(y Prof. J. A. McClelland on the 
The y rays might of course consist of a stream of uncharged 
particles ; but as the forces causing disintegration are probably 
of an electrical nature, this is not likely. 
Absorption of y Rays by Different Substances. 
Rutherford has measured the absorption of y rays by 
different substanees, and showed that the absorption is 
roughly proportional to the density of the substance. The 
absorption, however, increases more rapidly than the density, 
so that the ratio of the coefficient of absorption to the density 
is greater for the denser substances, being for lead about twice 
what it is for water. 
On account of the importance of this law of absorption, I 
have made some further experiments, which show that when 
the rays have passed through some thickness of the denser 
substances, the coefficient of absorption is diminished, so 
that the law that the coefficient of absorption is proportional 
to the density is then followed with remarkable closeness, 
Apparatus used. 
The brass tube a (fig. 2) is insulated and fitted with a 
paraffin cork ¢ through which passes the brass rod b joined 
by a wire to an electrometer. This wire is led along an 
earthed metal tube filled with paraftin. The other end of the 
tube a is closed with a very thin metal cap. 
TLR 
> LE 0 ROME 
SM 
CELLS 
iS) 
VW 
DM 
YN 
1 os A 
The radium is contained ina small vessel R, which is placed 
in a block of lead so that a diverging pencil of rays travels 
-along the tube a, ionizing the air between a and b. 
The tube a is kept at a high potential by joining it to a 
battery of small storage-cells. The current through the 
ionized gas is measured by the electrometer, the tube a 
being kept at a sufficient potential to produce the saturation- 
current. 
‘S 
Ss 
x 
{ 
