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fore we {hall merely refer the reader to Dr.Crawford’s Treatife (p. 453 and feq.) for thecalculations on this curious experiment.

It is alfo forcibly objected by Dr. Robifon*that it is highly improbable that the proportionbetween the adfual lofs of caloric and the dimi-nution of temperature (or the fpecific heat)remains the fame towards the extremity of thefcale of actual privation of heat as it does atthe higher temperatures, the form ftill remain-ing unchanged. So that, though the calculationsof fpecific heat may hold good experimentallyin all practicable cafes, it does not follow thencethat the inference deduced from them can beextended to the extreme hypothetical diftancewhich the determination of the aftual zerowould require. It is more analogous to otherphenomena, as Dr. R. obferves, to fuppofe thatin the temperatures nearer to that of abfoluteprivation, the quantities of heat neceffary forproducing equal elevation gradually diminifh,as in moft other progreffions of natural ope-rations. The hypothefis of Irvine and Crawfordalfo fuppofes an equal uniformity in the com-parative diminution of temperature in differentbodies, to allow of the fame point of abfolutecold for all fubftances in nature. But it isequally improbable that this uniformity holdsgood for different fubftances at all temperatures,and hence we cannot with any certainty inferthe abfolute heats of different bodies from theeftimation of the comparative fpecific heats thatare within the limits of experiment.

The evolution of caloric by the mixture ofdifferent fubftances will be further noticed inthe next fecfion concerning

The Sources of Caloric.

The two great fources of caloric that we areacquainted with, and whofe mighty operationsare conftantly before our eyes, are the fun’s raysand combuftion. The great proceffes in natureand art that demand a l'upply of this principlereceive it chiefly either from folar rays or culi-nary fire; but there are befides fome otheriource6 of caloric of confiderable extent andimportance, which cannot be arranged undereither of the foregoing heads, and require afeparate notice.

' Of Chemical Mixture and Change in Capacity.

The evolution of fenfible heat from any fub-flance or fubftances when their fpecific caloric,or capacity for heat, is diminifhing, has alreadybeen explained. Experiments on this head arethe exa£t converfe of the freezing proceffes bymixtures where the capacity is fuddenly en-

* Notes to B1

creafed; and it is fo feldom that chemical adftiontakes place between different fubftances, with-out a very fenfible change in the capacity forcaloric, that a rife or fall of the thermometerabove or below the arithmetical mean maygenerally be expected on the mixture of bodiesthat have an affinity for each other, and is con-fidered as a proof of intimate or chemical action.This fource of caloric, though feldom actingwith any great energy in natural proceffes, multin many inftances be able, by its conftant ope-ration, to afford an adequate fupply fo. veryimportant changes. Dr. Crawford has withgreat ingenuity of refearch traced to this fourceno inconfiderable portion of the heat perpetuallymaintained in the living animal above the com-mon temperature, having fliewn that the arterialblood has a greater capacity for heat than thevenous, and confequently that it muff, on thisaccount, give out fenfible caloric in the naturalprogrefs of the circulation. Likewife all theproceffes, natural or artificial, in which gaffesare abforbed and condenfed by liquids andfolids, or liquids by folids, muff be heat-evolvingproceffes from the diminution of capacity at-tending thefe changes of ftate.

Some of the moft powerful and familiar in-ftances of the evolution of heat from meremixture without a change of ftate, are, on add-ing water to alcohol, and to the nitric, andefpecially to the fulphuric acid. With the laft,the heat produced on mixing nearly equal por-tions is much above that of boiling water.

In all thefe inftances it has been attempted tobe fhewn by Drs. Irvine , Crawford, and others,that the evolution of caloric was very exactlyproportioned to the change in the fpecific caloric,and indeed the calculations on the fubject ofabfolute caloric require this proportion to holduniformly. So that according to thefe philofo-phers, the caloric can hardly be.faid to be chemi-cally united with the bodies that evolve it onmixture when the capacity is leffened, and theterm capacity is fomewhat better adapted totheir conception of the mode of union.

But it has been fhewn that a mere change ofcapacity will not account confiftently for all thecafes of evolution of heat on mixture, for thereis an utter want of tolerable correfpondence inall the calculations that have been made of thereal zero derived from very different fources.The vaft quantity of caloric given out in theflacking of well-burnt lime cannot be broughtto tally with the proportional quantity thatappears on mixing fulphuric acid and water (forck’s Le&urca.