2(5 ON THE ASSIMILATION OS CARBON. 



oxygen has any connexion with the process of assimilation, nor 

 have they the slightest relation to one another ; the one is a 

 purely mechanical, the other a purely chemical process. A 

 cotton wick, inclosed in a lamp containing a liquid saturated 

 with carbonic acid, acts exactly in the same manner as a living 

 plant in the night. Water and carbonic acid are sucked up by 

 capillary attraction, and both evaporate from the exterior part 

 of the wick. 



Plants living in a moist soil containing humus exhale much 

 more carbonic acid during the night than those growing in dry 

 situations ; they also yield more in rainy than in dry weather ; 

 these facts point out to us the cause of the numerous contra- 

 dictory observations made with respect to the change impressed 

 upon the air by living plants, both in darkness and in common 

 daylight ; but these contradictions are unworthy of considera- 

 tion, as they do not assist in the solution of the main question. - 



There are other facts which prove in a decisive manner that 

 plants yield more oxygen to the atmosphere than they extract 

 from it. These proofs may easily be obtained, without having 

 recourse to any peculiar arrangements, from observations made 

 on plants living under water. 



Pools and ditches, the bottoms of which are covered with 

 growing plants, often freeze upon their surface in winter, so 

 that the water is completely excluded from the atmosphere by 

 a clear stratum of ice ; under such circumstances 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 when the rays of the sun fall upon the ice ; they 

 are very small at first, but collect under the ice and form largei 

 bubbles. They consist of pure oxygen gas. Neither during 

 the night, nor during the day when the sun does not shine, are 

 they observed to diminish in quantity. The source of this 

 oxygen is the carbonic acid absorbed by the plants from the 

 water, to which it is again supplied by the decay of vegetable 

 substances 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 exhaled, is not again absorbed. 



