115 



CHAPTEK V 

 MOLECULAE AND ATOMIC ENERGY 



52. Dulong and Petit's Law for Gases 



THE theories of Clausius and Maxwell, developed in 

 Chapters II. and III., form, in the first place, a kind of 

 molecular mechanics ; but as heat is nothing else but the 

 mechanical motion of molecules, these theories are entitled 

 to form the basis of the laws of heat no less than of those 

 of mechanics. One of the immediate conclusions from it is 

 a theorem that exhibits a marked analogy with the law / 

 respecting the specific heats of solid bodies, which was dis- ~~ 

 covered by Dulong and Petit. 



If the temperature of a gas rises by 1 degree, the kinetic 

 energy ( 14, 16) 



E = 



which its molecules possess on the average, increases by 



a magnitude which, by what has gone before, is the same for 

 all gases. 



On the other hand, the law which Dulong and Petit 

 discovered for the specific heats of solid bodies may, as is 

 well known, be expressed in the form that, in order to heat 

 chemically different bodies to the same extent, the . same 

 amount of heat must be communicated to every atom ; and ^b 

 for this we generally say more shortly that the atomic heat ' 

 of all bodies is the same. 



If we remember that on our theory heat and energy are 

 identical, the analogy we have mentioned at once comes 

 into view. By a rise of temperature the energy of each 

 ATOM in the SOLID state increases by the same amount, and, 



i 2 



