It was ascertained by Hertz and his immediate successors that
light has a remarkable power of discharging negative
electrification from the surface of bodies—especially from
certain substances. For long no explanation of the cause of this
appeared. But the electron—the ubiquitous electron—is now known
with considerable certainty to be responsible. The effect of the
electric force in the light wave is to direct or assist the
electrons contained in the substance to escape from the surface
of the body. Each electron carries away a very small charge of
negative electrification. If, then, a body is
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originally charged negatively, it will be gradually discharged by
this convective process. If it is not charged to start with, the
electrons will still be liberated at the surface of the body, and
this will acquire a positive charge. If the body is positively
charged at first, we cannot discharge it by illumination.
It would be superfluous for me to speak here of the nature of
electrons or of the various modes in which their presence may be
detected. Suffice it to say, in further connection with the Hertz
effect, that when projected among gaseous molecules the electron
soon attaches itself to one of these. In other words, it ionises
a molecule of the gas or confers its electric charge upon it. The
gaseous molecule may even be itself disrupted by impact of the
electron, if this is moving fast enough, and left bereft of an
electron.
We must note that such ionisation may be regarded as conferring
potential chemical properties upon the molecules of the gas and
upon the substance whence the electrons are derived. Similar
ionisation under electric forces enters, as we now believe, into
all the chemical effects progressing in the galvanic cell, and,
indeed, generally in ionised solutes.
An experiment will best illustrate the principles I wish to
remind you of. A clean aluminium plate, carefully insulated by a
sulphur support, is faced by a sheet of copper-wire-gauze placed
a couple of centimetres away from it. The gauze is maintained at
a high positive
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potential by this dry pile. A sensitive gold-leaf electroscope is
attached to the aluminium plate, and its image thrown upon the
screen. I now turn the light from this arc lamp upon the wire
gauze, through which it in part passes and shines upon the
aluminium plate. The electroscope at once charges up rapidly.
There is a liberation of negative electrons at the surface of the
aluminium; these, under the attraction of the positive body, are
rapidly removed as ions, and the electroscope charges up
positively.
Again, if I simply electrify negatively this aluminium plate so
that the leaves of the attached electroscope diverge widely, and
now expose it to the rays from the arc lamp, the charge, as you
see, is very rapidly dissipated. With positive electrification of
the aluminium there is no effect attendant on the illumination.
Thus from the work of Hertz and his successors we know that
light, and more particularly what we call actinic light, is an
effective means of setting free electrons from certain
substances. In short, our photographic agent, light, has the
power of expelling from certain substances the electron which is
so potent a factor in most, if not in all, chemical effects. I
have not time here to refer to the work of Elster and Geitel
whereby they have shown that this action is to be traced to the
electric force in the light wave, but must turn to the probable
bearing of this phenomenon on the familiar facts of photography.
I assume that the experiment I have shown you is the most