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Arch Il

colour, archil, air and water

ARCH IL. A whitish lichen, growing upon rocks in the Canary and Cape Verd Islands, which 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 urine, or mix quick lime with the urine : 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 dearness, 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, slightly impregnated with the archil. The bloom thus communicated, soon decays upon exposure to the air. By the addition of a little solution of tin, this drug gives a durable dye ; its colour is at the same time changed toward a scarlet, and that 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 alcohol; it is the substance principally made use of for colouring the spirits of ther mometers. As exposure to the air destroys its colour upon cloth, the exclusion

of the air produces a like effect in those hermetically sealed tubes,—the spirits of large thermometers becoming in a few years colourless. The Abbe Nollet observes (in the French Memoirs for 1742), that the colourless spirit, upon breaking the tube, soon resumes its colour, and this for a number of times successively ; that a watery tincture of archil, included in the tube of ther mometers, lost its colour in three days ; and that in an open deep vessei it became colourless at the bottom, while the upper part retained its colour. A solution of archil in water, applied on cold marble, stains it of a beautiful violet or purplish blue colour, far more durable than the colour which it communicates to other bodies. There is a large establishment at Glasgow for an article of this kind, which is much esteemed; it is sold by the name of cudbear. Silks thus dyed with it are said to be very permanent, of various shades, from pink and crimson to a bright mazarine blue.