He several times noticed the deep blue liquid which nitrogen peroxide forms with cold water. He made many attempts to use nitric oxide as an antiseptic, especially for culinary purposes. But the gastronomic results with fowls and pigeons were not to his liking, although he says, “my friend Mr Magellan ... had not so bad an opinion of this piece of cookery as I had.” One cannot read Priestley’s description of his multifarious experiments without being struck with the number of occasions in which he just missed making discoveries of first-rate importance. It is obvious that he had obtained chlorine without recognising it, even before the news of Scheele’s discovery reached this country. He had also prepared, without knowing it, phosphoretted hydrogen and phosphorous acid. At times, however, he can follow a clue with remarkable perspicacity; as in his observation of the cause of the “flouring” of mercury, and in his discovery of a method of removing lead and tin from that metal. The subject of “dephlogisticated air” naturally continued to interest him, and he again returns to it in this volume, for he says:— “As it sometimes amuses myself it may perhaps amuse others to look back with me to the several steps in the actual progress of this investigation, some of which I overlooked in my last account of it.” He points out, as already stated, that he must have had the new gas in his hands as far back as November 1771, having obtained it from nitre. He admits that he had no particular view in making his crucial experiment of August 1, 1774, “excepting that of extracting air from a variety of substances by means of a burning lens in quicksilver, which was then a new process with me, and which I was very fond of.” He explains how he was led to his speculation that “this kind of air, and consequently of atmospherical air, which is the same thing but in a state of inferior purity,” consists “of earth and spirit of nitre.” “But,” he adds, “I have since seen reason to suspect that hypothesis, plausible as it appears. Indeed, some of my late experiments would lead me to conclude that there is no acid at all in pure air.” He then experiments with manganese, which Scheele, who independently discovered oxygen, had already employed, and finds that it yields the new air both when heated alone or with oil of vitriol. The production of oxygen from manganese was contrary to his expectations as the substances he had hitherto used, the _precipitate per se_ and the _red lead_ and the nitre, had all been subjected to “the influence of the atmosphere,” whereas “here was pure air from a substance which for anything that appeared had always been in the bowels of the earth, and never had had any communication with the external air.” This led to the surmise that possibly the expulsion of dephlogisticated air from such mineral substances