the Ions in Gases at Low Pressures. 563 



seconds, while the observations on the cloud measure the 

 intensity over an interval of a small fraction of a second. 



The value of e found by me previously for the ions pro- 

 duced by Rontgen rays was 6\7 x 10~ 8 : hence we conclude 

 that e for the ions produced by ultra-violet light is the same 

 as e for the ions produced by the Rontgen rays ; and as 

 Mr. Townsend has shown that the charge on these latter 

 ions is the same as the charge on an atom of hydrogen in 

 electrolysis, we arrive at the result previously referred to, that 

 the charge on the ion produced by ultra-violet light is the 

 same as that on the hydrogen ion in ordinary electrolysis. 



The experiments just described, taken in conjunction with 

 previous ones on the value of m/e for the cathode rays (J. J. 

 Thomson, Phil. Mag. Oct. 1897), show that in gases at low 

 pressures negative electrification, though it may be produced 

 by very different means, is made up of units each having a 

 charge of electricity of a definite size ; the magnitude of this 

 negative charge is about 6 x 10 -10 electrostatic units, and is 

 equal to the positive charge carried by the hydrogen atom in 

 the electrolysis of solutions. 



In gases at low pressures these units of negative electric 

 charge are always associated with carriers of a definite mass. 

 This mass is exceedingly small, being only about 1*4 x 10 -3 

 of that of the hydrogen ion, the smallest mass hitherto re- 

 cognized as capable of a separate existence. The production 

 of negative electrification thus involves the splitting up of 

 an atom, as from a collection of atoms something is de- 

 tached whose mass is less than that of a single atom. We 

 have not yet data for determining whether the mass of the 

 negative atom is entirely due to its charge. If the charge is 

 e, the apparent mass due to the charge supposed to be col- 

 lected on a sphere of radius a is %e 2 /f*a : hence m/e in this case 

 is e/dfia. Substituting the values of m/e and e found above, 

 we find that a would be of the order 10 -13 centim. 



We have no means yet of knowing whether or not the 

 mass of the negative ion is of electrical origin. We could 

 probably get light on this point by comparing the heat pro- 

 duced by the bombardment by these negatively electrified 

 particles of the inside of a vessel composed of a substance 

 transparent to Rontgen rays, with the heat produced when 

 the vessel was opaque to those rays. If the mass was 

 " mechanical," and not electrical, the heat produced should be 

 same in the two cases. If, on the other hand, the mass were 

 electrical, the heat would be less in the first case than in the 

 second, as part of the energy would escape through the 

 walls. 



