Measuring oxygen content of air. Image 2 of 2. Extinguished candle floating on water, underneath a bell jar. In this experiment, a bell jar was placed over a lit candle, limiting the amount of oxygen available for combustion. As the candle used up the oxygen within the bell jar, the water that had been previously displaced was able to enter the jar. The volume of water in the bell jar at the time when the candle extingused was used to calculate the percentage content of oxygen within the bell jar. The water has filled a fifth of the bell jar and so the oxygen content of air within the bell jar was 20%. See A510/270 for the start of the experiment.