344 Mr Crowther, On the Fatigue of Metals 
the dates at which the two plates were subsequently compared, 
the second column, the amounts of secondary radiation from the 
plate A 1 continuously exposed to radium, the third column the 
secondary radiation from the standard plate A n , and the last the 
ratio of secondary radiations from the two plates. 
It will be seen that, although the separate values obtained for 
the secondary radiation vary somewhat erratically from time to 
time, the ratio for the two plates as given in the final column of 
the table remains very nearly constant indeed, the greatest dis- 
crepancy only amounting to about one per cent. There is not the 
slightest evidence that there has been any appreciable alteration 
in the amount of secondary radiation given off by the plate A 1} 
in spite of its having been exposed to the ft and y rays from 
radium, at a distance of only 4 cms. for a space of over three 
months. 
The ft and y rays of radium having thus proved quite in- 
effectual to produce any fatigue in the secondary radiation from 
a metal plate, the experiment was now modified in order to 
enable the a-rays to be employed. The apparatus used is shewn 
in figure 2. The radium was spread uniformly over the bottom of 
Fig. 2. 
a shallow circular depression R, on a brass plate P, 4 cms. in 
diameter, and was covered in with a very thin piece of mica, 
in order to prevent the escape of emanation into the apparatus. 
The plates to be tested, were also 4 cms. in diameter, and 2 mm. 
in thickness, and screwed on to the rod r, by means of which they 
could be connected to a Wilson electroscope. The rod r, and the 
connecting wires, were enclosed in an earthed tube T, filled with 
sulphur, as air becomes too good a conductor when exposed to 
