as it was coloured red, could eafily be dlftinguifhcd
from the brine, remained perfectly feparated from
this heavier faline liquor 5 on which it repofed
quietly, without the fmalleil appearance cf any
tendency to mix with it.
I now filled, to the height of about 5 inches,
the void fpace between the outfide of the fmail jar
and the infide of the large jar in which it was
placed, with ice-cold water, mixed with a quantity
of ice, in pieces as large as walnuts, (pounded
ice would have obftrucled the view in obferving,
through the fides of the large jar, what paffed in
the fmaller) and when this was done, I very care
fully poured ice-cold olive oil* into the fmaller jar
till it covered the furface of the (tinged) frefh water
to the height of about an inch (fee Fig. 5. Plate IV.) ;
and placing myfelf near the table, in a fituation
where I had a diftincl: view of the contents of the
fmall jar, I fet myfelf to obferve the rcfult of the
Experiment.
After waiting above an hour without being able
to perceive the fmalleft appearance of any motion,
cither in the brine, or in the freili water, (the one
continuing to repofe on the other with the moil
perfect
* This oil ferved not only to keep the -water on which it repofed,
quiet ; but alfo to prevent any communication cf heat between it and
the air of the a^mofjphwe.
VOL. II. U u
328 Of the Propagation of Heat
perfect tranquillity, and without the fmalleft difpo-
iition to mix together) I left the room.
When I returned to it the next day, I found
things precifely in the flate in which I had left
them ; and they continued in this flate, without
the fmalleft appearance of any change, or of any
difpofition to change, during four days.
At the end of that time, thinking that any far
ther prolongation of the Experiment would be
quite ufelefs, I removed the fmall jar, taking care
not to agitate its contents, and placed it in the win
dow of a room heated by a German ftove.
In lefs than an hour I perceived that the brine
and the (tinged) freih water began to mix, and at
the end of 24 hours they were intimately mixed
throughout, as was evident by the colour of the
aqueous fluid on which the oil repofed ; which
now appeared to the eye to form one uniform mafs
of a light red tint.
I mail leave it to philofophers to draw their own
concluiions from the remits of this Experiment :
In the mean time, there is one fact which it feems
to point out that I ihall juft mention, which is not
only curious in itfelf, but may lead to very impor
tant difcoveries. ' It appears to me to afford ftrong
reafons to conclude that, were a lake but very deep^
its waters, near the furface, would neceffarily be
freih, even though its bottom mould be one folid
mafs of rock fait 1
Would it be ridiculous to make Experiments to
determine whether the water at the bottom of fome
very deep lakes is not impregnated with fait ?
Should
In Fluids. 329'
Should it be found to be actually the cafe, it might
prove an unexhauftible treafure in an inland coun
try, where fait is fcarce.
As mines of rock-falt are often found in the
neighbourhood of frefh lakes, it feems reafonable
to fuppofe that the waters of fuch lakes fliould
fomeiimes be in contact with ftrata of thefe mines ;
and when I firft began to meditate on the fubject,
I was much furprifed, not that the fait water
which may lie at the bottom of frefh lakes ihould
not already have been difcovered, for from the
firft I plainly perceived that nothing could happen
in the ordinary courfe of things that could bring it
to the light, or even afford any grounds to fufpecl:
its exiftence ; but, zsjlrata of fait mines frequently
lie higher than the mean level of the country, I was
furprifed that lakes of fait water fliould not more
frequently be found ; and as thefe reflections oc
curred to me after I had difcovered what appear
ed to me to be an evident proof of the wifdom and
goodnefs of the Creator in making all lakes in cold
countries/r<?/7j, I began to be alarmed for the fatal
confequences that might enfue, if, by chance, the
* fide of a lake fliould come into contact with a
mountain of fait ; as I faw might eafily happen.
