May 25, 1769

Jan Ingen-Housz

On this date, the Dutch physician and scientist Jan Ingen-Housz was elected to the Royal Society of London. He is best known today for showing that light is essential to photosynthesis and thus having discovered photosynthesis. He also discovered that plants, like animals, have cellular respiration.

In the summer of 1771, Joseph Priestley had carried out experiments with air and jars, noting that a closed jar would eventually kill a mouse and extinguish a candle, but vegetation (he used mint) would allow the mouse to live and the candle to burn. Although he did not have the official names of the “types” of air he was observing, Priestly had discovered that mice and candles need something (oxygen), and plants are capable of using other things in the air (carbon dioxide) to produce that something. In short, plants restore to the air whatever breathing animals and burning candles remove.  However, Priestly and others were unable to reproducibly demonstrate oxygen production by plants because they were unaware of the requirement for light in photosynthesis.

Probably motivated by Priestley’s publications on the subject, Ingen-Housz obtained a short leave of absence in 1779 from his post in Vienna, Austria in order to do research in England on plants during the summer months. He performed more than 500 experiments trying to determine why plants restore bad air and described the results in his exceptional book entitled Experiments Upon Vegetables, Discovering Their Great Power of Purifying the Common Air in the Sunshine and of Injuring it in the Shade and at Night, published in October 1779.

Underwater plants producing bubbles of oxygen.

In some of his experiments, Ingen-Housz placed plants underwater in a transparent container.  He found they gave off bubbles of gas only when placed in sunlight and that the bubbles gradually ceased when the plants were placed in darkness. He determined that it is not because of the warmth of the sun, and it is not the sun acting on its own, but the light of the sun reacting with the green parts (stalks and leaves) of the plants.

Once he realized a gas was being produced in the presence of light, Ingen-Housz collected it and conducted a series of tests to determine its identity. He eventually discovered that a smoldering candle would relight when it was exposed to the unknown gas. This showed that it was oxygen (known at that time as ‘dephlogisticated’ or ‘vital’ air).

In another experiment, Ingen-Housz put a plant and a candle into a transparent closed space. He allowed the system to stand in sunlight for two or three days. This assured that the air inside was pure enough to support a candle flame. But he did not light the candle. Then, he covered the closed space with a black cloth and let it remain covered for several days. When he tried to light the candle it would not light. Ingen-Housz concluded that somehow the plant must have acted in darkness like an animal. It must have breathed, fouling the air. Ingen-Housz quickly printed his book in London, allowing him to take along copies when he returned to Vienna.

The biochemistry of photosynthesis.

So why is Priestley until today a well-known name in the history of science, while Ingen-Housz is virtually unknown , except for a few historians of chemistry and botany? Ingen-Housz was a humble person, not interested in fame, pomp, or circumstance. Low-key and introverted, enjoying friendships, shying away from stardom, he stood in contrast to some of his fellow researchers of that time. For example, Priestley admitted in private that Ingen-Housz indeed had been the first to describe the beneficial power of plants in a letter to Giovanni Fabroni from 1779:

I have just read and am much pleased with Dr. Ingenhousz’ work. The things of most value that he hit upon and I missed are that leaves without the rest of the plants will produce pure air and that the difference between day and night is so considerable.

Priestley promised Ingen-Housz that he would rectify the situation in a later publication. But the attribution never appeared in print; Ingen-Housz was not even mentioned by Priestley. In the meantime, Priestley repeatedly claimed in public to have observed and published before Ingen-Housz and kept repeating this until 1800. In fact, never did Priestley give an accurate reference to Ingen-Housz’ work, never did Ingen-Housz’ name appear in the index of Priestey’s works. On the other hand, Ingen-Housz systematically referred to Priestley, with much respect.  Ingen-Housz refrained from disputing the claims of his rival colleagues, but they continued as they did, obfuscating Ingen-Housz’ rightful place in science as the discoverer of photosynthesis in the eyes of the historians and the public.



4 responses to “May 25, 1769

  1. Dear Professor Olsen:

    Dr. Harry Lang (NTID, Rochester, NY) and I are completing a paper on deaf entomologists and wish to seek your permission to use the image that we found in your WordPress blog. The observation that submerged plants produce gases was supposedly first publshied by Charles Bonnet, a deaf and eventually nearly blind man. We are submitting the paper to the scientific journal Terrestrial Arthropod Reviews.


    Jorge Santiago-Blay

    • Dear Jorge,

      You have my permission, although I have to admit that I myself “borrowed” the image from the Internet, so to speak. Since receiving your inquiry, I have done a Google search of the Internet to see where I obtained it, but I cannot seem to locate the image again. So, in the absence of the original source to go to, I guess I am the source of last resort, and I use that authority to give you permission to use the image.


      Bruce D. Olsen

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