"i 
: ford’s 
_o of a liqui 
— Dea bar exact 
tent 18 rendered latent in t ites a which he was 
‘ att 
acity for | 
HEAT. 
-gonverted into a liquid, he afterwards made on the con- 
into an elastic fluid. He also at- 
of heat which 
essentially aided by Mr 3 and the result oftheir 
“in ay was, that when water assumes the state of va- 
our, it absorbs 950° of caloric. * This number he ob- 
tained in two ways: Ist, By comparing the heat ne- 
cessary to yaise the temperature of a certain por- 
tion of water to the boiling point, with the effect 
produced by an equal addition of heat in after- 
wards Ae ee the water; and, 2dly, By finding 
what quantity of caloric was extricated, when steam 
ted into water by condensation. We have 
already mentioned, that when water is strongly com- 
pressed, as by being inclosed in Papin’s digester, its 
temperature may be raised far above the boiling point, 
without its assuming the aeriform state. In this case 
its tendency to evaporation is mechanically prevented, 
by the particles not being allowed to separate from each 
other ; and therefore as it cannot alter its form, its ca- 
pacity remains the same, and its caloric all continues 
to be in the uncombined state. But if the pressure be 
suddenly removed from the water, its particles now 
being at liberty to expand themselves, they unite to a 
ion of the heat, instantly assume the elastic state, 
and the remainder of the fluid sinks to 212°. 
» We have already explained, in a general way, the 
meaning of the term capacity for heat, and the differ- 
ence between the absolute and the specific heat of bo- 
dies ; but we must illustrate the subject with a little 
more minuteness. As a foundation for our reasoning, 
it may be assumed, that when equal quantities of the 
same substance, but at different temperatures, are mixed 
es the temperature of the mixture indicates that 
the arithmetical mean of the two ingredients. This, 
however, only applies to bodies of the same kind ; for 
when different substances are employed, it is impossible 
‘to predict what will be the temperature of the-mixture. 
Thus if a pound of water and a pound of mercury be 
mixed at different temperatures, the result will not 
be the mean temperature ; but it will be found that the 
mercury loses 28°, while the water gains enly 1°. 
Water is therefore said to have 28 times the capacity 
for heat that mercury has, because it requires 28 times 
. «as much to produce the same change of temperature ; 
or, to use a different, and perhaps a more correct phra- 
seology, we say that the specific caloric of water is to 
that of mercury as 28 to 1. Proceeding upon this prin- 
ciple, and taking water as a'standard of comparison, nu- 
merous experiments have been made on the specific 
heat of various substances, particularly by Crawford, 
Irvine, and Wilcke. 
The method employed by Crawford was to mix to- 
gether, at the same temperature, the substance to be 
examined and water, the specific heat of which is con- 
sidered as one, being that with which all the rest are 
compared. He then multiplied the weight of each 
body by the number of degrees between its original 
temperature, and the common temperature obtained by 
their mixture, and the capacities will be inversely as 
the products: (Ox Animal Heat, 2d edit. p. 96, et 
seq.) It isgenerally more convenient to employ a de- 
finite weight of the substance to be examined, than to 
measure it by. its volume; but when we examine the 
ific heat of equal volumes of different bodies, we 
find the same want of correspondence between the re- 
.* The late Mr Southern found, from numerous experiments, that the latent heat of a y 
295°, is 919°, 942%, and 950°. See Mr Watt’s Annotations on Dr Robison’s article Sream in Robison’s System 
sophy, vol. ij. p» 166.—Ep. 
685 
sults, so as to prove that the capacity for heat is a 
pro which bears no exact ratio to any other phy- 
sical quality. In the performance of these experiments, 
much dexterity is requisite, in order to ensure even a 
tolerable degree of accuracy; and after all, the results 
of different trials; made upon the same substance, will 
not always be found to correspond, yet there seenis to 
be no doubt of the correctness of the principle, and, in 
many cases, we may conceive that we arrive at a near a 
proximation to the truth. SeeCuemistry, vol. vi. p. 153. 
_ Mr Leslie has proposed another method of ascertain- 
ing the specific heat of bodies ; it consists in comparin 
the time which they occupy in cooling with the cool- 
ing of water placed under precisely the same circum- 
stances, The apparatus which he employed was a thin 
glass globe, furnished with a neck capable of receiving 
a delicate thermometer. He observed the tiie neces 
sary for water to cool a.certain number of de in 
this globe suspended ini the air, then he fills the instru- 
ment with another fluidyithe specific heat of which he 
wished to examine, and heats it to the same degree, and 
suspends it in air of the same temperature, and observes 
the time necessary for it to cool the same number of 
degrees. It was found, that, under the same cireum- 
stances, water cooled in 70 minutes, while oil required 
82 minutes only ; therefore, by estimating water at uni- 
ty, as the standard of comparison, ‘the specific heat of 
oil will be .46; if we estimate it by bulk, or .5 if we 
estimate it by weight. Inquiry, p. 170. 
Effects of 
Heat. 
—_——— 
Leslie's ex« 
periments. 
The great difficulty there is. in executing experi- isi 
ments, like those of Crawford’s, where the sebatantey ene 
are mixed together, and the specific heat calculated 
from the temperature of the mixture, induced Lavoisier 
-and Laplace to attempt to solve the problem by a new 
method. They proceeded upon the fact, that when 
ice is melted it must always absorb the same quantity 
of heat, and therefore that the» caloric which is disen- 
gaged from any botly, by a-change in its form,,or from 
the union of two or more bodies, may be accurately 
measured, by placing the substance in such a situation 
as that all the heat must necessarily pass into a stratum 
of ice; and from the amount.of water produced by the 
melting of the ice, the quantity of heat given out may 
be estimated. The apparatus. which they invented to 
accomplish this purpose, they called a calorimeter ; it 
consists of three, vessels inclosed one within another, 
so as to leave a cavity between each of them. Thesub- 
stance which is to be the subject of experiment is pla- 
ced in the innermost vessel ; the second is filled with 
pounded ice, and is proyided with a tube at the bottom 
through which all the water that is formed is conduct- 
ed into a suitable vessel, where it may be collected and 
measured. The outermost space is also filled with ice, 
in order to preserve the interior of the apparatus at 
the proper temperature: (Mem. Acad. Scien. 1780, P 
368.) This instrument is constructed upon scienti 
principles, and seems both simple and ingenious; yet 
we are informed that there are some circumstan- 
ces which interfere with its practical utility, and we 
believe it has never been employed except by the in- 
ventors. See Wedgewood in Phil. T'rans. 1784, p. 
371, et seq. ; and Cuemistry, vol, vi, p. 154. 
The capacity of gaseous bodies for heat, like that of 
liquids and solids, varies according to the nature of the 
individual gas ; but the determination of their respec- 
tive capacities requires a delicacy of experiment, pro- 
portionate to the difficulty of operating upon substances 
steam at the temperatures 229°, 270°, and 
of Mechanical Philo- 
place’s ex 
periments. 
Calorime- 
ter. 
PLATE 
cxLt 
Pig. 7. 
Capacity of 
the gases, 
