THE MEASURES AND THE NATURE OF HEAT. 501 



A pound of nitre contains about half its weight of dry acid, and the capa- 

 city of the acid, when diluted, is little more than half as great as that of 

 water ; the acid of a pound of nitre must therefore contain less heat than 

 a quarter of a pound of water : hut Lavoisier and Laplace have found,* that 

 the deflagration of a pound of nitre produces a quantity of heat sufficient to 

 melt 12 pounds of ice, consequently the heat extricated by the decomposition 

 of a pound of dry nitrous acid must be sufficient to melt 24 pounds of ice ; and 

 even supposing the gases, extricated during the deflagration, to absorb no 

 more heat than the charcoal contained, which is for several reasons highly 

 improbable, it follows that a pound of water ought to contain at least as 

 much heat, as would be sufficient to melt 48 pounds of ice, that is about 

 6720 degrees of Farenheit. 



In short, the further we pursue such calculations, the more we shall be 

 convinced of the impossibility of applying them to the phenomena. In 

 such a case as that of the nitrous acid, Dr. Black's term of latent heat t 

 might be thought applicable, the heat being supposed to be contained in the 

 substance, without being comprehended in the quantity required for main- 

 taining its actual temperature. But even this hypothesis is wholly inappli- 

 cable 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 tem- 

 perature 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 dimi- 

 nution 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 appro- 

 priate name, and a place in the order of the simplest elements has been 

 bestowed on it, appears to have caused the most eminent chemical philoso- 

 phers 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 quan- 

 tities, 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 equi- 

 librium of temperature : it has also been 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 con- 



. * Hist, et Mem. 1780, p. 355, H. 3. 



f Black's Lectures, 2 vols. 4to, Ed. See Lavoisier, Traite Elem. de Chimie, 1 789. 



