14 DISCOVERY OF PHOTOSYNTHESIS CHAP. 2 



the mob, whereupon the French RepubHc made him an honorary citizen, 

 and offered him refuge in France; however, he preferred to exile himself 

 to America, where he spent the last ten yesivs of his life in the little town 

 of Northumberland, Pennsylvania. 



Throughout his life, Priestley enjoj^ed playing about with gases; his 

 writings reveal him as one of the most skillful and successful experi- 

 mentalists of all times; but his powers of logic and analysis were reserved 

 for philosoph}^ and theology, and in the presentation of his experiments 

 he stuck to the old phlogiston theory — long after the new concepts of 

 Lavoisier had found general acceptance. Thus, the wide implications of 

 Priestley's greatest discovery — oxygen — escaped him; and he was even 

 less aware of the general import of his experiments with green plants, and 

 quite willing to leave their exploitation to others. The following quota- 

 tions (1772) show the extent of his contribution to the discovery of 

 photosynthesis : 



" I have been so happy as by accident to hit upon a method of restoring air which has 

 been injured by the burning of candles and to have discovered at least one of the restor- 

 atives which Nature employs for this purpose. It is vegetation. One might have 

 imagined that sinee common air is necessary to vegetable as well as to animal life, both 

 plants and animals had affected it in the same manner; and I own that I had that 

 expectation when I first put a sprig of mint into a glass jar standing inverted in a vessel 

 of water; but when it had continued growing there for some months, I found that the 

 air would neither extinguish a candle, nor was it at all inconvenient to a mouse which I 

 put into it. 



"Finding that candles would burn very well in air in whicli plants had grown a long 

 time ... I thought it was possible that plants might also restore the air which had been 

 injured by the burning of candles. Accordingly, on the 17th of August 1771 I put a 

 sprig of mint into a quantity of air in which a wax candle had burned out and found 

 that on the 27th of the same month another candle burnt perfectly well in it." 



This momentous observation was described in the Philosophical Trans- 

 actions of the Royal Society in 1772; and reprinted in the first volume of 

 Priestley's famous work, Experiments and Observations on Different Kinds 

 of Air, which appeared in 1776. 



Other experiments, described in the same ])aper, showed that plants 

 thrive particularly well in air made "obnoxious" by the exhalations of 

 animals. The discovery of the complementary character of the chemical 

 functions of plants and animals made a great impression on Priestley's 

 contemporaries, and brought him in 1773 the award of the Copley medal 

 of the Royal Society. However, Priestley did not return to the study of 

 air improvement by plants until 1777; and the first new results on this 

 subject appeared in his second physicochemical work, Experiments and 

 Observations Relating to Various Branches of Natural Philosophy, whose 

 first volume was published in 1779. These new observations were both 

 interesting and confusing. Several scientists abroad, notably Scheele, 



