PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 543 
salt of quinine which shall not be converted by common salt, by double decompo- 
sition, into a hydrochlorate, it must apparently be sought for among the combina- 
tions of quinine with very weak acids, the affinity of which for soda does not much 
help that of hydrochloric acid for quinine. It seems likely enough that such salts 
may exist; for though acetate or citrate of quinine decomposes hydrochlorate of 
soda, hydrochlorate of quinine is decomposed by carbonate of soda ; and it is probable 
that many vegetable acids behave like the carbonic in this respect. 
211. The blue dispersion of a solution of sulphate of quinine is destroyed by 
hydrobromic and hydriodic acids just as by hydrochloric. In the experiment, 
solutions of bromide and iodide of potassium were used ; but as a considerable ex- 
cess of sulphuric acid was purposely added to the solution of quinine, the potassa 
introduced would merely remain inert in the solution as a sulphate, without impeding 
the observation. The same experiment was tried with phosphate of quinine with the 
same result. 
212. It is stated in Turner’s Chemistry, that the play of colours observed in 
solutions of polychrome (/. e. esculine) is destroyed by acids, and heightened by 
alkalies. 'I’he destruction, or at least almost complete destruction, of the blue colour 
due to dispersed light in a decoction of the bark of the horse-chestnut, which is 
produced by acids, is readily observed ; but I could not perceive that the addition 
of alkalies in the first instance to a fresh solution made any difference one way or 
other. If the blue colour had previously been destroyed by an acid, it was restored 
by the alkali. If the horse-chestnut had never been examined chemically, these 
observations alone would indicate that in all probability the principle to which the 
blue colour was due was capable of entering into firm combination with acids, but 
did not combine with alkalies. It is, in fact, as we know, a vegetable base. 
213. A solution of nitrate of uranium in ether is insensible, as if some of the 
elements of the ether entered into firm combination with the oxide of uranium. In 
connexion with this circumstance, it is rather remarkable, that although the ether 
passes off by evaporation when the solution is left to itself in an open vessel, if heat 
be applied chemical action sets in, and the residue consists chiefly of a salt which 
has all the appearance of oxalate of uranium. This salt, when washed and ex- 
amined in the moist state, without very great concentration of light, was found to 
be insensible*. 
214. It is rare to meet with solutions so highly sensitive as those of quinine and 
esculine, but similar observations may be made on a great number of solutions, by 
employing suitable methods. The most searching method consists in forming a 
bright and tolerably pure spectrum, by transmitting the sun’s light through a very 
broad slit, or even leaving out the slit altogether. It is desirable to use a lens of 
only moderate focal length in connexion with the prisms. The solution having been 
placed in the spectrum, the acid, or other agent whose reactions it is desired to study, 
* See note I. 
4 A 
MDCCCLII. 
