of Radium , and Secondary Rays which they produce. 655 
paper. It may be pointed out in this connexion that the 
selective absorption properties of the elements probably do 
not change gradually with the atomic weight. Thus, 
aluminium and zinc possess approximately the same property 
of selective absorption, while that of carbon is different, 
though the ratio of the atomic weight of zinc to that of 
aluminium is about the same as the ratio of the atomic 
weight of aluminium to that of carbon. It will be observed 
that the existence of other rays or groups of rays than those 
dealt with in this paper, does not invalidate any of the 
deductions made. The existence of other rays or groups of 
rays when two groups of rays are compared, has the effect of 
making the properties of the latter appear less marked. 
Thus, it is evident that the larger the number of different 
kinds of rays in a beam of rays, the smaller will be the 
decrease in the strength of the beam when a screen which 
absorbs best one kind of rays is placed in the path of the 
beam. This indicates some measurements from which a 
rough indication of the distribution of the y rays amongst 
the different kinds of rays may be obtained, and which will 
now be described. The amount of secondary radiation from 
a zinc plate which penetrated a plate of lead 1*8 mm. thick 
was found to be equal to 795, in arbitrary units ; and the 
amount which penetrated a zinc plate 3 mm. thick (of about 
the same mass as the lead plate) was found to be equal to 
2258. Let the total radiation from the zinc plate be denoted 
by (A-fB), where A denotes the radiation of Group I. and 
B denotes the remaining radiation. Suppose that the lead 
plate (a substance of Group I.) allows the fraction u of the 
radiation A to penetrate the plate. Since the lead and zinc 
plates are approximately of equal mass, we may suppose as a 
first approximation that the same fraction w of the radiation 
B penetrates each plate, where w, it will be observed, must 
be larger than u. The amount of radiation measured when 
the zinc and lead plates were successively used as screen was 
therefore (wA + wB) and (uA + wW) respectively, and there- 
fore in this experiment 
uA + ioB = 795 and wA + nB = 2258. 
From these two equations we have 
wA—uA = 1463, 
and therefore 
wA-uA _ 1463 
wB + wA " 795' 
Now, if u = 0, that is, if all the radiation of Group I. is 
absorbed by the lead plate, the right-hand side of the last 
equation gives the ratio of the radiation of Group I. to the 
