SPECIFIC HEAT OF GASES 61 



By calculation, by deducting from the specific heat under 

 inconstant volume the heat converted into external work. 

 The calculation is very simple when the specific heat is 

 that at constant pressure (C p per kilogram -molecule). The 

 external work per 1 is then 



dAPV _ dzT _ 

 dT : dT ~~ 



In accordance with this we have these results : 



c p c v c p - c v 



O 3 ....... 6-96 4-96 2 



N 2 ....... 6.83 4.83 2 



C 10 H J6 ..... 68-8 66-8 2 



Thus the heat C. v serving for pure rise of temperature, 

 kinetically considered, causes two changes : in the mole- 

 cule, the velocity as measured by that of its centre of 

 gravity, is increased, and the external pressure conse- 

 quently raised ; in the atoms also the movements become 

 more rapid, but in consequence of this the relative dis- 

 tances of the atoms are increased, anticipating chemical 

 decomposition. Here again we may separate the first 

 factor by choosing a monatomic gas ; for in that case the 

 specific heat may be calculated directly from the kinetic 

 theory. 



According to the foregoing (Part II, p. 1 8) we have 



PV= 



where N is the number of molecules in volume V (in cubic 

 metres), M their mass, C their velocity : so that the kinetic 

 energy of the molecular movement 



and if NM is the mass of a kilogram-molecule the work 

 done in calories is 



I APV=$T. 



Hence for any gas the energy of molecular movement at 

 o is 3 x 273 = 819 calories. 



