50* VISCOSITY OF GASES 455 



This factor /3 need not be a pure fraction. If the wall is very 

 rough, many of the particles meeting it will be jerked back by its 

 unevennesses with an oppositely directed motion ; they undergo, 

 then, a diminution of their velocity v by an amount which may 

 rise to 2u. It may consequently be very possible that the particles 

 which strike upon the wall receive a mean motion which is 

 oppositely directed to their initial motion, as certainly in like 

 manner occurs at the edge of flowing water; and in this case 

 we should have to assume j3 > 1. According to Kundt and 

 Warburg's observations, which were discussed in 83, it 

 appears that 



/3 = |, 



whence the coefficient of external friction would have to be put 



The assumption herein contained, that the striking molecules 

 lose their whole velocity of translation and gain an opposite one, 

 is not in contradiction with the fact that the gases slide along 

 solid surfaces with a sensible speed ; for not all the molecules of 

 the surface layers of gas lose their velocity, but only those that 

 strike against the solid surface. 



