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Cavendish, Henry

water air heat studied

(1731–1810) British chemist and physicist: studied chemistry of gases and of air, water, and nitric acid; made discoveries in heat and electricity and measured the density of the Earth. Henry Cavendish: the only portrait, claimed to have been made without his knowledge by W Alexander. Uninterested in his appearance, he usually wore the same old lavender coloured coat.

As eldest son of Lord Charles Cavendish, Fellow of the Royal Society, and grandson of the 2nd Duke of Devonshire, Henry was wealthy and well-educated. His mother died when he was 2. He spent 4 years at Cambridge, took no degree and studied in Paris for a year before making his homes in London (for living, Gower Street; workshop and laboratory, Clapham; library, Dean Street, Soho). Thereafter he devoted his time and money to personal research in chemistry and physics. He had a most peculiar personality: although he enjoyed scientific friends and discussion, he otherwise avoided conversation to an extreme degree, especially with women. He was generous with money, but not to himself. He published only a part of his scientific work, although he was unperturbed by either jealousy or criticism. When he was 40 he inherited a large fortune, but he was not interested in it, although he did use part of it to form a library and apparatus collection. This was used by the public and by himself on the same terms, and characteristically was located well away from his house. He was described as ‘the richest of the learned and the most learned of the rich’ and as having ‘uttered fewer words in the course of his life than any man who lived to fourscore years’.

In 1766 he described methods for handling and weighing gases. He studied ‘fixed air’ (CO2 ), showing that it was produced by fermentation or from acid and marble, and he re-studied ‘inflammable air’ (H2 , which had been studied by ). He exploded mixtures of hydrogen and air with an electric spark, and found that no weight was lost and that the product ‘seemed pure water’ (1784). The volume ratio he found to be 2:1; and the synthesis of water in this way cast out the long-held idea that water was an element. These experiments also convinced Cavendish that heat was weightless. He examined air from different places, heights and climates, and showed it to be of nearly constant composition. He showed that nitric acid is formed by passing sparks through air (when N2 and O2 combine and the NO reacts with water). Cavendish, like , interpreted his results on the phlogiston theory and Cavendish thought hydrogen was phlogiston. Cavendish, unlike Priestley, realized that LAVOISIER’S theory would also explain his results. He noticed that a small residue (1%) of air remained after long sparking; this was later found by to be argon, a noble gas and a new element.

In physics, Cavendish used a method devised by to determine the gravitational constant ( G ) in 1798. Had earlier attempted to find the density of the Earth; Cavendish’s value for G (from which the Earth’s mass and density is easily calculated) gave a mean density of nearly 5.5 times that of water. ( obtained a slightly more accurate value, by the same method, a century later.) Since most rock has a density in the range 3–4, a metal core for the Earth could be deduced. Most of Cavendish’s work on heat and electricity was not published by him, but was revealed from his notes after 1879 by . He showed that Cavendish had distinguished between quantity and intensity of electricity and that he had measured the electrical conductivity of salt solutions. He had proved that the inverse square law ( law) holds (within 2%) by showing that no charge exists inside a charged hollow spherical conductor, a result which is consistent only with that law. He worked on specific and latent heat (possibly knowing of work) and believed heat to stem from ‘internal motion of the particles of bodies’. After 60 years of research, he chose (characteristically) to die alone. In his long lifetime this eccentric recluse achieved most in chemistry: notably in showing that gases could be weighed, that air is a mixture and that water is a compound–all fundamental matters if chemistry was to advance. His work in physics was equally remarkable but was largely without influence because much was unpublished. The famous Cambridge physics laboratory named after him was funded by a talented mathematical kinsman, the 7th Duke of Devonshire, in 1871.

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