106 CONDUCTION OF HEAT 287 



1O6. Theoretical Laws of the Conduction of 

 Heat 



The conclusion of the theory which we have found requires 

 the conductivity of a gas for heat to obey the laws which 

 hold for the coefficient of viscosity and for the specific heat. 



The kinetic theory of gases has led to the discovery, 

 which has afterwards been confirmed by experiment, that 

 the coefficient of viscosity of a gas is independent of its 

 pressure or density. It also follows from this theory that 

 the same value must be found for the specific heat at 

 constant volume, referred to unit mass of the gas, if the 

 experiment is made with a different volume or at a different 

 pressure ; for the number of molecules of a gas which are 

 contained in unit mass of the gas require under all circum- 

 stances the same addition of energy when this mass is 

 warmed by 1 degree without expanding, and therefore 

 without doing work. In agreement with this theoretical 

 result the experiments of Begnault 1 have shown that the 

 specific heats of air, hydrogen, and carbonic acid, measured at 

 constant volume, are independent of the pressure. 



Since, then, not only the coefficient of viscosity, but also 

 the specific heat, is independent of the pressure of the gas, 

 the theory leads to the law laid down by Maxwell and by 

 Clausius that the heat-conductivity also of a gas for heat 

 is not variable with its pressure. 



Eegnault has further found that the specific heat of 

 chemically simple gases is independent also of the tempera- 

 ture. Probably all gases whose molecules are composed of 

 only two atoms have this property, to judge from E. 

 Wiedemann's 2 observations. On the kinetic theory, 

 therefore, the heat-conductivity of a diatomic gas increases 

 with the temperature according to the same law as its 

 coefficient of viscosity. 



The laws we have cited for the specific heat do not, 

 however, hold without limitation. The more easily con- 

 densible gases and vapours do not obey these laws, at least 

 in all strictness. Hence, also, the theoretical laws of the con- 



1 M6m. de VAcad. de Paris, 1862, xxvi. 2 Pogcj. Ann. 1876, clix. p. 1. 