Shall I, or fliall I not attempt to give my reader
an idea of what I felt, when, meditating on the
fubjecl', and almoft beginning to repent of what
many, no doubt, have already condemned as the
foolifh dream of an enthufiaftic imagination, I faw,
all at once, that the moft effectual care had been
taken to prevent the evils I apprehended ; that
from
Of the Propagation of Heat
from the very conftitution of things, and the ordi
nary and uniform oprration of the known laws of
Nature, the permanent exiftence of a lake, SALT AT
THE SURFACE, is abfolutsly impoffible ; even though
it mould be furrounded on every fide by mountains
of fait ?*
Though the explofion of a volcano, an earth
quake, or any other great convulfion, by which the
fhores of a lake might be brought into contact with
a vaft mine of fait, might caufe the whole mafs
of its water to be fait for a time j yet, the evil
would foon effect its own remedy : the falling in
of the cnift of earth and ftones by which mines of
fait are every where found to be covered, (and
without which they could not exift) would very
foon cover the naked fait, and the water at the
furface of the lake would again become perfectly
frefh. Should, however, the lake be fo deep that
the temperature at the bottom fhould remain the
fame fummer and winter, without any fenfible va
riation, it is moft certain that its waters there (at
the bottom of the lake)- -would remain perfectly
faturated with fait for ever.
But are there not fome reafons to conclude that
the water at the bottoms of all 'very deep lakes ought
neceffarily to be fait, even in fituations where there
are no mines of fait near ?
The fea-fliells that are frequently found in high
inland fituations, as well as many other appear-
ances
* By the word Lake I mean, as is eafy to perceive, a collection of
water, in a high intend fituation, from which there is a conftant efflux.
in Fluids. 331
ances noticed by naturalifts, ftrongly indicate that
moft of our continents have been covered by the
waters of the ocean. Now if that event ever hap
pened however remote the period may be at
which it took place it feerns highly probable that
the fait water left at the bottoms of all deep lakes,
by the fea, on its retiring, muft be there now.
I cannot take my leave of this fubjecl: without
juft obferving, that the difcovery of the impoffibility
of the permanent exiftence of what w r e can plainly
perceive would be an evil, certainly ought not to
dlminijh our admiration of the wifdom of the great
Architect of the Univerfe.
332 Of the Propagation of Heat
CHAP. II.
-
Water made to congeal at its under Surface. Ob-
fervation refpecling the Formation of Ice at the
Bottoms of Rivers. Reafons for concluding that
Heat can never be equally diftributed in any
Fluid. Perpetual Motions occajioned in Fluids by
the unequal Diftribittion of Heat. An inconceiv
ably rapid Succejfion of Cdlifions among the in
tegrant Particles of Fluids is occofioned by the in
ternal Motions into which Fluids are thrown in the
Propagation of Heat. An Attempt to eftimate the
Number of thofe Collifions which take place in a
given Time. Thefe Inveftigations will greatly change
our Ideas refpecling the real State of Fluids appar
ently at reft. FLUIDITY may be called the LIFE
OF INANIMATE BODIES. Conjectures refpeffing
tbe VITAL PRINCIPLE in Living Animals ; and
the Nature of Phyfical STIMULATION.
tne mechanical operation may in
fact be, by which thofe effects are produced
that have given rife to the idea of the exiftence of
an attraction of affinity (a power different from
gravitation) between folid bodies and their liquid
menftrua, and between different portions of the
fame menftruum differently faturated ; the refult
of the foregoing Experiment (No. 57) proves that
two particles of water in combination with very
different
in Fluids. 333
different quantities of fea-falt ; or a particle of wa-
terfaturated with fait, and another perfectly free
from fait ; may be in contact with each other for any
length of time without mowing any appearance of
a difpofition to equalize the fait between them.
But mould we even admit as a fact, what this
Experiment feems to indicate, namely, that there
is no fuch thing as an attraction of prediledion be
tween folids and their folvents ; and that all thofe
motions which have been attributed to the action
of that fuppofed power, (as well as all other mo
tions which take place in Fluids,) are the imme
diate effects of gravitation acting according to im
mutable laws, and changes offpecific gravity by Heat ;
yet, there would {till remain one great difficulty in
explaining chemical folution. As all mechanical
operations require a certain time for their perform
ance ; and as the motion which is occaiioned in a
Fluid by a change of fpeciiic gravity in any indi
vidual particles of it, begins as foon as the change
begins to take place, if there be no attraction be
tween the particles of folid bodies and the parti
cles of their menftrua ; as Heat is fuppofed to be
generated or abforbed, or, to fpeak more properly,
both generated and abforbed, in the contact of
thole particles, and previous to the completion of
their chemical union ; how does it happen that
the particle of the menitruum whofe fpecific grav
ity is neceffarily changed by this change of tem
perature, does not immediately quit the folid, in
confequence of this change ; and before the procefs
f folution has had time to bz completed ?
