642 Mr. R. D. Kleeman on the Secondary Cathode 



by assuming that the radiation from aluminium decreases in 

 a greater proportion than that from lead, and the radiation 

 from each of the substances ~N m decreases in the same pro- 

 portion as that from aluminium. For in this case these 

 differences decrease when they are multiplied by a factor 

 which makes (Pb — Al) equal to its previous value. But it 

 will be seen that the decrease of the difference (C — Al) is 

 much greater than that of any of the other differences, and 

 cannot therefore be explained altogether in this manner. It 

 is evident that the radiation from carbon does not decrease 

 in the same proportion as that from aluminium, when the 

 thin lead screen is replaced by one of the above-mentioned 

 screens. If we assume that the radiation from carbon 

 decreases in a less proportion than that from aluminium, the 

 value of (k a — l) of the difference (C — Al) is numerically 

 decreased, since it is negative. In this case the value of 

 (C — Al) or (k a — 1)A1 would be much more decreased than 

 that of any of the other differences when they are multiplied 

 by a factor which brings (Pb — Al) or (Z^ — 1)A1 equal to its 

 previous value. Thus the rays that are most efficient in 

 producing secondary radiation from carbon are less absorbed 

 by a screen of iron, copper, zinc, or tin, than the rays that 

 are most efficient in producing secondary radiation from 

 aluminium and the substances !N«. 



We have seen that the rays that are most efficient in pro- 

 ducing secondary radiation from carbon are also less absorbed 

 by a screen of lead, mercury, or bismuth than the rays that 

 are most efficient in producing secondary radiation from lead. 



It will be profitable to place some of the foregoing con- 

 clusions side by side for comparison. "We have seen that the 

 decrease of (C — Al), when the thin lead screen is replaced 

 by a screen of iron, copper, zinc, or tin, is due, firstly, to the 

 radiation from carbon decreasing in a less proportion than 

 that from aluminium, and secondly, to the multiplying of 

 (C-Al) and (Pb-Al) by a factor which brings (Pb-Al) 

 to its previous value. This factor gives rise to the decrease 

 of the other differences, since the radiation from lead is 

 decreased with any one of the above changes of screen, in 

 a less proportion than that from aluminium and the other 

 substances. 



The decrease of the difference (C— -Al), when the thin 

 lead screen is replaced by a screen of lead, mercury, or 

 bismuth, is due to the decrease of (C — Al), produced by the 

 radiation from carbon decreasing in a less proportion than 

 that from aluminium, being greater than the increase pro- 

 duced by multiplying (C— Al) and (Pb— Al) by a factor 



