Oxygen
In the late 17th century, Robert Boyle found that air is needed for combustion. English chemist John Mayow added to this by showing that fire only needed a part of air. We now call this oxygen (O2). He found that a candle burning in a closed container made the water rise to replace a fourteenth of the air's volume before it went out. The same thing happened when a live mouse was put into the box. From this, he worked out that oxygen is used for both respiration and combustion.