474 Mr. H. M. Vernon 



on 



same freezing-mixture as the flask. It was found that, after 

 the passage of the current of air had been continued for two 

 hours, 19*1 per cent, of the available chlorine had been 

 removed. In another experiment, at — 6°, it was found that 

 33*8 per cent, was removed in two hours. The manganese 

 dioxide did not appear to be nearly so readily soluble in hydro- 

 chloric acid at these low temperatures as it is at ordinary tem- 

 peratures ; for it was found that some of it remained undissolved 

 even at the end of the two hours. It was therefore necessary 

 to ascertain how much of the dioxide had actually passed into 

 solution, as the undissolved dioxide would not enter into the 

 reaction at all. In these experiments at low temperatures, 

 therefore, after the passage of the current of air had been 

 stopped, the liquid in the flask was poured on to a small 

 filter, the filtrate passing into potassium-iodide solution, while 

 the undissolved dioxide remained behind. As the filtering 

 only took a few seconds, only a very small quantity of chlorine 

 would escape from the solution and so not be accounted for. 



III. The amount of chlorine removed from a solution of 

 dioxide in hydrochloric acid at —26° C. was then determined. 

 The flask and worm-tube used in the previous experiments 

 were surrounded by a freezing-mixture of crystallized calcium 

 chloride and pounded ice. The vessel containing this being 

 well covered up, the liquid was kept at a uniform temperature 

 throughout the experiment. It was found that in two hours 

 only 1*8 per cent, of the available chlorine had been removed. 

 During this time about 2 litres of air were drawn through the 

 solution. In the former experiments about 2*7 litres of air 

 were drawn through during the two hours. 



Another experiment, at —19 C, showed that 12*2 per cent, 

 of chlorine was removed in two hours. 



It thus appears that the stability of the dioxide solution is 

 very greatly increased as the temperature is lowered, and so 

 we might expect that at a still lower temperature it would not 

 undergo any decomposition whatever. It would not be possible 

 to try the action of hydrochloric acid on the dioxide at a much 

 lower temperature than —26°, as hydrochloric acid solidifies 

 at —29°, though when it contains chlorine or manganese 

 dioxide in solution it does not solidify till below this tem- 

 perature. 



It might be thought possible that the reason why no 

 chlorine is evolved from the solution is not that it is not 

 actually set free by the reaction of the dioxide and hydro- 

 chloric acid, but that it forms a solid hydrate with some of the 

 aqueous hydrochloric acid, and so is not removed from the 

 solution by the passage of the air. That this was not the case 



