542 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
dence of a more intimate union between quinine and hydrochloric acid than between 
quinine and the acids first mentioned, and to try whether the addition of hydrochloric 
acid to the solutions mentioned in the preceding paragraph would not destroy the 
blue colour. On trial this proved to be actually the case, so that even sulphuric acid 
is incapable of developing the blue colour in a solution of quinine in hydrochloric 
acid. 
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 redissolved 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 hydrochlorate of soda, giving rise to citrate of soda 
and hydrochlorate 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 ex- 
pelled from sulphate 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 and hydrochloric acids in atomic proportion. But 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 sufficient 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 be 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, ttie 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 hydrochlorate. 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 little consequence whether the sulphate, 
phosphate, acetate, or hydrochlorate be used, since the first thi-ee salts would be 
immediately converted by the common salt in the body into the hydrochlorate, 
and the small quantity of a neutral salt of soda resulting from the double decompo- 
sition could hardly, one would supppose, be worth considering. However, the com- 
mon quinine is associated with cinchonine, the reactions of which may be different. 
According to Sir John Herschel, the latter alkaloid does not exhibit the blue colour, 
and therefore the optical tests do not apply to it. If it be desired to obtain a soluble 
