120 MOLECULAR MOTION AND ITS ENERGY 54 



For this class of bodies, therefore, K K Q = H H Q , 

 and thus 



C -_c _ 2 

 c ~3' 



or, if this assumption is correct, the ratio of the two specific 

 heats must be 



Led by this consideration Kundt and W&ibuvg 1 have 

 submitted mercury vapour to an investigation, the result of 

 which completely confirmed the exactness of the theoretical 

 formula. They determined the ratio of the specific heats 

 from the speed of sound, which can be found from the wave- 

 length of a tone. By comparing the wave-lengths of the 

 same tone both in air and mercury vapour they found that 

 the ratio of the specific heats of mercury vapour is really 



^ = 1-67, 

 c 



if after Bontgen's 2 determination the value of this ratio 

 for atmospheric air is taken as 



-= 1-405. 



c 



Exactly the same behaviour has been also observed in 

 the case of the newly discovered gases, argon and helium, 3 

 and the conclusion has therefore been drawn that, in their 

 case too, each molecule consists of but a single atom. 



Since this fact, that C /c = If for a monatomic gas, 



is established by observation, we must assume that the 



theoretical hypothesis which led to its discovery corresponds 



/also to the truth. In mercury vapour, therefore, and in the 



* other monatomic gases, H = K, that is, there is no other 



nergy of any kind but that of the progressive motion of 



the molecules. 



1 Ber. der deutsch. Chem. Ges. Berlin viii. 1875, p. 945 ; Pogg. Ann. clvii. 

 1876, p. 353. 



2 Pogg. Ann. cxlviii. 1873, p. 580. 



3 Eayleigh and Eamsay, Proc. Roy. Soc. Ivii. 1895, pp. 282, 286; 

 Earn say, Gases of the Atmosphere, London 1896. 



