in THE DISCOVERY OF OXYGEN 39 



almost immediately by Joseph Priestley (1733 1804), a 

 nonconformist pastor of Leeds, who was at this time making 

 a large number of experiments with the object of finding out 

 what "airs," or gases, were formed when various substances 

 were heated. Having procured a new burning glass twelve 

 inches in diameter, Priestley " proceeded with great alacrity 

 to examine, by the help of it, what kind of air a great 

 variety of substances, natural and factitious, would yield, 

 putting them into vessels .... filled with quicksilver, and 

 kept inverted in a bason of the same." (Experiments and 

 Observations on Different Kinds of Air, 1774, II. 28; 

 A.C.R. VII. 8.) 



With this apparatus, on the ist of August, 1774, 

 Priestley endeavoured to extract air from " mercurius 

 calcinatus per se,," the RED CALX OF MERCURY prepared by 

 heating the metal gently in air. Having " found that by 

 means of this lens, air was expelled from it very readily," 

 Priestley proceeded to examine the product, and discovered 

 to his great astonishment that " a candle burned in this air 

 with a remarkably vigorous flame .... and a piece of 

 red-hot wood sparkled in it, exactly like paper dipped in a 

 solution of nitre, and it consumed very fast " (A.C.R. 

 VII. 10). He placed a mouse in a vessel filled with the gas 

 where " it remained perfectly at its ease another full half 

 hour," twice as long as it would have lived in ordinary air 

 (A.C.R. VII. 17). Later he had the curiosity to breathe 

 the gas himself, and fancied that his " breast felt peculiarly 

 light and easy for some time afterwards " (A.C.R. 

 VII. 54). Priestley also obtained the gas by heating other 

 substances, including red-lead, a red powder formed by 

 gently roasting white lead, or litharge, and capable, like the 

 mercury calx, of being decomposed when heated more 

 strongly. 



The only explanation which Priestley could give of his 

 discovery of a gas better and richer than air was that 



