It was not, however, until the following March (1775) (he having meanwhile been intent upon his experiments on the vitriolic air [sulphur dioxide]), that he ascertained the real nature of the new air, and was led “though very gradually ... to the complete discovery of the constitution of the air we breathe.” By trials with the nitrous air and with mice he found that the new gas was eminently fit for respiration: nitrous air reduced its volume to a greater extent than in the case of common air, and a mouse lived longer in it than it would in the same volume of common air. “Thinking of this extraordinary fact upon my pillow, the next morning I put another measure of nitrous air to the same mixture, and to my utter astonishment found that it was farther diminished to almost one-half of its original quantity.” Priestley now utterly missed his way for a time. He sought to get the new air from the various oxides of lead, but the fetish of phlogiston again led him wrong, and eventually by a train of reasoning which is fully set forth in the paper, but which need not here be repeated, there remained, he says, no doubt in his mind “but that _atmospherical air_, or the thing that we breathe, _consists of the nitrous acid and earth_, with so much phlogiston as is necessary to its elasticity; and likewise so much more as is required to bring it from its state of perfect purity to the mean condition in which we find it.” Priestley’s “complete discovery of the constitution of the air we breathe” was thus wholly erroneous: he was very far indeed from having a clear conception of its real nature. Priestley’s description of the main properties of oxygen is however accurate, and lecturers in chemistry are indebted to him for some striking experimental illustrations of them. “I easily conjectured,” he says, “that inflammable air would explode with more violence and a louder report by the help of dephlogisticated than of common air; but the effect far exceeded my expectations, and it has never failed to surprise every person before whom I have made the experiment.... The dipping of a lighted candle into a jar filled with dephlogisticated air is alone a very beautiful experiment. The strength and vivacity of the flame is striking, and the heat produced by the flame in these circumstances is also remarkably great.... Nothing would be easier than to augment the force of fire to a prodigious degree by blowing it with dephlogisticated air instead of common air.... Possibly _platina_ might be melted by means of it.