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Definition: Cavendish, Henry from Philip's Encyclopedia

English chemist and physicist, b. France. He discovered hydrogen and the compositions of water and air, and estimated the Earth's mass and density by a method now known as the 'Cavendish experiment'. He also discovered nitric acid (HNO3), the gravitational constant, measured the specific gravity of carbon dioxide (CO2) and hydrogen, and stated the inverse square law for the interaction of charged particles.

Summary Article: Cavendish, Henry (1731-1810)
From The Hutchinson Dictionary of Scientific Biography

Place: Italy

Subject: biography, physics

English natural philosopher whose main interests lay in the fields of chemistry and physics. His chief experimental work concerned gases, although he also carried out fundamental experiments concerning electricity and gravitation. He made the first determination of the gravitational constant and thereby obtained the first values for the mass and density of the Earth. He is also usually credited with the discovery of hydrogen. Cavendish was one of the few scientists to approach Newton's standard in both mathematical and experimental skills and was a major figure in 18th-century science. He devoted his entire life to the acquisition of knowledge, but published only those results that satisfied him completely. Most of his work, especially his experiments with electricity, were unknown for a hundred years or more, so the immediate impact of his work was far less than it might have been.

Cavendish was born in Nice on 10 October 1731. He was of aristocratic descent, his paternal grandfather being the Duke of Devonshire and his maternal grandfather the Duke of Kent. Cavendish attended Dr Newcome's Academy in Hackney, London, and then went on to Peterhouse College, Cambridge, in 1749. He left in 1753 without a degree, which was not an unusual occurrence at that time, and spent the rest of his life in London. His father encouraged his scientific interests and introduced him to the Royal Society, of which he became a member in 1760. Despite his active participation in the scientific community, Cavendish was a recluse and shunned most social contact, making no attempt to use a fortune of the order of a million pounds bequeathed to him.

Cavendish published his first paper, which demonstrated the existence of hydrogen as a substance, in 1776. He received the Copley Medal of the Royal Society for this achievement. His subsequent papers were few and far between, and included, most notably, a theoretical study of electricity in 1771, the synthesis of water in 1784, and the determination of the gravitational constant in 1798. He died alone in London on 24 February 1810.

Little is known of Cavendish's work until the late 1760s, when he began experimenting with ‘facticious airs’ (gases that can be produced by the chemical treatment of solids or liquids). He studied ‘fixed air’ (carbon dioxide) produced by mixing acids and bases; ‘inflammable air’ (hydrogen) generated by the action of acids on metals; and the ‘airs’ produced during decay and fermentation. He measured the specific gravities of hydrogen and carbon dioxide, comparing them with that of ‘common’ (that is, atmospheric) air.

In 1783 Cavendish found that the composition of the atmosphere is the same in different locations and at different times. He also found that a small fraction of ‘common air’ seems to be inert - a hundred years later William Ramsay was to show that this noble gas (rare gas) is mainly argon. A year later Cavendish demonstrated that water is produced when hydrogen burns in air, thus proving that water is a compound and not an element as had been suggested by early Greek scientists. By sending electric sparks through ‘common air’ he caused the nitrogen in it to combine with oxygen. When the gas produced was dissolved in water it produced nitric acid. He also showed that ‘calcareous earth’ dissolves in water containing carbon dioxide, to form what is now known as calcium bicarbonate (calcium hydrogencarbonate). He distinguished between the two oxides of arsenic, demonstrating that one contains more oxygen than does the other.

Cavendish's most important work in physics was on electricity and gravitation. His 1771 paper on the nature of electricity shows that he believed it to be an elastic fluid. He then worked on electricity for ten years, aiming to produce a sequel to Newton's Principia that would explain all electrical phenomena. But although this was his most concentrated research effort, Cavendish published nothing more about it. His fastidious attention to the details of his results and his thorough efforts to understand and unify all his observations frustrated this plan and he was not able to gain the overview that he sought. He tried unsuccessfully to uncover the relationship between force, velocity of current, and resistance, although he found that electric fields obey the inverse square law and was able to produce some valuable work on conductivity. Much of the work done by Michael Faraday, Charles Coulomb, and others during the next 50 years is foreseen in this early work by Cavendish, but none of his experiments were known until James Clerk Maxwell edited and published them in 1879.

During the latter part of the 1780s, Cavendish worked on the production of heat and determined the freezing points for many materials, including mercury. He relied on some of the early work he had done on latent heats. One of the practical outcomes from these experiments was the explanation for some anomalous readings obtained when using mercury thermometers at low temperatures.

The five papers that Cavendish published during the last 25 years of his life all had an astronomical theme. By far the most important of these appeared in 1798, when he announced his determination of Newton's gravitational constant, thereby deriving the density and mass of the Earth. Newton's law of gravitation contained two unknowns: the gravitational constant and the mass of the Earth. Determining one would give the other. In what has become known as the Cavendish experiment, the gravitational constant was found.

Cavendish used an apparatus that had been devised by John Michell (1724-1793). It consisted of a delicate suspended rod with two small spheres made of lead attached to each end. Two large stationary spheres were placed in a line at an angle to the rod. The gravitational attraction of the large spheres caused the small spheres to twist the rod towards them. The period of oscillation set up in the rod enabled Cavendish to determine the force of attraction between the large and small spheres, which led him to determine the gravitational constant for Newton's equation, and thus the density of the Earth (about 5.5 times that of water) and its mass (6 × 1024 kg/13 × 1024 lb). The sensitivity of this apparatus was extraordinary, for the gravitational force involved was 500 million times less than the weight of the spheres, and Cavendish's results were not bettered for more than a century.

Cavendish was a great scientist and was honoured by the naming of the Cavendish Laboratories at the University of Cambridge in his memory. His contributions to science are notable for their quality and diversity. Had he permitted all his results to be published, the rate of advancement of physical science would undoubtedly have been greatly accelerated. He stands today as one of the giants of modern science.

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