Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 5, BUR-CHI

4-52
CHEMISTRY.
tempera¬
ture.
209
In feme in-
-ftantane-
oufly.
210
In others
gradually.
Caloric, were thefe bodies expofed to a proper degree of tempe-
' v„"" ’ rature.
BniHes he 2’ r^e temPerature at which thefe changes are ef-
come fluid fefted in invariably the fame in the fame body. Thus,
at the fame a mafs of ice is converted into the date of liquid or of
degree of water, when it is expofed to a temperature above 3 2°;
and water, when it is raifed to the temperature of
212°, aflumes the ftate of vapour or of iteam. But
although this temperature is conftant in the fame bo¬
dies, it varies greatly in different bodies. Thus, fpi-
rit of wine and ether are converted into vapour at a
very low temperature, while mercury and the fixed
oils, to undergo this change, require a temperature
far above that which is neceffary for the converfion of
water.
3. Some bodies are inftantaneoully converted from
the folid to the liquid ftate. Thus ice, when the tem¬
perature is raifed, paffes immediately from the folid to
the fluid ftate. Other bodies undergo a gradual change.
They firft become foft, as in the inftance of melting wax,
and pafs through the different degrees of foftnefs, till
at laft they become perfeftly fluid.
4. It may perhaps now feem furprifing, that phe¬
nomena which were fo familiar Ihould have exifted fo
long without the true explanation of the changes which
bodies undergo, in palling from the folid to the liquid
or vaporous ftate, or from the fluid to the folid ftate.
The want of inftruments to meafure accurately the re¬
lative degrees of temperature at which thefe changes
take place, might perhaps be one caufe of the unfuc-
cefsful inveftigations of philofophers on this fubjeft.
But even after the invention and improvement of the
thermometer, it was long before the fecret, and we
may add, Ample caufe of thefe wonderful effe&s, was
fully afeertained. It was referved for the fagacity of
Dr Black to give the true explanation. The era of
the difeovery of this law, of fuch univerfal application
to the phenomena of nature, may be regarded as one
of the moft important in the hiftory of chemical fcience.
Great dif- Dr Black w^as always diftinguilhed for caution and
covery of precif10n in all his views j and as the progreflive fteps
by which this celebrated philofopher was. led to afeer-
tain the true caufe of fluidity afford us a fine ex¬
ample of fnnple and elegant inveftigation, we lhall de¬
tail the experiments by which it was eftablilhed in his
own words.
5. After ftating that the caufe of fluidity which had
been formerly given was unfatisfaftory, and inconfift-
ent with the phenomena, he obferves that u thefe phe¬
nomena, when attentively confidered, ftiew that fluidity
is produced by heat, in a very different manner from
that which was commonly imagined ; a manner, how¬
ever which, when underftoed, enables us to explain
many particulars relating to heat or cold, which ap¬
peared1, in the former view of the fubjeft, quite perplex¬
ing and unaccountable.
“ Fuidity was univerfally confidered as produced by
fuppofed to a addition to the quantity of heat which a body
!TfmaK-0contains, fwhen it is once heated up.to its melting
flition of point 5 and the returning of fuch body to a folid ftate,
as depending on a very fmall diminution of the quan¬
tity of its heat, after it is cooled to the fame degree;
that a folid body, when it is changed to a fluid re¬
ceives no greater addition to the heat within it than
what is meafured by the elevation of temperature indi¬
Dr Black.
212
Tluidity.
Caloric.
cated after fufion by the thermometer 5 and that, when Caloi
the melted body is again made to congeal, by a dimi-
nution of its heat, it fuffers no greater lofs of heat than
what is indicated alfo by the Ample application to it of
the fame inftrument.
“ This was the univerfal opinion on this fubjefl, fo Inconfi,.;
far as 1 know, when I began to read my lectures in thewidifo
univerfity of Glafgow', in the year 1757. But I foon
found reafon to object to it, as inconiiftent with many
remarkable fa£ts, when attentively confidered; and I
endeavoured to fhewq that thefe fadls are convincing
proofs that fluidity is produced by heat in a very dif¬
ferent manner.
“ 1 (hall now deferibe tBe manner in which fluidity
appeared to me to be produced by heat, and we ftiall
then compare the former and my view of the fubjeft
with the phenomena. j,
“ The opinion I formed from attentive obfervatien Calori
of the fa6ls and phenomena, is as follows: When ice, akfork;
for example, or any other folid fubftance, is changingfo!lds ^
into a fluid by heat, 1 am of opinion that it receives
much greater quantity ofheat than what is perceptible
in it immediately after by the thermometer. A great
quantity of heat enters into it, on this occafion, without
making it apparently warmer, when tried by that in¬
ftrument. This heat, however, muft be thrown into it,
in order to give it the form of a fluid 5 and I affirm,
that this great addition of heat is the principal and moft
immediate cauie of the fluidity induced. sijj
“ And, on the other hand, when we deprive fuch a Fluids tj
body of its fluidity again, by a diminution of its heat,“imPS
a very great quantity ofheat comes out of it, while it
is afluming a folid form, the lofs of which heat is not
to be perceived by the common manner of ufing the
thermometer. The apparent heat of the body, as mea¬
fured by that inftrument, is not diminilhed, or not in
proportion to the lofs of heat which the body actually
gives out on this occafion; and it appears from a num¬
ber of fafts, that the ftate of folidity cannot be induced
without the abftraftion of this great quantity of heat.
And this confirms the opinion, that this quantity of
heat, abforbed, and as it were, concealed in the compo-
fition of fluids, is the neceffary and immediate caufe
of their fluidity.
“ To perceive the foundation of this opinion, and
the inconfiftency of the former with many obvious fads,
we muft confider, in the firft place, the appearances
obfervable in the melting of ice, and the freezing of
water. 216
“ If we attend to the manner in which ice and fnow Proved
melt, when expofed to the air of a warm room, or ’
when a thaw fucceeds to froft, we can eafily perceive,?^—
that however cold they might be at the firft, they arewateIi
foon heated up to their melting point, or begin foon
at the it furface to be changed into w ater. And if
the common opinion had been well founded, if the
complete change of them into water required only the
further addition of a very fmall quantity of heat, the
mafs, though of a confiderable fize, ought all to be
melted in a very few minutes or feconds more, the
heat eontinuing inceffantly to be communicated from
the air around. Were this really the cafe, the confe-
quences of it would be dreadful in many cafesfor,
even as things are at prefent, the melting of great
quantities of fnow and ice occafions violent torrents,
and