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adopted to exprefs abfolute quantities of It. Thishas been done either by the thermometer, or bythe thawing of a giyen weight of ice.

Of Liquefaction and Congelation.

The oppoiite hates of Liquefaction and Con-gelation may be conveniently defcribed underthe fame head, as the circumftances attendingthe one will illuftrate the other, after which thefimilarly contra (led phenomena of evaporationand condenfation of vapour will be noticed.

During the liquefaction of any fubltance bymere heat, it has been already mentioned thata large quantity of heat is abforbed by the melt-ing fubltance which is eflential to the con-ftitution of the hate of liquidity, and does notraife the thermometrical temperature. Con-fequently if a folid fubltance, cold, and the famelubltance in a liquid ftate, and hot, be mixed inknown proportions, the temperature after mix-ture will not be the proportional mean of thetemperature of each, as would be the cafe ifboth were liquid, but will fall far Ihort of it: agreat portion of the heat of the hotter bodybeing taken up with melting the colder folidbody before its temperature can be raifed a Angledegree. Thus, equal parts of water at 32 0 , andwater at 212°, will on mixture give the tem-perature of 122 0 , or the arithmetical mean be-tween thefe two numbers; but-equal parts ofice at 3 2° and water at 212°, will give only a tem-perature of about 5 2°, the greater portion of theheat of the water being firlt employed in thaw-ing the ice before any rife of temperature what-ever can take place in the mixture.

Dr. Black’s original experiments * on theabforption of heat during the procefs of lique-faction may here be fhortly mentioned, onaccount of their elegant fimplicity, as they fervedas ground-works of his beautiful fyftem.

Two equal globular glafs veflels were filled,the one with five ounces of water, which wasafterwards frozen by furrounding it with fait andfnow, and the other with water limply cooleddown to 33°. That which contained the icewas allowed to remain in a warmer air till theice juft began to melt, at which time its tem-perature was as nearly as poflible 32 0 or 33 0 .The two veflels were then limply fufpendedfrom a wire frame fet in the middle of a largeroom without a lire or any notable draught ofair, and of the average temperature of 47 0 . The•veffels were therefore in every refpeft undernmilar circumftances of temperature within andwithout, and with fimilar contents, exceptingthat one contained ice and the other water. The

* Leisures on the E

progrefs of heating was then noted. In thatwhich held the water the thermometer rofe inhalf an hour to 40°; but in the other it took nolefs than 10 hours and a half before all the icewas melted, and the contents had reached to 40°.Confequently the rate of heating was 21 timesflower in the ice-veflel than in that which heldthe cold water; but the actual heat received bvthe former was nearly uniform the whole time,and therefore it required about 21 times asmuch heat to bring ice to the ftate of water andto raife it to 40°, as to raife ice-cold water tothe fame point. Or, to exprefs it by anotherform, if the water had continued to receiveas great an influx of heat as the ice, duringthe whole 21 half-hours, its temperature wouldof courfe have been raifed 21 times as much asduring the Angle half-hour, and as this was 7degrees, the whole would have been 21 X7=t47degrees. The difference between the gain oftemperature in the ice and the water with equalacceflions of heat, is about 140 degrees; andtherefore 140° is the expreflion of that quan-tity of thermometric temperature which isdeftroyed, or made latent , by the feparate ope-ration of the liquefaction of ice, the actualquantities of matter in the companion beingequal.

This calculation was confirmed by the experi-ment of melting ice in hot water, and com-paring the temperature of the mixture withwhat it would have been had ice-cold waterbeen ufed inftead of ice. When quantities ofwater of unequal temperature are mixed to-gether, the heat of the mixture is the exa£tmean of the difference Of temperatures and therefpeCtive quantities. In Dr. Black’s experi-ment, 119 parts of ice at 32 0 were melted by135 parts of water at 190°, to which mull beadded the heat communicated by the hot veffel,which he ellimated as equal to 8 more of thewater, making in the whole 143 parts. Nowthe gain of temperature produced by adding119 parts of water at 32 0 , to 143 parts ofwater at 190°, would have been 86 degreesabove 32 0 , or n8°of the fcale; (being found bymultiplying the difference of temperature intothe weight of the hotter water, and dividing bythe fum of the weights of both, for 1 5 44 t 4 3 =86). But the temperature relulting from themelted ice and hot water was only 21 degreesabove 32, or 53 0 of the fcale. ftherefore aquantity of heat had difappeared,. which wouldhave raifed the whole mafs (equivalent to 262parts of water) 65 degrees, being the difference

lemvnts of Chcmiftry.