DURABILITY. 99 



poles and posts in the ground break eventually at their 

 point of contact with it. 



The surface of wood exposed to flowing water becomes 

 slimy, owing to the action of bacteria, especially Lcptothrix, 

 but wood in this condition is very durable. 



Mechanical attrition by natural agencies occurs in rapid 

 watercourses, such as mountain-torrents carrying sand 

 and gravel, the continual impacts of which on wood wear 

 away its surface. A. similar action takes place near the sea- 

 shore, when the wind blows grains of sand against the wood, 

 as near sand-dunes. Planks and beams that are exposed to 

 the sand resist it best at their hard knots, which eventually 

 protrude beyond the rest of the wood in polished conical 

 projections. 



Wood is converted into peat, or becomes carbonised, after 

 lying for long periods of time in wet peat. It retains its 

 structure, but is converted gradually into soft peat or 

 eventually into lignite, but sometimes may be utilised as 

 bog-oak, etc., hardening considerably after it has been 

 removed from the bog and lias been dried. Wood from 

 forests that have been submerged by the sea resembles bog- 

 wood. In Japan whole forests have been destroyed by 

 volcanic eruptions and buried in ashes or lava. This wood 

 is at first of a silver-grey colour (Indai-wood of the Japanese), 

 but later becomes brown and loses its structure, forming a 

 homogeneous mass like jet (Umoregi). 



It is obvious that in many modes of utilisation of wood a 

 piece of wood may be exposed to two or even three kinds of 

 decay. Thus, bridge-posts are subject to grey decay above 

 water-level and to attrition below it, while a gate-post becomes 

 grey above ground, rotten near the surface of the ground, and 

 humified at its lower end. 



Usually a wood is held to be more durable the longer it 

 resists rottenness and humification ; this is natural durability, 

 as opposed to artificial durability, due to impregnation with 

 antiseptic substances. 



The natural durability of the wood of any species of tree 

 depends in the first place on its position in the tree ; heart- 

 wood is always more durable than sap wood, even heartwood 



H2 



