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LICHEN ROCELLA, 
Dyer's Lichen, or Orchall. 
Spec. Char. Frond cylindrical, solid, but little branched^ 
Tubercles alternate, powdery. 
This species of Lichen is the Ae/%viv of Dioscorides, and theFhy- 
cos Thalassion of Pliny. It is an indigenous plant, found on the 
maritime rocks of the south of England, particularly in Portland 
Island, but it does not grow in any great abundance. It grows very 
plentifully in the Levant, Canary Islands, &c. from whence it is 
imported into this country to supply the markets. The Canary 
Islands alone are said to produce two thousand six hundred quintals 
annually, aud from this abundance of orchall which they produce, 
the ancients named them the Purple Isles.* From this Lichen i& 
prepared the Argol, or archil, so much used as a dye-stuff. 
The Lichen Rocella is a small species, seldom exceeding two 
inches in height, and is firmly fixed to the rocks by a solid base, from 
which rises a tuft of small, round, smooth, acutely pointed stems, 
more or less branched, of a whitish grey, or brownish hue, and stud- 
ded towards their upper part with scattered tubercles, replete with 
white powder, which has been supposed to be the seed ; but the 
fructification of this plant, as well as many other species of the same 
tribe, is not well understood. 
The preparation of the archil from this species of Lichen was long 
kept a secret by the Dutcb,t who manufactured it into a paste, called 
by them Lacmus, or Litmus ; this was sold in square, hard, brittle 
masses, about an inch iu length, and half an inch in breadth and 
thickness. Archil is now prepared in this country, and large manu- 
factories of it are carried on in London and Liverpool. The Lichen, 
after being dried and cleaned, is reduced to powder in a mill ; it is 
* Mem. de I' Acad, des Inscriptions, torn. iv. p. 457. 
•{* The persons by whom archil, or litmus, was formerly prepared, with the Tiew to 
keep it a secret, gave it the name of tincture of turnsoh, pretending that it was ex- 
tracted from the turnsole, Heliotropium Earopeum. 
