PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 493 
the transmitted light. This law did not seem applicable to the orange dispersion 
exhibited by the solution just mentioned; but then it is to be remembered that the 
solution contained a quantity of chlorophyll, which produces absorption bands with 
such energy that it would naturally mask the bands which might be due to an- 
other colouring principle with which it was mixed. To try whether the law would 
be obeyed if the chlorophyll were got rid of, I boiled in water some portions of the 
root-and young shoots which had turned blue, chlorophyll being insoluble in water. 
The solution thus obtained was red, in small thicknesses pink, and dispersed copiously 
a yellow or rather orange light. On subjecting the fluid to prismatic analysis, a band 
of absorption was seen at the place expected. Since aqueous solutions of this nature 
are liable to decomposition, frequently decomposing before sunlight can be obtained 
by which to examine them, the red solution was concentrated by evaporation and 
purified by alcohol, in which the orange-dispersing principle is soluble, as had already 
appeared from the properties of the alcoholic solution. The alcoholic solution thus 
obtained remained unchanged, at least for a long time, and had the further advantage 
over the aqueous solution of presenting the sensitive principle more nearly in a state 
of isolation, though it was still contaminated by some principle which dispersed a 
whitish light under the influence of rays of high refrangibility. 
64. The blue colouring matter may be readily extracted by cold water, but is de- 
composed by boiling. The blue solution dispersed an orange light like the other, but 
the dispersed light could not be nearly so well seen, just as would be the case were 
the red orange-dispersing fluid mixed with an insensible blue fluid of a much deeper 
colour, so that the mixture of the two would be blue. And in fact when the blue 
fluid was changed to red by boiling the colour became far less intense. 
Archil and Litmus. 
65. It is stated by Sir David Brewster that a very remarkable example of internal 
dispersion, which had been pointed out to him by Mr. Schunk, is exhibited in an 
alkaline or in an alcoholic solution of a resinous powder produced from orcine by 
contact with the oxygen of the air. Not being able readily to procure a specimen of 
orcine, I tried archil, and obtained from it and litmus some very remarkable solutions. 
In the fluid state in which archil is sold, the colour is much too deep for convenient 
optical examination. When a small quantity of archil is diluted with a great deal of 
water, the diluted fluid is very sensitive. It is red by transmission, or in small thick- 
nesses purple, but exhibits by dispersive reflexion a pretty copious but rather sombre 
green. 
66. When the fluid was examined by different methods, it was found to disperse a 
little red, some orange, and a great deal of green. The red dispersion was so slight, 
that in observing by the third method it appeared doubtful whether there was any 
except false dispersion. It commenced in the red, when the active and dispersed 
lights had the same refrangibility, or nearly so. The orange dispersion commenced 
