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: 79·16 per cent of phlogisticated air (nitrogen). 20·84 per cent of dephlogisticated air (oxygen). This result does not differ materially from those obtained by the best modern analyses, which give, within very small variations: 79·04 per cent of nitrogen and argon, 20·96 per cent of oxygen, 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.” Cavendish chose processes for “phlogisticating” air in the course of which no fixed air should be produced. He therefore avoided the use of animal and vegetable materials, and confined himself to combustibles, such as sulphur or phosphorus, to the calcination of metals, the explosion of inflammable air, and the mixture of nitrous air. He adds as a suggestion, “Perhaps it may be supposed that I ought to add to these the electric spark; but I think it much more likely that the phlogistication of the air, and production of fixed air, in this process is owing to the burning of some inflammable matter in the apparatus.” We shall see later what magnificent results arose from this last mode of “phlogisticating” air.