308 DECAY 



DECAY OF WOODY FIBRE. 



The conversion of woody fibre into the sub- 

 stances termed humus and mould is, on account 

 of its influence on vegetation, one of the most re- 

 markable processes of decomposition which occur 

 in nature. 



Decay is not less important in another point of 

 view ; for, by means of its influence on dead vege- 

 table matter, the oxygen which plants retained 

 during life is again restored to the atmosphere. 



The decomposition of woody fibre is effected in 

 three forms, the results of which are different, so 

 that it is necessary to consider each separately. 



The first takes place when it is in the moist con- 

 dition, and subject to free uninterrupted access of 

 air ; the second occurs when 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 appreciable change ; but that when 

 brought into contact with air, in the moist condi- 

 tion 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. 



According 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 



