uence on the photo-electric effect, for while potassium is more
sensitive in blue light than sodium, the strong absorption of yellow
light by sodium makes it more than five times more sensitive to this
light than potassium. Stoletow, at an early period, called attention to
the connexion between strong absorption and photo-electric effects. He
showed that water, which does not absorb to any great extent either the
ultra-violet or visible rays, does not show any photo-electric effect,
while strongly coloured solutions, and especially solutions of
fluorescent substances such as methyl green or methyl violet, do so to a
very considerable extent; indeed, a solution of methyl green is more
sensitive than zinc. Hallwachs (_Wied. Ann._ 37, p. 666) proved that in
liquids showing photo-electric effects there is always strong
absorption; we may, however, have absorption without these effects.
Phosphorescent substances, such as calcium sulphide show this effect, as
also do various specimens of fluor-spar. As phosphorescence and
fluorescence are probably accompanied by a very intense absorption by
the surface layers, the evidence is strong that to get the
photo-electric effects we must have strong absorption of some kind of
light, either visible or ultra-violet.

[Illustration: FIG. 14.]

If a conductor A is placed near a conductor B exposed to ultra-violet
light, and if B is made the negative electrode and a difference of
potential established between A and B, a current of electricity will
flow between the conductors. The relation between the magnitude of the
current and the difference of potential when A and B are parallel plates
has been investigated by Stoletow (_Journal de physique_, 1890, 11, p.
469), von Schweidler (_Wien. Ber._, 1899, 108, p. 273) and Varley
(_Phil. Trans. A._, 1904, 202, p. 439). The results of some of Varley's
experiments are represented in the curves shown in fig. 14, in which the
ordinates are the currents and the abscissae the potentials. It will be
seen that when the pressure is exceedingly low the current is
independent of the potential difference and is equal to the negative
charge carried off in unit time by the corpuscles emitted from the
surface exposed to the light. At higher pressures the current rises far
above these values and increases rapidly with the potential difference.
This is due to the corpuscles emitted by the illuminated surface
acquiring under the electric field such high velocitie