114 NATURE OF THE RADIATIONS [CH. 
suitable for such a determination than the complex rays of radium. 
I have in consequence made some experiments with uranium rays 
to determine the dependence of absorption on the density. The 
results obtained are given in the following table: where 2 is the 
coefficient of absorption. 







1 O r 
Substance r Density Density 
Glass ... Bee 14:0 2°45 57 
Mica ... fists 14:2 2°78 5:1 
Ebonite Aa 6:5 114 57 
Wood ... res 2°16 “40 5:4 
Cardboard ... oul ‘70 5:3 
Iron =e. ae 44 78 56 
Aluminium ... 14:0 2°60 5:4 
Copper ae 60 8°6 7:0 
Silver ... arth 75 10°5 Fiat 
leads ie 122 11°55 10°8 
ARMIN See ae 96 ied 13°2 


It will be observed that the value of the absorption constant 
divided by the density is very nearly the same for such different 
substances as glass, mica, ebonite, wood, iron and aluminium. The 
divergences from the law are great, however, for the other metals 
examined, viz. copper, silver, lead and tin. In tin the value of X 
divided by the density is 2°5 times its value for iron and aluminium. 
These differences show that a law for the absorption of the ® rays 
depending only on the density does not hold for all substances. 
With an exception in the case of tin, the value of X divided by the 
density for the metals increases in the same order as their atomic 
weights. 
The absorption of the @ rays by matter decreases very rapidly 
with increase of speed. For example, the absorption of cathode 
rays in Lenard’s experiment (Joc. ct.) 1s about 500 times as great 
as for the uranium 8 rays. The velocity of the 8 rays of uranium 
was found by Becquerel to be about 16 x 10" cms. per sec. The 
velocity of the cathode rays used in Lenard’s experiment was 
certainly not less than 1/10 of this, so that, for a decrease of 
speed of less than 10 times, the absorption has increased over 
500 times. 
