138 DECAY OF WOODY FIBRE. 



CHAPTER XL 



Decay of Woody Fibre. 



The conversion of woody fibre into the substances termed hu- 

 mus and mould is, on account of its influence on vegetation, one 

 of the most remarkable processes of decomposition in nature. 



Decay is not less important in another point of view ; for, 

 by means of its influence on dead vegetable matter, the oxy- 

 gen retained by plapts di ring life is again restored to the atmo- 

 sphere. 



The decomposition of woody fibre is effected in three forms, 

 the results of which are different, so that it is necessary to con- 

 sider each separately. 



The first takes place when it is in the moist condition, and 

 subject to free uninterrupted access of air ; the second occurs 

 when the air is excluded ; and the third when the wood is covered 

 with water, and in contact with putrefying organic matter. 



It is known that woody fibre may be kept under water, or in 

 dry air, for thousands of years, without suffering any appreci- 

 able change ; but that when brought into contact with air, in the 

 moist condition, it converts the oxygen surrounding it into the 

 same volume of carbonic acid, and is itself gradually changed 

 into a yellowish-brown, or black matter, of a loose texture. Ac- 

 cording to the experiments of De Saussure, 240 parts of dry 

 sawdust of oak-wood convert 10 cubic inches of oxygen into the 

 same quantity of carbonic acid, which contains 3 parts, by weight, 

 of carbon ; while the weight of the sawdust is diminished by 15 

 parts. Hence, 12 parts, by weight, of water, are at the same 

 time separated from the elements of the wood. 



Carbonic acid, water, and mould or humus, are therefore the 

 products of the decomposition of wood. We have assumed that 

 the water is formed by the combination of the hydrogen of th« 



