10 CHAP. II. EVOLUTION OF PURE OXYGEN UNDER LIGHT 



the lower solubility of nitrogen, the nitrogen and oxygen 

 absorbed by water are as 2 : i. 



Table II. — Proportions of Gases present in iooo c.c. of Pond Water 



The intercellular spaces of the plant are rilled with 

 nitrogen and oxygen dissolved in water, which are carried 

 away with the oxygen evolved in photosynthesis. The gas 

 given out in two different experiments was analysed and 

 found to contain the following proportions of oxygen and 

 nitrogen in 10 c.c. of evolved gas. 



First experiment 

 Second experiment 



Oxygen, 4-29 c.c. ; Nitrogen, 5-71 c.c. 

 Oxygen, 4-2 ,, ; Nitrogen, 5-8 



The proportion of oxygen to nitrogen was thus about 4 • 2 

 to 5-8. 



No carbon dioxide was found in the evolved gas ; the 

 escape of C0 2 from its solution in water takes place only 

 when the quantity of the gas in solution is excessive. 



Evolution of Pure Oxygen 



The first problem is to secure that the evolved gas shall 

 be pure oxygen without any admixture of nitrogen, since 

 the photosynthetic activity can only be accurately estimated 

 by the rate of evolution of the pure gas. The difficulty has 

 been met by placing the plant in a beaker filled with pond- 

 water under the receiver of an air-pump. A partial vacuum 

 is produced which removes the air contained in the plant 

 and in the water ; this should be done slowly, for too sudden 



