138 MOLECULAR MOTION AND ITS ENERGY 59 



to the molecular energy E nearly as 2 : 3, and since all these 

 gases have diatomatic molecules, the mean energy e of an 

 atom is to the molecular energy E nearly as 1 : 3. This 

 class of gases, therefore, possesses the really remarkable 

 property that at equal temperatures not only are the values 

 of their molecular energy equal to each other, but also those 

 of their atomic energy, and, consequently, also their whole 

 eat-energy. These bodies, therefore, obey in the gaseous 

 state the law of Dulong and Petit, to which other sub- 

 stances in the solid state are subject. 



Not all diatomic gases seem to follow this law equally. 

 Even though HC1, HI, and perhaps HBr, obey it to 

 some extent, yet BrI, C1I, C1 2 , Br 2 , and I 2 exhibit very 

 considerable deviations from it. But we need not on this 

 account completely deny the validity of this law for diatomic 

 gases. For the substances last named are rather vapours 

 than gases, and it is therefore probable that with them the 

 ratio of the specific heats increases with rising temperature. 

 It is therefore not impossible that for all diatomic gases 

 the ratios 



- = 1-4 and ' = O33 

 c E 



would be found if the measurements were made at such 

 pressure and temperature that the laws of perfect gases were 

 exactly obeyed. 



For monatomic gases theory and observation agree (see 

 54) in giving 



C = 1-67 and e = 0. 

 c 



The idea is accordingly suggested that the value of the 

 ratio of the specific heats, as also those of the different 

 species of energy, depends, not on the material of the atoms, 

 but on their number. The observations quoted seem also 

 to indicate this ; at least the numbers for the triatomic 

 gases oscillate about the mean values 



C = 1-27 and = 0-5. 



C L 



It has therefore also been attempted to find a general 



