THE ACTION OF CHEMICAL AFFINITY. 
209 
too complicated a character to admit of a full understanding of the cause of the 
remarkable change in ratio. 
These two experiments suffice to prove the influence upon the production of this 
blue gallate, both of the nature and quantity of other substances present in the solu- 
tion at the same time. 
Quinine salts. 
In his elaborate paper “On the Change of Refrangibility of Light*,” Professor 
Stokes has shown that various acid salts of quinine exhibit that remarkable internal 
dispersion of light which is now known by the name “ fluorescence.” He mentions 
the acid sulphate, phosphate, nitrate, acetate, citrate, tartrate, oxalate, and hydro- 
cyanate, as giving rise to the phenomenon; while quinine dissolved in hydrochloric 
acid did not present any such appearance. He found, moreover, that the addition of 
hydrochloric acid, or chloride of sodium, to one of the fluorescent salts destroyed the 
colour. Hence he concluded, and no doubt correctly, that in these cases muriate of 
quinine was formed; and to obviate the objection that possibly the non-fluorescent 
salt in solution might be a sort of double salt, in which the quinine was combined 
with the hydrochloric and the other acid in atomic proportion, he devised the follow- 
ing elegant experiment. To a strong warm solution of neutral sulphate of quinine, 
which displays no fluorescence, a very small quantity of hydrochloric acid was added ; 
it produced the blue appearance; more hydrochloric acid was added; the blue was 
destroyed. This seemed intelligible only on the supposition that the small quantity 
of acid first added displaced an equivalent amount of sulphuric acid, which, com- 
bining with the undecomposed sulphate, formed the acid salt which displays fluo- 
rescence to such a remarkable degree ; and that the larger quantity of hydrochloric 
acid decomposed this again, setting free the sulphuric acid, and leaving the quinine 
in solution as hydrochlorate. 
From the manner in which Professor Stokes describes and comments on these 
experiments, it is evident that he imagined (as most others would have done) that 
the decomposition was perfect, and that, in the particular experiment just men- 
tioned, every particle of the quinine existed in the solution in the form of hydro- 
chlorate, on account of the stronger affinity of that acid for the base. He states, 
moreover, that “even sulphuric acid is incapable of developing the blue colour in a 
solution of quinine in hydrochloric acid.” If this be true, it evidently militates 
against the conclusions that double decomposition does not take place perfectly in 
solution, unless aided by the insolubility or volatility of one or more of the com- 
pounds produced, and that great mass counterbalances weak affinity. Accordingly, 
I repeated the experiments quantitatively, and performed some additional ones'f'. 
* Philosophical Transactions, 1852. 
t Since writing the above, my attention has been drawn to a paragraph in Professor Stokes’s second paper 
(Philosophical Transactions for 1853, p. 394), in which he remarks that the neutral hydrochlorate of quinine is 
not absolutely non-fluorescent, as first stated, and that the hydrocyana'te is like the hydrochlorate. 
