atoms ; and that the secondary cathode radiations might be 

 the negative particles thus set free. Let us suppose, provi- 

 sionally, that the particles, when set free, move at first in 

 the direction of the y stream, but are subsequently 

 scattered in the usual manner of jS rays. [It is here 

 that the absence of symmetry arises. On the pulse 

 theory the particles should go equally backwards and 

 forwards ; indeed, if they were ejected by atomic explosions, 

 the result of energy accumulated from passing pulses, as sug- 

 gested by J. J. Thomson in the case of X-rays, they would 

 move equally in all directions.] 



Wigger gives a table (Jahrbuch der Radioaktivitat, Bd. 

 ii., p. 431) showing that the y rays 

 are absorbed according to a density 

 law pretty strictly, except for small 

 thicknesses of substances of large 

 atomic weight. 



Assume this law to hold good, and 

 also assume for the present that the ab- 

 sorption of )8 rays follows the density 

 law. The latter is only roughly true, of 

 course ; but we may deal with quanti- 

 ties in a broad fashion first, and make 

 the proper amendments afterwards. 

 We can now compare the quan- 

 tities of cathode radiation which should emerge from 

 the far sides of two plates of different densities p and p' . Let 

 these be represented by AD and A' D' in the figure, and 

 let BG and B'C be corresponding strata of equal weight: 

 in fact, let AB / A'B' =BC j B'C = CD /'(" D' = p' / p. Let 

 the plates be crossed by equal pencils of y rays, as shown in 

 the figure. A certain quantity of y radiation is absorbed in 

 crossing BC. In the language of our present hypothesis we 

 should say that a certain number of y particles are stripped 

 of their positives, and the negative remainders go on. An 

 equal number of negatives are set free in B'C because the 

 two strata are of equal weight. Of those set free in BC 

 only a certain number emerge from the face D, because of 

 the absorption of the plate CD. Since CD and CD' are 

 of equal weight, a similar absorption occurs in the case of 

 the particles set free in 73' C . Thus the same number emerge 

 from each plate. Integrating for all effective strata, the 

 whole cathode radiations emerging from the two plates are 

 equal. 



We thus find that if the absorption of j3 and y rays both 

 followed the density law, the secondary cathode radiation on 

 the far side of a plate — we may call it the "emergence" radia- 



