NATURAL AND PHYSICAL SCIENCE, 1700-1800 307 



istry now dwells, namely, the discovery of oxygen. Joseph Priestley, 

 fearless reformer, Unitarian clergyman, and tireless experimenter in 

 natural philosophy, had already made important and interesting 

 discoveries when, in 1774, as stated above, he decomposed by 

 heat the reddish powder obtained by calcining mercury, and 

 collected and examined the gas given off. Candles and glowing 

 coals burned in this gas with extraordinary energy, and mice lived 

 in it under a bell glass even longer than in ordinary air. And, 

 since it was derived from a burnt, i.e. dephlogisticated, metal 

 and yet was colorless and odorless like ordinary air, Priestley 

 named it "dephlogisticated air." The following is his own 

 account of his work : 



There are, I believe, very few maxims in philosophy that have 

 laid firmer hold upon the mind than that air, meaning atmospheric 

 air, is a simple elementary substance, indestructible and unalterable 

 at least as much so as water is supposed to be. In the course of my 

 inquiries I was, however, soon satisfied that atmospheric air is not an 

 unalterable thing; for that, according to my first hypothesis, the 

 phlogiston with which it becomes loaded from bodies burning in it, 

 and the animals breathing it, and various other chemical processes, 

 so far alters and depraves it as to render it altogether unfit for inflam- 

 mation, respiration, and other purposes to which it is subservient; 

 and I had discovered that agitation in the water, the process of vege- 

 tation, and probably other natural processes, restore it to its original 

 purity. . . . 



Having procured a lens of twelve inches diameter and twenty 

 inches focal distance, I proceeded with the greatest alacrity, by the 

 help of it, to discover what kind of air a great variety of substances 

 would yield, putting them into the vessel, which I filled with quick- 

 silver, and kept inverted in a basin of the same. . . . With* this ap- 

 paratus, after a variety of experiments ... on the 1st of August, 

 1774, I endeavored to extract air from mercurius calcinatits per se; 

 and I presently found that, by means of this lens, air was expelled from 

 it very readily. Having got about three or four times as much as 

 the bulk of my materials, I admitted water to it, and found that it 

 was not imbibed by it. But what surprised me more than I can 

 express was that a candle burned in this air with a remarkably vigorous 



