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Theory:

Experiment 2 - Live mouse experiment:
 
In the burning process of the candle, oxygen gas was mostly converted into carbon-di-oxide. Without allowing the outside air to enter the glass jar, Priestley lifted the setup and placed a mouse inside it.
 
mouse.png
Placing a mouse inside the glass jar
 
Due to the insufficient amount of oxygen gas, the mouse died inside the glass jar after some time. As the mouse could not survive this situation, Priestley proved that oxygen was necessary for all living things to survive.
 
Experiment 3 - Mint plant experiment:
For the next experiment, a mint plant was introduced inside the bell jar by gently lifting it. The bell jar was maintained very much inside the water to ensure the prevention of outside air into it.
 
plant.png
Placing a mint plant inside the glass jar
  
Priestley thought that the mint plant would also die like the mouse. But instead, the plant survived. He also lit a candle after placing the mint plant inside, and the candle continued to burn.
 
Screenshot 2021-07-26 091901.png
A candle and a mint plant inside the glass jar
  
Experiment 4 - Synergy of plants and animals:
In the next experiment, Priestley again took a bell jar, converted the oxygen into carbon-di-oxide by burning the candle. Then, he placed a mint plant and a mouse into the jar. Priestley found both the plant and the mouse surviving since they had a synergy.
 
20.png
Survival of mint plant and mouse
 
Inference:
From all these experiments, it was observed that animals (mouse) consumed oxygen (\(O_2\)) and released carbon-di-oxide (\(CO_2\)) and plants (mint plant) consumed carbon-di-oxide and released oxygen.
 
Screenshot 2021-07-26 092043.png
Exchange of (\(O_2\)) and (\(CO_2\))
 
Jan Ingenhousz, during \(1730\ –\) \(1799\), proved that sunlight is essential for plant for the process of photosynthesis. The plants also purified the foul air released by the breathing animals or the burning candles. From all these experiments, it was evident that air was a composite mixture of many gases like oxygen and carbon-di-oxide.
Reference:
https://upload.wikimedia.org/wikipedia/commons/1/1f/Experiment_von_Priestley.jpg