the light. We may suppose that
under the action of the light some of the molecules are thrown into an
unstable state and explode, ejecting corpuscles; the light in this case
acts only as a trigger to liberate the energy in the atom, and it is
this energy and not that of the light which goes into the corpuscles. In
this way the velocity of the corpuscles would be independent of the
intensity of the light. But it may be asked, is this view consistent
with the result obtained by Ladenburg that the velocity of the
corpuscles depends upon the nature of the light? If light of a definite
wave length expelled corpuscles with a definite and uniform velocity, it
would be very improbable that the emission of the corpuscles is due to
an explosion of the atoms. The experimental facts as far as they are
known at present do not allow us to say that the connexion between the
velocity of the corpuscles and the wave length of the light is of this
definite character, and a connexion such as a gradual increase of
average velocity as the wave length of the light diminishes, would be
quite consistent with the view that the corpuscles are ejected by the
explosion of the atom. For in a complex thing like an atom there may be
more than one system which becomes unstable when exposed to light. Let
us suppose that there are two such systems, A and B, of which B ejects
the corpuscles with the greater velocity. If B is more sensitive to the
short waves, and A to the long ones, then as the wave length of the
light diminishes the proportion of the corpuscles which come from B will
increase, and as these are the faster, the average velocity of the
corpuscles emitted will also increase. And although the potential
acquired by a perfectly insulated piece of metal when exposed to
ultra-violet light would depend only on the velocity of the fastest
corpuscles and not upon their number, in practice perfect insulation is
unattainable, and the potential actually acquired is determined by the
condition that the gain of negative electricity by the metal through
lack of insulation, is equal to the loss by the emission of negatively
electrified corpuscles. The potential acquired will fall below that
corresponding to perfect insulation by an amount depending on the number
of the faster corpuscles emitted, and the potential will rise if the
proportion of the rapidly moving corpuscles is increased, even though
there is no increase in their velocity. It is interesting t