Mercuric Oxide: Red Precipitate from Mercuric Nitrate

Red oxide of mercury, the old 'red precipitate'. Thorpe (1912): boil mercury in nitric acid to mercuric nitrate, then heat the nitrate until it stops giving off nitrous fumes, but not so hot that the oxide itself breaks back into mercury and oxygen. Mercury and all its salts are poisons, and the fumes are deadly: outdoors, full protection, never alone. The picture: mercuric oxide in a tube (Wikimedia Commons).