in Natural History, is the name of a stone described by the ancients, and used by them in dyeing; perhaps from some vitriolic or alumino-salt contained in it, which served to enliven or fix the colours used by the dyers. It was light and spungy, resembling a pumice; and the whitest and lightest were reckoned the best. Pliny gives an account of the method of preparing it for the purpose of dyeing, which was by moistening it with urine, and then heating it red hot, and suffering it to cool.—This calcination was repeated three times, and the stone was then fit for use. Dioscorides recommended it in medicine after burning; he says it was drying and astringent.