350 
Mr Crowther, On. the Fatigue of Metals, etc. 
Conclusion. 
The results of the preceding experiments shew : 
(i) That the continuous impact of radium rays upon a 
metal plate does not cause any diminution in the amount of 
secondary radiation given out by the metal under the action of 
the rays themselves. 
(ii) That the continuous impact of the radium rays upon a 
metal plate does produce an alteration in the amount of secondary 
radiation given out by the plate under the action of X-rays, and 
ultra-violet light. 
(iii) That the fatigue so produced can be removed by 
repolishing the exposed surface. 
These results would seem to suggest that the cause of 
“ fatigue ” is not any alteration of the nature of the material 
of the plate, but some kind of alteration, possibly in the 
magnitude of the “ double layer ” effect, at the surface of the 
plate. 
The /3 and <y rays from radium penetrate a considerable 
distance into the metal, and even the a-rays penetrate to an 
appreciable extent. We should expect, therefore, that any altera- 
tion in the nature of the material of the plate would extend 
throughout a considerable thickness of material, in which case 
we should expect to get an appreciable alteration in the secondary 
radiation set up by the radium rays themselves. On the other 
hand, the secondary rays produced by radium rays, are very 
penetrating in character, and consequently the amount of them 
emerging from the plate would not be appreciably altered, by any 
alteration of the surface conditions. The secondary rays pro- 
duced by Rontgen rays and ultra-violet light, however, are com- 
paratively slow, and very easily absorbed. A difference in the 
surface conditions would, therefore, in their case greatly affect the 
amount of absorption, and consequently the number escaping from 
the plate. It is probably to their great penetrating power that 
the absence of fatigue in the case of the secondary radiation pro- 
duced by the radium rays themselves, is due. 
In conclusion, I wish to express my best thanks to Prof. 
J. J. Thomson, at whose suggestion these experiments were 
undertaken, for much kindly interest during the course of the 
work. 
