HENRY CAVENDISH (1731-1810), a chemist and natural philosopher, was the son of Lord Charles Cavendish, brother of the third duke of Devonshire, and of Lady Anne Grey, daughter of the duke of Kent. He was born at Nice on the 10th October 1731. Little is known about his early education. He was for some time at Newcombe's school at Hackney, and afterwards went to Cambridge. Probably his taste for experimental research was mainly acquired from his father, who gave some attention to meteorological observations, and whose very accurate determination of the depression of mercury in barometrical tubes has formed the basis of some of the most refined investigations of modern times. The morbid sensibility of his nature, which led him to shrink from society, would also have an influence in determining his choice of a scientific life; and he was free to follow his bent, as his allowance from his father was amply sufficient for his wants, and a large inheritance left him by one of his uncles put him in possession of abundant means for prosecuting his scientific investigations. In the latter part of his life, indeed, he was not less famed in his country for the great accumulation of his property than for his intellectual and scientific treasures. His merits in science were more generally understood on the Continent; and he was made, though not till he had passed the age of seventy, one of the eight foreign associates of the Institute of France. He resided principally at Clapham Common, but his library was latterly at his house in Bedford Square; and after the death of his librarian, he appointed a day on which he attended in person to lend any work to such men of letters as were either personally known to him or recommended by his friends. So methodical was he that he never took down a book for his own use without entering it in the loan book. In 1760 he became a member of the Royal Society. He was constantly present at the meetings of the society, as well as at the conversations held at the house of the president; and he dined every Thursday with the club composed of its members. Otherwise he had little intercourse with society, even with his own family. He saw only once a year the person whom he had made his principal heir. His dinner was ordered daily by a note placed on the hall table, and his female domestics had orders to keep out of his sight on pain of dismissal. His person was tall and rather thin; his dress was singularly uniform, although sometimes a little neglected. He had a slight hesitation in his speech, and an air of timidity and reserve that was almost ludicrous. He died unmarried on the 24th of February 1810, leaving a property in the funds of about £700,000, and a landed estate of £6000 a year. Some of his warmest admirers have expressed regret that no portion of that vast wealth was appropriated to scientific objects.

Henry Cavendish

For almost fifty years after Cavendish became a member of the Royal Society, he continued to contribute to the Philosophical Transactions some of the most interesting and important papers that have appeared in that collection; in which the precision of experimental demonstration, no less than the important scientific facts communicated, has been thought to have aided the further progress of chemical discovery. He may almost be called the founder of pneumatic chemistry, which had barely an existence when he began his researches. In a series of "Three papers, containing Experiments on Factitious Air," Phil. Trans., 1766, p. 141, he describes the apparatus used in processes of this kind, which he had improved by the occasional employment of mercury. By weighing a bladder filled with a known bulk of inflammable air (hydrogen), and then in a state of collapse, and by examining the loss of weight during the solution of zinc in an acid, he found the specific gravity of inflammable air to be about 1/11th of that of common air, a discovery which led to balloon experiments and projects for aerial navigation. He also observed that the gas obtained during the solution of copper in muriatic acid was rapidly absorbed by water, but did not inquire further into its nature. The second paper refers to fixed air (carbonic acid), which was found to undergo no alteration in its elasticity when kept a year over mercury, to be absorbed by an equal bulk of water or of olive oil, and by less than half its bulk of spirit of wine, to exceed the atmospheric air in specific gravity by more than one-half, and to render it unfit for supporting combustion even when added to it in the proportion of only 1 to 9. In the third part, the air produced by fermentation and putrefaction is examined, and is shown to be identical with the fixed air obtained from marble. It is also shown that the inflammable air emitted during putrefaction resembles that which is procured from zinc, although it appears to be a little heavier.

A paper on "Experiments on Air," Phil. Trans., 1784, p. 119, contains an account of two of the greatest discoveries that have ever been made in chemistry,—the composition of water, and that of nitric acid. Cavendish first establishes the radical difference of hydrogen from nitrogen, and then relates his experiments on the combustion of hydrogen with oxygen, which had partly been suggested by an observation of Mr Waltire, a lecturer on natural philosophy, and which prove that pure water is the result of the process, provided that no nitrogen be present. The second series of experiments shows that when phlogisticated air (nitrogen) is present in the process, some nitric acid is produced, and that this acid may be obtained from atmospheric air, by the repeated operation of the electrical spark. In another paper on "Experiments on Air," Phil. Trans., 1785, p. 372, the composition of nitric acid is further established, and it is shown that nearly the whole of the irrespirable part of the atmosphere is convertible into this acid, when it is mixed with oxygen and an electric spark is passed through the mixture,—the fixed air sometimes obtained being due to the presence of organic substances.

Besides the above, Cavendish contributed a number of other papers to the Philosophical Transactions. In an "Account of a New Eudiometer," Phil. Trans., 1783, p. 106, he attributes the great difference in the results of eudiometrical experiments with nitrous gas, or nitric oxide, to the different degrees of oxygenization of the acid that is formed. But he found that when the method employed was the same, there was no sensible difference in the constituent parts of the atmosphere under circumstances the most dissimilar,—the air of London, with all its fires burning in the winter, appearing as pure as the freshest breezes of the country. In "An Attempt to explain some of the principal Phenomena of Electricity by means of an Elastic Fluid," Phil. Trans., 1771, p. 584, his theory of electricity agrees with that which had been published a few years before by Aepinus, but he has entered more minutely into the details of calculation. The law of electric attraction and repulsion had not at that time been fully ascertained, but Cavendish inclines to the true supposition, of forces varying inversely as the square of the distance. In his "Observations on Mr Hutchin's Experiments for determining the degree of cold at which quicksilver freezes," Phil. Trans., 1783, p. 303, he denied to heat the character of a substance, and thought " Sir Isaac Newton's opinion, that heat consists in the internal motion of the particles of bodies, much the most probable,"—a view which it was one of the first of Sir Humphrey Davy's objects to confirm. The apparatus which Cavendish employed in his "Experiments to determine the density of the Earth," Phil. Trans., 1798, p. 469, had been invented and constructed many years before by the Rev. John Michell, who did not live to perform the experiments for which he intended it. The, method employed was to suspend by a vertical wire a horizontal bar, having a leaden weight at each end ; to determine the magnitude of the force of torsion by the time occupied in the lateral vibrations of the bar; and to measure the extent of the change produced in its situation by the attraction of two large masses of lead placed on opposite sides of the case containing the apparatus, so that this attraction might be compared with the weight of the balls, or, in other words, with the attraction of the earth. In this manner the mean density of the earth was found to be five and a half times as great as that of water.

There has been some difference of opinion as to the attitude of Cavendish towards the antiphlogistic theory of Lavoisier. Cavendish by no means dissented from the whole of that theory. In the "Experiments on Air," Phil. Trans., 1784, he quotes Lavoisier and Scheele with approbation, as having suggested the opinion "that dephlogisticated air and phlogisticated air are quite distinct substances, and that common air is a mixture of the two." Afterwards he says that "not only the foregoing experiments, but most other phenomena of nature, seem explicable as well, or nearly as well, upon this as upon the commonly believed principle of phlogiston." M. Cuvier has even asserted that the antiphlogistic theory derived its first origin from one great discovery of Cavendish, that of the nature of hydrogen, and owed its complete establishment to another, that of the composition of water.

Cavendish possessed a clearness of comprehension, and an acuteness of reasoning, which had been the lot of very few of his predecessors from the days of Newton. The splendid career of chemical investigation, which has since been pursued with a degree of success unprecedented in history, may be said to have been first laid open to mankind by his labours. [--]