ep 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 t