o compare
other cases in which corpuscles are emitted with the case of
ultra-violet light. When a metal or gas is bombarded by cathode rays it
emits corpuscles and the velocity of these is found to be independent of
the velocity of the cathode rays which excite them; the velocity is
greater than for corpuscles emitted under ultra-violet light. Again,
when bodies are exposed to Rontgen rays they emit corpuscles moving with
a much greater velocity than those excited by cathode rays, but again
the velocity does not depend upon the intensity of the rays although it
does to some extent on their hardness. In the case of cathode and
Rontgen rays, the velocity with which the corpuscles are emitted seems,
as far as we know at present, to vary slightly, but only slightly, with
the nature of the substance on which the rays fall. May not this
indicate that the first effect of the primary rays is to detach a
neutral doublet, consisting of a positive and negative charge, this
doublet being the same from whatever system it is detached? And that the
doublet is unstable and explodes, expelling the negative charge with a
high velocity, and the positive one, having a much larger charge, with a
much smaller velocity, the momentum of the negative charge being equal
to that of the positive.

Up to now we have been considering the effects produced when light is
incident on metals. Lenard found (and the result has been confirmed by
the experiments of J. J. Thomson and Lyman) that certain kinds of
ultra-violet light ionize a gas when they pass through. The type of
ultra-violet light which produces this effect is so easily absorbed that
it is stopped by a layer a few millimetres thick of air at atmospheric
pressure.

_Ionization by Collision._--When the ionization of the gas is produced
by external agents such as Rontgen rays or ultra-violet light, the
electric field produces a current by setting the positive ions moving in
one direction, and the negative ones in the opposite; it makes use of
ions already made and does not itself give rise to ionization. In many
cases, however, such as in electric sparks, there are no external agents
to produce ionization and the electric field has to produce the ions as
well as set them in motion. When the ionization is produced by external
means the smallest electric field is able to produce a current through
the gas; when, however, these external means are absent no current is
produced unless the strength o