24 OF THE ASSIMILATION OF CARBON. 



those substances already in the state of oxides, or to the oxida- 

 tion of such vegetable compounds as contain hydrogen in excess. 

 The fallen brown or yellow leaves of the oak contain no longer 

 tannin, and those of the poplar are destitute of balsamic consti- 

 tuents. 



The property possessed by green leaves of absorbing 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 ex- 

 posure 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 ele- 

 mentary 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 necessary to combine with 44*499 

 oxygen in order to form water, is J of this quantity, namely 

 5-56 ; it is evident, therefore, that oak wood contains -fa more 

 hydrogen than corresponds to this proportion. In Pinus larix, 

 P. abies, and P. picea, the excess of hydrogen amounts to -f , and 

 in Tilia europea to f . 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 wood 

 (Diospyros ebenum) 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, 



* When villages situated on the banks of rivers become inundated with 

 floods, this property of wood gives rise to much disease. The wood of the 

 floors and the rafters of the building become saturated with water, which 

 evaporates very slowly. The oxygen of the air is absorbed rapidly by 

 the moist wood, and carbonic acid is generated. The latter gas exercise; 

 a directly pernicious influence when present in air to the amount of 7 oj 

 8 per cent. 



