686 
Effects of of so subtle a nature. It has, however, been attempt- 
Heat. ed by Crawford, and prosecuted by him with much 
perseverance and ingenuity, and made the foundation 
of some very important doctrines, both in chemistry 
and physiology. It must indeed be confessed, that 
the extreme nicety of the process necessarily makes the 
conclusions liable to much uncertainty; yet the general 
results have been, for the most part, acquiesced in, al- 
though they have been considerably altered since they 
were originally brought forward. Of late, however, 
they have been controverted by MM. Delaroche and 
Delaroche Berard, who have published a very elaborate set of ex- 
and Be- eriments, which they performed upon the specific 
rard’s expe- heat of the gases, in which they employed an apparatus 
munents. = of a new construction. It is in fact a calorimeter ; but 
instead of measuring the heat which is extricated by 
Crawford's 
experi- 
ments, 
PLATE the quantity of ice which it will melt, they endeavour 
Fig. 12, 13, #0 ascertain what quantity of heat the gas will impart 
14, & | toa mass of water; and for this purpose a current of 
the gas is sent along a serpentine tube, which traverses 
a cylinder of water. The instrument appears to us to 
be very complicated, and to require a great share of 
manual dexterity in the operator, a circumstance which 
we always consider as very objectionable: (Ann. Chim. 
Ixxxv. 72.) But although their results differ considerably 
from those of Crawford, they e with him in the 
general principle, that the specific heat of the gases is 
different from each other, whether we attend to their vo- 
lumes or their weights, and that there is no relation be« 
tween the specific gravity of the gases and their specific 
heat. See Description of Plates at the end of th@ Volume. 
Capacity of — Besides the difference in their capacities for heat, 
—, which gases possess in consequence of a difference in 
i fs “" their nature, this property is also much affected by the 
degree of compression which they experience, or is in 
proportion to their density, It is consequently ob- 
served, both in experiments performed for the purpose, 
and in many natural processes, that the sudden con« 
densation of air generates heat; and, on the contrary, 
Heat ang ‘at sudden dilation produces cold. ~ Whenever an ex- 
cold by con- Pansion of air takes place, either from the removal of 
densing and Pressure, or from any other cause, the’ particles are 
rarefying more widely separated from each other, in consequence 
air, of their elasticity being now at liberty to exert itself’; 
and it would appear, that a quantity of caloric neces- 
sarily rushes in to supply the vacuity. This, although 
merely a mechanical view of the subject, appears to be 
rendered probable by the fact, that the emission and 
absorption of heat are respectively produced by causes, 
which can only be supposed to act, by bringing the 
oe of the gas nearer together, or removing them 
arther asunder. If a thermometer be placed in the 
receiver of an air pump, and the air be quickly ex- 
hausted, the mercury will sink several degrees; and if, 
on the contrary, air be rapidly condensed by the ap- 
propriate apparatus, the mercury is considerably ele- 
vated. (Dalton in Manch. Mem. vol. v.p.515,) The heat 
which is excited by condensing the atmosphere in the 
barrel of an air-gun is well known; and by employing 
it in the most advantageous manner, sufficient heat 
may be extricated to set fire to a piece of tinder. Ifa 
art of the apparatus be composed of glass, a flash of 
ight is perceptible, when the stroke of the piston is 
made with great force and rapidity. The effect of the 
rarefaction of air in promoting the absorption of heat, is 
Leslie's me- wel] illustrated in Professor Leslie's new method of form= 
Peat va ice. NG lee. (See our article Coxn, for a full account of this 
&*** process.) In this process, water is placed in a shallow 
dish over a vessel containing sulphuric acid ; the whole 
being enclosed in the receiver of an air-pump, and a 
HEAT. - 7 + 
vacuum produced. As the exhaustion proceeds, the 
quantity of heat abstracted from the water, im order to 
supply the evaporation, quickly converts the whole 
into ice. The sulphuric acid, in this case, promotes 
the effect, by its attraction for the aqueous vapour, 
which it absorbs as fast as it is grosested. A still 
more striking effect of evaporation been exhibited 
by Dr Marcet. He enclosed a quantity of mercury in Marcet’ 
a bulb of thin glass; this was wrapped up in cotton, Pe 
which was soaked in the very evaporable fluid, the sul- 
phuret of carbon ; the apparatus was then placed un- 
der the receiver of an air-pump, and the exhaustion 
produced, when the me was completely frozen : 
(Phil. Trans. 1818, 252.) . The ii ious i ent Wollas 
invented by Dr Wollaston, which he called the cryo- cryoph 
phorus, acts upon the same principle, It consists of a 
glass tube of some length, each extremity of which ter- paz 
minates in a globe about one inch in diameter, and ccuxxxvm 
bent at a right angle to the tube; one of the globes is Fig. 10. 
half filled with water, while all the remainder of the 
bonnet carefully exhausted of air. If the empty 
globe be plunged into a freezing mixture, the 
vapour with which it is filled is so rapidly condeaea, 
that a portion of the water in the second globe is im- 
mediately frozen. Phil. Trans. 1813, 71. b 
The met va’ i such as the steam of eve Watt on th 
are more expansible é permanent gases; and it capacity of 
has Steueatdas leit conceived, that they will acquire a ‘4% 
— degree of heat when they are permitted toen. 
ge themselves, by removing from them the pressure 
of the atmosphere. Mr Watt proved that this was the 
case, in some experiments which ‘he performed on the 
distillation of fluids in vacuo; a process which has been 
recommended as an economical project, in consequence 
of the smaller quantity of caloric which is necessary, 
under these circumstances, to convert the liquid into a 
vapour. But this he found was counterbalanced by the 
greater capacity for heat of the vapour, when in its 
most rarefied state. By taking off a considerable 
of the atmospherical pressure, water could be dist 
at a temperature of 100°; but its latent heat then ap- 
peared to be 1048°, instead of 840°, the latent heat of 
steam generated in the atmosphere. With the most 
perfect vacuum that could be formed, water may be 
distilled at 78°; but then the latent heat is still farther 
increased, so as to be about 1300°. Mr Watt estimated, 
that when water is converted into a vapour, under the 
ordinary pressure of the atmosphere, it is expanded ins 
to about 1800 times its former bulk. Phil. Trans. 1784, 
335 ; and Black’s Lectures, vol. i. p. 190. : 
Dr Crawford, from. his ag TE on the specific 
heat of bodies, was led to form the conclusion, that 
while their state remains unchanged, their specific heat 
continues to bear the same relation to their absolute 
heat. Taking water as the standard of comparison, if 
we can discover what proportion the specific heat of 
this fluid bears to mercury at the usual temperature of 
the atmosphere, we may suppose that the same propor- 
tion will exist atall temperatures. Upon this principle as 
was founded the proposal of Irvine, for ascertaining the the ca 
real zero, or that point of the thermometric BURR sera. 
which, if it be supposed to be sufficiently extended, bo- _ 
dies would be deprived of the whole of their heat. Thus 
it had been found by experiment, that the capacity of 
ice is to that of water as 9 to 10, and the actual “you 
tity of latent heat in water was estimated at 1400; 
therefore ten times 140°, or 1400° below the freezing 
point, was supposed to be the temperature at which wa- 
ter is absolutely deprived of all heat. (Nicholson’s Che- 
mistry, p. 16; Crawford On An, Heat, p. 435.) The 
