In the course of subsequent work on nitrous air Priestley had occasion to study its action on iron, whereby he says:— “A most remarkable and most unexpected change was made in the nitrous air,” the iron “makes it not only to admit a candle to burn in it, but enables it to burn with an _enlarged flame_.... Sometimes I have perceived the flame of the candle, in these circumstances, to be twice as large as it is naturally, and sometimes not less than five or six times larger; and yet without anything like an _explosion_, as in the firing of the weakest inflammable air.” Priestley in this manner obtained _nitrous oxide_, the properties of which he subsequently studied in some detail. In the paper which follows, _viz._, “On Air infected with the Fumes of Burning Charcoal,” he incidentally gains further insight into the nature of atmospheric air. By what he called throwing the focus of a burning mirror on charcoal suspended in air contained in a glass tube standing over water or mercury—a favourite method of his when he had occasion to heat a substance in a gas—he could observe the phenomena with great precision. He noticed the formation of the fixed air and determined the degree of diminution when the burning took place over water or over lime-water. “In this manner,” he says, “I diminished a given quantity of air one-fifth. Air thus diminished by the fumes of burning charcoal not only extinguishes flame, but is in the highest degree noxious to animals; it makes no effervescence with nitrous air, and is incapable of being diminished any farther by the fumes of more charcoal.... All my observations show that air which has once been fully diminished ... is not only incapable of any further diminution ... but that it has likewise acquired _new properties_, most remarkably different from those which it had before....” By heating pieces of lead and tin in air by means of a burning glass he observed the formation of a metallic calx, the volume of air was diminished, and it also “was in the highest degree noxious and made no effervescence with nitrous air.” The real significance of these phenomena was, however, wholly unperceived by Priestley, and phlogiston, as usual, led him astray. He had, of course, in all these experiments prepared _nitrogen_, and in a state of sensible purity. He imagined, however, that he had simply “phlogisticated” the air, the phlogiston coming from the charcoal and the metals, and that this phlogisticated air was imbibed by the water. An experiment described by Cavendish led Priestley to study the action of “Spirit of Salt” (hydrochloric acid) upon copper. As Cavendish had already stated, the gas so evolved “lost its electricity by coming into contact with water.” By collecting the gas over mercury Priestley was able to study its properties more exactly. From certain anomalies in the experiments he says:—