OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
187 
heat, or at most with the gentle warmth which rises in the ascending - current of air 
from an Arnott stove. If alcohol be not added, the application on paper must be 
performed immediately, since exposure to the air of the juices of most flowers (in 
some cases even for but a few minutes) irrecoverably changes or destroys their co- 
lour. If alcohol be present this change does not usually take place, or is much re- 
tarded ; for which reason, as well as on account of certain facilities afforded by its 
admixture in procuring an even tint (to be presently stated), this addition was com- 
monly, but not always made. 
164. Most flowers give out their colouring matter readily enough, either to alcohol 
or water. Some, however, as the Escholzias and Calceolarias, refuse to do so, and re- 
quire the addition of alkalies, others of acids, &c. When alcohol is added, it should, 
however, be observed that the tint is often, apparently, much enfeebled, or even dis- 
charged altogether, and that the tincture, when spread on paper, does not reappear 
of its due intensity till after complete drying. The temporary destruction of the 
colour of the blue heartsease by alcohol has been noticed in my former paper (Art. 
90.), nor is 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 always 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 darkness. 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 men- 
tioned, especially assisted by heat) destroys them for the most part rapidly, though 
some (as the colour of the Senecio splendens ) resist obstinately. Their destructibility 
by this agency, however, seems to bear no distinct relation to their photographic pro- 
perties. 
166. This is also the place to observe that the colour of a flower is by no means 
always, or usually, that which its expressed 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 intense variety of colour, commonly 
called by florists the Black Rose, gives a dark slate blue, as do also the clove car- 
nation 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 
Rheum ?), whose expressed juice imparts to paper a rich and most beautiful blue co- 
lour, whose elegant properties as a photographic material will be further alluded to 
hereafter *. 
* A semicultivated variety was used, having dark purple spots at the bases of the petals. The common red 
poppy of the chalk ( Papaver hybridum ) gives a purple colour much less sensitive and beautiful. 
2 B 2 
