e 34 - CHE M 
found that the capacity ofa given body for heat is leaft 
when folid, greater in the fluid llate, an. 1 greateil in the 
vaporous date. Thus for example, ice and water being 
expofedin equal quantities tofimilar heating matters, as 
before a fire, the ice wiil be melted without increife of 
temperature, while the water acquires 162° of Fahren¬ 
heit’s thermometer. Or equal parts of water at 162, and 
ice at 32’ being mixed, the ice will melt, and the whole, 
inltead of the mean temperature, will remain at 32°. In 
either cafe the ice requires 130 of heat, which produces 
no other effefl than rendering it fluid, and is not fhown 
by the thermometer- So likewife the condenfation of 
fleam, though little if at all hotter than boiling water, 
communicates much more heat to a refrigeratory, than 
the fame quantity of water equally hot, and therefore it 
contained more heat. The quantity of heat which con- 
ftitutes the difference between the feveral llates of the 
body, has been improperly called latcfit heat. 
Problem. The ratio of the capacities of the fame bo¬ 
dy in the folid and the fluid ftates, and alio the num¬ 
ber of degrees the fluid would increafe in temperature by 
the heat which would Amply melt the folid, being given ; 
it is required to determine the number of degrees of the 
lame thermometer, between the natural zero, or ahjolute 
privation of heat , and the temperature of the l'olid juft 
melting. 
The whole quantity of heat in the folid, when juft 
melting, will be denoted by the number of degrees of 
its temperature from the natural zero: and the whole 
quantity of heat in the fluid will be denoted by the fame 
number added to the number of degrees the temperature 
of the folid would have been railed by the heat applied 
to melt it, if its capacity had not been changed by melt¬ 
ing. This lalt number coniifts of the obferved increale of 
temperature in the fluid augmented in the inverfe ratio 
of the capacities. Now the capacities of the l'olid and 
fluid being as their whole quantities of heat, it will fol¬ 
low that 
The difference between the numbers exprefling the ca¬ 
pacities. 
Is to the number exprefling the capacity of the folid ; 
As the difference between their refpe&ive quantities of 
heat in the thermometrical degrees of the folid, 
Is to the number of degrees expreffmg its whole quan¬ 
tity of heat, or its temperature from the natural zero. 
This in the form of a practical rule, is. Multiply the 
number expreffmg the capacity of the fluid into the num¬ 
ber of degrees the fluid would have increnfed in tempe¬ 
rature by heat iufficient to melt the folid; divide this 
product by the difference between the numbers exprefling 
the capacities, the quotient will be the number of de¬ 
grees of temperature from the natural zero. From ex¬ 
periment, it appears, that the natural zero is 1268’ of 
Fahrenheit's fcale below o, or 1300 degrees below the 
freezing point of water. 
Corollary. The difference between the zero of any 
fcale, and the natural zero, being once determined from 
experiment, it will be eafy in all cafes, where any two 
of the three quantities, the capacity of the fluid, the ca¬ 
pacity of the lolid, and the number of degrees the fluid 
would be railed by beat futlicient to melt the folid, are 
given, to find the third. 
The foregoing theory of heat maybe applied to explain 
all the changes of temperature in bodies, from the Ut- 
moft violence of ignition to the moll intenle cold. For 
whenever by condenfation or freezing, or by a change in 
the chemical combinations of bodies, the capacities are 
diminilhed, a part of the heat contained will be applied 
in railing the temperature. And, on the contrary, cold 
will be produced whenever bodies are melted, or evapo¬ 
rated, or any chemical procefs goes forward, by which 
the cap icities are increafed. 
Tne diminution of temperature produced by evapora¬ 
tion, had been obferved by Marian, and accurately de¬ 
scribed by Richmannj but it was iirft referred to its pro- 
I S T R Y. 
per caufe by Dr. Cullen, in bis Eftay on the Cold pro¬ 
duced by evaporating Fluids. In the 78th volume of the 
Philofophical Tranfadlions, Dr. Darwin has related feve¬ 
ral experiments, to prove that a diminution of tempera¬ 
ture takes place when the air is mechinically expanded; 
and converiely, that the temperature riles when rhe air 
is condenied. He has applied this doitrine With great 
ingenuity to explain a variety of curious phenomena. 
