rties of so-called matter.

Scattering of Alpha Particles

This conception of the atom would doubtless fail of much support were
it not for certain experimental facts which lend great weight to it.
Certain suppositions can be based on this theory mathematically
reasoned out and tested by experiment. Predictions thus based on
mathematical reasoning and afterward confirmed by experiment give a
very convincing impression that truth lies at the bottom.

The first of these experimental proofs comes under the head of what is
known as the scattering of the alpha particles, a phenomenon which,
when first observed, proved hard to explain. If an alpha particle in
its escape from the parent atom should come within the influence of
the supposed outer electrical field of some other atom, it should be
deflected from its course and, the intensity of the two charges being
known, the angle of deflection could be calculated. For instance, if
it came to what might be called a head-on collision with the positive
central nucleus of another atom, it would recoil if it were itself of
lesser mass, or would propel the other forward if that were the
lighter.

The experiment is carried out by placing a thin metal foil over a
radio-active body, as radium _C_, which expels alpha particles with a
high velocity, and counting the number of alpha particles which are
scattered through an angle greater than 90 deg. and so recoil toward their
source. This has been done by a number of investigators and it has
been found that the angle of scattering and the number of recoil
particles depend upon the atomic weight of the metal used as foil. For
example, if gold is used, the number of recoil atoms is one in
something less than 8,000.

Taking the atomic weight of gold into consideration, Rutherford
calculated mathematically that this was about the number which should
be driven backward. But he went further and calculated also the number
which should be returned by aluminum, which has an atomic weight of
only about one-seventh that of gold. Two investigators determined
experimentally the number for aluminum and their results agreed with
Rutherford's calculations.

The metals from aluminum to gold have been examined in this way. The
number of recoil particles increases with the atomic weight of the
metal. Comparing experiment with theory, the central charge in an atom
corresponds to about one-half the atomic weight multiplied by the
charge on an electr