OV THE MEASURKS AKD THE NATURE OF HEAT. 653 



SuTfstance, withovit being comprehended in the quantity required for main- 

 taining its actual temperaturCi But even this hypothesis is wholly inap- 

 plicable to the extrication of heat by friction, where all the qualities of the 

 substances concerned remain precisely the same after the operation, as before 

 It. If any further argument were required in confutation of the opinion, that 

 the heat excited by friction is derived from a change of capacity, it might 

 be obtained from Mr. Davy's experiment on the mutual friction of two 

 pieces of ice, which converted them into water, in a room at the tempera- 

 ture of the freezing point: for in this case it is undeniable that the capacity 

 of the water must have been increased during the operation ; and the heat 

 produced could not, therefore, have been occasioned by the diminution of 

 the capacity of the ice. 



This discussion naturally leads us to an examination of the various theories 

 which have been formed respecting the intimate nature of heat; a subject 

 upon which the popular opinion seems to have been lately led away by very 

 superficial considerations. The facility with which the mind conceives the 

 existence of an independent substance, liable to no material variations, except 

 those of its quantity and distribution, especially when an appropriate name, 

 and a place in the order of the simplest elements has been bestowed on it, 

 appears to have caused the most eminent chemical philosophers to overlook 

 some insuperable difficulties attending the hypothesis of caloric. Caloric has 

 been considered as a peculiar elastic or ethereal fluid, pervading the substance 

 or the pores of all bodies, in different quantities, according to their different 

 capacities for heat, and according to their actual temperatures; and being* 

 transferred from one body to another upon any change of capacity, or upon 

 any other disturbance of the equilibrium of temperature: it has also beert 

 commonly supposed to be the general principle or cause of repulsion ; and in 

 its passage from one body to another, by radiation, it has been imagined by 

 some to flow in a continued stream, and by others in the form of separate 

 particles, moving, with inconceivable velocity, at great distances from each 

 other. 



The circumstances which have been already «tated, respecting the produc- 

 tion of heat by friction, a^ipear to afford an unanswerable confutation of the 

 VOL. I. 4 m 



