Electric Field of the «, /3, and Secondary Rays, 725 
Thus these penetrating rays came from a depth of about 
6 cms. of wood or 4 cms. of slate. It is difficult to decide 
whether these rays are high velocity scattered primary /3rays, 
or secondary negative rays, or secondary 7 rays due to the 
primary y rays, or secondary 7 rays due to the expulsion of 
secondary /3 rays, or secondary 7 rays due to the stopping of 
primary /3 rays. There is some evidence that they are 
secondary 7 rays, but it is not conclusive. The paper by 
A. S. Mackenzie, in this Magazine, July 1907, shows that 
there are remarkable conditions, depending on direction, when 
7 rays traverse matter. 
In the experiments made by Cooke on the penetrating- 
radiation due to the earth, he found that brick gave rise to a 
very penetrating radiation. As clay generally contains radium 
it was natural to attribute the effect observed by Cooke to the 
presence of radium in the brick. It is probable, however, 
that the penetrating rays were due to the secondary radiation 
set up in the bricks by the radium and thorium in the ground, 
and not chiefly by the minute amount in the bricks employed. 
I have not succeeded in obtaining these penetrating rays from 
lead ; they appear to arise in the lighter substances. 
Electrical Method. 
By an important experiment it was proved by Lenard* 
that the cathode rays could be accelerated or retarded in an 
electric field, when the particles moved along the lines of 
force. He found that the value of e/m remained nearly 
constant, whilst the change in the velocity amounted to more 
than 20 per cent, for a difference of potential of 29,000 volts. 
It seemed possible to apply a similar method to the a and 
(3 rays from the radioactive substances, and to the secondary 
rays produced by the (3 and 7 rays. 
a Rays. 
A metal plate, 10 X 8 cms., was charged negatively and 
exposed to the emanation from thorium hydroxide until an 
active deposit of maximum activity was obtained. The plate 
was then placed parallel and opposite to the thin aluminium face 
of the gold-leaf electroscope, and was insulated on a block of 
paraffin so that it could be charged to about 30,000 volts, 
either positive or negative, by a Wimshurst influence-machine. 
One pole of the machine was earthed, a Ley den-jar was used 
to steady the potential of the other, and the potential, which 
* Lenard, Wied. Ann. xlv. (1898). 
Phil. May. S. 6. Vol. 15. No. 90. June 1908, 3 C 
