54 ^-^'■- Henry on the MaterialHy ofHeah 



Nor is its deficiency of gravity fufficient to exclude it from 

 the clafs of material fubftaiices. Such nicety of -arrangement 

 might, with equal propriety, lead us to deny the materiality 

 of light, the gravity of which has never yet been proved ; 

 for, bcfides the experiments of Mr. Michell, which failed in 

 afcertaining this property of light, we have feveral chenuciU 

 fac'^s tending to the fame conclufion. Thus Mr. Cavendifli, 

 after firing a mixture of hydrogenous and oxygenous gafes in 

 a clofe velfel, a procefs during which much light is always 

 emitted, found not the fmallelt diminution of weight. 



To have completed this defence of the material nature of 

 heat, it would have been proper to have pointed out the cir- 

 cumftances in which the pluvnomena of heat differ from the 

 known and acknowledged phasnomena of motion. At pre- 

 fent, however, I have not leifure to purfue the fubjeft at much 

 length ; and though feveral points of difagrcement would 

 doubtlfTs be found, I fliall mention only one of the moft 

 marked and decifjve. 



Motion is an attribute of matter, independently of which 

 it cannot poflibly fubfift. If, therefore, the phaenomena of 

 heat can be fliown to take place where matter is not prefent, 

 we fliall derive from the fac^ a conclufive argument againft 

 that theory of heat which affigns motion as its caufe. Now, 

 in the experiment of Count Humford, before alluded to., heat 

 pafled through atorricellian vacuum, in which, it need hardly 

 be obferved, nothing could be prefent to tranfport or propa- 

 gate motion. This experiment, in my opinion, decidedly 

 proves that heat can fubfift independently of other matter, 

 and confequently of motion ; in other wordSj that heat h a 

 dyitniJ and peculiar bodji *, 



* The argument at p. 49, which is the bafis of my objcftions to the 

 commonly employed mode of afcertaining Ijiecitic caloric, I fear, is not fa 

 fully and clearly ftatcd as the abftrufc nature of the fubjeft requires. 



AfiTuming two bodies, A and B, to be at the point of privation of tem- 

 perature, or to pofftls no free caloric whatfijever, the quantity of conibined 

 caloric in each, according to Dr. Crawford's theory, is <r/!)Y/?.V proportional 

 to the quantities of heat necelfary to produce equal elevations of teir,v>era- 

 ture in the two bodies. Thus, if to attain a given temperature, A require 

 csloric as 20, and B only as to, the combined caloric of A, before this 

 addition, is inferred to have borne to that of B the rafip of 2 to j. But it 

 might, with equal or peihaps greater probability, have been affu^iTicd that 

 the combined caloric of A and B is hivctfrly proportional to the quantities 

 of heat required to produce a given temperature ; that A, for example, to 

 attain a certain tciT.perature, has abforbed more caloric than B, becaufc iu 

 A Icfs caloric exillcd previoully in a llatc of chemical union. 



IX. Uepcri 



