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soon iillor, lie (U'scrilu'd liis iliscovery to Liivoisii-r. Priestley, witli tlu- other 

 chemists <>!' his viiiie, lield to tlie tlieorv of plilogiston, and expressed liis discovery 

 in terms of thai theory. In aeeordanee with tliat theoiy he called oxygen dephlo- 

 gisticated air, and uitrosicn, or in general, air which had lost the power of 

 supporting comhustion, whether pure nitrogen or not. phlogisticated air. The 

 thought conveyed i)y these terms was that air possessed a certain capacity for 

 ahsorhing the phlogiston which was supposed to he given off during comhustion, 

 hut that ordinary air already contained a considerahle amount of phlogiston. If 

 this phlogiston were removed the capacity to take it U[) again would, of conrsi', he 

 increased, and the resulting suhstance which we call oxygen could projierly l)e 

 called dephlogisticated air, while nitrogen, which was supposed to have taken up 

 all the phlogiston which it could hold, was called phlogisticated air. It is evident, 

 at onie, that while tiie honor of the discovery of oxygen really helongs to Priest- 

 ley, the new substance was not to hii^i a separate and distinct element in any such 

 sense as we now understand it, but was rather a sort of modified air. The theory" 

 of phlogiston dealt chiefly with outward appearances and qualitative phenomena, 

 and the time had now come when the theory was inadequate and a hindrance to 

 further progress. Lavoisiei' seems to have l)een the only chemist of the time who 

 recognized this. After Priestley had told him of his discovery he repeated the ex- 

 periments for himself, and soon came to a comparatively clear and correct view of 

 the composition of air, and the real nature of oxidation and combustion. But while 

 even at this early date he must have begun to see that fjie theory of phlogiston 

 was unnecessary, and probably fallacious, his open conflict with the theory does 

 not seem to have begun till several years later. He contented himself with a 

 description of his experiments and explanation of his results, rather ignoring 

 than directly combatting the prevailing theory. He had acquired reputation by 

 this time as a careful experimenter and as one thoroughly acquainted with the 

 history and theories of his science. He was recognized, therefore, when the time 

 came, as oni' competent to criticise current theories, and as one whose criticism 

 must, at least, receive respectful attention. 



During the ten years that followed, from 1775 to 1785, Lavoisier busied him- 

 self almost exclusively with experiments more or less closely connected with 

 (•ombustion and oxidation. Gradiially he proved, by careful experiments made 

 with a great number of different substances, that ordinary combustion consists in 

 all cases of a combination with oxygen. He showed that "fixed air" is formed 

 by the combustion of the diamond and of charcoal; that phosphoric acid, accord- 

 ing to the nomenclature of the period which followed, is formed by the combus- 

 tion of phosphorus, and also by its oxidation with nitric acid; that both sulphurous 



