Chapter 2 



THE DISCOVERY OF PHOTOSYNTHESIS * 



1. Precursor: Stephen Hales 



Stephen Hales (1677-1761), the minister-naturalist of Teddington, 

 England, an illustrious contemporary of Newton, made numerous ex- 

 periments on the evolution and absorption of gases by different sub- 

 stances of animal or vegetable origin, and decided thereupon that air 

 must be an important constituent of all organic matter. (He meant by 

 this, that air, one of the four original elements of Aristotle, must be added 

 to the list of alchemistic "first principles" — mercury, oil, salt, sulfur — 

 which were supposed at that time to contribute to the composition of all 

 material bodies.) Discussing further, in his Vegetable Staticks (1727), the 

 importance of leaves for plants. Hales wrote: "Plants very probably draw 

 through their leaves some part of their nourishment from the air," and 

 he added, "may not light also, by freely entering surfaces of leaves and 

 flowers, contribute much to ennobling the principles of Vegetables?" 



2. The Background : The Birth of Pneumochemistry 



No further elaboration of Hales' vision was possible until the existence 

 and nature of different kinds of "air" became known through the work 

 of the great "pneumochemists" of the second half of the eighteenth 

 century. Black discovered "fixed air" (that is, carbon dioxide) in 1754; 

 Scheele prepared chlorine in 1774, and found in 1773 that atmospheric 

 air is composed of two gases, one inert and the other capable of main- 

 taining combustion. (This observation was not made public until 1777, 

 thus depriving Scheele of the priority in the discovery of oxygen.) In 

 1775, Priestley obtained "dephlogisticated air" (that is, oxygen) from 

 mercurous oxide; and later the same author described nitrous oxide, sulfur 

 dioxide, hydrochloric acid gas, and carbon monoxide. "Inflammable 

 air" (that is, hydrogen, although, for a while, methane was often con- 

 fused with it) was discovered by Cavendish in 1766; in 1784, the same 

 author proved that water is a compound of hydrogen and oxygen. Be- 

 tween 1772 and 1782, Lavoisier discovered the composition of the air 

 and propounded the new doctrine of oxidation and respiration, interpret- 

 ing the.se processes as combinations of different substrates with oxygen, 



* Bibliography, page 27. 



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