146 NATURE OF THE RADIATIONS [CH. 
and include electrons travelling with a speed of more than 95 per 
cent. of that of light. The apparent mass of an electron would 
increase rapidly as the speed of light is approached, and for the 
velocity of light the mass should be infinite and the path unaffected 
by a magnetic field. It does not seem improbable that some of 
the 8 rays of radium are projected with a velocity very nearly 
equal to that of light, and thus it is possible that the y rays 
may really consist of electrons expelled with velocities which still 
more nearly approach that of light. The great increase of pene- 
trating power is to be expected on account of the rapidly increasing 
energy of the electron as the speed of light is approached. An 
objection to this hypothesis lies in the experimental observation 
that there appears to be no gradual passage from the stage of 
penetrating deviable rays to non-deviable very penetrating rays. 
It is also possible that the y rays may consist of uncharged 
particles projected with great velocity. Such an hypothesis would 
account for the relative conductivity of gases and for the non- 
deviation of the rays in a magnetic field. It would also account 
for the great penetrating power of the rays, since a small uncharged 
particle moving through matter would probably not be absorbed 
as rapidly as a charged particle of the same mass and velocity. 
Nevertheless, sufficient experimental data are not yet available to 
distinguish definitely between the three hypotheses discussed above. 
eyes tl We 
SECONDARY Rays. 
100. Production of secondary rays. It has long been 
known that Réntgen rays, when they impinge on solid obstacles, 
produce secondary rays of much less penetrating power than the 
incident rays. This was first shown by Perrin and has been 
investigated in detail by Sagnac, Langevin, Townsend and others. 
Thus it 1s not surprising that similar phenomena should be 
observed for the radiation from radio-active substances. By 
means of the photographic method, Becquerel' has made a close 
1 ¢@. R. 132, pp. 371, 734, 1286. 1901. 
