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Proceedings of Royal Society of Edinburgh. [sess. 
conductivity. Similarly, red amorphous non-conducting selenium 
is gradually converted into the black crystalline conducting form, 
and red crystalline non-conducting mercuric sulphide changes to 
the black amorphous conducting modification (5, 6). These changes 
are permanent. 
The common characteristic in these simple cases is thus an 
increase in the electric conductivity of the substances exposed to 
light. 
Chemical Combination of Elementary Substances . 
It may he doubted whether, strictly speaking, any instances of 
such photo-chemical action are known. 
The classical example which most naturally suggests itself is that 
of a mixture of hydrogen and chlorine gases. It would appear, 
however, that the presence of water is essential to this action of 
light. It is certain that the gaseous mixture is much more sensi- 
tive to the action of light when moist. 
Liquid hydrogen chloride, according to the researches of Grove, 
is not an electrolyte. On the other hand, in aqueous solution 
hydrogen chloride has a very high degree of electrolytic con- 
ductivity. 
Reduction of Simple Metallic Compounds . 
Here also it is matter for doubt whether, strictly speaking, any 
instances of photo-chemical action falling under this head are 
known. Nitrate of silver, as ordinarily prepared, blackens on 
exposure to light, but the pure nitrate of silver prepared by Stas 
remained perfectly white for years, although kept in an ordinary 
white glass bottle. That admixture with certain foreign substances 
greatly increases the sensitiveness of compounds of silver to light 
is a familiar phenomenon in photography. 
The increase in electric conductivity of a film of silver haloid 
when exposed to light, is well illustrated in the actinometer devised 
by Arrhenius (7). 
The oxides of gold and silver are stated to give off oxygen on 
prolonged exposure to light and to leave residues containing the 
highly-conducting metals (8, 9, 10). Similarly, mercurous oxide 
