| the same. 
present known, may all be referred to rarefaction, 
evaporation, and liquefaction, induced by chemi- 
cal action. We shall briefly notice, in order, 
each of these methods of reducing the tempera- 
ture of bodies. 
If an aerial fluid be suddenly enlarged in volume, 
by the removal of any mechanical pressure to which 
it may have been subjected, its temperature is per- 
ceptibly diminished. Thus, a thermometer exposed 
to the stream of air which issues from an air-gun, 
has its temperature reduced 5 or 6 degrees; and a 
similar effect is produced when it is enclosed in the 
receiver of an air-pump, during the process of ex- 
haustion. The diminution of temperature by rarefac- 
tion, is well exemplified by the celebrated fountain 
of Hiero, at the mines of Chemnitz in Hungary, 
where the air in a large receiver is compressed by a 
column of water, equal to the weight of about 8 at- 
mospheres. On turning a stop-cock, the air sud- 
denly makes its escape, and during its expansion, 
deposits the watery vapour which it held in solution, 
in the form of snow, or covers the inside of the tube 
through which it rushes, with slender spicule of ice. 
In most cases, however, this method of producing 
cold is very limited, and scarcely capable of being 
applied to practical purposes. 
Evaporation is of more extensive influence in re- 
ducing the temperature of bodies, than rarefaction. 
Every person must have felt, that when the hand 
has been dipt in water and then exposed to a current 
of air, the parts which had been moistened soon be- 
came much colder than the parts which had been 
allowed to remain dry, though the cooling medium is 
Dr. Cullen appears to have been the first 
who ascribed the difference of effect to the evapora- 
tion of the fluid. He observed, that a thermometer, 
the bulb of which had been dipt in a fluid, and then 
exposed to a current of air, always indicated, so long 
as it remained moistened, a lower temperature than 
the air itself; and that the reduction of temperature 
was greatest when the thermometer had been dipt in 
the most volatile fluids. If the bulb of the instru- 
ment be surrounded with a piece of linen well moist- 
ened with ether, and afterwards exposed to a free 
current of air, the temperature is reduced from 50° 
to near zero. ‘The effect with alcohol is consider- 
ably less, and with water it is limited to four or five 
degrees. Water may, in this manner, be frozen, by 
| pouring a small quantity of it into a slender tube 
which is surrounded with linen moistened in ether, 
| and then swinging the tube rapidly round the head 
by means of a string attached to it. When water is 
made to ooze slowly through porous unglazed earth- 
en vessels, it presents a very extensive surface to the 
atmosphere, and as it suffers a proportional degree of 
evaporation, its temperature is kept considerably be- 
low that of the air. If one of these porous vessels 
be allowed to imbibe water for some time, it retains, 
when the water is poured out of it, a considerable 
quantity of moisture; and as it now presents both an 
interval and an external surface to the air, the eva- 
poration is greatly increased. The absorption of 
caloric for the conversion of the water into vapour is 
increased in an equal degree, and consequently a 
greater reduction of temperature is obtained. Ac- 
cordingly, liquors are very conveniently cooled, by 
placing the bottles which contain them, in these un- 
glazed earthen vessels, previously well moistened 
with water. Wet cloths wrapped round bottles 
produce the same effect. In India, where the con- 
version of water into ice is the sole employment of 
many individuals, the water to be frozen is exposed 
during the night in flat unglazed earthen vessels, 
which are placed upon the worst conductors that can 
be procured, such as sugar-canes, dried stems of 
COLD. 
maize, &c. The dryness of the air causes a very 
considerable evaporation, and the consequent reduc- 
tion of temperature is such, that a thin film of ice is 
formed towards the morning, though the temperature 
of the air may have been all the time above the freez- 
ing point. The quantity of ice obtained is greater 
when the weather is warm and dry, than when it is 
moist, though in the latter case the temperature of 
the air may have been actually lower. 
The third method of producing cold which we 
proposed to consider, is liquefaction induced by 
chemical action. The solution of salts in water, 
by the transition to the fluid state, is always ac- 
companied with a considerable diminution of tem- 
perature. When nitre is added to water, at the 
ordinary temperature of the atmosphere, until the 
water is saturated, the temperature is reduced 15 or 
16 degrees; and a still greater degree of cold is ob- 
tained by the solution of muriate of ammonia. But 
of all the salts, nitrate of ammonia seems to undergo 
the greatest reduction of temperature during its solu- 
tion. Ifit be mixed, in the state of a fine powder, 
with an equal weight of water, the temperature is 
reduced from 50° to 4°. As the production of cold, 
by freezing mixtures, depends partly upon the rapi- 
dity with which the substances employed pass into 
the liquid state, and partly upon the increase of ca- 
pacity for calorie which is acquired by that transition, 
it might be expected that a greater reduction of tem- 
perature would be obtained, by dissolving salts in 
acids than in water. Accordingly, by dissolving a 
pound and a half avoirdupois of Glauber’s salts (sul- 
phate of soda) in 12 cunces of nitrous acid, previously 
diluted with 6 ounces of water, Mr. Walker obtained 
a diminution of temperature from 50° to —1°; when 
sulphuric acid, diluted in the same proportion, was 
employed, the reduction was from 50° to 5°; and 
three parts of the same salt by weight reduced to a 
fine powder, and dissolved in two parts of muriatic 
acid, sunk the thermometer from 50° to zero. The 
reduction of temperature is still greater, when dif- 
ferent salts are employed at the same time. Thus, 
nitrate of ammonia 6 parts, phosphate of soda 9 parts, 
and diluted nitrous acid 4 parts, all by weight, reduce 
This is a diminution of 71°, and is the greatest de- 
gree.of cold that can be procured by a single mixture 
yet known. Mr. Walker succeeded in freezing mer- 
cury, by cooling, in a succession of these mixtures, 
the ingredients by whose liquefaction the congelation 
was ultimately to be produced, though the tempera- 
ture of the air at the commencement of the experi- 
ment was 45°. 
tained by artificial means. The greatest cold 
that has hitherto been observed in the open air, 
does not exceed —50; and probably the extreme 
range of the inferior temperatures which occur 
in nature, is but little below that point. The 
| temperature of a particular place on the surface | 
of the earth is determined by a variety of cir- 
| cumstances, some of which are regular in their 
operation, others accidental. Among the former 
of these, may be stated, the direct influence of 
the solar rays, and the latitude; among the lat- 
ter, the winds, evaporation, and perhaps the evo- 
lution or absorption of heat by operations going 
on in the central regions. Even the causes of 
temperature which we have denominated regular, 
are not absolutely so: the heat produced by the 
\ 
the temperature of the mixture from 50° to —21°. | 
The degrees of natural cold which occur in the | 
more inhospitable regions of the globe, are far | 
surpassed by the reductions of temperature ob- | 
841 
