262 Dr. Thomas Thomsons Experiments on the 



0-0569 or nearly ^th, and the heat evolved is 178°. If 

 Dr. Irvine's theory were correct, the heat evolved w^hen 1 

 atom oil of vitriol is mixed with three atoms water, being 

 208°, that evolved when 1 atom oil of vitriol is mixed with 

 2 water should be 178° instead of 219°; 1 atom with 5 should 

 be 155° instead of 178°. 



The same want of coincidence will be found, if we ex- 

 amine the whole table from beginning to end. It is clear 

 then that the heat evolved when oil of vitriol and water 

 combine, is not the consequence of mere diminution of 

 specific heat. 



It was observed, a good many years ago, by Dulong and 

 Petit, that when the atomic weight of a simple substance 

 is multiplied by its specific heat the product is a constant 

 quantity. In a paper of mine published in the third volume 

 of the Records of General Science, I have given a consider- 

 able number of examples of this law, and showed, by a 

 pretty copious induction, that the constant quantity is 

 0*375. The obvious consequence deducible from this law 

 is, that the same absolute quantity of heat exists in com- 

 bination with every atom of a simple body, and that the 

 difference of the specific heats of different simple substances 

 is owing to a difierence in the atomic weight. 



In the same paper I have shown that when the atomic 

 weight of a compound body is multiplied by its specific heat, 

 the product is always a multiple of 0*375 by a whole 

 number, which number depends upon, or at least is con- 

 nected with, the number of atoms of which the compound 

 body is composed. If the number multiplying 0*375 were 

 equal to the number of atoms in the compound body, then 

 it would follow that every atom of the compound body 

 retains all the heat with which it was combined when in 

 an isolated state, or that the compound retains all the heat 

 that existed in its constituents. If the multiple be less 

 than the number of atoms of which the body is composed, 

 then it follows, that the compound contains less heat than 

 existed in its elements before combination, and the difference 

 between the multiple and the number of atoms gives us the 

 proportion of heat which is wanting. On the other hand, 

 if the multiple be greater than the number of atoms, the 

 heat in the compound is greater than in its simple elements. 



