Jan Ingenhousz discovered photosynthesis. He showed that green plants in sunlight convert carbon dioxide into oxygen, which they release from the undersides of their leaves.
Moreover, he discovered plant respiration, discovering that in heavy shade or darkness, plants consume oxygen, converting it to carbon dioxide.
Beginnings
Jan Ingenhousz was one of three children born to Maria Beckers, the wife of Arnold Ingenhousz in the town of Breda in the Dutch Republic.
Maria died before Jan reached his first birthday and Jan’s sister died in infancy.
Jan and his older brother were raised by their father, Arnold Ingenhousz, who was a prosperous leather merchant and pharmacist.
In 1743, the British Army helped defend the Dutch Republic against a French invasion. The Ingenhousz family became friendly with John Pringle, the British Chief Medical Officer, who no doubt obtained supplies from Arnold Ingenhousz’s pharmacy. Pringle took note that Jan was a boy of high intelligence – we’ll meet Pringle again soon.
Jan attended Catholic schools and showed extraordinary talent in Latin and Classical Greek, which he spoke as comfortably as his native Dutch.
I’m Not Too Young!
Beginning at age 16, Jan Ingenhousz studied medicine at the Catholic University of Leuven. When he first arrived there, the authorities realized he was only sixteen. He was asked by the Principal to confirm his knowledge of Greek and Latin. The boy, annoyed with the challenge, took the Greek Bible from the table in front of him and said: “Tell me which passage to translate into Latin.” He was given a passage, which he immediately translated with great skill. There were no more challenges.
The Graduate
After graduating, age 23, in July 1753, Ingenhousz took more medical and scientific courses in Paris, Leyden, and Edinburgh before establishing a flourishing medical practice in his home town of Breda in 1757.
A physician by day, he was a scientist by night, carrying out mainly electrical experiments. The weird sounds and lights coming from his home during the hours of darkness led some of the townspeople to speculate that Ingenhousz was on good terms with the Devil.
As a result of his experiments, Ingenhousz published several papers and became a recognized name in scientific circles.
London and Vienna
In 1764, after Ingenhousz’s father’s death, their old family friend John Pringle got in touch, inviting Ingenhousz to London. Pringle was now personal physician to King George III, who had knighted him: he was now “Sir John Pringle”.
In London, Pringle introduced Ingenhousz to several illustrious scientists, including Joseph Priestley and Benjamin Franklin.
Ingenhousz and Franklin became firm friends – they had shared interests in electricity and smallpox inoculations (Franklin had lost a son to smallpox,) – and had much to talk about.
Fighting Smallpox with Inoculations
Ingenhousz became increasingly interested in inoculation as a defense against smallpox – the disease killed about a third of those it infected, and epidemics were common.
Inoculation consisted of taking powdered, dried crusts from the scabs on victims’ skins and scratching it into the skin of healthy people. If healthy people were scratched too deeply with too much powder, they were likely to become smallpox victims themselves. The skill of the inoculator was to scratch with enough powder, not too deeply, so that healthy people’s immune systems learned how to defeat smallpox.
In 1766, Ingenhousz took a job as an inoculator at London’s Foundling Hospital, where four or five children a day out of 400 died of smallpox. Inoculating new arrivals saw the number of deaths drop to about one in three hundred.
London’s Foundling Hospital
On April 1, 1768, at the request of the Austro-Hungarian Empire’s Imperial family, Ingenhousz departed for Vienna. The Imperial family had been devastated by smallpox in the preceding years and the Empress, who had survived with scars, wanted all of her family inoculated.
Ingenhousz obliged. He was successful in his work and was well-rewarded – among other gifts, he was granted an immediate lifetime pension of 5,000 florins a year, making him financially independent. He continued in Vienna as court physician and began a wider campaign of inoculation.
Photosynthesis in Green Plants
In January 1778, age 47, Ingenhousz returned to London on temporary leave. He gave the 1778 Baker Lecture to the Royal Society, talking about electricity.
