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CAVENDISH.

A GREATER contrast between two men of science, both eminent benefactors to the same branch of knowledge, can hardly be imagined than Cavendish offers to Priestley. He was thoroughly educated in all branches of the Mathematics and Natural Philosophy; he studied each systematically; he lived retired from the world among his books and his instruments, never meddling with the affairs of active life; he passed his whole time in storing his mind with the knowlege imparted by former inquirers and in extending its bounds. Cultivating science for its own sake, he was slow to appear before the world as an author; had reached the middle age of life before he gave any work to the press; and though he reached the term of fourscore, never published a hundred pages. His methods of investigation were nearly as opposite as this diversity might lead us to expect; and in all the accidental circumstances of rank and wealth the same contrast is to be remarked. He was a duke's grandson; he possessed a princely fortune; his whole expenditure was on philosophical pursuits; his whole existence was in his laboratory or his library. If such a life presents little variety and few incidents to the vulgar observer, it is a matter of most interesting contemplation to all who set its just value upon the cultivation of science, who reckon its successful pursuit as the greatest privilege, the brightest glory of our nature.

Henry Cavendish was born at Nice, whither his mother's health had carried her, the 10th of October, 1731. He was the son of Lord Charles Cavendish, the

late Duke of Devonshire's great uncle, by the daughter of Henry Grey, Duke of Kent. His family, aware of the talents which he early showed, were anxious that he should take the part in public life which men of his rank are wont to do, and were much displeased with his steady refusal to quit the studies which he loved. An uncle, disapproving of the course pursued towards him made him his heir; and so ample a fortune came into his possession that he left at his death a million and a quarter of money.* The Mathematics, and the various branches of Natural Philosophy, were the chief subjects of his study, and of all these sciences he was a consummate master.

The discoveries of Black on carbonic acid and latent heat, appear to have drawn his attention to the cultivation of pneumatic chemistry; and in 1766 he communicated to the Royal Society his experiments for ascertaining the properties of carbonic acid and hydrogen gas. He carried his mathematical habits into the laboratory; and not satisfied with showing the other qualities which make it clear that these two aëriform substances are each sui generis, and the same from whatever substances, by whatever processes, they are obtained; nor satisfied with the mere fact that one of them is heavier, and the other much lighter, than atmospheric air, he inquired into the precise numerical relation of their specific gravities with one another and with common air, and first showed an example of weighing permanently elastic fluids: unless, indeed, Torricelli may be said before him to have shown the relative weight of a column of air and a column of mercury: or the common pump to have long ago

*M. Biot's article in the Biog. Univ. makes him the son of the Duke of Devonshire, and states his yearly income at £300,000 sterling, and yet gives the property he left at only £1,200,000-so that he must have spent £300,000 a-year, and also dissipated five millions. Such errors seem incredible.

Three papers containing experiments on factitious air. Phil. Trans. 1766, p. 141.

compared in this respect air with water. It is, however, sufficiently clear, that neither of these experiments gave the relative measure of one air with another: nor, indeed, could they be said to compare common air with either mercury or water, although they certainly showed the relative specific gravities of the two bodies, taking air for the middle term or common measure of their weights.

The common accounts in chemical and in biographical works are materially incorrect respecting the manner in which Mr. Cavendish was led to make his great experiment upon the composition of water in 1781 and the following years. It is said, that while making his experiments on air in 1765 and 1766, he had observed for the first time, that moisture is produced by the combustion of inflammable air, and that this led him, sixteen or seventeen years later, "to complete the synthetical formula of water, and to find that the moisture that he had before observed was simple water.' Nothing can be more erroneous than this whole statement. In Mr. Cavendish's paper, of 1766, upon fixed and inflammable airs, there is not one word said of the moisture formed by the combustion; and respecting inflammable air, the experiments are confined entirely to its burning or exploding, to its specific gravity, and to its production. The paper of 1784 is, in fact, entitled 'Experiments upon Air,' and it commences with stating, not that those experiments were undertaken with any view to the water formed by burning inflammable air, but that they were made "with a view to find out the cause of the diminution which common

* Penny Cyclopædia, vol. vi., p. 392. This and other similar accounts are plainly given by some persons who never read Mr. Cavendish's writings. But a still greater error occurs in them: they represent him as having first shown that fixed and inflammable airs are separate bodies from common air; whereas Dr. Black, in his Lectures from 1755 downwards, showed this distinctly by his experiments, proving clearly that these gases have nothing in common with the atmospheric air (vol. ii., p. 87, 88).

air is well known to suffer by all the various ways in which it is phlogisticated, and to discover what becomes of the air thus lost or consumed;" and the author adds, that besides, "determining this fact, they also threw light on the constitution and means of production of dephlogisticated air." Instead of referring to any former observation of his own either in 1766, or subsequently, on the moisture left by burning inflammable air, he expressly refers to Mr. Warltire's observation of this moisture, as related by Dr. Priestley: and both Mr. Warltire's observation and Dr. Priestley's publication were made in 1781. Upon this observation Mr. Cavendish proceeded to further experiments, with the view of ascertaining "what becomes of the air lost by phlogistication." For this purpose, he introduced a portion of hydrogen gas into a globe or balloon of glass, sufficiently strong to resist the expansive force of the combustion which had often been observed in mines, and also in experiments upon a smaller scale, to produce an explosion. He adapted to the globe two wires of metal, fixing them in air-tight sockets, and bringing their points within a short distance of each other in the inside of the globe; so that, by an electrical machine, he could send the spark or the shock from the one point to the other, through the gases mixed together in the globe. He found that the whole of the hydrogen gas disappeared by the combustion thus occasioned, and a considerable portion also of the common air. Water was, as usual, found in small quantity, and an acid was also formed. He then weighed accurately the air of both kinds which he exposed to the stream of electricity, and he afterwards weighed the liquid formed by the combustion; he found that the two weights corresponded with great accuracy. It was difficult to resist the inference that the union of the two airs had taken place; and it might further have been inferred that the latent heat which held them in an elastic state had been given

out, forming the flame which was produced; and that water was formed by the union of the two airs, having, of course, less latent heat than was required to keep them in a gaseous state; but Mr. Cavendish did not approve of this manner of stating the conclusion which Mr. Watt had adopted, because of doubts which he had respecting the nature of heat.* The residue of the combustion, however, was two-fold: there was an aëriform body left in the glass vessel, as well as liquid in the bottom. This was much smaller in volume than the air which had filled the globe before the combustion, because the hydrogen gas and part of the common air had disappeared. This aëriform residue was also of a different nature from common air; it was found to be the phlogistic air of Priestley; the azotic air of Rutherford: and the air consumed in burning the hydrogen gas must, therefore, be the vital air or oxygen gas of the atmosphere. By another experiment he more fully ascertained this: for, burning oxygen gas with hydrogen gas, nearly the whole aëriform contents of the globe disappeared, and water, equal in weight to the two gases taken together, remained as the produce of the combustion, but still an acid was formed, unless in some cases, when very pure oxygen gas was used.

Thus was effected the important discovery of the composition of water, which Watt had inferred some time before from a careful examination of the similar facts collected by former experimentalists; one of whom, Warltire, had even burned the gases in a close vessel, and by means of electricity. The conclusion arrived at by Mr. Cavendish from his capital experiment was, in his own words, that "dephlogisticated air is in reality nothing but deplogisticated water, or water deprived of its phlogiston, or in other words, that water consists of dephlogisticated air united to

* Page 140.