1896-97.] Prof. John Gibson on Photo-Chemical Action . 305 
is said to be decomposed by light into metallic mercury and mer- 
curic oxide : 
Hg 2 0 = HgO + Hg; 
while mercurous chloride is split up into a mixture of metallic 
mercury and mercuric chloride. 
Oxidation of Simple Metallic Compounds . 
Lead oxide, under the influence of light, absorbs oxygen from 
the air, being ultimately converted into peroxide of lead (11). 
Red lead (Pb 3 0 4 ) and peroxide of lead (Pb0 2 ) are both, rela- 
tively to lead oxide, good conductors of electricity, and with this 
conductivity their use in secondary batteries is connected. 
Peroxide of manganese is another instance of a conducting 
metallic oxide. It is formed when manganous hydrate is exposed 
to air. Red light is said to accelerate and violet light to retard 
this oxidation (12). This latter statement is, however, admittedly 
problematical, and a consideration of the difficulty of arranging 
proper conditions for a crucial experiment makes it appear very 
doubtful. 
Changes Produced in Aqueous Solutions . 
Perhaps the most important example of this class of change 
is chlorine water. It is at best a very poor electrolyte, but the 
solution containing hydrochloric acid, which results from the action 
of light on chlorine water, has great electrolytic conductivity. It 
has been stated that this reaction is a reversible one, and it would 
be a matter of interest to determine experimentally whether this 
reversion can take place under conditions which involve a diminu- 
tion of electrolytic conductivity. 
Bromine water behaves similarly to chlorine water, while the 
very dilute solutions of iodine, which alone are obtainable, appear 
to be unaffected by light. 
Sulphurous anhydride in aqueous solution, and in absence of 
oxygen, gives, on exposure to light, sulphuric acid and sulphur. 
Hero also increase of electrolytic conductivity is obvious (13, 14). 
Gaseous hydrogen iodide remains quite unchanged if kept in 
complete darkness, but decomposes on exposure to sunlight. 
