134 PHYSICAL SCIENCE 



of a single expansion. Each ion will then be the 

 nucleus of a water-drop ; and, since the amount 

 of water left in the air must be just that required 

 for the equilibrium of saturation, the quantity of 

 water removed by the falling cloud can be calcu- 

 lated. This amount of water is constant for a 

 given expansion, and the number of ions present 

 must therefore be the factor which determines the 

 size of the drops. Minute drops, the constituent 

 parts of the artificial cloud or fog under considera- 

 tion, fall very slowly, and Sir George Stokes 

 showed long ago how their size may be calculated 

 from the rate of their fall. The cloud settles down 

 at a steady, well-marked pace, which can readily 

 be observed by watching the upper surface as seen 

 in Fig. 28. This measurement gives the average 

 size of each drop ; and, since the total mass of all 

 the drops can be calculated from the expansion, 

 the total number of drops, and therefore of ions, 

 can be deduced approximately. 



Sir J. J. Thomson used this method to deter- 

 mine the electric charge on a gaseous ion. The 

 current through the gas is given by the product 

 of the number of ions, the charge carried by each, 

 and the velocity with which they move. The 

 velocity, as we have said, can be determined for a 

 known electro-motive force ; and, by measuring 

 the resultant current with an electrometer, and 

 finding the number of ions by Wilson's method, the 

 ionic charge was estimated as 3.4 x lO"^^ electro- 

 static units. Within the limits of experimental 

 error it was found to be the same as the charge 

 on an ion in liquid electrolysis, and this result was 

 obtained also by Townsend in another way. The 

 importance of this conclusion will appear later. 



