art. The Greeks thought that the explanation of the universe
was that an infinite number of these atoms had been moving and mixing in
an infinite space during an infinite time, and had at last hit by chance
on the particular combination which is our universe.

This was too simple and superficial. The idea of atoms was cast aside,
only to be advanced again in various ways. It was the famous Manchester
chemist, John Dalton, who restored it in the early years of the
nineteenth century. He first definitely formulated the atomic theory as
a scientific hypothesis. The whole physical and chemical science of that
century was now based upon the atom, and it is quite a mistake to
suppose that recent discoveries have discredited "atomism." An atom is
the smallest particle of a chemical element. No one has ever seen an
atom. Even the wonderful new microscope which has just been invented
cannot possibly show us particles of matter which are a million times
smaller than the breadth of a hair; for that is the size of atoms. We
can weigh them and measure them, though they are invisible, and we know
that all matter is composed of them. It is a new discovery that atoms
are not indivisible. They consist themselves of still smaller particles,
as we shall see. But the atoms exist all the same, and we may still say
that they are the bricks of which the material universe is built.

[Illustration: _Photo: Elliott & Fry._

SIR ERNEST RUTHERFORD

One of our most eminent physicists who has succeeded Sir J. J. Thomson
as Cavendish Professor of Physics at the University of Cambridge. The
modern theory of the structure of the atom is largely due to him.]

[Illustration: _Photo: Rischgitz Collection._

J. CLERK-MAXWELL

One of the greatest scientific men who have ever lived. He
revolutionised physics with his electro-magnetic theory of light, and
practically all modern researches have had their origin, direct or
indirect, in his work. Together with Faraday he constitutes one of the
main scientific glories of the nineteenth century.]

[Illustration: _Photo: Ernest H. Mills._

SIR WILLIAM CROOKES

Sir William Crookes experimented on the electric discharge in vacuum
tubes and described the phenomena as a "fourth state of matter." He was
actually observing the flight of electrons, but he did not fully
appreciate the nature of his experiments.]

[Illustration: _Photo: Photo Press_

PROFESSOR SIR W. H. BRAGG

One of the most distinguished p