He worked out how the world breathes — and the world went on breathing without learning his name.
In the summer of 1779, in a rented house at Southall Green outside London, a Dutch physician spent three months drowning leaves. He held sprigs under water in glass vessels, set them in sunlight and in shade, and counted the bubbles that rose from their undersides. By his own reckoning he ran more than five hundred experiments in that one season, and out of them came three plain facts nobody had put together before: a green leaf in sunlight sweetens the air, the same leaf in darkness spoils it, and only the green parts do the work. That is photosynthesis — the reaction that puts oxygen into the sky and carbon into every living body on Earth.
He was already celebrated for something else entirely. Eleven years before, the Empress Maria Theresa had summoned him to Vienna to cut live smallpox into the arms of her own children, and he had made them live. Courts knew him as a royal physician; Franklin and Jefferson wrote to him as a friend. The leaf work was a private summer's labour, published once in English and then largely left to Joseph Priestley's larger fame. Two centuries on, the discovery is in every biology syllabus on the planet. The discoverer is a footnote.
Breda to Vienna to a Wiltshire lawn
The lifeline
1730
Born 8 December in Breda, a fortress town in the Dutch Republic
1753
Doctor of medicine at Leuven, aged 22; then Leiden, Paris, Edinburgh
1768
Called to Vienna; inoculates the imperial family against smallpox
1779
One summer, 500+ experiments — and the leaf gives up its secret
1784
Watches charcoal dust jitter on alcohol and writes down what he sees
1799
Dies at Bowood House, Wiltshire, aged 68; buried at Calne
By the numbers
The measure of him
500+
Experiments run in the single summer of 1779, by his own count
1779
The year a green leaf was shown to make the air we breathe
43
Years before Robert Brown that he described specks jittering in fluid
1769
Elected Fellow of the Royal Society — a decade before the leaf
What one physician settled
The work
1768
Smallpox, inoculated at court. Summoned to Vienna after the epidemic that tore through the Habsburgs, he variolated the imperial children — live smallpox in a small, calculated dose, three decades before Jenner's safer cowpox vaccine.
1779
Experiments upon Vegetables. The title says the finding out loud: plants have a great power of purifying the common air in the sunshine, and of injuring it in the shade and at night. Priestley had shown plants could restore air; Ingenhousz showed when, and which parts.
1784
The jitter. Powdered charcoal on the surface of alcohol, twitching without cause. He recorded it forty-three years before Robert Brown's study gave the motion its name.
1789
How heat travels. Wax-coated metal rods pushed into a hot bath; the wax melts furthest along the best conductor. The apparatus still carries his name in school laboratories.
1796
What a plant is made of. In his essay on the food of plants, he argued that a tree's bulk comes not from the soil beneath it but from the air around it — carbon, pulled out of the sky.
Eclipsed, then quietly restored
Recognition
In his lifetime
Known at court as the doctor who saved the Habsburgs, not as a discoverer
Priestley's earlier, vaguer result on plants and air took the credit
A Catholic from the Dutch Republic, he took his degree abroad at Leuven
His findings scattered across English, French and German editions
Died in a patron's country house, far from Breda and from Vienna
Today
The 1779 experiments are a founding text of plant science
The Ingenhousz apparatus is still built to teach heat conduction
A bronze bust stands in the arcaded court of the University of Vienna
A plaque in Calne, Wiltshire, marks the town where he died
Google marked his 287th birthday, 8 December 2017, with a doodle
The town that made him
Breda
He was born on 8 December 1730 in Breda, a walled garrison town on the river Mark in Brabant — a place of sieges, drill and quiet Catholic households in a Protestant republic. His father dealt in leather and dabbled in remedies; the son wanted the university. The Dutch chairs were closed to him, so he crossed into the Austrian Netherlands and took his medical doctorate at Leuven at twenty-two. From that provincial start he moved through Leiden, Paris and Edinburgh, then London, then the Hofburg in Vienna — and finally to a rented house at Southall Green, where the whole planet's chemistry turned on a jar of pondwater and a leaf.
22
His age when Leuven made him a doctor of medicine
3
States that claimed him: the Dutch Republic, Austria, Britain
68
Years from Breda to Bowood — the whole span of his life
"The sun by itself has no power to mend air without the concurrence of plants."
— Jan Ingenhousz, Experiments upon Vegetables, 1779
