62 PHYSICAL PROPERTIES AND COMPOSITION 

 The specific heat at constant volume is accordingly 



and at constant pressure 



C p = C. + 2 = 5. 



For mercury, it is well known, these numbers have been 

 confirmed, and are now regarded as the test of a monatomic 

 gas. For if the gas were polyatomic both C and C p would 

 be increased by the calories required for accelerating the 

 atomic movements and for separating the atoms, and we 

 should have 



C v = $ + a C p = 5 + a. 



And accordingly, as was stated above, C v and C p are always 

 found greater than 3 and 5 for polyatomic gases. On the 

 other hand the ratio 



Cp = 5+_a 



C v 3 + a 



can be determined from the velocity of sound, and was found 

 by Kundt and Warburg for mercury, in accordance with 

 theory, to be 



whilst for other gases 



Cp __ 5 + a 

 C 9 3+a 



as e. g. in the case of oxygen O 2 



C p : C v = 6-96 : 4-96 = 1-4. 



The criterion has lately been of great value as applied to 

 the gases discovered by Ramsay, Argon, Helium, Neon, 

 Metargon, and Krypton, all of which are found to be mon- 

 atomic. 



There is another test as well. The specific heat is inde- 

 pendent of temperature for the monatomic gases, whilst for 

 polyatomic, according to Le Chatelier 1 , it is made up of 



1 Zeitschr. f. Phys. Chem. i. 456. 



