Investigation of the Nature of the 7 Rays. 669 
substituted for an iron one the change was 1 : 1*04. In 
illustration of the effect of the second disturbing factor 
mentioned above, we have found that, other things being- 
equal, the substances of small atomic weight give the most 
secondary radiation, in a general way ; and it may be no 
coincidence that in some cases we have found Sn and Fe to 
give surprisingly small amounts. This is in agreement with 
what is to be expected ; for it is clear, on consideration of 
the argument already given, that the greater the /3-ray 
absorption of a substance in proportion to its density, the lets 
" emergence" radiation should issue from it. Some observers 
have found Sn and Fe to possess exceptional absorbing powers. 
We do not wish, however, to lay any stress upon these last 
observations, some of which we may not have interpreted 
correctly ; but we mention them in order to show that the 
inequalities that are found to exist between the emergence 
radiation of various substances promise to be reducible to 
order as soon as the difficulties of interpretation have been 
surmounted. 
Let us now consider the cathode radiations on the front 
sides of the plates. Of the cathode particles set free in BC 
and moving at first in the direction of the 7 rays, a certain 
proportion, say p, is returned by what is beyond. These move 
towards the face A, and a certain number of them succeed 
in reaching it and emerging therefrom. In the case of the 
other plate the proportion returned is p' ; the absorption in 
B'A' is the same as in BA because the weights are the same. 
Comparing the two plates stratum by stratum, we find that 
the "incidence" radiation of one plate is to the incidence 
radiation of the other plate as p to p r . Now p and p' are 
the well-known constants of the ft rays. 
When a stream of 7 rays is allowed to fall upon a plate, 
the cathode radiation which issues from the place of inci- 
dence must be divisible into two parts. One consists of 
scattered ft particles derived from the stream of such particles 
which was travelling with the 7 rays before incidence, and 
which was formed during the previous transit of the screens 
employed — solid, liquid, or gaseous. This part is scattered to 
an extent which depends on the atomic weight of the plate, 
according to the usual (McClelland's) law of ft particles. 
The other part is originated in the plate itself in the manner 
just described, and the amount of it is also regulated ac- 
cording to the /3-ray law. When, therefore, observers have 
measured the secondary radiation due to y rays, and have 
found a law corresponding to that for ft rays, the reason of 
the correspondence has been that they really were measuring 
