ARCHILLA, ROCCELLA, ORSIELLE, is a whitish moss which grows upon rocks, in the Canary and Cape de Verde 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 between 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 it is called Archil; in the latter, Lacmus or Litmus.