of the Solar Spectrum on Vegetable Colours. 13 
that by any means a singular instance. In some, but in very few 
cases, it is destroyed, so as neither to reappear on drying, nor 
to be capable of revival by any means tried. And in all cases 
long keeping deteriorates the colours and alters the qualities 
of the alcoholic tinctures themselves, so that they should al- 
ways be used as fresh as possible. 
165. If papers tinged with vegetable colours are intended 
to be preserved, they must be kept perfectly dry and in dark- 
ness. A close tin vessel, the air of which is dried by quicklime 
(carefully enclosed in double paper bags, well pasted at the 
edges to prevent the dust escaping), is useful for this purpose. 
Moisture (as already mentioned, especially assisted by heat) 
destroys them for the most part rapidly, though some (as the 
colour of the Senecio splendens) resist obstinately. Their de- 
structibility by this agency, however, seems to bear no distinct 
relation to their photographic properties. 
166. This is also the place to observe that the colour of a 
flower is by no means always, or usually, that which its ex- 
pressed juice imparts to white paper. In many cases the tints 
so imparted have no resemblance to the original hue. Thus, 
to give only a few instances, the red damask rose of that in- 
tense variety of colour, commonly called by florists the Black 
Rose, gives a dark slate blue, as do also the clove carnation 
and the black holyoak; a fine dark brown variety of Sparaxis 
gave a dull olive green; and a beautiful rose-coloured tulip, 
a dirty bluish green; but perhaps the most striking case of 
this kind is that of a common sort of red poppy (Papaver 
Etheum?), whose expressed juice imparts to paper a rich and 
most beautiful blue colour, whose elegant properties as a pho- 
tographic material will be further alluded to hereafter*. 
167. This change of colour is probably owing to different 
causes in different flowers. In some it undoubtedly arises 
from the escape of carbonic acid, but this as a general cause 
for the change from red to blue, has, I am aware, been con- 
troverted+. In some (as is the case with the yellow Ranun- 
culi) it seems to arise from a chemical alteration depending 
on absorption of oxygen; and in others, especially where the 
expressed juice coagulates on standing, to a loss of vitality or 
disorganization of the molecules. The fresh petal of a single 
flower, merely crushed by rubbing on dry paper, and in- 
stantly dried, leaves a stain much more nearly approximating 
to the original hue. This, for example, is the only way in 
* A semi-cultivated variety was used, having dark purple spots at the 
bases of the petals. The common red poppy of the chalk (Papaver hy- 
bridum) gives a purple colour much less sensitive and beautiful. 
4 Nicholson’s Journal. 
