But these wonders of the atom are only a prelude to the more romantic
and far-reaching discoveries of the new physics--the wonders of the
electron. Another and the most important phase of our exploration of the
material universe opened with the discovery of radium in 1898.
In the discovery of radio-active elements, a new property of matter was
discovered. What followed on the discovery of radium and of the X-rays
we shall see.
As Sir Ernest Rutherford, one of our greatest authorities, recently
said, the new physics has dissipated the last doubt about the reality of
atoms and molecules. The closer examination of matter which we have been
able to make shows positively that it is composed of atoms. But we must
not take the word now in its original Greek meaning (an "indivisible"
thing). The atoms are not indivisible. They can be broken up. They are
composed of still smaller particles.
The discovery that the atom was composed of smaller particles was the
welcome realisation of a dream that had haunted the imagination of the
nineteenth century. Chemists said that there were about eighty different
kinds of atoms--different kinds of matter--but no one was satisfied with
the multiplicity. Science is always aiming at simplicity and unity. It
may be that science has now taken a long step in the direction of
explaining the fundamental unity of all the matter. The chemist was
unable to break up these "elements" into something simpler, so he called
their atoms "indivisible" in that sense. But one man of science after
another expressed the hope that we would yet discover some fundamental
matter of which the various atoms were composed--_one primordial
substance from which all the varying forms of matter have been evolved
or built up_. Prout suggested this at the very beginning of the century,
when atoms were rediscovered by Dalton. Father Secchi, the famous Jesuit
astronomer said that all the atoms were probably evolved from ether; and
this was a very favoured speculation. Sir William Crookes talked of
"prothyl" as the fundamental substance. Others thought hydrogen was the
stuff out of which all the other atoms were composed.
The work which finally resulted in the discovery of radium began with
some beautiful experiments of Professor (later Sir William) Crookes in
the eighties.
It had been noticed in 1869 that a strange colouring was caused when an
electric charge was sent through a vacuum tube--the walls of the glass
tube began to glow with a greenish phosphorescence. A vacuum tube is one
from which nearly all the air has been pumped, although we can never
completely empty the tube. Crookes used such ingenious methods that he
reduced the gas in his tubes until it was twenty million times thinner
than the atmosphere. He then sent an electric discharge through, and got
very remarkable results. The negative pole of the electric current (the
"cathode") _gave off rays which faintly lit the molecules of the thin
gas in the tube_, and caused a pretty fluorescence on the glass walls of
the tube. What were these Rays? Crookes at first thought they
corresponded to a "new or fourth state of matter." Hitherto we had only
been familiar with matter in the three conditions of solid, liquid, and
gaseous.
Now Crookes really had the great secret under his eyes. But about twenty
years elapsed before the true nature of these rays was finally and
independently established by various experiments. The experiments proved
"that the rays consisted of a stream of negatively charged particles
travelling with enormous velocities from 10,000 to 100,000 miles a
second. In addition, it was found that the mass of each particle was
exceedingly small, about 1/1800 of the mass of a hydrogen atom, the
lightest atom known to science." _These particles or electrons, as they
are now called, were being liberated from the atom._ The atoms of matter
were breaking down in Crookes tubes. At that time, however, it was
premature to think of such a thing, and Crookes preferred to say that
the particles of the gas were electrified and hurled against the walls
of the tube. He said that it was ordinary matter in a new
state--"radiant matter." Another distinguished man of science, Lenard,
found that, when he fitted a little plate of aluminum in the glass wall
of the tube, the mysterious rays passed through this as if it were a
window. They must be waves in the ether, he said.
[Illustration: _From "Scientific Ideas of To-day_."