ISOLATION OF AN ION MILLIKAN. 



249 



ten which was irregular. The drop shown in Table I is perhaps the 

 best illustration of the case under consideration which we have 

 observed. It jdelds a value of e x which is 4 per cent too low to fall 

 on the curve of figure 2. This is as large a departure from this 

 curve as we have thus far obtained. 



Table XIII. 



THE CORRECTION OF STOKES S LAW. 



The simple form of Stokes's law, which has been used in obtaining 

 the values of e x involves the assumption that there is no slip at the 

 bounding surface between the medium and the drop, or that the 

 coefficient of external friction between oil and air is infinite. From 

 the standpoint, of the kinetic theory this surface slip, though in 

 general very small, is, strictly speaking, never zero, and to take it 

 into account a term must be introduced into the equation of motion 

 which is proportional to the ratio between the mean free path of the 



