Electric Field of the a, /3, and Secondary Rays. 733 
The third column gives the secondary radiation actually 
observed from the blocks used as radiators. The fourth 
column shows the differences per cent, on the mean value 
when the radiators were charged to 30,000 volts, first positive 
and then negative. In the fifth column are the differences 
per cent, for a higher potential, obtained with the dynamo- 
static machine. In the last column are the values of the 
coefficients of absorption by aluminium, 0*021 cm. thick, 
requoted from the first part of this paper. In this experiment 
the bricks w r ere slightly moistened to make them conductors, 
and the paraffin was covered with aluminium foil, '00031 cm. 
thick. This thin layer of aluminium did not affect the 
radiation to a measurable extent, and it made the surface a 
conductor. 
By a comparison of the fourth and fifth columns it appears 
that it becomes increasingly difficult, as the potential rises, 
to magnify r the percentage differences for opposite charges 
of the radiator. The values of the coefficients of absorption 
in Table I. are also in accord with the view that together 
with the slower electrons, which form the bulk of the radia- 
tion from the lighter substances, there are also swifter 
electrons, which are with greater difficulty absorbed by 
matter or affected by an electric field. 
These experiments as a w r hole indicate that the secondary 
rays from various substances have a distinctive group velocity 
depending on the density or atomic weight of the substance 
of the radiator. The values of the secondary radiation follow 
the order of the atomic weight, as showm by McClelland : 
it is now seen that the values of \ are also in inverse order 
of the secondary radiations, and that the percentage changes 
of ionization, due to the reversal of the electric field, are in 
that same inverse order. 
Hence, secondary rays are in the main intrinsic, released 
from the atoms of the radiating substance, w T ith distinctive 
group velocities depending on the density of the radiator. 
It appears that the secondary rays are for the most part not 
due to dispersed primary rays, which have entered the 
radiator, and by changes of path re-emerged ; but it is not 
improbable that the secondary rays may include an unknown 
fraction of such primary rays, Thus /3 and <y rays appear 
merely to release the electrons, wdiich issue from the radiators 
with velocities possibly depending on those which they had 
in the radiator *. 
* Since -writing- the above I nave received a paper by Bragg and 
Madsen (Trans of R. Soc. of S. Australia, 2 Jan. 1908; Phil. Mag. May 
1908) advocating a different view. 
