Nat. Ord. Lichenes. Officinal. Rocella tinctoria, litmus, Dub. Litmus.

Syn. Orseille (F.), Oricello (I.), Orciglia (S.).

This is an indigenous lichen, found in Portland Island: but, as an article of commerce, it is obtained from the Levant;

1 Manuel, p. 150. 2The bark of the root of the tree is a powerful purgative.

and also the Canary Islands1, which produce annually 2600 quintals. It is a small species, seldom exceeding two inches in height, and is firmly fixed to the rocks by a solid base. From this base rises a tuft of worm-like stems, round, acutely pointed, often curved, more or less branched, smooth, of a white grey or brownish hue, and studded about their upper part with scattered tubercles, replete with white powder, which have been thought the seeds; but the fructification of this species is not well understood.

From this lichen is prepared the Dutch argol or archil of commerce. It was known to the ancients, being the of Dioscorides, and the Phycos thalassion of Pliny. Its use as a dye was, however, lost; till it was again accidentally discovered by a merchant of Florence, in 13002, observing that urine gave the lichen a fine violet colour. The preparation was long a secret, and was confined to Florence and Holland: but it is now known in England, and large manufactories of it are carried on in London and Liverpool. The lichen, after being dried and cleaned, is reduced to powder in a mill resembling an oil-mill.8 It is then mixed in a vat with one half of its weight of pearl-ash, and moistened with human urine; fermentation soon succeeds, and is kept up by stirring, and by successive additions of urine, until the colol of the material changes first to red, and then to blue. In this state it is mixed with a third of its weight of good potassa, and spread out to dry.4 Chalk is sometimes added to it, but with no other view than to increase the weight.5 It is generally sold in the form of cakes, but sometimes in that of a moist pulp.

Qualities.-Prepared argol has a slight violet odour, arising from orris-root, with which it is always mixed, and a mawkish taste, leaving some degree of pungency in the mouth. When moist, the form of the lichen is evident in the pulp. It communicates to water and to alcohol a beautiful violet colour, which, however, is very evanescent: all acids, and salts with an excess of acid, change it to red, which is again destroyed, and the blue restored by the addition of alkalies; and even exposure to the air of a room, in which many people are assembled, reddens the watery infusion. The tincture is least liable to change when kept, if it be reddened by an acid, and kept in close vessels.

1 The ancients named the Canaries the Purple Isles, from the abundance of orchilla which they yielded.-Mem. de l'Acad. des Inscriptions, torn. iv. p. 457. It forms the only article of vegetation on the northernmost of the Ilhas desertas, near Madeira.

2 Thomson's Chymistry, 4th ed. vol. v, p. 284. Bancroft, on Philosophy of Colours, 2d ed. p. 292.

3 Sometimes it is not ground, but prepared in the entire state.

4 Nicholson's Journal, 4to. ii. 311.

5 Archill is chiefly used by the dyers, and in times of scarcity it has been sold at 1000l. sterling the ton. It is often mixed with the Lichen fuciformis.

Use.-This species of lichen is said to have been "administered medicinally, with an intention of allaying the tickling cough attendant on phthisis, and in hysterial coughs1;" but we must suppose the recent lichen is meant, or before it has undergone any preparation as a colouring matter. We know of no other use of the prepared lichen than as a dye-stuff, or as a chymical test of the presence of acids; and it is certainly-the most delicate.