616 Mr. A. 8S. Eve on the Ionization produced in 
aluminium *85 mms. thick. This reduced the ionization- 
current from 100 to 22, and a screen of double thickness to 
20°6. The electroscope was then filled with air and various 
gases, and the observations were repeated. As the deflexions 
of the leaves were only about a quarter of their former 
value, it was necessary to run the bulb for a considerable 
time. The results were not as accurate as in the first 
instance. Although 78 per cent. of the rays had been cut off, 
the values obtained agreed with those previously determined. 
There cannot be any wide divergence between the relative 
conductivities for primary and secondary rays. In the case 
of the secondary radiation caused by the rays, the interior 
aluminium screen of °85 mms. thickness reduced the ionization- 
current from 100 to 79, and a double thickness from 100 
to 70. These screens had a negligible effect if placed on the 
outside of the electroscope. The absorption coefficients may 
therefore be calculated on the assumption that the primary 
rays are unaffected. If I, be the initial intensity, and [, be 
the intensity after penetration through thickness t, then » 
the coefficient of absorption may be calculated from the 
equation I,=Ije-*’. 
The values of X for secondary rays from lead penetrating 
aluminium were found to be 
The former were therefore about five times as penetrating as 
the latter. : ae 
The curves in fig. 3 indicate that secondary Réntgen rays 
are absorbed in a few centimetres of air, except at low 
pressures, whilst the straight line in fig. 5 shows that the 
y rays pass the air with little absorption. This agrees with 
Becquerel’s results obtained by photographic methods. 
By interposing lead screens outside the electroscope the 
penetrating powers of the primary rays were also compared. 
The values of X for lead with Réntgen rays were 24°6 for the 
first 1:7 mms. and 21°0 for the thickness 1°7 to 3:4. There 
is a similar decrease in the value of X for lead with y rays 
from ‘64 to *47, as the thickness ranges from 2 to 96 mms. 
Taking mean values, it may be said ‘that y rays are about 
40 times as penetrating as the Réntgen rays used. 
Whilst these experiments have een in progress, papers 
have appeared in the Annalen der Physik (Nos. 6, 7, 1904) 
by F. Paschen. In the first he describes the repetition, 
with y rays, of the experiments made by Strutt in the case 
of B rays (Phil. Mag. 1903, [6] vi. p. 538). : 
