48 PATHFINDERS OF PHYSIOLOGY 



Priestly was thirty years old when Franklin was sixty. Priestly 

 like Franklin was well informed on a variety of subjects. He wrote 

 learnedly on politics, religion and on science, particularly on pneu- 

 matic chemistry. Boswell dubbed him a "literary Jack-of-all-trades," 

 and he was busy with proof sheets until the day of his death. His 

 pamphlets on politics and religion were so much opposed by the orth- 

 odox theologians of his day that they answered his arguments by 

 burning his house and dispoiling his belongings, a peculiar way that 

 the so-called orthodox theology has had in the past of dealing wih 

 those bold intrepid spirits who have dared to stand for what they 

 believed to be the truth. His home surroundings in Birmingham 

 became so unpleasant that in self-defense he set sail for America, 

 here to breathe the atmosphere of civil and religious freedom. He 

 was offered the professorship of chemistry in the University of Phila- 

 delphia, but the following year moved to Northumberland, a town on 

 the Susquehanna, a hundred and thirty miles northwest of Philadel- 

 phia. He lived and worked until his death, which occurred in Feb- 

 ruary, 1804. 



Priestly endeavored to change back to its original condition, air 

 that had been breathed, or which had failed to support the flame of a 

 candle. He eventually succeeded by means of vegetation. First he 

 experimented by placing a sprig of mint into a glass jar standing in- 

 verted over a vessel of water. Parenthetically, Priestly invented the 

 pneumatic trough, which has been found so convenient in experiment- 

 ing with gases. When the sprig of mint had been growing some 

 months, the air within the vessel would not extinguish a flame nor 

 act deleteriously to small animals, such as the mouse, placed therein. 

 The growing plant really contributed to the flame or the animal that 

 was placed in the vessel. Further experiment showed that a growing 

 plant placed in a vessel in which a flame had been extinguished would 

 in time render the atmosphere in the jar capable of supporting either 

 flame or animal life. This lead him to conclude: "That plants, in- 

 stead of affecting the air in the same manner with animal respiration, 

 reverse the effects of breathing and tend to keep the atmosphere 

 sweet and wholesome when it is become noxious in consequence of 

 animals either living and breathing or dying and putrifying in it." 



Priestly's researches might have been more fruitful in results 

 had he not been dominated by the phlogiston theory, a term devised 

 by^ Stahl. Phlogiston, from phlogistos, burnt, was a hypothetical 

 principle of fire regarded as a material substance. Every combustible 

 substance was a compound of phlogiston and the phenomenon of com- 

 bustion was due to a separation of the compound into its component 

 elements. 



Priestly was able to obtain the same gas by heating mercuric 

 oxide, and from red precipitate. But he could not get away from 

 the phlogiston theory. Air supported combustion because it took up 

 phlogiston given out by the burning body. Common air supported 

 combustion in proportion as it was free from phlogiston. He pre- 

 pared oxygen in 1774, that is he discovered that the gas he prepared 

 was part of the common air, which supported life and combustion. 

 Venous blood was blood laden with phlogiston. Blood exposed to de- 

 phlogisted air gave up its phlogiston and became bright arterial blood. 



