuppofe. And to begin with Flame.
It has been already fo plainly demonftrated, that it is
compofed of Parts very fmall, and which move with
the greateſt Celerity, that it is fuperfluous to ſay any more
about it.
20. Whencel
20. We fee alfo, that there arifes very bright Sparks up-
it is that on ſtriking a Flint against Steel, or two Flints againſt each
Sparks arife,
pon ftriking other, or an Indian Cane againſt a common one, or by
or rubbing ftrokeing the Back of a Cat in the Dark, when the Wea-
two hard Bo- ther is dry and cold, and in a Multitude of other Things.
The Cauſe of all which, is only this, that fome of the
Particles of theſe Bodies being entangled between others
when they are ftruck, acquire in flying off, a Motion like
that of Flame, by which they in like manner puſh forward
the ſmall Globules of the fecond Element.
dies against
each other.
of rotten
21.The Cauſe
21. There is fome fort of rotten Wood, and of Fishes,
of the Shining when they begin to be corrupted, which fhine very bright.
Wood, and of Now a Body cannot putrify or be corrupted, but by the
fome Fishes Motion of its Parts, fome of which fly off (as is evident
shat are cor- in rotten Wood, from the Largeneſs of its Pores, and
rupted.
from its Lightneſs, which render it different from what
it was before, as a Coal, and the Wood out of which it
is made differ from each other.) We muſt own therefore,
1. And in a Multitude of other
Things) Thus likewife Amber :ub-
bed very hard in the Dark; Quick-
filver fhaken in aVacuum; and a Glaſs
our of which the Air is exhauſted,
if it be turned round very quick and
rubbed, will ſhine bright, not by im-
pelling or preffing upon the Parti-
cles of the fecond Element, for there.
is no fuch Thing; but by fending
forth fmall Particles which are the
very Light it felf.
that
Chap. 27. of NATURAL PHILOSOPHY.
105
that the Motion of the Parts which we fuppofe in lumi-
nous Bodies, is to be found here alſo.
I
· 22. Of the
22. It is not ſo eaſy to tell certainly, what fort of
Light of
Motion that is, which makes fome Worms and Flies to Glow-worms.
fhine in the Dark: However it is very probable, that
fome fort of Matter is exhaled out of theſe Infects, like
the Sweat of other Animals, and that this puſhes the
Matter of the fecond Element; and this is confirmed
from hence, that they ceafe to fhine as foon as they.
are dead.
23. Of the
Light of the
23. The Sun and the Stars are the moft luminous Bo-
dies of any that we know; but by reafon of their great Sun and
Diſtance, it is impoffible to make appear by any Experi- Stars.
ments taken near them, that all their Parts are in Moti-
on; all that we can affirm, is only this, that we do not
obferve any Thing to the contrary: And fince they pro-
duce the fame Effects in us, that Flame does, we
ought to think, that they refemble it in that by which
thefe Effects are produced, viz. in the Motion of their
Parts.
deceived in
buncle and
24. If it were true, what they fay of a Carbuncle and 24.That Na-
a Diamond, viz. that they fhine in the Dark; I thould turalifts are
freely own, that I am miſtaken in all that I have faid what they re-
about Light; for there is no Probability, that Bodies fo late of a Car-
hard, fhould be compoſed of Parts which feparately are a Diamond.
in any Sort of Agitation. But it is certain, that theſe
are only idle Stories, told without any Proof, and receiv-
ed by credulous Perfons, for I have often times experien-
ced the contrary my ſelf.
a
25. Tis true indeed, That a Diamond fhines very bright 25. What
in a darkiſh Place; but the Reaſon of this is, because it the Bright-
ness of a Di-
is fo cut, that the Sides reflect all the Light which they amond con-
receive towards the fame Part, as fhall be more fully fifts in.
explained * afterwards, when we come to treat of the
Refraction of Light.
26. We have lately had an Account from England, that
2 fome Diamonds rubbed in the Dark, have fhined fo
bright for a thort time, that a Word or two might be
read by Light of them. I have not obſerved this in
|
any
1. Is to be found here alfo.) The
famous Mr. Boyle made an Experi-
ment of this Matter, which is very
well worth taking notice of. He
put a piece of rotten Wood into the
Air Pump,which was in a manner ex-
tinguiſhed and ceaſed to ſhine, when
the Air was exhaufted; but upon let- | Dark) See Art, 20, above.
ting the Air in again, it feemed to be
new lighted, and ſhined as before. See
the Philofophical Tranfa&t. Numb. 31
For this was true Flame, and like
all other Flame cannot be preferved
without Air.
2. Some Diamonds rubbed in the
Diamonds
* Sect. 46.
26. Of the
Light of a
when it is
Diamond,
rubbed.
*
+
206
Part I.
ROHAULT's SYSTÉM
27. Of the
Boulogn-
Stone.
28. The
Reason of
this Stone's
Shining.
29. A Con-
firmation
hereof.
Diamonds that I have tried; however it may be true,
without contradicting any Thing that I have hitherto
wrote. For the Rubbing may raiſe fome Agitation, if
not in the Parts of the Diamond, yet at leaſt in ſome
Matter contained in the Pores of it, which continuing in
Motion in the fame manner as the Flame in the Pores of
a burning Coal, may for fome time puſh the ſecond Ele-
lement which is all round it, and diſpoſe it to raiſe a ſmall
Senfation of Light.
27. Though we have no Jewels which fhine in the
Dark, yet we have a Stone that is truly luminous: This
Stone was accidentally found by an Italian Chymift near
Boulogn in a hollow Place caufed by a Torrent. After
having put it into a Fire for fix Hours, he took it out,
and let it cool; and when it had been expoſed to the Light
of the Air for fome time, upon carrying it afterwards in-
to the Dark, he firft perceived it to look like a Fire-
Coal covered over with a few Ashes. I have feen fome
ſhine near half a quarter of an Hour, after which their
Light vanished, but by expofing them to the Light of the
Air for a ſhort time, we could make them ſhine again when
we pleaſed.
28. The Reafon hereof very probably is, that the Fire
has made this Stone extremely porous, fo that among the
Parts which are almoſt wholly disjoined from each other,
there may be fome fo eafy to be put in Motion, that
the Light of the Air alone is capable of agitating them,
and they may be fo difpofed to retain this Motion, that
they may keep it after they are removed from amongſt
the luminous Bodies, which put them in Motion; and
this is confirmed from hence, that when this Experiment
is often repeated, theſe Parts exhale, and the Stone quite
lofes its fhining Quality; which Quality cannot be prefer-
ved above four or five Years, though the Stone be
carefully fhut up in a Box, where no Light can come
at it.
29. For a further Confirmation of what has been faid,
we may obſerve, that if this Stone be kept too long in
the Fire, or though it be kept in it but fix Hours, yet
1. So easy to be put in Motion)
In much the fame manner may the
Phosphorus be accounted for; (the
manner of preparing it, is at large
explained by the famous Mr. Boyle,
to whom I refer you,) for it is
very probable, that fome fulphureous
Parts of the Urine, prepared over a
very hotFire, are fo volatile and eaſy to
be
put in Motion, that they are turn-
ed into a kind of Flame, by the
Agitation of the groffer, or perhaps
of the finer Air.
if
Chap. 27. of NATURAL PHILOSOPHY.
207
if the Fire be very hot, all the Parts of it which cannot
reſiſt the Fire, may be carried off, and then the remain-
ing Parts may be fo heavy, as not to be ſhaked by the.
Light; in which cafe the Stone ought not to fhine, and
fo we find by Experience.
30. Having thus fhown the Truth of thoſe three Things
which comprehend the Whole of our Conjecture, about
Primitive or Original Light, concerning what they call
fecondary or derivative Light, we obferve firft; that be-
cauſe it does not confiſt in the actual Motion of the fub-
tle Matter which fills the Pores of tranfparent Bodies, but
only in the Tendency or Difpofition which this Matter
has to Motion; it neceffarily follows, that luminous Bo-
dies, be they never fo diftant, ought to propagate their
Force, and to affect our Senfes in a Moment of Time;
becauſe the Matter which is pushed, being extended eve-
ry way without Interruption, like a very long Stick; the
luminous Body cannot pufh forward the neareſt Part of
it, but at the fame time it muft impell the furtheft Part
likewife.
30. That
Light ought
to be propaga-
red in a Me-
ment to all
Distances.
the Rays of
31. But perhaps fome may think, that this Train of 31. Dif
'A
Matter which is extended from one Point of the lumi- ficulty about
nous Body, to a Point of the Object which it illumi- the A Gion
nates, and which is called a Ray of Light, may more pro- Light.
perly be compared to a Thread than to a Stick, becauſe its
Parts are not fo firmly connected together, as thoſe of a
Stick are; and fo
and ſo it may be conceived, that as we can
move one end of a Thread, without moving in the leaſt
the other End, fo the luminous Body may impel the Mat-
ter of the fecond Element to which it is applied, with-
out neceffarily continuing that Impreffion to any great
diſtance. However, if we confider, that the World is
full of Matter, and that a Ray of Light is always fur-
rounded by a great many others, which hinder it from
bending, as a Thread does which is not furrounded
by others, we fhall be of Opinion, that every Ray of
1. To affect our Senfes in a Mo-
ment) It appears now from the Phe-
nomena of Jupiter's Satellites, which
get into the Shadow of Jupiter a
little fooner than they ought to do,
when the Earth approaches towards
Jupiter; and on the other hand,
come out of the Shadow a little la-
ter than they ought to do, when
the Earth departs from Jupiter (as
many eminent Aftronomers have ob-
ferved) that Light (which is a real
Body) is not propagated in a Mo-
ment of Time, but takes up about
feven Minutes in coming from the
Sun to the Earth, which is about
50000000 of Miles (See Newt. Opt.
p. 252.) What furprifing Things
follow from Lights not being pro-
pagated in a Moment, but in a certain
Space of Time. You may fee in the
Notes on Part II, Ch. xxy. Art. 3
Light
1
Part I
268
ROHAULT'S SYSTEM
I
Light ought to propagate the Force of the luminous
Body in the fame manner, as if it were as ſtiff as a Stick.
32. That a
32. In order to explain what is difficult in this Matter,
Body may
let us compare this Action of the ſecond Element which
propagate its
Action thro' tranfmits Light, to the Action of Water contained in a
an interme long thick Tube ftopped at the lower End; and then let
us confider, that all the fmall Threads of which this grofs
Column of Water is compofed, do every one in parti-
cular prefs with its whole Weight upon the Bottom; and
that if we pour in never fo little Oil, it will prefs
upon the Bottom in the fame manner as if we had pour-
ed it upon a ſtiff Stick.
diate Liquor.
33. That to
Action it is
tained in any
Veſſeld
33. If this Compariſon does not ſeem juft, becauſe in
propagate this this Inftance the Water is contained in a Veffel; take
not neceffary another: Suppoſe the Surface of the Earth, inſtead of
the Liquor being unequal and rough as it is now, were round and
Should be con- fmooth, and imagine it to be covered all over with Wa-
ter to a certain Height; then would every Point of the
Earth's Surface be preffed upon by the whole Weight of
the Thread of Water which correfpends to it; how com-
pare the Action of the Rays of Light to the Action of
this Water, and you will find, that they are capable of
acting in the fame manner, as if they were as ſtiff as a
Stick.
34. Why the
Action of
Light grows
weaker, the
more diftant
the luminous
Body is.
Tab. IV.
Fig. 3.
34. It is true however, and muſt be granted, that there
is fome Difference between theſe two Things: For the
Threads of the Water approach nearer and nearer to each
other, and tend to the fame Center, whereas the Rays of
Light go from the Center and fpread themſelves towards
the ſpherical Superficies which we may conceive all
round them: But this Difference will only be of uſe, to
ſhow us the Reaſon of a very remarkable Property of
Light; which is, that the Impreffion of the luminous Bo-
dy does not come entire to the Object; but is weaken-
ed and diminiſhed a little, according as it fpreads it ſelf, and
proportionably to its Diſtance from the Center of Action.
In order to explain this, let us fuppofe the Tube ABC,
which grows wider towards the Top, to be filled with
Water as high as DE, and that afterwards with a Sy-
ringe we put as much Water in at the End A of this
Tube, as will fill the Space AFG, which is of a confide-
rable Height, but of a fmall Breadth. It is certain, that
this Addition of Water, will raiſe up the Water at HI a
1. Ought to propagate the Force)
To propagate it indeed, but not
in ftraight Lines; as Light is really
propagated. See the Notes on Art. 15.
above.
little,
Chap. 27. of NATURAL PHILOSOPHY.
209
little, but that it will be fcarce fenfibly raiſed at DE. Now
this explains the Nature of Light perfectly well. For as
we cannot ſay that the Water at DE is not raiſed at all,
but only that it is raiſed but a very little: So we may
conclude, that the further the Rays of Light are diftant
from the luminous Body, the weaker they are; which a-
grees with Experience.
Tab.iv.
35. Now as we are certain, that a Body in Motion al- 35.Why
ters its Determination when it meets with another Body Light meet-
ing with cer-
that refifts it: So likewiſe we may conclude, that Light, tain Bodies
when it falls upon the Surface of a folid Body ought to be ought to be
turned back or reflected. Thus for Example, if the fmall reflected.
Globules which are in the Line CD reprefent the Parts
of the ſecond Element compofing a Ray of Light, which
falls upon the folid Body AB, its Action ought to be con-
tinued towards E, along the Line DE, in ſuch a manner,
as that the Angle of Reflexion BDE ought to be equal
to the Angle of Incidence ADC, that is, this Action ought to
be propagated in the fame Lines that the Globule C would
deſcribe, if it were alone, and moved in the Line CD:"
For
1. Upon the Surface of a folid Body)
The Reflexion of the Rays of Light
is cauſed, not by falling upon the
Parts themſelves of the reflecting
Body, but by a certain Power equal-
ly diffuſed all over the Surface of the
Body, whereby it acts upon the Ray
to attract or repel it, without imme-
diate Contact; by which fame Power
in other Circumftances the Ray is re-
fracted and by which fame Power
it is at firft fent forth from the lu-
cid Body; as the fore-cited admira-
ble Perfon has demonftrated by many
Arguments.
;
I. Though thofe Glaffes which we
call plain and poliſhed, do indeed
appear to the Eye to have a ſmooth
uniform Surface; yet in reality, (fince
poliſhing is nothing elfe but wear-
ing away and breaking the Protu-
berances of the Glafs, with Sand,
Putty, or Tripoly) their Surfaces are
very far from being plain and finooth:
Now if the Rays of Light were re-
flected by impinging on the folid
Parts of the Glafs, their Reflexions
could not be fo exact and regular,
as we find they are; nay, the Kays
ought to be difperfed all Ways, al-
moft as much by the best polifhed
Glaſs, as by the rougheft. See Newt
Opt, p. 240.
II. If the red and blue Rays
which are ſeparated by a Priſm (the
manner of doing which, See in the
Notes on Art. 65. below.) be all of
them caft on a fecond Prifm,' in fuch
manner, that they are all alike inci-
dent upon it; the fecond Prifin may
be fo inclined to the incident Rays,
that thoſe which are of a blue Colour,
fhall be all reflected by it, and yet
thofe of a red Colour (though falling
with the fame Obliquity) pretty co-
piously tranfmitted. Now if the Re-
flexion be caufed by the impinging
of the Rays upon the Parts of the
Glafs how comes it to pafs, that
when all the Rays fall with the fame
Obliquity, the Blue fhould wholly
impinge on the folid Parts, fo as to
be all reflected, and yet the red find
Pores enough in the fame Place to
be in a great meaſure tranfmitted ?
Pag. 239.
III. Where two Glaffes touch one
another, there is no fenfible Reflexi-
on, and yet there is no Reafon why
the Rays fhould not impinge on the
Parts of Glafs as much when con-
tiguous to other Glafs, as when con-
tiguous to Air. Ibid.
IV. When the Top of a Water-
bubble, made by the working up of
Soap and Water, by the continual
P
Fig. 4.
210
Part I
ROHAULT'S SYSTEM
J
For it is evident, that the Globule D ought to have a
Tendency, and to be diſpoſed to go where it would really
go, if its Power were put into act. And fince this Glo-
bule, upon meeting with the Body AB, would neither go
towards G, nor towards H, but only towards F, it muſt
be allowed, that it is the Globule F only which is impel-
led
·
fubfiding and exhaling of the Water
grows very thin; there is no mani-
feft Reflexion, not only at the
leaſt Thickneffes, but alfo at many
other Thickneffes of the Bubble con-
and yet
tinually greater and greater;
in the Superficies of the thinned Bo-
dy, where it is of any one Thickneſs,
there are as many folid Parts for the
Rays to impinge on as where it is
of any other Thickneſs. Ibid.
V. If the red and blue Rays fe-
parated by a Prifm (the manner of
doing which, as was faid before, you
may fee in the Notes on Art. 65. be-
low) be afterwards caft diftin&tly and
fucceffively upon a thin Plate of
any tranfparent Matter, whofe Thick-
nefles grow continually greater and
greater (fuch as a Plate of Air con-
tained between a plain Glaſs, and a
Glafs that is a little gibbous, fuch as
the Object-Glafs of a long Telef-
cope) this Plate in the very fame
Part of it will reflect all the Rays
that are of one Colour, and tranfmit
all thoſe that are of another Colour;
in different Parts of it, it will tranfmi:
Rays of the fame Colour at one
Thickneſs, and reflect them at ano-
ther, and this by innumerable Fitts.
Now it is not any way to be ima-
gined or conceived, that it can ſo
happen by chance, that in the very
fame Part of the Plate, and with the
very fame Obliquity of the Rays,
all the Rays that are of one Colour
fhould impinge upon the folid Parts,
and all the Rays that are of another
Colour fhould hit upon the Pores
only; and that in different Parts of
the Plate, in one Place the blue
Rays fhould all impinge upon the
Parts of the Body, and the red Rays
run all into the Pores, and in ano-
ther Place where the Plate is a little
thicker or a little thinner, on the
contrary the blue Rays only fhould
run all into the Pores, and all the
red Rays impinge upon the Parts.
Pag. 240.
1
VI. In the Paffage of Light out
of Glaſs into Air there is a Reflexi-
on as ftrong as in its Paffage out
of Air into Glaſs, or rather a little
ftronger, and by many degrees ftron-
ger than in its Paffage out of Glafs
into Water. And it feems not pro-
bable, that Air fhould have more re-
flecting Parts than Water or Glafs.
But if that fhould poffibly be fup-
pofed, yet it will avail nothing, for
the Reflexion is as ftrong or ftronger
when all the Air is removed from
the further Surface of the Glaís, as
when it is adjacent to it. p.237. Now
if any one ſhould imagine accord-
ing to the Opinion of Cartes, that
the fubtle Matter at the further Sur-
face of the Glafs is denfer than any
other Matter whatſoever, and upon
that Account more ſtrong to reflect
Light than any other Bodies; be-
fides that we have before demon-
ſtrated, that that Matter is only a ficti-
tious Thing; and that if we fhould
allow this Matter, and its Power to
reflect Light, the Light could not be
propagated by it at the Beginning,
but muft immediately be all reflected
back upon the lucid Body as foon
as it is fent forth from it; befides
thefe I fay, he will be convinced of
the Fallity of this Fiction by the
following Experiment.
VIII. If Light in its Paffage out
of Glafs into Air be incident more
obliquely than at an Angle of 40 or
41 Degrees, it is wholly reflected,
if lefs obliquely, it is in great mea-
fure tranfmitted. Now it is not to
be imagined, that Light, at one De-
gree of Obliquity, ſhould meet with
Pores enough in the Air to tranf-
mit the greater Part of it; and at
another degree of Obliquity, fhould
meet with nothing but Parts to re-
flect it wholly; eſpecially, confider-
ing, that in its Paflage out of Air
into Glafs, how oblique foever be its
Incidence, it finds Pores enough in
the Glaſs, to tranſmit a
great Part of
it. If any Man fuppofe, that it is
not reflected by the Air, but by the
outmoſt fuperficial Parts of the Glafs,
that will appear to be falfe, by ap-
plying
:
→
Chap. 27. of NATURAL PHILOSOPHY.
And this is
led by it, and which receives its Action.
confirmed by Experience. For when the Light falls up-
on the Surface of any Opake and folid Body, as Gold or
Steel, we ſee its Rays are reflected, and the Angle of
this Reflexion is equal to the Angle of Incidence.
are no tranf·
parent Bodies
36. Now this being fo in one folid Body, fuchas Gold 36. That there
or any other Metal; as it is a general Truth, it ought to
extend to all Sorts of folid Bodies, and the Light ought but reflect
to be reflected in Angles equal to thoſe of their Incidence. Some Rays of
Wherefore fince the Pores of two tranfparent Bodies which
touch each other, cannot exactly anſwer to one another;
plying Water or Oil behind fome,
part of the Glafs inftead of Air.
For fo in a convenient Obliquity of
the Rays ſuppoſe of 45 or 46 De-
grees, at which they are all reflected
where the Air is adjacent to the.
Glaſs, they will be in great meaſure
tranfmitted where the Water is ad-
jacent to it; which argues, that their
Reflexion or Tranfmiffion depends
on the Conftitution of the Air and
Water, or Oil behind the Glafs, and
not on the ſtriking of the Rays up-
on the Parts of the Glafs, viz. that
the Rays are not reflected till they
get to the further Surface of the
Glafs, and begin to go out of it.
For if when they are going out of
it, they fall upon Oil or Water,
they go on, becauſe the Attraction
of the Glaſs is almoft ballanced and
rendred ineffectual, by the contrary
Attraction of the Liquor that ſticks
to it. But if the Rays, which go our
of the further Superficies, go into a
Vacuum, which has no attractive
Force, or into Air which has very
little, and therefore cannot ballance
the Attraction of the Glafs, and ren-
der it ineffectual, then the Attraction
of the Glafs reflects them, by draw-
ing and bringing them back. And
this is ftill more evident, by laying
together two Priſms of Glafs, or two
Object-Glaffes of very long Telef-
copes, the one plain, the other a
little convex, and fo compreffing
them, that they do not fully touch, nor
are too far afunder. For the Light
which falls upon the farther Surface
of the firft Glafs, where the Inter-
val between the Glaffes, is not above
the Ten hundred thousandth part of
an Inch, will go through that Sur-
face, and through the Air or Vacuum
between the Glaffes, and enter into
the fecond Glafs. But if the fecond
4
Glafs be taken away, the Light
which goes out of the fecond Sur-
face of the firft Glaſs into the Air
or Vacuum that is between the Glaf-
fes, will not go on forwards, buz
turns back into the firft Glaſs, and
is reflected. From whence it is
evident, that the Rays are drawn
back by the Power of the firſt
Glafs, there being nothing elfe
to turn them back. p. 238, and
347. And hence it is alfo manifeft,
as was before obferved, that the
Rays are not reflected by any fub-
tle Matter or Æther, becauſe tha:
Matter ought to reflect them not at
all the lefs, when the fecond Glaſs
is fo placed as not quite to touch the
firſt, than when it is quite taken
away.
Lastly, If any one ſhould ask;
becauſe we have afcribed the Reflexi-
on of Rays to the Action of the
whole Superficies of Bodies, with-
out immediately touching them;
how it comes to pafs, that all Rays
are not reflected by all Superficies:
but while fome are reflected, others
are refracted and enter in: This ex-
cellent Perfon fhows, that there are
certain Vibrations (or fome fuch
kind of Property) both in the Bo-
dies themfelves, and in the Rays of
Light, impreffed upon the Rays,
either by the Action of the Body
which emits them, or by the A&i-
on of fome other Bodies; whence
it comes to pafs, that thofe Rays
which are in that Part of their
Vibration which confpires with the
Motion of the Parts of the Body,
enter into the Body, and are tranf-
mitted by Refraction; and thofe
which are on the other Part of their
Vibration, are reflected. See Nemt.
Opt. p. 255.
P
2
and
Light.
211
Part I
ROHAULT'S SYSTEM
37. How the
as they pass
out of one
and therefore many of the Pores, of Air for inftance, may meet
with the folid Parts of Water, Glafs, or Chryftal; it is im-
poffible, but that tranfparent Bodies muft reflect fome
part of the Light which falls upon their Surface; and they
muſt reflect ſo much the more, as the Rays fall more
oblique, becauſe in that Pofition they meet with more
of the folid Parts of the tranfparent Body upon which they
fall.
37. Let us now confider, what will happen to Rays that
are refracted Pass out of one tranfparent Medium into another, upon
whofe Surface they fall obliquely. We foreſee that they
ought to be refracted agreeably to what was faid before
transparent concerning Refraction, becauſe theſe tranſparent Bodies
being of a different Nature, the one may afford an eaſier
Paffage to the Light than the other, and fo the Rays ought
to be lefs inclined, or nearer to the Perpendicular on that
Side which more eafily admits them.
Medium into
another.
38. The har-
der a tranf-
parent Body,
fo much the
eafter will the
Light pals
through it.
38. Nor are we to think, that a tranfparent Body will
afford fo much the eaſier Paffage to Light, by how much
the eaſier it yields to other groffer Bodies which make
Way for themfelves, by removing its Parts: Juſt the
contrary: For as the Paffages for Light are already made,
1. That they ought to be refracted)
The Rays are refracted, not by fal-
ling upon the very Superficies of Bo-
dies, but without immediate con-
tact, by that very fame Power by
which they are emitted or reflected,
exerting it felf differently in diffe-
rent Circumſtances, as may be de-
monftrated by the fame Arguments
as were before made uſe of about
Reflexion without Contact, and alfo
by the following ones.
1. Because when Light goes out of
Glaſs into Air, as obliquely as it
can poſſibly do, if its Incidence be
made ftill more oblique, it becomes
totally reflected. For the Power of
the Glass, after it has refracted the
Light as obliquely as is poffible, if
the Incidence be made ftill more
oblique, becomes too strong to let
any of its Rays go through, and
by confequence caufes total Reflex-
zons.
2. Becauſe Light is alternately
reflected and tranfmitted by thin
Plates of Glafs for many Succeffi-
ons accordingly as the Thickness of
the Plate increases in an arith-
metical Progreffion. For here the
Thickness of the Glafs determines,
whether that Power by which Glaſs
acts upon Light ſhall cauſe it to be
reflected, or suffer it to be trans-
mitted.
3. Because these Surfaces of tranf-
parent Bodies which have the great-
eft refracting Power, reflect the great-
eft quantity of Light. Newt. Opt.
p. 244.
4. Becauſe, although the Forces of
Bodies to reflect and refract Light, are
very nearly proportional to the Den-
yet
fities of the fame Bodies;
unctuous and fulphureous Bodies
refract more than others of the
fame Denfity. For the Rays act.
with greater Force upon thofe Bo-
dies to fet them on Fire, than they
do upon others; and thefe Bodies
act upon the Rays again with
greater Force by mutual Attraction
to refract them. p. 245. &c.
Laftly. Becaufe, not only the Rays
which are tranfmitted through Glafs
are reflected; but alfo thoſe which
are near the Extremities of it in
Air or in a Vacuum, or even thofe
which are near the extreme Parts
of any opake Bodies (as the Edges
of Knives, &c.) are bent by the At-
traction of the Body. p. 293, &c.
it
Chap. 27. of NATURAL PHILOSOPHY.
it can move fo much the eaſier as the Parts of the Bo-
dy through which it paffes, are more difficult to be put
out of their Places; becauſe it is the lefs liable to loſe
its Motion in paffing, in the fame manner as a Bowl
will run eaſier upon the firm hard Ground, than upon
foft Ground, or upon the Grafs. And thus as Water
is in fome Senfe harder than Air, and Glafs harder than
Water, and Chryſtal harder than Glaſs, it follows, I hat
Light ought to pafs more eafily through Water, Glafs
and Chryftal, than through Air; and its Rays ought to
be leſs inclined, or to approach nearer to the Perpendi-
cular in theſe Bodies than in Air.
