34 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
the opposite extremities (or rather, as we ought to express it, in the opposite regions) 
of the spectrum, will naturally give rise to many interesting speculations and conclu- 
sions, of which those I am about to state will probably not be regarded as among the 
least so. We all know that colours of vegetable origin are usually considered to be 
destroyed and whitened by the continued action of light. The process, however, is 
too slow to be made the subject of any satisfactory series of experiments; and, in con- 
sequence, this subject, so interesting to the painter, the dyer, and the general artist, 
has been allowed to remain uninvestigated. As soon, however, as these evidences of 
a counterbalance of mutually opposing actions, in the elements of which the solar 
light consists, offered themselves to view, it occurred to me, as a reasonable subject 
of inquiry, whether this slow destruction of vegetable tints might not be due to the 
feeble amount of residual action outstanding after imperfect mutual compensation, in 
the ordinary way in which such colours are presented to light, i. e. to mixed rays. 
It appeared therefore to merit inquiry, whether such colours, subjected to the un- 
compensated action of the elementary rays of the spectrum, might not undergo 
changes differing both in kind and in degree from those which mixed light produces 
on them, and might not, moreover, by such changes indicate chemical properties in 
the rays themselves hitherto unknown. The want of sunshine has alone prevented 
me from pushing these inquiries to the extent to which, it will appear from the result 
of the only trials I have made, they well deserve to be pursued. 
90. One of the most intense and beautiful of the vegetable blues is that yielded by 
the blue petals of the dark velvety varieties of the common Heartsease ( Viola tri- 
color). It is best extracted by alcohol. The alcoholic tincture so obtained, after a 
few days keeping in a stoppered phial, loses its fine blue colour, and changes to a pallid 
brownish red, like that of port wine discoloured by age. When spread on paper it 
hardly tinges it at first, and might be supposed to have lost all colouring virtue, but 
that a few drops of very dilute sulphuric acid sprinkled over it, indicate, by the beau- 
tiful and intense rose colour developed where they fall, the continued existence of the 
colouring principle. As the paper so moistened with the tincture dries, however, the 
original blue colour begins to appear, and when quite dry is full and rich. The tinc- 
ture by long keeping loses this quality, which does not seem capable of being restored. 
But the paper preserves its colour well, and is even rather remarkable among vege- 
table colours for its permanence in the dark, or in common daylight. 
91. A paper so tinged (No. 599.), of a very fine and full blue colour, was exposed 
to the solar spectrum concentrated, as usual, (October 11, 1839,) by a prism and 
lens ; a water-prism, however, was used in this experiment, to command as large an 
area of sunbeam as possible. The sun was poor and desultory; nevertheless in half 
an hour there was an evident commencement of whitening from the fiducial yellow 
to the mean red. In two hours and a half, the sunshine continuing very much in- 
terrupted by clouds, the effect was marked by a considerable white patch extending 
from the extreme red to the end of the violet, but not traceable beyond that limit. 
