Electric Field of the a, /3, and Secondary Rays. 735 
on this view. The result obtained by him was not for a wide 
range of thickness, amounting only to 0*5 mm. of aluminium, 
but within that limit he proved that the (3 rays from radium 
E continued to move with undiminished speed until they 
were actually absorbed. There is no doubt that after passage 
through matter the swifter electrons emerge, because the 
slower are first of all absorbed. The question remains an 
open one, whether the /5 particle loses velocity during its 
passage through a considerable thickness of matter. The 
question is always complicated by the attendant train of 
secondary rays which everywhere arise in the path of the 
particle through matter from the moment of its emission to 
the time of its complete absorption. In the case of the « 
particle, Rutherford has clearly proved that velocity is lost 
during passage through matter. 
In conclusion, one more experiment may be described. 
Fourteen milligrams of radium bromide were enclosed in a 
cylindrical block so that the y rays passed through 2'5 cms. of 
lead before emerging to the air. The lead block was insulated, 
and placed before the electroscope. The potential was 
changed from + to —30,000 volts. The difference of the 
ionization observed was 3 per cent, of the mean value. The 
inner curves (fig. 2), otherwise obtained, almost represent 
the results of this experiment. The total ionization observed 
was 4 mainly due to <y rays, and the change of ionization, 
on reversal of the sign of potential, was due to the electrons, 
which are projected from the surface of matter traversed by 
the 7 rays*. 
This paper must be considered as preliminary, and there 
are some criticisms which may be applied to the method and 
to the arrangement of apparatus. Thus it is difficult to 
estimate the effect of the scattering and absorption by air 
of the rays employed, but it is not relatively large. Again, 
it might be expected that some of the ions generated outside 
the electroscope near the active plate would be accelerated 
in the electric field sufficiently to enter the electroscope. 
This either did not take place, or the effect was relatively 
too small to be noticed. 
It is hoped that these experiments may be extended to 
other substances and that higher potentials will be employed. 
The electric method may throw some light on the ratio of the 
amounts of secondary X rays and of negative rays from 
* Phil. Mas:. Dec. 1904. 