Air into
VVater,
Tab. IV.
Fig. 5.
39. This may be tried many Ways; I will fhow you 39. An Ex-
one that feems to me very evident. I caufed a Brafs periment of
the Refracti
Box ABCD to be made, with a Cover to it of the
on of Light in
fame Metal. The Bottom BC was a Piece of Venice palling out of
Chryftal, under which I glued a piece of Paper, with
feveral Marks made upon it at Pleaſure. I expofed this
Box to the Rays of the Sun, that a Ray, fuch as FE
might pafs the Cover at the Hole E, and looking un-
derneath, I obferved the Point,G, which the Ray came
to; then without altering the Situation of the Box, which
was full of Air only, I filled it with Water, which I
poured in at the Hole M; then I obferved, that the
Ray did not come fo far as G, but only to L, fo
that it was nearer the Perpendicular HI, than it was be-
fore.
•
VVater into
Air.
Tab. IV.
Fig. 6.
40. Now to find whether a Ray paffing out of Water 40. An Ex-
into Air be turned from the Perpendicular, we may make periment of
the Refracti
uſe of a very common Experiment. We may put any on of Light
Body, a piece of Money fuppofe, at the Bottom of a paffing out of
hollow Veffel, which contains nothing but Air; then we
may move our Eye B back, till the Edge of the Vef-
fel juft hides the Object A; then let the Veffel be fil-
led with Water after which, the Object without ha
ving changed its Place, will begin to appear by the Ray
CB, which coming from A by C, will be bent, and re-
moved from the Perpendicular ECF, whereas otherwiſe the
Ray would have gone ftreight on to D.
1. That Light ought to pass more
cafily) Mr. LeClerc has committed a
furprizing Miftake here. Therefore,
fays he, the greater the Refiftance of
the Body is upon which the Ray falls,
So much the more does it recede
from the Perpendicular, and the lefs
the Refiftance, the iefs does it receđe.
P
+
Wherefore a Ray falling upon Wa
ter out of Air, goes further from the
Perpendicular; on the contrary, a
Ray coming out of Water into Air ap-
proaches nearer to the Perpendicular;
becauſe Air refifts it less than Water.
Phyf. Book V. Chap. viii. Sect. 17.
Contrary to all Experience.
41. Be-
3
14
Part I
ROHAULT'S SYSTEM
Fig. 7.
1
!
41. Of the
41. Becauſe Refraction will be of great Ufe hereafter,
Refraction of it is worth while to explain the Nature of it fully, by
Light paffing
confidering how it is made, when Light paffes out of
through a
Glass Prifm. Air into Glaffes of various forts of Figures. Suppofe
Tab. IV. then, in the firft Place a triangular Prifm ABC, up-
on one side of which, fuppofe AB the Ray DE falls
obliquely. From what was faid before concerning the
Rays paffing out of Air into Glafs, it follows, that it
ought not to go on in a ftreight Line to F, but to G,
in order to approach nearer the Line HEI, which is
fuppoſed to be drawn through the Point E, upon which
the Ray falls, and to be perpendicular to the Surface
AB. After which, the Ray EG paffing obliquely out
of Glafs into Air, ought not to go directly to L, but
to M, becauſe it is turned from the Perpendicular
NGO.
42. Of the
through a
Tab, IV.
Fig. 8.
42. Suppoſe now a Lens or a Glaſs convex on both
Refraction of Sides, fuch as is repreſented by the Figure 2B3K, and
Light palling imagine a great many parallel Rays, fuch as AB, CD,
Convex Light. EF, to fall upon its Surface; now in order to find out
how theſe Rays ought to be refracted, we muſt firſt
draw through the Points B, D, F, Lines perpendicular
to the Glaſs, that is, the Lines ABK, HDI, LFM,
tending towards the Point G, which I fuppofe to be the
Center of the Superficies 2B3. This being done, we
may confider, that the Ray AB, being in the Perpendi-
cular it felf, ought not be at all refracted as it paffes out
of Air into Glafs, but to go on directly towards K, where
it falls again perpendicular upon the Superficies of the
Air 2K3 (becauſe it comes from the Point R, which is
the Center of this Superficies) and therefore it will con-
tinue to go ftrait on ftill towards G, without any Refra-
ction. But as to the other Rays, fuch as CD, and EF,
becauſe they do not fall perpendicularly, it is evident,
that they will not go directly to O and N, but will ap-
proach nearer to the Perpendiculars HI, LM, and go to
Qand P, and by this means they will tend towards the
Ray ABK; and becauſe, having drawn the Lines, TQL,
SPM perpendicular through the Points P and Q, that is,
the Lines which tend to the Point R, we find that the
Rays DQ, FP fall obliquely on the Surface of the Air,
we conclude, that they will be refracted, and go from
the Perpendicular. So that DQ will not go directly to X
but to G, and FP alfo will not go directly to V, but to
|- Bud Triangular Prism) See the Notes on Art. 65, below.
the
Chap. 27. of NATURAL PHILOSOPHY.
21S
the fame Point G. The fame may be demonftrated of
the Rays, that fall on the other Side of AB, which will be
bent fo, as to interfect the firſt, fomewhere near the Point
G; thus we fee, that it is the Property of a Convex-Glaſs,
to collect together the Rays of Light which fall parallel
upon it.
the Rays
Placec.
44. Of the
Refraction of
Light paffing
through a
Concave-
Glafs.
Tab. IV.
43. If whilft the Glaſs remains in the fame Situation,
43. Of the
parallel Rays fall upon it from fome other Place, we fhall Refraction of
find that they will meet together in fome other Point, and which come
not in G; thus if they come from the right Side of thoſe from different
before drawn, they will meet on the left Side, viz. near
Y; and on the contrary, if they come from the left
Side, they will meet on the right Side fomewhere near Z.
44. Let us confider in the Third Place, A Glafs that
is thinner in the Middle than at the Edges, that is, a Glafs
concave on both Sides, fuch as is repreſented by GBHIMK,
and ſuppoſe the parallel Rays, AB, CD, EF, to fall up-
on it. Now in order to fee how they ought to be refracted,
let us erect Perpendiculars at the Points B, D, F, where Fig. 9.
they enter the Glaſs: This being done; fince the Ray
AB coincides with the Perpendicular, it will enter the
Glaſs as far as M without any Refraction, where becauſe
it falls perpendicularly upon the Superficies of the Air, it
will no more be refracted at going out, than it was at
entring into the Glafs, and confequently it will go direct-
ly to L. But becauſe the Ray CD falls obliquely upon
the Surface of the Glaſs, it will not go directly to P, but
will turn to Q, becauſe it tends towards the Perpendicu-
lar NDO; and becauſe the Ray DQ falls obliquely up-
on the Surface of the Air alfo, it will not go directly to
T, but will be refracted towards V, becauſe it goes from
the Perpendicular RQS. So likewife if we examine the
Ray EF, we ſhall find by the like Way of Reafoning, that
it will go to Y, and from thence to Z. Whence we fee,
that it is the Property of a Concave-Glafs 2 to difperfe the Rays
which fall parallel upon it.
1. Somewhere near the Point G)
For the Rays are not collected to-
gether exactly into the
Tab. IV. fame Place, and the Fo-
Fig. 8.
cus is not in a Point, but
in a fmall Line, that is,
in part of the Line KG, ſo that ſome
of the Rays meet with each other
nearer the Point K than others of
them. Thus for Inftance, if the
Glafs be equally gibbous on both
Sides that Line will be of the
whole Thickneſs BK. See Hugen's
Diopt. Prop. 27. P. 94. and Bar-
row, Sect. V.
2. To difperfe the Rays) In fuch
a manner that they may feem to
come from a fmall Line,
or fuch Part of the Line Tab. IV.
AB as the foremention- Fig. 9.
ed fmall Line was, into
which they were gathered in paffing
through a Convex-Glafs.
P 4
45. Let
216
Part 1
ROHAULT's SYSTEM
45. How the
Light is re-
fracted in
paffing thro'
a Glafs that
many Super-
ficies.
has a great
Tab. V.
Fig. 1.
46. VVherein
45. Let us confider in the Fourth Place, a Glafs cut
with feveral Surfaces on the one Side, but plain on the
other, fuch as is reprefented by the Figure ABCDETS,
and fuppofe the Rays FG, HI to fall parallel upon it.
Draw Perpendiculars in the Points G and I; then be-
caufe, from what was before faid, thefe Rays ought to go
towards the Perpendiculars, we are fure that they will
bend towards K and Q; and becauſe they again fall ob-
liquely upon the Surface of the Air ST, we conclude that
they will be refracted a fecond Time; fo that GK will
tend towards L, and IQ towards M; and becauſe all the
parallel Rays that fall upon the fame plain Superficies, are
equally inclined to it, they will be equally refracted, and
confequently will be parallel when they come out, fo that
thoſe which fall upon the Superficies BC will go along
with the Ray KL, and thofe which fall upon AB,
CD, DE, will go along with the Rays QM, PN,
and RO.
46. So that if the Surface TS were covered with an
the Leftre of opake Body which receives all the Rays of Light that fall
precions
Stones con-
upon the Superficies AB, BC, CD,
fifts.
47. Of the
Refraction of
Glafs.
Tab. V.
Fig, 2,
none of them will come upon the p, it is evident, that
SQ and RT, and
confequently they will look darker; whereas the Part QR
receiving all the Light which falls upon every one of the
Surfaces ought to appear very bright; and herein con-
fifts the Luftre of a Diamond and other precious Stones
which are any way tranfparent. For they will not fhine,
unless they be cut with a great many Superficies in
fuch a Manner as to turn the Rays of Light towards
one Place at the Bottom, where is a fmall Plate of
Gold or Silver to receive the Light, and reflect it back
to our Eyes.
47. Laftly, Let us ſuppoſe a plain Glafs of equal Thickness
Light paffing every where, fuch as ABCD, upon which the parallel
thro' a plain Rays, EF, GH, IL, if they fall obliquely, fall with equal
Obliquity, fo that they are equally refracted, by approach
ing every one of them towards the Perpendicular, and
therefore go to M, O, and Q, being ftill parallel, and
confequently equally inclined to the Surface BC; whence
it follows, that in paffing into Air, they recede equally
from their Perpendiculars, and fo continue always paral-
lel. But we muft obferve here, that the Rays EF, GH,
IL, which incline towards the Right, when they firſt en-
ter into the Glafs, are inclined as much towards the Left,
when they come out of it: So that we may fay, the Glass
L
un-
Chap. 27. of NATURAL PHILOSOPHY.
217
I undoes that by the fecond Refraction, which it did by
the Firſt. 2
48. Since Light not only fhines, but heats alfo, we 48. That all
may here add, that though we cannot perceive any Ine- Sorts of Light
quality in the Action of luminous Bodies, but that they producing
are capable of
feem to impell uniformly the ſecond Element which fur- Heat.
rounds them, towards thofe Bodies which terminate their
Action; yet Reaſon fhows us, that they act more ſtrong-
ly at fome times than at others; not only becauſe their
Parts are not all equal, nor are they always the fame
which are applied to the fame furrounding Matter to im-
pell it; but alſo becauſe this Action is at firſt commu-
nicated to a tranſparent and liquid Medium, the Parts of
which continually move out of their Places. And this
cauſes the ſmall Globules of the fecond Element to im-
prefs a kind of Trembling upon the Parts of the Bodies
to which they are impelled by the luminous Bodies; and
becauſe Heat confifts in fuch a kind of Agitation, it
follows, that all luminous Bodies ought to produce fome
Heat.
49. However, it may happen that this Heat may not 49. VV hy we
be at all perceivable, either becauſe of the Weakneſs of do not feel the
Heat of fome
the luminous Body, or becauſe the Organ upon which luminous Bo-
it acts is hotter than it. Thus if coming from a Fire we dies.
expoſe our ſelves in a cold Night to the Rays of the
Moon, we fhall find it very cold; becauſe in fuch Cir-
cumſtances, we give more Heat to the Air which ſurrounds
us, than that does to us.
50. And as the Sun is very bright, fo ought it to raiſe so. The fur-
the moſt fenfible Heat in us; and fo we find by Ex-prifing Power
of the Sun's
perience every Day that it does; nay to that Degree, Heat.
that when its Rays are collected by a concave-Glafs, they
will not only fet combuftible Bodies on which they fall,
on Fire, but will melt Metals, Stones, and Flints,
1. Undoes that by the ſecond Re-
fraction) We must have a Care of
thinking, that the ſecond
Tab. V. Refraction fo undoes the
Fig. 2.
first, that the Object is
feen in its true Place; for
the Ray BQ extended backwards
will not coincide with the Ray LI,
but fall to the right Hand of it, and
that fo much the more, the thicker
the Glafs is. But as to Colours, the
fecond Refraction does indeed undo
the firft. See the Notes on Art. 65.
2.That double and irregular Re-
fraction of Island Chryftal, where-
by not only the oblique Rays are
feparated into two Parts on the fame
Superficies by a double Refraction;
but alfo thofe that fall perpendicular-
ly are half of them retracted like-
wife, is very different from all thoſe
hitherto explained: The Explication
of this you may fee in Newt. Opt.
p. 331.
which
218
Part I.
ROHAULT's SYSTEM
51. That the
is not the im-
which are very difficult to melt with Fire; as I my felf
have ſeen.
51. Having fufficiently explained the Nature of Light,
Coloured Body and the common Properties of it; the first Thing that
mediate Canfe We obferve concerning Colours, is, that they are not per-
of the Senfa- ceived by the immediate Application of the coloured Ob-
ion of Colour.
ject to the Organ of Senſation: From whence it follows,
that it does not of it felf excite in us that Senſation of
Colour which we have upon looking on it; for we cer-
tainly know, that one Body cannot act upon another with-
out immediate Contact; but whatever there may be in
the coloured Object, in which its Colour confifts, we
muſt think, that it acts thereby upon fome Medium which
it finds, and by that Means acts afterwards upon our Or-
gan of Senfation.
52. That it is
tion of Colours
in us.
52. If the coloured Object only had been confidered,
the different which generally is at reft, when it affects the Senfes, I
Modification doubt the manner of its acting upon the Medium would
of the Rays of
Light that
never have been diſcovered, and confequently we ſhould
confes the dif- never have known diſtinctly what Colour confifts in. But
ferent Senfa- if we obferve, that ſuch Bodies are not to be perceived
in the Dark, and that in order for them to appear co-
loured, it is neceſſary for them to have fome Light, the
Nature of which is to be reflected, when it meets with
a Body which it cannot penetrate; it is eaſy to conclude,
that it is the Light which acts upon our Organ of Sen-
fation to make us perceive any Colour, and that the whole
Action of the coloured Body confifts in giving it fome Mo-
dification which it had not before.
1. Some Modification which it had
not before.) In order to explain the
Nature of Colours we must ob-
ferve,
(1.) That it is found by Experi-
ence, that the Rays of Light are
compounded of Particles different
from one another: that is, which
are (as is highly probable) fome lar-
ger and fome fmaller.
(2.) That a Ray, fuch as FE,
falling upon a refracting Superficies
in a dark Room, is not
Tab. IV. refracted whole to L, but
Fig. 5.
as it were ſplit into a
great many fmaller Rays,
fome of which are refracted to L,
others of them to fome other Points
betwixt L and G: That is, (as is
very probable likewife) thoſe Par-
ticles of Light which are ſmalleſt,
I
53. This
are the eafieft of all, and the moſt
turned out of a ſtraight Line towards
L, by the Action of the refracting
Superficies; and the rest of them,
according as they exceed each other
in Bignels, are more difficultly, and
lefs turned out of a right Line, to
the Points betwixt G and L.
(3) Thofe Particles of Light
which are moft refracted, make a
fmall Ray of a Violet Colour; that
is (as is very likely) the ſmalleſt
Particles of Light, feparated from
the reft in this manner, excite the
fhorteft Vibrations in the Tunica Re-
tina, to be propagated from thence
along the folid Fibres of the optick
Nerves into the Brain, there to ex-
cite the Senfation of Violet Colour,
the darkeft and the faintest of all
Colours. And thofe Particles which
are
Chap. 27. of NATURAL PHILOSOPHY.
219
53. This being fuppofed, there cannot be an eaſier 53. That the
Way to come at the certain Knowledge of the Nature Roughness of
the Superfi-
of Colours. For fince Light is nothing elſe but a parti- cies of a Body
cular Motion of the fmall Globules of the fecond Ele- does alone
modify the
ment, or at leaſt a Difpofition to a particular Sort of Mo-Action of
tion; nothing more is requifite for the underſtanding of Light.
Colours, but only to examine the different Modificati-
ons which this Motion is capable of, and to find out what
there is in the Bodies which we call coloured, to cauſe
thefe Modifications. Now the firſt Thing which offers it
felf, and which is the moft fimple Modification, is this,
viz. That this Motion cannot but be weak, if all the
Rays
are refracted leaſt, they make a ſmall
Ray of a red Colour; that is, the
biggeſt Particles of Light, excite the
longeſt Vibrations in the Tunica Re-
tina, in order to raife the Senfation
of a red Colour, the brighteft of all
Colours; and the other Particles are
alfo every one feparated into fmall
Rays, according to their Bignefs and
Refrangibility, in order to excite
intermediate Vibrations, which raiſe
the Serfations of intermediate Co-
lours. Much in the fame manner, as
the Vibrations of Air, according to
their different Bignefes, caufe Seafa-
tions of different Sounds.
(4.) The Colours therefore of thoſe
fmall Rays, fince they are not acci-
dental Modifications of them, but
connate, original, and neceffary Pro-
perties of them, confifting (as is
highly probable) in the different
Magnitudes of them, are permanent
and unchangeable; that is, fuch as
cannot be altered by any future
Refraction, Reflexion, or any other
Modification.
(5.) As the Rays of different Co-
lours begin in this manner to be fe-
parated by the fingle Refraction of
one Superficies; fo that Separation
is much more compleated (fo as ve-
ry eafily to be perceived by our
Senfes) by that double Refraction
(the Firft being increafed by the Se-
cond) which is made in the two
Sides of a Triangular-Glafs Prifm,
(the Phænomena of which are fully
explained in the Notes on Art. 65.
below) and in the double Refracti-
on made in the Superficies of Glaffes
of other Figures, according as their
Superficies are further from being
parallel to each other, fuch as the
Object Glafles of Teleſcopes,
.
(and this is the Reaſon why they
cannot be made perfect, vix. becaufe
of the Separation of the coloured
Rays. See the Notes on Chap. xxxiii.
Art. 28.)
(6.) As the Rays of different Co-
lours are feparated by the Refracti-
ons of Priſms, and other thick Bo-
dies, fo are they likewife feparated
in another manner, in very thin
Plates of any tranfparent Matter.
For all Plates, which are thinner than
a certain determinate Thickneſs,
tranſmit the Rays of all Colours, and
reflect_none; but as their Thickneſs
increaſes in an Arithmetical Progref-
fion, they begin to reflect, firſt, kays
that are intirely Blue; then Green,
Yellow, Red, in order; and again,
Blue, Green, Yellow, Red; but more
and more faint and mixed; till at
laft, when they come to a certain
Thickneſs, they reflect the Rays of
all Colours throughly mixed together,
just as they fell upon them, and
thefe make White. And in that
Part of the thin Plate where it re-
Alects any Colour, for Inftance,
Blue, it always tranfmits the con-
trary Colour, viz. Red, or Yellow:
For the Truth of all which Phæno-
mena, found out by numberless Ex-
periments, and for the Calculation
of what Thickneſs the Plate ought
to be, to reflect particular Colours,
and for the Reafons why Plates of
particular Thickneſſes reflect parti-
cular Colours in this manner: See
the eminent Sir Ifaac Newton moſt
clearly difcourfing in his Opt. Book II.
(7.) All natural Bodies are made
up of very thin tranfparent, fmall
Plates; which, if they be fo regu-
larly difpofed, with regard to each
other, that there is no Reflexions
ΟΣ
220
Part I.
ROHAULT's SYSTEM
1
54.V Vhat
the Nature
Banfifts in.
Rays of Light which fall upon an Object in a certain
Order, and in a certain Quantity, be not reflected back
in the fame Order, nor in the fame Quantity towards one
determinate Place of the Medium where the Eye is fixed:
And we are fure, that this muſt neceffarily happen, if the
very ſmall Particles of the illuminated Body are fo diſpo-
fed, as to make a rough and uneven Superficies; for then
the Rays which come as it were parallel from the lumi-
nous Body, fall upon fuch a Superficies with all forts of
Obliquities, and therefore are ſcattered and reflected all
Ways; and this is the Reaſon why the Eye does not re-
ceive the Light with its full Force; but only a certain
fmall Number of Rays are determined by this Superfi-
cies to come to the Place where the Eye is fixed; and
hence we may conclude, that there is fome particular Co-
lour which confifts only in the Roughness of the Surface of
the coloured Body, and which gives no other Modification to
the Light, but only this, that it reflects it all ways indiffe-
rently in the fame manner as it received it.
54. Now as this is the leaft Modification of Light that
can be; fo the Body which cauſes it ought to reſemble
of Whiteness the luminous Body as much as poffible, that is, it ought
to excite in us the Senfation of Whiteness, which comes
the neareſt to Light of any Colour. And this is confirm-
ed by Experience; for the white Colour of Eftamps Sand
is found ro confift in this, that every Grain does thus re-
flect any Ray of Light all Ways. For when we look up-
on any of the Grains with a Microfcope, they have no
Colour at all, but are transparent, like fmall Pieces of
Chryſtal of all Shapes, or like little Diamonds which af
or Refractions in their Interftices,
then they conftitute a transparent
Body. But if their Interftices be fo
large, and filled with fuch Matter, or
fo empty (proportionably to the Den-
fity of the Parts themfelves) that
there are feveral Reflexions and Re-
fractions made within the Body,
then that Body is Opake. (See Art.5.
above) Further, thofe opake Bodies
which are made up of the thinneſt
fmall Plates of all, are Black; and
thoſe that are made up of the thick-
eft finall Plates, or of fuch as are
of very different Thickneffes, and
are therefore fitted to reflect all Co-
lours; fuch as the Froth of Water,
thefe are VVhite; and thoſe which
are made up of fmall Plates, the
most of which are of fome interme-
diate Thickneſs, are therefore Blue,
Green, Yellow, or Red, viz. by re-
flecting not all the Rays of that
Colour, but more of thofe than of any
other Colours, the greateſt Part of
which other, they either fuffocate,
and by intercepting them, extinguish
them quite, or elfe they tranfmit
them; whence it is, that fome Li-
quors (for Inftance, an Infufion of
Lignum Nephriticum) appeared Red
or Yellow by a reflected Light, and
Blue by a tranfmitted Light; and
Leaf-Gold appears Yellow when
looked upon,
but Green or Blue when
looked through.
ford
Chap. 27. of NATURAL PHILOSOPHY
ford fuch a Paffage to the Light, that they reflect it all.
Ways in the fame manner as they received it.
55. That
55. We may further conjecture, nay, we may be af
fured, that the Effence of Whiteness confifts in nothing elſe Roughness is
fufficient to
but the Roughness of the white Body, if we confider, that cafe VVhite
we cannot make fome Bodies rough, but they will alſo nefs.
become white at the fame Time, nor take away their
Roughness, but we muſt likewiſe take away their Whiteness.
Thus Goldfmiths make Silver white, by putting it firſt
into the Fire, to take off all the Drofs and Dirt which
foils it; and then dipping it in boyling Water, into which
they caſt a certain Quantity of Tartar and common Salt
(which are corrofive Bodies, and proper to make the Su-
perficies of Silver rough and uneven.) And to take off the
Whiteness, they do nothing more but rub the Silver with
what they call a Blood-ftone, which is very hard and
fmooth; which by preffing upon the Part it is applied
to, muft neceffarily deprefs the Parts which ftick up, and
raiſe the Parts which fink in, that is, take off the Rough-
nefs.
white Body
56. As we take it for granted, that a white Body does 56.hy
not abſorb any of the Rays, but that its Superficies re- looks fo, when
flects them all Ways indifferently, it follows, that we can- viewed every
not place the Eye any where, but that it will receive pret- VVay.
ty near the fame Number of Rays as if it were placed
any where elſe; and confequently the Body ought to ap-
pear white from what Side foever it is viewed. But the
Cafe of plain poliſhed Bodies, fuch as Looking-Glaffes, is
different; for when they receive the parallel Rays of Light
from one Side only, they can reflect them to the other
Side only, where they may dazzle the Eye, but they will
not reflect Rays to any other Part.
Nature of
57. As Black is contrary to White, there is no doubt 57.Of the
but that the Effence of Blackness confifts in the contrary Blackness.
to that of Whiteness. Wherefore, as it is neceffary, in
order for a Body to look White, that it ſhould reflect the
Light which falls upon it towards all Parts in the fame
manner as it receives it, fo that there can be no Place,
but that a fufficient Quantity of Rays muft affect our
Eye: So likewife ought we to think, that in order to per-
ceive Blackness, there muſt come no Rays at all to the
Eye; and confequently the Bodies which we call Black,
and which appear fo to our Senfes, abſorb all the Rays in
fuch a manner, that they reflect none of them to make
any Impreffion upon the Eye: And becaufe a Body can-
not deſtroy the Motion of another Body, but by gaining
*
it
222
Part I
ROHAÜLT's SYSTEM
58.VVhy a
great many
Bodies that
fo.
it it felf, it is eaſy to conceive, that the Parts of Black Bo-
dies are very fine and broken, so as to be easily fhaken.
58. And this is confirmed from hence. Firft, That
Darkness, that is, thofe Places where Bodies having no
are not Black, Light falling upon them, can reflect no Rays to the
do yet appear Eyes, I appears Black. Secondly, Shadows, or thofe Pla-
ces, which, by reafon of the Interpofition of fome opake
Body, do not receive the Rays of Light from the lumi-
nous Body, or receive but a few of them, appear Black.
Laſtly, A well-polished Body, which does receive a great
many Rays of Light, but reflects them to the Side op-
pofite to us, appears Black.
59. VVhy
Vood when
it is burnt to
a Coal, turns
Black.
60. That all
the Parts rf
Black.
59. Theſe Things being allowed, it will not ſeem ſtrange,
that Flame which is fo bright, ſhould convert White Wood
into a Black Coal. For it is manifeft, that the Wood has
loft a great many of its Particles, which ferved to nou-
rifh the Flame; wherefore the greateſt Part of the
remaining ones are fo 2 difunited, and eaſily fhaken,
that they abforb almoſt all the Light that falls upon
them.
1
60. I fay, the greateſt Part only are difunited and ea-
a Coal are not ly to be put in Motion, and not all of them; for it may
happen, that the fineſt Particles which are on the Out-
fide of the Coal, may be like Down to cover the more
folid Parts, and fuch as are capable of reflecting a fuffi-
cient Quantity of Rays of Light: And thus we fee, that
after the Fire has carried off all that it can confume of the
Coal; there yet remains a great many Parts which com-
pofe the Cinder, which are pretty folid, for they appear of
a whitiſh Colour.
61.That,cx-
teris paribus,
61. Becauſe the Particles of Black Bodies are more dif
Black Bodies united than thoſe of White Bodies, it follows, that they
contain leſs of their own proper Matter in the fame Bulk
than VVhite. than theſe other. And becauſe the more a Body has of
heavy Matter, the heavier ought it to weigh, therefore
ought to
weigh less
1. Appears Black) This is taken
out of Ariftotle's first Book of Colours.
