August 17, 1771 (a Saturday)

Alone in a sealed jar, a mouse would die from exhaled CO2. But as scientist Joseph Priestley observed in 1771, adding a mint plant allows the mouse to thrive. In this proof of photosynthesis, the mint absorbed CO2, retained carbon for growth, and released oxygen. Two centuries later humans tried—and failed—to survive in a sealed environment in Arizona's Biosphere 2.

On this date, Joseph Priestley began an experiment in which he discovered photosynthesis, although he did not give it that name. He described his experiment in 1772 in a paper entitled “Observations on Different Kinds of Air”:

…I flatter myself that I have accidentally hit upon a method of restoring air which has been injured by the burning of candles, and that I have discovered at least one of the restoratives which nature employs for this purpose. It is vegetation. In what manner this process in nature operates, to produce so remarkable an effect, I do not pretend to have discovered; but a number of facts declare in favour of this hypothesis…

One might have imagined that, since common air is necessary to vegetable, as well as to animal life, both plants and animal had affected it in the same manner, and I own that I had that expectation, when I first put a sprig of mint into a glass-jar, standing inverted in a vessel of water; but when it had continued growing there for some months, I found that the air would neither extinguish a candle, nor was it at all inconvenient to a mouse, which I put into it.

…Accordingly, on the 17th of August 1771, I put a sprig of mint into a quantity of air, in which a wax candle had burned out, and found that, on the 27th of the same month, another candle burned perfectly well in it. This experiment I repeated, without least variation in the event, not less than eight or ten times in the remainder of the summer.

Priestley’s experiment also demonstrated the key processes of the natural carbon cycle. Although Priestley could not name the gases responsible, the fire and respiration used up oxygen and gave off carbon dioxide. The mint reversed both processes. Photosynthesis took up the carbon dioxide, converted it into plant tissue, and gave off oxygen as a by-product.

Priestley’s experiment thus has ecological implications for today, since the world is just a bigger jar. Tens of billions of tons of carbon a year pass between land and the atmosphere: given off by living things as they breathe and decay and taken up by green plants, which produce oxygen. A similar traffic in carbon, between marine plants and animals, takes place within the waters of the ocean. And nearly a hundred billion tons of carbon diffuse back and forth between ocean and atmosphere.

1917 image of Athbasca Glacier, Jasper National Park, Canada, courtesy National Archives of Canada, from the Wheeler Survey; 2005 image by Gary Braasch. Receding glaciers are visible evidence of global warming.

Compared with these vast natural exchanges, the few billion tons of carbon that humans contribute to the atmosphere each year seem paltry. Yet like a finger on a balance, our steady contributions are throwing the natural cycle out of whack. The coal, oil, and natural gas that drive the industrial world’s economy all contain carbon inhaled by plants hundreds of millions of years ago — carbon that now is returning to the atmosphere through smokestacks and exhaust pipes, joining emissions from forests burned to clear land in poorer countries. The atmosphere’s carbon dioxide level has risen by some 30 percent since Priestley’s time. It may now be higher than it has been in at least 20 million years.

Carbon dioxide is foremost in an array of gases from human activity that increase the atmosphere’s ability to trap heat, resulting in ongoing global warming.


  • Allen, J.F. and W. Martin, “Evolutionary biology: Out of thin air,” Nature 445: 610-612 (8 February 2007)
  • Appenzeller, Tim, “The case of the missing carbon”, National Geographic Magazine (Feb 2004) , vol 205, number 2, p 88.

4 responses to “August 17, 1771 (a Saturday)

  1. Pingback: Different Kinds of Air | Secret Gardener

  2. Perfect crystallization of the tender process that has allowed life in what was our perfect crystal container—Now so horribly awry that humans have created a new geological epoch in which, –to put it simply– things don’t work right anymore—-
    I’d like to ask your permission to include this, with all attributions to both you and your sources of course, in a post on my blog.
    —-A great expansion of, and historical context for, a brief word on the subject in a previous post:
    Thank you for bringing it forward.

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