110 ORD. LIV. Alge. ROCCELLA TINCTORIA, 
The preparation of the Archil from this species of lichen was long kept a 
secret by the Dutch, who manufactured it into a paste, called by them 
lacmus or litmus ; and the persons by whom it was formerly prepared, with 
a view to engross the manufacture, gave it the name of tincture of turnsole, 
pretending that it was extracted from the turnsole, (Heliotropium europeum). 
This substance (Litmus) was sold in square, hard brittle masses, about an 
inch in length, and half an inch in breadth and thickness. Archil is 
now prepared in this country, and large maufactories of it are carried on in 
London and Liverpool. The lichen, after being dried and cleaned, is re- 
duced to powder in a mill; itis then mixed in a vat, with one half its weight 
of pearl-ash, and moistened with human urine: fermentation soon ensues, 
and is kept up by stirring, and by successive additions of urine, until the co- 
lour of the mass changes, first to red and then to blue. _In this state it is 
mixéd 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 its 
weight, but we are told that it is sometimes mixed with the lichen fucifor- 
mist It is usually sold in the form of cakes, but sometimes in that of moist 
pulp. 
Sensible Qualities, $c. of Archil. Prepared Archil has a violet odour, 
which is said to be derived from a certain quantity of Orris-root, with which 
it is always more or less mixed; its taste is mawkish, with some degree of 
pungency. It communicates both to water and to alcohol a beautiful violet 
colour, which changes to red by the addition of any acid; this red colour is 
again destroyed, and replaced by the violet, by adding a portion of any of 
the alkalies. Hence, it is used in chemistry as a delicate test, to detect the 
presence of acid or alkaline substances. Paper which has been dyed with 
litmus, changes to red by acids, and has its blue colour immediately restored 
by an alkali : the oxygen of the air, also, in a short time, destroys stuffs dyed 
blue with archil. 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 red- 
dened by an acid, and kept in close vessels. 
* Nicholson’s Journal, vol. ii. p.311. 
+ This species is said to vie in richness of colouring-matter with the common Orchal, 
while the plant attains a much larger size; this lichen occurs very sparingly on the rocks 
of the south of Europe, but it is said to abound in the East Indies, especially on the 
shores of Sumatra. 
