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A poisonous compound, HgS, having two forms:
a. Black mercuric sulfide, a black powder obtained from mercury salts or by the reaction of mercury with sulfur, used as a pigment.
b. Red mercuric sulfide, a bright scarlet powder occurring naturally or prepared by heating mercury with sulfur, that is sensitive to heat and light and is used as a pigment. Also called cinnabar, vermilion.