Joseph Priestley demonstrated that green plants purify the foul air produced by breathing animals and burning candles.

Background:
During the 18th century, the quality of air in closed spaces was a matter of concern. People noticed that the air inside closed rooms often became stale and unpleasant, especially when many people were present or when candles were burned. It was also known that plants could survive in sealed containers for extended periods of time, suggesting that they were somehow able to refresh the air.

Experiment by Joseph Priestley:
Joseph Priestley, an English scientist, conducted a series of experiments to investigate the role of plants in purifying the air. In 1771, he placed a sprig of mint in a transparent container and sealed it. He then observed that a candle burned in this container for a longer time compared to a similar container without the mint sprig. This indicated that the mint was able to restore the air's ability to support combustion.

Key Findings:
Through his experiments, Priestley made several significant findings:

1. Plants produce oxygen: Priestley discovered that plants have the ability to release a substance that supports combustion, which we now know as oxygen. This was a groundbreaking discovery and led to the understanding of the process of photosynthesis.

2. Plants purify the air: Priestley demonstrated that plants are capable of removing the foul air produced by breathing animals and burning candles. They do this by absorbing carbon dioxide, which is produced during respiration, and releasing oxygen.

3. Importance of plants in closed spaces: Priestley's experiments highlighted the importance of having plants in closed spaces to maintain a fresh and healthy atmosphere. This was particularly relevant in situations where a large number of people were present or when candles were burned, as these activities consumed oxygen and released carbon dioxide.

Impact and Significance:
Priestley's experiments paved the way for further studies on the role of plants in purifying the air and understanding the process of photosynthesis. His findings also had practical applications, such as the use of plants in enclosed spaces, like hospitals or submarines, to maintain air quality. Today, we continue to recognize the importance of plants in reducing air pollution and improving indoor air quality.