Long before the discovery of radium led to the recognition of the electron
as the common constituent of all the bodies previously described as
chemical elements, the minute particles of matter in question had been
identified with the cathode rays observed in Sir William Crookes' vacuum
tubes. When an electric current is passed through a tube from which the air
(or other gas it may contain) has been almost entirely exhausted, a
luminous glow pervades the tube manifestly emanating from the cathode or
negative pole of the circuit. This effect was studied by Sir William
Crookes very profoundly. Among other characteristics it was found that, if
a minute windmill was set up in the tube before it was exhausted, the
cathode ray caused the vanes to revolve, thus suggesting the idea that they
consisted of actual particles driven against the vanes; the ray being thus
evidently something more than a mere luminous effect. Here was a mechanical
energy to be explained, and at the first glance it seemed difficult to
reconcile the facts observed with the idea creeping into favour, that the
particles, already invested with the name "electron," were atoms of
electricity pure and simple.
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