D a Cullen had obierved that a thermometer fufpended 
in the receiver of an air pump, finks always two or 
three degrees while the air is exhaufted. 
1 The grand queftion, whether heat be merely a vibra¬ 
tion of the parts of bodies, or a peculiar fluid, is not de¬ 
cided. If heat be merely vibration, it will be fcarcely 
poflible to account for its not being univerfaily commu¬ 
nicated to bodies according to their mafl'es, as the efta- 
blilhed laws of motion require; but if heat be a peculiar 
fluid, rhe notion of a greater or lefs capacity for that 
fluid, whole variations of denfity will be the caufe of 
change of temperature, will very naturally account for 
the different quantities required to be imbibed or given 
out by bodies of equal weight, before a like denfity or 
temperature can be produced in all. Neither will it be 
at all difficult, according to this hypothelis, to give ve¬ 
ry probable accounts of what happens when bodies change 
their Hates of folidity, fluidity, or vapour. But the va¬ 
rious theories rel'pedting beat, conlidered as matter, and 
a component part of bodies, are not fufiiciently grounded 
ondecifive fahts, to admit cf a curlory difeuflion, or in¬ 
deed to be ranked with the cftabiifhed doftrines collected 
and arranged in this place; though it muff be allowed 
that feveral of them do honour to the genius and abilities 
of their inventors. The late Dr. Black of Edinburgh, 
Profeffor Wilcke of Stockholm, Dr. Irwine of Glalgow, 
Dr. Crawford of London, are among the leading names 
of philofopliers who invented and illuftrated this excel¬ 
lent theory; and it is fincerely to be vriihed, that fome 
cotemporary writer would fettle their reipedtive claims 
before the lapl'e of time fliall have rendered it difficult. 
There are two opinions concerning the change of tem¬ 
perature which bodies undergo when they change their 
llate or their mode of combination. By luch changes it 
is found that heat is either ablorbed or given out; or to 
fpeak more unexceptionably, the alterations of tempera¬ 
ture are either lets or greater than would have been in¬ 
ferred from general reafoning. Some philofophers lay, 
that the capacities of the bodies are changed, and there¬ 
fore require more or lefs heat to occafion limilar muta¬ 
tions of temperatuie than they did befoie ; others affirm, 
that the heat which dilappears or appears has no relation 
to the capacity, but is either received in combination, as 
a principle of bodies, or given out as luch. 1 hefe poii- 
tions are not matter of opinion, but relate to fafts, about 
which philofophers will acquire more knowledge by ex¬ 
periment than by realoning. If the natural zero be de¬ 
termined truly by Dr. Ii wine’s theorem, noted above, 
and the capacities of various bodies in their ftates of io- 
lidity and fluidity, be found from direft experiment, the 
corollary to that theorem wiil give the number of degrees 
the fluid would be railed by heat that would Amply melt 
the folid. If this deduftion (hould be found in all cales 
to agree with the fads, the former opinion is true ; but 
if not, there is a portion of heat not accounted for, which, 
if heat be matter, may probably be a princjple of bodies. 
There are two methods of determining the capacities 
of bodies for caloric. The one confifts in mixing toge¬ 
ther equal quantities of different bodies at different tem¬ 
peratures, and oblerving the temperature of the mixture 
as foon as it becomes uniform. The capacities of the 
bodies are in this cale inverfely, as the changes of tem¬ 
perature. The other method coniifts in railing the tem¬ 
perature of different bodies to,the fame degree, enclofing 
equal quantities of them feparately in a lphere of ice, 
and colleding the quantity of water Whidi is obtained 
from the thawing of the internal furface of the ice, while 