Chap. i. There are three VVays that
Black appears to us. VVhere we
cannot fee at all, it is naturally Black.
Or where there is no Light brought
to our Eyes. Or where the reflected
Light is very rare and small; and
thus Shadows appear Black.
2. Difunited and eaſily ſhaken)
And they very eafily and ftrongly
make other Bodies, to which they
are applied, of a Black Colour, be-
cauſe the very fmall Particles of the
Coal, the Number of which is very
great, cafily cover over the groffer
Particles of other Bodies. But this
Opinion, concerning the Nature of
Blacknefs, in general is very much
confirmed from hence, viz. that
Black Bodies are fooner heated; and
îf wetted, grow fooner dry than
White, as is confirmed by certain
Experiments. See Art. 62.
We
Chap. 27. of NATURAL PHILOSOPHY,
223
we ought to conclude, that cæteris paribus of two equal
Bodies, the one Black, and the other White, the latter
ought to weigh more than the other; Wherefore the Wood
ought to weigh more than the Coal; and a piece of White
Marble more than a Piece of Black, of the fame Bignefs.
62. Having thus explained the Nature of White and 62. VVhy the
Black, we fhall eaſily underſtand the Reaſon why the Rays Sun, collected
Rays of the
of the Sun collected by a Convex-Glaſs, will not burn at by a Convex-
all, or burn with greater Difficulty White Bodies; but will Glass, burn
eafily kindle Black Bodies, though they be both combufti- easier than
ble. For it is evident, that the White Body which re- they do
flects all the Rays that fall upon it, is not fhaked by
them, and that the Black Body which abforbs and
choaks all the Rays, therefore abſorbs them becauſe
it receives all their Motion; by which Means it begins to
grow hot, and at laſt takes Fire.
63. Hence we ſee the Reaſon of a Fact which we
fhould not know but by Experience; which is, that
White Bodies weary the Sight, and Black ones refreſh it.
For we cannot look upon White, but we muſt receive the
Impreffion of a great Quantity of Rays, which fatigues the
Sight, whereas we fee Black when no Rays come to us,
which refreſhes it.
Black Bodies
VVhite.
63.VV
White Bodies
Sight, and
Black Ones
refresh it.
weary the
64. VVhat
eft Bodies of
64. From all which it follows, that thofe Bodies are
the whiteft which reflect all Ways, and with the fame are the whit-
eft and black-
Force, all the Light which falls upon them; and on the
contrary, that thoſe Bodies are the blackeft, which abſorb all.
the Light the moft that can be. Such we have reaſon to
believe black Velvet to be, becauſe the ſmall Threads of Silk
of which it is made, are like Briftles, and fo placed as to be
as rough as poffible; wherefore it is the blackeſt Thing
in the World.
65. As to the Modifications of the Rays of Light,
which excite in us the Senfation of other Colours; as
Red, Yellow, and Blue, we ought to think that they confift
in this, viz. that the fmall Globules of the fecond Ele-
ment, which compofe the Rays that are reflected from
all fuch Bodies, have not fo much Force or fo great a Dif-
pofition to go on in a ftreight Line, as the Globules of the
Rays which are reflected from white Bodies, and therefore
inſtead thereof, they are fome way turned about their own
Centers; and fo part of the Force which they had be-
fore to go on in a ftreight Line, is beftowed upon this
Motion. Which may be juſtified from hence, that we
cannot conceive what other Alteration than this can hap-
pen
65. Of the
Nature of o-
ther Colours.
224
Part I
ROHAULT's SYSTÉM
}
Ι
a trian-
pen to the Rays of Light, in paffing through
gular Glafs Prifm; and yet we fee, that by going through
this Prifm, they are capable of exciting in us the Senfation
of Red, Yellow, and Blue.
1.A triangular Glafs Prifm) Becaufe
the Experiments of a triangular
Prifm, are as it were the Touch-
ſtone by which every Hypothefis,
and every Theory, concerning the
Nature and Properties of Colours, is
to be examined and tried; I fhall
not think it too much trouble briefly
to
enumerate here the principal
Phænomena as they are explained
by the famous Sir Ifaac Newton all
along in his Opticks. 1. Then, the
Rays of Light tranfmitted through
a Priſm, paint an Image upon the
oppoſite Wall, diſtinguiſhed into va-
rious Colours, the Chief of which
are, Red, Yellow, Green, Blue, and
Violet. 2. This Image is not round,
but when the Angle of the Priſm is
about 60 or 65 Degrees, five times
as long as it is broad.
3. Thoſe
Rays which make a Yellow Colour,
deviate more from a ftreight Line,
than thoſe which make a Red; and
thoſe which make a green Colour,
deviate more than thofe that make
a Yellow,&c. and thoſe which make
a violet Colour deviate moſt of all.
4. If the Priſm, through which the
Rays are tranfmitted, be fo turned
about its Axis, that the Red, Yel-
low, Green, &c. Rays fall in order
through a ſmall Hole upon another
Priſm, about twelve Foot diftance,
and be turned another Way; the
Yellow, &c. Rays, though they fall
with the fame Incidence upon the
fecond Prifm as the Red do, yet
they will not be turned upon the
fame Place as the Red, but will be
carried further towards that Part,
to which the Refraction is made.
Further, if in the Place of the fe-
cond Priſm they be received by a
Glafs that is a little gibbous, the
Yellow, Green, &c. Rays, every one
in their Order, will meet in a Fo-
cus fooner than the Red,
5. The
Colours of the coloured Rays, well
feparated, (the manner of doing
which, you may ſee in Newt. Opt.
p. 54, &c.) cannot be deftroyed, ñor
any Way altered by repeated Re-
fractions. 6. The Colours of co-
loured Rays cannot be at all altered,
66. But
by paffing through a Place that is
Light, nor by croffing each other;
nor by the Confines of a Shadow;
nor by reflecting them from any
natural Bodies in a Place dark every
where elfe. 7. All the coloured
Rays together, collected, either by
feveral Priſms, or by a Convex or
Concave-Glaſs, make White; but
when feparated, after croffing each
other, they all exhibit their own
Colour. 8. If the Rays of the Sun,
fall upon the inward Superficies, of
the Priſm, with the greateſt Obli-
liquity that any of the Rays can be
tranfmitted at, thofe that are reflect-
ed will be Violet, and thoſe which
are tranfmitted will be Red. 9. If
there be two Priſms, the one filled
with a red Liquor, and the other
with a Blue; the two Priſms clap-
ped together will be opake, though
if they be both filled with a red or
blue Liquor, they will be tranfpa-
rent when clapped together. 10. All
natural Bodies, but eſpecially White,
when looked at through a Priſm,
appear to be bordered on one Side
with a red and yellow Colour, and
on the other Side Side with a Vio-
let and Blue. 11. If two Prifms be
fo placed, that the Red of the one,
and the Purple of the other, be
mixed on a fitted Piece of Paper,
furrounded with Darkneſs, there will
be a pale Image; which if it be
looked upon through a third Priſm
at a due Diftance, will appear dou-
ble, Red and Purple. 12. So like-
wife, if two Sorts of Powder, the
one perfectly Red, and the other
perfectly Blue, be mixed together,
and any fmall Body be dawbed
thick with that Mixture, it will
appear to the Eye through a Priſm,
to have two Images, a red and a
blue One.
Theſe are the moſt general Phæno-
mena of the Prifm; (to reckon up
all the Particulars which are worth
obferving, would be endleſs) from
which it appears at firft Sight, that
the Colours cannot confift in the
turning round of the Globules only,
according to Cartes, nor in the Ob-
liquity
Chap. 27. of NATURAL PHILOSOPHY.
225
66. But for the clearer undertanding hereof; let the 66. Of the
Side BC of the Priſm ABC be covered all over with
fome opake Body, except the Place DE, where there
is to be a Hole in the opake Body for fome of the Rays fing through a
FI, GL, coming from the
liquity of the Pulſes of the aetheril
Matter, as Mr. Hock thought, Mi
crog. Obfer. 9, nor in the Light being
thick and rare or flower moved; as
the famous Barrom conjectures,
Lect. 12. towards the End.
thefe and all other Phænomena of
Colours, are very eafily and clearly
explained, by the true Theory of
that incomparable Perfon fo often
cited.
-
But
For Firft. The Rays of Light
tranfmitted through a Prifm, paint an
Image upon the oppofite Wall, diftin-
guiſhed into various Colours: Becauſe
the coloured Rays are feparated by
Refraction. Thus the blue Rays, for
Tab. XXII.
Inftance, marked with
Sun FG to pass through;
which,
ther, if the Prism through which the
Rays are tranfmitted, be fo turned
about its Axis, that the Red, Yellow,
Green, &c. Rays, fall in order thro”
a fmall Hole upon another Priſm a-
bout twelve Foot distance, and be
turned another Way; the Yellow, &c.
Rays, though they fall with the Jame
Incidence upon the Second Prifm as
the Red do, yet they will not be turn-
ed upon the fame Place as the Red,
but will be carried further towards
that Part, to which the Refraction is
made. Further, if in the Place of
the ſecond Prism, they be received by
a Glaſs that is a little gibbous, the
Tellow, Green, &c. Rays, every one
in their order, will meet in a Focus
than the Red : Becauſe the
Yellow Rays are more refracted than
.
the Red, and the Green than the
Yellow, and the Blue and Violet moft
of all.
the prick'd Line, whichfooner
Fig.1. Fig.2 begin to be feparated in
the Side ca of the Prifm abc (and
alfo in the firft Superficies of the
Globe of Water abc) from the reſt
by the firft Refraction in dd; are ſe-
parated ftill more in be, the other
Side of the Priſm (and alfo in co-
ming out of the Globe abc) by a
fecond Refraction towards
Fig. 4. the fame part in ce: But,
Fig. 3.
on the contrary, in the
plane Glafs abcf (and al-
fo in the Priſm glo placed in ano-
ther Situation, the blue Rays, which
begin to be feparated from the reſt
in the firft Superficies in dd, go out
parallel in the other Superficies, the
Refraction being made the contrary
Way, that is, they are mixed
again with the Colours of the other
Rays.
Secondly. This Image is not round,
but about five times as long as it is
broad: Becaufe fome Rays are more
refracted than others, and therefore
they reprefent a great many Ima-
ges of the Sun like one Image drawn
into a great Length.
Thirdly and Fourthly. Thofe Rays
which make a yellow Colour, deviate
more from a freight Line, than thofe
which make a Red, and thoſe which
make a green Colour, deviate more
than those that make a Tellow, &c.
and those which make a violet, Co-
eurs deviate most of all; And fur-
Fifthly and Sixthly. The Colours
of the coloured Rays well feparated,
cannot be destroyed, nor any Way al-
tered, by repeated Refractions, nor
by paffing through
by paſſing through a light Place, nor
by croffing each other, nor by the Con-
fines of a Shadow,nor by reflecting them
from any natural Bodies, in a Place
dark every where elfe: Becauſe their
Colours are not Modifications arifing
from Refraction, but immurable
Properties belonging to their Na-
ture.
Seventhly. All the coloured Rays
together, collected either by feveral
Prifms, or by a convex or concave
Glaſs, make White; but when Sepa-
rated after croffing each other, they
all exhibit their own Colour: For as
the Ray, before it was divided into
feveral Parts by Refraction, was
White; fo by thofe Parts being
mixed together again, it becomes
White again; and the coloured
Rays, when they unites do not de-
ftroy one another, but are only mix-
ed together, And hence it is, that
Red, Yellow, Green, Blue, and Violet
Powders mixed together in a certain
Proportion, are fomewhat Whitiſh 3
that is, are of fuch a Colour as ari-
fes from a Mixture of White and
Black,
Action of
the Rays of
Light paf
Glafs Prifm.
Tab. V.
Fig. 3.
}
!
K
!
e26
Part 1.
ROHAULT'S SYSTEM
→
which, according to what was faid before, will be refract
ed in fuch a manner, that the Ray FI will tend towards
M, and from thence to N, and GL will go to O, and
from thence to P. Whence it is to be obferved, that
FI, GL are therefore turned out of the Way in this
manner, becauſe the fmall Globules at their entring into
the Glafs, find an eafier Paffage this Way, that is towards
the right Hand, than towards the Left. Thus for in-
ftance; Let STV be one of thefe Globules, we muſt
“
Black, and would be entirely White,
if ſome of the Rays were not ab-
forbed So likewife if a round piece
of Paper be painted with all thofe
Colours diftinct from each other, and
in a certain Proportion, and then
turned very quick round upon its
Center, that by the Swiftnefs of the
Motion, all the Species of Colours
may be mixed together in the Eye;
the particular Colours will immedi-
ately vaniſh, and the Paper will look
all of one Colour, which is a Medi-
um betwixt White and Black.
Eighthly. If the Rays of the Sun
fall upon the inward Superficies of
the Prism, with the greatest Obliqui
ty that any of the Rays can be tranf-
mitted at, those which are reflected will
be Violet, and those which are tranf-
mitted, will be Red: Becaufe the
Rays, fince they were coloured be-
fore they were refracted at all, and
the more they are capable of being
refracted, the fooner are they re-
flected alfo; are feparated in this
manner.
Eleventhly and Twelftbly. If two
Prisms be so placed, that the Red of
the one, and the Purple of the other,
be mixed on a Piece of Paper fitted
and furrounded with Darkness; there
will be a pale Image, which if it be
looked upon through a third Prism,
at a due Distance, will appear dou
ble, Red and Purple So likewife, if
two Sorts of Powder, the one per
fectly Red, and the other perfectly
Blue, be mixed together, and any
Small Body be dambed thick with
that Mixture, it will appear to the
Eye, through a Prism, to have two
Images, a Red and a Blue one
caufe the Red Rays, and the Purple
or Blue ones are feparated by an
unequal Refraction.
Be
Moreover, thirteenthly. If the Rajs
which are tranſmitted through a gib-
bous Glafs, be received upon a Piece
of Paper before they meet in the Fo-
cuss the Confines of Light and Shadow
will ſeem tinged with a red Colour,
but if beyond the Focus with a Blue:
Becauſe in the former Cafe, the Red
Rays, which are fomewhat lefs re-
fracted, are uppermoft, but after
crofting in the Focus, the Blue are
fo.
Ninthly. If there be two Prisms,
the one filled with a Red Liquor, and
the other with a Blue, the two Prifms
clapped together, will be opake, tho'
if they be both' filled with a Red or
a Blue Liquor, they will be tranf
parent when clapped together Be
caufe one of them tranfmit none
but Red Rays, and the other none
but Blue, therefore when put toge-ed
ther, they can tranfmit none at all.
1
Fourteenthly. If the Rays that
go through one half of the Pupil be
intercepted by any opake Body put
clofe to the Eye, the Extremities of
the Objects beyond, will appear ting-
ין
with Colours, as they do through a
Prifm, but not fo vivid. Becaufe the
Rays which are tranfmitted through
the other part of the Pupil, are fepa
into Colours by Refractions
and will not be diluted by the Mix-
ture of the intercepted Rays, which
would have been refracted the con-
trary way: And hence it is, that a
Body which looked at through two
Holes in a Piece of Paper, appears
double, appears tinged with Colours
Tenthly. All natural Bodies, but
efpecially white ones, when looked at
through a Prism, appear to be border-rated
ed on one Side, with a Red and Tel-
low Colour, and on the other Side with
a Blue, and Violet. Becauſe thoſe
Borders are the Extremities of whole
Images, which the Rays of every
Species, according as they are more
o lefs refracted, exhibit at a greater
or lefs diftance from the true Place of alfo.
the Objects
יד
1
think
Chap. 27. of NATURAL PHILOSOPHY.
227
think that the Superficies AB determines it to move to-
wards S rather than towards V, and confequently to turn
about its Center in the order of the Letters STV, which
it will continue to do the whole Length of the Line
IM. And becauſe when it is come to M, where it un-
dergoes a Refraction towards the right Hand; this is a
Reaſon why it ſhould be turned about again in like man-
ner; therefore it muſt be acknowledged, that the ſmall
Globules which come out of the Glafs towards N, are fo
modified, that befides the Difpofition they have to move
ftreight along, they have a Difpofition alfo to turn about
their own Centers.
1
Tab. V
Fig. 3.
67: What
was affirmed of the Globules of the Ray 67. That the
FIMN is to be underſtood alſo of thoſe of the Ray Shadow can-
GLOP and of all the other intermediate Rays. But af- fes divers Mo-
difications in
ter the fecond Refraction, which is made at the Surface thefe Rays.
BC, we find on the one Hand, that the ſmall Globules of
the Ray MN are turned about in the fame manner as
they were at firſt, from a new Caufe; for the Shadow
on the Side D flackens the Motion of the Globule M on
the fame Side; and the Rays which are between IMN
and LOP being ftronger than the other, prefs upon the
Side Q of the fame Globule, and becaufe they move the
fame Way as it turns, they quickens its Motion on this
Side: And on the other Hand, we are affured, that the
Globules of the Ray GLOP, have the Rotation which
they had acquired from thefe two Refractions hindred by
Two Things. Firſt, From the Shadow which hinders
them on that Side on which they were moft ftrongly
impelled, and retards their Motion. And, Secondly, Be-
cauſe they are impelled on the other Side, by Rays that
are ftronger, and which imprefs a Motion upon them,
contrary to that of their Rotation.
+
ܕ܆
Blue.
Tab. V.
68. Having thus confidered the feveral Alterations, and 68. What the
the Reaſons of thofe Alterations which may happen to Modifications
the Rays of Light in their Way to the opake Body NP; are which
of thofe Rays
we find, that the Globules which fall near N are turned cause Red and
round with a greater Force, than that with which they are mo- Tellow, and
ved on in a ſtraight Line; and on the contrary, that the
Globules of the Rays which fall near P, move on in a
ftraight Line, with a greater Force than that with which they
turn round, their Centres. And, Laftly, That there inter-
mediate Rays, about X, have pretty near the fame Force
to turn round, as to move ftraight along. But by Experience.
we find, that we fee Red in N, Blue in P, Tellow in X,
Orange between N and X, and Green between X and P;
Q 2
whence
Fig. 3.
228
Part I
ROHAULT'S SYSTEM
69. What the
Colours of co-
loured Bodies
confift in.
70. That co-
whence it appears what the particular Difpofitions of the
Globules which compofe the Rays of Light are, to excite
in us thefe Senfations.
69. Now there are two Things in the Objects which
we call coloured, which may cauſe the fame Modificati-
ons in the Light, as thofe acquired in paffing through a
Prifm. For, Firſt, Their Particles may be fo tranfparent,
that the Rays of Light may penetrate a little Way into
them, and be refracted, before they are reflected: Se
condly, (and which may produce the fame Effect, and
be the Caufe of the Colours of different Objects) Their
Particles may be fo fmall and uneven, that the Globules
of the Rays of Light which fall upon them, may com-
municate fome of their Motion to them, and by that
Means they may be turned round and reflected back, in
the fame manner as a Ball thrown with great Force up-
on the Grafs, is ftopped a little by the Spires and turn-
ed round.
#!
70. Neither can it be doubted, but that fome of the
loured Bodies Particles of coloured Bodies are really tranſparent, as may
are in fome be ſeen by the Help of a Microſcope, in all kinds of Sand,
transparent. Flint-ftone, Marble, Sugar, Silk, Wool, Hair, Herbs, and
an infinite number of other Bodies.
meaſure
71. That the
coloured Bo-
71. And that the Particles are very fmall and broken,
Surfaces of is evident, not only from hence, that coloured Bodies ap-
dies is made pear coloured when viewed all Ways, but is further con-
rough by co- firmed from the manner in which Colours are made by the
Louring them. Dyers. For, fince Brafil-Wood, Indian-Wood, Indico, Yel-
72. That the
Particles of
black Bodies
are more bro-.
ken than thofe
of any other
coloured Bo-
dics.
low-Weed, &c. will not tincture any Thing with a red,
violet, blue, yellow, &c. Colour, unless there be fome
Allum mixed with them, we muſt conclude, that this pe-
netrating corrofive Body infinuates itſelf into the Pores
of the Cloth, and dilates them; whereby there is Room
made for the Water to enter tinctured with the ſeveral
Colours, which fink into the Cloth in fuch a manner, as
to leave fome on the Superficies, which caufes a kind of
Roughneſs, and makes it capable of all the different Mo-
difications of Light.
72. After what has been faid concerning Dying, it is
neceffary to make one particular Obfervation about Black;
and that is, that becauſe the Roughnefs, in which this
Colour confifts, muſt be the greateſt that can be, to ex-
tinguifh all the Rays; therefore in dying Cloth of a Black
Colour, Allum and Nut-galls are not fufficient alone;
1. VVhich we call coloured) See above on Art. 52.
1
but
Chap. 27. of NATURAL PHILOSOPHY.
229
but there muſt be Vitriol inſtead of Allum, which is more
corrofive than Allum; and further, to make the Vitriol
corrode the more, they put the Cloths to be died into
the Copper, and leave them for fome time in the boiling
Liquor; whereas in dying of other Colours, they only
dip the Cloth ſeveral times into the Liquor, which is but
juſt warm.
73. Since the Particles of black Bodies are the moſt
uneven, it is eaſy to imagine, that Cloths and other Stuffs
of this Colour muſt tear and wear fooner than thofe of
any other Colour.
73. Why
black Cloth
than other.
wears fooner
74.Why
Cloths of light
Colours will
dye of darker
74. Further, if we confider, that the darkeſt Colours
require that the Particles ſhould be the fmalleft that can
be; it is evident, that we may eafily make a light Piece
of Cloth of a dark Colour, becauſe it requires only to Colours, but
have its Superficies made rough; but becauſe it is very trary.
difficult to make it ſmooth again; therefore Cloth of a
dark Colour, can very hardly be died of a Lighter.
not the con-
Things of the
the fame
75. Now, when I fpeak of the Particles of coloured 75. It is not
Bodies, I mean only the very ſmalleſt of all; many Hun- necessary that
dreds of which may be united together differently, in or- feme Colony
der to compofe groffer Parts which may be of very dif- Should have
ferent Figures, in the fame manner as different Buildings Tale
may be formed of Bricks, which are all alike. Thus we
know, that coloured Bodies act upon the Eyes by their ſmal-
left Particles, and upon the Tongue by thofe which are lar-
ger, and compoſed of the other; whence we conclude,
that Things of the fame Colour have not neceffarily the
fame Tafte.
left Particles
of any Body
the Colour is
altered alſo.
1
76. Since there are two Sorts of Particles in the fame 76. By alter-
Body, this fhows us, that if we make any Alteration in ing the mal-
the fmaller Sort, the Colour muſt be changed likewife.
And fo we experience it in Herbs. bruifed in a Mortar; and
in Colours which Painters grind upon a Stone, fuch as
Vermilion and Orpiment. But if the Body be fuch, that
the ſmalleſt Particles of it cannot be altered, neither can
the Colour be changed; as we fee in fome Paints, which
are not fo eaſy to be altered as thofe mentioned; efpecially
as Herbs, whofe Particles have before a proper Motion
of their own, as being in fome meafure liquid, which
helps to dafh them against each other, and to fe-
parate them into fmaller Particles, than they would o-
therwiſe be.
Q3
77. From
230
Part I
ROHAULT'S SYSTEM
ご
​77. How a
77. From what has been faid concerning coloured Bo-
white Body dies, and particularly concerning a white Body; we may
ought to ap-
pear, which infer, that if there fall no other Rays of Light upon a
receives Rays white Body, but thoſe that are caft upon it by another
already mo-
Body which has already modified them, the Rays will
dified.
not be altered at all by the white Body, but reflect-
ed back to the Eye with the fame Modification; fo
that the Body inſtead of appearing White, will appear
of the Colour of that Body from which it received the
Rays.
}
78. A curi-
ment.
78. We may be convinced of this by a very curious
pus Experi- Experiment, which it is not very difficult to make. The
Way of doing it is this. Let all the Windows of the
Room be fhut up clofe, except a very fmall Hole, through
which the Rays reflected from the Objects on the out-
fide, may enter in; then let the Rays fall upon a white Cloth,
or any other white Body, and it is pleaſant to fee the diffe-
rent Colours of the Objects which are painted upon it.
79. Why the
Same Place,
each other's
Effects.
79. This Experiment perhaps may raife a Difficulty in
Actions of the Minds of fome, who may imagine, that different
different Ob- Rays, and differently modified, paffing through the fame
jects tranf
mitted thro' Hole, muft hinder one another, and confound their re-
one and the fpective Actions: But it will not be hard to get clear of
do not destroy this Difficulty, if they confider in the firft Place, the vaft
number of Pores that there is in the leaft Quantity of
Air, or of any other tranfparent Body, which afford a
Paffage for an infinite number of Rays, if I may for
fpeak, without difturbing one another. But that which
is principally to be confidered, and which takes away
the Difficulty intirely, is, that the Light, or the Co-
lours, does not confift fo much in actual Motion, as in
a Tendency to Motion, or a Preffure. Now it is eafy
to apprehend, that an infinite number of thefe fort of
Actions, different from each other, may be tranfmitted
through the fame Point without confounding one ano-
ther. For inftance, fuppofe a Force equal to a hundred
Pound Weight, applied to the Point A, of the ſtraight
Line AB, pufhing it towards B, where we fuppofe alſo,
that there is a Body able to refift this Force. The Line
AB could not move at all according to the Direction of
AB, much leſs can it bend towards C or D, becauſe
it is ftraight; but the leaft Force that can be, will bend
it towards any Side whatſoever. Thus if any Force in C
pufhes it by E towards D, if it be but the Force of
one Pound, it will bend it towards D: But if we fuppofe
another Force in D which can refift that of a Pound,
Tab. V.
Fig. 4:
;
this
Chap. 27. of NATURAL PHILOSOPHY.
235
this will hinder the Line AB from bending, ſo that the
Force which is at A, fhall tranfmit its Action whole and
entire to B, without being diſturbed by the Force which
is at C: and the Force which is at C fhall tranſmit its
Action to D, without the leaſt hindring the Continuance
of the Action along AB. So likewife we may imagine
a Force at F equal to five Pounds acting upon a Body
at G.
The fame Point E therefore may ferve to tranf
mit as many Actions as we will, without at all confound-
ing them.
lours are not
80. After what I have already faid: I have but one 8o. That Co-
Thing more to remark concerning the Diſtinction that is rightly diftin-
ufually made of Colours; viz. that fome of them are guifhed, into
true or real Colours, fuch as thofe of Tapiſtry, and o- true and
fulfe, real
thers falfe or feeming Colours only, fuch as thofe feen and apparent
through a Glass Prifm. But I don't fee any Foundation for Colours.
this Diſtinction, becauſe the Reality is juft the fame in
each of them: For if the Senfation of Colour which we
have upon viewing a Piece of Tapiſtry be real; that
which we have in looking through a Prifm is as real; for
the Prifm is as real a Thing as the Tapiſtry. And in-
deed it is the fame Light which caufes us to perceive
the Colours through the Prifm, as caufes us to perceive
the other.
that
1
81. If any one, in order to fupport that Diftinction of 81. That we
make a falfe
Colours which we have juft now rejected, replies; Judgement of
there is at leaſt ſome falfe Appearance in looking through all Colours.
a Priſm, becauſe we apply the Colours that we fee, to
Objects where they are not: To this I anfwer, that the
Fault is not in our Sight, but only in the Judgement
which we make afterwards. And if this were fufficient
to conclude, that thefe are falfe Colours; we may
for the fame Reaſon fay, that all other Colours are
falfe likewife, becauſe we equally falfely refer the Sen-
fations which are caufed in us by them, to the coloured
Objects.
there is as
ifing Co-
lours into fix-
82. Nor have they fucceeded any better, who owning 82.That
all Colours to be equally real, have yet diftinguished them little Reason
into fixed and flying; giving the Name fixed to thofe for diftin
which the other called real; and the Name flying to thoſe
which the other called falfe: For if the Eye continue ne-
ver fo long applied to the Prifm, and during that Time
the Light intervene in the fame manner, we fhall always
fee the fame Colours; fo that theſe are no leſs fixed and
durable, than thofe of a Piece of Tapiſtry.
