54 OF THE ASSIMILATION OF CARBON. 



been generated by the decay of vegetable matter. 

 Rain and well water, and also that from other 

 sources, invariably contains carbonic acid. — Plants 

 during their life constantly possess the power of 

 absorbing by their roots moisture, and, along with 

 it, air and carbonic acid. Is it, therefore, surprising 

 that the carbonic acid should be returned unchanged 

 to the atmosphere, along with water, when light 

 (the cause of the jfixation of its carbon) is absent? 



Neither this emission of carbonic acid nor the- 

 absorption of oxygen has any connexion with the 

 process of assimilation ; nor have they the slightest 

 relation to one another; the one is a purely me- 

 chanical, the other a purely chemical process. A 

 cotton wick, inclosed in a lamp, which contains 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 which live in a soil containing humus exhale 

 much more carbonic acid during the night than those 

 which grow in dry situations ; they also yield more 

 in rainy than in dry weather. These facts point out 

 to us the cause of the numerous contradictory 

 observations, which have been made with respect to 

 the change impressed upon the air by living plants, 

 both in darkness and in common daylight, but 

 which are unworthy of consideration, 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 atmo- 

 sphere than they extract from it ; these proofs, 

 however, are to be drawn with certainty only from 

 plants which live under water. 



When pools and ditches, the bottoms of which 

 are covered with growing plants, freeze upon their 

 surface in winter, so that the water is completely 

 excluded from the atmosphere by a clear stratum of 

 ice, small bubbles of gas are observed to escape, con- 



