t 35* 1

■which the nitrous acid may dissolve preferably to the silverwhich it already holds dissolved. This is plainly the cafein bones, horns, hair, marble, and several other bodieswhich are stained by the silver solution; though there arealso some stones which are stained by it, as agate, in whicha substance soluble by the acid has not yet discovered itself.

It may be observed, that the production of a dark co-lour from the action of the sun is not peculiar to solutionof silver, or to a combination of this solution with solubleearths. When bismuth is dissolved in the nitrous acid,and afterwards precipitated by dilution with water, theprecipitated powder, exceedingly white, soon becomesdark coloured in the fun, so as to require great care, indrying and keeping it, to preserve the whiteness, for whichthat preparation is valued. Mercurius dulcis, a combina-tion of quicksilver with the marine acid, suffers a likechange. The effect, however, is less considerable herethan in the silver liquor; for though both preparations be-come dark, I have not observed blackness produced ineither.

III. Black from Lead and Sulphur.

Lead, a metal which of itself makes a blackish markon paper, yields colours more approaching to blackness insome of its dissolutions and combinations with other bo-dies. Solutions of lead made in acids, dropt upon paperor other white subjects, communicate no stain; but onbeing exposed to sulphureous vapours, or washed over withalcaline solutions of sulphur, the parts moistened with thesolution of lead become immediately yellow, and soon afterof a deep brown or black, according as the liquors weremore or less saturated with the matters dissolved in them.The production of this colour has not been applied toany important use, being regarded chiefly as a matter of

curiosity,