Q4
↓
83. All
ed
and flying.
A
شتر
232
Part I
ROHAULT'S SYSTEM
83. That
there
is no
the one and
the other.
at
83. All the Difference that is to be found in the Ob-
jects that raife in us any Senfation of Colour, is only this;
all between that fome of them, fuch as the Prifm, feem to require
that the Eye fhould be fixed in a certain Place, out of
which there is nothing to be feen; whereas others, fuch
as Tapiftry appear of the fame Colour, which way fo
ever they are looked upon. However, if we confider the
Matter a little more clofely, it is certain, that the Prifm,
and the Tapiſtry, agree in this; that the fame Parts of the
Tapiſtry which reflect the Light to the Eye when it is
in any certain Place, does not reflect the fame to it,
when it is removed ever fo little out of that Place;
and the only Reaſon why we perceive the fame Colour
when we change our Place is, becauſe inſtead of theſe
Parts, thofe Parts that are next to them, and which are
exactly like them, reflect the Light in the fame manner.
If therefore the Eye were fixed in one certain Pofition,
from whence it fhould fee fome particular Places of the
Tapiftry of fome particular Colours, and God fhould
annihilate all the other Parts of the Tapiftry, ſo that
they could not at all reflect any Light; in the Place
where the Eye is, it would continue to fee
lours, but if it fhould change its Place, they would im-
mediately diſappear.
84. Of the
changeable
Colours.
}
offame Co-
84. This being well underſtood, there will be no great
Nature of Difficulty in explaining thoſe Colours which we call
Changeable, fuch as we obferve in a Duck's Neck, or in
a Pidgeon, or in a Peacock's Tail: For it is eafie to con-
ceive, that the Parts of thefe Bodies are placed in fuch
order, that thoſe of them which are proper to modify
the Light after one particular manner, are difpofed to re-
flect it to one certain Place; and thofe that modify it
in another manner, reflect it to another Place. Thus,
if the Eye be in the Place where the Rays come,
which caufe the Senfation of Red in us, then the
Ob-
ject appears Red, and if it be placed where the Rays,
which caufe Yellow are reflected, the Object appears
Yellow.
85. A Com- 85. This is confirmed from hence; that Workmen have
parison of found out a Way to make Stuffs of a changeable Colour,
changeable
Colours with by making the Warp of a Light Colour, and the Striking
Things made of a Colour not quite fo Light: But what moſt refem-
bles the Objects to which we afcribe thefe changeable
Colours, are thofe channelf'd Tables which reprefent
different Sorts of Things, according as they are viewed
from different Places: For one of thefe Tables, when
by Art.
it
Chap. 27. of NATURAL PHILOSOPHY.
233
it is looked directly upon, repreſents a Cafar's Face; when
looked upon on the Right Side, it repreſents a Cat, and on
the Left Side a Skeleton. Thus, as they are different
Parts of the Table which make thefe different Reprefen-
tations, fo likewiſe are they different Parts of the Pidgeon
which caufe us to fee different Colours.
:
86. If after what has been faid concerning the Nature 86. The re
and Properties of Light and Colours, there remains any maining Pro-
further Difficulty, it will be folved afterwards, when we Light and
perties of
have particularly examined the Nature of Vifion: And Colours can-
this is what I fhall proceed to; which I the more
'not be under-
readily do, becaúfe the following Parts of this Treatife have explain-
frood, till we
of Natural Philofophy, depend, in fome meaſure, upon ed the Na-
Obfervations made by the Help thereof, fo that it is
ture of Vifion.
neceffary to know all the Circumftances, of this Sort
of Senfation, which is the most wonderful of any that we
are poffeffed of: I fhall begin with a Deſcription of the
Eye, and to avoid Tedioufnefs, I fhall mention only thoſe
Things which belong particularly to this Subject.
!
CHA P. XXVIII.
A Defcription of the ETE.
WHILST fro thd it, hide us
THILST the Eye is inclofed in the Head of any
Animal, the Bones which furround it, hinder us
from feeing what Figure it is of; but when it is taken
out, we find it is round, and fuch as is repreſented
the Figure ABCDEF. FABC is the fore-part of it, or
that which fticks out; CDEF that part which is incloſed
in the hollow Bone of the Head.
in
I. Of the
Figure of the
Eye.
Tab. V.
Fig. 5.
2. AB is a tranfparent Part of that particular Coat of 2. Of the
the Eye, which is called the Tunica Cornea.
of
3. BCDEFA is the reft of this Covering, the Parts of
which, that are next to A and B, are called the White
the Eye.
Tunica Cor-
nea.
3. Of that
which joins
to it, or the
VVhite of the
4. AILB is the Tunica Uvea, in which there is is a Hole Eye.
IL, which is called the Pupil.
1
4. Of the Tu-
nica Uvea,
and the P-
5. MN, MN, are certain black Filaments, which are
called the Ciliary Ligaments; there is a certain foft and pil
tranfparent Body called the Chryſtalline Humour which is of the Ci-
fufpended upon them.
liary Liga-
ments.
6. The Space QQQ is filled with a tranfparent Liquor, 6.0f the A-
which is very fluid like Water, and for that Reaſon is called queens Hu-
the Aqueous Humour.
7. NONP
mour.
1
}
234
7. Of the
Chryftalline
Humour.
8. Of the Vi-
treous Hu-
THONY
3. Of the Op-
{
and the Re-
ļ
ROHAULT's SYSTEM Part I.
7. NONP is a tranſparent Body of the Figure of a
Lens, a little more convex on the Superficies NPN than'
on NON, which, becauſe it is a little hard, is called the
Chryftalline Humour.
8. The reft of the Cavity of the Eye RRR is filled
with a flimy Matter, almoſt like the White of an Egg,
which is more tranſparent than either the Aqueous or the
Cryſtalline Humour, and is of middle Confiftency betwixt
them, (for it can eaſier be compreffed than the Chryſtalline,
and yet it is not fo fluid as the Aqueous Humour ;) and
this is called the Vitreous Humour.
9. DEGH is a Part of the Optick Nerve, whofe Ca-
tick Nerve, pillaments TS, beginning in the Brain, and reaching to the
Eye, form at the Bottom of it a curious Piece of Net-
work which Phyſicians call the Retina.
tina.
10. That the
Infide of the
Eye is black.
11. Of the
the Eye.
10. I purpofely forbear mentioning the Number and
Names of the feveral Coats with which the Eye is co-
vered, becauſe they are not of any particular Ufe in ex-
plaining the Nature of Vifion; but I must not omit to
take notice, that the Superficies of thefe Coats are all
Black in thofe Places which are over-againſt the Bottom of
the Eye.
II. The whole Body of the Eye is encompaffed with
Muscles of fix Mufiles, four of which are called Right, and the other
two Oblique. Every Nerve, which is thought to be the
Original of the feveral Right Muſcles, is derived immedi-
ately from the Brain, from whence it comes along through
a little Hole in the Bone of the Head, and divides it
ſelf into theſe Muſcles, every one of which is inſerted
into fome Part of the Coat of the Eye, fuch as that here
marked F, in fuch a manner, that of theſe four Muſcles,
the Firſt is above, the Second below, and the other Two
on each Side this Coat. And as the oblique Mufcles have
their Origin alſo in the Brain, their Nerves are bent round,
ſo that they ſeem to come from that Corner of the Eye
which is next the Ear, and one of them ſpreads over the
Top, and the other along the Bottom of the Eye, and fo
crofs the four right Mufcles, and then are inferted into
the Bone of the Nofe.
12. The Use
of the Muf
cles of the
Eye.
12. There is no one Muſcle in the whole Body, but
what is fometimes filled with a certain Liquid like very
thin and fine Air, which comes to it from the Brain along
the Nerve which belongs to it. This Liquid is what Phy-
ficians call the Animal Spirits, which cannot fwell the
Muſcle without ſhortning it or leffening the Length be-
twixt
1
24
Chap. 28. of NATURAL PHILOSOPHY.
235
"""
twixt the Origin and the Place into which it is inferted.
Thus when the right Mufcle which is above, is filled
with Spirits, the Eye muft neceffarily be lifted up, and
when the Three other right Muſcles are filled in their
Turns, they ſerve either to turn the Eye downwards, or
to the Right, or to the Left Side. But what is very
remarkable here, is, that if theſe four Muſcles be filled
all at the fame time, they will alter the Figure of the
Eye a little, and make it flatter than it was before. But
as to the oblique Mufcles, I am not of the fame Opi-
nion with thofe Phyficians, who fay, that they ferve to
turn the Eye round like a Pulley: I rather think, that
they are filled both together with Spirits, and by that
Means fhortned, and ſo they preſs upon the Eye and al-
ter its Figure, in fuch a manner, that the fore-part of
it is made more gibbous, and the hinder-part funk a lit-
tle deeper in, and this makes a greater Diſtance between
the Chryſtalline Humour and the Retina.
13. To theſe Alterations of the Eye we may add, that 13. That the
the Pupil is capable of dilating and contracting it felf. Pupil is ca-
pable of being
And thus we find, that it dilates it felf, when we are in dilated.
Places where there is but a little Light, and when we
try to look at a great Diſtance; and on the other Hand,
it contracts it felf when we are in a very light Place, or
look at an Object very near.
two Optick
Nerves.
14. Laftly, we may obferve, that if the two Optick 14.Of the
Nerves be purfued to the Origin of them, we fhall find,
that after they come into the Skull, they approach nearer
and nearer to each other, till at laft their Coats are mixed
together, and they become one and the fame, but after-
wards they are ſeparated again, and then enter into the
very Subſtance of the Brain, after which we fee them no
more. Wherefore to add any Thing further about this
Matter, would have no Similitude of Truth; unleſs it
were to account for certain Phænomena which otherwiſe
could not poffibly be explained.
;
1
CHAP.
236
Part I.
ROHAULT's SYSTEM
1. What is
meant by Vi-
fion, and that
about it.
f
}
A
CHA P. XXIX.
How Vision is commonly explained.
RISTOTLE has faid nothing in particular as to the
manner how Vision is performed; for though the
Ariftotle has Title of the Seventh Chapter of his Second Book of the
Said nothing Soul, concerning Vision, feems to promife treating of this
Matter fully; yet he fays nothing more of it, but only
this; that the Object must act upon the Medium in order
to have its Action tranfmitted to the Organ of Sight.
It is true indeed, that he fays further in the Twelfth
Chapter of the fame Book; that in every Senfation we
receive the images of the Things, but not the Mat-
ter, in the fame manner as Wax receives the Impreffion
of the Seal, without retaining any part of the Seal it ſelf:
but here likewiſe, what he fays is as general and looſe, as
what he faid in the forecited Place; and the Compari-
fon which he makes, does not at all thow us how fo
great a Number of Parts of which the Object is compo-
fed, can be diſtinctly perceived at the fame Time, nor
how we can know the Situation, Distance, Bignefs, Figure,
Number, Motion or Relt of the Objects which are in
our View.
2. The Opi-
2. The Followers of Ariftotle faw plainly, that he fell
nion of the
Ariftotelians very much, fhort of teaching what one would wish to
about Vision. know upon this Subject; and this has put them upon
trying to find how his Doctrine was to be underſtood.
Thus taking the Word Image, which he ſpeaks of in the
forecited Place, in the literal Senfe; they affirm, that the
vifible Object impreffes an Image upon the Air which
furrounds it; that this Image impreffes another a little
lefs upon the Air beyond it, and this impreffes a Third,
a little leſs ſtill, and fo they go on till there is one im-
preffed on the Chryftalline Humour of the Eye, which
they pretend is the principal Organ of Vifion, or that
Part of the Body which the Soul makes immediate uſe
of to cauſe Senfation. Theſe are what they call inten-
tional Images or Species; and in order to explain their Man-
ner of Production, they fay, that the Objects caufe them
in the fame manner, as our own Image is produced in a
Looking-Glafs.
3. From
Chap. 29. of NATURAL PHILOSOPHY.
237
+
3. From what has been already faid, it fufficiently ap- 3. That the
pears, that I agree with Ariftotle himself; but I can by Ariftotelians
do not at all
no Means come in with his Followers in this Thing of explain the
their intentional Species, the Nature of which feems to me Nature of
inconceivable, and has all along put their Underſtandings onal Species.
upon the Rack. And it is a mere Sophifm to pretend to
eſtabliſh their Opinion upon the Inftance of a Looking-
Glafs, becauſe reflected Images are harder to be explain-
ed than direct.
their intenti-
4. There is no need of mentioning all the Abfurdities 4. The Ab-
confequent hereupon, in order to fhow that there is no furdity of
thefe Species.
fuch Thing as intentional Species. It fhall fuffice only to
obferve; that if They are diminiſhed in the manner they
fay, it will follow, that when an Object is feen at ten
Yards diftance, the Specias of it is only as little again, as
when it is feen at five Yards diftance; that is, an Object
of fix Foot in Length in the one Cafe, will appear of three
Foot in Length in the other Cafe. Wherefore if the Eye
and the Object be within five Yards of each other, it
can receive but a very fmall Part of fuch a Species, and
confequently we could fee but a very fmall Part of the
Object; but this is contrary to all Experience, for we
can fee fuch an Object intire at fuch a Diſtance, nay, at
a much leſs. If they fay, that thefe Species diminiſh o-
therwiſe when the Eye is nearer to them, than when it
is further off; they muſt allow then, that a Thing inani-
mate, and which acts neceffarily, has however Under-
ſtanding enough to proportion its Action, fo as to per-
form the fame Thing at different Diſtances. Which be-
ing abfurd, it follows, that the Foundation upon which
their Species is eſtabliſhed, is abfurd alſo.
on is not per-
5. It is not only without Reaſon, but contrary to Rea- 5. That Vifi-
fon, to affirm, that Vision is perfected in the Chryftalline formed in the
Humour, and that the Vitreous Humour behind it, is of Chrystallins
the fame Ufe as the Quickfilver behind a Looking-Glaſs, Humor.
viz. to terminate the Action of the viſible Object: For
doubtless, the Object ought to continue its Action thro'
the Vitreous Humour, which being one of the moſt
tranſparent Things that we know of in the World, can-
not reaſonably be compared to Quickfilver, which is very
opake. To this we may add, that fince the Chryftalline
Humour is found in both Eyes, and two Species are
formed by it at the fame Time, if That were the prin-
cipal Organ of Viſion, it would follow, that we muit al-
ways fee the Object double, when we look upon it with
both Eyes at once.
6. This
238
Part I:
ROHAULT's SYSTÉM
6. Neither is 6. This laſt Reafon fhows alfo how falfe the Opinion
it performed of fome Philofophers is, who affirm the Retina to be the
in the Retina. principal Organ of Vifion.
7. That it is
7. As to the Opinion of thofe who contend that this
not performed Senfation ariſes from hence, that the Action of the Object
in the Place, is carried to the Place where the Optick Nerves meet; this
where the Op- is confuted by the Experience of Anatomifts, who have
tick Nerves
meet.
found thefe Nerves feparated in the dead Bodies of fome
Men, who, when they were alive, faw Things in the fame
manner as others do.
I. How the
ancient Phi-
lofophers came
CHA P. XXX.
Of the Paffage of the Light through the Humours
of the Eye.
I
Think that moſt of thoſe who have endeavoured
to explain the Nature of Viſion, have run into great
to be mistaken Miſtakes, principally from hence, becauſe they attempt-
upon this Sub- ed too many Things at a Time, and did not obferve
ject of Vision. any Method or Order: Their Miftake will be a Help to
us, if, upon obferving, that Vifion is a Confequence of
the Action of the Object upon both the external and in-
ternal Organs; we, in the firſt Place, inquire, how the
Rays of Light, which are the Means by which any Objects
are feen, are received by the Humours of the Eye.
2. That it is
Sufficient to
confider only
Some few of
thofe many
Rays which
come from
2. Let us fuppofe, for Inftance, Z to be the Eye, and
ABC the Object; there is no Doubt, but that every
Point, that is, every the ſmalleſt viſible Part of this
Object, fends forth Rays all Ways through the Air, to
every Place where it can be feen; but becauſe thoſe
every Point only which paſs through the Pupil are of any ufe to
of an Object. caufe Vifion, we will examine thoſe only which fall up-
on that Part of the Tunica Cornea which anſwers direct-
ly to the Pupil: Thus, in order to examine the Action
of the Point B, it is fufficient to confider fome few
of the Rays which come from this Point, fuch as BD,
BE, BF.
Tab. VI.
3. That fome
of the Rays
tom of the
3. Now becauſe the Ray BD is perpendicular to the
go to the Bot- Superficies EDF, it will not be at all refracted in paffing
out of the Air into the aqueous Humour, wherefore it
will continue on in a ftraight Line to H, where falling
again perpendicularly upon the Superficies of the Chryf
Eye without
any Refraction
at all.
tal-
Chap. 30. of NATURAL PHILOSOPHY.
239
talline Humour, it must go on ftill directly to M; and
here falling again likewife perpendicularly upon the Su-
perficies of the Vitreous Humour, it must go directly
to the Point O in the Bottom of the Eye.
4. But the Ray BE not falling perpendicular upon the 4. Of the Re-
Superficies EDF, where it is to pafs out of Air into fraction of
Some other of
Water, it ought to be refracted, and to go towards the the Rays, and
Perpendicular EP, and confequently it will tend to fome how they
Point of the Superficies of the Chryſtalline Humour,ſuppoſe from one Part
which come
meet againin
one Point in
the Retina.
G, which is fomewhat nearer H, than it would be without of an Object,
fuch Refraction: Again, the Ray EG likewiſe, not being
perpendicular to the Superficies GHI, through which it is
to paſs out of the Aqueous Humour into a denſer Me-
dium, it ought to be refracted again, and go towards the
Perpendicular GR, and confequently to arrive at fome
Point of the Vitreous Humour, as L, which is nearer to M
than if there had been no Second Refraction: Laftly, Be-
cauſe the Ray GL is alſo inclined to the Superficies LMN,
through which it is to paſs from a denfe Medium to one
that is much rarer, it muſt be refracted, and go from the
Perpendicular LT, the Pofition of which is fuch, you
fee, that the Ray, by going from the Perpendicular, ap-
proaches towards the Ray BDO, and we may conceive
it refracted in fuch a manner, that it fhall go to the
fame Point that the Ray BDO went to, that is, to the
Point O. So likewife if we confider the Ray BF,
fhall find that the Refractions will carry it from F to
I, and from I to N, and that at laft it will meet the o-
ther Two at O. And fince the Rays which fall betwixt
BE and BF, are not quite fo much refracted as they
themſelves are, it is evident, that they cannot do otherwiſe
than meet all together in the fame Point O. Thus we
fee, that the Point B acts upon the Bottom of the Eye,
in the fame manner, as if the Pupil were of no Breadth,
and as if there were to come but one Ray with a Force
equal to the Forces of all them that are contained between
BE and BF.
we
5. Now if we confider the Rays which come from any 5. That the
other Point of the Object, as from A, we fhall find, that Rays which
come from
all thoſe which enter into the Eye, will be refracted different
in fuch a Manner, as almoft to meet all together in Points of the
the fame Point X. And fo likewife thofe Rays which Object, fall
come from any other Point between A and B, they will different
meet very near together in fome Point of the Bottom Points of the
of the Eye between X and O. So that we may affirm
in general, That every Point of the Object, acts very near-
upon as many
Retina.
#
ly
240
Part I
ROHAULT's SYSTEM
6. That the
Rays which
come from
Some Points,
do not reunite
So exactly as
those which
come from
Some other
Points.
7.That if the
way be alter-
ed, the Re-
fractions
could not re-
unite upon
tances.
Tab. VI.
ly but upon one and the fame Point in the Bottom of the
Eye, and on the other Hand, That every Point of the
Bottom of the Eye receives very nearly the Impreffion of one
Point only of the Object.
6. I fay very nearly, not exactly. For if the Superfi-
cies EDF, GHL, LMN, were of fuch a Curvature, as
to carry the Rays from one fingle Point, fuch as B, to
another fingle Point fuch as Ŏ, exactly; it would be
impoffible for them to unite the Rays which come
from any other Point fuch as A, becauſe every o-
ther Point is differently fituated from B with refpect to
the Eye.
7. Now we may obferve, that if the Object be re-
Eye could no moved further from the Eye, in fuch a manner that the
Point B continues always in the Line BD, and the
Shape or Difpofition of the Eye be no ways altered; the
Rays which come from the Point B to the Pupil, will not
the Retina, the diverge ſo much, or be at quite fo great a Diſtance from
Rays which each other as they were before; wherefore in entring the
come from
Three Superficies EDE, GHI, LMN, they will be re-
Objects at all
Sorts of Diffracted in fuch a manner, as to reunite a little nearer to
the Chryftalline Humour than the Point O is. On the
other Hand; if the Object be removed nearer to the Eye;
becauſe the Rays which come from the Point B in or-
der to pass through the Pupil, diverge more than they
did, their Refractions will cauſe them to meet beyond the
Point O. And the Object may be ſo very near the Eye,
that the Rays which come from any one of the Points,
may diverge fo much, as never to unite at all. In all which
Cafes, 'tis plain, there would be no one Point of the Ob-
ject, that would not affect too large a Portion of the Bot-
tom of the Eye; and confequently the Action of each
Point, would be confounded by that of the Point which
is next unto it.
8. Of the Al-
in the Eye, in
order to reu-
nite them.
8. This is what would happen, if the Figure of the
teration made Eye could not be altered; but to remedy all theſe In-
conveniences Nature has fo formed the Eye, that it can
become flatter or longer to fuch a Degree, as to adjuſt
it felf to the different Diftances that we would view the
Object at. Wherefore when we would look upon an
Object at a greater Diſtance, than it could be feen di-
ftinctly at when the Eye is of the ufual Figure, it is
then made flat by the Help of the four right Muf-
cles, all which acting together, pull it towards the Bot-
tom of its Ball, and the Retina is by this means near
enough to the Chryftalline Humour, to be exactly in
the
Chap.
245
of NATURAL PHILOSOPHY.
the Place where all the Rays which come from any one
Point of the diftant Object are reunited. And when we
would look upon an Object that is very near, the Eye
is lengthened by the Help of the oblique Muſcles which
encompaſs it, and by being fwelled, comprefs it; and then
the Diſtance between the Chryſtalline Humour and the
Retina becomes greater, that the Rays which come from
any fingle Point of the Object which is fo near, may be
reunited in a ſingle Point upon the Retina. If, therefore,
there remains any Confufion which Nature has not pro-
vided a Remedy for, it is only in refpect to the Action
of thoſe Rays which come from an Object that is too
near the Eye, at two or three Inches diftance, fuppofe;
but this is needlefs, or at leaſt, not neceffary to be reme-
died; for as Sight was given us principally to take Notice
of Things at a Diſtance, and there is very feldom any Oc-
cafion for feeing Objects fo very near, Nature has not pro-
vided for it.
are altered in
?
9. This Approaching and Receding of the Chryftalline 9. That the
Humour with refpect to the Bottom of the Eye, is fone- Eyes of Birds
ceffary in order to fee diftinctly, that becauſe it cannot a different
be performed by Mufcles in fome Birds, the Coats of manner.
whofe Eyes are almoft as hard and inflexible as Bones,
Nature has provided another Way. For there are pla-
ced in the Eyes of fuch Birds certain black Filaments,
that are not in the Eyes of Men or other Animals, by
which the Chryſtalline Humour is connected with the Bot-
tom of the Eye, and by which it can be made to draw
nearer to, or remove further from the Retina.
10. A re-
10. It is obfervable, that the firft of the three Refra-
ctions which the Rays of Light undergo, in paffing thro' markable Ob-
the Humours of the Eye, is not to be found in Fishes the Eyes of
fervation of
who live in the Water, becauſe the Rays are already in Fishes.
an aqueous Medium, when they begin to enter into the
Eyes of Fifhes. And this feems to be a Reaſon why the
Want of this Refraction fhould be compenfated fome
other Way. And fo we find it is; for Nature has made
the Chryftalline Humour of Fiſhes Eyes more convex,
infomuch, that it is almoft as round as a Globe, and
not of the Figure of a Lens, as it is in other Animals.
11. As moſt antient Perfons grow lean and thin by 11. That the
Age, fo their Eyes grow flat and more funk than when Images of
they were younger. Now in this Figure of the Eyes, the are near, is
Objects that
Rays which come from an Object very neat, come to very confuſed
the Retina before they are reunited; wherefore they im- in old Men.
preſs but a confuſed Image upon it; fo that it is impoffi-
R
ble
242
Part I.
ROHAULT's SYSTEM
12.That thofe
Eyes which
a confused
Impreffion of
Objects that
are at a Di-
Stance.
ble for fuch Sort of Eyes to receive any diſtinct Image,
except when the Object is at a fufficient Diftance.
12. On the other Hand, fome Perfons have by Nature
Eyes that are longer and more gibbous than thoſe of
are very large other Men; in which the Diſtance betwixt the Chryf-
and stick out,
receive only talline Humour, and the Bottom of the Eye, is likewife
greater than ufual: In thefe, the Rays which come from
one Point of an Object further off than ordinary, are reuni-
ted alfo, before they come at the Retina, and then are ſepa-
rated again, ſo that they ſpread themſelves a little upon the
Bottom of the Eye. Whence it comes to paſs, that
thefe Sort of Eyes can receive only a confuſed Image
of Objects that are at a Diſtance;
Diſtance; and have a diftinct
Image of thofe only that are near.
1. That per-
felt
CHAP. XXXI.
What we mean, when we say, that the Images of
the Objects are impressed upon the Organs of
Sight.
Images of WHEN
HEN we once clearly underſtand, that every
fingle Point of the Object acts upon one fingle
wifihle Objects Point only of the Bottom of the Eye which anſwers
on the Retina. directly to it; and on the other hand, that every Point
of the Bottom of the Eye receives the Impreffion of
but one Point only of the Object; it is not difficult to
conceive that the whole Objet acts upon a certain Part
of the Retina, which is as exactly of the fame Shape
with it, as could be drawn upon a Cloth by the moſt
skilful Painter. We can yet further conceive, that this
Part of the Retina does fill more perfectly reſemble the
Object, becauſe it receives as many different Preffures
in all its ſeveral Parts as there are different Colours, or dif-
ferent Degrees of Light in the feveral Parts of the Ob-
ject. And becauſe we call that an Image, or a Species,
which has any Refemblance to the Thing which it re-
prefents, we call that Part of the Retina upon which
all the Rays of the Object fall by that Name, and
fay, That it impreffes its Image on the Bottom of the Eye.
2. There
Chap. 31. of NATURAL PHILOSOPHY.
243
2. There is no need of fearching after any other Re- 2.Wherein
ſemblance in this Image, than what has been mentioned. this Image is
different from
For if we would make any further Compariſon betwixt the Object.
it and the Object, we ſhall find them very different. And
firſt herein they differ, that a Body is always repre-
fented by a Superficies, and fometimes a Superficies by a
Line, and fometimes a Line by a Point: Secondly, The
Situation is different, for the upper Part of the Object
is painted upon the lower Part of the Eye, and the right
Side of the Object upon the left Side of the Eye, &c.
Laſtly, They differ in Magnitude, for a very large Object
is repreſented upon a very ſmall Part of the Eye.
