LICHENS. 181 
and brilliant hues to French ribbons, which are so 
much admired. The common yellow wall-lichen 
(Parmelia parietina), so abundant everywhere, 
yields a beautiful golden yellow crystallizable 
colouring matter called chrysophanic acid, which is 
identical with the yellow colouring matter of rhu- 
barb ; and like orchil-litmus, it may be used as a 
test for alkalies, as they invariably change its yel- 
low colour into a vivid red tint. A beautiful and 
valuable crimson pigment, occasionally employed 
by artists, is the product of a dark-brown shrubby 
lichen (Cornicularia aculeata), very common on the 
hills ; while the common stone lichen (Parmelia 
saxatilis), which forms grey rosettes on almost 
every wall, rock, and tree, is still collected abun- 
dantly by the Scottish peasantry, under the name 
of stane-raw, to dye woollen stuff of a-dirty purple 
or reddish-brown colour. On the low rocks, on 
the summits of all the loftiest Highland hills, there 
is a curious leafy lichen (Parmelia fahlunensis) 
found abundantly, scorched apparently by the sun 
into a black cinder. Of all lichens, this species, 
judging from its outward colour and appearance, 
would seem to be the least capable of yielding 
colouring matter; and yet, when treated in the 
ordinary way, it yields a brilliant pink, cherry, or 
claret colour, which in France has been applied to 
so many useful purposes, that the lichen in con- 
