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it milky, and precipitating a part of the dissolved silvernIt is to be observed also, that if the liquor touches theskin, it has the fame effect thereon as on the matter to bestained, changing the part moistened with it to an indeli-ble black.

III. Marble.

It is difficult to introduce into marble a true blackcolour. Solution of silver links deep into the stone,sometimes an inch or more ; but the colour it communi-cates, at first reddish or purplish, deepens only to a brown..Mr. du Fay, in the Memoirs of the French Academy forthe years 1728 and 1732, gives two methods of stainingmarble of a blue colour, approaching more or less toblack according to its deepness, and not ill resemblingthose which are naturally found in some marbles : one iswith essential oil of thyme digested in volatile spirit ofsal ammoniac, the other with tincture of archel. Whenthe oil of thyme is digested with the volatile spirit, it be-comes first yellow, then red, then violet, and at last of adeep blue. In six weeks digestion it had acquired a paleblue, and in this state gave little colour to marble : afterstanding for six months, it' was deepened almost to ablack blue, and being now applied on warm marble, gavethe stain desired.

With regard to archel, a tincture of it in water is i ap -plied on cold marble, and renewed as it evaporates, tillthe colour is sufficiently deep. Though the. colour ofarchel is very perishable on cloth, yet in marble it appearsto be more durable. Mr. du Fay fays he saw pieces ofmarble stained with it, which in two years were not sen-sibly changed. The colour however, though made verydeep, is far from being a true black, being rather a darkpurplish blue.

The.