55-2 LECTURE LII. 



ill the return of the substance to its former state, but nothing has yet been 

 advanced which renders this opinion improbable; and tlie estimation of the 

 natural zero, which is deduced from this doctrine, may at least be considered 

 as a tolerable approximation. 



if, how-ever, we attempt to deduce the heat produced by friction and by 

 combustion from changes of the capacities of bodies, thus estimated, we shall 

 find that the comparison of a very few facts is sufficient to demonstrate the 

 imperfection of such a theory. Count Rumford found no sensible difference 

 between the capacities of solid iron and of its chips; but if we even suppose, 

 for the sake of the argument, that the pressure and friction of the borer had 

 lessened the capacity of tlie iron one twelfth, so as to make it no greater than 

 that of copper; we shall then find that one twelfth of the absolute heat of 

 the chips, thus abraded, must have amounted to above 60 000 degrees of 

 Fahrenheit, and consequently that the natural zero ought to be placed above 

 700 000 degrees below the freezing point, instead of 14 or 1500 only. It is, 

 therefore, impossible to suppose that any alteration of capacities can account 

 for the production of heat by friction : nor is it at all easier to apply this 

 theory correctly to the phenomena of combustion. A pound of nitre contains 

 about half its weight of dry acid, and the capacity 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: but 

 Lavoisier and Laplace have found, that the deflagration of a pound of nitre 

 produces a quantity of heat sufficient to melt twelve pounds of ice, conse- 

 quently 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 char- 

 coal contained, which is for several reasons higlijy 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 Fahren- 

 heit. 



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 might 

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



