180 FIRST FORMS OF VEGETATION. 
ing it, estimating its market price at 3s. 4d. per 
stone of 22lbs. It appears also to have been an 
article of commerce in Derbyshire; the price there 
given to the collector, who could gather from 20 
to 30 pounds per day, being Id. per pound. This 
source of remunerative employment in Britain has 
now ceased, as the lichen is chiefly imported from 
Norway and Sicily, where it occurs in greater pro- 
fusion than with us, and is said to contain a larger 
proportion of colouring matter. The dye produced 
by the cudbear is quite equal to orchil, and is cap- 
able of being so modified as to give any tinge of 
purple or crimson. It is never employed by itself 
to give fast colours to cloth, but merely for the 
purpose of improving the hues already imparted. 
It is sold to the dyers in the form of a purple 
powder. Schunk, in his analysis of this plant, dis- 
covered a colourless crystalline acid, called ery- 
thric acid, which is soluble in alkaline solutions, 
and converted by them into orcine and carbonic 
acid, and which, under exposure to the air, ac- 
quires first a red and at length a fine deep violet 
tint. 
A species closely connected with the cudbear, 
and often growing together with it on the same 
rock, is very extensively employed in the south of 
France. This is the famous Perelle d’Auvergne 
(Lecanora parella), which imparts those beautiful 
