CONDUCTION THROUGH GASES 147 



explains the large currents which can be carried 

 by the electric arc or spark discharge. 



These considerations indicate that, in the ex- 

 periments we are now describing, the limit of the 

 second stage, in which some but not all of the 

 negative ions are stopped by the magnetic field, 

 gives the distance at which those ions coming 

 from the surface of the zinc plate just fail to get 

 across the space between the plates. The expres- 

 sion given above then leads directly to a value for 

 ejin, the ratio of the ionic charge to the ionic mass. 

 Thomson found as the result 7.3 x lo^ a number 

 which agreed well with that which he deduced 

 for cathode rays, namely, J.J y> lo^ 



With the negative ions produced by the inci- 

 dence of ultra-violet light on a zinc plate, it is easy 

 to repeat C. T. R. Wilson's experiments on the 

 formation of clouds round ions as nuclei, and thus 

 to determine the value of e, the electric charge 

 associated with the same ions for which ejin has 

 already been obtained. The result shows that, 

 as always, the charge is the same as the charge 

 on an ion in liquid electrolytes ; and therefore for 

 the ions due to ultra-violet light, as for the cathode 

 ray particles, the mass must be much less than 

 that of the hydrogen atom. The result has been 

 confirmed by Lenard, who used a somewhat 

 different type of apparatus. 



In all these investigations the existence of 

 particles much smaller than the smallest of the 

 hitherto indissoluble chemical atoms is clearly 

 indicated. Since the beginning of the nineteenth 

 century the chemical atom has been the ultimate 

 unit in which our conception of matter has been 

 expressed. The sixty, seventy, or eighty different 



