COLCHIS. 



731 



war, the Roman general raised one Aristarchus to the 

 throne of Colchis. His reign, however, seems to have 

 been but short ; for Pharnaces II. son of Mithridates, 

 soon after subdued the kingdom of Colchis, whilst 

 Csesar was wasting his time in the embraces of Cleopa- 

 tra ; but when that leader again returned to the field, the 

 conqueror of Colchis lost the fruits of his victory. Our 

 attentiou is not again called to the Colchians, till the 

 reign of Trajan, to whom they willingly submitted. It 



is probable, that the king which they had at this time, 

 was allowed to reign in subjection to the Romans ; but 

 the country was soon deprived of this shadow of royalty, 

 and without being even formed into a province by any of 

 the emperors, it fell under the jurisdiction of the Praetors 

 of Bithynia and Pontus. We shall only add, that Col- 

 chis shared in the fortunes of the eastern empire, when it 

 was overturned by its barbarous invaders, and now forms 

 a province of Asiatic Turkey. (N) 



Colchis. 



COLD. 



Cold. 



production 

 of cold by 

 rarelac- 



tiuu. 



Production 

 of cold by 

 rvapora. 

 liva. 



V^oLD, is a peculiar sensation excited in animals, when 

 substances at an inferior temperature are applied to their 

 organs of feeK:ig. It is also used to denote a certain 

 pnnciple or power residing >n bodies, by the operation 

 of which the sensation is produced. It is still doubtful, 

 whether this pri: ciple cught to be regarded as a distinct 

 condition of matter, or as a mere modification of caloric ; 

 th< iv h we are disposed to consider it in the latter point 

 of view. 



The means, with which we are acquainted, of reducing 

 the temperature of bodies, are much mure limited in their 

 effects, than those of increasing it. But many import- 

 ant facts connected with the production of cold have al- 

 ready been discovered ; and though we can scarcely ex- 

 pect ever to acquire the same extensive command 'jver 

 the lower, as we already possess over the higher tempera- 

 tures, we have reason to hope, that since w<- k'.ow the 

 circumstances upon which refrigeration depends, pro 

 cesses of cool.ng may yet be discovered still more effec- 

 tual than any that have lutlierto been employed. 



The various methods of producing cold, which are at 

 present known, may all be referred to Rarefaction. Eva 

 porztion, and Liquefaction, induced by chemical action. 

 We shall consider, in oider, each ot these methods of re- 

 ducing the temperature 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 perceptibly dimi 

 Dished. Thus, a thermometer exposed to the stream of 

 air which issues from an air gun, lias it temperature re- 

 duced 5 or 6 degrees ; and a similar effect is produced 

 when it is inclosed in the receivirof an air pump, during 

 the process of exhaustion. The diminution of tempera- 

 ture by rarefaction, is welt exemplified by the celebrated 

 fountain of Hiero, at the mines of Cjiemnitz in Hungry, 

 where the air in a large receiver is compressed by a column 

 of water, equal to the weight ot -about 8 atmospheres. 

 On turning a stop-cock, the air suddenly makes its escape, 

 and during its expansion, deposits the watery vap.mr 

 which it held in solution, in the form of snow, or covers 

 the inside of the tube through which it rushes, with slrn 

 dcr spiculae of ice. In most case-, how. ver, this method 

 of producing cold is very limited, and scarcely capabl'- of 

 being applied to practical purposes. 



Evaporation is of more extensive mfluei.ce in reducing 

 the temperature of bodies, than ran/faction. Every person 

 must have felt, that when the hand has been dipt in wa- 

 ter and then exposed to a current of ir, tne parts which 

 had been moistened soon became much c< Ider than the 

 parts which had been allowed to remain dry, though the 

 cooling medium is the same. D-- Cu ten appears to have 

 been the first who ascribed the d ffen :.te of . ffe..t to the 

 evaporation of the fluid. He observed, that a thermo- 



meter, 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 j 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 sur- 

 rounded with a piece of linen well moistened with ether, 

 and afterwards exposed to a free current of air, the tem- 

 perature is reduced from 50 to near zero. The effect 

 with alcohol is considerably less, and with water it is li- 

 mited to four or five degrees. Water may, in this man- 

 ner, be frozen, by pouring a small quantity of it into a 

 slender tube which is surrounded with linen moistened in 

 ether, and then swinging tlu- tube rapidly round the head 

 by means of a string attached to it. 



When water is made to ooze slowly through porous 

 unghzed earthen vessels, it presents a very extensive sur- 

 face t<< the atmosphere, and as it suffers a proportional 

 degree of evaporation, its temperature is ktpt consider- 

 ably below that of the air. If one of these porous ves- 

 sels be allow d to imbibe water for some time, it retains, 

 when the water i-. poured out of it, a considerable qtian- 

 tit) of mo:sturo ; a<id 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, hy placing the bottles which contain them, in 

 these unglazed earthen vessels, previously well moistened 

 with water. Wet cloths wrapp-.-d round bottles produce 

 the same effect. In India, where the conversion of wa- 

 ter 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 mar//. , &c. The dryness of the air causes 

 a very considerable evaporation, and the consequent re- 

 duction ot temperature is such, that a thin film of ice 

 is formed towards the morning, though the tempera- 

 ture of the air may have been all the time above the free- 

 zing 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. 



A:: liquids assume the state of vapour, at a very redu- 

 ced te uperature, when the pressure of the atmosphere is 

 removed from their surface, it might be expected, that a 

 more rapid as well as a greater production of cold would 

 be obtained by inclosing .hem in the receiver of an air- 

 pump, and then exhausting the air. Accordingly, if a 

 small tube, about a quarter of an inch in diameter, and 

 four or iive inches long, be nearly filled with water, and 



Freezing of 

 water by 

 the evapo. 

 ration of 

 ether in the . 

 open air* 



Method of 

 procuring 

 ice in lu- 

 dia. 



Freezing of 

 water by 

 the evapo* 

 ration of 

 ether un- 

 der an ex- 

 hausted re. 

 crivcr. 