A con-
334 Qf the Propagation of Heat
A confideration of the effects of the vis inertia
of the particle of the menftruum whofe fpecific
gravity is thus changed, and alfo of the vis inertia
of the reft of the Fluid, and the refiftance it muft
oppofe to the motion of its individual folitary par
ticles, would furniili us with arguments that might
be employed \vith advantage in removing this dif
ficulty ; but I fancy that the refult of the Exper
iment of which I mall prefently give an account
will be more fatisfactory than any reafoning, un-
fupported by facts, that I could offer on the
fubjecl:.
When a doubt arifes with regard to the poffibil*
ity of any operation of a peculiar kind, which is
fuppofed to take place, in any procefs of nature
among thofe infinitely fmall integrant particles of
bodies which efcape, and muft ever efcape, the cog
nizance of our grofs organs, however they may be
affifted by art, the fhorteft way of deciding the
queftion is to put the known powers of nature in
action under fuch circumftances that the effects
produced by them muft {how, unequivocally,
whether the fuppofed operation be poflible, or not :
and if it be found to be poilible in one cafe, we
may then argue with lefs diffidence on the proba
bility of its actually taking place in the fpecific
cafe in queftion.
It has been abundantly proved by the Experi
ments of M. de Luc, and by thofe of my friend
Sir Charier* Blagden, that, when water, in cool
ing, has arrived at the temperature of about 41 F.
its condemnation with cold ceafes ; and it begins to
expand ;
in Fluids. 335
expand ; and continues to expand gradually as its
temperature goes on to be farther diminiftied, till
it is changed to ice. Availing myfelf of that molt
important difcovery, I made the- folio wing Exper
iment.
Experiment, No. 58.
Having poured mercury, at the temperature of
60, into a common glais tumbler, till this Fluid
flood at the height of about an inch ; I then poured
about twice as much water (at the fame tempera
ture) upon it ; and placing the tumbler in a fhal-
low earthen dim, furrounded it to the height of
the level of the furface of the mercury with a
freezing mixture of fnow and common fait. Hav
ing done this, I was very curious indeed to fee in
what part of the water ice would firft make its ap
pearance. Could it be at the upper furface of it f
That appeared to me to be impoffible ; for the
Experiment being made in a room warmed by a
German ftove, the temperature of the air which
repofed on that furface was confiderably above the
point at which water freezes.
Could it be at its lower furface, where it refted
on the upper furface of the mercury i If that
Ihould happen, it would fhow, that, notwithftand-
ing the diminution of the fpecilic gravity of the
water in palling from the temperature of 41 to
that of 32 ; and the tendency which this diminu
tion gave it to quit the furface of the mercury
from the inftant when, in being cooled by a con
tact with it, it had palled the point of 41 5 yet,
. II. W w there
33^ Of the Propagation of Heat
there was time iufficient for the congelation to be
completed before the particle of water fa cooled could
make its efcape.
The reader will naturally conclude from what
was faid in the preceding page, that it was merely
with a view to the determination of that lingle
fact, that this Experiment was contrived ; and he
will perceive by the remit of it that my expecta
tions with regard to it were fully anfwered.
Ice was not only formed at the bottom of the yya-
fer, at its under furface, where it was in contact
with the cold mercury 5 but, I found on repeating
the Experiment, and varying it, by previoufly
cooling the mercury in the tumbler to about 10,
that boiling hot water, poured gently upon it, was
inftarftly frozen, and gradually formed a thick
cake of ice, covering the mercury ; though almoft
the whole of the mafs of the unfrozen water, which
refted on this ice, remained nearly boiling hot.
This Experiment not only determines the point
for the decifion of which it was undertaken ; but
alfo enables us to form a jufl opinion refpecting a
matter of fact which has been the fubject of a good
deal of difpute.
Though many accou'nts have been publifhed of
ice found at the bottom of rivers, yet doubts
have been entertained of the poffibility of its being
formed in that fituation. From the refult of the
foregoing Experiment it appears to me that we may
fafely conclude, that, if after a very long and a very
fevere froft, by which the furface of the ground
has not only been frozen to a confiderable depth,
but
in Fluids. .. 337
but alfo cooled fevcral degrees below the freezing
point, a river fliould overflow its banks, and cover
the furface of ground previoitfly fo cooled, ice
would be formed at the bottom of the water : but
all the Experiments that have been made on the
congelation of water fliow the abfolute impofiibil-
ity of ice being ever formed, in any country, at the
bottom of a river which conftantly fills its banks,
or which never leaves its bed expofed, dry, to the
cold air of the atmofphere.
