CAL

( 215 ) CAL

preflure as that above. After a while the cockwas fuddenly opened, the excefs of air rufhedout, and the mercury again rofe in a few fecondsto its former level, and the cock was imme-diately clofed. The preflure from the conden-fation was therefore removed, but the coldthereby produced dill remained in the air of thereceiver, though, for the reafons above men-tioned, it was counteracted in the air of thetube by the heat it would inftantly abforb fromthe glafs itfelf. The column of mercury thenbegan again to fink down the tube (the cockbeing all the while clofed) and became ftationarywhen it had fhortened of the length of the aerialcolumn by T 'o- This was obvioully owing tothe expansion and preflure of the air in the bodyof the receiver by the reftoration of the equili-' brium of heat through the Tides of the veil'd,which added in the fame manner as- the firfl:condenfer, and accordingly, when the cock wasagain opened, air ruflied cut of the receiver, andthe mercury finally rofe to its firfl; level, dhenext experiment was the reverfe of the former.The column’ of mercury was firfl; let down towithin one-fourth of the length of the tube from,the fealed end : three-iourths of the air was thenexhaufted from the receiver (which was knownby the air in the tube expanding till the mercuryreached the top), the ftop-cock was opened fora few inftants till the mercury had fubfided toits former level; it was then fhut, and the mer-cury now rofe in the tube for five or ten fecondsto more than T V .of its original height, owing tothe cooling of the air confined within the body,of the receiver, and which had acquired its heatby the fudden condenfation o-n opening the ftop-cock. Accordingly on again opening the ftop-cock, air rufhed in, and the mercury fubfidedto its former level. In each experiment a changeof about T ‘o of the bulk of the air of the tubewas produced after the equilibrium of mechani-cal preflure had been reftored, and confequentlythe whole of this is to be attributed to theoperation of the encreafed or diminifhed heat;and as a change of 50 degrees of heat is known,to produce this quantity of effe£t on air, it is tobe inferred that this change, above or below thecommon temperature, actually takes place inthe receiver in the above experiments-. Thecondenfation of aqueous vapour is very con-fiderable in the firft experiment, where con-denfed air is fuddenly rarefied, the fudden cold-caufing this watery precipitation ; but this doesnot contribute to the production of the cold,but has a contrary effedt, as indeed mult always

' 1 Ph. Toi

happen when a vapour is converted to a liquidform.-

It has been already mentioned that in theeftimation of the fpecific caloric of bodies theyare always fuppofed to retain the fame form,that is, folid, liquid, or gafeous, for it is onlythen that it can be aflumed that equal incre-ments of heat produce equal changes of ther-mometrical temperature. But a knowledge ofthe fpecific heat of any fubftance will give aready method of calculating the latent heat, orthe quantity of caloric abforbed during the i'uc-ceflive pafiage from foliditv to the Hate of liquidor vapour. Several experiments on the latentheat of melted metals are given by Dr. Irvine,junr. £ Bifmuth will furnifh an example. Thismetal when juft in fufion, or when a portion ofthe melted mafs has begun to congeal, raifes thethermometer to 476°. A quantity of it perfectlyfluid, and amounting to 1555 grains, was pouredinto a veflel holding 2236 grains of water at62°. The heat of the mixture was 86.25°,therefore the water had gained 24.25 degrees,and the bifmuth had loft 389.75 degrees: andhence 1555 grains of water (that is, equal in-weight to the bifmuth) would have gained 34*degrees. Now the fpecific caloric of folid bif-muth compared with water is previoufly knownto, be .042, fo that the fame quantity of heat,that will raife water .042° will raife bifmuth i°,and confequently what will raife water 34° willraife bifmuth 809.5°. ® ut ^ le bifmuth actuallyloftordy ai temperature, , a.Vt.& confequently

the difference between 389.45° and 809.5°, or419.75°, will exprefs the quantity of latent heatin the melted metal, or that which is loft whenmelted bifmuth- at 476° is changed into folidbifmuth at the fame temperature. In adfualexperiment however feveral precautions arerequired and correftions neceflary, both for thequantity of water diflipated in fleam when amelted metal is poured into it, and for the final!paiticles of metal which may-be carried oft’ oroxydated, and alfo for the heat acquired by theveflel in which the experiment is made. Befidesit is well afeertained that the mercurial ther-mometer becomes very fenfibly inaccurate as ameafure of equal accefiions of heat when thetemperature is very high.

To obviate the objections that arife in fomecafes from the ufe of thermometer as a meafureof heat, a very ingenious application was madeby Mefl'rs. La Place and Lavoifier, of. anotherdirect eft’eft of heat, in order to eftimate re-,fpective quantities of this fubtle, imponderous-•. vol. ix.