ABSORPTION OF GASES. 75 



hence, since the amount of a gas dissolved from a mixture 

 is proportionate to the relative volume of it in the mixture 

 multiplied into its coefficient of solubility, the proportions of 

 these gases dissolved by the cell-sap of the cell under con- 

 sideration will be, at a temperature of I5C. and 760 mm. 

 pressure, 



Oxygen 0-6183 



Nitrogen .....1-1692 



Carbon dioxide O'O/j-OO 



and these may be assumed to be the proportions in which 

 these gases are at first absorbed by the cell. 



But it has been abundantly proved that whereas when the 

 cells of a plant are saturated with nitrogen no further ab- 

 sorption of this gas takes place, the absorption of oxygen 

 and carbon dioxide is continuous provided that the con- 

 ditions are favourable. The total quantity of these gases 

 absorbed in a given time is far greater than that of nitrogen ; 

 in fact the amount of nitrogen absorbed by a plant is nil, 

 inasmuch as the cell-sap is saturated with it from the first. 

 From this we may conclude that oxygen and carbon 

 dioxide undergo chemical change after their absorption, that 

 they enter into the metabolic processes of the living cells, 

 whereas nitrogen does not. 



Absorption of Oxygen. 



That plants absorb oxygen is a fact which has long been 

 known. Scheele and Priestly both found that, under certain 

 conditions which they did not investigate, plants deteriorate the 

 quality of air ; but Ingenhousz was the first to clearly define 

 the relation of the plant to the atmosphere. With a more 

 developed chemical science at his disposal, de Saussure was 

 able to establish definitely the importance of oxygen for the 

 life of plants, and his results have been confirmed by all sub- 

 sequent observers. 



There are, however, lowly organised plants which can 

 live, for a considerable time at least, without being supplied 



