But this was not enough. It was necessary for Cavendish to show that, so far as he could ascertain it experimentally, all the phlogisticated air was capable of combining with dephlogisticated air to form nitre. This he next proceeded to do.
“As far as the experiments hitherto published extend, we scarcely know more of the phlogisticated part of our atmosphere than that it is not diminished by lime-water, caustic alkalies, or nitrous air; that it is unfit to support fire or maintain life in animals; and that its specific gravity is not much less than that of common air; so that though the nitrous acid, by being united to phlogiston, is converted into air possessed of these properties, and consequently, though it was reasonable to suppose that part at least of the phlogisticated air of the atmosphere consists of this acid united to phlogiston, yet it might fairly be doubted whether the whole is of this kind, or whether there are not in reality many different substances confounded together by us under the name of dephlogisticated air. I therefore made an experiment to determine whether the whole of a given portion of the phlogisticated air of the atmosphere could be reduced to nitrous acid, or whether there was not a part of a different nature from the rest, which would refuse to undergo that change. The foregoing experiments, indeed, in some measure decided this point, as much the greatest part of the air let up into the tube lost its elasticity; yet, as some remained unabsorbed, it did not appear for certain whether that was of the same nature as the rest or not. For this purpose I diminished a similar mixture of dephlogisticated and common air in the same manner as before, till it was reduced to a small part of its original bulk. I then, in order to decompound as much as I could of the phlogisticated air which remained in the tube, added some dephlogisticated air to it, and continued the spark until no further diminution took place. Having by these means condensed as much as I could of the phlogisticated air, I let up some solution of liver of sulphur to absorb the dephlogisticated air; after which only a small bubble of air remained unabsorbed, which certainly was not more than 1⁄120th of the bulk of the phlogisticated air let up into the tube; so that, if there is any part of the phlogisticated air of our atmosphere which differs from the rest, and cannot be reduced to nitrous acid, we may safely conclude that it is not more than 1⁄120th part of the whole.” We shall afterwards see that this is a marvellously close estimate. There is actually 1⁄84th part of the supposed nitrogen of the air which will not combine with oxygen when sparked with it in presence of potash.
But there still remained, in Cavendish’s opinion, one point unproved. It was still conceivable that the potash might contain some “inflammable matter” which would diminish the air on sparking, and therefore oxygen nearly pure was sparked in presence of potash; but only a very small diminution of volume occurred, owing probably to some nitrogen present as an impurity in the oxygen. Water was substituted for potash with the same result; but if litmus was added to the water the colour was discharged, and lime-water introduced into the tube gave a cloud, showing that “the litmus, if not burnt, was at least decompounded, so as to lose entirely its purple colour and to yield fixed air; so that, though soap-lees cannot be decompounded by the process, yet the solution of litmus can, and so very likely might the solutions of many other combustible substances.”
Such are the chemical researches of Cavendish. Of all experimenters on the subject he was undoubtedly the greatest, though Mayow and Scheele were near rivals. But his researches were so complete that it is scarcely possible to criticise. He was not content with partial results: every point was proved and re-proved, and every possibility of erroneous conclusion was allowed for. It is curious that he did not employ the balance to check his results. Had he done so he could not have remained an adherent of the phlogistic theory. Although, as we have seen, he was perfectly acquainted with the method in which his results were interpreted by Lavoisier, he chose the old well-trodden path leading to the wilderness of distorted facts. Lavoisier tried to repeat Cavendish’s experiments, but without success; and in 1788 the last part of his Experiments on Air was published, in which he recorded the successful repetition by a Committee of the Royal Society of the conversion of nitrogen into nitric acid by the electric spark in presence of oxygen and potash.
His remaining papers deal with meteorological and astronomical subjects. One, published in 1790, refers to the height of a remarkable aurora seen in 1784; another to the civil year of the Hindoos; and another to a method for reducing lunar distances. And in 1798 his famous memoir on the density of the earth appeared. It would be quite beyond the province of this book to enter into any detail regarding it; but it may be remarked in passing that the method consisted in measuring, by means of a torsion balance, the attraction of one leaden ball for another, and that recent experiments, made with the utmost refinement, have barely altered the number which he obtained, 5·4, to 5·527.
His last paper, on an improvement in a machine for dividing astronomical instruments, was published in 1809, the year before his death.
Nothing has been said here regarding the rival claims of Watt to the discovery of the composition of water, and little need be said. The discovery was made by both in 1784, yet Cavendish visited Watt at Birmingham in 1785, and was apparently on the best of terms with him; and Watt, as proved by Cavendish’s diary, showed him many of his devices connected with the steam-engine. There can be no doubt that Watt had also discovered that when hydrogen and oxygen are exploded together water is the sole product, but he coupled the phenomenon with views involving the material nature of heat, or caloric, as it was then called, which Cavendish repudiated.
Cavendish’s later work was carried out in a villa at Clapham, which was fitted as a laboratory, workshop, and observatory, but he had a town-house near the British Museum, at the corner of Gower Street and Montague Place. He had also a library in Dean Street, Soho, which was available for any scientific man who chose to present himself. So singular were Cavendish’s habits that when he wished a book he went to this house and borrowed it as from a public library, giving a receipt for it.
Of all men, Cavendish was probably the most singular, but there can be no question of his extraordinary genius.