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JOSEPH PRIESTLEY discovers photosynthesis ? ..."the injury which is continually done to the atmosphere by the respiration of such a large number of animals ... is, in part at least, repaired by the vegetable creation" (1). ? ...In fact, Joseph Priestley discovered that plants exposed to sun light could "renew" air made "bad" by combustion by placing a small green branch of mint in a vessel of water inside a bell jar in which a candle had extinguished itself through burning. The mint was left in the jar for about one week. Then, he placed another burning candle in the jar and saw that the new candle remained lit for a quite long period of time. He repeated the experiment but put a mouse in the jar instead of the burning candle. This experiment led Priestley to conclude that the air regenerated by the mint "would neither extinguish a candle, nor was it all inconvenient to a mouse which I put into it". His work has provided most likely the first reasonable observation of how the air remained purified despite the burning of a fire or the breathing of a animal. ...However, Joseph Priestley was a zealous defender of the phlogiston theory created by George Ernest Stahl (1660- 1734) and eagerly followed by Henry Cavendish (1731-1810) and Karl Wilhem Scheele (1742-1786). According to the the phlogiston theory, which lasted for more than a century, the phlogiston (named from the Greek word for "fire-stuff") was present in every combustible substance from which it escaped when the substance burned. ...Therefore, Priestley interpreted his experiments with the burning candle, the sprig of mint and the mouse according to the phlogiston theory. The same he did in 1774 when he discovered the gas that was named later "oxygen" while working with mercuric oxide ("mercurius calcinatus per se"). He thought erroneously that this gas was the one in the atmosphere with which phlogiston combined when escaping from the burning substance. Thus, he called the new gas "dephlogisticated air", that is, a gas devoid of phlogiston. This question was at last clarified by Antoine Laurent Lavoisier (1743-1794) when he developed in 1774 the new theory of combustion, a concept which Joseph Priestley never accepted till the end of his life. ...A few years later, in 1782, Joseph Senebier showed that the noxious gas identified by Priestley as being produced by animal and plants in the dark (i.e., CO2) stimulated the release of oxygen by plants exposed to light. Senebier had thus shown before the end of the eighteenth century that CO2 and oxygen were participants in photosynthesis. ? ...It is finally interesting to emphasize the ironical side of this quest. In short, Joseph Priestley discovered, or invented, photosynthesis and interpreted his first photosynthesis experiments using the phlogiston theory, a set of concepts which were actually totally wrong. ? (1) J. R. Partington, A Short History of Chemistry, 3rd ed, Dover Publications, New York, 1989. M.
Fragata, July 18, 2004..
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