Absorption of a Rays. 721 
through which it is passing, because it contains some 
thousands of electrons and ionizing collision is so much the 
more probable. But a collision between an electron of the 
flying atom of the aray and an electron of the atom traversed, 
can have very little effect on the motion of the a atom as a 
whole. All the electrons of the « ray which do not go very 
close to electrons of the stationary atom, are practically 
undeflected ; and probably even one which does, is steadied 
by its connexions with its fellows. 
Deductions from this hypothesis harmonize very well with 
observed effects. In the first place, penetration of matter 
must be inversely proportional to the density of the matter. 
In the second place, there should be no scattered or secondary 
radiation as in the case of the 8 electrons, and this is found 
to be true. Again, since the « rays are undeviable by 
collision, a stream of them should be as sharply defined after 
traversing a thin material plate as it was before ; which 
Becquerel found to be the case. Again, « rays emitted at a 
certain speed should reach a certain distance from the parent 
body which is proportional to the initial energy ; so that an 
ionization chamber should be unaffected if just out of range, 
and much affected if just within it, as Madame Curie found. 
Also the @ particle should penetrate much further than the 
8 particle of the same initial velocity, since the former only 
loses its motion on account of the expenditure of energy on 
ionization, whereas the latter is liable also to deflexion by 
collision. This appears to be true from the experiments of 
Lenard and Rutherford. It is also easy to see that even if 
the « particle is uncharged when it leaves the parent body, 
it must immediately become positive, since in traversing an 
atom it is just as likely to lose one of its own electrons as to 
take one away from the atom traversed. This agrees with 
Rutherford’s statement made in his Bakerian Lecture. It 
may very well be, however, that the loss of a second electron 
is much more unlikely than the loss of the first. 
On the other hand, it is clear that on this hypothesis no 
exponential law can express the phenomena of the absorp- 
tion of the « rays, and that if an exponential law is found to 
be approximately realized, the hypothesis must furnish some 
explanation of the fact. This test it can, I think, satisfy. 
In the first place, however, it must be pointed out that in 
Rutherford’s experiment, wherein he showed that the absorp- 
tion curve was approximately exponential, the radioactive 
material was scattered over the floor of the ionization chamber. 
But if this hypothesis is correct, it is simpler to investigate 
the matter in a rather different way. Suppose a narrow 
