242 Mr Kaye, The Selective Absorption of Rontgen Rays. 
potential difference of about 25,000 volts between the cathode 
and anode. Hence the Rontgen rays worked with were distinctly 
soft in character, and were largely cut down by quite thin sheets 
of metal. The aluminium window would let through all but the 
very softest rays. 
The amounts of Rontgen radiation from the different metals 
were first compared when no additional screen was used to cut 
down the rays, so that they had only to penetrate the thin 
aluminium window to reach the ionization chamber. These rela- 
tive values (calling platinum 100) are given in the third column 
of the table of results. No very evident relation is apparent 
between these numbers and the atomic weights of the anticathode 
metals, e.g. lead, silver and nickel, are all out of place. Perhaps 
this result is not altogether surprising when the heterogeneous 
character of the Rontgen rays is taken into account. It will be 
noticed that with the exception of aluminium, there is not a 
great deal of difference between the amounts of radiation from 
any of the other metals. 
Eelative Kadiation 
Atomic 
Weight 
Anti- 
E 
cathode 
No 
screen oi 
At. wt. 
screen 
A1 
pt 
Cu 
Ni 
Fe 
207 
Pb 
93 
(R) 
109 
88 
24 
41 
26 
'53 
195 
Pt 
100 
100 
100 
24 
41 
25 
•51 
108 \ 
(154)/ 
Ag 
77 
86 
66 
21 
35 
22 
(•56) 
64 
Cu 
87 
35 
29 
35 
33 
12 
•55 
58-7 
Ni 
89 
33 
24 
26 
33 
11 
•56 
56 
Fe 
83 
30 
19 
6 
29 
30 
•54 
27 
A1 
20 
14 
9 
3 
9 
5 
•52 
Screens of aluminium 25 mm., '5 mm., and 1 mm. thick were 
then inserted in turn in the path of the Rontgen rays. The first 
thickness of screen alters the relative amounts of radiation con- 
siderably, but very little further change is produced by thicker 
screens. Indeed the numbers for the ‘5 mm. and the TO mm. 
screens are almost identical. 
