34 OF THE ASSIMILATION OF CARBON. 



they are very small at first, but collect under the 

 ice and form larger bubbles. They consist of pure 

 oxygen gas. Neither during the night, nor during 

 the day when the sun does not shine, are they ob- 

 served to diminish in quantity. The source of this 

 oxygen is the carbonic acid dissolved in the water, 

 which is absorbed by the plants, but is again sup- 

 plied to the water, by the decay of vegetable sub- 

 stances contained in the soil. If these plants absorb 

 oxygen during the night, it can be in no greater 

 quantity than that which the surrounding water 

 holds in solution, for the gas, which has been ex- 

 haled, is not again absorbed. The action of water- 

 plants cannot be supposed to form .an exception 

 to a great law of nature, and the less so, as the 

 different action of aerial plants upon the atmosphere 

 is very easily explained. 



The opinion is not new that the carbonic acid of 

 the air serves for the nutriment of plants, and that 

 its carbon is assimilated by them ; it has been 

 admitted, defended, and argued for, by the soundest 

 and most intelligent natural philosophers, namely, 

 by Priestley, Sennebier, De Saussure, and even by 

 Ingenhouss himself. There scarcely exists a theory 

 in natural science, in favour of which there are 

 more clear and decisive arguments. How, then, are 

 we to account for its not being received in its full 

 extent by most other physiologists, for its being 

 even disputed by many, and considered by a few as 

 quite refuted ? 



