Henry Cavendish FRS (10 October 1731 – 24 February 1810) was a British natural philosopher, scientist, and an important experimental and theoretical chemist and physicist. Cavendish is noted for his discovery of hydrogen or what he called "inflammable air". He described the density of inflammable air, which formed water on combustion, in a 1766 paper "On Factitious Airs". Antoine Lavoisier later reproduced Cavendish's experiment and gave the element its name.
A notoriously shy man, Cavendish was nonetheless distinguished for great accuracy and precision in his researches into the composition of atmospheric air, the properties of different gases, the synthesis of water, the law governing electrical attraction and repulsion, a mechanical theory of heat, and calculations of the density (and hence the weight) of the Earth. His experiment to weigh the Earth has come to be known as the Cavendish experiment.
Henry Cavendish was born on 10 October 1731 in Nice, where his family was living at the time. At age 11, Henry attended Hackney Academy, a private school near London. At age 18 (on 24 November 1748) he entered the University of Cambridge in St Peter's College, now known as Peterhouse, but left three years later on 23 February 1751 without taking a degree (a common practice). He then lived with his father in London, where he soon had his own laboratory.
His first paper, "Factitious Airs", appeared in 1766. Soon after the Royal Institution of Great Britain was established, Cavendish became a manager (1800) and took an active interest, especially in the laboratory, where he observed and helped in Humphry Davy’s chemical experiments.
Cavendish published no books and few papers, but he achieved much. Several areas of research, including mechanics, optics, and magnetism, feature extensively in his manuscripts, but they scarcely feature in his published work. Cavendish is considered to be one of the so-called pneumatic chemists of the eighteenth and nineteenth centuries, along with, for example, Joseph Priestley, Joseph Black, and Daniel Rutherford. By combining metals with strong acids, Cavendish made hydrogen (H2)—which he called "inflammable air"—by dissolving metals in acids.
Although others, such as Robert Boyle, had prepared hydrogen gas earlier, Cavendish is usually given the credit for recognizing its elemental nature. Also, by dissolving alkalis in acids, Cavendish made "fixed air" (carbon dioxide), which he collected, along with other gases, in bottles inverted over water or mercury. He then measured their solubility in water and their specific gravity and noted their combustibility. Cavendish was awarded the Royal Society’s Copley Medal for this paper. Gas chemistry was of increasing importance in the latter half of the 18th century and became crucial for Frenchman Antoine-Laurent Lavoisier’s reform of chemistry, generally known as the chemical revolution.
In 1783 Cavendish published a paper on eudiometry (the measurement of the goodness of gases for breathing). He described a new eudiometer of his own invention, with which he achieved the best results to date, using what in other hands had been the inexact method of measuring gases by weighing them. He next published a paper on the production of water by burning inflammable air (that is, hydrogen) in dephlogisticated air (now known to be oxygen), the latter a constituent of atmospheric air (phlogiston theory).
Cavendish concluded that dephlogisticated air was dephlogisticated water and that hydrogen was either pure phlogiston or phlogisticated water. He reported these findings to Joseph Priestley, an English clergyman and scientist, no later than March 1783, but did not publish them until the following year. The Scottish inventor James Watt published a paper on the composition of water in 1783; Cavendish had performed the experiments first but published second. Controversy about priority ensued.
In 1785 Cavendish carried out an investigation of the composition of common (i.e., atmospheric) air, obtaining impressively accurate results. He conducted experiments in which hydrogen and ordinary air were combined in known ratios, and then exploded with a spark of electricity. Furthermore, he also described an experiment in which he was able to remove, in modern terminology, both the oxygen and nitrogen gases from a sample of atmospheric air until only a small bubble of unreacted gas was left in the original sample.
Using his observations, Cavendish observed that, when he had determined the amounts of phlogisticated air (nitrogen) and dephlogisticated air (oxygen), there remained a volume of gas amounting to 1/120 of the original volume of nitrogen. By careful measurements he was led to conclude that, "common air consists of one part of dephlogisticated air [oxygen], mixed with four of phlogisticated [nitrogen]".
Following his father's death, Henry bought another house in town and also a house in Clapham Common, to the south of London. The London house contained the bulk of his library, while he kept most of his instruments at Clapham Common, where he carried out most of his experiments. The most famous of those experiments, published in 1798, was to determine the density of the Earth and became known as the Cavendish experiment.
The apparatus Cavendish used for weighing the Earth was a modification of the torsion balance built by Englishman and geologist John Michell, who died before he could begin the experiment. The apparatus was sent in crates to Cavendish, who completed the experiment in 1797–1798 and published the results. The experimental apparatus consisted of a torsion balance with a pair of 2-inch 1.61-pound lead spheres suspended from the arm of a torsion balance and two much larger stationary lead balls (350 pounds). Cavendish intended to measure the force of gravitational attraction between the two. He noticed that Michell's apparatus would be sensitive to temperature differences and induced air currents so he made modifications by isolating the apparatus in a separate room with external controls and telescopes for making observations.
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