HOGG. — FRICTION IN GASES AT LOW PRESSURES. 1 5 



gases depends upon the proportion in which the gases are mixed. 

 Meyer shows that the coefficient of external friction is given by 



pmNQ, 



where m is the molecular weight of the gas ; TV is the number of 

 molecules per unit volume ; O is the mean molecular speed ; and ft is 

 a constant depending upon the solid surface. He gives some experi- 

 mental evidence to show that /3 is independent of the gas. 



In the case of a mixture of gases where there are Ni molecules of 

 one kind and 2V 2 molecules of another, in each unit volume we have, 

 if N is the total number of molecules in the unit volume, 



and the mean molecular weight is given by 



m = 



N 



where m l and m 2 are the molecular weights of the two gases mixed. 

 Since the temperatures of the two gases are the same, 



^A 2 = m<£l£ = m£l 2 . 

 Therefore, 



mn = wAV -Tf H iff. 



If Boyle's Law holds, which seems a fair inference from the results given 

 above, then we may write 



m£l = w?A V — H i 



p mi p 



where p\ and p 2 are the partial pressures and p is the whole pressure 

 under the given conditions. If ft is independent of the nature of the 

 gas it follows that the ratio of the external friction of the mixture to 

 the external friction of the gas whose partial pressure is p 1} if it were 

 in the vessel alone, is 



t»t ^ i/?h m 2 pi 

 iVfaAV — + 



p m x p _ N a/pi m 2 pi 

 Z7T\ at- V 77 "•" ~ ' TT" 



NmVt 

 NimSli ~ lS\miili Ni T p ' m x p 



