500 LECTURE LIT. 



mixed with a pound of water at 172 of Fahrenheit, the whole excess of 

 140 is absorbed in the conversion of the ice into water, and the mixture is 

 reduced to the temperature of 32 ; and, on the other hand, when a pound 

 of ice freezes, a certain quantity of heat is evolved which is probably 

 capable of raising the temperature of a pound of water 140, or that of 140 

 pounds a single degree. Dr. Crawford found, by means of other experi- 

 ments, that a quantity of heat capable of raising the temperature of water 

 9 would raise that of ice as much as 10 : hence he inferred that the capa- 

 city of ice was T 9 V as great as that of water, and that if this capacity, 

 instead of being reduced to -j- 9 ^, had been wholly destroyed, the quantity of 

 heat extricated would have been 10 times as great, or about 1400, which 

 has, therefore, been considered as the whole quantity of heat contained in a 

 pound of water at 32, and the beginning of the natural scale has been 

 placed about 1368 below the zero of Fahrenheit. Dr. Irvine makes the 

 capacity of ice still less considerable, and places the natural zero about 

 900 degrees below that of Fahrenheit. 



If direct experiments on the quantities of heat, required for producing 

 certain elevations of temperature, in different states of the same substance, 

 compared in this manner with the emission or absorption of heat which 

 takes place while those changes are performed, agreed with similar experi- 

 ments made on different substances, there could be no objection to the 

 mode of representation. But if it should appear that such comparisons 

 frequently present us with contradictory results, we could no longer con- 

 sider the theory of capacities for heat as sufficient to explain the pheno- 

 mena. With respect to the simple changes constituting congelation and 

 liquefaction, condensation and evaporation, and compression and rarefaction, 

 there appears to be at present no evidence of the insufficiency of this theory ; 

 it has not perhaps yet been shown that the heat absorbed in any one change 

 is always precisely equal to that which is emitted in the return of the sub- 

 stance to its former state, but nothing has yet been advanced which renders 

 this opinion improbable ; and the estimation of the natural zero, which is 

 deduced from this doctrine, may at least be considered as a tolerable 

 approximation. 



If, however, 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 differ- 

 ence 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 the 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. 

 * Ph. Tr. 1798, p. 80. 



