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for heat, and the numbers that exprefs thecapacities are inverfely as the degrees of tem-perature to which the feveral fubftances areraifed by equal quantities of heat. Thus in theinftance above given, equal quantities of heatwill raife mercury 28 degrees and water 1, andtherefore the fpecific caloric or capacity of calo-ric of the two liquids is inverfely as 28:1, orwhen water is reckoned as unity, mercury willbe -jV* As all thefe numbers are comparativewith each other, and do not exprefs any abfolutequantity of heat, it is neceffary and convenientto affume a common ftandard of comparifon,and here, as in the cafe of fpecific gravity, wateris the ftandard fixed upon, and aflumed to beunity. The fubftances compared are alfo gene-rally eftimated by equal weights and not meafures,but the former may be readily converted intothe latter by multiplying into their fpecific gra-vities. Thus as mercury is (in round numbers)14 times as heavy as the fame bulk of water,epual bulks of the two liquids would give 14pounds of mercury to one pound of water, andif thefe are mixed at the refpedtive temperaturesof 69 and 40, the mixture will be about 49^,making the refpedtive capacities of water andmercury as 1 to \ with equal bulks.

Two data mull be aflumed in this calculation,the one, that equal degrees of the thermometerwill meafure equal increments of heat; and theother, that the fpecific caloric of bodies remainsthe fame in any temperature, provided the formis unchanged. It has been already mentionedthat the expanfion of mercury is nearly, thoughnot rigoroufly, uniform in all the common tem-peratures at which experiments are made, thatis for a confiderabje diftance above and belowthe points of freezing and boiling water : andthe very accurate experiments of Crawford fhewthat much reliance may be placed on the generalpofition that the capacities for heat are equal inthe fame fubftance at every temperature, whilethe form remains the fame.

The general formula for eftimating the fpecificcaloric of bodies from the temperature refultingfrom the mixture of two fubftances at unequaltemperatures, whatever be their refpeftive quan-tities, is the following, but it is convenient thatone of thefe fubftances be water, as it is theftandard of comparifon, and its fpecific calorieis therefore reckoned as = 1. Multiply theweight of water by the difference between itstemperature before mixture, and that of themixture itfelf. Alfo multiply the weight of the

■ Crawford. * Thompfon’s Chemiftry, vol. i. when

• Enquiry on H

other fubftance by the difference between itstemperature and that of the mixture. Dividethe firft pro duel by the fecond, and the quotientwill exprefs the fpecific caloric of the other fub-ftance, that of water being 1. For example,fuppofe 20 ounces of water at 105° be mixedwith 12 ounces of fpermaceti oil at 40°, andthe temperature after mixture be 90°. Thenmultiply 20 by 15 (the difference between 105°and 90°) = 3®o: and multiply 12 by 50 (thedifference between 40° and 90°) = 600: and300 divided by 600 will give ^ or .5, which isthe fpecific caloric of the oil, compared to thatof water, which is 1. The fame rule holds whenthe water is the colder of the two-

The above rule gives the fpecific caloric ofequal weights of the two fubftances : to make itapply to equal bulks the fpecific caloric of thefubftance compared with water mult be mul-tiplied by its fpecific gravity. Thus in the aboveinftance if the fpecific gravity of the oil be .92,the fpecific heat for equal bulks will only be .4 6,for this converfion into equal bulks is in factthe fame as diminifhing the weight, when oil isthe fubjedt of experiment. Therefore when thecomparifon of equal weights is converted intothat of equal meafures or bulks, thofe fubftancesthat are lighter than water will have the numberexprefling the fpecific caloric diminijhed, andthofe that are heavier will have it encrcafed.

Many precautions are required in thefe ex-periments to enfure tolerable accuracy, and theeffedt of the veffel itfelf mull be eftimatedfeparately by previous experiment. m

The fpecific caloric of folids was afeertainedvery early in this enquiry by Wilcke of Sweden ,and publifhed in the Stockholm Academy Tranf-adtions for 1772. By ingenious calculations*he found firft the quantities of water requiredto produce a fimilar'effect with that of the folidemployed, whence the fpecific caloric was after-wards deduced.

Mr. Leflie fuggefts 0 an elegant and Amplemode of determing the fpecific caloric of bodiesfrom obferving their relative times of coolinga certain number of degrees comparatively withwater in fimilar circumftances. A very thinglafs globe, with a narrow neck to receive thebulb of a fine thermometer, is a convenientveffel for this purpofe. Firft fill it w Jth hotwater, and obferve the time required to cool acertain number of' degrees, eftimating alwaysfrom the temperature of the room, or better agiven number of degrees above it, and not from

they are accommodated to Fahrenheit’s thermometer,

»t> P- 340, 34*.