60 MOLECULAR AND ATOMIC ENERGY 143 



This remark can, however, be valid only so far as the 

 question concerns the phenomena which are conditioned by 

 heat alone. By this is only intended to be meant that 

 addition of heat can bring about no other motions than 

 those named. But that other kinds of motion can be pro- 

 duced in the molecules and atoms of gas by other causes is 

 shown by the spectra of incandescent gases.' The motions 

 inside a molecule, which are perceptible as light, are easily 

 brought about by electrical or chemical forces. But simple 

 heating in gases does not cause them to radiate red or white 

 light like solids or liquids. Hot gases, of course, send out 

 both dark and luminous radiation ; but the radiation which 

 they emit, purely in consequence of their being heated, is 

 very much less than that which comes from gases in com- 

 bustion ; and under all circumstances gases radiate heat in 

 much less degree than solids or liquids. With this re- ^ 

 markable fact the results of the kinetic theory agree most 

 excellently. 1 



It is not for all diatomic gases that the value of the ratio 

 of the specific heats is independent of the temperature and 

 equal to 1'4 ; for many of these gases it is smaller and is 

 variable with the temperature. This fact can be interpreted 

 in two ways. In the first place, the constant h, which 

 expresses the ratio of the energy spent on internal work to/' 

 the energy of translation, need not be equal to for these 

 gases ; and if it is greater than zero, the ratio of C to c is 

 less than 1*4. 



Another, though not essentially different, possible ex- 

 planation might be found in the assumption that the 

 number of degrees of freedom q is not 5, but 6. There 

 is, indeed, a sixth kind of movability if we drop the assump- 

 tion that the two atoms of the molecule must remain at an ' 

 invariable distance from each other. With the value h = 0, 

 the ratio would approach the value 



- = 1-33, 



c 



1 B. v. Helmholtz, Licht- und Warmestrahlung verbrennender Gase, 

 1890, p. 64. 



