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fundamental photographic experiment which it is now in our power to make.

We must first ask from what substances can light liberate electrons.  There are many—­metals as well as non-metals and liquids.  It is a very general phenomenon and must operate widely throughout nature.  But what chiefly concerns the present consideration is the fact that the haloid salts of silver are vigorously photo-electric, and, it is suggestive, possess, according to Schmidt, an activity in the descending order bromide, chloride, iodide.  This is, in other words, their order of activity as ionisers (under the proper conditions) when exposed to ultra-violet light.  Photographers will recognise that this is also the order of their photographic sensitiveness.

Another class of bodies also concerns our subject:  the special sensitisers used by the photographer to modify the spectral distribution of sensibility of the haloid salts, e.g. eosine, fuchsine, cyanine.  These again are electron-producers under light stimulus.  Now it has been shown by Stoletow, Hallwachs, and Elster and Geitel that there is an intimate connection between photo-electric activity and the absorption of light by the substance, and, indeed, that the particular wave-lengths absorbed by the substance are those which are effective in liberating the electrons.  Thus we have strong reason for believing that the vigorous photo-electric activity displayed by the special sensitisers must be dependent upon their colour absorption.  You will recognise that this is just

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the connection between their photographic effects and their behaviour towards light.

There is yet another suggestive parallel.  I referred to the observation of Sir James Dewar as to the continued sensitiveness of the photographic film at the lowest attained extreme of temperature, and drew the inference that the fundamental photographic action must be of intra-atomic nature, and not dependent upon the vis viva of the molecule or atom.  In then seeking the origin of photographic action in photo-electric phenomena we naturally ask, Are these latter phenomena also traceable at low temperatures?  If they are, we are entitled to look upon this fact as a qualifying characteristic or as another link in the chain of evidence connecting photographic with photo-electric activity.

I have quite recently, with the aid of liquid air supplied to me from the laboratory of the Royal Dublin Society, tested the photo-sensibility of aluminium and also of silver bromide down to temperatures approaching that of the liquid air.  The mode of observation is essentially that of Schmidt—­what he terms his static method.  The substance undergoing observation is, however, contained at the bottom of a thin copper tube, 5 cm. in diameter, which is immersed to a depth of about 10 cm in liquid air.  The tube is closed above by a paraffin stopper which carries a thin quartz window as well as the sulphur tubes through which the connections pass.  The air within is very carefully dried by phosphorus