THE ACTION OF CHEMICAL AFFINITY. 
2J1 
conclusion ; or rather, that if this source of error had not existed, the general con- 
clusion would have been somewhat more strikingly brought out. 
The influence of mass is very apparent in the experiment just described, but in 
order to observe it in the opposite direction, a considerable quantity of sulphuric acid 
was added to liquids where one equivalent of sulphate of quinine had been mixed 
with sufficient hydrochloric acid or chloride of sodium to render the fluorescence just 
invisible. The blue instantly became apparent in each case. 
In order to examine this matter still more closely, some neutral hydrochlorate of 
quinine was prepared by dissolving the organic alkali in the acid, and evaporating 
gently to dryness. Its solution diluted showed a mere trace of blue, which was 
removed on the addition of a drop or two of free hydrochloric acid. Portions of this 
solution were mixed with all the acids mentioned by Stokes as giving fluorescent 
compounds with quinine. The addition of sulphuric, nitric, phosphoric, acetic or 
oxalic acid, instantly reproduced the blue colour in a very marked manner. Citric 
or tartaric acid added in very large excess also produced the blue, but it was faint. 
I failed to detect any change on the addition of a considerable amount of hydro- 
cyanic acid. 
Similarly, a solution of sulphate of soda in considerable excess was added to an 
acid solution of hydrochlorate of quinine. A very perceptible amount of blue made 
its appearance. This also is in perfect consonance with what might theoretically 
be expected, and indicates that not only had the commixture of sulphate of soda 
converted a portion of the hydrochlorate of quinine into sulphate, but the free hydro- 
chloric acid had decomposed some of the sulphate of soda, liberating sulphuric acid, 
which had combined with the quinine salt to form the bisulphate. That a mixture 
of neutral sulphate of quinine with sulphate of soda does not give the blue tint, 
unless some free acid be added, was verified by previous experiment. 
Stokes also states that iodide or bromide of potassium added to a solution of 
bisulphate of quinine, or the acid phosphate, destroys the fluorescence. On exami- 
ning these reactions, I found that these haloid salts behaved precisely as the chloride 
did. A solution containing bisulphate of quinine had its blue tint gradually dimi- 
nished by the addition of either the bromide or the iodide of potassium ; and, where 
the fluorescence had been thus rendered barely if at all perceptible, it was restored 
on the addition of dilute sulphuric acid. 
This accumulation of evidence all goes to prove that quinine follows the same laws 
as the substances previously considered. 
Other fluorescent Organic Substances. 
The beautiful fluorescence exhibited by a solution of the inner bark of the horse- 
chestnut is due to the presence of the vegeto-alkali eesculine in a free state. When 
combined with hydrochloric acid it loses its peculiar optical properties, which are 
MDCCCLV. 2 G 
