how photosynthesis was discovered

The Mechanism

Joseph Priestley was born on 24 March 1733 in Fieldhead, near Birstall in the West Riding of Yorkshire, into a family of dissenting Calvinist clothiers. He trained at the dissenting Daventry Academy, took the ministry of a chapel in Suffolk in 1755, and by 1770 was a Presbyterian minister in Leeds — where his back garden happened to abut a public brewery, from which he obtained a constant free supply of the "fixed air" (carbon dioxide) fermenting off the beer vats. It was in this small back-garden laboratory in Meadow Lane, Leeds, that in August 1771 Priestley did the experiment that founded the modern understanding of plants. He took two glass bell jars. In one he sealed a lighted candle. It burned for a few minutes and went out. In another he sealed a mouse. It suffocated in a few minutes. He concluded that both the candle and the mouse had "injured" the air — respired something out of it. He then placed a sprig of mint inside a third bell jar of similarly "injured" air, sealed it, and left it in a sunny window for ten days. When he tried a burning candle in the jar afterwards, the candle burned. When he sealed a mouse in it, the mouse breathed. "The air had been perfectly restored by the plant," he wrote. Priestley published the observation in 1772 in the Royal Society's *Philosophical Transactions*: "Observations on Different Kinds of Air" (*Phil. Trans.* 62, 147–264) — a paper for which he was awarded the Copley Medal that same year. The result had a beautiful moral shape for a natural theologian: plants and animals were mutually necessary in the divine economy of air. But Priestley did not know what the plant was doing, and he could not always replicate his own results. Sometimes the mint restored the air; sometimes it did not. He blamed differences in the health of the plant. The real variable he was missing was found by another man. Jan Ingenhousz was born on 8 December 1730 in Breda, in the Dutch Republic, the second son of a leather merchant. He took his medical doctorate at Leuven in 1753 and moved to England in 1764 in his early thirties, where he came to the attention of Sir John Pringle, physician to Queen Charlotte, and became a leading British advocate of smallpox variolation. In 1768, Empress Maria Theresa of Austria — having lost two children and a daughter-in-law to smallpox — summoned him to Vienna to variolate her surviving children, including the future Emperor Joseph II. He succeeded, was rewarded with a lifetime pension and a court appointment as physician to the imperial family, and by 1778 was 47 and prosperous enough to spend the summer of 1779 in England as the private guest of Lord Shelburne at Bowood House in Wiltshire — where Priestley, since 1772, had been the family librarian and "literary companion." At Bowood over the summer of 1779, in a purpose-built glass laboratory on the grounds, Ingenhousz repeated Priestley's experiment more than five hundred times under systematically varied conditions. He replaced Priestley's mint with fresh sprigs of many plants, moved the bell jars into full sunlight, into shade, and indoors after dark, and measured the air chemically after each trial. He found the missing variable in a fortnight. It was light. In the sun, the green parts of the plant — only the green parts — produced "dephlogisticated air" (oxygen). In darkness, the same green plant did the opposite: it produced "fixed air" (carbon dioxide), like an animal breathing. The roots, the flowers, and the fruit did the second thing at all hours. Ingenhousz published in October 1779 a book with the title (in English, in London, in his own translation from the Latin): *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.* The 302-page volume, printed by P. Elmsly on the Strand, is the founding document of the science of photosynthesis. The next two decades filled in the rest of the chemistry: Jean Senebier in Geneva showed in 1782 that the CO₂ was the input, Nicolas-Théodore de Saussure showed in 1804 that the water was too. But the fact — that green plants, in light, use the sun's energy to invert the direction of animal respiration; the fact on which the whole of the Earth's atmosphere and every calorie in every organism now depends — was established at Bowood House in a set of glass bell jars in the summer of 1779. Ingenhousz spent much of the 1780s in Vienna as a court physician, was a personal correspondent of Benjamin Franklin, and died on 7 September 1799 at Bowood at the age of 68, of a lingering illness while visiting his old friend and rival Priestley's former English patron. Priestley himself, by then a religious dissenter driven out of England by a mob that burned down his Birmingham house in 1791, died in Pennsylvania in 1804 at 70.