140 NATURE OF THE RADIATIONS [CH. 
for A =0, since g=0 when v=0. q should thus be proportional 
to v. This conclusion is contrary to the experimental results, for 
it has been shown, at any rate for the 8 particles, that the 1oniza- 
tion per unit distance decreases with increase of velocity. 
The variation of ionization with distance thus cannot be due 
entirely to the gradual retardation of the particles. It seems 
probable that it is due to one of the following causes :— 
(1) absorption of the projected particles in their passage 
through matter ; ; 
(2) neutralization of some of the charges carried by the pro- 
jected particles. 
It can be shown that the number of a particles which are able 
to produce “scintillations”” on a zine sulphide screen 1s diminished 
by the interposition of a metal screen. The hypothesis (2) seems 
more probable than (1), for it 1s difficult to see how masses, possess- 
ing such an amount of kinetic energy as the a and @ particles at 
the moment of their expulsion, can be completely stopped by a 
single collision, unless the velocity of the particles has already been 
greatly reduced by their passage through matter. On the second 
hypothesis, the particles after losing their charges may still keep 
moving in their path with a high velocity, but it is to be expected 
that they would not be nearly as efficient in ionizing the gas as a 
charged particle of equal mass moving with the same velocity. 
Their existence would not be recognized by ordinary methods 
unless they produced an appreciable ionization by collision with 
the molecules. Thus, it is possible that, in addition to the a and 
8 charged particles, there may be a stream of uncharged particles 
moving through the gas with great velocity, the existence of which 
has not yet been detected. 
This gradual neutralization of the charges on the projected 
particles, and the consequent mability of the particles to produce 
ions in their path, are probably responsible for most of the so-called 
“absorption” of the rays in traversing matter whether solid, liquid, 
or gaseous. If, in addition, the speed of the projected particles is 
gradually decreased by their passage through matter, as the mini- 
mum velocity required to produce ions is approached, the particles 
which still retain their charge will decrease in ionizing power, and, 
