12 Sir J. F. W. Herschel on the Action of the Rays 
not affect it. And under the action of the spectrum it is dis- 
coloured (though much more slowly) by the same, or nearly 
the same rays which are effective in the case of guaiacum. 
The colour of paper tinged with the juice of the common red 
stock is not affected when dry by any heat short of what suf- 
fices to scorch the paper, but when wet (as when exposed to 
steam) it is speedily discharged. ‘There are few, if any ve- 
getable colours indeed which long resist the combined ef- 
fects of heat and moisture, even when light is excluded, still 
less when admitted*. 
Of the Colours of Flowers in general under the action of the 
Spectrum. 
163. In operating on the colours of flowers I have usually 
proceeded as follows :—the petals of the fresh flowers, or rather 
such parts of them as possessed a uniform tint, were crushed 
to a pulp ina marble mortar, either alone, or with addition of 
alcohol, and the juice expressed by squeezing the pulp ina 
clean linen or cotton cloth. It was then spread on paper with 
a flat brush, and dried in the air without artificial 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 colour. If alcohol be present this change does 
not usually take place, or is much retarded; for which reason, 
as well as on account of certain facilities afforded by its ad- 
mixture in procuring an even tint (to be presently stated), 
this addition was commonly, but not always made. 
164. Most flowers give out their colouring matter readily 
enough, either to alcohol or water. Some, however, as the 
Eschnolzias and Calceolarias, refuse to do so, and require 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 discharged altogether, 
and that the tincture, when spread on paper, does not reap- 
pear of its due intensity till after complete drying. ‘The tem- 
porary destruction of the colour of the blue heartsease by al- 
cohol has been noticed in my former paper (Art. 90.), nor is 
* On the effects of light, air, and moisture at common temperatures, as 
discolouring agents on several dyeing materials, I may refer to M.Chevreul’s 
elaborate memoir (Acad. R. des Sciences, tom. xvi.). M.Chevreul’s experi- 
ments, however, relate to the action of light simply as it comes from the sun 
without prismatic separation, and have therefore little or nothing in com- 
mon with the objects of this paper. 
