EXHALATION OF OXYGEN BY PLANTS. 33 



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 

 day-light, 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 at- 

 mosphere 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, 

 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 ; 



D 



