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colour the more perfectly by virtue of its blackness, andwhich thus enables us to judge and compare the colourand fineness of metallic compositions, with much morecertainty than could be done by viewing them in the mass:the black flint so called, which performs the fame officewith the touchstone, and being harder than the commontouchstones, answers better for the hard metals: the com-mon black marbles, used for many kinds of ornamentalworks: the more rare black marble, called lapis obsidianusor opsianus, which, in virtue of the very high polish it re-ceives, was made into mirrours by the Greeks and Romans:the black gallinazo stone, described by D’Ulloa, whichanswered the same purposes among the Indians of Perubefore the conquest of that kingdom by the Spaniards: theblack jet, and other substances of the same class, which areformed into many elegant toys: the plain and variegatedblack agates, pebbles, crystals, &c. which are cut by thejeweller. The stone called tourmalin, remarkable for thesingular phenomena it exhibits in some electrical experi-ments, is in its rough state of a full black colour, thoughwhen polished it looks rather of a brown black: I havebeen informed by a skilful jeweller that he had seen ablack diamond, cut and set in a ring; though perhaps theexamination made of it was not so rigorous as could bewished for determining its being truly of the diamond kind.However this may be, a black tinge in bodies of this classis looked upon as an imperfection or foulness, although,when the diamond is cut, a black foil placed under it im-proves its beauty: for the rose diamonds, the collet, orsocket in which the stone is set, is specked with ivoryblack in little dots; and for brilliants it is all over blacked.

Among vegetables and animals, blackness, though fre-quent, is less diffused, or of more limited extent. Theblack colour of the stalks and feeds of certain plants, that

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