The day Joseph Priestley discovered oxygen
However, his studies on air showed that it is actually a composition of lots of different gasses. He managed to isolate eight gasses, one of which was oxygen. He called it dephlogisticated air, leaving Antoine Lavoisier to name it oxygen.
He managed to isolate oxygen by focussing light onto a jar of mercuric oxide. The resulting gas kept a mouse alive for longer than the other gasses and made a flame burn brighter for longer.