Early Experiments: Priestley and Ingenhousz
Before photosynthesis could be understood chemically, it had to be observed. In 1770, Joseph Priestley (1733–1804) used a candle, a mouse, a mint plant and a sealed bell jar to show that green plants restore air that burning candles and breathing animals spoil. A few years later, Jan Ingenhousz (1730–1799) added the missing ingredient: sunlight.
This quiz tests your grasp of these two classic experiments — the observations, the reasoning, and the conclusions each investigator could legitimately draw.
- Priestley’s bell-jar observations and his hypothesis
- The chronology: plant experiments (1770) before the discovery of oxygen (1774)
- Ingenhousz’s light/dark comparison and his aquatic-plant bubble experiment
- Why only the green parts of a plant release oxygen
Read each question carefully — several options are historically plausible but not supported by the experiments themselves.
Q1. In which year did Joseph Priestley perform the series of experiments that revealed the essential role of air in the growth of green plants?
Priestley performed these experiments in 1770. Note that his discovery of oxygen came later, in 1774.
Q2. What did Priestley observe when a candle was left burning in a closed space such as a bell jar?
Priestley observed that a candle burning in a closed space soon gets extinguished, which led him to conclude that burning damages the air.
Q3. According to the lesson, what happened to a mouse placed in a closed space?
Priestley noted that a mouse would soon suffocate in a closed space, just as a candle is extinguished — both damage the air.
Q4. What conclusion did Priestley draw from the candle and mouse observations?
Priestley concluded that a burning candle or an animal that breathes the air, both somehow damage the air.
Q5. What happened when Priestley placed a mint plant in the same bell jar as the mouse and candle?
With the mint plant present, the mouse stayed alive and the candle continued to burn — the key observation of the experiment.
Q6. Which statement best expresses Priestley's hypothesis?
This is Priestley's hypothesis as stated in the lesson: plants restore to the air whatever breathing animals and burning candles remove.
Q7. Why would Priestley need to rekindle the candle after a few days?
Rekindling the candle was the test: if the air had been restored, the candle would burn again. It was the way of checking the state of the enclosed air.
Q8. In which year did Priestley discover oxygen, and how does this relate to his plant experiments?
Priestley discovered oxygen in 1774, which is after his 1770 plant experiments. His initial interpretation of damaged and restored air was therefore framed without knowledge of oxygen.
Q9. What was the main limitation of Priestley's account that Jan Ingenhousz addressed?
Ingenhousz supplied the missing ingredient: he showed that sunlight is essential for the plant process that purifies the air fouled by burning candles or breathing animals.
Q10. How did Ingenhousz modify Priestley's basic setup?
Using a similar setup to Priestley's, Ingenhousz placed it once in the dark and once in the sunlight, thereby isolating light as the variable.
Q11. In Ingenhousz's elegant experiment with an aquatic plant, what was observed in bright sunlight?
In bright sunlight, small bubbles were formed around the green parts of the aquatic plant.
Q12. What did Ingenhousz observe in the dark in his aquatic-plant experiment?
In the dark, no bubbles formed. This contrast with the bright-sunlight result showed that the process depends on light.
Q13. What did Ingenhousz later identify the bubbles from his aquatic-plant experiment to be?
Ingenhousz later identified these bubbles to be of oxygen.
Q14. What did Ingenhousz conclude about which parts of a plant can release oxygen?
Ingenhousz showed that it is only the green part of the plants that could release oxygen.
Q15. Which pair of lifespan dates is correctly matched to the two investigators?
Joseph Priestley lived from 1733 to 1804, and Jan Ingenhousz from 1730 to 1799.