Henry Cavendish(1731–1810)

Henry Cavendish was born at Nice on the tenth of October 1731, the eldest son of Lord Charles Cavendish and Lady Anne Grey, the grandson on his father's side of William Cavendish, second Duke of Devonshire, and on his mother's of Henry Grey, Duke of Kent. His mother died when he was two; he was raised in London by his father, himself a careful experimental philosopher and a fellow of the Royal Society, who put him at eleven into Dr Newcome's Academy at Hackney and then to Peterhouse, Cambridge in 1749. He left Cambridge after three years without taking a degree, in the manner of well-born young men of his generation, and returned to London to work in the small laboratory his father kept at the family house in Great Marlborough Street.

He inherited a fortune that put him beyond the need to earn a living, and gave the next sixty years to the patient laboratory work that his temperament made him fit for. He kept almost no company, declined all society outside the weekly meetings of the Royal Society Club and the Royal Society itself, and built first at Great Marlborough Street and later at a large house on Clapham Common a private laboratory that contemporaries thought the best-equipped in Europe. Sir Humphry Davy, who saw it after Cavendish's death, called it without qualification the finest in the world.

His first great paper, the Three Papers Containing Experiments on Factitious Airs of 1766, was the work that isolated and identified inflammable air, what we now call hydrogen, as a distinct chemical substance, and established the procedures for collecting and measuring gases on which the new chemistry of Lavoisier, Priestley and Black would be built. In 1783 and 1784 he showed by careful combustion experiments that hydrogen and oxygen burn to form pure water, proving water to be a compound and not, as the older chemistry had held, an element. The result settled one of the founding questions of the new science.

The work for which he is now best remembered came in 1797 and 1798, when at the age of sixty-six he carried out at the Clapham house the experiment proposed by the late John Michell, who had bequeathed him the apparatus. A pair of small lead spheres at the ends of a six-foot horizontal rod were suspended from a torsion fibre, and the gravitational pull of two large lead balls of 350 pounds each on the small ones was measured by the twist of the fibre. From the deflection Cavendish calculated the mean density of the Earth as 5.48 times that of water, accurate to within one per cent of the modern figure, and thereby gave the world the first quantitative measure of the gravitational constant and of the mass of the planet. He had, as the newspapers later put it, weighed the Earth in his garden.

He died at Clapham on the twenty-fourth of February 1810 and was buried in the Cavendish family vault at Derby. A vast accumulation of unpublished electrical work was found among his papers and edited a generation later by James Clerk Maxwell, who at the new Cavendish Laboratory at Cambridge, founded in 1874 in his memory by his cousin the seventh Duke of Devonshire, discovered that Cavendish had anticipated Coulomb's law and Ohm's law by decades. The Cavendish name in modern science carries the weight of the building from which the electron, the neutron, the structure of DNA and a third of twentieth-century physics would later come.