EVOLUTION OF CARBONIC ACID DURING THE NJGHT. 25 



upon the presence of constituents, in part soluble, and in part 

 insoluble, such as resin and other matters, containing a large 

 proportion of hydrogen : the hydrogen of such substances being 

 in the analysis of the various woods added to that of the true 

 woody fibre. 



It has previously been mentioned that mouldering oak wood 

 contains carbon and the elements of water, without any excess 

 of hydrogen. If, in its present state, its further decay does not 

 alter the volume of the air, it is certain that in the beginning of 

 the process the result must have been different, for the amount 

 of hydrogen present in the fresh wood has been diminished, and 

 this could only have been effected by an absorption of oxygen. 



Most vegetable physiologists have connected the emission of 

 carbonic acid during the night with the absorption of oxygen 

 from the atmosphere, and have considered these actions as a true 

 process of respiration in plants, similar to that of animals, and, 

 like it, having for its result the separation of carbon from some 

 of their constituents. This opinion has a very weak and un- 

 stable foundation. 



The carbonic acid, which has been absorbed by the leaves 

 and by the roots, together with water, ceases to be decomposed 

 on the departure of daylight ; it is dissolved in the juices which 

 pervade all parts of the plant, and escapes every moment 

 through the leaves in quantity corresponding to that of the water 

 which evaporates. 



A soil in which plants vegetate vigorously, contains a certain 

 quantity of moisture indispensably necessary to their existence. 

 Carbonic acid, likewise, is always present in such a soil, 

 whether it has been abstracted from the air, or has been gene- 

 rated 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 

 ahsorbing by their roots moisture, and, along with it, air or car- 

 bonic acid. Is it, therefore, surprising that the carbonic acid 

 should be returned unchanged to the atmosphere along with 

 water, in the absence of light ; for this is known to be the cause 

 of the fixation of its carbon 1 



Neither this emission of carbonic acid nor the absorption of 

 3 



