684 
Effects of two following, as those the most deserving our atten- 
Heat. 
Capaci 
for aed 
Specific 
heat. 
tion : 
1. When solids become liquid, and liquids become 
elastic fluids, a quantity of heat, previously latent, be- 
comes sensible, and may be now measured by the ther- 
mometer, although before it was not capable of affecting 
this instrument. On the contrary, when vapours are 
converted into liquids, or liquids into solids, a portion 
of heat, which was previously in the uncombined state, 
is now absorbed or rendered latent. In the former 
process, therefore, warmth is produced; in the latter, 
cold is generated. 
2. Substances which are at the same temperature ac- 
tually contain different quantities of heat. Ice just be- 
fore liquefaction, and the water which is formed, are 
both at the temperature of 32°, and steam is no hotter 
than the boiling water from which it is produced ; so 
that the water and the steam both contain a large quan- 
tity of heat, in the combined or latent state, which they 
have absorbed in order to cause the fusion of the one, 
and the vaporization of the other. This property in 
bodies, of indicating the same temperature when the 
contain different quantities of heat, is denominated their 
capacity for heat ; while the quantity of heat which they 
require to bring them to the same temperature is called 
their specific heat, in opposition to the real quantity, 
which is stiled their absolute heat. 
HEAT. 
ably below that of the by min Effe 
cu aes eae elas 
tinually going forwards; and by the 4 
ment of this process, even ice is fe ins : 
quantity, for the purposes of domestic economy, at a 
temperature some degrees above the ‘point. 
By means of the evaporation of ether, water is easily 
frozen at the medium temperature of our climate, and’ 
indeed a degree of cold may be ted far below that ° 
of the pane 2 A still greater diminution of tem- 
perature is produced by a solution of certain salts ;* a cir-: 
cumstance which depends upon the heat that is 
pre pteoorsacrd eer conversion of a solid into a lix 
uid, an ly by mixture possessing a greater 
depacity for heat than the ingredients in their ee 
state. To these two circumstances, the change of state 
which bodies ex see and ers in their ca-" 
ity, as d ing either upon this alteration in their 
Rates ob Sabah? the ers cornbinations into which they - 
enter, we ascribe the increase of tem re which oc- 
curs in a variety of chemical operations, where caloric 
is not introduced from any external source. The heat, 
which is excited by combustion, is to be attributed to 
the latter cause, the of capacity in the substan< 
ces; the mcrae of combustion, cco gr the car« 
bonic acid, being supposed to possess a capacity 
than the carbon Sod aged before their combination. 
It would carry us far beyond our prescribed limits, 
were we to give an account of all the experiments 
which were performed by Black, for the purpose of 
establishing his theory, and of repelling the objections 
- The cases in which the conversion of a body from eat pr 
the liquid to the solid form actually produces an in- duced 
crease of temperature, are not very numerous ; but there solidif 
are not unfrequent instances in which the rate of cool- 4" 
€old produ. 
against it, They consisted, in the first place, in com- 
paring the effect produced by the same quantity of heat 
upon bodies which possessed different capacities, as ice 
and water. When the same weight of ice and water, 
both at the temperature of 32°, were suspended in an 
atmosphere of 47°, the water rose to 39° in 30 minutes, 
while the ice required ten hours to become liquid, and 
to acquire the same temperature. The ice therefore was 
21 times as long as the water in acquiring the same de- 
gree of warmth, and we may conclude that it would 
absorb 21 times as much caloric. While the number of 
degrees gained by the water was no more than seven, 
the number that entered the ice would be 147° ; but of 
these only seven were employed in warming the sub- 
stance, the rest was expended in melting it. Hence we 
should say, that when ice is converted into water, 140° 
of caloric are rendered latent. The same general con- 
clusion was obtained by a different process, When two 
equal portions of water, at unequal temperatures, are 
mixed together, the mixture indicates the mean tem- 
perature of the two portions; thus a pound of water 
at 32°, and a pound at 172°, produce a mixture which 
indicates 102°; but if we mix a pound of ice at 32°, 
with a pound of water at 172°, the ice is melted, but 
the temperature of the fluid is not raised. Hence the 
- 140° of heat are all employed in thawing the ice, and 
thus becomes latent. 
The absorption of heat that. takes place when solids 
ced by eva- are melted, or liquids evaporated, and the discharge of 
porstion, 
heat that occurs in the reverse operations, are often 
employed in different processes, for the purpose of cool- 
ing or warming bodies. The effect of evaporation in 
generating cold is well known, both as it affects the 
sensations, and as it actually reduces the temperature 
of substances. In hot climates, fluids and various ar- 
ticles of diet are preserved at a temperature consider- 
ing is obviously retarded by this process. And in no 
operation is this more remarkable than in the natural 
freezing of water. When water is exposed to the in- 
fluence of external cold, its temperature sinks in 
portion as the heat is removed from it, until it arrives 
at 32°; the refrigeration is then suspended, but the 
water begins to become solid, and the temperature re- 
mains stationary, until the freezing is completed ; then 
the cooling recommences, and is continued until the 
ice arrives at the temperature of the surrounding me- 
dium. The freezing of water, however, under icus 
lar circumstances, affords an example of the actual evos 
lution ef heat. . This takes place in that case which has 
been described above, where water has been cooled be-~ 
low 82° without becoming solid ; if it be then agitated, 
or a small spicula of ice be introduced into it, the 
eongelation takes place with great rapidity, and the 
portion of water which still remains fluid instantly 
rises to 82° An experiment of an analogous kind 
may be performed on the crystallization of salts. By 
proper management, a warm saturated solution of the 
sulphate of soda may be cooled for several degrees, 
without any of it beginning to crystallize ; if the vessel 
be then slightly shaken, the certo suddenly com- 
mences, pa om large part of the salt immediately be- 
comes solid, in consequence of which the temperature 
will be very sensibly raised... The general law, both in 
this and the reverse operation, is, that the most power~ 
ful effect, either of heating or cooling, takes place when 
the process is performed with the most rapidity, when 
the substance that has its state or its capacity cha 
has its equilibrium with the neighbouring. bodies: re- 
stored in a short space of time, and therefore most per- 
ceptibly affects their temperature. 
The same kind of experiments which Black perform- 
ed to establish the absorption of heat, when a solid is 
* See Walker's experiments on freezing mixtures, Phil. Trans, 1801, p. 1205 end our article Coxn, vol. yi. p» 733, 734. 