By reflecting on the various confequences that
ought to follow from the peculiar manner in which
Heat appears to be propagated in Fluids, we are led
to conclude, that it is almoft impoffible that any
Fluid expofed to the action of light mould ever be
throughout of the fame temperature, though its
mafs be ever fo fmall ; and that the difference in
the Heat of its different particles inuft occaiion
perpetual motions among them.
Suppofe any open vefTel, as a common glafs
tumbler for inflance, containing a piece of mo
ney, a fmall pebble, or any other fmall folid opaque
body, to be filled with water, and expofed in a
window, or elfewhcre, to the action of the fun's
rays. As a ray of light cannot fail to generate
Heat when and where it is flopped or abforbed,
the rays, which, entering the water, and pailing
through it, impinge againft the fmail folid opaque
body at the bottom of the vefiel, and are there ab
forbed, muft neceffarily generate a certain quantity
of Heat ; a part of which will penetrate into the
interior parts of the folid, and a part of it will be
communicated
33 8 Of the Propagation of Heat
communicated to thofe colder particles of the water
which repofe on its furface.
Let us fuppofe the quantity of Heat fo commu
nicated to one of the integrant particles of the wa
ter to be fo fmall, that its effect in diminishing the
fpecific gravity of the particle is but juft fufficient
to caufe it to move upwards in the mafs of the li
quid with the very fmalleft degree of velocity that
would be perceptible by our organs of fight, were
the particle in motion large enough to be vifible.
This would be at the rate of about one hundredth
fart of an inch in a fecond.
This velocity, though it appears to us to be flow
in the extreme, when we compare it with thofe
motions that we perceive among the various bod
ies by which we are furrounded, yet, we ihall be
furprifed when we find what a rapid fucceflion of
events it is capable of producing.
If we fuppofe the diameter of the integrant par
ticles, or molecules of water, to be one millionth
fart of an inch (and it is highly probable that
they are even lefs*) in that cafe, it is moft cer r
tain that an individual particle, moving on in a
quiefcent mafs of that Fluid with the velocity in
queftion, namely, at the rate of part of an inch
in i fecond, would run through a fpace equal to
ten
* Leaf gold, fiich as is prepared and fold by the gold-beaters,
is not four times as thick as the diameter here affumed for the integrant
particles of water. Thefe leaves of folid metal have been found by
computation to he no more than TT _ I __- of an inch in thicknefs.
How much lefs muft be the diameter of the integrant particles ef
gold ?
in Fluids. 339
ten thoufand times the length of Its diameter in one
fecond, and confequently, would come into contact
with at ieatt fix hundred thoufand different particles
of water in that time.
Hence it appears how inconceivably mort the
time muft be that an individual particle, in mo
tion, of any Fluid, can remain in contact with
any other individual particle, not in motion, againft
which it flrikes in its progrefs, however flow that
progrefs may appear to us to be) through the qui-
efcent mafs of the Fluid !
Suppoiing the contact to laft as long as the
moving particle employs in paffing through a fpace
equal to the length of its diameter which is evi
dently all that is poffible ; and more than is prob
able ; then, in the cafe juft Hated, the contact
could not poffibly laft longer than T -^~-^ part of a
fecond ! This is the time which a cannon bullet,
flying with its greateft velocity, (that of 1 600 feet
in a fecond) would employ in advancing 2 inches.
If the cannon bullet be a nine pound Vr, its diame
ter will be four inches ; and if it move with a ve
locity of 1600 feet (m 19200 inches) in a fecond,
it will pafs through a fpace juft equal to 4800 times
the length of its diameter in i fecond. But we
have feen that a particle of water moving * of
an inch in a fecond actually pafTes through a fpace
equal to 10000 times the length of its diameter in
that time : Hence it appears that the velocity ivith
which the moving body quits the fpaces It occupies is
more than twice as great in the particle of water,
as in the cannon bullet !
There
Of tie Propagation of Heat
There is one more computation which may be of
life in enabling us to form more juft ideas of the
fubject under conflderation, and furely too much
cannot be done to enlighten the mind, and aflift the
imagination, in our attempts to contemplate thofe
invifible operations of nature which nothing but
the iharpeft ken of the intellectual eye will ever be.
able to detect and feize.
