IN LIGHT IN THE PRESENCE OF OXYGEN. CREIGHTON. 53 



The air thus removed was replaced by air from outdoors, 

 which first passed through a U-tube containing potassium 

 hydroxide. The air, after passing through this solution of 

 potassium hydroxide divided, and half went to one solution and 

 half to the other. After passing over these solutions it went, 

 as stated, through the potassium hydroxide in the U-tubes, A 

 and B, and then met in a common tube leading to the aspirator. 

 By these mea:ns it was very easy to pass the same quantity of 

 air over each solution. A diagram of the apparatus is given in 

 Fig. 1. After the last trace of the iodine had disappeared the 

 amount absorbed could easily be estimated. The use of the 

 second smaller IJ-tubes marked B in the diagram was to make 

 sure of the complete absorption of the iodine. 



After the solution had been exposed to the action of sunlight 

 for seven weeks, the solution in the bottle that was not coloured,, 

 became colourless. The solution in the amb^ 1 coloured bottle 

 still contained considerable quantity of iodine, and it was not 

 for nearly another seven weeks that its colour entirely dis- 

 appeared. This shows that the change of the iodine is acceler- 

 ated by light, and that its loss cannot probably be totally 

 accounted for by evaporation. On examining the U-tubes con- 

 taining the caustic potash solution, it was found that the first 

 one, A, contained iodine, while there had been none absorbed in 

 the second smaller tube, B, showing that no escape of iodine 

 had taken place. 



The amount of iodine carried away by the air and absorbed 

 by the solution of potassium hydroxide was next determined. 



When iodine is absorbed in potassium hydroxide, there is 

 formed five molecules of potassium iodide to one of iodate. The 

 solution of hydroxide was acidified with sulphuric acid ; and as 

 some of the iodide might have oxidised to iodate, a little iodide 

 was added to ensure complete decomposition of the iodate, then 

 a few cc. of starch solution added, and the liberated iodine 

 determined by means of sodium thiosulphate solution. 



