COLD, is a peculiar sensation excited in animals, when substances at an inferior temperature are applied to their organs of feeling. It is also used to denote a certain principle or power residing in bodies, by the operation of which the sensation is produced. It is still doubtful, whether this principle ought to be regarded as a distinct condition of matter, or as a mere modification of caloric ; though we are disposed to consider it in the latter point of view.
The means, with which we are acquainted, of reduc ing the temperature of bodies, arc much more limited in their effects, than those of increasing it But many im portant facts connected with the production of cold have already been discovered ; and though we can scarcely expect ever to acquire the same extensive command over the lower, as we already possess over the higher zemperatures, we have reason to hope, that since we know the circumstances upon which refrigeration de pends, processes of cooling may yet be discovered still stoic effectual than any that have hitherto been em Illoyed.
The various methods of producing cold, which are at present known, may all be referred to Rarefaction, Eva poration, and Liquefaction, induced by chemical action. We consider, in order, each of these methods of reducin;; tic temperature of bodies.
If an erial fluid be suddenly enlarged in volume, by the removal of any mechanical pressure to which it may have been suLiected, its temperature is perceptibly dimi nished. Thus, g 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 i when it is in the receiver of an air pump, during the process of exhaustisn. The diminution of tempera ture by rarefaction, is well exemplified by the celebrated fountain of Hiero, at the mints 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 atmos pheres. On turning a stop-cock, the sir suddenly 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 spicuLz of ice. In most cases, however, this method of producing cold is very limited, and scarce ly capable of being applied to practical purposes.
Evaporation is of more extensive influence in reduc ing 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 became much colder than the parts which had been allowed to remain dry, though the cooling medium is the same. Dr Cullen appears to
have been who ascribed the difference of effect to the evaporation of the fluid. He observed, that a ther mometer, 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 instrument be surrounded with a piece of linen well moistened 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 considerably 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 earthen vessels, it presents a very extensive surface to the atmosphere, and as it suffers a proportional degree of evaporation, its temperature is kept consider ably below that of the air. If one of these porous ves sels be allowed to imbibe water for some time, it retains, when the water is poured out of it, a considerable quan tity of moisture ; and as it now presents both an internal and an external surface to the air, the evaporation 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. Accordingly, liquors are very conveniently cooled, by placing the bottles which contain them in these unglazed earthen vessels, previously well moistened with water. Wet cloths wrapped round bot tles 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 arc placed upon the worst conductors that can be procured, such as sugar canes, dried stems of maize, &c. The dryness of the air causes a very considerable evaporation, and the consequent reduction 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 freezing 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.