ARCHILLA, ROCHELLA, ORSIELLE, is a whitish moss which grows upon rocks, in the Canary and Cape Verd islands, and yields a rich purple tincture, fugitive indeed, but extremely beautiful. This weed is imported to us as it is gathered. Those who prepare it for the use of the dyer, grind it betwixt stones, so as to thoroughly bruise, but not to reduce it into powder; and then moisten it occasionally with a strong spirit of urine, or urine itself mixed with quicklime: in a few days it acquires a purplish red, and at length a blue colour. In the first state it is called archil; in the latter, Lacmus or Litmus.

The dyers rarely employ this drug by itself, on account of its dearth and the perishableness of its beauty. The chief use they make of it is, for giving a bloom to other colours, as pinks, &c. This is effected by passing the dyed cloth or silk through hot water lightly impregnated with the archil. The bloom thus communicated soon decays upon exposure to the air. Mr Hellot informs us, that by the addition of a little solution of tin, this drug gives a durable dye; that its colour is at the same time changed towards a scarlet; and that it is the more permanent in proportion as it recedes the more from its natural colour.

Prepared archil very readily gives out its colour to water, to volatile spirits, and to spirit of wine; it is the substance principally made use of for colouring the spirits of thermometers. As exposure to the air destroys its colour upon cloth, the exclusion of the air produces a like effect in these hermetically sealed tubes, the spirits of large thermometers becoming in the compass of a few years colourless. M. l'Abbe Nollet observes (in the French Memoirs for the year 1742), that the colourless spirit, upon breaking the tube, soon refurnes its colour, and this for a number of times successively; that a watery tincture of archil, included in the tubes or thermometers, lost its colour in three days; and that, in an open deep vessel, it became colourless at the bottom, while the upper part retained its colour. See Colour-Making, no 38.

A solution of archil in water, applied on cold marble, strains it of a beautiful violet, or purplish blue colour, far more durable than the colour which it communicates to other bodies. Mr du Fay says he has seen pieces of marble stained with it, which in two years had suffered no sensible change. It sinks deep into the marble, sometimes above an inch; and at the same time spreads upon the surface, unless the edges be bounded by wax or other like substances. It seems to make the marble somewhat more brittle.

Linnaeus informs us, in the Swedish Transactions for the year 1742, that the true archil moss is to be found on the western coasts of England. It has been for a considerable time past prepared by Messrs Gordons at Leith, from a species found in the Highlands of Scotland.