532 Mr. R. E. Hughes on Water as a Catalyst. 



thus strips of highly glazed note-paper, ordinary filter-paper, 

 Swedish filter-paper, and vegetable parchment were cut, 

 soaked in the same KI solution, and exposed side by side to 

 bright sunlight. It was found that after some hours a grada- 

 tion of tints was thus obtained, the deepest being that on the 

 glazed paper, whereas the tint of the parchment was almost 

 imperceptible. Hence this change is perhaps due to the 

 traces of chlorine invariably present in glazed paper. The 

 depth of tint in the same paper was affected by the surface on 

 which the paper lies. Thus, if lying on blotting-paper the 

 change was very minute, slightly more perceptible on glass or 

 polished surfaces, and most evident when on wood or other 

 rough surface. This chemical decomposition also took place 

 in the dark but more slowly; further, once the paper was 

 quite dry, no perceptible deepening in tint was observed. A 

 solution of potassium iodide may be kept in sunlight for an 

 indefinite period, provided it is not exposed to the atmo- 

 sphere. This change is doubtless due to organic matter or 

 ozone present in the atmosphere, but is dependent on the 

 presence of moisture ; whereas the staining of paper by this 

 solution is due to the chlorine present or other constituent of 

 the glaze, and is also dependent on the presence of moisture. 



Silver nitrate behaves closely similarly, as also to a lesser 

 degree does platinum chloride. 



Some experiments were then made to determine the 

 question whether dried hydrochloric-acid gas has any action 

 on dry silver nitrate. 



The gas was passed through a tube containing copper 

 filings (to remove chlorine), then through a series of drying- 

 tubes containing strong H 2 S0 4 , and finally over P 2 5 con- 

 tained in a tube. This dried gas had no action on dried blue 

 litmus-paper. The silver nitrate was contained in a porcelain 

 boat, and had been previously heated to incipient fusion. 

 The gas was allowed to pass slowly through for about two 

 hours. A very slight change of colour was produced in the 

 silver salt, and a slight change did take place. By weighing 

 before and after, the amount of change was determined and 

 found to be 1*7 per cent, of the theoretical amount of change. 

 This experiment was conducted at the temperature of the 

 laboratory. An experiment conducted at 100° showed that 

 l'O per cent, of the theoretical amount of change had taken 

 place. 



The action of dried HC1 gas on dried manganese dioxide 

 was next investigated. This Mn0 2 was prepared from a pure 

 manganous sulphate by treatment with bromine and caustic 

 soda. It was thoroughly washed and dried. 



