200 ON THE MATERIALITY OF CALORIC*, 



the impoflibility eftablilhed, it would yet remain to be proved, 



that the evolved caloric does not proceed from an internal 



fourcej and this can only be done by an accurate comparifon 



of the quantity of caloric in bodies, before and after friction. 



and the experi- Now, in inftituting this comparifon, it is implied, that we 

 ments for decid- * r ...... , , - , . .. , . . 



in- capacities are P°" els means ot determining the ablolute quantity 01 caloric in 



questionable, bodies, and that we can compare quantities of caloric with as 

 much certainty as we can obtain from an appreciation by 

 weight or by meafure. Such perfection however does not, I 

 apprehend, belong to the prefent flate of our knowledge re- 

 fpefting heat ; for I have always been difiruftful of that part of 

 the doctrine, which affigns the ratio of heat latent in bodies. 

 The grounds of this diftrult I (hall ftate pretty fully ; for, if it 

 can be proved that we have no accurate conceptions of quan- 

 tity, as appertaining to heat, all arguments againft its materi- 

 ality, derived from fuppofed determinations of its quantity, 

 mull be inconclusive. 

 It Is afferted that The only clear conceptions which the mind has of quantity, 

 conceptions""*" are derived either from a comparifon of the magnitude, or of 

 quantity are the gravity of bodies. In the inftance of caloric, both thefe 

 from magnitude mo ^ es of menfaration fail us> W e cannot eftimate the bulk of 



a fubftance which eludes our grafp and our vifion ; nor have 

 we yet fucceeded in comparing its gravity with that of the 

 grofler kinds of matter, which it furpalTes in tenuity beyond all 

 neither of which comparifon. Our notions of the quantity of caloric are derived, 

 in caloric. not ^ rom ^ ucn firnple judgments, but from complicated pro- 



cefles of reafoning, in the fteps of which, errors, fatal to the 

 whole, may perhaps fometime appear. 

 Caloric is pecu- Whatever be the nature of caloric, whether it be a body fui 

 ments being* de- generis, or a quality of other bodies, its effects are peculiar and 

 ■oted by expan- appropriate ; and, like all other fads, bear a proportion to 

 the energy of their caufe. Expanfion, for example, it is proved 

 by experiment, keeps pace with the actual increments of heat ; 

 and on this principle is founded the thermometer, the great 

 agent in the acquirement of all our ideas respecting heat, both 

 abfolute and relative. The competency of this inftrument, 

 however, to afford information of the quantity of caloric, is 

 limited by the following cirqumftances : 

 The thei-mome- \jt y| )e mercury of the thermometer indicates only the 

 abfolute quanti- W^jj of heat which it has itfelf acquired, and by no means 

 ties, that contained in furrounding bodies. 2dly t The fcale of ex- 



panfion 



