Such a life demands our pity; yet, if an object of human life is to give pleasure to its possessor, we can hardly say that Cavendish’s was a failure. Ordinary mortals have a craving for the sympathy of their fellows; Cavendish appears to have been devoid of any such sensation. Indeed, his experiments were in many cases not published until long after they had been made. He appears to have carried on his work for his own information, and to have been indifferent to the impression which his labours made on his fellow-men. Yet his inquiries cover a more extensive field than those of almost any other man of science. They begin with experiments on arsenic, by which he endeavoured to determine the difference between the element arsenic and its two oxides. He held that arsenic acid was more thoroughly “deprived of phlogiston” than arsenious acid (i.e. more highly oxidised); and on the same occasion he studied the effect of the addition of air to nitric oxide, produced by the action of nitric acid on the element arsenic and on arsenious oxide. His next experiments related to heat; and had he published them, he would doubtless have anticipated Black in his discovery of latent heat. His paper on “Factitious Airs,” published in the Philosophical Transactions for 1766, deals with the properties of hydrogen, carbon dioxide, and the gases produced by the destructive distillation of organic substances. As we shall see later, he supposed that hydrogen, generated by the action of acids on metals, came out of the metal, and was an unknown principle in combination with phlogiston, if indeed it was not phlogiston itself; and this idea is not absurd, for many metals, and indeed a very large number of minerals, evolve hydrogen when heated, the gas having been “occluded” in their pores.

In 1772 he communicated privately to Dr. Priestley the results of a series of experiments dealing with nitrogen. To prepare it, he passed air repeatedly over red-hot charcoal, and absorbed the resulting carbon dioxide in potash. The residue was nitrogen. His description of it is:—“The specific gravity of this air was found to differ very little from that of common air; of the two it seemed rather lighter. It extinguished flame, and rendered common air unfit for making bodies burn in the same manner as fixed air, but in a less degree, as a candle which burned about 80 seconds in pure common air, and which went out immediately in common air mixed with 6⁄55ths of fixed air, burned about 26 seconds in common air mixed with the same proportion of this burnt air.”[23] He named it, as usual, “mephitic air,” and it is certain that, although Cavendish did not publish his results, his discovery was not later in date than Rutherford’s. Dealing next with the phenomena observed when that curious fish, the torpedo, produces shocks, he ascribed them to the discharge of electricity, and he was the first to distinguish between intensity, or potential, and quantity of electricity, a distinction now familiar to all.

It was in 1777 that he commenced his beautiful “Experiments on Air,” the first account of which was published in 1783. They led to the discovery of the constant quantitative composition of the atmosphere, of the compound nature of water, and of the composition of nitric acid, and pointed the way to the recent discovery of argon.

In determining the composition of the atmosphere, Cavendish made use of nitric oxide in presence of water, as a means of removing oxygen. This process, originally devised by Mayow, was rediscovered by Priestley, who employed it to ascertain the “goodness” of various samples of air; in Cavendish’s hands it became an accurate quantitative method. The title of his paper, published in the Philosophical Transactions for 1783, is “Of a new Eudiometer.” The term “eudiometer,” signifying “measurer of goodness,” was devised when it was supposed that ordinary air presented considerable variations in its power of supporting respiration and combustion, according to the seasons, and according to the place from which it was collected. Dr. Ingenhousz had found a greater absorption when air from near the sea-coast was tested by Priestley’s method with nitric oxide, than when town-air was employed; and he ascribed the salubrious nature of sea-air to its being richer in “vital air.” The Abbé Fontana, too, had made similar experiments, and had come to similar conclusions. Cavendish modified Fontana’s apparatus, rendering it capable of giving more accurate results; and during the last half of the year 1781 he analysed the air collected on sixty days, some fine, some wet, and some foggy. He also collected air from different localities, sometimes at Marlborough Street, sometimes at Kensington, which was then a country village. The results of his analyses establish as the composition of air, freed from carbon dioxide by potash:

This result does not differ materially from those obtained by the best modern analyses, which give, within very small variations:

after absorption of carbon dioxide, ammonia, and water-vapour.

In the following year, 1784, Cavendish published the first of his great memoirs, entitled Experiments on Air. His experiments were made principally “with a view to find out the cause of the diminution which common 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 condensed.”