te of our knowledge,
be settled within fixed limits that meet with general agreement.

[Illustration: MAKING THE INVISIBLE VISIBLE

Radium, as explained in the text, emits rays--the "Alpha," the "Beta"
(electrons), and "Gamma" rays. The above illustration indicates the
method by which these invisible rays are made visible, and enables the
nature of the rays to be investigated. To the right of the diagram is
the instrument used, the Spinthariscope, making the impact of radium
rays visible on a screen.

The radium rays shoot out in all directions; those that fall on the
screen make it glow with points of light. These points of light are
observed by the magnifying lens.

A. Magnifying lens. B. A zinc sulphite screen. C. A needle on whose
point is placed a speck of radium.

The lower picture shows the screen and needle magnified.]

[Illustration: THE THEORY OF ELECTRONS

An atom of matter is composed of electrons. We picture an atom as a sort
of miniature solar system, the electrons (particles of negative
electricity) rotating round a central nucleus of positive electricity,
as described in the text. In the above pictorial representation of an
atom the whirling electrons are indicated in the outer ring. Electrons
move with incredible speed as they pass from one atom to another.]

[Illustration: ARRANGEMENTS OF ATOMS IN A DIAMOND

The above is a model (seen from two points of view) of the arrangement
of the atoms in a diamond. The arrangement is found by studying the
X-ray spectra of the diamond.]

As we have said, there are other fundamental existences which give rise
to more complex problems. The three great fundamental entities in the
physical universe are matter, ether, and energy; so far as we know,
outside these there is nothing. We have dealt with matter, there remain
ether and energy. We shall see that just as no particle of matter,
however small, may be created or destroyed, and just as there is no such
thing as empty space--ether pervades everything--so there is no such
thing as _rest_. Every particle that goes to make up our solid earth is
in a state of perpetual unremitting vibration; energy "is the universal
commodity on which all life depends." Separate and distinct as these
three fundamental entities--matter, ether, and energy--may appear, it
may be that, after all, they are only different and mysterious phases of
an essential "oneness" of the universe.

Sec. 9

The Future

Let us, in conclu