210 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
There is another entirely different argument, which seems to sup- 
port the neutral-pair hypothesis. 
The a, 8, and y rays all ionize the gases which they traverse. It 
has just been shown by Kleeman? that the ionization per atom due 
to 8 and y rays is nearly proportional to the ionization per atom 
due to a rays (and, therefore, approximately proportional to the 
volume, as I have shown, Proc. Roy. Soc. of S. A., Oct., 1906; Phil. 
Mag., March, 1907). The figures for the heavier atoms are rather 
larger for the 8 than the a rays, and still larger for the y rays. It is 
known that the ionizations due to X rays differ considerably from 
those due to y rays when the X rays are soft, but approximate to 
them when the X rays are hard. 
All this fits in excellently with the theory that all four types of rays 
are material. Take the a particle first, since its circumstances are the 
most simple. It moves directly through the atoms, without scatter- 
ing or transformation. It liberates ions in the form of 8 rays as it 
goes, approximately according to the volume law. The @ ray is also 
a charged particle, and it is readily to be supposed that it would, if 
its whole motion were rectilinear, liberate ions according to the same 
law (comparing atom with atom) as the a particle, though the num- 
bers would be less. But the 8 particle is lable to scattering, and each 
act of scattering generally implies an increase in the path of the 
particle in the gas, and increased ionizing power since its speed is a 
little diminished. Now, scattering is proportional to the atomic 
weight, whilst the ionization is more nearly proportional to the square 
root of the atomic weight. Thus a heavy atom is the cause of more 
than its proper amount of lonization; and so we find in Kleeman’s 
table that the ionizations of the atoms Cl, Br, and I are rather higher 
than in the case of the a particle. Again, the y particle is liable to 
resolution into its elements, with a relatively large amount of ioniza- 
tion. Since this transformation is chiefly effected by impact with 
heavy atoms, these latter will be the cause of a disproportionately 
large ionization, as compared with the a rays; and this is also shown 
by ieamnent s figures. Passing on to X rays, we find a further illus- 
tration of this effect, until we come to very soft rays, when we find 
that the heavy atoms are the occasion of exceedingly large ionization. 
(“ Conduction of Electricity through Gases,” 2d ed., p. 800.) There 
is a good continuity in all these phenomena, with gradual diver- 
gences just where we should expect them. The a, B, y, and X rays 
all produce the same primary ionization, comparing atom with atom, 
and differ only in the effects due to scattering and transformation ; 
4Mr. Kleeman has been good enough to inform me of his results by letter; 
but I believe I am at liberty to quote them, since he has, I understand, recently 
read a paper on the subject before the Royal Society. 
