62 



REPORT — 1889. 



.show that, whatever the agent may be which decolourises the per- 

 manganate, it readily becomes inactive on standing in the dark, bnt that 

 it is again formed by exposing the liquid to light ; this process can be 

 repeated indefinitely. In these experiments the temperature was kept 

 below 25°C. Another experiment was made to determine to what ex- 

 tent evaporation and condensation of the liquid due to changes in tem- 

 perature bring about the formation of the peroxide. Acidified water 

 was exposed in two flasks : in one case the flask was rendered impervious 

 to light by a covering of opaque paper, whilst the other flask was exposed 

 to light. After one week's exposure to bright sunshine (Aug. 20 to 

 Aug. 27) the liquid was tested with permanganate, when it was found that 

 50 CCS. water fi'om the flask in the dark registered 1 cc. permanganate 

 (50 CCS. pure acidified water required 1 cc. permanganate), showing 

 absence of peroxide of hydrogen. 50 cos. of water from the exposed flask 

 registered 2'4 ccs. permanganate, subtracting 1 cc. for pure water = 

 1"4 cc. due to the peroxide ='00047 per cent. 



Table sJioiving the Action of Liijlit on Water and Oxygen. 

 Ea'jjosure July 28 to August 30. 



Dry and moist oxygen were exposed to light, but no trace of ozone or 

 hydrogen peroxide was detected. 



These experiments seem to show that hydrogen peroxide is formed 

 when oxygen and liquid water are exposed to sunshine, and tbat the rays 

 of high refi'angibility are influential in bringing this change about. 



The influence of -moisture in bringing about the oxidation of hydrogen 

 chloride is now easily explained, for it seems probable that when a 

 saturated solution of the chloride is exposed to light in presence of 

 oxygen, hydrogen dioxide is first formed, and it is this substance which 

 decomposes the hydrogen chloride. Further, the oxidation of the 

 sulphides and of certain pigments by light is explained, though no clue is 

 afforded to rhc explanation of reductions by light, since in many instances 

 this takes place in dry air. 



Numerous experiments are being made in other directions, but these 

 are not sufiiciently advanced for publication. 



