EVOLUTION OF CARBONIC ACID DURING THE NIGHT. 53 



wood (Diospyros Eherium) the oxygen and hydrogen 

 are in exactly the same proportion as in water. 



The difference between the composition of the 

 varieties of wood, and that of simple woody fibre, 

 depends, unquestionably, upon the presence of con- 

 stituents, in part soluble, and in part insoluble, such 

 as resin and other matters, which contain a large 

 proportion of hydrogen : the hydrogen of such sub- 

 stances being in the analysis of the various woods 

 superadded 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. But the propor- 

 tions of its constituents must necessarily have been 

 different, if the volume of the air had not changed 

 during its decay, because the proportion of hydrogen 

 in those component substances of the wood which 

 contained it in excess is here diminished, and this 

 diminution could only be effected by an absorption 

 of oxygen, and consequent formation of water. 



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 respi- 

 ration 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 unstable 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 day- 

 light ; 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, con- 

 tains a certain quantity of moisture which is indis- 

 pensably necessary to their existence. Carbonic 

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

 whether it has been abstracted from the air or has 



