_X_ rays discovered by Roentgen and
are a form of energy analogous to light. The beta rays are particles
of negative electricity or electrons. With these, then, we have some
degree of familiarity. But what are the alpha rays? An answer to this
question should make clearer the character of the changes taking
place, and should give some insight into the composition and mechanism
of the atom.

The Alpha Rays

It has already been stated that these alpha rays are similar or
analogous to the canal rays, but this advances the matter very little,
as the nature of these canal rays has not been fully determined. The
full identity with them, if proved, should have an important
theoretical bearing.

Alpha Rays Consist of Solid Particles

In the first place, these alpha rays have been found to be made up of
solid particles, that is, of what we are accustomed to call matter.
Since it has become more and more difficult to draw a clear
distinction between matter and energy, it would perhaps be better to
say that these particles appear to have some of the properties
hitherto attributed solely to matter. The best evidence that these
particles are of atomic mass is furnished by their deflection in
electric and magnetic fields.

Electrical Charge

It is not of first importance to discuss this or other proofs of the
material nature of these particles. That they carry a charge of
positive electricity is, however, a fact of very great import. The
value of this charge has been carefully determined by a number of
investigators working with different sources of the alpha particles
and has been found to be 9.3 x 10^{-10} electrostatic units
(.000,000,000,93 e.s.). From the consideration of the charge upon an
electron previously obtained by J. J. Thomson and others, it was
concluded that the alpha particle carried two unit positive charges;
the fundamental unit charge, therefore, is half this value, or
4.65 x 10^{-10} e.s.

Helium Formed from Alpha Particles

To determine the nature of the alpha particle a crucial experiment was
carried out by Rutherford and Royds, which was described as follows:

[Illustration: FIG. 7.--APPARATUS USED IN EXPERIMENT BY
RUTHERFORD AND ROYDS.]

A large quantity of radium emanation was compressed into a fine glass
tube _A_, about 1.5 cm. long. This tube, which was sealed to a larger
capillary tube _B_, was sufficiently thin to allow the alpha particles
from the emanation and its produc