662 Dr. Sadler and Mr. Steven on an Apparent Softening 



results similar to that for copper were obtained. On the 

 other hand, if cut down by substances of low atomic weight, 

 which chiefly scatter incident radiation, e. g. C, Al, &c, or 

 by substances of higher atomic weight, e. g. An, in which 

 the characteristic radiation is not excited except by very 

 penetrating beams, the results were similar to those given in 

 Table I. 



It might be supposed that these effects were due to 

 secondary radiation superposed on the primary beam, but 

 direct experiment showed that when the beam was cut down 

 by copper or iron at S, the amount of secondary radiation 

 entering either of the electroscopes E 1 or E 2 would not 

 account for 1 per cent, of the observed ionization. 



The use of the charcoal cooled by liquid air as an aid to 

 exhaustion had however its disadvantages. After the bulb 

 had been running for a considerable time the discharge 

 tended to become intermittent, and the readings obtained 

 were very irregular. The charcoal was therefore dispensed 

 with, and the pump alone depended on for exhaustion. Under 

 these new condition*, the discharge at any stage appeared much 

 more regular, and there was the added advantage of being; 

 able to investigate the rays at an earlier stage in the 

 exhaustion. By continuing the pumping, higher stages of 

 exhaustion could be reached, and the process, although 

 slower, proved more reliable. 



The beam was now tested by a thin sheet of aluminium 

 ('00305 cm.), and the absorption was found both before and 

 after cutting down the beam by iron ('00124 cm.). With a 

 moderately penetrating beam the absorption by the aluminium 

 was considerably greater after transmission through the iron 

 than before transmission, and this apparent softening was 

 greater as the initial primary beam became still harder. 

 This is clearly shown in the following table : — 



Table III. 



Per cent, absorp- 

 tion by Al v-00305) 

 of initial 

 beam. 



48-8 

 45-5 

 39-4 

 30-4 

 211 



Per cent, absorp- 

 tion by Al (-00305) 

 after cutting down 

 primary beam 

 by iron. 



o.rv 

 57 "2 

 507 

 40-0 

 396 



Per cent, absorbed 



by the iron sheet 



(•00124 cm.). 



Per 

 in 



th 



cent, increase 

 absorption by 

 e aluminium. 



65 







14-2 





67 







25-8 





67-6 







28-7 





638 







513 





54-5 







85-0 





