To find out what happens when a candle burns in air.
Invert a dry gas jar over a burning candle and watch what happens. Carefully remove the candle by lifting the gas jar slightly, light it again and invert the same gas jar over it. Watch what happens and try to explain why it does. Now stand the burning candle in water in a glass trough. Invert another dry gas jar over it as shown in. Note the level of water in the jar at the start and at the end of the experiment.
In the first part of the experiment, you will observe that the light goes out after some time. In the second part, the light goes out almost immediately, that is, the remaining air in the jar does not support burning. A mist is also formed on the sides of the jar. In the second part of the experiment you will observe that the level of water in the jar falls at first, the candle goes out after some time and that the level of water rises in the jar.
From this simple experiment, one thing is clear only a portion of the air is used up in burning. Therefore air is made up principally of two gases: one that supports combustion, i.e. makes things burn, and one that does not. We noticed in the experiment that a mist is formed in a jar inverted over a burning candle. Since we do not allow any moisture to enter the jar from outside, then it must have been formed from the burning candle.