As a child, Joseph Priestley noticed something that would influence the rest of his life: If you keep a bug in a jar for too long, it will die.

At the time Joseph Priestley didn’t understand why that was true—nobody did. No one really studied the air; it was something everyone took for granted. After all, air is invisible. Why would you study nothingness, when there was so much to explore in the world that you could see?

As an adult, Joseph Priestley resumed his childhood experiments in his basement utility sink. He would time how long it took certain things to die if they were trapped under glass. He learned that nothing survived in that closed environment for very long. He also tried to burn candles in the limited atmosphere, and found that they would extinguish themselves in moments, seeming to burn up whatever kept them going.

One day, Joseph Priestley turned his eye on a new living thing. In mid-May of 1771, he popped a little mint plant under the glass, and waited to see how long it would take to run out of “air.” Joseph Priestley thought it would take days, or maybe even a few weeks for the plant to die, because it wouldn’t need very much air, but in late June, although the plant had shriveled a little, it was not dead, in fact it was growing upward still!

This was very puzzling for Joseph Priestley. How had the plant survived? He turned again to his experiments. A candle was placed in the jar with the plant. It would burn, much longer then the candles without a plant in the jar. Then Joseph Priestley tried it with a mouse he had caught in his kitchen. The mouse lasted a very long time compared with the mouse without the plant.

Through this experiment, Joseph Priestley discovered both oxygen and nitrogen.

He wrote several times to his friend Ben Franklin, and together they began thinking about life on earth and how plants sustain mammal life, and vice versa. No one had ever before suspected that trees and other plants took carbon dioxide from our bodies, and replenished the air with oxygen. In fact, during the time of Joseph Priestley and Ben Franklin, many people thought trees were poisonous! It was considered wise and fashionable to cut down all the trees near your house to clean the air around it. Of course, the opposite is true!

Joseph Priestley had many serendipitous moments of discovery in his life. Serendipity is an accidental discovery. In this case, Joseph Priestley was looking for the reason that living things “used up” air. But what he found instead was that plants create oxygen from the carbon dioxide that we exhale, one of the most important discoveries in history!