e speed of more than a thousand
miles an hour, each passing something like 6,000,000,000 of its
neighbours in the course of every second. Yet this particle of gas is
a thinly populated world in comparison with a particle of metal. Take
a cubic centimetre of copper. In that very small square of solid matter
(each side of the cube measuring a little more than a third of an inch)
there are about a quadrillion atoms. It is these minute and elusive
particles that modern physics sets out to master.

At first it was noticed that the atom of hydrogen was the smallest or
lightest of all, and the other atoms seemed to be multiples of it.
A Russian chemist, Mendeleeff, drew up a table of the elements in
illustration of this, grouping them in families, which seemed to point
to hydrogen as the common parent, or ultimate constituent, of each. When
newly discovered elements fell fairly into place in this scheme the idea
was somewhat confidently advanced that the evolution of the elements was
discovered. Thus an atom of carbon seemed to be a group of 12 atoms of
hydrogen, an atom of oxygen 16, an atom of sulphur 32, an atom of copper
64, an atom of silver 108, an atom of gold 197, and so on. But more
correct measurements showed that these figures were not quite exact, and
the fraction of inexactness killed the theory.

Long before the end of the nineteenth century students were looking
wistfully to the ether for some explanation of the mystery. It was the
veiled statue of Isis in the scientific world, and it resolutely kept
its veil in spite of all progress. The "upper and limpid air" of the
Greeks, the cosmic ocean of Giordano Bruno, was now an established
reality. It was the vehicle that bore the terrific streams of energy
from star to planet across the immense reaches of space. As the atoms of
matter lay in it, one thought of the crystal forming in its mother-lye,
or the star forming in the nebula, and wondered whether the atom was not
in some such way condensed out of the ether. By the last decade of the
century the theory was confidently advanced--notably by Lorentz and
Larmor--though it was still without a positive basis. How the basis was
found, in the last decade of the nineteenth century, may be told very
briefly.

Sir William Crookes had in 1874 applied himself to the task of creating
something more nearly like a vacuum than the old air-pumps afforded.
When he had found the means of reducing the quantity of gas in a tube
u