As he was doing these experiments, Priestley made an enormously important observation. A flame went out when placed in a jar in which a mouse would die due to lack of air. In other words, he had a flame and mouse in a sealed container. As time went on, the mouse died and the flame went out. We know now that that is because the mouse couldn’t breathe because there was no oxygen. The flame went out for the same reason, since oxygen is needed for combustion. He did another experiment when he put a live plant in the container with the candle and the mouse. When the plant was exposed to sunlight, it would “refresh” the air, allowing the candle to burn down to a stub and the mouse to live on happily. Perhaps, Priestley wrote, “the injury which is continually done by such a large number of animals is, in part at least, repaired by the vegetable creation.” He didn’t know at the time that he just witnessed photosynthesis . His research would pave the way for future scientists to look into this further.
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