Adjective
Of, pertaining to, or derived from, mercury; containing mercury; -- said of those compounds of mercury into which this element enters in its lowest proportion.
Source: Webster's dictionaryHe had produced oxygen gas by heating mercuric oxide and various nitrates by about 1772. Source: Internet
Oxygen was discovered by Joseph Priestley in 1774 when he focused sunlight on a sample of mercuric oxide and collected the resulting gas. Source: Internet