an 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_."

DETECTING A SMALL QUANTITY OF MATTER

In the left-hand photograph the two pieces of paper exactly balance. The
balance used is very sensitive, and when the single word "atoms" has
been written with a lead pencil upon on