210 PHENOMENA DEPENDENT ON MOLECULAR PATHS 82 



differs but inconsiderably from it, and involves instead of G 

 the smaller arithmetical mean fl of the molecular speeds. 



Multiplication of this number into the loss of momentum 

 of a molecule as found above gives 



/3mNClv = sv 



for the total loss of momentum experienced per unit area 

 per unit time by a gas which flows along a solid wall with 

 the velocity v ; or, more shortly expressed, this expression 

 gives the friction of the gas per unit area per unit time on a 

 solid body. 



The coefficient which comes into the formula, viz. 



s = 



is the constant of the external friction of the gas ; the 

 formula shows that the theory is in agreement with the law, 

 mentioned already in 81, which Kundt and Warburg 

 deduced from their observations, viz. that the external 

 friction is proportional to the density p = mN. 



For the coefficient of slip we have 



f = 77/5= 0-30967 L/J& 

 1-23868 L/A 



which is therefore simply proportional to the free path of 

 the molecules ; in denser gases, accordingly, as experiment 

 has proved, the slip on a solid surface is vanishingly small, 

 and it can in general be shown and measured only in rarefied 



83. Comparison of the Theory with Experiment 



The observations of Kundt and Warburg confirm most 

 excellently the law that immediately follows from the 

 foregoing formula, viz. that f, just like L, is inversely 

 proportional to the density and the pressure of the gas. I 

 forbear citing here in fulness the series of numbers given 

 by them, and limit myself to a setting forth of their con- 

 clusions. 



From every three or four observations under different 

 pressures made with the same arrangement of apparatus they 