3. And the further diftant the Object is, fo much the
lefs is this Part of the Bottom of the Eye; as is evident
in the Figure of the Eye C, where the Space HI, which
receives the Image of the Object FG, is less than the
Space DE on which the Object AB, which I fuppofe
equal to FG, is impreffed; and this very nearly in the
fame Proportion, as the Diſtance of FG from the Eye is
greater than the Diſtance of AB.
3. The far-
ther the Ob-
ject is diftant
from the Eye,
the leffer is its
Tab. VII.
Image.
Fig. 2.
ges may be
4. Whoever confiders ever fo little of what we have 4. An Expe-
before laid down, concerning the Nature of Light and riment where-
by these Ima-
Colours, cannot but be of our Opinion, That the Ima-
ges of Objects are in this manner impreſſed on the Bottom of Jeen.
the Eye But he may be further convinced of it from Ex-
perience; for if, after having darkned all the Windows of
a Room, over-againſt which are fome bright Objects,
we make a Hole in the Window Shut, and place in it the
Eye of an Animal, freſh killed, firſt taking off neatly all the
Membranes which the Bottom of the vitreous Humour
is covered with, and put an Egg-Shell in their ſtead to
hold this Humour in, and you will fee upon the Egg-
Shell a diſtinct Picture of all the Objects that are with-
out.
5. But becauſe there are fome Difficulties to make this 5. An artifi
Experiment fucceed well; I have thought that the fame cia! Eye for
the fame pur-
Thing might be done, by making a large artificial Eye, rofe.
which I accordingly tryed: The opake Coats, or Tunicks,
were all made of thick Paper, except the Retina, which
was made of a very white thin Piece of Vellum; in the
Room of the Tunica Cornea, I put a tranfparent Glaſs, and
inſtead of the Chryftalline Humour, was a Piece of Chryf
tal of the Figure of a Lens, but more flat, than this
Humour; for fince there was nothing in this Machine
but Air, in the Places of the aqueous and vitreous llu-
mours, a little lefs Convexity was fufficient to produce.
R 2
the
244
Part I.
ROHAUL
ROHAULT's SYSTEM
!
6. How to fee
the Image of
an Object in
this artificial
Eye.
7. The first
the Refractions required: And becauſe it was very diffi-
cult to flatten or lengthen this artificial Eye, in the man-
ner the natural Eye is done by the Muſcles, I placed the
Vellum in fuch a manner, that it could be moved back-
ward or forward, at pleaſure.
6. This artificial Eye being fo placed in the Window
of a Room, that the Glafs which repreſents the Tunica
Cornea, may be directly againſt ſome Objects that are ve-
ry much illuminated; we ſhall not only fee the Images
of them impreffed upon the Vellum, but we may alſo
obferve all the moſt minute Particularities,which we before
collected from Reafon. Thus we may obferve,
7. First, That it is at one particular Diſtance only of
Obfervation, the Vellum from the Chryftal Lens, that the Image will
appear the moſt diftinct that is poffible.
8. The fecond 8. Secondly, That this Image is not fo diftinct in the ex-
Obfervation. treme Parts, as in the Middle.
9. The third 9. Thirdly, That if the Vellum be too near the Lens;
Obfervation. the Image will be lefs, and very much confuſed.
10.The fourth
10. Fourthly, That if it be too far, the Image will be
Obfervation. larger, but all confufed likewiſe.
11. The fifth
II. Fifthly, That the diftinct Image of any Object, is
Obfervation. fo much the lefs, as the Object is more remote.
12. The fixth
12. Sixthly, If a certain Diſtance between the Lens and
Obfervation. the Vellum, be requifite to make a diftinct Image of an
Object at a moderate Diſtance; the Vellum muſt be mo-
ved a little nearer, fo that the Diſtance of the Lens from
it may be lefs, if we would have a diftinct Image of a-
another Object, which is at a confiderably further Di-
ftance.
13. The fe-
vation.
13. Seventhly, When the Vellum is at a proper Di-,
venth Objer ftance, to reprefent diftinctly an Object which is at a
great Diftance, fuppofe an Hundred, or Two hundred
Yards; there is no need of altering it, in order to repre-
fent, as diftinct as is poffible, any Objects that are at a
ftill greater Diſtance.
14. The eighth
14. Eighthly, The nearer the Object is to this artificial
Obfervation. Eye, the further muft the Vellum be removed from the
15. The ninth
Lens.
15. Ninthly, When the Object is too near this arti-
Obfervation. ficial Eye, it is impoffible to get any diftinct Image,
let the Vellum be removed to what Diſtance we
will.
•
16. It
Chap. 31. of NATURAL PHILOSOPHY.
248
twixt this ar-
16. It is to be obſerved, that in thofe Cafes where any 16. The dif
Alteration muſt be made in the Eye, in order for the I- ference be
mage to become diftinct, this Alteration is much lefs in tificial Eye,
the Eyes of Animals, the Coats of which are flexible, and the natu-
ral Eye.
than in this artificial Eye. For in Animals, the length-
ning or fhortning the Eye being always attended with a
greater or leſs Convexity of the Cornea, the Figure of
this Coat contributes its Part in producing that Effect
which in the artificial Eye wholly depends upon the Length
or Shortness of it. Thus, if when the artificial Eye has re-
ceived a diſtinct Image of a diftant Object, another Object
be placed before it at fuch a nearer Diſtance, that in order
to have the Rays which come from every Point of it re-
united, the Eye ought to be made One hundredth Part
longer than it is; the Vellum muſt be removed juſt ſo
much further from the Lens: But in a parallel Cafe of
the natural Eye, it is not requifite that That fhould be
lengthened a hundredth Part of the Whole, becauſe the
Tunica Cornea being more gibbous than it was before,
caufes greater Refractions, and fo makes the Rays reunite
ſooner than they would otherwiſe do.
17. The Image of an Object impreffed on the Eye of 17. That the
an Animal, being received in a Place where the Capilla- Capillaments
of the Optick
ments, of which every Optick Nerve is compofed, meet Nerves,
each other; it is very probable, that this Image is fo im- tranfmit the
preffed, that the Rays do not move thefe Capillaments Object to the
Action of the
fide-ways, but always fall directly upon the Extremities of Brain.
them. To which, if we add; That the Impreffion which
is made upon the Extremity of every one of thefe Capil-
laments, is communicated from one End to the other,
we may conclude, that the Image of the Object is tranſ-
mitted intire to that Place where thefe Capillaments end
in the Brain.
18. And becauſe we have no Senfation, when thofe 18. That the
Parts of the Body are any way affected, in which there Place where
Brain is the
are no Nerves; it is very probable, that the Nerves are the Soul per-
neceffary to Senfation. And becauſe we have no Senfa- ceives.
tion likewife, when any Object makes an Impreffion up-
on a Nerve, if its Communication with the Brain be hin-
dred, or if the Brain it felf be affected with any particu-
lar Diftemper; therefore it is reaſonable to think, that the
Nerves are not the immediate Organs of the Soul, but
that they are fo formed by Nature, as to tranfmit the
Impreffion which they receive, to that Place in the Brain
where the Origin of them is, and where probably the im-
mediate Organ of the Soul's Senfation is.
R 3
19. How-
246
Part I.
ROHAULT's SYSTEM
19. That
there is a
19. However, we may further obſerve, that there be-
Part of the ing Two of a Sort, of almost all the Parts of the Brain,
Brain which they cannot all of them indifferently be thought the im-
is the princi- mediate Organ of the Soul. On the contrary, it is high-
pal Organ of
the Soul.
ly probable, that fince we have but one Senſation only,
though two Impreffions are made by the Object upon
the external Organs of the Senſes which are affected,
that there is likewife one particular Place in the
Brain where theſe two Impreffions meet. Which that
Place is, may be very difficult to determine; but whe-
ther it be that fmall Gland which Phyſicians call the
Conarium, or whether it be any other Part of the Brain, it
is hardly to be conceived how they can thus unite, with-
out fuppofing fomething equivalent to what is now
faid.
20. A Conje-
cture about
the Continua-
tion of the
Tab. VII.
Fig. 2.
20. Beſides the manifeft Refemblance which there is
betwixt the two Eyes; I imagine there is another yet,
which cannot be difcerned by the Senfes, which conſiſts
Capillaments in this, that the Number of Capillaments in one Optick
of the two
Nerve, is equal to the Number of Capillaments in the
Optick
Nerves. other Optick Nerve. Thus (to make the Thing eaſier)
if we fuppofe the Optick Nerve of the Eye A to contain
five Capillaments, the Extremities of which are CDEFG;
it is reaſonable to think, that there is the fame Number
in the Nerve of the Eye B, the Extremities of which are
HIKLM. I imagine alfo, that the Extremities E and K,
which are in the Middle of the Reft, are exactly at the
End of the Optick Axes, that is, at the Ends of the Lines
TE, VK, which país through the Centers of the Pupil,
the Chryftalline Humour, and the Body of the Eye; and
that the reft are placed fo regularly about thefe, that we
may take ſeparately all the Capillaments of one Eye in
order, and affociate them with thofe in the other Eye ta-
ken in the fame Order, fo as to make up a great Num-
ber of Pairs, which may be called Sympathetick: Thus
beginning with the Capillaments C and H, which are
moft on the Left Hand, I make them the firſt Pair; the
other Pairs are DI, EK, FL, GM. I am alfo of Opinion,
that each Pair of Sympathetick Capillaments end in the
1. In the fame Point of that Part
of the Brain) This Conjecture is not
yet confirmed, by cutting open the
Brain. But be that as it will; the Ca-
pillaments CH, DI, EK,
&c. may very properly
be called Sympathetick.
For whether the Pairs of
Tab. VII.
Fig. 2.
Nerves meet in the Brain or no, it
is evident, that two Images of every
Object imprefled upon thofe Capil-
laments must be feen in the very
fame Place (becauſe the Optical Axes
meet each other) that is, muſt be-
come one; and therefore the Object
appears fingle.
fame
t
A
**
Chap. 31. of NATURAL PHILOSOPHY.
247
fame Point of that Part of the Brain which raifes a Sen-
fation in the Soul; as you fee in the Figure, where the
Pair CH meet in the Point O of, the principal Organ
X, the Pair DI in the Point P, the Pair EK in the Point
Q, the Pair FL in the Point R, and the Pair GM in
the Point S.
21. This being fuppofed. I conceive that when we
would look upon an Object, we turn our Eyes to it
in ſuch a manner, that the two Optick Axes meet at
the Point which we fix our Attention principally up-
on. Thus the Rays TE, VK, coming from that Point,
and falling upon the Sympathetick Capillaments E and
K, the two Impreffions which they make there, are re-
united in one Point only, viz. in the Point Q
So
likewife the Part of the Object which is on the right
Hand, ſhakes the Sympathetick Capillaments D and I,
the Impreffions, of which are carried to P. And again,
the Part of the Object which is on the left Hand acts
upon the Sympathetick Capillaments F and L, and their
Impreffions unite in the Point R, and fo of the reft.
So that though there be two Images impreffed upon
Eyes, yet there is but one impreffed upon that Part of
the Brain X which we here fuppoſe to be the immedi-
ate Organ of Vifion.
the
21. How the
Object acts
upon the im-
mediate Or-
gan of the
Soul.
Tab. VII.
Fig. 2.
not the imme-
22. What has been already faid of the Images which 22. An evi-
dent Proof,
vifible Objects imprefs upon the Eye, being well un- that the
derſtood; it cannot but be a ftill greater Surprise, Chryftalline
that the Ariftotelians and almoft all Phyficians fhould Hamer is
be ſo miſtaken, as to affirm, that theſe Images are im- diate Organ
preffed upon the Chryftalline Humour, and go no fur- of Vision.
ther;
for it will evidently appear, that the different Im-
preffions of the diverfe Points of the fame Object, are all
confuſed there.
CHAP
R 4
248
Part I
ROHAULT's SYSTEM
}
1. What is
CHAP. XXXII.
How Vision is performed.
FTER having traced the material Image of the Ob-
pacant by Vi-Aject, or the Impreffion which it makes upon the ex-
fion
2. How the
mage of the
Soul.
ternal Organs, to the Brain, I come next to explain how
this raiſes in us an immaterial Image, or that Senfation in
which Sight properly confifts, and to fhow the Reaſons
why it is clear and diftinct; and alfo how, we perceive the
Place, Situation, Diſtance, Magnitude, Figure, Number, and
the Motion or Reft of fuch Objects.
2. In order to underſtand how this immaterial Image
immaterial I- is formed in us, I must remind you of a certain Truth
Object is which has been fufficiently demonftrated before, and that
formed in the is, That fuch is the Nature of our Soul, that particular
Motions of the Body to which it is united, are the Oc-
cafions of particular Perceptions in it: Now different Parts
of the Object, act diftinctly upon different Parts of the
Bottom of the Eye, and their Impreffions being tranf
mitted to that Place of the Brain which is the principal
Organ of the Soul, it is eafy to apprehend, that the Soul
muſt have as many diftinct Senfations raiſed in it, at the
fame Time, and without any Confufion, as every one of
them excites different Motions.
3. Whence it
is that this I-
mage is fo
clear.
clear as that
3. It is manifeft alfo, that this immaterial Image, ought
to be fo much the more vivid or clear, as the Object
fends forth more Rays of Light which are received by
the Eye, for by this means the Impreffion made upon
the Organ will be ſo much the ſtronger And the Large-
nefs of the Pupil contributes likewife to this Clearness, be-
cauſe it affords Room for more Rays that come from the
fame Point of an Object to imprefs the Image on the
Bottom of the Eye.
A. That the
4.
It is true, that if we confider the Action of one
Image of a
Point of the Object only, we muſt ſay, that the Senfa-
Body at a
Distance tion ought to be weaker or more obfcure in proportion to
ought to be as the greater Diſtance of the Body, becauſe the Rays of Light
of a Body which come from one Point of it diverge, and therefore
which is near. fewer of them enter into the Pupil when the Eye is far of,
than when it is near. But we know that one Point off
the Object does not act alone, but always acts in com-
pany with a great many others, and the whole Image of
the
Chap. 32. of NATURAL PHILOSOPHY.
249
the Object is impreffed upon fo much a lefs Space on
the Retina, as the Diſtance of the Object from the Eye
is greater. Thus if one viſible Point, at the Diſtance of
two Miles, fend to the Pupil but half the Rays that it
would do if it were but at a Mile distance only, this is
made good by fome other viſible Points that are near it,
which fend their Rays upon the fame Capillament of the
Optick Nerve, where one fingle Point of a nearer Object
would fend its Rays; wherefore the Viſion ought to be
as ftrong and vivid.
appear more
5. To this we may add, that becauſe we open the 5. Why di
Pupil of the Eye a little more when we look upon Ob- Rant Objects
jects that are at fome diſtance, than when we look at clear to us.
thofe which are near; therefore we take in more Rays
from any Point than we do when the Pupil is not fo
wide, and this makes the Senſation more clear. And thus
we find, that a Mountain looked upon at fome diſtance
does not appear of fo dark a Colour as when we are
nearer it.
6. As to the Diftinctness of Vision, that evidently de- 6. How Ob-
pends upon the Refraction of the Rays; and it is then jets appear
diftinct.
as diftinct as poffible, when the Refraction is fo made, as
that all the Rays which come from one and the fame
Point of the Object, meet together exactly in one and
the fame Point of the Bottom of the Eye: But
this never is preciſely fo, but in thofe Rays which come
from that Point of the Object which is at the Extremi-
ty of the Optical Axis; for it is evident, that thofe
Rays which come from the other Points, are reunited
fo much the lefs, exactly one than another, as they are
more diſtant from this Axis; wherefore we cannot at the
fame time have the moft diftinct Senfation but in this
Place alone, and the reft will be more confufed.
7. This being fo, it follows from what was before de-
7. Why old
monftrated concerning the confufed Impreffion of an jets that are
Men fee Ob-
jects
Object that is near, on the Eye of an Old-Man; that near them ve-
he muſt ſee fuch a near Object very confuſedly; and thus ry confuſed.
we fhall eſcape the Error of thofe, who are of Opinion,
that the Confufednefs in the Sight of Old-Men, arifes
from hence; that the Faculty of Seeing, or the Senſe of
Seeing is weaker in them than in others. And indeed it
is very furprizing, and very lucky, that at a Time when
the Doctrine of Refractions was not at all known, A-
riſtotle ſhould hit upon faying, that if an old Man had
the Eye of a young Man, he would fee as the young
Man does; which is the fame Thing as to fay; that the
Fault
250
Part I.
ROHAULT's SYSTEM.
8. Why fome
Perfons fee
Objects that
are at a Dif-
tance confu-
fedly.
9. Another
Canfe of the
Diftin&iness
•f Viſion.
any
Fault in the Sight of an old Man, does not ariſe from
Defect in the Faculty of Seeing, but only from fome De-
fect in the Organs.
8. On the other Hand we are affured, that thoſe Per-
fons, whofe Eyes are longer and more gibbous than ordi-
nary, receive a diſtinct Impreffion only of thofe Objects
which are near; and a confuſed Impreffion of thofe that
are diſtant: Whence it is eaſy to conclude, that fuch Per-
fons muſt fee Objects that are near them diftinctly, and
thoſe that are at a Diſtance confufedly.
9. The Diftinctnefs of Vifion depends alfo upon the
Largenefs of the Space which the Impreffion of the Ob-
ject takes up in the Bottom of the Eye, where there ought
to be at leaſt as many Extremities of the Capillaments of
the Optick Nerve, as there are different fenfible Parts
in the Object which fends forth the Rays, in order for
every one of them to make a diftinct Impreffion. For
if the Rays which come from two different Parts of the
Object, meet together in two different Points of the fame
Capillament, it is the fame Thing, as if they met in one
Point, becauſe they cannot communicate two different
Motions to this Capillament at the fame time. And this
is the Reaſon why Objects, that are at a very great Di-
ſtance, becauſe their Images are impreffed on a lefs Space,
are ſeen but confufedly.
10. Further, if this diſtant Object be compofed of a
10. Why Ob-
jects, whose great many different Parts which are of different Colours,
Parts are of it is evident, that if ſeveral of theſe Parts act together
different Co-
Lours, appear upon the fame Capillament, that which is of the brighteſt
at a Distance Colour is the only one that will be feen, becauſe the
of the fame Capillament will receive the Impreffion only of this Part.
And thus we fee in a Meadow where there are a great
many white Flowers mixed with a vaſt Number of green
Spires of Grafs, at a Diſtance it looks all White.
Colour.
II. How we
refer our Sen-
fation to ex-
ternalThings.
11. If it had never been obferved, that we fometimes
have no Perception, when we would have ſome, and at
other Times have a Perception, when we would not, we
fhould not have been fo ready to have connected our
Judgement with our Senfation, and Senſation would only
have been fimple Perception: But when we had once
made this Reflexion, our Senfation muſt neceffarily be a
compound Perception. And if we had been more wary
in our Judgement at firft, fo as not to have affented to
any Thing of which we had not a clear Perception, all
that we could plainly have inferred, is, that fomething
concurred with us to caufe Senfation. But having been
dif-
Chap. 32. of NATURAL PHILOSOPHY.
25x
37
1
ferently accuſtomed from the Beginning, and over hafty
in our Judgement, we have drawn a different Confe-
quence; and look upon the Senſation, which now upon
more mature Deliberation, we acknowledge only as an
accidental Mode of exiſting, to be without us, and there-
fore we refer it to external Objects; and we have ſo of-
ten made this Judgement, that we are accuſtomed to do
it without any Difficulty, and without the leaſt Suſpicion
of its not being conformable to Truth.
we
Reafon why
we do this.
12. We have been confirmed in this Errour about Vi- 12. Another
We obſerve, that when an
fion by another Miftake.
opake Body is put between the Object and our Eye,
then ceaſe to fee it: From whence we ought to con-
clude, that the Thing which concurs with us to excite
Senſation, is beyond the opake Body, and being no longer
able to act upon our Organs, we ceafe to have the Sen-
fation we had before. But inſtead of reaſoning in this
manner, we imagine, that the Senfation which we have
of Light or Colour, that is, the Light or Colour which
we perceive, is beyond that Body, and fo carrying our
Imagination as far as the Object it felf, we go as it
were out of our felves, along the Line in which we
receive the Impreffion of the Object, and afcribe our
own Senfation to it, that is, the Colour which we per-
ceive.
13. The fame Thing that leads us to refer the whole 13. How we
Senfation which we have of an Object to fomething without Situation of
us, leads us alfo to refer all the particular Senfations of an Object.
which it is compofed, in the fame manner, in ſtraight
Lines, according to the Direction in which we receive
the Impreffions from different Parts of the Object: Thus
the Impreffion which is made in the lower Part of the
Bottom of the Eye, coming to us in the higheſt of all
the Lines by which the Object raiſes any Senfation in us;
it is along this Line that we refer the particular Senfati-
on which ariſes from it. So likewife we refer to the
loweſt Part of the Object, that Senfation which arifes from
the Impreffion made by it, on the higheſt Part of the
Bottom of the Eye. And hence it is, that though the
whole Image which the Object impreffes on the Bottom
of the Eye be inverted, yet when we look upon the Ob-
ject through a fimple uniform Medium, this hinders not
but that it appears in its true Situation; that is, the
immaterial Image makes the Object appear to us as
it is.
14. The
252
Part I.
ROHAULT'S SYSTEM
14. How we
perceive its
Diſtance.
15. Another
For
14. The Knowledge of the Diſtance of an Object,
as well as that of the Situation of it, depends upon our
referring our Senfation to fomething without us.
our regard being chiefly upon the Pofition of the two
optical Axes, and the Motion of the right Muſcles of
our Eyes by a natural Way of Reaſoning, fhowing us
very near, the Relation or Inclination which theſe two
Axes have to each other, and at what Diſtance from
us they meet together; it is to this Diſtance that we
refer our Senfation, that is, to the fame Place where the
Object is. Wherefore if at any Time we are deceived
in the Judgement we make of the Diſtance of any Ob-
ject, when we look upon it with both Eyes, it is be-
cauſe we do not know exactly at what Diſtance the
Optical Axes meet.
15. And if we make uſe of but one Eye, we can
Way to know know the Distance of an Object, provided we move
the Distance
of an Object. from one Place to another; for we have fome kind of
Memory of the Poſition of the Optical Axis in the firſt
Station, when we really attend to the Pofition of it in
another Station, fo that we imagine two Optical Axes,
though there be indeed but one, and by that means
guefs at the Diſtance where they meet; and to this we
refer the Object.
16. A Third
of an Object.
16. Since we cannot incline the Optical Axes to each
Way to know other in a certain manner, in order to make them meet
the Distance
at one Point of an Object which is at a certain Di-
ftance from us, but at the fame Time, we muſt put
each Eye into a particular Difpofition or Figure, necef-
fary to fee diftinctly at that Diſtance; we may prefume
that Nature has fo ordered the Muſcles of the Eyes,
that they neceffarily procure both thefe Effects at the
fame Time: And that this is fo, we fhall have no Doubt,
if we obſerve, that they who ſee but with one Eye,
move their Eyes in the fame manner to look upon Ob-
jects at different Diſtances, as they who fee with both
Eyes. So that it is fufficient, if our Eye be fo flatten-
ed or lengthened in a particular manner by the Action
of the Muſcles, as to cauſe fome Alteration in the Brain,
which puts the Soul upon conceiving the Pofition of the
Optical Axes: And fince the perceiving this Difpofition
is the moſt natural Argument to make us know the Di-
ftance of an Object, it follows, that the lengthning or
flattening the Eye is alone fufficient to difcover this
Diſtance.
17. But
Chap. 32. of NATURAL PHILOSOPHY.
253
eafier to be
upon it with
17. But becauſe the Alteration of the Shape of one 17. That it is
Eye only, when we make uſe of it, to fee diftinctly at deceived in
different Diſtances, is not ſo fenfible, as the Alteration of the Judge-
the Situation or Pofition of the two Eyes, when in ment we make
of the Dif
order to look at different Diſtances, we turn them dif- tence of an
ferently that we may make the two Optical Axes meet Object, when
we look upon
in the fame Point; therefore we are not to think, that it with but
this latter Alteration is ſo exactly made, when it is deter- one Eye, thau
mined by the other, as if it were cauſed by that Atten- when we look
tion which we have when we look with both Eyes up- both Eyes.
on the fame Point of an Object. And this is the Rea-
fon why we are more apt to be deceived in the Judge-
ment we make of Distance, when we ufe but one Eye
than when we uſe both. And indeed if we try to touch ì an
Object at three or four Foot diſtance, with the End of a
Stick of about the fame Length, we ſhall find, that if
we look at it but with one Eye, we fhall miſs touching
it two or three times together; whereas if we look at it
with both Eyes, we fhall touch it the firſt Time.
18. Whatever the Alteration be, which is made in the 18. That it is
easier for us
Eyes when we look upon Objects at unequal Diſtances, to be deceived
it is certain, that That Alteration cannot be at all fenfi- in or Judge-
ble, when the Diſtance is ſuch, that the neareſt Object is ment of great
Distances
a great Way off; wherefore we muſt be very liable to be than of small.
more deceived in our Judgement of great Diſtances than
of ſmall.
19. Beſides the two forementioned Means of judging 19. That the
of the Diſtance of Objects, which are the principal ones, Diftinctness
or Confused-
there is yet fome others: As Firft. Having often obfer- nefs of the I-
ved, that an Object appears more confufed the further it mages of Ob-
is diftant from us, we make this a Rule of determining in judging of
jects, help us
the Diſtances of Bodies, fo that according as they appear their Dij
more or less confuſed do we imagine them to be at a
greater or leſs Diſtance.
tance.
20. So likewiſe, becauſe we have often obferved, 20. The fame
that an Object looks of a brighter Colour, the further it Thing allo
follows from
is removed from us; therefore when we fee an Object their being
of a brighter Colour than it uſes to appear of when it is more or less
near; we conclude, that it is at a great Diſtance from
us.
1. An Object at three or four Foot
Distance) It is to be obferved, that
the Stick muſt not be thruſt directly
upon the Object, but moved ob-
liquely, in the fame manner, as if,
when a Ring is turned Side-ways to
the Eye, we would try to run a
Stick through it; as is juftly re-
marked by Malbranch in his En-
quiry after Truth. Book I. Chap. ix.
Sect. 3•
bright.
1
21. The
;
254
Part I.
ROHAULT's SYSTEM
!
21.That we
4
21. The Situation is another Means ftill of knowing
know the Di- the Diſtance of Objects. For, of thofe Things which
Stance by the
Situation al- we imagine to be lower than our Eye we judge them to
So.
be fartheft diftant which affect the Eye, with the higheſt
Rays; and on the other Hand, of thofe Things which
we imagine to be higher than our Eye, we judge them
to be fartheſt diſtant which affect the Eye with the low-
eſt Rays.
22. The In-
a great many
makes us
22. Further, the Interpofition of a great many other
terpofition of Objects, between us and the Object we look at, makes
other Bodies, us think, that the Diſtance is greater than otherwiſe
we ſhould; becauſe the Diſtance which we conceive
think, that
the Object is
to be betwixt every one of them, is the Meaſure which
at the greater we compute the Diſtance of the Object by: Thus
Distance. in the Inftance of the Moon, when it is at the highest
above the Horizon, and we look at it through the
Air only in which there are no other vifible Objects,
we imagine it to be nearer to us, than when it riſes or
fets, becauſe at thofe Times, there are a great many
intermediate Objects upon the Earth, between us
23. How we
and it.
23. When we know the Situation and Diſtance of
come to know an Object, by joining theſe together, we form a Judge-
the Bignefs of ment of the Bignefs of it; For, becauſe we imagine the
Objects.
