90 DETERMINATION OF THE MASS [CH. vm. 



moreover care must be taken to ensure that all the 

 desired nuclei are utilised, and not only a portion. 



The number of drops found in a certain experiment, 

 by this means, was about 30,000 to the cubic centi- 

 metre ; the total quantity of water which went to 

 form them being about the two-hundredth part of a 

 milligramme. The number of drops is of course 

 equal to the number of nuclei. Wherefore the nuclei 

 are counted. 



Result. 



The result of the execution of this ingenious 

 counting process is that the absolute charge and the 

 absolute mass of an electron are at length directly 

 determined. Hitherto we have determined by many 

 and various ways the ratio e/m and the speed u. We 

 have likewise been able to determine Neu and Ne 

 and Nm^ 2 , as already explained. Now at length we 

 have determined N ; and at once the terms in the 

 ratio e/m are disentangled. 



e comes out, as suspected, in all cases, the regular 

 ionic charge, of the order of magnitude 3 x 10~ 10 

 electrostatic, or 10~ 20 electromagnetic units. Hence, 

 while m comes out for positive carriers and for all ions 

 the appropriate mass of the atoms present, or in 

 some cases greater than this, by reason of the forma- 

 tion of molecular aggregates, for the negative 

 carriers set free by ultra-violet light, and for the other 

 cases where e/m= 10 7 , the masses come out definitely 

 of the order 10~ 27 grammes; or about -7-^0 th part of 

 the smallest and lightest previously known quantity 

 of matter, viz., an atom of hydrogen. 



The existence of masses smaller than atoms is 

 thus experimentally demonstrated, and a discovery 

 clinched of epoch-making importance. 



