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.