C A L

CAL

( 210 )

eloth is fpread out in a keen wind at a tempera-ture a little above freezing, the water by rapidlyevaporating foon cools the damp cloth belowthe freezing point, and makes it hard and ftiffwith the formation of ice within its pores. Butthe cold produced by evaporating ether is ftillmore linking. A little of this Angularly eva-porable fluid poured on the back of the handflies off immediately, and leaves the hand ex-ceffively cold. If a thermometer bulb is keptconftantly damp with ether, the mercury finksrapidly many degrees below 3 2°, and if theevaporation is affifted by agitation, or by avery ftrong draught of air, the mercury may belowered to a much greater cold than any naturalto this climate, except in very rare inftances.This is readily performed in a few minutes witha fmall delicate mercurial thermometer: if thebulb be wrapped round with a little tow, foakedby once dipping it in ether, and blown uponwith a hand bellows held a little diftance off,it will then fink as low as o°, or even a degreeor two lower.

The evaporation of all fluids is regulated asmuch by the denfity of the air with which theyare in contact as by the degree in which theyare heated, and in a dire£t ratio to each other;fo that the denfer the atmofphere prefling onthem, and the higher is the evaporable or theboiling heat required. Hence it follows thatby diminilhing the preffure, liquids will boil ata much lower temperature than their naturalpoint of ebullition. Thus it is found that atthe top of high mountains liquids boil muchfooner than at the bottom. Sauflure found thatwater boiled at the top of Mont Blanc at 187°Fahr. inftead of 212°, and other liquids in pro-portion. The fame effect is produced artificiallyby inclofing the liquid, already heated to a cer-tain point, under the receiver of an air-pumpand exhaufting the air. If water fcalding hot beput in this fituation, a few ftrokes of the pumpwill make it boil vehemently, but this lafts onlya fhort time, on account of the cold producedby the evaporation, which lowers the tempera-ture immediately. A further exhauftion renewsthe ebullition, a lefs heat being now required,till the cold of the evaporation has again loweredthe temperature ; and thus the alternate boilingand cooling may be renewed for a confiderabletime.

Ether, which evaporates fo readily under thecommon atmofpherical preffure and temperature,and which boils at about 102° In the open air,is brought to ebullition at a low heat, fimply byremoving the preffure, and the cold thus gene-

rated (as already mentioned) is very intenfe.This furnifhes a very pretty experiment. Put alittle ether (well rectified by wafliing with water)into a thin vefiel of glafs or metal, inclofed inanother vefiel, with a little water interpofedbetween the two. Set the whole under the'receiver of an air-pump, and exhauft the airbrifkly. The ether will very foon begin to boilvehemently and rapidly evaporate, and the coldthereby produced will in a few minutes freeze -the water on the outfide of the inner vefiel, and'may be taken out as a coating of ice adhering"to it. As fome of the ether fpurts out duringthe ebullition, it fhould be prevented from mix-ing with the water on the outfide, which wouldretard its congelation.

Advantage is taken of the cold produced byevaporation to manufacture ice in the tropicalclimates, where the natural temperature is neverfo low as 3 2°. At Benares, in the Eaft Indies,very porous earthen veffels are employed, andthe water conftantly and flowly oozing throughthem produces by its evaporation fufficient coldin the night to form a thin coating of ice. (Seethe article Freezing.) In Spain , and many ofthe Mediterranean countries, water is kept con-ftantly cool by being kept in porous earthenjarS, called in Spain Alcarazzas, and in EgyptBalaJJis , which furnifh a conftant evaporationfrom their furface. Bottles of wine and otherliquors are cooled by being fet in thefe jars withwater interpofed.

The evaporation conftantly going on from thefurface of animal bodies in the form of infenfibleor fenfible perfpiration, is the principal coolingprocefs to check the accumulation of heat in thefyftem, and it is well known by the feelings ofevery one how much this evaporation is en-creafed (in the healthy body) by external tem-perature, exercife, and other circumflances thatencreafe the production of heat. The remark-able experiments of Sir C. Blagden, Dr. Fordyce,and others, in an exceffively heated room, {hewthat the living animal can exift in a dry heat evenabove 212°, that of boiling water, and that theinternal temperature remains notwithftandingnearly at its natural point of about 98°, theenormous excefs of external temperature beingcounteracted by the profufe perfpiration. (Seethe article Respiration .)

The conftitution of fome of the permanentgaffes, or thofe that remain in a gafeous ftatein the temperature of the atmofphere, {hewsthe combination of latent caloric neceflary tothe gafeous ftate, though not in fo marked amanner as the vapours. Several of the gafes