Manganese Tetrachloride. 471 



seemed to demonstrate the existence of Mn 2 Cl 6 , yet he did not 

 clearly show that the tetrachloride had no existence. Expe- 

 riments were therefore made with a view to elucidating this 

 question. 



If, as Pickering concludes, the action of hydrochloric acid 

 upon manganese dioxide is expressed by the equation 



2Mn0 2 + 8HCl = Mn 2 Cl 6 + Cl 2 + 4H 2 0, 



then it is evident that, if such really be the case, when the 

 dioxide is dissolved in hydrochloric acid, half of the total avail- 

 able or loosely combined chlorine present ought to be evolved 

 very quickly, while the other half ought to come off very much 

 more slowly, as manganese sesquichloride only decomposes 

 moderately slowly at ordinary temperatures. Also it would 

 follow that the rate of evolution of the first half of available 

 chlorine should be almost unaffected by the temperature at 

 which the solution takes place ; whilst, as Pickering has 

 clearly shown by his experiments, the rate of decomposition 

 of the higher chloride of manganese, whatever it may be, 

 is very much diminished by lowering the temperature, and is 

 correspondingly increased by raising the temperature of solu- 

 tion. In fact, the only ways in which temperature would 

 affect the rate of evolution of the first half of the available 

 chlorine would be that (1) the dioxide would dissolve more 

 slowly at lower than at higher temperatures, and (2) chlorine 

 is more soluble in hydrochloric acid at lower temperatures, 

 and hence less of it would be evolved from the solution even 

 if it might exist in it in the free state. 



The Amount of Decomposition taking place in the 

 Dioxide Solution. 



Experiments were therefore made to ascertain : — 



(1) Whether, on solution of the dioxide in hydrochloric 

 acid, a considerable quantity of chlorine is imme- 

 diately evolved ; and whether, after half the available 

 chlorine has been evolved, the rate of evolution 

 becomes much slower. 



(2) To what extent the rate of evolution of the chlorine is 

 affected by lowering the temperature. 



The method adopted consisted in drawing a slow current of 

 air through a known quantity of the dioxide dissolved in a 

 measured volume of concentrated hydrochloric acid, which 

 was kept at a constant temperature. The air then passed 

 through potassium-iodide solution. The iodine thus liberated 

 by the chlorine was estimated by titration with standard 



