52 OF THE ASSIMILATION OF CARBON. 



with exclusion of light, then moistened with water, 

 and placed under a glass globe filled with oxygen, 

 they are found to absorb that gas in proportion as 

 they change in color. The chemical nature of this 

 process is thus completely established. The diminu- 

 tion 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 vege- 

 table compounds which contain it in excess. 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 w^ood, 

 dried by exposure to the atmosphere and then moist- 

 ened, converts the oxygen into carbonic acid, with- 

 out change of volume ; fresh wood, therefore, absorbs 

 most oxygen. 



MM. Petersen and Schodler have shown, by the 

 careful elementary analysis 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 I of this quantity, namely, 5*56 ; it is evident, 

 therefore, that oak wood contains ^ more hydrogen 

 than corresponds to this proportion. In Pinus 

 Larix, P. Abies, and P, picea, the excess of hydro- 

 gen amounts to }, and in Tilia europcBa to \, The 

 quantity of hydrogen stands in some relation to the 

 specific weight of the wood; the lighter kinds of 

 wood contain more of it than the heavier. In ebony 



