Measuring oxygen conctent of air. Image 1 of 2. Lit candle floating on water, underneath a bell jar. The candle requires oxygen to keep burning and will extinguish once the oxygen supply is used up. In this experiment, a bell jar has been placed over the candle, limiting the amount of oxygen available. Initially, the water in the glass bowl is displaced by the air in the bell jar, but as oxygen is used up, the water takes its place. Once the oxygen runs out, the candle will extinguish and the oxygen content of the air can be calculated as a percentage. See A510/271 for the outcome of the experiment.