_Joseph Priestley, born in Yorkshire, England, March 13, 1733, was a Unitarian minister. In 1774 he discovered oxygen, which he called “dephlogisticated air.” Because of his liberal political ideas he was persecuted by his countrymen, and in 1794 emigrated to Northumberland, Pennsylvania, where he lived until his death, February 6, 1804._ THE DISCOVERY OF OXYGEN[15] Presently, after my return from abroad, I went to work upon the _mercurius calcinatus_, which I had procured from Mr. Cadet; and, with a very moderate degree of heat, I got from about one-fourth of an ounce of it, an ounce-measure of air, which I observed to be not readily imbibed, either by the substance itself from which it had been expelled (for I suffered them to continue a long time together before I transferred the air to any other place) or by water, in which I suffered this air to stand a considerable time before I made any experiment upon it. In this air, as I had expected, a candle burned with a vivid flame; but what I observed new at this time (November 19), and which surprised me no less than the fact I had discovered before, was, that, whereas a few moments agitation in water will deprive the modified nitrous air of its property of admitting a candle to burn in it; yet, after more than ten times as much agitation as would be sufficient to produce this alteration in the nitrous air, no sensible change was produced in this. A candle still burned in it with a strong flame; and it did not, in the least, diminish common air, which I have observed that nitrous air, in this state, in some measure does. But I was much more surprised, when, after two days, in which this air had continued in contact with water (by which it was diminished about one-twentieth of its bulk) I agitated it violently in water about five minutes, and found that a candle still burned in it as well as in common air. The same degree of agitation would have made phlogisticated nitrous air fit for respiration indeed, but it would certainly have extinguished a candle. These facts fully convinced me, that there must be a very material difference between the constitution of air from _mercurius calcinatus_, and that of phlogisticated nitrous air, notwithstanding their resemblance in some particulars. But though I did not doubt that the air from _mercurius calcinatus_ was fit for respiration, after being agitated in water, as every kind of air without exception, on which I have tried the experiment, had been, I still did not suspect that it was respirable in the first instance; so far was I from having any idea of this air being, what it really was, much superior, in this respect, to the air of the atmosphere.