Manganese Tetrachloride. 477 



not quite stopped even after nearly five hours. The first two of 

 these three curves show that the gas is evolved quite regularly 

 till the last few cub. centim., when the rate of evolution becomes 

 much slower. The reason of this is evidently, that at the points 

 where the forms of the curves change, the greater part of the 

 tetrachloride present has been decomposed into manganous 

 chloride and chlorine, and that most of the gas which is 

 evolved after this is evolved from the hydrochloric acid in 

 which it is mechanically dissolved. Chlorine was found to 

 have about the same degree of solubility in hydrochloric acid 

 as in water ; at 50°, therefore, it dissolves *9 vol. and at 75° 

 •2 vol. of chlorine. At the commencement of the evolution of 

 this " residual " chlorine the acid will be considerably super- 

 saturated with it, but the greater part present will be expelled 

 from the solution in the course of an hour or two. We may 

 therefore expect the volume of " residual " gas to bear some 

 proportion to the temperature as well as to the volume of 

 hydrochloric acid employed. In the experiments of the first 

 two curves, representing the evolution at 75° and 50 o, 8, 

 5 cub. centim. of hydrochloric acid were used. We see, 

 however, that the volume of residual gas is much greater at 

 the lower temperature than at the higher, it being about 

 10 cub. centim. in one case and only 4 cub. centim. in the 

 other. In the curve representing the evolution at 31°*5, 

 8 cub. centim. of hydrochloric acid were used. Here the 

 rate of decomposition of the tetrachloride is so slow that it is 

 not possible, from the form of the curve, to tell where the 

 greater part of it has undergone decomposition and where 

 residual gas begins to be evolved. They evidently overlap 

 each other, so as to form one regular curve. Though the 

 form of this curve is regular, it nevertheless shows that the 

 rate of decomposition of the tetrachloride solution is much 

 slower at the end than at the beginning of the experiment, 

 after allowing of course for the amount of tetrachloride actually 

 existing in the solution. The reason of this will be made 

 evident further on. It will be noticed how much more slowly 

 the chlorine comes off from the solution when it is left alone 

 than when a current of air is drawn through it, as in the 

 previous experiments, allowance being made for the difference 

 of temperature. Thus at 18° it was found that in four hours 

 70*5 per cent, of the chlorine was removed by a current of air 

 from the dioxide solution, whilst here only about 85 per cent, 

 of chlorine was evolved from the solution in four hours at 

 31°*5. The reason of this is partly that a certain amount of 

 the chlorine remains dissolved in the solution without being 

 evolved, but it is also probable that the passage of the current 