In London, be became intrigued by Joseph Priestley’s research with plants and air: Priestley had shown that different types of “air” existed. “Good air” supported respiration in animals and burning flames. “Bad air” did not.
In France, Antoine Lavoisier soon demonstrated that “good air” was air rich in oxygen, while in “bad air” the oxygen had reacted to form carbon dioxide – both combustion and animal respiration make carbon dioxide from carbon and oxygen.
Joseph Priestley discovered that plants thrive in air rich in carbon dioxide and that they release oxygen.
Sir John Pringle was full of praise for Priestley’s discovery:
“…no vegetable grows in vain, but that from the oak of the forest to the grass in the field, every individual plant is serviceable to mankind… [because it] cleanses and purifies our atmosphere.”
Ingenhousz was fascinated by Priestley’s experiments, amazed that plants actually thrived in “bad air”. He was even more amazed that plants restore “bad air” so that again a candle would burn brightly in it.
“I was struck with admiration: and I could not read afterwards, but with a kind of ecstasy.”
Ingenhousz did over 500 experiments in the summer months of 1779, working morning until night. He discovered that plants need sunlight to convert “bad air” to “good air”, or in modern terms, covert carbon dioxide to oxygen. Using heat from a fire in the absence of sunlight he showed that light, not heat is needed for plants to produce oxygen.
“I observed that plants not only have a faculty to correct bad air in six or ten days, by growing in it, as the experiments of Dr. Priestley indicate, but that they perform this important office in a complete manner in a few hours. This wonderful operation is by no means owing to the vegetation of the plant, but to the influence of the light of the sun upon the plant. This office is not performed by the whole plant, but only by the leaves and the green stalks that support them.”
Moreover, Ingenhousz correctly identified that sunlight causes plants to release oxygen from the undersides of their leaves. He carried out his experiments by placing leaves in water and collecting the gas bubbles they released. He tested a large number of different plants, including aquatic plants, which he found to be exceptionally good at generating oxygen.
Soon after his discovery that in sunlight green plants convert carbon dioxide to oxygen, he found that in darkness or heavy shade, they do the opposite: like animals, they take in oxygen and release carbon dioxide. Overall, the rate that plants make oxygen in sunlight is much greater than the rate they consume it in darkness.
“I am far from thinking that I have discovered the whole of this salutary operation of the vegetable kingdom; but I cannot but flatter myself, that I have at least proceeded a step farther than others, and opened a new path, for penetrating deeper into this mysterious labyrinth.”
Honors
1769: Elected Fellow of the Royal Society of London
1786: Elected member of the American Philosophical Society
Many other prestigious societies from Europe’s cities asked Ingenhousz to become a member, but he declined.
Personal Details and The End
In November 1775, Ingenhousz married Agatha Maria Jacquin. Agatha was also Dutch, the sister of a professor of botany and chemistry in Vienna. Ingenhousz was 44 years old and she was 40. They had no children.
In retirement, Jan Ingenhousz returned to the United Kingdom. He died, age 68, at Bowood House, near the town of Calne, Wiltshire, on September 7, 1799. He was buried in the churchyard of St Mary the Virgin, Calne. His wife died in 1800.
“Mr. Ingenhousz is one of the small number of working physicists who have the fertile talent to pursue individual topics with admirable effort and combine each new phenomenon harmoniously with those already known.”
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Further Reading
Jan Ingenhousz
Experiments upon vegetables, discovering their great power of purifying the common air in the sun-shine, and of injuring it in the shade and at night. To which is joined, a new method of examining the accurate degree of salubrity of the atmosphere.
P. Elmsly and H. Payne, London, 1799.
Geerdt Magiels
From Sunlight to Insight: Jan IngenHousz, the Discovery of Photosynthesis & Science in the Light of Ecology.
ASP Editions, September 1, 2010.
J.M. Ingen Housz, N. Beale, E. Beale
The life of Dr Jan Ingen Housz (1730–99), private counsellor and personal physician to Emperor Joseph II of Austria.
J Med Biogr. Vol. 13 (1): pp.15–21, 2005