Theory:

Till \(18th\) century, people thought that air is a fundamental particle of matter. But Joseph Priestley in \(1774\), experimented and showed that air is a composition or mixture of gases and not an elementary substance.
Priestley's experiment:
Experiment 1 - Glowing candle experiment:
Priestley took a tub of water and placed a lighted candle on it. He then covered the set-up with a glass jar. He filled the tub with water to seal the pathway of air around it.
 
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Lighted candle covered with a glass jar
 
After a short span of time, the lighted candle got extinguished due to the absence of air.
 
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Extinguished candle inside a glass jar
 
Again Priestley tried to light the candle by focussing the sun rays on it with a magnifying glass. He repeated this lighting process many times without opening the sealed glass jar but could not succeed in it.
 
Discovery of oxygen gas:
 
The candle could not be relit because it was clear that some constituent in the air was being used for burning. This constituent was completely used up by the burning flame and was converted into another substance when the flame went out after the burning process.
 
In this experiment, Joseph Priestley identified a colourless and highly reactive gas that was necessary for burning. This gas was later named 'Oxygen' by Antoine Lavoisier, a French chemist.