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 sulphate^1 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