About two hundred years ago, it was believed that many substances contained a chemical named phlogiston, which was thought to be taken out of matter by food consumption and absorbed into the air. When an animal was confined in an airtight container, it lived as long the air could absorb the phlogiston. When the phlogiston ran out, or the air could not absorb any more, the animal would die. .
An English clergyman and scientist named Joseph Priestly became interested in the phlogiston concept, and attempted some experiments with plants and animals. He used mint plants and found out that they would not die. He started to doubt the phlogiston theory and tried the experiment with other plants, and got the same results. The plants did not die. He found that if a mouse or a burning candle were in the same airtight container, that they would not die or stop burning, as long as the plant was in there too. He believed that the plant could remove the phlogiston from the air, and provide enough air for breathing and combustion.
Although Priestly had a good theory, it was discovered to be wrong by a French chemist named Lavoisier, who said that the plants did not remove something from the air, but to put something back into it. He called this substance oxygen. Oxygen is removed from air, through food consumption and combustion. .
Another of Priestly's problems with the phlogiston theory was the fact that it only worked some of the time, and many times the experiment failed. In 1779, an Austrian scientist, Jan Ingenhouz discovered that the plants only restored oxygen "in clear daylight, or in sunshine- and only the green stalks and leaves were capable of this reaction. This helped to show that light was required and only the green parts produced oxygen. Other scientists helped to discover that the oxygen was a byproduct of the reaction, and water and carbon dioxide were the original reactants, with sunlight as a catalyst for the reaction.