LICHEN ROCELLA. 
183 
then mixed in a vat with one-half its weight of pearl-ash, and mois- 
tened with human urine ; fermentation soon succeeds, and is kept 
up by stirring, and by successive additions of urine, until the colour 
of the mass changes first to red and then to blue. In this state it is 
mixed with a third of its weight of good potass, and spread out to 
dry.* Chalk is sometimes added to it, but with no other view than 
to increase the weight. It is usually sold in the form of cakes, but 
sometimes in that of a moist pulp.f 
Sensible Qualities, &c. of Archil. Prepared archil has 
a violet odour, derived from the orris root, with which it is always 
more or less mixed ; its taste is mawkish with some degree of pun- 
gency. It communicates both to water and alcohol a beautiful 
violet colour, which changes to red by the addition of any acid ; the 
red colour is again destroyed, and replaced by the violet, by adding 
a portion of any of the alkalies. Litmus is used in chemistry as a 
test to detect the presence of acid or alkaline substances. Paper 
stained with litmus changes to red by acids,^ and has the blue colour 
restored by an alkali. By the addition of a little solution of tin, 
archil gives a durable dye of a fine scarlet colour. It is least liable 
to change when reddened by an acid, and kept in close vessels. 
Medical Properties, &c. Formerly this species of Lichen 
was used medicinally in disorders of the lungs, attended with tick- 
ling coughs, &c. It is not now considered to possess any remedial 
efficacy, and the only use to which it is appHed, is in the preparation 
of archil, the uses of which we have already mentioned. 
OS. Litmus. 
* Nicholson's Journal, toI. ii, p. 311. 
t We may mention that another species of Rocella, the Rocella Fusiformis, is said 
to vie in richness of colouring matter with the common orchal!, while the plant attains 
a ranch larger size. This species occurs sparingly on the rooks of the soulii oi Europe, 
but it is said to abound in the East Indies, especially on the shores of Sumatra, 
