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is discharged by acids, and restored again by the sulphure-ous vapour or solution.

Calces of lead, melted with sulphur, form a black orblackish mass, which proves an useful matter for takingcasts from medals, being considerably more tough thansulphur alone. For this purpose, equal parts of miniumand flowers of sulphur are put in an iron ladle over thesire, till they soften into the consistence of pap, and arethen kindled with a piece of lighted paper, and stirred forsome time : the vessel being afterwards covered close andcontinued on the fire, the mixture becomes fluid in a fewminutes, and is then poured upon the medal previouslyoiled and wiped pretty clean. This process, communi-cated to me by a friend, I have often tried with satisfaction.The casts are very neat; the colour, sometimes a prettydeep black, and sometimes a black grey, according to dif-ferent circumstances in the fusion ; they are very durable,and when soiled may be washed clean again with spiritof wine.

There are other metals also which produce a black co-lour with sulphureous bodies. When a solution of silverin aquafortis is added to a solution of sulphur made inalcaline ley, the silver and sulphur unite and precipitatetogether in the form of a black powder. Quicksilver andsulphur, by being barely rubbed together in a mortar, be-come black, and hence this mixture, commonly made formedicinal uses, is called the mineral ethiops. But as thesekinds of compositions afford nothing of importance forthe art of colouring black, it would be needless in thisplace to consider them more particularly.

IV. Black from the combination of other colours.

In the three foregoing articles we have seen blacknessgenerated from the action of certain bodies on one an-other,