Extremities of an Object, to be contained between two
ftreight Lines coming from the Eye, which diverge from
each other in proportion to their Diſtance; therefore
we eaſily conceive what the Bignefs of the Object is at
a given Diſtance. So that if at any Time we are de-
ceived in our Judgement of the Bignefs of any Object,
it is becauſe we are firft deceived in our Notion of
its Diſtance. Thus, becauſe we cannot truly compre-
hend the Diſtance of the Moon or Sun from us, there-
fore no Imagination can reprefent thofe Bodies to us fo
great as they really are.
24. Why the
Stars feem
bigger to us
when they are
in the Hori
xon.
24. And this is fo true, that the Stars feem to us
fomewhat larger, when the Interpofition of vifible Ob-
jects which are between them and us, helps us to ima-
gine their Diſtance to be greater; For it is not ow-
to the Interpofition of Vapours, as the Ancients
thought,
ing
To the Interpofition of Va-
pours, &c.) Since the Angle under
which the Moon appears when in
the Horizon, is not greater than
ordinary, it is evident, that nothing
ought here to be afcribed to the
Refraction of the Vapours. And
that this Angle is not greater than
ordinary, is clear from hence; that
though every particular Part of the
Horizon (as well the Diſtances of
the Stars from each other as the Stars
them-
Chap. 32. of NATURAL PHILOSOPHY
259
thought, that makes the Stars to appear of different Big-
neffes, as if the Rays which came from the Extremities
of them to the Eye of the Spectator were by that means
refracted, fo as that he fhould fee them under a bigger
Angle. For modern Aftronomers who have meaſured the
Angles under which the Stars appeared, when they were
in the Horizon, and when they were at their greateſt
Altitude in the fame Day, I have always found them the
fame.
appear larger
25. It is to be obſerved alfo, that very luminous or 25. That per
bright Objects muft needs appear bigger than they would bright Objets
do if they were not fo bright. For if the Image which than they
they imprefs upon the Bottom of the Eye, affects not ought to de.
only a certain Number of Capillaments, but fpreads it
felf to the Extremities of other Capillaments which are
about it, it is the fame as if it had covered them al-
fo; becauſe the Rays have fo great a Force that all
thefe Capillaments are moved by them, and not at all
hindred by the Motion of thoſe Rays which come from
the other furrounding Bodies which affect the fame
Part, but are very faint; therefore a bright Body ap-
pears fo much the bigger, as it takes up part of the
Object which is not fo bright, whofe Rays are ſwallow-
ed up by it.
!
when looked
pear as mucha
26. We may add ftill further; that the Impreffion of 26. Why she
a very luminous Body may be fo ftrong as to extend it fixed Stars
felf all round to fome Capillaments, which no Rays at at through a
all come to from the luminous Body; in which Cafe, Telescopes up-
it is manifeft, that the Object muft appear much bigger diminished,as
than it would do, if its Light were more faint. And other --Objects
it is certain, that we fee the fixed Stars in this man- appear mag-
ner; becauſe if we weaken their Action, by artificial-
nifyed.
ly contracting the Pupil, and looking at them through
a Hole made in a Card with a Needle, 2 they
appear much leſs. But that which moſt ſurpriſes thoſe
who
1
:
themfelves; may the Stars, when
they feem to be larger, feem alfo
to take up more of the Space which
furrounds them;) though, I fay,
every Part of the Horizon feems to
be equally inlarged; yet the whole
Circle cannot contain any more
than 360 Degrees; wherefore, Bo-
dies in the Horizon are not feen
under a greater Angle, but every De-
gree in the Horizon feems greater
than in the Meridian.
1. Have always found them the
fame) Nay, they have found the Di-
ameter of the Moon, when at the
higheft, a little bigger, than when
the rifes or fets. See Malbranch's
Search after Truth, Book I. Chap. ix.
Sect. 3.
2. They appear much less) Nay.
that the fixed Stars, by reafon of
their immenſe Diſtance, are but like
Points only, except that their Light
is a little dilated by Refraction, is
evident
256
Part I
ROHAULT'S SYSTEM
27.The know-
ing the Big
ness of an Ŏb-
ject, helps us
much in
judgeing of
its Distance.
28. How we
know the Fi-
who fee not the Reafon of this, is, that when we look
at the Stars with a Teleſcope, they appear as much di-
miniſhed as other Objects appear inlarged by it; and for
this fole Reaſon, becauſe hereby the Force of their Rays
is very much weakned.
27. It is certain alfo, that as the Knowledge of
the Diſtance helps us to find out the Bigneſs, ſo like-
Wife the knowing of the Bignefs helps us to conceive
the Diſtance. Thus, when we know that a Man is
about five or fix Foot high, when we fee him to
appear but very little, we conclude him to be at a great
Diſtance.
28. It would be fuperfluous to fhow particularly how
gure of an Ob- We know what Figure any Object is of, after what has
been ſaid concerning knowing the Situation, Diſtance, and
Bignefs of its Parts; for the Knowledge of its Figure
confifts in theſe.
ject.
29. Why we
See an Object
Single, when
we look at it
with both
Eyes.
30. Why an
Object ap-
29. Nor is it difficult, after what has been ſaid, to
give a Reafon why an Object appears fometimes fin-
gle and fometimes double; for it is evident, that an Ŏb-
ject muſt appear fingle, when it fo affects the Sympa-
thetick Capillaments of the two Optick Nerves, as to im-
prefs but one Image upon the Brain.
30. And this is confirmed from hence, That if we
pears double. prefs either of our Eyes with our Finger, fo as to make
it receive the Image of the Object on a different Part
from what it would do by the common Motion of the
Muſcles; as it is certain, that the Images which are then
impreffed on the two Eyes, do not fall upon the Sympa-
thetick Nerves, nor reunite in the Brain, fo we cannot fail
to fee the Object double.
31. Another
Object double.
31. So likewife, if we look very intently upon a par-
Way to See an ticular Object, and at the fame time another Object be
placed nearer or further off, which confequently cannot
impreſs its Image on the Sympathetick Capillaments of the
two Optick Nerves; in this cafe it muſt impreſs two I-
mages on that Part of the Brain which is the immediate
evident from hence, that when they
are about to be eclipfed by the
Moon, when they enter into its Bo-
dy, their Light does not decreaſe
gradually (as that of the Planets does)
but vaniſhes all at once, and at the
End of the Eclipfe, it appears again
all at once.
I. But one Image upon the
Brain) See the Notes on Chap. xxxi.
Art. 20.
Organs
Chap 32. of NATURAL PHILOSOPHY.
257
|
Organ of Vifion, and therefore it muſt be feen
double.
5
32. Having feen how we come to know the Situa- 32. How we
tion, Diſtance, Magnitude, and Number of Objects by perceive Mo-
our Sight; nothing more remains but to examine how tion and Reft.
we know whether they be in Motion or at Reft. Now it
is not difficult to conceive, that we know a Body to be
in Motion; firft, when its Image appears fucceffively
applied to different Images of certain Objects, which
we do not compare with any other, but imagine to be
immoveable; or when we find that we muſt turn our
Head or our Eyes in order to have the Object always
at the End of the Line, along which we carry our
principal Attention; or laftly, when, if we move nei-
ther our Eyes nor our Head, we find it is gone out of
that Line. The contrary to all which makes an Ob-
ject appear to us to be at reft.
Tab. VII.
Fig. 2.
1. It must be ſeen double) It may |
be further obferved here, that if
the Object now mentioned, be pla-
ced beyond the Point where the Op-
tical Axes meet, it will
then appear double in
fuch a manner, that of
the two Images, that
which is on the right Hand is
feen with the right Eye, and that
on the left Hand with the left
Eye; but if the Object be on this
Side, that Point, then the Image
which is on the right Hand will
be feen with the left Eye; and the
Image on the left Hand with the
right Eye. The Reafon of which
is, becauſe in the former Cafe the
Object impreffes its Image on
HIK the left Side of the right
Eye, and therefore is feen by it
on the right Hand, and on EFG
the right Side of the left Eye,
and therefore is feen by it on the
left Hand In the latter Caſe it
impreffes its Image on KLM the
right Side of the right Eye, and
therefore appears to it on the lett
Hand; and on CDE the left Side
of the left Eye, and therefore ap-
pears to it on the right Hand.
What furprising Things follow from
this Obfervation, may be feen in
the Notes on the following Chap-
ter.
S
CHAP.
258
Part I.
ROHAULT'S SYSTEM
+
CHAP. XXXIII.
Of DIOPTRICKS.
1.That our
Opinion a-
bout Vision
may be con-
1.
IN
N order to prove the Truth of fome of thofe Suppo-
fitions which we have made about Vifion; we ought
now to confider, whether or no all thofe Things, which up-
firmed by the on theſe Suppofitions ought to come to pafs, when we
Examination look through different Sorts of Perſpective-Glaffes or
Sorts of Per- upon Looking-Glaffes, be agreeable to Experience; for
Spective- this will be a great Proof of the Truth of thofe Sup-
Glaffes and
Looking-
pofitions.
Glaffes.
of different
2. We will begin with Perſpective-Glaffes, and firft
2. Why an let us confider that Sort called Multiplying-Glaffes, fuch
Object is mul-
tiplied when as that in the Figure ABCD. Now it is evident in the
Looked at thro' firft Place, that without this Glaſs, the Eye E would fee
a multiplying the Object F, by means of the Rays which come from
Glass.
Tab.VIII. F to G; and becauſe the Superficies BC is here parallel
to the Superficies AD, which is oppofite to it, and there-
fore the Refraction which the Rays fuffer when they
enter into the Glaſs, is deſtroyed by the Refraction made
at their coming out; it follows, that the Eye ought not-
withstanding, to receive the Impreffion of the Object in
the fame Place G, where it would have received it if
there had been no Glafs, and for this Reafon it ought
ftill to ſee the Object in F. It is alfo certain, that
the Object F, would make an Impreffion upon an Eye
placed in N by the Rays which it would fend thither,
if there were no Glafs between; but becauſe theſe Rays
now meet with the Superficies AB, by which they are
fo refracted, that when they come out of the Glaſs,
they enter into the Pupil of the Eye E, and afterwards.
go on in fuch a manner as to fall upon that Part of
the Bottom of the Eye marked I, where they impreſs
fuch an Image as an Object placed in M would do;
therefore this caufes the Eye at the fame Time that it fees
the Object F in its true Place, to fee it alſo in M.
So likewife the Rays which would excite Vifion in the
Eye, if it were placed in O, and no Perfpective-Glafs
intervened, being in this Cafe refracted by the Super-
ficies CD, fo as to imprefs an Image of the Object F
on the Part of the Eye marked H, where an Object
placed
Chap. 33. of NATURAL PHILOSOPHY.
259
placed in L would make its Impreffion if there were
no Glafs; it follows, that the Eye E ought to fee yet
another Object Fin L. In a Word, it is eafy to infer,
that the Eye muft fee the Object F in all thofe Pla-
ces, where the ftreight Lines termináte, which coming
from the Pupil, pafs through the feveral Sides of the
Glafs, by which the Rays of the Object are ſo re-
fracted as afterwards, to make an Impreffion of it upon
the Retina.
ed.
3, I have nothing further to add to this, but only that fome- 3. Why it
times the Object when looked at through the Sides AB, fometimes ap-
CD may appear differently coloured from what it does pears colour-
when looked at through the Side BC; the Reaſon of
which is, becauſe the Rays which come from the Object
through the Sides AB, CD, are refracted pretty much in
the fame manner, as they are by a Prifm, which has been
explained before.
Points, are
a Convex
4. Let us now examine a convex Glafs fuch as that 4. How Rays
in the Figure CDEF. Now it is to be obſerved, that that come
as it is the Property of this Glafs to collect into a Point
from different
the Rays which fall parallel upon it; fo is it the Pro- refracted in
perty of it, to collect into a Point, likewife the feveral paſſing thre³
Rays that fall upon it from any fingle Point of an Ob- Glas
ject, with this Condition, that the Point where they are
reunited is fo much the further diftant from the Glafs,
as the Point from which the Rays feparate is nearer to
it; and this latter Point may be fo near, that the Rays
which proceed from it, may never be reunited at all,
but become parallel or fomewhat diverging when they
come out.
Tab. X.
may make the
5. This being fuppofed, if the Object AB be at a pro- 5. How a con-
per Diſtance from the Glafs, all the Rays which come vex Glass,
from every Point of this Object, may be reunited again Image of the
in as many other Points. For instance, the Rays which Object conf-
come from the Point A may be collected together in H, ſed
and thoſe which come from the Point B, may be col-
lected together in G. Now if the Eye were placed in
the Point I, it is certain, that becauſe the Rays which
convey the Image to it from every Point are converging,
that is, enter into the Eye with a Tendency to unite toge-
ther; therefore I fay it muft neceffarily be, fince the Re-
fractions of the three Humours of the Eye are made in
the uſual manner, that by means hereof thefe Rays muſt
unite together fomewhat nearer to the Chryftalline Hu-
mour than they would otherwife have done. Wherefore
$ 2
if
260
{ Part I.
ROHAULT'S SYSTEM
6. How it
makes old
Men See more
diftinct.
ject appear at
4 greater
if this Eye be the Eye of a young Man, which cannot
flatten it felf beyond what is requifite to fee Objects di-
ftinctly, whofe Rays fall upon it as it were parallel, it is e-
vident, that ſuch a Perſon will fee Objects ſo much the
more confufedly as the Rays which fall on the Eye have
a greater Tendency to unite together more on this Side
the Retina.
6. But if it be the Eye of an old Man, which by the
common Decay of Age is become flatter than the Eyes
of other Men; becauſe the Reaſon of fuch a Perfon's
ſeeing Objects confufedly is, that the Rays which come
from any Point in an Object are not reunited when they
come at the Retina, which they fall upon fooner than
they ſhould do, therefore a Convex-Glaſs makes them
fee diftinctly; for it makes the Rays more converging, and
fo helps the Humours of the Eye to reunite them juſt
when they come at the Retina.
7. Why it 7. The Distance of an Object looked at through fuch
makes an Ob- a Glafs, ought to appear greater, becauſe the Difpofition
of the Rays which come from any Point is fuch, as
Distance. cauſes the Eye to put it felf into fuch a Figure, as
occafions the Mind to imagine the Diſtance greater.
And this is the Reaſon why we think the Object
to be further off, if we be not prejudiced before-hand
in our Opinion of the Place where it really is.
1
1. Why we think the Object to be
further off) Here the famous Dr. Bar-
row propoſes a very great Difficulty
in his Optical Lectures, viz. the 18.
towards the End. However, fays he,
I will not leave off, till I have propo-
Sed to you a very great Difficulty (out
of the Sincerity I owe to you, and to
Truth, by no means to be diffembled)
which is contradictory to that Opinion
which I have been recommending to
you, at least cannot be folved by it.
It is briefly this. Let the Point A
be expoſed to the Lens
Tab. X. CDEF, at fuch a Di-
ftance, that the Rays may
be fo bent as to tend towards uniting
Somewhere in the Artis HD, and let
the Point H be the Place where they
meet, or the Image of the Point A as
we have all along before affericd, viz.
the Focus; between this Point and the
Glass V, let us fuppofe the Eye to be
I ask, in what
any where placed.
Place ought the Point A to appear
|
8. As
to be. In the Nature of Things it
cannot be ſeen behind at the Point
H (because every Impreffion that af-
fects the Senfe, comes from the oppo-
fite Part, vìz. A) and it is contrary to
Experience alfo. Now it ſeems to fol-
loco, from the Doctrine we have laid
down, that it ſhould appear to be
before us, and at the greatest Di-
Stance poffible (a Distance exceeding
any that we can imagine). For the
lef's diverging the Rays that come
from any Object are, so much the fur-
ther diftant do we conceive it to be
(if we be not prejudiced concerning
its Distance before-hand ;) and that
Object which fends forth parallel Rays
we imagine to be the most diftant
that can be. In Reaſon therefore,
one would think, that when the Rays
come from the Object converging, it
should appear, if it were poſſible, at
a greater Distance yet. But in this
Cafe it may be asked in general,
what is it that determines the ap-
parent
Chap. 33 of NATURAL PHILOSOPHY.
261
8. As to the Situation, that will appear the fame as ufu- 8. Why it
makes the Ob-
al, and the fame as if we look at the Object without
the Glaſs, becauſe the Eye fees the right Side of the its true Sita-
To
parent Place of the Point A, and
makes it appear sometimes nearer,
and fometimes further off, and al-
ways in the fame Proportion.
which Scruple we can give no An-
fwer from the Analogy of any Thing
that has been hitherto faid, only that
the Point A ought always to appear
to be at the greatest Distance. But
Experience shows the contrary, viz.
that it appears at different Diſtan-
ces, according to the different Pofition
of the Eye between the Points F and
H, and ſcarce ever (if at all) at a
further Distance than the Point A
really is; but many times it appears
much nearer; nay, the more the
Rays which come to the Eye con-
verge, the nearer the Image of the
Object approaches. Thus, if the Eye
be placed in the Point V, the Point
A will feem to be very nearly in its
true Place; if the Eye be moved
backward to T, the Image will feem
to approach nearer; and it will ap-
pear fill nearer, if the Eye be in I or L,
and fo by degrees till the Eye be placed
fomwhere near H, where the Object
will appear very near, and begin to
vanish confusedly. All which fecm
to contradict our Arguments and O-
pinions, or at leaſt, do not very well
agree with them. And this Expe-
riment not only contradicts 0747 No-
tion, but all other that I know of,
equally. It feems so much to over-
throw that antient and common one,
which is more akin to ours than
any other, that the learned Tacquet
was forced thereby to renounce that
Principle (upon which alone, almost
all his Catoptricks depend) as uncer-
tain, and not to be depended upon,
whereby he overthrew his own Do-
Arine----In the preſent Cafe there
is ſomething that lies deep hid in the
Subtlety of Nature, which perhaps can-
not be difcovered, till we understand
the Nature of Vifion more perfectly.
Concerning which, I confefs, I have
not yet been able to think of any
Thing to flatter my ſelf with, much
lefs to give my ſelf entire Satisfa-
dion. I therefore leave this Dif-
ficulty with you, and wish you better
Object ation.
Succeſs in folving it. Thus far the
famous Dr. Barrow,
And indeed it muſt be acknow-
ledged, that there is a very great
Difficulty here. For it is evident,
that a Candle, the Rays coming from
which, are collected together, and
made to converge by a convex Glaſs,
however near, we, by a furprizing
Miſtake in our Judgement, conceive
it to be, does notwithſtanding af-
fect the Eye when it is placed in I
or L, exactly in the fame manner,
as it would do, if thofe very Rays
came indeed from an infinite Di-
ftance, as will appear by the follow-
ing Obfervations.
Firf, If the Lens be fo broad,
that we can ſee the Candle through
it with both Eyes at the fame time,
though we endeavour all we can to
make our Optical Axes diverge to a
diftant View, yet the Candle will
never appear fingle, but always
double; in fuch a manner double,
that of the two Images of the Can-
dle, the right Hand one will ap-
pear on the right Hand, and the
left Hand one, on the left Hand.
Whence it is moſt manifeft, that the
Place from whence we ought to judge
the Rays come, is beyond that where
the optical Axes meet, be it at never
fo great a Diſtance ;
that is, the
Candle will affect the Eye in the
fame manner as if it were at an in-
finite Distance. See the Notes on
Chap, xxxii. Art. 31
•
Neither can it be faid here, that
the Candle is not therefore ſeen dou-
ble, becauſe it is feen, as it were, at
an infinite Diftance; but that it is
only an accidental Thing, and effect-
ed by the Interpofition of the Glafs.
For if we look through a concave
Glaſs, it does not appear double ;
and it may be feen fingle through a
Convex-Glafs, if either the Eye, or
the Candle, be fo near the Glafs, that
the Rays fall upon the Eye, not
converging, but only lefs diverging;
in which Cafe, fuch Glaffes are of
great Ufe to render the Sight more
diftin&t.
.
$ 3
Secondly,
262
Part I.
ROHAULT's SYSTEM
}
Object B, by means of the Ray VI, which is on the right
Hand of the Ray SI, by means of which, it fees the left
Side A.
Secondly, The Reaſon of the Ap-
pearance of a Candle in this man-
ner when looked at through a con-
vex Glaſs, is exactly the fame, as
that of a Candle feen erect when
the Kays are reflected by a concave
Looking-Glafs. In both Cafes the
Rays are converging; in both Ca-
fes the Object feems equally near.
Now in a concave Glaſs, if when
the Image is feen ere behind
the Glafs, a Stick or a long Reed
be fo put between the Candle and
the Superficies of the Glafs as to
ftand perpendicular to the Glafs,
the Image of that Stick ought to
appear of an infinite Length be-
hind the Glafs (as Tacquett has de-
monftrated in his Catoptricks, Book ill.
Prop. 22. and as the thing it
felf ſhows us); and yet the Image
of the Candle muft neceflarily ap-
pear beyond the Image of this
Stick; however near therefore we,
through Prejudice, judge the Image
of the Candle to be when alone, it
is yet evident, that it does really
affect the Eye, as if there were an inti-
nite diftance between. And the fame
must be faid of a convex Glafs.
Now here is the_great_Difficulty
(as the learned Perfon before-men-
tioned obſerved) how it comes to
pafs, that when the Rays fall
up-
on the Eye as if they came really
from an infinite Diftance, yet the
Candle does not feem (as one would
expect) to be as remote as poffi-
ble, but always very near, though
fometimes nearer than other, and
that in a certain and conftant Pro-
portion.
Now having confidered this Diffi-
culty on all Sides, I at laft found out
the following Solution of this ſurpri-
zing Phænomenon.
First, Becauſe we cannot judge of
the Diſtance of the Candle by the
meeting of the optical Axes (for
in this Cafe, thofe Axes can never
meet at all at the Candle, as was
before demonftrated ;) and becaufe
the Judgement which we make of
the Diſtance of Objects by one Eye
only, is always the worſt and moſt
uncertain, and becauſe the true Di-
ftance of the Candle is known be-
fore; therefore from Prejudice and
Prepoffeflion, it muſt always feem
to be pretty near to us. To which
we may add, that we cannot by our
Sight perceive any Diſtance, how
great foever it be, if there be no-
thing in the intermediate Space:
Thus the Body of the Sun, though
we very well know, that it is at
an immenſe Diſtance from us, yet
it feems very near; and were it
not that we imagine to our felves,
from the Concavity of the Heavens,
a certain Radius of a Sphere, we
ſhould think it fill much nearer.
Thus if we look at the Sun
through a very long Tube, which
hinders our feeing any other Bo-
dies, it feems to be at the End of
the Tube.
Secondly, It ought alfo to appear
fometimes nearer than other, and
that in a certain and conftant Propar-
tion. For when the Eye is placed
near the Glaſs, as in V, the Candle
feems further off (as by the Laws
of Opticks it ought to do) than it
does without the Glafs; now if the
Eye be removed backward gradual-
ly, the common Refraction of the
Rays will be fuch, that the Candle
muſt neceffarily feem larger and
brighter, in the fame Proportion as
the Eye recedes from the Glafs. Now
this Largenefs and Brightneſs is the
Reaſon why it feems nearer and
almoft clofe to the Eye.
And this is confirmed from hence,
that if the Rays of the Candle are
firft tranfmitted through a concave
Glafs (that the Bignefs and Bright-
nefs of it may be diminiſhed) and
then by pathing through a convex
Glafs they be made to converge (as
when we look through an inverted
Teleſcope of two Glaffes) then we
eaſily imagine the Candle to be at
a very great, and almoft infinite
Distance.
9. But
Chap. 33. of NATURAL PHILOSOPHY,
263
9. But this Object will appear fomewhat bigger, becaule 9. Why it
the Rays VI, SI, as they enter into the Eye, are incli- makes the Ob-
ject appear
ned to each other with a larger Angle, than they are be- bigger.
fore they were refracted by the Glafs, fo that they
ſeeming to come from the Places 2 and 3, impreſs
an Image of the Object upon the Eye as big as if they
poffeffed all the Space between 2 and 3.
10. How it
may make the
Object appear
and more com-
"fused.
Tab. X.
10. If the Eye be placed in L, the Rays which come
to it from any Point are ſtill more converging; and
therefore if the Sight were confuſed before, it will be fill bigger
much more ſo now. And becauſe the Rays XL, and TL,
which come from the two Points A and B of the Object,
make a ſtill greater Angle than SI, VI, they muſt make
the Object appear yet bigger. Whence it ſhould feem
to follow, that the Vifion fhould not be fo clear, but
more obſcure; becauſe the Rays which imprefs the Image
of the Object on the Eye taking up a larger Space up-
on the Retina, each Capillament of the Optick Nerve
receive fewer of them in Proportion: However it is
certain, that we can then fee as clearly as if the Image of
the Object were finaller. For there are a greater Num-
ber of Rays, which come from every Point, and which
are difpofed by the Glaſs to reunite, that enter into the
Pupil when it is fo placed as to fee the Object very
large, than when it is placed where the Object appears
fmaller.
I
11. How it
11. So likewife if the Eye be placed in Y, the Object
ought to appear very bright and clear, becauſe all the may make the
Object appear
Rays which come from any Point of the Object, and wholly confu
fall upon the whole Superficies of the Glaſs do then en- fed.
ter into the Pupil; but it muft, notwithſtanding this,
appear very confuſed, becauſe the Rays being already col-
lected together when they are about to enter into the
Eye, are refracted afterwards by the feveral Humours
of it, and fo are by that means difperfed again; fo that
thoſe which come from the fame Point of the Ob-
ject, impreſs an Image on a great many of the Capil-
laments of the optick Nerves, upon which the Rays
which come from other Neighbouring Points imprefs
their Image alfo, and this makes the Image of the Object
wholly confused.
1. Are refracted afterwards) Are
difperfed again when they come at
*
1
the Bottom of the Eye,
12. If
264
Part I
ROHAULT'S SYSTEM
{
12. How it
confused,
12. If the Eye be placed in M, the Object muſt ne-
may make the ceffarily appear inverted; for we fee the left Side A by
Image appear
inverted and means of the Ray HM which is on the right Side of
GM, by which we fee the right Side of the Object.
It muſt alſo neceffarily appear confufed; as well becauſe
the Rays which come from any Point, as A, cannot be
exactly collected together at all beyond the Glafs, fo
that the Eye cannot put it felf into any Figure which
will reunite all the Rays that come from H; as be-
cauſe when the Rays really come from H as from
one Point only, they fall fo diverging upon the Eye,
that it cannot lengthen it felf enough to reunite them
upon the Retina. The Firſt of theſe Two Reaſons
fhows us, that in this Cafe it is impoffible for the Eye
to judge what Diſtance the Object is at; and that it
feems in that Place in which we before-hand imagine it
to be.
1. That it feems in that Place) |
Here we meet with another Diffi-
culty, concerning the Place in which
the Image ought to appear, almoft
as great as the former, which Mr. De-
chales propofes in this manner,
Book II. Prop. 11. of his Dioptricks.
There is, fays he, always a very great
Difficulty in explaining the manner
how the Eye fees the Plate of the
Object, but in this Cafe
Tab. X, there is a very particular
Difficulty, because Reafon
and Experience do not feem to agree
together, nay, the Experience here is
contrary to other Experiments alfo.
For it is evident from Experience,
that the Object AB is not ſeen in
the Place of its Image, viz. in GTH,
when the Eye is placed in M, for I
have tried That a hundred Times,
and turned the Glaffes all Ways in
order to find if I could poſſibly make
it fucceed fo. However, according to
Reafon, it ought without all Doubt to
be ſeen in the Place of the Image, viz.
in GTH. For when the Object AB
affects the Eye by the Rays of its
Image, it should feem as if it ought
So to affect the Eye as if it were
in GTH. For if the Point A, for
Inftance, were in H, it would send
forth Rays from H to the Eye in M;
And though it be in its proper Place
viz. in the Point A, yet it fends
13. If
forth Rays in the fame manner as
if they came from the Point H;
therefore it seems as if it should
affect the Eye in the fame manner
as if it were in the Point H.
