83 



VISCOSITY OF GASES 



211 



have deduced a magnitude, denoted by a, whose relation to 

 the coefficient of slip is given by 



aD/f=2 x 760, 



where D denotes the distance in centimetres between the 

 discs of the apparatus employed with Maxwell's method. 

 By means of this formula I have calculated the following 

 values of the coefficient of slip from the numbers given on 

 pp. 544 and 545 of the memoir cited : 



In this table all the numbers represent centimetres. 



In the last column of the table I have put the values of 

 the free paths already obtained in 78 and 79 from the 

 observations on internal friction, and comparison of these 

 with the values of f, in the third column, shows in all 

 these cases that the coefficient of slip is nearly equal to the 

 free path, or 



ftmL, 



From this we obtain for the coefficient /3, which was intro- 

 duced as in some sort a measure of the roughness of the 

 surface where the friction takes place, 

 = 1-23868 - 1J. 



By means of this assumption, which corresponds with the 

 theoretical requirement < j3 < 2, complete agreement be- 

 tween theory and experiment is obtained. 



Later experiments, undertaken by Warburg 1 to measure 

 the external friction by experiments on transpiration also, 

 confirm this behaviour, but gave smaller values for the co- 

 efficient of slip f, and therefore larger values for /3. The 

 numbers lately found byBreitenbach, 2 on the contrary, 

 agree very well with those given above. 



1 Pogg. Ann. 187G, clix. p. 399. 2 Wied. Ann. 1899, Ixvii. p. 826. 



p 2 



