736 Changes in Velocity of a, /3, and Secondary Rays. 
radiators exposed to primary Rontgen rays*. The investiga- 
tion o£ transmitted rays, and particularly of the rays from 
hot and cold bodies, should prove interesting when this 
method is applied. 
Some of the work done overlaps that by Allen, with 
magnetic and electric deflexion methods. I trust that full 
justice has been done in making references to his valuable 
paper. 
Summary. 
1. The coefficients of absorption by aluminium of the incident 
secondary radiations, due to the /3 and y rays of radium, 
produced from various substances have been determined. The 
lighter substances emit negative rays not only less in 
quantity than the dense, but these rays are initially more 
readily absorbed. (Allen.) 
2. Some substances, such as brick, slate, wood, paper, and 
carbon, give rise to penetrating incident secondary rays 
originating from several centimetres depth, which are either 
secondary <y rays or high velocity negative rays. After 
screening, the secondary radiation from brick or paper may 
exceed that from a heavy substance, such as lead. 
3. In an electric field, with the lines of force parallel to 
those of direction of motion, the velocity of the a, rays can 
be increased or diminished, and there is apparently a slight 
variation of range. 
4. The same method may be applied to primary j3 rays or 
to secondary rays due to (3 and <y rays. 
5. The amount of secondary radiation is known to follow 
an order depending on the atomic weights of the radiators. 
It is shown by the method herein described that the velocities, 
regarded as a group, as well as the initial absorptions by 
aluminium, follow the same order, so that the main secondary 
negative rays are projected with velocities, which are a 
function of the density, or the atomic weight, of the radiating 
substance, the velocities being greater for the more dense. 
The main group of electrons therefore emerge with less 
velocity from the lighter substances. 
* Recent experiments have been made, using the electric method 
described in this paper, with the secondary rays due to (1) X rays and (2) 
y rays. In the case of X rays the cathode secondary rays are so rapidly 
absorbed by the air that they do not reach the electroscope. Hence a 
change from -f to —50,000 volts in the potential of the secondary 
radiator made no difference in the ionization current. Perhaps there was 
a slight effect in the case of lead. The secondary rays, due to y rays, 
gave percentage changes about equal to those observed in the secondary 
rays due to both /3 and y rays of radium. 
