358 DECAY OF WOODY FIBRE. 



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

 table matter, the oxygen which plants retained dur- 

 ing life is again restored to the atmosphere. 



The decomposition of w^oody 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 \vhen it is in the moist con- 

 dition, 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 appreciable change ; but that when 

 brought into contact with air, in the moist con- 

 dition 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.* 



It has already been mentioned, that pure woody 

 fibre contains carbon and the elements of water. 

 Humus, however, is not produced by the decay of 

 pure woody fibre, but by that of wood which contains 

 foreign soluble and insoluble organic substances, be- 

 sides its essential constituents. 



The relative proportions of the component elements 

 are, on this account, different in oak wood and in 

 beech, and the composition of both of these differs 

 very much from woody fibre, which is the same in 

 all vegetables. The difference, however, is so triv- 

 ial, that it may be altogether neglected in the con- 

 sideration of the questions which will now be brought 

 under discussion ; besides, the quantity of the for- 

 eign substances is not constant, l^ut varies according 

 to the season of the year. 



* According to the experiments of De Saussure, 240 parts of dry 

 saw-dust of oak wood convert 10 cubic inches of oxygen into the same 

 quantity of carbonic acid, which contains 3 parts, by weight, of car- 

 bon ; 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. — L. 



