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■matter. The thermometer meafures heat bythe expanfion of the fluid employed (which isgenerally mercury); the Calorimeter of La Placeand Lavoilicr meafures it by the quantity of iceat 3 2°, which it melts under fimilar circum-ftances. This method appears at firft fight tobe capable of very great accuracy, for ice be-ginning to melt is invariably at 3 3°, and the.quantity of water produced muft, if accuratelycollected, be an exact meafure of relative quan-tities of caloric. The calorimeter is a hollow.cylindrical .veffe! made of tinned iron, and con-taining three diftinct concentric cavities fepa-rated from each other by circular partitions of•the fame material. The object intended is toplace tire heated body in contact with or con-tiguous to ice, fo infulated that no heat can getto it but that of the fubject of experiment.Tlie inner cavity is intended for the heatedbody; the middle one contains the ice to bemelted by it, and for readier communicationthe partition is perforated all over; and theouter cavity alfo contains ice, which is intendedto detain the heat of the external air, and pre-vent it from reaching the ice of the middlecavity, whilft the partition between the twokeeps entirely feparate the melted water of each.A lid ingenioufly contrived with correfpondingpartitions {huts over the whole. At the bottomare two pipes, one communicating with theouter, the other with the middle cavity. Thewater from the melted ice drains off throughthem, that from the outer cavity is not noted,but that from the middle is carefully colleftedand weighed. Some fmaller precautions are re-quired, and an extenfion of the apparatus toparticular purpofes, u which need not here be.enumerated.

As the quantities indicated by the calorimeterare only proportional, fome ftandard of compan-ion mult be fixed upon, and this the inventorshave determined to be the quantity of caloricneceffary to melt a pound of ice, which thereforeis called 1. This is equivalent to the effeift ofa pound of water at 135 0 above 3 2°, or 167°,which will alfo melt a pound of ice. Theexample given by Lavoifier of the application ofthe inftrument to other bodies may be hereinferted. A quantity of iron rolled in narrowflips, and weighing 7.7070319 pounds, washeated in boiling water to 207.5°, or 175.5above 32°, and introduced within the innercavity of the calorimeter. In cooling to 32° itmelted 1.109795 pounds of water. Hence it isfound by dirett proportion that if 175.5 degrees

of caloric above 32° will melt 1.109795 poundsof ice:: 135 degrees above 32° would melt0.85384 pounds. And 135° above 32 0 , or 167°,is the temperature to which a pound of watermuft be raifed to melt a pound of ice. There-fore by equal Ioffes of temperature , 1 pound ofwater will melt 1 pound of ice, and 7.7.070319pounds of iron will melt 0.85384 pounds ofice; or (dividing 0.85384 by 7.7070319) X poundof iron will melt .1109 pound of ice. There-fore the quantity of caloric given out by iron, isto that given out by water, at equal differencesof temperature, as .1109 to 1.

In meafuring temperatures near the utmoftlimit of tire fcale of the mercurial thermometerand upwards, the calorimeter may certainly bemade a valuable inftrument; but it appearsfrom fome experiments by the late Mr. Wedge-wood, v that it is fcarcely poffible to feparatewith accuracy the melted water from the iceremaining in the middle cavity of the inftru-ment, and even, as it ftiould feem, a portion ofthe thawed ice again freezes, fo as to throwmuch uncertainty on this method of eftimatingrelative quantities of caloric.

The attempts that have been made by feveralvery accurate experimentalifts to weigh caloricneed not be particularly deferibed. Fordyce,Lavoifier, Rumford, and Higgins, have eachinftituted experiments with great care, but therefults of none prove unexceptionably that anyweight can be attributed to caloric, or that anyfubftance within the reach of human inftru-ments weighs lefs when cold than when hot,no other change having taken place than themere lofs of caloric. In balances that are ad-jufted with extreme accuracy, the vicinity of ahot body to one end of the fcale beam willreadily expand it enough to deftroy for a timethe equilibration; and if this caufe of error isavoided by allowing a confiderable time to elapfebetween the weighing of the fubftance beforeand after the lofs of heat (it being at the tem-perature of the atmofphere in each cafe) a flightchange may probably have taken place in theweight of the air, amounting to a fenfible dif-ference of the barometer, which will caufe auadditional difficulty in thefe delicate refearches.Even if fome afcertainable weight could befound to a confiderable quantity of caloric, it isobvious that the balance could never be prac-tically employed as a fubllitute to the ther-mometer.

Of Abfolute Caloric.

The fpecific caloric of a body has been {hewn

* Mem. de 1 ’Acad. for 1780, er Layoifier’s Chemiftry, p. 4SI. ’ Phil. Tranf. vol. 74.