On August 1, 1774, an experiment conducted by the British clergyman Joseph Priestley focused sunlight on mercuric oxide (Cinnabar) contained in a glass tube, which liberated a gas he named “dephlogisticated air”. He noted that candles burned brighter in the gas and that a mouse was more active and lived longer while breathing it. After breathing the gas himself, Priestley wrote: “The feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards.” Priestley published his findings in 1775 in paper titled “An Account of Further Discoveries in Air”, which was included in the second volume of his book titled Experiments and Observations on Different Kinds of Air. Because he published his findings first, Priestley is usually given priority in the discovery of Oxygen.
Cinnabar is mercury sulfide, HgS. Priestly used mercuric oxide, HgO. Mercuric oxide can occur in nature as the rare mineral montroydite, but it’s usually made synthetically.