MOLECULAR WEIGHT OF GASES 25 



Most of the elements studied are, like hydrogen, diatomic ; 

 only a few have more atoms, as phosphorus P 4 , arsenic As 4 , 

 sulphur S 8 . 



Monatomic elements. Mercury. Especially interesting 

 are the cases in which the elementary molecule is, according 

 to the above reasoning, to be regarded as monatomic. This 

 was first shown to be probable for mercury ; since then 

 other metals (zinc, cadmium, potassium, sodium) have been 

 studied in the state of vapour, and appear to behave 

 similarly; and the newly discovered elements, argon and 

 helium, belong to the same category. Mercury, moreover, 

 was the subject of an important experiment by Kundt and 

 Warburg, which, by the aid of a deduction from the kinetic 

 theory, confirms the conclusion of monatomicity. The 

 experiment refers to the ratio between the specific heats 

 at constant pressure and constant volume. The latter has 

 obviously the smaller value, and in the case of monatomic 

 gases represents the increase in kinetic energy of the 

 molecular movement ; in polyatomic gases it includes also 

 an increase in the atomic movements, more difficult to 

 calculate. According to the previous equation 



PV= NMC 2 

 the kinetic energy 



INMC* = f PF, 



so that its increase per degree is 



laPF, 



where a = - - 



273 



If free expansion (at constant pressure) takes place, we 

 are concerned with the specific heat at constant pressure, 

 and additional heat is required to perform external work, 

 which obviously amounts, per degree, to 



aPV. 



The ratio of the two specific heats is therefore 



faPF+aPF: f aPF =5:3 = 1-67, 



