DE SAUSSURE 23 



in the common air. In his introduction to the German translation of 

 Ingen-Housz' pamphlet, von Humboldt communicated analyses which 

 purported to show the constant occurrence of carbon dioxide in common 

 air, in concentrations from 0.7 to 1.4%. The true carbon dioxide content 

 of the air — which is fairly constant at 0.03% in the open country, but 

 may rise considerably above this value in the neighborhood of populated 

 places, industrial establishments and active volcanoes — was established 

 early in the next century by Dalton, de Saussure, and Boussingault 

 (c/. the review by Letts and Black, 1900). 



After the appearance of Ingen-Housz' Food of Plants, the problem of 

 oxygen liberation by illuminated plants became merged with the problem 

 of carbon assimilation from the air and the synthesis of organic matter. 

 The two terms under which this process is generally known, ''photosyn- 

 thesis" and "assimilation" have their origin in these two aspects of the 

 problem. Neither is entirely satisfactory, since "photosynthesis" could 

 etymologically mean any synthesis under the influence of light, and "as- 

 similation" (or even "assimilation of carbon") covers an even wider 

 variety of phenomena. We shall use the term "photosynthesis" because 

 it has practically acquired a very definite meaning and is only seldom ap- 

 plied to any photochemical reaction other than the synthesis of organic 

 matter by plants in light. 



In the light of Ingen-Housz' realization of the nutritive role of "air 

 refining" by plants, we can again rewrite equation (2.2), in the form 



plant 

 (2.3) Carbonic acid + light > organic matter + oxygen 



7. The Participation of Water : de Saussure 



Quantitative treatment soon proved equation (2.3) to be incomplete. 

 The recognition of this incompleteness came when weighing was added to 

 volume measuring in the study of photosynthesis. This progress was due 

 to another scholar from Geneva, Nicolas Theodore de Saussure (1767- 

 1845) (son of the physicist w^ho invented the hygrometer), a quiet and 

 retiring man and a skillful and conscientious experimentalist. His results 

 were published in 1804, under the title, Recherches chimiques sur la vegeta- 

 tion. This is the first modern book on plant nutrition, full of careful 

 analyses of gases, humus, and ash, and almost devoid of any speculations, 

 or even hypotheses. The measurements of de Saussure proved definitely 

 the correctness of Ingen-Housz' doctrine of aerial nutrition, and showed 

 what elements the plants acquire from the soil. They confirmed the 

 surmise of Senebier, that i)lants find enough nourishment in the small 

 amount of carbon dioxide regularly present in the air, and showed that 

 this is the only source of their carbon supply. De Saussure made the first 

 comparison of the amounts of carbon dioxide absorbed and of oxygen 



