54 ON CARBON. 



which the gas is absorbed. The chemical 

 nature of this process is thus completely esta- 

 blished. 



There are other facts which prove in a de- 

 cisive manner that plants yield more oxygen 

 to the atmosphere than they consume, but 

 these proofs must be drawn from those plants 

 which live under water. 



'' When pools and ditches, the bottoms of 

 which are covered with plants, freeze on their 

 surface in winter, so that the water is com- 

 pletely excluded from the air by a stratum of 

 ice, small bubbles of gas are observed to escape 

 continually during the day from the points of 

 the leaves and twigs. These bubbles are seen 

 most distinctly during the day when the sun's 

 rays fall on the ice, they are very small at 

 first, but eventually form large bubbles. These 

 consist of pure oxygen gas, and neither during 

 the night nor during the day even, when the 

 sun does not shine, are they observed to dimi- 

 nish in quantity. The source of this oxygen 

 is the decomposition of the carbonic acid gas 

 by the plant, the carbon of which is assimilated, 

 and the oxygen evolved, and the water is again 

 supplied with carbonic acid by the continuous 

 decay of vegetable substances contained in the 

 soil." 



The principal sources of carbon, and the 

 manner in which it is assimilated by plants in 

 the beautiful system of nature have now been 

 considered, and yet many questions may arise 

 respecting it; one of the most obvious is, that 

 if plants are intended by nature to perform the 



