Conduction of Heat by Rarefied Gases. 205 



(2) In absorbing and evaporating again the fraction / of 

 the incident molecules with velocities equal on an average to 

 the velocity of the body. 



His way of reckoning can be applied, with some little 

 modifications — also to the case of conduction of heat ; I have 

 found by these means : 



1^ _^ 2-/ ao) 



7 i« . v» /' ' ' ' : ' 



where /i is the coefficient of viscosity and p the density of the 

 gas. 



By introducing the mean length of free path, after Meyer, 

 as equal to 



/JL7T \/2tt 

 A,= =- j 



4 y/pp 

 this will be 



15 2-A 



Now it is easy to see that Maxwell's supposition about the 

 reflected and evaporated molecules is equivalent to the sup- 

 position made before in formula (9) if /3 is put equal to 

 1 — /. Then the last formula turns out to be : 



»-£(>+&> (•« 



quite analogous to the one deduced before in (8), but with 

 somewhat larger numerical coefficients. 



Also in respect to several other phenomena, these two 

 theories give somewhat different numerical results ; the actual 

 state of gases has been found usually to be intermediate be- 

 tween them ; probably here also this will be the case. 



At any rate, it is a very satisfactory result that both theories 

 agree in proving the existence of a discontinuity of tempera- 

 ture, as expressed in (1), and the proportionality of the factor 

 7 to the mean length of free path of molecules in the gas ; 

 exactly the conclusion drawn from the experiments in § 7 (4). 

 This perfect agreement between the experimental facts and 

 the kinetic theory of gases could be considered as anew strong 

 evidence in favour of the latter — if such evidences were 

 wanted any more. 



12. A very suggestive fact is the great difference found 

 in my experiments between y/X in air and in hydrogen (1*70 

 and 6'96). It would not be surprising to find the factor ft 



Phil. Mag. S. 5. Vol. 46. No. 279. Aug. 1898. Q 





