h what is
known of the physics of chemical activity. Finally, as will be
seen later, it is hardly adequate to account for the varying
degrees of stability which may apparently characterise the latent
image. Still, there is much in Bose's work deserving of careful
consideration. He has by no means exhausted the line of
investigation he has originated.

Another theory has doubtless been in the minds of many. I have
said we must seek guidance in some photo-physical phenomenon.
There is one such which preeminently connects light and chemical
phenomena through the intermediary of the effects of the former
upon a component part of the atom. I refer to the phenomena of
photo-electricity.

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

204

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