- 2 - 



of which he provided by his own discovery of oxygen. But in the absence 

 of adequate concepts concerning heat, work and energy, Priestley recog- 

 nized that combustion, respiration and photosynthesis presented many 



baffling aspects in addition to the gaseous exchange processes that are 



12 5 

 involved ' ' . 



With conflicting views on religion, on scientific observations, 



and on scientific interpretation, Priestley was also intensely concerned 



with the stirring political changes that were taking place in France and 



in the American Colonieso At the time of his discovery of oxygen, he 



probably could not foresee that these incongruent interests would drive 



him to emigrate to the United States just 20 years later. Nor could he 



know, even by the time of his death in I80i4., that science was destined 



to rank with religion and government in its effect upon human affairs. 



Complexity of Oxygen Revealed by Analysis 



TB.th the development and application of analytical methods, Priest- 

 ley's dephlogisticated air was found to be our most abundant chemical 

 element. It comprises about 89 per cent of the waters of the earth, 

 about half of the rocks and soils, and about one fourth of the atmosphere 

 by weight „ It has become the standard to wnich the chemical atomic 

 weights of all the other elements are referred. As a standard reference 

 for these chemical atomic weights, oxygen was given the atomic weight of 

 16 exactly. But after a century of intensive chemical investigations, 

 the value of the atomic weight assigned to oxygen was fo\xnd to be in 

 error. This error, like the erroneous concept of phlogiston, was based 

 upon limited experimentation and upon concepts that appeared reasonable 

 in the light of the available information. The error in the presumed 



