714 



Prof. L. T. More on the Fatigue of 



temporary effect, as the plates recover their original activity 

 if the Rontgen rays are stopped for a time. Iron and lead 

 required a rest of less than an hour, but the effect on nickel 

 and zinc was more lasting, as they still showed some fatigue 

 after resting until the following day. 



Polished Metal Plates in Air. 



The same plates were then polished with emery and with 

 silk to a mirror surface and tested as soon as possible, allowing 

 only time enough, before taking the first set of readings, for 

 the plates to lose the heat which might have been developed 

 by the friction. This delay, I think, is important, since if 

 the fatigue is at all dependent on surface-films of gas and 

 gas occluded in the metal, these would be strongly affected 

 by changes of temperature. 



Table II. 



Exposure. 



Iron x . Iron 2 . 



Ratio to 



Standard ' Ratio. 

 Plate. 



Lead. 

 Ratio. 



Nickel. 

 Ratio. 



hours 



1 „ 



100 100 

 105 106 

 103 108 

 102 109 



109 



1 



100 

 103 

 105 

 101 

 101 



100 



98 



101 



98 



2 „ 



3 „ 



4 „ 





The effect of the Rontgen rays is here just the reverse, as 

 the secondary radiation for iron and lead rises to a maximum. 

 Unfortunately, the experiments could not be continued long 

 enough to see whether the ratios finally became less than the 

 initial value. While nickel does not show this increase in 

 radiation, it remains practically unaffected and does not 

 fatigue as it did before the surface was polished. 



In the following experiments the trial plates were immersed 

 in gn ses, at atmospheric pressure, other than air. 



Polished Plates in Coal-gas. 



Both the trial and standard plates were well polished and 

 placed in their respective boxes. The windows of both were 

 closed with aluminium-foil free from holes, and the box con- 

 taining the trial plate was sealed air-tight with wax. A slow 



