When a gas contained in a vessel is brought to a sufficient state of
rarefaction the molecules cease to be subject to the laws of diffusion,
but move back and forth in straight lines from side to side of the
vessel. Their courses can be affected by electric discharge, which can
cause them to all impinge upon one of the electrodes, the positive one,
producing luminous effects. The path, if referred to the negative
electrode, tends to be normal to its surface, so that the resultant path
may be curved, as the stream of molecules go to the positive electrode.
The fanciful name of molecular bombardment is given to the phenomenon,
the luminous effect being attributed to the impinging of the molecules
against the positive electrode as they are projected from the positive.
The course of the molecules is comparable to the stream of carbon
particles from the positive to the negative electrode in an arc lamp.
(See Matter, Radiant.)