Radioactivity of the Atmosphere. 145 
be calculated. The results for aluminium foil of average 
thickness ‘00038 em. are shown in fig. 6. The lower one is 
for the excited activity transferred to leather and the upper 
one for that on a lead wire. The ordinates give the per- 
centage of rays unabsorbed, and the abscissee the number of 
layers traversed. It will be seen that there is a small differ- 
ence in the penetrating power of the two. 
In fig. 7 are plotted two curves, showing the penetrating 
power of the excited activity from (1) a thick piece of felt, and 
(2) a thin piece of leather? Curve II. follows closely a G.P. 
with the thickness up to about 10 layers, the radiation fall- 
ing to half value after passing through about 2 layers of 
aluminium. After 10 layers there is a marked difference in 
the curve, 5 additional layers having only a small effect. 
With 25 layers -8 per cent. of the rays was still unabsorbed. 
The portion of the curve between 10 and 25 layers follows 
roughly a G.P. 
These results show the presence of a more penetrating 
kind of radiation. Curve I. for the felt shows the same 
effect, except that there isa greater percentage of penetrating 
rays than in the case of the leather. 
This difference can be explained on the assumption that 
there are two kinds of radiation given off—one, the a radia- 
tion, being practically all absorbed in about 10 layers of the 
aluminium foil, and a more penetrating 8 radiation. Now, 
the greater part of the excited activity on the felt would be 
carried into the interior by the ammonia, and hence the 
radiation would have to penetrate through a considerable 
thickness of felt before reaching the surface. The @ rays 
will thus be largely absorbed, and the radiation at the surface 
consist of a greater per centage of 8 rays, which pass through 
more easily. This also explains why such a small amount of 
energy is given off from the felt compared with that given 
off from the leather, since in the former case the a@ rays are 
mostly absorbed. The & rays represent about 99 per cent. of 
the total energy radiated ; the @ rays are half absorbed in 
about 15 to 18 layers of the aluminium. 
The excited activity from the ashes from radioactive cotton 
was tested in the same way, and showed the presence of 
8 rays, though not in so large a percentage as the leather 
and felt, The three cases are compared in the following 
table. 
If the radiation consists of rays of a homogeneous character, 
then the intensity I, after passing through a distance d of the 
Phil. Mag. 8. 6. Vol. 7. No. 38. Feb. 1904. L 