To this Difficulty, this famous
Perfon anſwers, That the Body AB is
indeed really feen by the Eye Min
the Place of its Image GYH; but
becauſe it can be feen only by one
Eye at a Time, therefore by a mi-
ftaken Judgement, we imagine it to
be further from us. Thus far He.
I have oftentimes fo ordered the
Glafs, that the Object AB (which
ought to be a Candle) may be ſeen
with both Eyes N and P at the
fame Time. If it be a very large
Glafs the Candle may very eafily be
feen with both Eyes at the fame
Time.
Having therefore made exact Ob-
fervation of this Matter through ſuch
a Glaſs, I affirm, that the Body AB
is ſeen by the Eyes NP exactly in the
Place of its Image GYH
For if the optical Axes Tab. X,
be fo directed, as to meet
in the Superficies of the Glafs, the
Candle will always be feen double,
and in fuch a manner double, that
the right Hand Image is feen by the
left Eye, and the left Hand Image
by the right Eye. Whence it is moft
manifeft, that the Image is placed
within
1
Chap. 33. of NATURAL PHILOSOPHY.
265
Object may
13. If the Eye be fuppofed in N, the Second of theſe 14. How the
Reasons will not take Place, and therefore the Object
ought to be ſeen a little more diftinct, but always verted and
inverted, for the Reaſon above-mentioned. And as to less confused.
the Bignefs of it, we judge of that by the Largeneſs of
the Angle made by the Rays, which come from the Ex-
tremities of the Object, at their Entrance into the Eye,
compared with the Diſtance which we imagine it to be at.
But it muſt not here be omitted, that the Space OP
and QR, through which the Rays which come from each
Extremity of the Object diffufe themſelves, is fo much
the greater as it is further diftant from Y, where the
Rays which come from every Point of the Object meet.
And this make the Space QP, where the Eye receives
the Impreffion of the two Extremities A and B at the
fame Time, to be fo much the bigger alfo; fo that there
is a large Space for the Eye to move about in, where it
will always fee the whole Object.
appear in-
diftinct.
14. Hitherto we ſuppoſed the Object to be ſo far re- 14. How it
moved from the convex Glafs, that the Rays coming.
may be made
to appear very
from it might eaſily be reunited in the Bottom of the
Eye; let us now fuppofe it ſo near the Glafs, that the
Rays which come from any one Point of it, have no
Tendency towards uniting together, after they are paffed
through it, but are only made much leſs diverging than
they were before: Let us fuppofe alfo, the Eye to be at
fuch a Diſtance from the Glafís, that the Refractions which
are made at the Entrance into each of the Humours be
fuch, as will cauſe the Rays which come from any fin-
gle Point of the Object, to unite again in one Point up-
on the Retina; in this Cafe it is evident, that the Viſion
muſt be exceedingly diftinct. For, befides that the Rays
which come from different Points of the Object, do not
at all confound each other, the whole Image impreffed
by them is fo large, that there is a fufficient Number
of Capillaments of the Optick Nerve, to cauſe the Soul
to perceive a great many Particulars, which it would
within the Place of Concourfe of
the optical Axes, that is, between the
Glaſs and the Eye, viz. in GYH. See
the Notes on Chap. xxxii. Art. 31.
But further, if the optical Axes be
fo directed as to meet on this Side
the Glafs, the Candle will be feen
fingle, and manifeftly on this Side
the Glafs.
But in the former Cafe, where the
optical Axes were directed to a
Point further diftant, becauſe the
Image of a Candle does not termi-
nate the Sight like a folid Body,
and becauſe we were beforehand pre-
judiced concerning the true Place of
it, therefore it feems to be at a greater
Diſtance.
1
{
other-
266
Part I.
ROHAULT's SYSTEM
If. Concern-
ing Micro-
Scopes.
16. How a
concave
Glafs refrails
the Rays
which come
Object.
Tab. XI.
otherwife have taken no notice of, if the Image had been
fo fmall, that the Rays which came from two adjoin-
ing Points of the Object, had been forced to meet to-
gether in two different Points of one and the fame Ca-
pillament.
15. Upon this Foundation it is, that thofe fmall
Glaffes which we call Microfcopes, are made. They con-
fift of one Glafs only, which is fo convex, that if a
Flea, or any other fmall Object be placed at about
an Inch Diſtance from the Eye, and the Glafs be put
between them, it will cauſe the Rays which come from
any fingle Point of fuch a fmall Object, and which di-
verge very much, to diverge afterwards fo little, that
the ordinary Refractions of the Humours of the Eye,
will determine them to unite in one Point on the Retina.
By this Means the Eye which without a Glafs cannot
fee any Object diftinctly which is nearer than a Foot
Diſtance from it, may be made to fee one which is
twelve Times nearer it. From whence it follows,
that the Diameter of the Image which this Object im-
preffes upon the Retina is twelve times larger, and con-
fequently, that the whole Superficies is a Hundred and
Forty Four times as large, as it would be, if the Ob-
ject were at a Foot Diſtance; wherefore fince it ex-
tends it ſelf upon a Hundred and Forty Four times as
many Capillaments of the Optick Nerve as it would
otherwiſe do, the Object cannot but be feen very di-
ftinctly.
16. Let us now examine a concave Glafs, fuch as
that in the Figure CDEFGH, the Property of which
is, according to what was before ſaid, to make the
Rays which it receives from any fingle Point of an
from different Object, to become more diverging than they were be-
Points of an
fore they paffed through the Glafs. Thus the Rays
which come from the Point A, and fall upon that
Part of the Glafs marked VX, fpread themſelves after
they are paffed through it, from R to Z; and thoſe
which come from the Point B, and fall upon the fame
Space VX, extend themfelves through the Space YT.
Further, it is alfo the Property of a concave Glafs, fo
to incline the Rays, which come from two different
1. Thoy confift of one Glaſs only) |
There are fome which confiit of fe-
veral Glaffes, that are much more
nice. What and how furprizing
1
Things have been found out by
the Help of thefe Microſcopes, may be
feen in Mr. Hook's Micrography, and
in others.
Points
Chap. 33. of NATURAL PHILOSOPHY.
267
Points of the Object, to each other; that when they
meet together, they make a lefs Angle than they would
do, if they had not paffed through fuch a Glafs. For
inſtance, the Ray MI which comes from the Extremity
of the Object A, and the Ray LI which comes from
the other Extremity B, make fo fmall an Angle, viz.
MIL, that they ſeem to come from the Places mark-
ed N, O.
17. Whence it follows, that if the Eye be placed in I 17. How it
and look upon the Object AB, it will fee it confufedly may make the
Becauſe the Rays which come from every Point, are fo Viſion confu-
diverging, that the Refractions of the Humours of the
Eye cannot make them unite in fo many Points upon
the Retina.
Sed.
See very di-
18. However, there may be fome Eyes fo much long- 18. That it
er and more gibbous than ordinary, as to reunite the may make
Rays which they receive from any fingle Point of a di- fome Perfons
ſtant Object, before they come to the Retina, ſo that finetly.
they can ſee only near Objects diftinctly; they there-
fore who have fuch Sort of Eyes as thefe, may make
good uſe of a concave Glafs to fee diftant Objects
diſtinctly with; becauſe by this Means the Rays which
come from any fingle Point of the Object are made fo
diverging, that the large Refractions made by the Humours
of fuch Eyes, do not reunite them before they come at
the Retina.
19. How it
times make
19. If an Eye of the ordinary Figure be placed at a
greater Diſtance from the Glafs, as at P, it will fee fome- may ſome-
what more diſtinctly, becaule the Rays which fall upon the Sight lefs
the Pupil from any fingle Point of the Object are lefs and fometimes
diverging than they were in I; and on the other Hand, more confused.
an Eye too long or too gibbous will fee it ſo much the
more confufedly as the Point P is further from the Glaſs,
becauſe the Rays which come from any fingle Point of
the Object, being lefs diverging, the Refractions made in
the Eye, determine them to meet before they come to the
Retina.
20. But whatſoever the Figure of our Eyes be, whe-
ther they are fitted to fee Objects that are near, or fuch
as are at a Diſtance; whoever makes uſe of fuch a
Glafs will fee the Object in its true Situation; for the
Rays which caufe us to fee the right Side of the Ob-
ject, come to us from the right Side; and thofe which
cauſe us to ſee the left Side, come from the left Side.
21. As
20. That it
hems the Ob-
situation.
ject in its true
1
!
7
268
Part I.
ROHAULT's SYSTEM
f
21. That it
pear nearer to
21. As to the Distance, it makes that feem less than
makes it ap- it really is, becauſe when the Rays which come from
any one Point, enter into the Humours of the Eye,
they diverge juft as much as they would do, if they
did indeed come from a Point of an Object much
745.
22. That it
pear lefs.
nearer.
22. And as to the Bignefs; becauſe the Extremities
makes it ap- of the Object are ſeen by Rays which make a leſs Angle
than they would make without a Glafs, it follows, that it
must appear much lefs.
23. That it
equally clear.
23. Becauſe the Rays which come from any Point
makes it look of the Object are made more diverging by paffing
through a concave Glafs, it follows, that fewer of
them can enter into the Pupil, than if they had not
paffed through the Glafs; however the Vifion ought
not to be the lefs clear upon this Account; becauſe
this is made good by the Image being impreffed on a
lefs Space of the Retina, fo that every Capillament
of the Optick Nerve is fufficiently fhaked to caufe
us, when we look through fuch a Glafs, to ſee the
Object as clear as when we look on it without a
Glafs.
24. That it
Object to be
24.
To what has been hitherto faid concerning the
makes a large concave Glafs, we may add, that the Space RT, which
Space for the contains the Rays that come from the two Extremi-
ties of the Object, being very large, it follows, that
the Eye may fee the Object entire in any Part of this
large Space.
feen in.
25. Concern-
25. One of the beft Inventions of our Age, is that of
ing Telescopes. Telescopes. For by the Help of them we have not on-
ly diſcovered fome Particulars in the Stars, which were
not obſerved before, but they ſhow us alfo a Multitude
of new Stars in the Heavens, which we cannot fee
without them, nor fhould we ever have come to the
Knowledge of them otherwife. They were indeed firft
diſcovered by Chance; but the Invention appeared fo
furprizing, and ſo uſeful, that the greateſt Genius's have
laboured hard to bring them to the higheſt Perfection
poffible. I cannot therefore forbear explaining the Na-
ture of them in this Place; and the fo doing will very
much confirm all that has been hitherto faid about Vi-
fion. They confift commonly of two Glaffes, fixed to
each End of a Tube: That Glafs which is at the End
next the Object, and is for that Reafon called the Ob-
ject-Glafs, is a little convex, and the other Glafs which
is
}
Chap. 33. of NATURAL 269
F
PHILOSOPHY.
I
is at the End of the Tube next the Eye, and is therefore
called the Eye-Glafs, is on the other Hand, very con-
cave, that is, much thinner in the Middle, than at the
extreme Parts.
26. The Object-Glafs caufes all the Rays which come 26. The Pro-
from every fingle Point of the Object, to unite together ve- perty of the
Object-Glafs
ry nearly in as many different Points, on a Superficies
which we are to fuppofe on this Side the Glaſs, at a
greater or leſs Diſtance from it, according as the Glaſs
is more or less convex; now becauſe the Rays which
come from different Points of the Object, cross one ano-
ther as they paſs through the Glaſs, it is eaſy to con-
ceive, that they paint fuch a Sort of an Image upon this
Superficies as we have before fhown they do upon the
Retina, and that it is fo much the larger, as the reuniting
of the Rays, caufes it to be at a greater Diſtance from
the Glaſs If therefore the Bottom of the Eye were put
in the Place of this Superficies, and it were poffible for
the Humours of it not to make any Refractions; we
ſhould have a very large Image impreffed on the Retina,
by Means of this fingle Glafs, and it would fall upon fo
great a Number of the fmall Capillaments of the Op-
tick Nerve, which would receive "diftinctly the Impreffi-
on of every ſmall Part of the Object, that it would
be impoffible but that the Vifion must be very di-
ftinct.
:
27. But becauſe the Humours of the Eye cannot be 27. The Pra-
hindred from caufing the ufual Refractions, they must perty of the
neceffarily fo refract the Rays which come from every
Eye-Glass.
Point of the Object, and which had before a Tendency
to unite together, that they will unite before they come
at the Retina, and then feparating again, will imprefs a
confuſed Image upon that Tunick. Now the Eye-Glass
is fo fitly placed between the Object-Glafs and the Place
where it would make the Rays meet; that it cauſes thoſe
which come from any Point of the Object converging,
to become parallel, or rather a little diverging; but yet
it does not hinder the Rays which come from different
Points, from being as much difperfed as they were when
they croffed each other in paffing through the Object-
Glafs. And thus the Refractions neceffarily made by the
1. Concave) There are alfo Te-
leſcopes confifting of two, three,
or four convex Glatles; Concern-
ing which, fee Regis Phyficks, Book
VIII. Part. II. Chap. xxxix, xl, xli.
Hu-
270
Part I.
ROHAULT''s SYSTEM
}
Humours of the Eye, inftead of being injurious, as they
were without this Glaſs, become very uſeful with it; for
they unite thoſe Rays which this Eye-Glafs difperfed;
and by this Means the Image which the Object impref-
fes on the Retina becomes perfectly diftinct, and at the
fame Time very large. Whence it follows, that the
Object is feen diftinctly and fo much the bigger as the
Rays which come from any one of theſe Points, are
lefs diverging, and make us think it at a greater
Diſtance.
28.Why these 28. The beſt Curvature that can be of the Superfi-
Glaffes, the
cies of Glaffes for Teleſcopes, is, 2 that of an Hyperbo-
longer they
are, make the la, or any fuch like Figure, and not the Curvature of a
Sight So much Sphere. But Workmen have not yet been able to make
the more
Scure.
обо
their
1. So much the bigger as the Rays
which come from any one of these
Points are lefs diverging, and make
us think it at a greater Distance.)
For
That is, by how much the Rays
of every Pencil being lefs difperfed,
make it appear further off.
the further the Object ſeems to be
from us, the more do we neceffari-
ly imagine the Pencils of Rays, which
crofs one another as they pafs thro'
the Object-Glaſs, to divaricate, that
is, the Object feems ſo much the
bigger.
2. That of an Hyperbola, or any
Such like Figure, &c.) Cartes took a
great deal of Pains about theſe fort
of Figures, and about the manner
of poliſhing Glaffes, but with no
great Succefs. For it is evident,
that Spherical Glaffes, as they can
be more eafily and more accurately
made, than Elliptical or Hyperboli-
cal ones; fo are they to be prefer-
red before fuch upon this Account,
becauſe they do more exactly re-
tract the Pencils of Rays which are
out of the Axis of the Glafs. And
indeed, it is not to be afcribed
to the Unfitneſs of the Figures
of the Glaffes, but to quite other
Caufes, that Teleſcopes cannot be
made abfolutely perfect and com-
pleat. The Two Principal of which
Caufes are thefe.
First, The unequal Refraction of
the Rays themſelves; (See the Notes
on Chap. xxvii. Art. 52.) by which
means neither the Eye-Glafs (which is
Convex) can be made of Spheres
fmall enough to magnify the Ob-
ject; nor the Object-Glaſs of a fuf-
ficient Aperture, to render the Ob-
ject bright and diftinct, but every
Thing will immediately be tinged
with Colours, and confounded by
the unequal Refraction of the Rays.
For the eminent Sir Ifaac Newton
has ſhown, that the Difference be-
tween the Refraction of the leaſt
and moſt refrangible Rays, is about
the Twenty ſeventh Part of the
whole Refraction of the mean re-
frangible Rays; and that the Focus
of the most refrangible Rays is
nearer to the Object-Glass than the
Focus of the leaft refrangible ones
by about a Twenty feventh Part
and a Half of the whole Diſtance
between the Object-Glaſs and the
Focus of the mean refrangible Rays.
(Opt. p. 74.) And therefore the
greateft Errours which arife from
the fphærical Figure of the Glass, are
very much lefs than the Errours
which arife from the unequal Re-
fraction of the Rays themſelves;
nay, in fome Cafes, the Proportion
is as great between them, as 1200
to 1 (pag. 89.) From whence it
abun-
Chap. 33. of NATURAL PHILOSOPHY.
271
their Glaffes of any other Curvature but that of a
Sphere, of which they take fo fmall a Part, that it does
not fenfibly differ from an Hyperbola. But then there
is this Inconvenience attends it, that there does not fall fo
many Rays upon it from any one Point of the Object, as
there would do if the Glafs were larger; and conſe-
quently all the Rays which come from the whole Object,
and which ſpread themſelves upon a large Portion of the
Retina, ſhake but a very few of the Capillaments of the
Optick Nerve; and this is the Reaſon why we fee Things
more obfcurely, than when we do not uſe ſuch a Glafs;
and the longer fuch Glafs is, and the fewer the Rays are
which come upon the Pupil from any Point of the Ob-
ject, fo much the weaker and more obfcure muſt that
Object appear.
abundantly appearing, that not the
Spherical Figure of the Glaffes, but
the different Refrangibility of the
Rays themſelves, is the Caufe why
Teleſcopes have not hitherto been
made abfolutely perfect and com-
pleat, and that there can be no Re-
medy for this Incovenience by any
way figuring or polishing refracting
Glaffess this excellent Perfon, at
length invented, and agreeable to
Experiments, propofed the manner
of making a Teleſcope which ſhould
cauſe the Object to be feen by Re-
flexion: Concerning the Conftructi-
on and Ufe of which Inftrument,
See Optic. pag. 95.
|
copes which have larger Apertures.
For the Rays of Light which paſs
through diverfe Parts of the Aper-
ture tremble each of them apart,
and by means of their various, and
Sometimes contrary Tremors, fall at
one and the fame Time upor
different Points in the Bottom of the
Eye, and their trembling Motions
are too quick and confused to be per-
ceived feverally. And all these illu-
minated Points conftitute one broad
lucid Point, compofed of thofe many
trembling Points confufedly and in-
fenfibly mixed with one another by
very short and Swift Tremors, and
thereby cause the Star to apear broad-
Secondly, If the Theory of making er than it is, and without any Trem-
Telescopes could at length be fully bling of the Whole. Long Telescopes
brought into Practice, yet there would may cauſe Objects to appear brighter
be certain Bounds, beyond which Te- and larger than fhort ones can do,
Lefcopes could not perform. For the but they cannot be fo formed as Zo
Air through which we look upon the
take away that Confusion of the Rays
Stars, is in a perpetual Tremor ; which arifes from the Tremors of
as may be seen by the tremulous Mo-
the Atmosphere. The only Remedy is
tion of Shadows caft from high a moft ferene and quiet Air, fuch as
Towers, aud by the twinkling of the may perhaps be found on the Tops of
fixed Stars. But theſe Stars do not the higheſt Mountains above the grof-
twinkle when viewed through Telef-fer Clouds. Newt. Opticks p. 98.
4
{
$
CHAP.
72
Part I
ROHAULT'S SYSTEM
1
3
1. Ofthe dif-
ferent Sorts
of Looking-
Glaffes.
2. The com-
Looking-
Glaffes.
BEST
CHA P. XXXIV.
Of Looking-Glaffes.
ESIDES plain Looking-Glaffes, which are every
where uſed, there are two other Sorts, viz. Convex
and Concave ones, not to mention thoſe which are com-
pounded of theſe three Sorts, which are capable of being
infinitely diverfify'd.
T
par-
2. Each Sort of Looking-Glaffes has indeed its
mon Property ticular Property or Manner of reprefenting the Object;
of all Sorts of but in this they all agree, that they fo reflect the Rays
of Light, that the Angle of Incidence is equal to the An-
gle of Reflexion, and that the reflected Ray is not in the
leaft turned afide, either to the right Hand or to the Left;
that is to fay, the incident and reflected Rays are al-
ways in the fame Plane which is perpendicular to the
Superficies of the Glaſs; whence it follows, that though
the vifible Object fends forth from every Point a Multi-
tude of Rays which are reflected by the whole Superfi
cies of the Glafs, yet a determinate Number of them
only can come to the Eye when it is fixed in a certain
Place.
1. The incident and reflected Rays
are always in the fame Plane which
is perpendicular to the Superficies of
the Glass) This Property wonder-
fully perplexed the famous Dr. Bar-
rom; you will not easily find any
good and clear Account of this Mat-
ter amongst the Writers of Opticks ;
almost every Thing that they alledge
with relation to it, is either begging
the first Principle, or elſe labours un-
der fome incomprehenfible Obfcurity;
nor do I much wonder that this ſhould
be the Cafe of those who always con-.
fider a Ray of Light as one continued
Streight Lines which if granted, I
can scarce believe it poſſible to affign
any good Reafon for this Thing. I
therefore think that a Ray of Light
is not a mere Line, but a Body endued
with all the Dimenfions; fo that it may
be cylindrical or prifmatical, &c.
Lect. I. Sect. 11. But there do not
1
feem to be any neceffity of recurring
to the Figure of the Rays; it is all
one whether they be cy-
lindrical or prifmatical, Tab. II.
whether they be folid Bo- Fig. 6.
dies or indivifible Lines.
For let GBL be the Superficies of
the Earth (which I fuppofe to be
plain and fmooth) A the North,
the South, AB a Ray of Light. Now
it is evident, that this Ray of Light
is carried with a double Determina-
tion, the one AG downwards to
the Earth, the other AH directly
to the South; the firft Determina-
tion is refifted by the Superficies of
the Earth, the other is not; the Ray
therefore ought to go on directly to
the South with this Determination,
that is, in a Plane perpendicular to
the Superficeies of the Earth; nor can
it turn towards the Eaft in an ob-
lique Plane.
3. This
!
Chap 34 of NATURAL PHILOSOPHY.
273
the Object to.
Fig. 3.
3. This being fuppofed, let AB be a plain Looking- 3. How a
Glafs, by Means of which the Eye C fees the Object plain Look-
DE; having drawn from any Point at Pleafure, fup- makes any
ing-Glafs
pofe D, the Line DIL perpendicular to the Superficies one Point in
of the Glaſs, we fhall fhow that this Point D ought to be seen.
be ſeen in the Point L of this Perpendicular, ſo that the Tab. VII.
Diſtance IL, which we imagine it to be at behind the
Glafs, fhall be equal to the Line ID; for it is eaſy
to demonſtrate, that the Rays DF, DG, by which the
Point D affect the Senfe, are fo reflected in the Lines
FC, GH, that they enter into the Pupil CH, as if
they really came from the Point L; fo that this di-
verging of the Rays caufes the Eye to put it felf
into fuch a Shape, as gives occafion to the Soul to ima-
gine that it fees the Object really in the Point L.
4. And as the Point D was taken at pleaſure, what 4. That the
has been faid concerning that, ought equally to be un- whole Object
derſtood of all other Points of the Object; and therefore it pear as far
is evident, that when we look upon an Object in a beyond a plain
plain Looking-Glaſs, the whole Image ought to appear
as far behind the Glass, as the Object is placed before
it..
alfo to
Glass as it is
placed on this
Looking -
Side of it.
5. That a
plain Look-
ing-Glass
ought to make
5. It is further evident, that this Object ought
appear of the fame Bignefs, as if it were really placed in LM:
For the Space which the Image feems to take up, is com-
prehended between two parallel Lines which are at the the Object ap-
fame Diſtance from each other as the Extremities of the pear of its
Object are.
true Bignefs.
1
pear in its
6. Laftly, This Object ought fo to appear in the Look- 6. That it
ing-Glaſs, that the upper Part fhould be ſeen above, and ought to ap-
the right Side on the right Side, and fo of the reft. true Situati-
Thus the Part D, which is higher than E being feen by 07.
the Rays of Incidence DF, DG, and by the reflected
Rays FC, GH, which feem to come from the Point
L; and the lower Part E being feen by the Rays of
Incidence EN, EO, and by the reflected Rays NC, OH,
1. For it is easy to demonftrate, &c.) |
For the Angle DFI to the An-
gle CFB and the An-
Tab. VII. gle CFB to the An-
Fig. 3.
gle IFL, therefore the
Angle DFI to the
Angle IFL and the Angles at I
T
are right, and the Side IF common.
Therefore the Triangles DFI and
IFL are fimilar and equal. In like
manner the Triangles DGI and
IGL are fimilar and equal: There-
fore the Triangles DGF and FGL
are fimilar and equal, Q.E. D.
H
which
'Tab. VII.
Fig. 3.
¿
J
274
Part I
ROHAULT's SYSTEM
ว
7. That it is
the fame
Thing whe-
ther we look
upon the Glass
with one Eye
or with both.
8. That a
ought to make
Distance be-
}
which feem to come from the Point M; we refer the
Senfation which we have of the Point D to the Place L,
and that which we have of the Point E to the Place M,
which is lower than L.
7. What has been faid concerning one Eye, ought
equally to be underſtood of the other. And indeed if
we fuppofe the Spectator principally attentive to look
upon the Point L, it will eafily appear, that his two Op-
tical Axes, will be fo inclined to each other, that they
will feem to meet in the Point L. Whence it follows,
that the Rays which come from every Point of the Ob-
ject to enter into one of the Eyes, feem to come from
the fame Points beyond the Glafs, from whence the Rays
ſeem to come which cauſe every Point of the Object to
be ſeen by the other Eye.
8. As to a convex Looking-Glafs, fuch as that in the
convex Look- Figure repreſented by ABC, by Means of which the Eye
ing-Glass
Dfees the Object EF, it is eaſy to apprehend, that it
the Object ap- fo reflects the Rays which fall upon it from any Point of
pear at a lefs the Object, fuch as EB, EG, that the reflected Rays
BD, GH diverge juſt as much as if they really came
Glafs, than it from the Point I, which is at a much leſs Diſtance be-
is on this Side. bind the Glafs than the Object is before it: And this
is the Reaſon why we fee the Image much nearer than
when we look upon a plain Looking-Glaſs.
hind the
Tab. IX.
Fig. 1.
9. That it
qught to ap-
9. Further, the Point L from whence the Rays MD,
NH, ſeem to come, by which we fee the Point F, 2 is
pear smaller. fo near the Point I, that IL appears much leſs than EF,
that is, a convex Looking-Glafs makes the Object ap-
pear much leſs than it really is.
10. That it
pear in its
10. But though in this a convex and plain Looking-
ught to ap- Glafs differ from each other, yet they agree in another
trne Situati- Particular, viz. that they both make the Object to be
ſeen in its true Situation, as appears from hence, that
07.
1. It is ealy to apprehend, &c.) |
This may eafily be demonftrated, if
we draw
draw a ſtraight
Line BG reprefenting
a plain Looking-Glafs,
and compare it (as to
the Situation) with the Tangents of
Tab. IX.
Fig. 1.
the Points B and G.
2. Is fo near the Point 1) There
I,)
are two Reaſons of this. First, Be-
cauſe the Image in this Glafs, by
reafon the Rays of every Pencil
are more difperfed, is not fo far
diftant from the Vertex of the
Angle of Vision as in a plain Look-
ing-Glafs. Secondly, Becauſe chis
Angle of Vifion is therefore lefs,
becaufe the Portion of the Glafs upon
which the Rays that are reflected
to the Eye, fall, is lefs than in a
plain Looking-Glaſs.
the
Chap. 34. of NATURAL PHILOSOPHY.
275
the Rays EBD, EGH, by which the Eye fees the
Point E are higher than the Rays FMD, FNH, by
which it fees the Point F, which is the lower Part
of it.
