206 PHENOMENA DEPENDENT ON MOLECULAK PATHS 81 



more the differences of the velocity of flow in the different 

 layers are blotted out, by which the difference between the 

 velocity of the gas, moving as a more compact mass, and that 

 of the solid body must necessarily be increased. The slip 

 therefore increases with increase of internal friction just 

 as with decrease of external friction, while it would be 

 diminished as well by decrease of the internal as by increase 

 of the external friction. 



A measure for the amount of the external friction we 

 might obtain in the same way as that for internal friction, 

 viz. by means of a coefficient of friction, which measures the 

 intensity of the friction exerted per unit area of the surface 

 if the difference of the two velocities is the unit of velocity. 

 But after our explanation of the relation between friction and 

 slip another method commends itself as no less convenient, 

 viz. to introduce instead, as a measure of the slip, a 

 coefficient of slip, as Helmholtz and von Pietrowski 1 

 have first defined it for liquids. By the coefficient of slip, 

 which we denote by , is understood the ratio of 97, the 

 coefficient of internal friction, to s, the coefficient of external 

 friction, or 



ITS*/* 



so that f appears a really suitable measure of the slip which 

 is increased by increase of 77 and diminution of g. 



The older investigations of the friction of gases, both 

 those founded on observations of oscillations and those 

 directed to the measurement of transpiration, had agreed in 

 showing that the value of the coefficient of slip is vanish- 

 ingly small, so that f might be put equal to 0. The 

 external friction, therefore, in the circumstances under 

 which those experiments were carried out, is infinitely 

 greater than the internal. An essential advantage was in 

 consequence gained for the determination of the internal 

 friction, since, as no slip occurred, the external friction 

 might be left out of account in the working out of the 

 observations. 



But when Kundt and Warburg undertook to test 



1 Wiener Sitzungsber. 1860, xl. p. 607. 



