The Evolution of Oxygen. 97 



(1804) who first attempted to obtain quantitative data as to the 

 relation between the amount of oxygen evolved and the carbon- 

 dioxide absorbed by tlie assimilating plant. 



Saussure found that the volume of oxygen evolved in a given 

 time by the plant was less than the volume of carbon dioxide 

 absorbed, and he c ame to the conclusion that part of the oxygen 

 of_the_carhon^dioxide was used in assimilation. It is strange that 

 Saussure should be so often quoted as the discoverer of the fact 

 that the volume of oxygen given out by the assimilating plant is equal 

 to the carbon dioxide absorbed. Thus even Sachs (1882 or see 1887) 

 says " As/aii essential point, it is at the same time to be insisted 

 upon here that the volume of oxygen evolved is equal to the volume 

 of the carbon dioxide taken in, as de Saussure and, later and more 

 exactly, Boussingault have already established." It is therefore 

 worth while to quote Saussure's conclusion in his own words, " II 

 resulte de toutes ces experiences, que les plantes, en decomposant 

 le gaz acide carbonique, s'assimilent une partie du gaz oxyg^ne qui 

 y est contenu." 



As Saussure's work is perhaps not always easily accessible, it 

 may be worth while to give his actual results here. 



Saussure placed a suitable number of plants in a large vessel 

 containing an artificial atmosphere comprising about 21% oxygen 

 and the rest nitrogen, to which was then added carbon dioxide. 

 The plants were then exposed to sunlight on a number of successive 

 days (6 to 18) and at the end of the period the gas in the vessel 

 was analysed. 



The following numbers were obtained with 7 plants of periwinkle 

 (Vinca minor). 



Before. After. 



Nitrogen ... 4199 c.c. ... 4338 c.c. 



Oxygen ... 1116 „ ... 1408 „ 



Carbon dioxide 431 „ ... „ 



5746 5746 



Oxygen evolved ... ... ... 292 c.c. 



Carbon dioxide absorbed ... 431 „ 



Oxygen absorbed by plant ... 139 „ 



In the following table are summarised all Saussure's experi- 

 ments on this subjeet. 



