"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. Electricity was found, or certain eminent physicists thought they had found, that electricity per se had inertia. So the windmills in the Crookes' vacuum tubes were supposed to be moved by the impact of electric atoms."