342 Mr Crowther, On the Fatigue of Metals 
The plate was exposed to the action of the rays at a distance 
of 4 cms. The measurements were made with the radium 25 cms. 
away from the plate. 
The first experiments were made with a view to observing any 
rapid change that might occur in the secondary radiation from 
exposure to radium. Accordingly, the plate to be tested was 
mounted on the block W, and carefully polished, first with sand- 
paper, then with flour-emery, and finally, with a clean silk rag. 
The plates were of sufficient thickness to stop practically the 
whole of the /3-rays, that is in general about 2 mm. The plate 
was placed in position under the ionization chamber and a reading 
made as quickly as possible after polishing. The plate was then 
exposed at close quarters to the radium, readings being made 
at frequent intervals during the rest of the day. The following 
table (Table I.) will give an idea of the nature of the results 
obtained. The first column gives the total length of exposure to 
radium in minutes ; the second gives the rate of leak of the 
electroscope, in arbitrary units, and hence affords a measure of the 
secondary radiation from the plate, which in the particular case 
recorded below was of lead. 
Table I. 
Length of Exposure 
Secondary Radiation 
mins. 
15 
100-0 
40 
100-0 
72 
100-6 
86 
101-0 
110 
100-9 
192 
100-8 
210 
100-4 
It will be seen that the ionization in the chamber B, and 
hence the radiation from plate A, remained quite constant to 
within one per cent, during the whole course of the exposure. The 
experiment was repeated, in various ways, but always with the 
same result. 
It was determined, therefore, to arrange for a continuous 
exposure to radium rays, for a prolonged period of time. 
Accordingly a lead plate was carefully mounted and polished, 
and the secondary radiation from it measured. It was then 
removed and exposed to the /3 and y rays from the radium at 
a distance of only 4 cms. Readings were taken daily of the 
