n :  either of two forms of a slightly water-soluble crystalline poisonous compound HgO:  a  :  a yellow finely divided powder obtained usu. by precipitation from solutions of mercuric chloride and sodium hydroxide and used in antiseptic ointments  b  :  a bright red coarse powder obtained by precipitation from hot solutions or by heating a nitrate of mercury and used similarly to the yellow form .
 
 
 
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