President's Address. 23 



absorption of anything from without. This fixed air of 

 Black, it is right to mention, was really first discovered by 

 Helmont, who called it gas silvestre. Helmont's work, how- 

 ever, had been long forgotten by Black's time, so that that 

 philosopher really made the discovery anew. The composi- 

 tion of this gas — which Black called fixed air, because it 

 exists in a fixed or solid form in the alkaline carbonates — 

 was first properly demonstrated by Lavoisier, who showed it 

 to consist of carbon and oxygen. 



Somewhat less than twenty years after Black published the 

 results of his researches on quicklime and other alkaline sub- 

 stances, Joseph Priestley was busily engaged in investigating 

 the changes produced on air by the breathing of animals, and 

 by the burning of combustible substances in it. He found 

 and proved conclusively that both respiration and combustion 

 altered the character of the air, and also lessened its bulk. 

 He also investigated the effects which living plants produce 

 on air, and among other things he found, to his astonishment, 

 that living plants had the power of undoing, so to speak, 

 what the respiration of animals or the burning of combus- 

 tibles had done. He found, for example, that a candle after 

 having burned a certain time in a fixed quantity of air 

 became extinguished, and that the residual air — which would 

 no longer support combustion, nor enable an animal to breathe 

 — was restored to its original condition by the action of a 

 living plant. These experiments, which were made about 

 the year 1772 or 1773, were preparing the way for his great 

 discovery, which was made in 1774. On the first day of 

 August in this year, Joseph Priestley discovered oxygen — a 

 discovery deemed so important as to cause this memorable 

 day to be called the birthday of modern chemistry. The 

 mode in which the great discovery was made is well known. 

 Some oxide of mercury, or red precipitate as it was then 

 called, was heated in a closed glass vessel by means of the 

 sun's rays, concentrated with a lens. The oxide was decom- 

 posed, and oxygen was given off. The name oxygen, by 

 which the gas is now universally known, emanated from 

 Lavoisier; Priestley called it dephlogisticated air. By his 

 numerous admirers Priestley has been called the father of 



