80 VISCOSITY OF GASES 205 



Now that the formula has been confirmed, let us, as a 

 simple application of it, calculate the transpiration-coefficient 

 of atmospheric air, that is, the ratio of its coefficient of 

 viscosity 77 to that of oxygen 77^ Putting p l jp = O21 and 

 Pzlp = 0-79, in accordance with the known ratio of mixture 

 of oxygen and nitrogen in air, and also m l 31'76 for 

 oxygen and m 2 27*88 for nitrogen, and lastly, Graham's 

 value T72/ 7 ?! 0'873 for the transpiration-coefficient of 

 nitrogen, we obtain for the transpiration-coefficient of 

 atmospheric air 



7? / 7?1 = 0-898, 



which is in complete agreement with the mean value 

 obtained from Graham's observations, viz. 0-899. 



81. Friction of Gases on Fixed Bodies 



A gas experiences frictional effects, not only when two of 

 its layers flow by each other with different speeds, but also 

 when it streams along the surface of a fixed body, or of a 

 body which moves within it ; this friction causes the more 

 quickly moving part to be retarded, and the more slowly 

 moving part to be accelerated. This external friction of 

 gases has been investigated and measured by Kundt and 

 Warburg. 1 



The existence of a friction between a gas and a solid 

 body assumes that they are moving unequally fast, so that 

 the gas does not adhere firmly to the solid surface, but slips 

 along it. The friction, which diminishes the difference of 

 the velocities by accelerating the slower and retarding the 

 quicker, diminishes this slip on the surface, so that only 

 with very weak or moderately strong external friction is a 

 sensible slip perceptible, while with more considerable 

 friction it almost disappears, and at last entirely ceases. On 

 the other hand, the amount of slip depends also on the 

 magnitude of the internal friction. The stronger this is, the 



for the numerator and denominator are equal only when p.,fp - 2-664, which 

 does not correspond to any possible combination. TE.] 



1 Pogg. Ann. 1875, civ. pp. 337, 525. Previously published in abstract in 

 Monatsber. d. Berl. Akad. 1875, p. 160. 



