This theory is furthermore closely allied to the theory of ionisation, and, like this latter, is based on the concept of the electron.

Cathode rays are electrons in rapid motion.
The phenomena produced both inside and outside a Crookes tube are, however, generally complex.

In Lenard's first experiments, and in many others effected later when this region of physics was still very little known, a few confusions may be noticed even at the present day.
At the spot where the cathode rays strike the walls of the tube the essentially different X rays appear.

These differ from the cathode radiations by being neither electrified nor deviated by a magnet.

In their turn these X rays may give birth to the secondary rays of M.
Sagnac; and often we find ourselves in presence of effects from these last-named radiations and not from the true cathode rays.
The electrons, when they are propagated in a gas, can ionise the molecules of this gas and unite with the neutral atoms to form negative ions, while positive ions also appear.