564 Prof. J. J. Thomson on the Masses of 



Hitherto we have been considering only negative electrifi- 

 cation ; as far as our present knowledge extends positive 

 electrification is never associated with masses as small as 

 those which invariably accompany negative electrification in 

 gases at low pressures. From W. Wien's experiments on the 

 ratio of the mass to the electric charge for the carriers of 

 positive electrification in a highly exhausted vacuum-tube 

 (Wied. Ann. lxv. p. 440), it would seem that the masses with 

 which positive electrification is associated are comparable 

 with the masses of ordinary atoms. This is also in accordance 

 with the experiments of Elster and Greitel (Wied. Ann. xxxviii. 

 p. 27), which show that when positive ions are produced by 

 an incandescent platinum wire in air they are not affected to 

 anything like the same extent as negative ions produced by 

 an incandescent carbon filament in hydrogen. 



It is necessary to point out that the preceding statements 

 as to the masses of the ions are only true when the pressure 

 of the gas is very small, so small that we are able to deter- 

 mine the mass of the carriers before they have made many 

 collisions with the surrounding molecules. When the pres- 

 sure is too high for this to be the case, the electric charge, 

 whether positive or negative, seems to act as a nucleus around 

 which several molecules collect, just as dust collects round an 

 electrified body, so that we get an aggregate formed whose 

 mass is laro-er than that of a molecule of a gas. 



The experiments on the velocities of the ions produced by 

 Rontgen or uranium rays, by iiltra-violet light, in flames or 

 in the arc, show that in gases at pressures comparable with 

 the atmospheric pressure, the electric charges are associated 

 with masses which are probably several times the mass of a 

 molecule of the gas, and enormously greater than the mass 

 of a carrier of negative electrification in a gas at a low 

 pressure. 



There are some other phenomena which seem to have a 

 very direct bearing on the nature of the process of ionizing a 

 gas. Thus I have shown (Phil. Mag. Dec. 1898) that when 

 a gas is ionized by Rontgen rays, the charges on the ions are 

 the same whatever the nature of the gas : thus we get the 

 same charges on the ions whether we ionize hydrogen or 

 oxygen. This result has been confirmed by J. S. Townsend 

 (" On the Diffusion of Ions," Phil. Trans. 1899), who used an 

 entirely different method. Again, the ionization of a gas by 

 Rontgen rays is in general an additive property; i. e., the 

 ionization of a compound gas AB, where A and B represent 

 the atoms of two elementary gases, is one half the sum of the 

 ionization of A 2 and B ? by rays of the same intensity, where 



