A HISTORY OF SCIENCE 



story of this important discovery is probably best told 

 in Priestley's own words : 



" 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 sub- 

 stance, indestructible and unalterable, at least as much 

 so as water is supposed to be. In the course of my in- 

 quiries 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 be- 

 comes 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 un- 

 fit for inflammation, respiration, and other purposes to 

 which it is subservient ; and I had discovered that agi- 

 tation in the water, the process of vegetation, and prob- 

 ably 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 quicksilver, and 

 kept inverted in a basin of the same. . . . With this ap- 

 paratus, after a variety of experiments, ... on the ist of 

 August, 1774, I endeavored to extract air from mer- 

 curius calcmatus per se; and I presently found that, by 

 means of this lens, air was expelled from it very read- 

 ily. 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 can- 



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