refpetling Heat or Caloric. 125 



veffel whofe capacity is known or can be come at ; in others, 

 as in a wet piece of wood, a brick, or other fybftance, the 

 quantity muft be found out by other means : but in every 

 cafe where water is added to another fubftance, which is 

 made thereby to expand exactly in the dire6l ratio of the 

 quantity of water added (if there be any fuch), the quantity 

 may be determined by measuring the compound, and de- 

 dueling therefrom the original volume of the other fubftance. 

 IVould it be a ! 'Jurd to talk of meafuring the matter, fluid, or 

 fubjlance, called heat, in ajimilar manner? 



When a thermometer is applied to any fubftance of a 

 higher temperature than itfelf, it is, by the operation of the 

 general law, foon brought into equilibrium with that fub- 

 ftance as to heat ; and we fay, " it has rifen fo many de<- 

 grees." We are habituated to this mode of fpeaking, and 

 fatisfy ourfelves, without any more inquiry, that the pheno- 

 menon requires no further inveftigation ; and as to the ac- 

 companying phenomena, we generally overlook them alto- 

 gether. When we find that the mercury has increafed in 

 volume, would it be abfurd to aflc this fimple queftion ? Is 

 the increafe to be attributed not merely to the addition of 

 heat, but to the addition of a quantity equal in bulk to the 

 increafe of volume acquired by the mercury P I think it ex- 

 tremely probable that the amount of increafe or diminution 

 of the volume of any fubftance, when heat is added or ab- 

 ftraclcd, is the real bulk of the heat fo added or abftracled. 

 That I may be clearly underftood, I fhall illuftrate my mean- 

 ing bv a comparifon : 



If to a cubic inch of a compact piece of gum and water 

 new unknown quantities of water be added, who would ever 

 think of wire-drawing the mixture through a tube, and ex- 

 prcvTing the refult in degrees of no known quantity, nor re- 

 ferable to any determinate meafure ? The cafes to me appear 

 perfectly parallel. 



To a mixture of gum and water we add water, and the 

 volume of the mixture is increafed; and to a mixture of 

 mercury and heat we add heat, and the volume of that 

 mixture is increafed. 



When 



