Physicists did not take long to discover that the radiation from radium
was very like the radiation in a "Crookes tube." It was quickly
recognised, moreover, that both in the tube and in radium (and other
metals) the atoms of matter were somehow breaking down.
However, the first step was to recognise that there were three distinct
and different rays that were given off by such metals as radium and
uranium. Sir Ernest Rutherford christened them, after the first three
letters of the Greek alphabet, the Alpha, the Beta, and Gamma rays. We
are concerned chiefly with the second group and purpose here to deal
with that group only.[3]
[3] The "Alpha rays" were presently recognised as atoms of helium
gas, shot out at the rate of 12,000 miles a second.
The "Gamma rays" are _waves_, like the X-rays, not material particles.
They appear to be a type of X-rays. They possess the remarkable power of
penetrating opaque substances; they will pass through a foot of solid
iron, for example.
The "Beta rays," as they were at first called, have proved to be one of
the most interesting discoveries that science ever made. They proved
what Crookes had surmised about the radiations he discovered in his
vacuum tube. But it was _not_ a fourth state of matter that had been
found, but a new _property_ of matter, a property common to all atoms of
matter. The Beta rays were later christened Electrons. They are
particles of disembodied electricity, here spontaneously liberated from
the atoms of matter: only when the electron was isolated from the atom
was it recognised for the first time as a separate entity. Electrons,
therefore, are a constituent of the atoms of matter, and we have
discovered that they can be released from the atom by a variety of
agencies. Electrons are to be found everywhere, forming part of every
atom.
"An electron," Sir William Bragg says, "can only maintain a separate
existence if it is travelling at an immense rate, from one
three-hundredth of the velocity of light upwards, that is to say, at
least 600 _miles a second, or thereabouts_. Otherwise the electron
sticks to the first atom it meets." These amazing particles may travel
with the enormous velocity of from 10,000 to more than 100,000 miles a
second. It was first learned that they are of an electrical nature,
because they are bent out of their normal path if a magnet is brought
near them. And this fact led to a further discovery: to one of those
sensational estimates which the general public is apt to believe to be
founded on the most abstruse speculations. The physicist set up a little
chemical screen for the "Beta rays" to hit, and he so arranged his tube
that only a narrow sheaf of the rays poured on to the screen. He then
drew this 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.