s that when they
strike against the molecules of the gas through which they are passing
they ionize them, producing fresh ions which can carry on additional
current. The relation between the current and the potential difference
in this case is in accordance with the results of the theory of
ionization by collision. The corpuscles emitted from a body under the
action of ultra-violet light start from the surface with a finite
velocity. The velocity is not the same for all the corpuscles, nor
indeed could we expect that it should be: for as Ladenburg has shown
(_Ann. der Phys._, 1903, 12, p. 558) the seat of their emission is not
confined to the surface layer of the illuminated metal but extends to a
layer of finite, though small, thickness. Thus the particles which start
deep down will have to force their way through a layer of metal before
they reach the surface, and in doing so will have their velocities
retarded by an amount depending on the thickness of this layer. The
variation in the velocity of the corpuscles is shown in the following
table, due to Lenard (_Ann. der Phys._, 1902, 8, p. 149).

+------------------------------------+--------+----------+-----------+
| | Carbon.| Platinum.| Aluminium.|
+------------------------------------+--------+----------+-----------+
| Corpuscles emitted with velocities | | | |
| between 12 and 8 X 10^7 cm sec. | 0.000 | 0.000 | 0.004 |
| between 8 and 4 X 10^7 cm sec. | 0.049 | 0.155 | 0.151 |
| between 4 and 0 X 10^7 cm sec. | 0.67 | 0.65 | 0.49 |
| | | | |
| Corpuscles only emitted with the | | | |
| help of an external electric | 0.28 | 0.21 | 0.35 |
| field. +--------+----------+-----------|
| | 1.00 | 1.00 | 1.00 |
+------------------------------------+--------+----------+-----------+

If the illuminated surface is completely surrounded by an envelope of
the same metal insulated from and completely shielded from the light,
the emission of the negative corpuscles from the illuminated surface
would go on until the potential difference V between this surface and
the envelope became so great that the corpuscles with the greatest
velocity lost their energy before reaching the envel