11. Why a
greater di-
it,
ftance behind
than it is
Tab. IX.
Fig. 2.
at before it.
II. As to Vifion made, in looking upon a concave
Looking-Glaſs, it may be diverfify'd feveral Ways ac- concave Look-
ing-Glass
cording as the Eye and the Object are in different Po- makes the
fitions. Let us fuppofe a concave fpherical Looking- Object ap-
Glaſs, whofe Center is about the Point T; and let us pear at a
imagine in the firft Place, that by Means thereof the Eye
D fees the Object EF which is pretty near the Superfi-
cies of it. This being fuppofed, the Rays EB, EG which
come from the Point E, are fo reflected to the Pupil,
that BD, GK diverge but very little, and feem to come
from the Point H, which is at a much greater diſtance
beyond the Glafs, than the Object is on this Side of it. I
And this makes us refer the Image of it to a greater
diſtance than if we look on a plain Looking-Glafs, and
to a ſtill greater than when we look on a convex Look-
ing-Glaſs.
12. How it
on and much
12. As to the Rays which come from different Points
of the Object, they are in this Cafe fo reflected, that may make the
Image to ap-
thoſe which affect the Senfe from the upper Part of the
pear in the
Object, are higher than thoſe which affect the Senfe Jame Situati
from the lower Part of it; thus the Rays BD, GK, larger than
which cauſe the Senfation of the Point E, are higher than the Object.
the Rays ID, LK, which cauſe the Senſation of the Tab. IX.
Point F; and thefe Rays ID, LK, feeming when they
enter into the Pupil as if they came from the Point M,
are the Cauſe of feeing the Point F as if it were in M.
And becauſe HM is much bigger than EF, it fol-
lows that the Object ought not only to appear in its true
Situation, but alſo much bigger than it really is.
Fig. 2.
Image appear
inverted.
Tab. IX.
13. The Rays EN, FO, as they go towards the Glafs 13. How it
divide more and more from each other; wherefore if may make the
they be continued backwards, they muſt meet together
fomewhere in the Point P; and afterwards dividing a-
gain that which was uppermoft, will be lowermoft, and
that which was lowermoft will be uppermoft; whence
we cannot but conclude, that if an Object be in QR,
1. And this makes us refer the
Image, &c.) See the Notes on Chap.
*xxiii. Art. 7. for the Cafe of
T
2
!
the concave Looking-Glaſs here is
the fame as that of the convex Glaf
there.
it
Fig. 2.
276
Part I
ROHAULT''s SYSTEM
ง
14. How it
be that
may
it muſt appear inverted; but becauſe the Rays which
ought to affect the Senfe from any fingle Point of it,
fall in fuch a manner upon the Superficies of the Glaſs
that as they are reflected to the Eye, they cross one
another in feveral Places between the Glafs; and fo
cannot be reunited in one Point upon the Retina, there-
fore the Viſion must be very confuſed.
14 If the Eye be placed exactly in the Center of a
concave Looking-Glafs, it can fee nothing but the Pupil;
the Pupil only for thoſe Rays only which fall perpendicularly on the
can be seen.
15. How the
Object may
appear very
large.
Tab. IX.
Fig. 2.
16. How it
may appear
abfolutely con-
fused.
fpherical Superficies, are reflected to the Center; and
thofe Rays only which come from the Center fall per-
pendicularly upon the Superficies; wherefore the Rays
which go from the Pupil and fall upon the whole Su-
perficies of the Glafs, return from thence to the Eye
again, which must therefore ſee the Pupil ſpread all over
the Glafs.
15. If the Object EF continues in its Place, and the
Eye be moved to X, between the Rays BD, GK, pro-
longued; it is evident, that it will ftill fee the Point E
by means of fome of thofe Rays which it faw it by be-
fore; but it will not fee the Point F, by Means of the
Rays ID, LK, which came to it from the Part IL of
the Looking-Glafs; inftead of which, thofe which fall
from F upon Y, and go from thence to X will make
the Point F to be feen, and confequently it will feem
to be fomewhere in Z, and fo the Object will appear as
large as HZ.
16. If the Eye continues in D, and the Object EF be
removed backward to P, the Rays which come from
every Point of it, and fall upon any Part of the Glaſs
as BG, will be lefs diverging than they were before.
Wherefore after Reflexion they will become converging,
and more difpofed to unite, when they enter into the
Eye, than they ordinarily are, and fo muft really unite
before they come at the Retina, which will make the
Vifion confufed. But it will be ftill more confufed if the
Eye be in that Place where the Rays which come from
every Point of the Object meet together again; for theſe
Rays at their Entrance into the Eye will begin to be fe-
1. Separated by Refraction) They
are feparated, not by Refraction, but
merely by receding from the Point
where they croſs each other.
parated
Chap. 34. of NATURAL PHILOSOPHY.
2779.
parated by Refraction, and will be feparated more and
more by the Humours of it.
17. If the Object remain in P, and the Eye be remo- 17. Another
ved a little from the Place where the Rays which Reason of its
appearing
come from every Point of the Object reunite, the confused.
Rays when they enter into the Pupil, will diverge too
much; wherefore becauſe the Eye cannot lengthen
it felf enough, the Object will appear confuſed here
alfo
18. But if the Eye be moved fo far backward from 18. How the
that Place where the Rays reunite, that the Rays which object may be
enter into it, be not too much diverging, the Vifion the Eye and
Seen between
ought then to be diftinct; and what is here very re- the concave
markable, and the moſt furprizing Effect of a concave Looking-
Glafs.
Looking-Glaſs, is this; that becauſe we are accuſtomed
to refer our Senfation to the Place from whence the
Rays which affect the Eye from every Point of the
Object ſeem to come, therefore the Image must appear
between the Glass and the Eye; fo that if a drawn Sword
be preſented before the Glafs, we fhall fee the Blade
come out from the Glaſs, and grow longer and longer
as we approach nearer to it; becauſe the Rays which
come from every Point of the Object, the nearer it
is, are the lefs inclined to each other after Reflexion,
and therefore meet together at fo much the greater Di-
ftance.
1. The Phænomena of a concave
Looking-Glafs, may be very pro-
perly reduced to five Cafès.
Tab. IX.
Fig. 2
First, Let the Arrow or the Can-
dle EF be near the Glaſs. Now
becauſe the Pencils
EBGKD, FILKD do
not crofs each other,
wherefoever the Eye be,
placed, whether it be near or at a
diftance; therefore the Image HM
ought always to appear erect. And
becauſe the Rays of thofe Pencils
are reflected, not converging to each
other, but only lefs diverging, there-
fore the Candle ought to appear to
be at a certain Distance beyond the
Glafs.
Secondly, Let the Candle be in
the very Center T. Then becauſe
all the Rays fall perpendicularly
19. It
Tab. IX.
Fig. 2.
upon the Glafs, they
muft neceffarily be all
reflected to the Center
it felf; therefore where-
ever the Eye is placed, out of the
Center or any of the Lines tending
to the Center, it is evident, that it
cannot fee the Candle at all in the
Glafs.
Thirdly, Let the Eye be in the
Center T. Then becauſe no Rays
but thoſe which fall perpendicular-
ly are reflected to the Center
therefore the Eye can fee nothing but
its own Image fpread all over the
Glafs.
Fourthly, Let the Candle QR be
further diftant from the Glafs, and
the Eye KD further diftant alſo.
Then becauſe the Pencils 00, RN, -
croſs each other, it is evident, that
T3
the
278
Part I.
ROHAULT's SYSTEM
19. That Ob-
jects do not
paint their I-
Looking-
Glaffes.
*
19. It may be obferved here, that they have been
very much miſtaken, who have affirmed, that visible
mages on the Objects paint their Images upon the Superficies of Looking-
Superficies of Glaffes; for every Thing there is fo confufed, that there is
no one Part of the Glaſs but receives Rays from all Parts
of the Object at the fame Time; and indeed it is cer-
tain that all Objects which we fee by the Help of a
Looking-Glaſs, do not impreſs their Image any where
elfe but on the Bottom of the Eye, unleſs when we fee
them by Means of a concave Looking-Glafs, under the
Circumftances mentioned in the foregoing Articles; and
in that Cafe it is certain, that the Image impreffed by
the Image of the Candle ought to
appear inverted to the
Eye KD. And becauſe
the Rays of every Pen-
Tab. IX.
Fig. 2.
the Glaſs the Image of
the Candle ought to ap- Tab.XVII.
pear; for fince the Rays Fig. 5.
of every Pencil con-
verge towards each other, that is,
do not come from any given Point,
but as it were from an infinite Di-
ſtance, to enter into the Eye; and
fince thoſe reflected Rays BM, SM
do not meet with their reſpective
Perpendiculars of Incidence DT, FL,
(from which meeting the Place of
the Image is always determined)
there remains nothing to judge of
the Diſtance of the Image by but
mere Prejudice.
cil are reflected con-
verging, and after meeting fome-
where in a Focus, go from thence
diverging to the Eye; therefore the
Image will not appear beyond the
Glafs, but on this Side of it, in
that Focus. So likewiſe, in another
Figure, becauſe the Pencils GD, BC
croſs each other, it is
Tab.XVII. evident, that the Image
Fig. 3.
of the Candle GB ought
to appear inverted to
the Eye in Q; and alfo on this
Side the Glafs, and not beyond it,
becauſe the Rays of every Pencil
crofs one another in a Focus, as was
before explained. But why in this
Caſe we ſhould not imagine it to be
very near, (unleſs we look pery in-feen in the Place where the reflected Rays
rently upon it) when it is really ve-
ry near, See the Notes on Chap. xxxiii.
Art. 12. for the Cafe is the fame
here as in the Perfpective Glaſs
there.
Fifthly, Let the Candle GB be at
fome Distance from the Glass, and
the Eye M very near it. Then
becauſe the Candle GB
is feen by other Pen-
cils GHM, BCM which
do not crofs each other;
it is manifeft, that the Image of GB
ought to appear erect again, but more
confufed.
Tab.XVII,
Fig. 3.
But in this Cafe it is particularly
to be obſerved, that the Eye M hath
no way to judge either in what
Place, or at what Diſtance behind!
Catropticks
Book III,
Prop. 30.
It was very ill
therefore in * Tac-
quet, after he had fo
well demonftrated
under this Head; that the reflected I-
mage in any Looking-Glafs is always
meet with their Cathetus of Incidence,
(the Cathetus of Incidence is a Line
drawn from any Point in the Objec
perpendicular to the Glafs) to except
this laft Cafe as contradicting this Ax-
iom; whereas it is no ways contra-
dictory to it. For when the Eye is
in fuch a Pofition, as to receive the
reflected Rays before they meet with
their Catheti of Incidence, the Image
cannot be feen where they meet,
becauſe they don't meet any where ;
neither is it feen in any other certain
Place; but it affects the Eye as if it
came from an infinite Diffance; in
the fame manner as when the Rays
come converging out of a Perfpe-
Etive-Glafs. See the Notes on Chap.
xxxiii. Art. 7,
the
Chap. 34. of NATURAL PHILOSOPHY.
279
the Object, is not upon the Superficies of the Glafs, but
in the Air, in the Place where we imagine we ſee the
Object, and where the Rays which come from every Part
of it, are united after Reflexion. I
1. Befides fuch Looking-Glaffes,
where we look upon one Superficies
only, we may alſo confider Perfpe-
ctive-Glaffes, or certain clear Glaſſes,
as Looking-Glaffes confifting of two
Superficies; according to the Variety
of which, there is alſo a wonderful
Variety of reflected Images. For not
only the firft Superficies which re-
ceives the incident Rays out of Air,
but alſo the ſecond Superficies which
receives the Rays going out of Glafs
into Air, exhibits a reflected Image,
as may be feen by placing a Candle
before fuch a Glass.
First then, let a Candle be placed
before a Glaſs which is plain on both
Sides; then the Images reflected by
each Superácies, will both be feen
erect and exactly like each other, ex-
cepting only, that That which is re-
flected by the farther Superficies will
feem a little more obfcure, becauſe a
great many of the Rays have already
been reflected by the firft Super-
ficies.
Secondly, Let the Glaſs be plain
on the one side, and convex on the
other, then if the Candle be placed
before the convex Superficies, the I-
mage will be reflected erect by each
Superficies (unleſs the Glafs be of
fuch a Thickneſs, and the Fore-fide
of it fo convex, that the Rays in
paffing through it are made converg-
ing, and after having been reflected
by the plain Superficies, and paffing
a fecond Time through the convex
Side, meet in a Focus before they
come to the Eye; in which Caſe
the Image from the latter plain Su-
perficies will be feen inverted) but
that which is from the firſt and
convex Superficies, will appear lefs.
But if the Candle be placed before
the plain Superficies, then the Image
reflected from the firft Superficies
will be erect again, and that from
the further Superficies, which is con-
cave within, will be reflected in-
verted, and will alfo feem to be
much nearer to the Eye, than that
from the firſt and plain Superfi-
cies.
Thirdly, Let the Glaſs be plain on
one Side, and concave on the other.
Then if the Candle be placed before
the concave Superficies, the Image
reflected from the firft Superficies
will be inverted, and that from the
further one, erect. But if the Can-
dle be placed before the plain Su-
perficies, the Images reflected from
each Superficies will be erect, but
that from the further one, which is
convex within, will appear lefs.
Fourthly, Let the Glafs be con-
cave on one Side, and convex on
the other. Then if the Candle be
placed before the concave Superfi-
cies, the Images by each Superfi-
cies will be inverted; but if before
the convex Side, they will be both
erect.
Fifthly, Let the Glafs be convex
on both Sides. Then the Image of
the Candle placed before it, will al-
ways be reflected erect by the firſt
Superficies; and always inverted by
the other Superficies, which is con-
cave within.
Laftly, Let the Glaſs be concave
on both Sides. Then the Image of
the Candle placed before it, will al-
ways be reflected by the firft Su-
perficies inverted, and always erect
by the latter which is convex
within.
T 4
CHAP.
J
289
ROHAULT's SYSTEM Part I,
1. Of the
Rays which
we fee dart
downwards
dic.
{
CHAP. XXXV.
A Solution of fome Problems concerning Viſion.
TH
HOUGH I have been very large upon this Sub-
ject of Viſion, yet I doubt not but that I have
upwards and paffed over a great many curious Queſtions, the Solu-
from a Can- tion of which, may perhaps be fomewhat difficult to
thoſe who are not well acquainted with our manner of
Explication. That this Treatife therefore may be as
little defective as poffible, and to fhow the Ufefulneſs
of it, I fhall here propofe fome of thefe Sort of Que-
ries; and leave the Excellency, at leaſt the Truth of
our Hypothefis to be judged of, by feeing how eaſy it
is to refolve them. And Firſt, I ask; Whence it is,
that when we look upon a lighted Candle at a little Di-
ftance with our Eyes winking, there feem to come Rays of
Light from the Flame of the Candle, and dart upwards
and downwards into the Air? And whence is it also,
that if an opake Body be put between the Eye and the
Place where we fee the uppermost Rays, we still continue to
fee them, and on the contrary, ceafe to fee the lowermoft
Tab. IX. Rays? In order to understand the Reafon of theſe Pha-
nomena, let us confider the Eye A, the Eye-lids of
which H, I, are fo near each other, that there is only
a very narrow Paffage left, through which the Rays
which come from the Candle BCD paſs to imprefs its
Image on the Part of the Retina EFG in the manner
above explained: Further, it is to be obſerved, that
the Parts H and I (which are uſed to touch one ano-
ther when the Eye is clofe fhut,) are fo fmooth, that
they reſemble two fmall convex Looking-Glaffes,
which reflect the Rays of Light falling upon them, to-
Fig. 3.
1. Two finall convex Looking-
Glaffes) The Rays in this Cafe, are
not reflected by the inward Super-
ficies of the Eye-lids themfelves, in
the manner of Looking-Glafles, but
are refracted by the Humour which
fticks to the out-fide of them; in
explaining all the reft of this Pheno-
menon, the Reaſon is the fame.
wards
Chap. 35. of NATURAL PHILOSOPHY.
281
wards the Retina, to the Parts of it EK, FL, which
otherwife would not have been affected but by Objects
which are about BM and CN. Wherefore the Impref
fion made upon EK cauſe the Appearance of bright
Rays, which we refer to the Place BM, and the Im-
preffion made on GL caufe the Appearance of the
Rays which we imagine to be in CN. But that which
is moft worthy of Obfervation here, is, that the Part
of the Flame B, which illuminates the lower Eye-lid I
by Rays which are reflected to the upper Part of the
Retina LG, cauſe the Appearance of the lower Rays
CN; wherefore if an opake Body OP be put between
the Eye and upper Part of the Flame, we fhall ceaſe
to fee the lower Rays, and continue to fee the upper
ones, becauſe they are feen by Means of the Rays
CH, which come from the Bottom of the Flame, and
which are not intercepted. And all the Difference that
we fhall find in thefe upper Rays, is this; that where-
as before they ſeemed to be in BM, they will now
ſeem to be on this Side the opake Body OP. But
when the Eye is open as ufual, that is, when the Eye-
lids come no nearer than S and T, we ought not to
fee thefe Rays of Light; becauſe the Rays which fall
upon thoſe Places which we now compared to Looking-
Glaffes, enter but a little Way into the aqueous Humour
at furtheſt, and are hindred from going any further by the
Uveous Tunick.
round.
2. Whence is it that when a Fire-brand is turned round, 2.0f a Fire-
we fee a Circle of Fire through which it paffed? The Rea- brand turned
fon of this, is, becauſe the Fire-brand makes a circular
Impreffion upon the Retina, and the Motion of it being
very quick, fome of the Impreffion made at firſt remains
till it returns again.
3. From this Phænomenon we may draw this Con-
clufion, that though Viſion is made in an Inftant, it does
however continue fome fhort Space of Time.
3. That the
Scufe of Sec-
ing continues
fome time.
4. Whence is it that a Cannon-Ball, or any other black 4. Why we
Body, paffing very quick before a white Wall, cannot be cannot at all
fee fome Bo-
perceived at all? The Reafon is, becauſe a black Bo- dies which
dy making no Impreffion upon the Eye; the Ball in- move very
terrupts the Rays of Light reflected from the Wall, fo quick.
very little, that the Motion which thefe Rays excited
in the Eye juft before, is continued in it for fo fhort a
Time.
5. Why
282
5. Why Some
Perfons can
ROHAULT'S SYSTEM
Part I.
5. Why do fome Perfons fee distinctly at a certain Di-
fee Objects ftance only, and fee confusedly at a greater or leffer Di-
diftinctly, at Stance? It is becauſe they are fo accuſtomed to look
certain Di- at that Diſtance, that the Mufcles by which the Figure
Bance only.
of the Eye is altered, are grown ftiff, and uncapable
of performing their Office; in the fame manner as the
other Muſcles of the Body are uncapable of moving
the Members of it, if they have not been exerciſed
for a long Time. To which we may add; that the
Tunicks which contain the three Humours of the Eye,
are fo hardened, that they will not fo eafily yield as
before.
6. Of Viſion
made with a
Needle.
6. Whence is it that an Object which appears confu-
through a Hole fed, when we look at it too near, may be feen very di-
Stinctly at the fame Diſtance through a Hole made with a
Needle in a fine Card, or a Piece of Paper? The Rea-
fon is, becauſe the Eye then receiving a lefs Quantity
of Rays from every Point of the Object, each of them
paints its Image but upon a very fmall Space, fo that they
which come from two neighbouring Points, do not con-
found each other's Actions. 2
7. Whence
1. Because they are so accustom-fall upon X, and therefore it will
ed, &c.) This often happens to fome
particular Sort of Workmen, as En-
gravers, &c. and ought to be look'd
upon as a particular Sort of Dif-
temper.
Tab. VI.
to be
2. It may alfo here be enquired;
Why a very small opake Body fuf-
pended in the Middle of an Hole be-
tween the Eye and a great many
Lights, is multiplied fo, as
feen before every Light? The Rea-
fon is, becauſe the Rays croſs one
another in that Hole, and are in-
tercepted by the fmall opake Bo-
dy. Let us immagine
GHILN to be the Eye,
PEDFQ the fmall Hole
in the Paper, HD the fmall opake
Body fufpended in the Middle of
the Hole; and A, B, C, three Can-
dles. This being fuppofed, the Bo-
dy HD will intercept the Ray
BO; then the Shadow of that Bo-
dy will fall on O, and therefore
the Body it felf will be feen in B ;
fo likewife it will intercept the
Ray AX; fo that its Shadow will
be feen in A. Laftly, it will al-
fo intercept the Ray CY, whofe
Shadow will fall on Y, and there-
fore it will be feen in C. Neither
is it neceffary that an opake Body
fhould be fufpended in a Hole at
all: For fince the Rays that come
from a great many lucid Bodies,
crofs one another in the Tunica
Cornea, if you fix your Eyes upon
a Fire of burning Coals, and put
a very flender Iron-rod cloſe to your
Eye, it will be greatly multiplied,
and feen as it were before every
Coal.
Secondly, Why an Object is feen
double when looked at with one Eye
through two Holes made in a Pa-
per close to cach other? In order
to account for this Effect, it is to
be obferved, that the Objects are
never feen double, but when all
the Rays of the fame Pencil, meet
together before they come to the
Bottom of the Eye, or after they
are paffed beyond it. In order to
have thefe Rays meet together be-
fore
Chap. 35. of NATURAL PHILOSOPHY.
283
couched of
7. Whence is it that those who have been couched for 7. Why they
Cataracts, can fee but confufedly afterwards, and why do who have been
they want very large convex Glaffes in order to fee di- Cataracts
ftinctly? Before we refolve this Queſtion, it is to be ob- want large
ferved, that a Cataract is not a Pearly Subftance form- magnifying-
Glaffes.
ed between the Aqueous and Chryſtalline Humours, as
has been long imagined, but is an Alteration made in
the Chryſtalline Humour it felf, which has thereby in-
tirely loft its Tranfparency and is become opake, if not
through the whole Subftance of it, yet at leaſt in ſome
Part of it; which may very eaſily be, for this Humour
is compofed of a great many Membranes one upon
another, which become viſible when it is boiled. Whence
Now
fore they arrive at the
Tab. X. Bottom of the Eye, let us
fuppofe CDE to be the
Pupil of a young deep Eye, the mid-
dle Part of which D is covered by
the finall Interftice between the
Holes of the Paper; and let OQNPR
be the Bottom of the Eye.
becauſe this opake Body intercepts a
great many of the Rays, and for
that Reafon makes all the Pencils
hollow, that is, without any Rays
in the Middle of them, it is evi-
dent that the Point A is feen in
the Place marked 2 by the extreme
Rays HR, and a few others near
them, and in the Place marked
3
by the Rays HQ, HN, whereas, o-
therwiſe it would have been ſeen
only confuſedly in A by the mid-
dle Rays P, and thoſe which fur-
round them. And becauſe the fame
Thing happens in every other Point
of the Arrow, it shows that it
ought fo to appear double, that
when the right Hole DE of the o-
pake Body which covers the Pupil is
topped, the left Image OQ, and
the Arrow on the right Side dif-
appear; and if the left Hole be ftop-
ped, the right Image and left Ar-
row diſappear. But if on the other
Hand, we fuppofe the Eye to be old
and flat, fo that the Bottom of it is
not OQNPR, but very near GYH,
and that the Rays of every Pencil
arrive at the Bottom of the Eye be-
fore they are collected into a Point,
the Arrow will be feen double again,
but fo that the Images of it upon
ſtopping the Holes by Turns, will
difappear in the contrary manner to
what they did before. Further, by
the fame Argument we may collect,
that if there be a great many Holes
inſtead of Two, there ought to be
a great many Images of the Object
feen. Laftly, Why the Body which
appears double in this manner, ap-
pears to be edged with Colours alfo,
may be feen in the Notes on Chap.
xxvii. Art. 65. towards the End.
Thirdly, Why, if there be two
Candles A and B ſo placed, that
through the Hole S, only the Candle
A can be ſeen with the right Eye
F, and only the Candle B with the
left Eye D's when both the Eyes are
open together, is there one Candle only
feen, as if it were in H; but the
Candles must be both of the ſame
Heighth, and at the fame Time no
opake Bodies must be seen with which
the true Places of the Candles A and
B may be compared? The Reafon
hereof, is, That becauſe one Candle
only can be feen by each Eye; and
one Eye only makes a very bad
Judgement of the true Diſtance of
Objects; each of thefe Candles are
therefore feen nearer than it really
is, the one in the Line AF, and
the other in the Line BD, and there-
fore they feem both to unite in the
common Place H as if they were
but one.
it
284
Part I.
ROHAULT's SYSTEM
when we are
it follows, that when the Cataract is taken away, the
whole Chryftalline Humour is taken away, or at leaſt,
is made flatter or lefs convex than it was before:
Now if this Humour be lefs convex than it was before,
the Rays which the Eye receives from every Point of
the Object will not be fo much refracted, or will not
incline fo much to each other, as to be able to unite
together when they come at the Retina; and this muſt
make the Viſion confuſed. But this may be remedied by
the Help of a very convex Glafs, which makes the Rays
that were before diverging, become converging when they
enter into the Eye.
A
8. Why we fee 8. Why do Divers, when they are under Water, fee all
confufedly Things confufedly, unless they make use of very convex
under Water. Glaſſes? The Reafon is, becauſe the Rays of Light
which come to them from the Object, are very little
refracted in paffing out of Water into the Aqueous Hu-
mour of the Eye, fo that thofe Rays which come from
the fame Point, are not united together when they fall
upon the Retina; and this is remedied by very convex
Glaffes.
9. Why if we
with one Eye
cannot fee a-
ita
9. Lastly, Whence is it, that if we but one Eye, and
look intently look intently with the other, upon a fmall Object, which is
upon a small at fix Foot Distance, fuppofe, we cannot at the fame Time
Object, we fee another fmall Object which is at a little more than
nother small half a Foot Diſtance from it; though we can fee it, if it
Object which be a little nearer, or a little further off? The Reaſon
is very near is, becauſe when this other fmall Object is at the
Place where it cannot be ſeen, it impreffes the Image
exactly on that Part of the Bottom of the Eye where
the Optick Nerve enters in, and where the Separati-
on of the Capillaments of this Nerve is made, in or-
der to fpread themſelves every Way, and cover the
Bottom of the Eye; fo that this Image has no Ef-
fect, becauſe it does not fall upon the Extremities of the
Capillaments of the Optick Nerve, which is neceffary in
order to Sight, as has been before explained.
10. That it is
fornetimes
worth while
to take the
10. There are innumerable other Queſtions upon this
Subject that might be asked; but they who rightly
underſtand the Nature of Vifion, will find it no great
Pains to find Difficulty to refolve themfelves, and the Pains which
they take in finding out the Solution of them, will
make them have a clearer Notion of them, and render
them more familiar: And as to thoſe who are unca-
out the Truth.
pable
Chap. 35. of NATURAL PHILOSOPHY,
285
pable of underſtanding them, or who will not be at
any Pains; it is to no purpofe to attempt to fatisfy
them, by explaining a great Number of Queſtions.
Wherefore I fhall here conclude this firft Part; which
is fufficient to content all reaſonable Perfons, and to
open the Minds of fuch, that they may for the future
proceed in a right Method of difcovering the Truth,
and avoiding Error, which are the Two Things we ought
principally to have in View in all humane Sciences. For
the Exactneſs and Improvement of Reafon, together
with fuch a Freedom and Openeſs of Mind, as may ren-
der it capable of judging fincerely and impartially, and
of clearing it felf of all Difficulties, are incomparably
more to be valued than the Knowledge of all the Sciences
in the World.
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:
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