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Saluce who " demonstrated " that air in which candles had burned 

 out was vitiated by the heat, and could be restored by exposure 

 to extreme cold, and of Hales, who, as late as 1769, taught that 



The scientific successor of Van Helmont was Joseph Priestley, 



Born in 1773, he became pastor of the church at Needham at the 

 age of twenty-five, but was forced to leave the place because of 

 his Unitarian tendencies. He was versed, not only in Latin and 

 Greek, but also in Hebrew, Arabic, Chaldee and Syriac. Among 

 his writings one finds such titles as Theory of Languages, Ora- 

 tory and Criticism, The Constitution and Laws of England, 

 Matter and Spirit, Comparison of Heathen and Christian Philos- 

 ophy, The Doctrine of Necessity, The French Revolution, On the 

 American War. Laughed at in France for being a Christian, he 



atheist. After being attacked by a mob which tore down his 

 house in Birmingham, because of his sympathy with the French 



ings there, as every one feared that the house in which he dwelt 

 would be torn down. Shunned by members of the Royal 

 Society, he took refuge in America, and made discoveries enough 

 in science to make half a dozen men famous. 



" The interrogation point," said DeCandolle, " is the key to all 

 the sciences." With this key Priestley unlocked the door that 

 led to the discovery which became the foundation of both chem- 

 istry and physiology, the discovery of oxygen gas. This dis- 

 covery was celebrated at the grave of Priestley, in Northumber- 

 land, Pa., on August I, 1875, as the starting point of modern 

 chemistry. It was Priestley, also, who discovered the osmosis of 

 gases through a bladder membrane. He rejected Van Helmont's 

 term "gas," as being a needless introduction of a new term, and 

 in its stead employed the word air in a generic sense. 



The discovery of oxygen, in the year 1775, is described in his 

 " Treatise on different kinds of air." Chemists in that day knew 

 that the atmosphere contained "fixed air" (carbon dioxide), 

 "phlogisticated air" (nitrogen), and "phlogiston," a term used 



