76 Prof. J. A. McClelland on the 
divergence from the law disappears in the case of more 
penetrating radiation. 
This is similar to what Eve (‘ Nature,’ March 10th, 1904) 
has found for Réntgen rays. The ratio of the coefficient of 
absorption to the density varies between very wide limits 
when rays from a moderately weak tube are used, the ratio 
being greater for the denser substances. LHve finds that for 
gases, at any rate, this ratio is more nearly a constant when 
more penetrating Rontgen rays are used. 
Rutherford has determined the coefficient of absorption of 
y rays for various substances (‘ Nature,’ p. 818, 1902). His 
numbers are given for comparison for substances which occur 
in the above table :— 
x / 
| Substance. . r. Tenses | 
SE eA Tee a Lye tl PPD MS ve a 
Niece? < EES, ay | 033 033 | 
feelin: Smee | ‘086 | 035 | 
| DG. os ata aaac eee ‘280 | 036 
Be ee eet OY | ‘77 | 068 
jg Sn 92 ‘068 | 
| 
’ 
The numbers for water and glass are almost identical with 
those in the above tables; for lead and mercury they are 
greater even than those in column I. 
As mentioned at the beginning of the paper, the fact that 
the absorption of y rays is proportional to the density of the 
absorbing substance was in favour of these rays being charged 
particles, like other radiation which follows approximately 
the same law. We cannot, however, base much.on this, as 
according to Hve’s work the same law probably holds for 
penetrating Rontgen rays ; and apart from their absorption, 
the weight of evidence is certainly in favour of the y rays 
being similar to Réntgen says. 
We have rather to consider why charged particles travelling 
with great velocity and Roéntgen rays should follow the same 
law of absorption. Prof. Thomson (‘Conduction of Hlec- 
tricity through Gases,’ p. 312) has pointed out the significance 
of this law of absorption in the case of charged particles. If 
the particle be small compared with the molecule of the 
absorbing substance, we must consider collisions as taking 
place not with the molecule as a whole, but with the con- 
stituents of which it is composed. If, then, all substances are 
built up of the same primordial atoms, the absorption by 
different substances should be proportional to the number of 
