B84 TRANSACTIONS OF SHCTION A. 
Thé character of tlie effects produced by the a and £ particles is inost 
simply studied in gases. The a particle has such great energy of motion that 
it plunges through the molecules of the gas in its path, and leaves in its train 
more than a hundred thousand ionised or dissociated molecules. After 
traversing a certain distance, the a particle suddenly loses its charac- 
teristic properties and vanishes from the ken of our observational methods. 
It no doubt quickly loses its high velocity, and after its charge has been 
neutralised becomes a wandering atom of helium. The ionisation produced 
by thea particle appears to consist of the liberation of one or more slow velo- 
city electrons from the molecule, but in the case of complex gases there is no 
doubt that the act of ionisation is accompanied by a chemical dissociation 
of the molecule itself, although it is difficult to decide whether this. dis- 
sociation is a primary or secondary effect. The chemical dissociation 
produced by a particles opens up a wide field of investigation, on which, so 
far, only a beginning has been made. 
The B particle differs from the a particle in its much greater power 
of penetration of matter, and the very small number of molecules it ionises 
compared with the a particle traversing the same path in the gas. It is 
very easily deflected from its path by encounters with the gas molecules, 
and there is strong evidence that, unlike the a particle, the B particle can 
be stopped or entrapped by a molecule when travelling at a very high speed. 
When the great energy of motion of the a particle and the small 
amount of energy absorbed in ionising a single molecule are taken into 
consideration, there appears to be no doubt that the a particle, as Bragg 
pointed out, actually passes through the atom, or rather the sphere of 
action of the atom which les in its path. There is, so to speak, no time 
for the atom to get out of the way of the swiftly moving a particle, 
but the latter must pass through the atomic system. On this view, the 
old dictum, no doubt true in most cases, that two bodies cannot occupy the 
same space, no longer holds for atoms of matter if moving at a sufficiently 
high speed. 
There would appear to be little doubt that a careful study of the effects 
produced by the a or # particle in passing through matter will ultimately 
throw much further light on the constitution of. the atom itself. Work 
already done shows that the character of the absorption of the radiations 
is intimately connected with the atomic weights of the elements and their 
position in the periodic table. One of the most striking effects of the 
passage of 8 rays through matter is the scattering of the B particles, 7.e., 
the deflection from their rectilinear path by their encounters with the 
molecules. It was for some time thought that such a scattering could not be 
expected to occur in the case of the a particles in consequence of their much 
greater mass and energy of motion. The recent experiments of Geiger, 
however, show that the scattering of the a particles is very marked, and is 
so great that a small fraction of the a particles, which impinge on a screen 
of metal, have their velocity reversed in direction and emerge again on the 
same side. This scattering can be most conveniently studied by the method 
of scintillations. It can be shown that the deflection of the a particle from 
its path is quite perceptible after passing through very few atoms of matter. 
The conclusion is unavoidable that the atom is the seat of an intense electric 
field, for otherwise it would be impossible to change the direction of the 
particle in passing over such a minute distance as the diameter of a 
molecule. 
Tn conclusion, I should like to emphasise the simplicity and directness 
of the methods of attack on atomic problems opened up by recent discoveries. 
As we have seen, not only is it a simple matter, for example, to count the 
number of a particles by the scintillations produced on a zinc sulphide 
screen, but it is possible to examine directly the deflection of an individual] 
