The day Joseph Priestley discovered oxygen
Until Joseph Priestley came along to the scientific scene in 1774, the general idea was that air was just a single entity.
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 used a container turned upside down to catch gasses given off by a range of experiments like heating the air with flame and putting a plant inside the jar.
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.