82 THE FITNESS OF THE ENVIRONMENT 



and Petit in 1819. The law which bears their 

 name consists of the statement that in the 

 case of elementary substances the product of 

 specific heat and atomic weight is a constant, 

 — roughly, 6.4. Certainly this so-called law 

 is a mere approximation, and some elements, 

 notably carbon, silicon, and boron, at the 

 ordinary temperature, depart widely from its 

 requirements, but in the main the approxi- 

 mation holds good. Later the researches of 

 Neumann, Gamier, Cannizaro, and especially 

 of Kopp made possible an extension of the law 

 to compounds. 



It is evident that the law of Dulong and 

 Petit amounts to the statement that for all 

 elementary substances the quantity of heat 

 which is required to change the temperature 

 of every atom, regardless of its nature, is a 

 constant. A brief discussion will serve to 

 make this plain. According to the law the 

 specific heat of an element varies inversely 

 as its atomic weight, diminishing as the atomic 

 weight increases, so that the product of the 

 two quantities remains constant. But of 

 course the number of atoms per gram of sub- 

 stance also varies inversely as the atomic 

 weight. Hence the specific heats of elemen- 

 tary substances and the number of atoms per 

 gram are always roughly proportional, which 



