378 ON THE CHANGE OF REFRANGIBILITY OF LIGHT.
207. That the quinine was not decomposed when the blue colour due to sulphate of quinine was destroyed by hydrochloric acid, but only differently combined, was shown by adding a solution of carbonate of soda, which produced a white precipitate : and when this was collected on a filter, washed, and re-dissolved in dilute sulphuric acid, it exhibited the blue colour as usual.
208. The addition of a solution of common salt, instead of hydrochloric acid, to the solutions mentioned in Art. 205, likewise destroyed the blue colour. In the case of sulphuric acid this is only what might have been confidently anticipated ; but we should not perhaps have expected that quinine in combination with a weak acid, such as citric, would decompose hydro-chlorate of soda, giving rise to citrate of soda and hydroehl orate of quinine; yet this appears to be the nature of the reaction.
209. It might perhaps be supposed that the sulphuric acid was only partially expelled from sulphate1 of quinine by hydrochloric acid, and that the salt in solution was really a sort of double salt, in which the same base, quinine, was combined with sulphuric arid hydrochloric acids in atomic proportion. Hut if so, it is probable, though not certain, that the same salt would be formed on adding hydrochloric acid to a solution of disulphate of quinine, even though the quantity were not stiHicient to combine with the whole of the disulphate. On this supposition, if hydrochloric acid were added by small quantities at a, time to a solution of disulphate of quinine, the blue colour ought not to he developed; and when acid enough had been added it ought to be incapable of being developed by the addition of sulphuric! acid; whereas, if the whole of the sulphuric; acid be expelled by hydrochloric acid, the blue colour ought to be first developed, by the conversion of a portion of the disulphate of quinine into a sulphate, and then destroyed, on the addition of more acid, by the conversion of the sulphate into a hydroehlorate. On trying the experiment with a solution of disulphate of quinine in warm water, it was found that the blue colour was actually first developed and then destroyed.
210. A practical conclusion which seems to follow from these results is, that in the employment of quinine in medicine it is of