112 NATURE OF THE RADIATIONS [CH. 
the mass of the electron is practically constant. The increase of 
mass becomes appreciable at about half the velocity of light, and 
increases steadily as the velocity of light is approached. Theo- 
retically the mass becomes infinite at the velocity of light, but 
even when the velocity of the electron only differs from that of 
light by one part in a million, its mass is only 10 times the value 
for slow speeds. 
The above results are therefore in agreement with the view 
that the mass of the electron is altogether electrical in origin and 
can be explained purely by electricity in motion. The value of 
e/m,, for slow speeds, deduced from the results was 1:84 x 10’, 
which is in very close agreement with the value obtained by 
Simon for the cathode rays, viz. 1°86 x 10’. 
If the electricity carried by the electron is supposed to be 
distributed uniformly over a sphere of radius a, for speeds slow 
2 
compared with the velocity of light, the apparent mass m, =; = 
Therefore G= 2e@ ey 
3m, 
Taking the value of e as 1:13 x 10-™, a is 1-4 x 10~8 cms. 
Thus the diameter of an electron is minute compared with the 
diameter of an atom. 
77. Absorption of the 8 rays by matter. The absorption 
of the 8 rays by matter can readily be investigated by noting the 
variation of the ionization current in a testing vessel when the 
active matter 1s covered by screens differmg in material and thick- 
ness. When the active matter is covered with aluminium foil of 
thickness ‘1 mm., the current in a testing vessel such as is shown 
in Fig. 16, is due almost entirely to the 8 rays. If a uranium 
compound is used, it is found that the saturation current decreases 
with the thickness of matter traversed very nearly according to an 
exponential law. Taking the saturation current as a measure of 
the intensity of the rays, the intensity J after passmg through a 
thickness d of matter is given by 
Lf 
md enh, 
0 
where J is the constant of absorption of the rays in unit thickness 
