PRIESTLEY 13 



and this concept rapidly displaced the old phlogiston theory of Stahl. 

 In 1781 Lavoisier showed that "fixed air" is a compound of carbon and 

 oxygen. Such was the background of rapid progress in the chemistry of 

 gases between 1750 and 1775, which made the discovery of photosynthesis 

 possible, yes, almost inevitable. 



Of three men whose names are associated with this discovery, Priestley, 

 Ingen-Housz and Senebier, two were clerics, like Hales, and one a physi- 

 cian; all were typical amateur naturalists of the Age of Enlightenment. 

 There their similarity ended; for it would be difficult to find men more 

 unlike each other than the militant nonconformist minister, Joseph 

 Priestley, the pompous, brilliant, court physician, Jan Ingen-Housz, who 

 was equally at home in Amsterdam, London, Paris and Vienna, and Jean 

 Senebier, a plodding, provincial pastor from the pious and savant town 

 of Geneva. 



3. The Purification of Air by Plants: Priestley 



Joseph Priestley (1733-1804) was undoubtedly the greatest scientist 

 of the three, although he considered science the least important of his 

 many occupations. His foremost interest was in theology and philoso- 



FiG. 1. — Joseph Prie.stley. 



phy; his ardent nonconformism brought him into perpetual conflict with 

 authorities and into disrepute as a sympathizer with the French Revolu- 

 tion. In the stormy days of 1791, his house in Birmingham was sacked bj' 



