30 OF THE ASSIMILATION OF CARBON, 



diminution of the gas which occurs, can only be 

 owing to the union of a large proportion of oxygen 

 with those substances which are already in the state 

 of oxides, or to the oxidation of the hydrogen, in 

 those vegetable compounds which contain it in ex- 

 cess. The fallen brown or yellow leaves of the oak 

 contain, no longer, tannin, and those of the poplar 

 no balsamic constituents. 



The property which green leaves possess, of ab- 

 sorbing oxygen, belongs also to fresh wood, whether 

 taken from a twig, or from the interior of the trunk 

 of a tree. When fine chips of such wood are placed 

 in a moist condition, under a jar filled with oxygen, 

 the gas is seen to diminish in volume. But, wood, 

 dried by exposure to the atmosphere and then 

 moistened, converts the oxygen into carbonic acid, 

 without change of volume ; fresh wood, therefore, 

 absorbs most oxygen. 



MM. Petersen and Schodler have shown, by the 

 careful elementary analyses of 24 different kinds of 

 wood, that they contain carbon and the elements of 

 water, with the addition of a certain quantity of 

 hydrogen. Oak wood, recently taken from the 

 tree, and dried at 100 C. (212 F.), contains 49.432 

 carbon, 6.069 hydrogen, and 44.499 oxygen. 



The proportion of hydrogen, which is necessary 

 to combine with 44.498 oxygen in order to form 

 water, is ^ of this quantity, namely 5.56 ; it is evi- 

 dent, therefore, that oak wood contains ^ more 

 hydrogen than corresponds to this proportion. 



