ally he can only make mental
pictures, guesses or hypotheses, which he tries to fit to the facts, and
discards when they will _not_ fit.

At present, after nearly twenty years of critical discussion, there are
two chief theories of the structure of the atom. At first Sir J. J.
Thomson imagined the electrons circulating in shells (like the layers of
an onion) round the nucleus of the atom. This did not suit, and Sir E.
Rutherford and others worked out a theory that the electrons circulated
round a nucleus rather like the planets of our solar system revolving
round the central sun. Is there a nucleus, then, round which the
electrons revolve? The electron, as we saw, is a disembodied atom of
electricity; we should say, of "negative" electricity. Let us picture
these electrons all moving round in orbits with great velocity. Now it
is suggested that there is a nucleus of "positive" electricity
attracting or pulling the revolving electrons to it, and so forming an
equilibrium, otherwise the electrons would fly off in all directions.
This nucleus has been recently named the proton. We have thus two
electricities in the atom: the positive = the nucleus; the negative =
the electron. Of recent years Dr. Langmuir has put out a theory that the
electrons do not _revolve round_ the nucleus, but remain in a state of
violent agitation of some sort at fixed distances from the nucleus.

[Illustration: PROFESSOR SIR J. J. THOMSON

Experimental discoverer of the electronic constitution of matter, in the
Cavendish Physical Laboratory, Cambridge. A great investigator, noted
for the imaginative range of his hypotheses and his fertility in
experimental devices.]

[Illustration: _From the Smithsonian Report_, 1915.

ELECTRONS PRODUCED BY PASSAGE OF X-RAYS THROUGH AIR

A photograph clearly showing that electrons are definite entities. As
electrons leave atoms they may traverse matter or pass through the air
in a straight path The illustration shows the tortuous path of electrons
resulting from collision with atoms.]

[Illustration: MAGNETIC DEFLECTION OF RADIUM RAYS

The radium rays are made to strike a screen, producing visible spots of
light. When a magnetic field is applied the rays are seen to be
deflected, as in the diagram. This can only happen if the rays carry an
electric charge, and it was by experiments of this kind that we obtained
our knowledge respecting the electric charges carried by radium rays.]

[Illustration: _Reprod