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to be the proportional quantity of caloric necef-fary to raife its temperature any given numberof degreeSj compared with that required to raifean equal weight of any other body, and efpeciallywater, which is the ftandard generally agreedupon. The proportions between thefe refpectivequantities are affirmed to be the fame at anya&ual temperature, provided the refpectiveftates remain unchanged, that is, provided folidsremain folid; liquids, liquid; and gaffes, gafi-form. For example, it requires twice as muchcaloric to raife tin to any given point as it doesto raife lead to the fame, or, expreffed in theproportions to water, the fpecific caloric of tin,according to Dr. Crawford, is .070, and of leadis .035, and thefe proportions are fuppofed tobe the fame from the lowed poffible tempera-ture to any degree fhort of the melting point oftin, which is the mod eafily fufible of the two.Confequently, if they could be put in a lituationto be deprived of all their caloric of tempera-ture, the tin would lofe exaftly double thequantity that the lead would do, and thereforeat all equal temperatures the abfolute quantityof heat contained in tin is double that of thelead.

This leads to the enquiry into the real zero , orthe point of abfolute cold meafured by an equi-differential thermometer, for though fuch afhermometrical meafurement is practically im-poffible, the fcale may be hypothetically extendedindefinitely to any didance below the aftualzero. It may be premifed that there is no reafonto fuppofe that any degree of cold, natural orartificial, ever obferved, has approached theterm of abfolute privation of temperature. For-merly the zero of Fahrenheit was fuppofed tobe this point, but an adtual cold of at lead 40°lower is now procurable without much difiicultyin the congelation of mercury, and by particularmeans a cold of much greater intensity may beproduced, though the meafurement below thisbecomes very uncertain, for want of an unex-ceptionable liquid for the thermometer whenmercury becomes ufelefs. Even in the modintenfe cold ever produced, pure alcohol remainsfluid, and the permanent gaffes retain theirgafeous form, but from analogy we may reafon-ably fuppofe that alcohol owes its fluidity to thecombination of caloric, and perhaps that thegaffes, themfelves might be brought to a liquidor folid date by abdraftion of heat. Howeveras the lead computation of the real zero placesit at lead five times the didance between freez-ing mercury and boiling water below any

obferved cold, no material objection can arife tothefe computations from the probable effect ofabfolute cold on the hitherto uncongealablebodies.

The late Dr. Irvine , of Glafgow, was the firdwho indituted a theorem for the determinationof the real zero, and this fubjedt has been fur-ther examined by Crawford, Gadolin , and manyother very accurate experimenters. Unfortu-nately there exid fuch material objections tothe data which mud be affirmed as the bafes ofthefe calculations that a fhort notice in this,place will fuffice.

Dr. Irvine ’s mod ingenious theory is foundedon the comparifon between the fenfible heatgiven out when a fubdance paffes -from a dateof greater to a lefs capacity for caloric, and thedifference between the capacity in each date:thus it is afcertained that about 140 degrees ofheat are given out when water is converted toice (or, what amounts to the fame, that ice at32°, in thawing abforbs as much heat as wouldraife water at 32 0 to about 17 2°) and the capacityof ice is to that of water as 9 to 10. Watertherefore contains as much abfolute caloric asice, and 140 degrees additional; and it is af-fumed that the abfolute caloric of water com-pared to that of ice, is as their fpecific calorics,or as lot09 : and hence it follows that 140° isone-tenth of the abfolute quantity contained inwater, or that the entire quantity is 1400°. Orin other terms, the difference of the capacitiesbefore and after the change, is to the capacityafter the change, as the fenfible heat produced isto the whole number of degrees of heat whichthe body contained previous to the alteration iriits form. .

Another method given by Dr. Irvine of afcer-taining the quantity of abfolute heat is by thedifference of temperature that generally refultsfrom the mixture of two bodies, compared withtheir refpective capacities for heat. Sulphuricacid and water are very convenient examples,as the fenfible heat evolved on mixture is veryconfiderable. To render it a meafure of theabfolute heat, it ffiould be produced merelyfrom the capacity for heat of the mixture beinglefs than the mean capacities of the two beforemixture, relative to their refpedtive weights. wIt unfortunately happens that experiments madewith different mixtures by no means correfpondor approach fufliciently to a correfpondence toenable us to affume the neceffary datum that'the change of temperature only arifes from achange in the capacity after mixture, and there-

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