Cavendish , Henry

(1731–1810) English chemist and physicist

Cavendish, who was born in Nice in the south of France, was the son of Lord Charles Cavendish, himself a fellow of the Royal Society and administrator of the British Museum. Henry was educated at Cambridge University (1749–53), but left without a degree. Following this he devoted the rest of his life to science. He inherited from his uncle a vast fortune with which he built up a large library and financed his scientific interests. Throughout his life he was an eccentric recluse, appearing only rarely in public and then chiefly at scientific meetings. He communicated with his housekeeper by a system of notes and was such a misogynist that he ordered all his female domestics to keep out of his sight.

Cavendish's first published work was Three Papers containing Experiments on Factitious Airs (1766). In these he clearly distinguished hydrogen (inflammable air) and carbon dioxide (fixed air) as gases separate from common air. Some of the work on fixed air duplicated that of Joseph Black, little of which had been published, but Cavendish was the first to weigh gases accurately.

Much of Cavendish's work remained unpublished in his lifetime and he is now known to have anticipated or come very close to several major discoveries. His electrical studies, which were edited by Clerk Maxwell in 1879, following the discovery of his notebooks and manuscripts, included the clear distinction between electrical quantity and potential, the measurement of capacitance, and the anticipation of Ohm's law (1781). He had the concept of specific heat in 1765 but the work was not published. In 1778, working on the effect of water vapor on the compressibility of air, he arrived at what is essentially the law of partial pressures (see John Dalton). One important physical investigation that was published was the determination of the mean density of the Earth (1798) by means of the torsion balance in what became known as the Cavendish experiment.

In his chemical work Cavendish came close to the concepts of equivalent weights and multiple proportions but he was not a generalizer and the concepts only became explicit in the works of others. His most illustrious and controversial work was his synthesis of water. The paper Experiments on Air (1784) reported his researches on exploding hydrogen with oxygen and air. He concluded that air consisted of a mixture of oxygen and nitrogen in a ratio of 1:4 and that hydrogen and oxygen mixed in proportions of 2:1 yielded their own weight of water. This work was carried out in 1781, and although Cavendish's priority is quite clear a dispute ensued between James Watt, Antoine Lavoisier, and Cavendish. It was discovered from this work that water is not an element but a compound.

The reason for Cavendish's three-year delay in publishing his work on water was the persistent discovery of nitric acid (then called nitrous acid) in the water after sparking hydrogen and air. In further experiments he accomplished the conversion of nitrogen to nitric acid by sparking over alkali, which formed potassium nitrate. This synthesis was the basis of the commercial production of nitric acid until 1789. In the course of his work on gases Cavendish refined the eudiometer and his measurements of the oxygen content of air showed it to be the same everywhere.

On his death Cavendish left over a million pounds sterling to his relatives. From this the endowment of the famous Cavendish Laboratory was made to Cambridge University in 1871.