78 DETERMINATION OF THE MASS [CH. vm. 



vessel attached to an electrometer of known capacity, 

 it is not difficult to estimate the total quantity of 

 electricity which enters the vessel in a given time, 

 that is to say, to determine Ne ; but, again, how are 

 we to discriminate e, from N ? 



Another thing that is comparatively easy to deter- 

 mine, especially in such cases as leak from a negative 

 surface under the action of ultra-violet light, or the 

 conductivity of air induced by the influence of Rontgen 

 rays, is the total current transmitted ; viz. the quantity 

 New, the quantity of electricity conveyed per second 

 between electrodes immersed in the air, and main- 

 tained at a sufficiently high difference of potential 

 to cause all the corpuscles or the ions present to 

 take part in conducting the current. 



We may consider the following quantities experi- 

 mentally determined, by researches carried on at 

 the Cavendish laboratory and elsewhere, and so far 

 already described or indicated in the preceding three 

 chapters:- gfm 



u 



Ne 

 Nw 



New 



But still we have not described a method of 

 measuring separately either e ox m: only methods 

 of measuring their ratio. 



If only it were now possible to count the corpuscles 

 or electrons, to determine the number N which are 

 started into existence, or which enter the hollow 

 vessel or which take part in conveying the current in 

 the case of a leak by ultra-violet light, we should no 

 longer have to guess at the actual value of e and of m 

 separately, but should have really determined them. 



