720 Prof. W. H. Bragg on the 
If the encounter be not too close, both the deflexion and the 
amount of energy given by the moving to the stationary 
electron are inversely propor tional to the energy of the former. 
The actual deflexion which an electron is likely to suffer in 
going through an atom, must not of course be calculated on 
the supposition that the atom consists merely of so many 
electrons, taken as at rest: the positive charge must be 
brought in, and the fact that the atom’s electrons are also in 
motion. Indeed, it may be calculated that the more simple 
but defective supposition is unable to explain fully the pene- 
trative power which the electron certainly possesses. Never- 
theless, it is clear that the 8 rays are liable to deflexion 
through close encounters with the electrons of the atoms ; 
and therefore the distance to which any given electron is 
likely to penetrate before it encounters a serious deflexion is 
a matter of chance. This, of course, brings in an exponential 
law. Such a law was, for example, obtained by Lenard in 
his original experiments on the cathode rays, which pene- 
trated a thin aluminium window in the wall of the tube in 
which they were formed. By the aid of stops he isolated 
a small pencil of rays whose intensity, after they had traversed 
a certain distance of air, he measured by ‘aid of a phos- 
phorescent screen. If such a jet of electrons be projected 
into the air, some will go far without serious encounter with 
the electrons of the air molecules ; some will be deflected at 
an early date from their original directions. The general 
effect will be that of a stream whose borders become ill- 
defined, which weakens as it goes, and is surrounded by a 
haze of scattered electrons. Ata certain distance from the 
source all definition is gone, and the force of the stream is 
spent. 
There is a second cause of the gradual “ absorption”’ of 
a stream of $8 rays. Occasionally an electron in passing 
through an atom goes so near to one of the electrons of the 
atom as to tear it from its place, and so to cause ionization. 
In doing so, it expends some of its energy. It is easy to 
calculate from the data that Durack has given as to the 
number of new ions made in each centimetre, and Townsend 
as to the amount of energy required to make a new pair of 
ions, that the 8 ray must slow down considerably towards 
the end of its course ; and the slower it goes, the more 
liable it is to deflexion. 
On the other hand there is, in the case of the « ray, only 
one cause of “absorption.” The a ray is a very etfective 
ionizer, and rapidly spends its energy on the process. It is 
of course far more likely than the @ ray to ionize an atom 
