Manganese Tetrachloride. 481 



rnanganous chloride, just as, in an analogous case, Wurtz 

 showed that phosphorus pentachloride dissociated to a very 

 much slighter extent when volatilized in vapour of phosphorus 

 trichloride than when volatilized alone. In the case of the 

 solution of manganese dioxide in hydrochloric acid, we have 

 unstable molecules of the tetrachloride continually dissociating 

 into manganous chloride and two atoms of chlorine, and the 

 molecules of manganous chloride thus formed continually, but 

 to a smaller extent, uniting with the atoms of chlorine present, 

 to form the tetrachloride again. If, therefore, a large excess 

 of MnCl 2 is present, it is obvious that much less of the chlorine 

 formed by the decomposition of the tetrachloride will escape 

 recomposition with Mn01 2 molecules, and so less chlorine will 

 be evolved from the solution. Also it is evident that the 

 addition of the first molecule of MnCl 2 will cause a greater 

 increase in the amount recovered than the addition of the 

 second and other molecules, but at the same time a curve 

 expressing the amount recovered for each addition of MnCl 2 

 would not change its form suddenly after the addition of the 

 first molecule. 



We thus arrive at an explanation as to why the amount of 

 chlorine removed from a solution of the dioxide in hydro- 

 chloric acid was considerably greater in the first hour than in 

 the second, and this was greater than in the next two hours. 

 It is because that, as the original MnCl 4 begins to be decom- 

 posed, a corresponding quantity of MnCl 2 is formed. There- 

 fore, the more MnCl 4 decomposed, the more MnCl 2 is formed, 

 and so the more stable becomes the remnant of MnCl 4 still 

 left undecomposed. The same explanation also applies to the 

 curve for the rate of evolution of chlorine from the dioxide 

 solution at 31°*5. 



It might be concluded that if the dioxide be allowed to 

 dissolve in hydrochloric acid saturated with chlorine, less de- 

 composition would, for a similar reason, take place, and there- 

 fore more dioxide would be recovered. In practice, it was 

 found that the amount of Mn0 2 recovered was raised scarcely 

 at all by this means. This is probably due to the fact that 

 Mn(Jl 2 is unable to recombine with molecules of chlorine to 

 form the tetrachloride, but only with nascent chlorine set free 

 by other decompositions of MnCl 4 molecules. 



It was found experimentally that hydrochloric acid con- 

 taining MnCl 2 in solution did not dissolve any more chlorine 

 even at —32° than hydrochloric acid alone, and also the 

 solution was not darkened in colour. No higher chloride 

 could therefore have been formed. It is scarcely possible to 

 try the action of nascent chlorine upon manganous chloride 



Phil. Mag. S. 5. Vol. 31. No. 193. June 1891. 2 M 



