388 Mr. J. L. Hogg on Friction in 



To be sure it does not follow that the decrement of a 

 mixture of hydrogen and mercury vapour, in such propor- 

 tions that the partial pressures of the two are the same, is 

 simply the sum of the two decrements obtained when the 

 gas and vapour are separate. If we accept the expression 

 deduced by Meyer* for the external friction of a gas and 

 apply the same method in considering external friction of 

 mixtures as he does in dealing with the internal friction of 

 mixtures, we shall be able better to understand how the 

 external friction of a mixture of gases depends upon the 

 proportion in which the gases are mixed. Meyer shows that 

 the coefficient of external friction is given by 



where m is the molecular weight of the gas, N is the number 

 of molecules per unit volume, 12 is the mean molecular speed, 

 and /3 is a constant depending upon the solid surface. He 

 gives some experimental evidence to show that /3 is inde- 

 pendent of the gas. 



In the case of a mixture of gases where there are N x 

 molecules of one kind and N 2 molecules of another, in each 

 unit volume, we have, if N is the total number of molecules 

 in the unit volume, 



N = N 1+ N 2 , 



and the mean molecular weight is given by 



^i w i + N 2 ?n 2 



m = 



N 



where m x and m 2 are the molecular weights of the two gases 

 mixed. 



Since the temperatures of the two gases are the same, 



m^Q,] 2 = ??2 2 f2 2 2 = m n 2 . 

 Therefore, 



r» o . /-Nl m 2 N 2 



If Boyle's law holds, which seems a fair inference from the 

 results given above, then we may write 



V p ■ mi p 



where p x and p 2 are the partial pressures and p is the whole 

 pressure under the given conditions. If /3 is independent of 



* ' Kinetic Theory of Gases/ p. 210 (Eng. ed.). 



