734 Prof. A. S. Eve on Changes in Velocity in an 
If we take, for any radiating substance, the square root of 
the double ordinate, or percentage difference, at 20,000 or 
at 30,000 volts, and multiply by the actual secondary radia- 
tion from the plate, we get, with no great accuracy, a 
constant. 
For example, with + 34,000 volts :— 
Substance. 
Percentage 
Difference. 
Square 
Root. 
Secondary 
Radiation. 
Product. 
Carbon 
Paraffin 
Brick 
Aluminium . . . 
Iron 
35-6 
41-0 
34-0 
35-8 
24-6 
165 
5-96 
640 
5-84 
5-98 
4-96 
4-06 
69 
65 
79 
80 
95 
113 
410 
415 
470 
476 
470 
460 
Lead 
This approximate rule emphasises the relationship between 
the velocity of the secondary rays and the amount of secondary 
radiation. The rule requires further investigation before it 
can be affirmed. 
At some distance from the origin the curves shown on 
fig. 3 crudely resemble the parabola a? = %, where k 
is a constant for each radiator. In the case of the less 
dense substances, it is found that when either the potential, or 
the distance between radiator and electroscope, is increased, 
there is not symmetry about the x axis, but the positive 
ordinates are numerically less than the negative ordinates for 
the same abscissa. With a sufficiently high positive voltage 
the ionization could be reduced to zero, and a very large 
negative potential would cause each curve to approach an 
asymptote parallel to the x axis. 
Some preliminary experiments were made to compare the 
quantity and quality of the secondary rays from hot iron 
plates and from hot bricks. The results were but slightly 
different, if at all, from those obtained from cold bricks and 
cold iron. Observations were made on radiators from a red- 
heat to the temperature of the room. 
It is usual to suppose that the secondary rays, which start 
from some depth within the radiator, must lose velocity on 
their passage through matter before they make their escape. 
The remarkable experiment of H. W. Schmidt * throws doubt 
* Phys. Zeit. June 1, 1907. 