As fucceecling events which fall under the cog
nizance of our fenfes cannot be diftiriguimed if
they happen oftener than about ten times in a fec-
ond,* it appears that when a particle of water
moves in a quiefcent mafs of that fluid at the rate
of T ~ part of an inch only, in one fecond, its fuc-
ceeding collifions with the different particles, at
reft, of that fluid, againfl which it ftrikes as it
moves on, mull be fo inconceivably rapid that no
lefs than one thoufand of them muft actually take
place, one after the other , in the fliorteft fpace of
time that is perceptible by the human mind.f
After
* This afiertion, in as far, at leafl, as it relates to objects of fight,
may be proved by the following eafy experiment : Let a wheel, with
any known number of fpokes, be turned round its axis with fuch a ve
locity as fhall be found necefiary, in order that the fpokes may difappear
or become invifible. From the velocity of the wheel, and the number
of fpokes in it, the facl will be decided.
f It probably will not efcape the obfervation of my learned read
ers, that the velocity which I have here afligned to the fingle particls
of water, moving upwards in that fluid in confequence of a change
of its fpecific gravity by Heat, though apparently very fmall,
(-r^- part of an inch in a fecond) is, however, moft probably con-
fiderably greater, in fail, than any individual folitary particle of that
fluid could poflibly acquire,- in the fuppofed circumftancc*, by any
change of temperature, however great, owing to the refinance which
would
n
Fluids.
After we have patiently examined the refult of
thefe investigations, and the imagination has be-
come familiarized with the contemplation of the
interefting facts they prefent to it, how much will
our ideas be changed with regard to the real ftatc
of fluids apparently at reft ! They will then ap
pear to us to be, what no doubt they really arc in
fact, an affemblage of an infinite number of infi
nitely fmall particles of matter moving continu
ally, or without ceafing, and with inconceivable
velocities*
We mall then conflder fluidity as the life of hi"
animate bodies, and congelation as thefaep of death ;
and we mall ceafe to afcribe active powers, or
exertions of any kind, to dead moiionlefs matter.
But what fhall we think of the vital principle in
living animals ? Does not their life alfo depend
on the internal motions in their fluids, occafioned
by an unequal diftribution of heat ? And is not
Jtimulation^
would neccflarily be oppofed to its motion by the quiefccnt particles
of the fluid. Aware of this objeiSlion, and being defirous of being
prepared to meet it, I took fome pains to compute, by the rules laid
down by SIR ISAAC NEWTON in his Principh y book ii. fe6t. vii. what
the greateft velocity is that a folitary particle of water fuppofed to be
Ts-o-gTrecT ^ an * n k * n diameter) could poflibly acquire by a given
charge of its fpecific gravity : And I found that if the fpecific gravity
of water at the temperature of 32 F. be taken at 1.00082, and its fpe
cific gravity at 80, at 0.99759, as lately determined by accurate exper
iments, then, a fmgle particle of water at the temperature of 80,
fituatcd in a quiefcent mafs of that fluid at 32, the greatcil velocity
this hot particle could acquire in moving upwards in confeqnence of its
comparative levity would be that of TTTTT P art f an ^ Bcn in * fecond.
This is at the rate of about one inch and an half in I hour. But it is
evident, that when great numbers of particles unite and form current-:.
they will make their way through the quiefcent fluid with greater faci'-
"y, and confequently will move failer.
342 Of the Propagation of Heat
Jtimulation> in all cafes, the mere mechanical effect
of the communication of Heat ?
It is an opinion which we know to be as old as
the days of MOSESJ that the life of an animal refidss
in its blood ; and it is highly probable that it dates
from a period ftill more remote* It was lately re
vived by an anatornift and phyfiologift, (now no
more,) who was eminently di&inguifhed for fa*
gaclty ; and it appears to me that the late difcov-
eries refpecling the manner in which Heat is prop
agated m Fluids tend greatly to elucidate the fub-
jecb, and to give to the hypothecs a high degree
of probability.
According to this hypothecs (as it may now
be explained) every thing that increafes the ine
quality of the diflribution of the Heat in the mafs of
the blood (even though it mould not imme
diately augment its quantity) ought to increafe
the intenfity of thofe actions in which life confifts.
But are there not many finking proofs that this is
the cafe in fact ?
Do not rejj)iration 9 dlge/lion, and infenflble per*
fpiratlon all tend evidently (that is to fay, accord
ing to our affumed principles, with regard to the
manner in which Heat is propagated in Fluids)
- to produce, and to perpetuate this inequality of
heat in the animal fluids ? And do we not fee
what an immediate and powerful effect they have
in increasing the intenfity of the action of the pow