subjected to Radium Rays. 
343 
secondary radiation, the plate and radium being replaced in 
proximity to each other, as soon as possible after each reading. 
It was found that considerable fluctuations occurred in the 
values obtained from day to day. Corrections were applied to 
the leak in the ionization chamber B, for pressure and temperature 
variations, on the assumption that the amount of ionization in 
a gas produced by rays of given intensity was proportional to the 
density of the gas. Even so, however, the values obtained for the 
secondary radiation varied by as much as 5 °/ o on different 
occasions, which was considerabty more than the errors of reading. 
These fluctuations, which were probably due to atmosphere 
variations (possibly to changes in the humidity), were sufficient to 
mask any small change in the radiation from the plate A. It was 
necessary, therefore, to somewhat modify the method. Accordingly 
two exactly similar plates of the same metal were taken, and mounted 
in exactly the same way, on wooden supports, and carefully 
polished. The one plate, A l say, was exposed to radium after 
measuring the secondary radiation from it, in exactly the same 
way as before. The second, A 0 , served as a standard plate, and 
was only exposed to radium radiation for a sufficiently long time 
to enable readings to be made of the secondary radiation from it, 
and then only at a considerable distance. 
The secondary radiation from the two plates was compared 
immediately after polishing, and then at intervals of a few days, 
the plate A x being meanwhile exposed continuously to radium 
rays at close quarters. 
Table II. 
Date 
Secondary Radiation 
Ratio AJAq 
Exposed Plate A x 
Standard Plate A 0 
Nov. 6th ... 
105-6 
98-0 
1-077 
Nov. 19th ... 
104-6 
97-6 
1-072 
Nov. 23rd ... 
108-0 
100-8 
1-072 
Nov. 28th ... 
106-7 
98-7 
1-081 
Dec. 12th ... 
103-6 
96-3 
1-078 
Jan. 11th ... 
104-2 
98-0 
1-065 
Jan. 23rd ... 
100-0 
93-9 
1-066 
Feb. 6th ... 
104-2 
97-8 
1-068 
Feb. 19th ... 
104-4 
97-9 
1-070 
The results obtained with lead plates are given in Table II. 
The plates were polished on Nov. 6th. The first column gives 
23—2 
