224 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



which sets in on the exposure of the wood surface to 

 the alternate wetting and drying in contact with the 

 atmosphere : it is known that wood oxidizes under 

 such circumstances, and we may be sure that wounds 

 are no exception to this rule. The surface of the 

 wood gradually turns brown, and the structure of the 

 timber is destroyed as the process extends. 



The difficulty always arises in Nature, however, 

 that mould-fungi and bacteria of various kinds soon 

 co-operate with and hurry these processes, and it is 

 impossible to say how much of the decay is due to 

 merely physical and chemical actions, and how much 

 to the fermentative action of these organisms. We 

 ought not to shut our eyes to this rich field for investi- 

 gation, although for the present purpose it suffices to 

 recognize that the combined action of the wet, the 

 oxygen of the air, and the fermenting action of the 

 moulds and bacteria, &c., soon converts the outer 

 parts of the wood into a mixture of acid substances 

 resembling the humus of black leaf-mould. 



Now as the rain soaks into this, it dissolves and 

 carries down into the wood below certain bodies which 

 are poisonous in their action on the living parts of the 

 timber, and a great deal of damage may be caused by 

 this means alone. But this is not all : as soon as the 

 decaying surface of the wound provides these mixtures 





