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volution ;' * On the American War ;' besides twenty volumes of 

 tracts in favour of Dissenters and their Rights. His general works 

 fill twenty-five volumes, of which only five or six are on scientific 

 subjects; his publications being in all 141, of which only seventeen 

 are scientific. When residing at Leeds Priestly's house immediately 

 adjoined a brewery, which led him to make experiments upon the 

 fixed air copiously produced during the process of fermentation. 

 These experiments resulted in his discovering the important fact 

 that atmospheric air, after having been corrupted by the respiration 

 of animals, and by the burning of inflammable bodies, is restored to 

 salubrity by the vegetation of plants ; and that, if the air is exposed 

 to a mixture of sulphur and iron-filings, its bulk is diminished be- 

 tween a fourth and a fifth, and the residue is both lighter than 

 common air and unfit to support life ; this residue he termed ' phlo- 

 gistic air,' afterwards called azotic or nitrogen gas.* For these 

 experiments the Copley medal was awarded to him in 1773 by the 

 Royal Society. The following year to this, from experiments with 

 minium or red lead, Priestly made his great and important discovery 

 of oxygen gas. This was followed by his discovering the gases of 

 muriatic, sulphuric, and fluoric acids, ammonial gas and nitrous 

 oxide gas. He also discovered the combination which nitrous gas 

 forms suddenly with oxygen ; diminishing the volume of both in 

 proportion to that combination ; and he thus invented the method 

 of eudiometry or the ascertainment of the relative purity of different 

 kinds of atmospheric air. 



In considering the great merits of Priestly as an experimentalist, 

 it must not be forgotten that he had almost to create the apparatus 

 by which his processes were to be performed. He for the most part 

 had to construct his instruments with his own hands, or to make 

 unskilful workmen form them under his own immediate direction. 

 His apparatus, however, and his contrivances for collecting, keep- 

 ing, transferring gaseous bodies, and for exposing substances to 

 their action, were simple and effectual, and they continue to be still 

 used by chemical philosophers without any material improvement. 

 Although Priestly was the first to discover oxygen, and thus give 

 the basis of the true theory of combustion, he clung all his life with 

 a wonderful pertinacity to the Phlogistic Theory,f and nothing in 

 after life would make him give it up. In 1773 Priestly accepted an 

 invitation from Lord Shelbourne (afterwards first Marquis of Lans- 

 downe), to fill the place of librarian and philosophic companion, 

 with a salary of 250Z., reducible to 150Z. for life should he quit the 

 employment ; 40Z. a-year was also allowed him for the expense of 



* Discovered at the same time by Dr. Rutherford of Edinburgh. 



f The Phlogistic Theory explained the phenomena of combustion by suppos- 

 ing the existence of a hypothetical substance termed Phlogiston, the union of 

 which with bodies made them combustible, and the disengagement of which was 

 the occasion of combustion. 



