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XLVIII. On the Colour of Salts in Solution, each constituent of 
which is coloured. By J. H. Guavstonz, Ph.D., F.RS. &e.* 
[With a Plate.] 
prBou the general rule, that a particular base or acid has the 
same effect on the rays of light, with whatever it may be com- 
bined in aqueous solution, it may be inferred that when two bodies 
combine, each of which has a different influence on the rays of light, 
a solution of the salt itself will transmit only those rays which are 
not absorbed by either, or im other words, those which are trans- 
mitted by both. Thus if a red acid and a blue base combine, 
the resulting salt will certainly not be purple, but it may present 
the colour of some ray intermediate between red and blue—per- 
haps green. 
“This, indeed, was laid down by me as a general rule in a paper 
read before the Chemical Society, “On the Use of the Prism in 
Qualitative Analysist ;” but only one instance was given, namely 
chromate of copper. Since then I have examined several salts 
of the character above described, and many haloid salts, which 
have led to some unexpected, and I think suggestive results, 
The method employed was that described in the paper already 
referred to. It is briefly as follows:—The solution to be ex- 
amined is placed in a hollow wedge of glass, which is interposed 
between the eye of the spectator and a narrow slit in the window- 
shutter, im such a manner that the thin line of light is seen 
traversing the different thicknesses of liquid. This line of light 
is then analysed by placing. a good prism between the hollow 
wedge and the eye. In this way it is seen at once what rays 
are absorbed by increasing thicknesses of the solution. The 
diagrams in Plate IJ. give the appearances thus presented; the 
lowest portion representing the prismatic spectrum as seen 
through the thinnest possible stratum of liquid, the higher por- 
tions showing the gradual absorption of different rays. The 
diagrams are not coloured, but the fixed lines will indicate with 
far greater accuracy than colour would, the different parts of the 
spectrum transmitted. The least refracted, that is, the red ray, 
is always to the right hand; then follow of course the orange, 
yellow, green and blue, while the violet ray is on the extreme 
left {. 
* Communicated by the Author; having been read at the Meeting of 
the British Association at Dublin, 1857. 
+ Quart. Jown. Chem. Soc. vol. x. p. 79. 
{ Since writing the above, my attention has been directed by the Abbé 
Moigno to a description contained in his Répertoire d’Optique Moderne, 
3™¢ partie, p. 1258, of some experiments by J. Miiller, in which he has 
represented the transmission of light through varying thicknesses of a 
