to remark that these
phenomena are in bad agreement with the strain theory of Bose. We
have, in fact, but the one resource--the allotropic modification
of the haloid--whereby to explain all these orders of stability.
It appears to me that the elasticity of the electronic theory is
greater. The state of the ionised system may be very various
according as it arises from continued rhythmic effects or from
unorganised shocks. The ionisation due to X-rays or to friction
will probably be quite unorganised, that due to light more or
less stable according to the gradual and gentle nature of the
forces at work. I think we are entitled to conclude that on the
whole there is nothing in Wood's beautiful experiments opposed to
the photo-electric origin of photographic effects, but that they
rather fall in with what might be anticipated according to that
theory.

When we look for further support to the views I have laid before
you we are confronted with many difficulties. I have not as yet
detected any electronic discharge from the film under light
stimulus. This may be due to my defective experiments, or to a
fact noted by Elster and Geitel concerning the photo-electric
properties of gelatin.

212

They obtained a vigorous effect from Balmain's luminous paint,
but when this was mixed in gelatin there was no external effect.
Schmidt's results as to the continuance of photo-electric
activity when bodies in general are dissolved in each other lead
us to believe that an actual conservative property of the medium
and not an effect of this on the luminous paint is here involved.
This conservative effect of the gelatin may be concerned with its
efficacy as a sensitiser.

In the views I have laid before you I have endeavoured to show
that the recent addition to our knowledge of the electron as an
entity taking part in many physical and chemical effects should
be kept in sight in seeking an explanation of the mode of origin
of the latent image.[1]

[1] For a more detailed account of the subject, and some
ingenious extensions of the views expressed above, see
_Photo-Electricity_, by H. Stanley Allen: Longmans, Green & Ca.,
1913.

213

PLEOCHROIC HALOES [1]

IT is now well established that a helium atom is expelled from
certain of the radioactive elements at the moment of
transformation. The helium atom or alpha ray leaves the
transforming atom with a velocity which varies in the different
radioactive elements, but which is alw