472 



Mr. H. M. Vernon on 



sodium-thiosulphate solution. It was accordingly first neces- 

 sary to see how far the passage of a current of air was capable 

 of removing chlorine dissolved in strong hydrochloric acid. In 

 the table below will be found a series of determinations of the 

 percentage of chlorine removed on that present in the solution, 

 under varying conditions as to temperature, degree of saturation 

 of the hydrochloric acid, volume of air drawn through the 

 solution, and time during which the passage of the air lasted. 

 The air was caused to take up the same temperature as the 

 hydrochloric acid through which it was drawn, by allowing it 

 to first pass through a worm-tube surrounding the flask con- 

 taining the hydrochloric acid, the whole apparatus being im- 

 mersed in water or a freezing- mixture, as the case might be. 



No. 



of 



Exp. 



Time of 

 passage 

 of Air. 



Volume, 



in litres, 



of Air drawn 



through. 



Temperature. 



Amount of 



Chlorine present 



on the saturating 



quantity. 



Percentage 



of 



Chlorine 



removed. 



1. 



10 min. 



•7 



o 



21 



100 



87*8 



2. 



10 „ 



•7 



21 



47 



77-5 



3. 



10 „ 



•7 



21 



231 



73-8 



4. 



10 „ 



•3 



21 



72-8 



80-0 



5. 



10 „ 



•2 



-14 



100 



56-8 



6. 



15 „ 



•3 



-14 



2-4 



48-3 



7. 



50 „ 



1-0 



7 



22-1 



99-7 



It will be seen that in a very short time the greater part of 

 the chlorine is removed from the solution by the passage of 

 the current of air, whether it be moderately fast or slow. In 

 the last experiment, where the current was continued for a 

 longer time, the hydrochloric acid was found to contain only 

 the merest trace of chlorine. As in the experiments to be 

 described below, the current of air was usually continued for 

 two hours or more, it may be concluded that the amount of 

 chlorine removed from the solution of manganese dioxide in 

 hydrochloric acid gives a very good estimate of the amount of 

 chlorine actually set free. 



Both anhydrous and hydrated manganese dioxide were 

 used in these experiments. The anhydrous variety was pre- 

 pared by igniting pure manganese nitrate, washing, and 

 drying at 100° C. It was found to contain almost the theo- 

 retical quantity of manganese dioxide (99'2 per cent.). The 

 hydrated variety was prepared by dissolving commercial 

 dioxide in hydrochloric acid, and precipitating by pouring the 



