his sheaf of rays out of its course with a magnet, and he
accurately measured the shift of the luminous spot on the screen where
the rays impinged on it. But when he knows the exact intensity of his
magnetic field--which he can control as he likes--and the amount of
deviation it causes, and the mass of the moving particles, he can tell
the speed of the moving particles which he thus diverts. These particles
were being hurled out of the atoms of radium, or from the negative pole
in a vacuum tube, at a speed which, in good conditions, reached nearly
the velocity of light, i.e. nearly 186,000 miles a second.

Their speed has, of course, been confirmed by numbers of experiments;
and another series of experiments enabled physicists to determine the
size of the particles. Only one of these need be described, to give the
reader an idea how men of science arrived at their more startling
results.

Fog, as most people know, is thick in our great cities because the
water-vapour gathers on the particles of dust and smoke that are in the
atmosphere. This fact was used as the basis of some beautiful
experiments. Artificial fogs were created in little glass tubes, by
introducing dust, in various proportions, for supersaturated vapour to
gather on. In the end it was possible to cause tiny drops of rain, each
with a particle of dust at its core, to fall upon a silver mirror and be
counted. It was a method of counting the quite invisible particles of
dust in the tube; and the method was now successfully applied to the new
rays. Yet another method was to direct a slender stream of the particles
upon a chemical screen. The screen glowed under the cannonade of
particles, and a powerful lens resolved the glow into distinct sparks,
which could be counted.

In short, a series of the most remarkable and beautiful experiments,
checked in all the great laboratories of the world, settled the nature
of these so-called rays. They were streams of particles more than a
thousand times smaller than the smallest known atom. The mass of each
particle is, according to the latest and finest measurements 1/1845 of
that of an atom of hydrogen. The physicist has not been able to find any
character except electricity in them, and the name "electrons" has been
generally adopted.

The Key to many Mysteries

The Electron is an atom, of disembodied electricity; it occupies an
exceedingly small volume, and its "mass" is entirely electrical. These
electrons a