CAVENDISH, HENRY (I 731—I 81O), English chemist and physicist, elder son of Lord Charles Cavendish, brother of the 3rd duke of Devonshire, and Lady Anne Grey, daughter of the duke of Kent, was born at Nice on Oct. Io, 1731. He was sent to school at Hackney in 1742, and in 1749 entered Peterhouse, Cam bridge, which he left in 1753, without taking a degree. He appears to have spent some time in Paris with his brother Frederick during the following years, and apparently occupied himself in the study of mathematics and physics. Until he was about 4o he seems to have enjoyed a very moderate allowance from his father, but in the latter part of his life he was left a fortune which made him one of the richest men of his time. He lived principally at Clapham Common, but he had also a town house in Bloomsbury, while his library was in a house in Dean street, Soho; and there he used to attend on appointed days to lend the books to men who were properly vouched for. He was a regular attendant at the meetings of the Royal Society, of which he became a fellow in 176o, and he dined every Thursday with the club composed of its members. Otherwise he had little intercourse with society; indeed, his chief object in life seems to have been to avoid the attention of his fellows. With his relatives he had little intercourse, and even Lord George Cavendish, whom he made his principal heir, he saw only for a few minutes once a year. His dinner was ordered daily by a note placed on the hall table, and his women servants were instructed to keep out of his sight on pain of dismissal. In person he was tall and rather thin ; his dress was old-fashioned and singu larly uniform, and was inclined to be shabby about the times when the precisely arranged visits of his tailor were due. He had a slight hesitation in his speech, and his air of timidity and reserve was almost ludicrous. He was never married. He died at Clapham on Feb. 24, 1810, leaving funded property worth £ 700,000, and a landed estate of f 8,000 a year, together with canal and other property, and L50,000 at his bankers'. He was buried in the family vault at All Saints' church, Derby; in 1927 this church be came the cathedral church of the new diocese of Derby and it was decided to erect a monument there to Henry Cavendish.
Cavendish's scientific work was wide in its range. The papers he himself published form an incomplete record of his researches, for many of the results he obtained only became generally known years after his death ; yet the Institute of France in 1803 chose him as one of its eight foreign associates. His first communication to the Royal Society, a chemical paper on "Factitious Airs" (Phil. Trans., 1766), consisted of three parts, a fourth part remaining unpublished until 1839, when it was communicated to the British Association by Canon W. Vernon Harcourt. This paper dealt mostly with "inflammable air" (hydrogen), which he was the first to recognize as a distinct substance, and "fixed air" (carbon di oxide). He determined the specific gravity of these gases with reference to common air, investigated the extent to which they are absorbed by various liquids, and noted that the air produced by fermentation and putrefaction has properties identical with those of fixed air obtained from marble. He introduced new re finements into his experiments, such as the use of drying agents and the correction of the volume of a gas for temperature and pressure. In the following year he published a paper on the analysis of one of the London pump-waters (from Rathbone place, Oxford street), which is closely connected 'with the memoirs just mentioned, since it shows that the calcareous matter in that water is held in solution by the "fixed air" present and can be precipitated by lime. In 1783 he described observa tions he had made to determine whether or not the atmosphere is constant in composition ; after testing the air on nearly 6o different days in 1781 he could find in the proportion of oxygen no difference of which he could be sure, nor could he detect any sensible variation at different places. Two papers on "Ex periments on Air," printed in the Phil. Trans. for 1784 and 1785, contain his great discoveries of the compound nature of water and the composition of nitric acid. Starting from an experi ment, narrated by Priestley, in which John Warltire fired a mixture of common air and hydrogen by electricity, with the result that there was a diminution of volume and a deposition of moisture, Cavendish burnt about two parts of hydrogen with five of com mon air and noticed that the only liquefiable product was water. In another experiment he fired, by the electric spark, a mixture of hydrogen and oxygen in a glass globe, similar to the apparatus now called "Cavendish's Eudiometer," and again obtained water. Proceeding with these experiments he found that the resulting water contained nitric acid. In the second of the two papers he gives an account of the methods by which the composition of nitric acid was discovered ; he observed also that a very small fraction, about one one-hundred and twentieth, of the "phlogisti cated air" of the atmosphere differed from the rest, and in this residue he doubtless had a sample of the inert gas argon which was only recognized as a distinct entity more than zoo years later. It may be noted here that, while Cavendish adhered to the phlogis tic doctrine, he did not hold it with anything like the tenacity that characterized Priestley ; thus, in his 1784 paper on "Experiments on Air," he remarks that not only the experiments he is describ ing, but also "most other phenomena of nature seem explicable as well, or nearly as well," upon the Lavoisierian view, but he did not accept it and continued to use the language of the phlogistic theory. Experiments on arsenic, published for the first time in 1921, showed that Cavendish had investigated the properties of arsenic acid about ten years before Scheele.
Cavendish's work on electricity, with the exception of two papers containing relatively unimportant matter, remained in the possession of the Devonshire family until 1879, when the papers were edited by James Clerk Maxwell as the Electrical Researches of the Hon. Henry Cavendish. This work shows that Cavendish had anticipated the researches of Coulomb, Faraday and others. He investigated the capacity of condensers and constructed a series of condensers with which he measured the capacity of various pieces of apparatus using the "inch of electricity" as the unit of capacity. He discovered specific inductive capacity and measured this quantity ; he showed that electric charges are con fined to the surface of a conductor and that the inverse square law of force between charges holds to within 2%. Cavendish intro duced the idea of potential under the name of "degree of electri fication," in a paper published in 1771, under the title "Attempt to explain some of the principal phenomena of electricity by means of an elastic fluid." He investigated the power of different sub stances to conduct electrostatic discharges (Phil. Trans., and completed an enquiry which amounted to an anticipation of Ohm's law.
Cavendish took up the study of heat, and had he published his results promptly he might have anticipated Joseph Black as the discoverer of latent heat and of specific heat. He published a paper on the freezing point of mercury in 1783 and in this paper he expresses doubt of the fluid theory of heat.
Cavendish's last great achievement was his series of experiments to determine the density of the earth (Phil. Trans., 1798). The apparatus he employed was devised by the Rev. John Michell, though he had the most important parts reconstructed to his own designs. (See GRAVITATION.) The figure he gives for the specific gravity of the earth is 5.48, but in fact the mean of the 29 results he records works out at 5.448. Other publications of his later years dealt with the height of an aurora seen in 1784 (Phil. Trans., 1790), the civil year of the Hindus (Id. 1792), and an improved method of graduating astronomical instruments (Id. 1809). Cav endish also had a taste for geology, and made several tours in England for the purpose of gratifying it.
A life by George Wilson (1818-59), printed for the Cavendish Society in 1851, contains an account of his writings, both published and unpublished, together with a critical enquiry into the claims of all the alleged discoverers of the composition of water. Some of his instruments are preserved in the Royal Institution, London, and his name is commemorated in the Cavendish Physical Labora tory at Cambridge, which was built by the 7th duke of Devonshire.
The remainder of Cavendish's papers were placed at the disposi tion of the Royal Society by the Duke of Devonshire. In 1921 the previously published work, together with a number of unpub lished experiments, appeared under the title : The Scientific Papers of the Honourable Henry Cavendish, F.R.S.; Vol. I., The Electri cal Researches, revised with preface and notes by Sir J. Larmor. Vol. II., Chemical and Dynamical, edited by Sir Edward Thorpe, with additions by Dr. Charles Chree and others.