The first gas which Priestley investigated was “nitrous gas,” or, as it is now named, nitric oxide. It had previously been prepared by Mayow (see p. 25) by the action of nitric acid on iron; and Mayow had made the important observation that when it was introduced into ordinary air confined over water, the volume of the air was decreased, and a rise of temperature occurred. But Mayow did not apply his discovery to the analysis of air, though he rightly conjectured that the reason of the decrease in volume of the latter was due to combination between the nitric oxide and his “fire-air particles.” It was left for Priestley to rediscover this fact, and to apply it to the analysis of air, or, as he expressed it, to the determination of its “goodness.” Priestley’s use of a mercurial trough enabled him to collect and investigate various kinds of airs, among others “marine acid air” or gaseous hydrogen chloride, a gas differing entirely in properties from ordinary air. This made his mind familiar with the thought that different kinds of air exist, not necessarily modifications of atmospheric air. He had previously from his experiments come to the conclusion that “atmospheric air is not an unalterable thing, for that the phlogiston with which it becomes loaded from bodies burning in it, and animals breathing it, and various other chemical processes, so far alters and depraves it, as to render it altogether unfit for inflammation, respiration, and other purposes to which it is subservient; and I had discovered that agitation in water, the process of vegetation, and probably other natural processes, by taking out the superfluous phlogiston, restore it to its natural purity. But I own I had no idea of the possibility of going any farther in this way, and thereby procuring air purer than the best common air.”