ABSTRACT
Puzzling observations In about 1754, Charles Bonnet (1720-93) observed that a brightly lit leaf under water gave off bubbles of gas. Almost twenty years later, Joseph Priestley (1733-1804) carried out a famous experiment which would now be considered unethical. He put a mouse in a closed jar and timed how long it took before it collapsed. He found that, if he added a green plant to the jar, the mouse survived for much longer. In a similar and somewhat more ethical experiment, he put a lighted candle in a jar with a plant. It burnt for some time before it went out. Twenty-seven days later, Priestley focused light onto the wick of the candle, using a magnifying glass, and found that it re-ignited. From this he proposed that plants return to the air whatever it was that the breathing animals and the burning candles remove. We now know that this gas is oxygen.
