43 On the Materialify of iJetL 



Gmin^'Rumford alfo afcertained that the fpecific heat of irdrt 

 was not dnninilhed when converted bv a borer into turning?, 

 aiid coaieqaentlv when it had been the f'ource of much tem- 

 perature. In explanation of thefe fafts we may be allowed 

 to aiTume the communication of caloric from furrounding 

 bodies, till this communication has been demonftraied to be 

 impoftible. But even were the impoffibility eftaljlifiied, it 

 would yet remain to be proved, that the evolved caloric does 

 not proceed from an internal fource;. and this can only be 

 done by an accurate comparifon of the (|uantitv of caloric in 

 bodies before and after friAion, Now, in inftituling tnis 

 comparifon, it is implied that we pofltfls means, of determin- 

 ing the abfoliite quantity of caloric in bodies, and thatw e 

 can compare quantities of caloric with as much certainty as 

 we can o!it;iin liora an appreciation bv weight or by meafure. 

 Such ptMieciion, however, does not, \ apprehend, belong to 

 ■ the prt;fent llateof our knowledge refpefting heat j for I have 

 always been dillruftful of that part of the doctrine which af- 

 fgns the ratio of heat latent in bodies. The grounds of this 

 dillrufr I Ihall ftate pretty fully ; for, if it can be proved that 

 we have no accurate conceptions of quantity, as appertaining 

 ro heat, all arguments againft its materialitv, derived from 

 ilippofed determinations of its quantity, mufl be inconclufive. 



The only clear conceptions which the mind has of quan- 

 tity, are derived either from a comparifon of the magnitude 

 or of the gravity of bodies. In the inftance of caloric, both 

 thefe modes of menfuration fail us. We cannot eftimate the 

 bulk of a fubfhmce 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 furpaffes in tenuity 

 beyond all comparifon. Our notions of the quantity of ca- 

 loric are derived, not from fuch (imple judjrments, but from 

 Complicated procelfes of reafoning, in the Iteps of which, 

 errori? fatal to the whole may perhaps fometimes appear. 



Whatever be the nature of caloric, whether it be a body 

 fui geums, or a quality of other bodies, its efle6i"s are pecu- 

 liar and appropriate; and, like all other effetl?, bear a pro- 

 portion to the energy of their caufe. Expanlion, for example, 

 it is proved by experiment, keeps pace with the aftual incre- 

 ments of heat ; and on thisprinciple is founded the thermo- 

 meter, the great agent in the acquirement of all our ideas re- 

 fpecling heat, both abfolute and relative. The competency nf 

 this inftrument, however, to afiord inforuiation of the quantity 

 of caloric, is liiaited by the following circumlhuioes : 



itl:. The mercury of the thermometer indicates only the 



quantity of heat which it has itfelf acquired, and by no niean,^ 



9 ' that 



I 



