98 PROPERTIES OF WOOD. 



temperature, insolation, etc., turns grey. The pale tints of 

 freshly utilised wood, as in fences, become darker when its 

 tannin is oxidised, and though this may increase its durability 

 temporarily, the wood becomes gradually grey owing to the slow 

 decomposition of its external layers. The lignin is dissolved 

 first, leaving a substance that is richer in cellulose. The 

 exposed cells are gnawed by various species of wasps in order 

 to obtain wood-pulp for their nests. 



Softwoods turn grey earlier than hardwoods, and spring- 

 wood is attacked before summer-wood, hard knots and resinous 

 parts of the wood offering most resistance. Local climate 

 affects this decomposition greatly, for in maritime and 

 mountain climates wood (e.g., shingles on roofs and walls of 

 buildings) becomes grey much sooner than in dry continental 

 countries. Thus, in North America, Weymouth pine shingles 

 last for about five years near the Atlantic coast, but for ten 

 years and more in the Prairies. Planed planks resist greyness 

 longer than planks that are merely sawn. 



Humification, or eremacausis, affects wood when it is 

 exposed constantly to atmospheric humidity and obtains 

 insufficient supplies of oxygen. Wood buried in the ground, 

 or in mines, shafts, ship-cabins, cellars, inside hollow trees, 

 etc., is exposed chiefly to this form of decay, which, besides 

 being due to chemical decomposition, is also assisted by 

 fungi ; the final product is damp, powdery, brown mould. In 

 such places, according to Mayr, fungi appear whenever the 

 relative humidity of the air attains 70 per cent. ; when the 

 air is less moist and fungi are absent the wood decomposes 

 more slowly. 



Rottenness is the decomposition of wood by the agency of 

 fungi, when the wood being supplied fully with atmospheric 

 oxygen is exposed also from time to time to humidity. All 

 wood in contact with the ground is in this condition, 

 e.g., floors, posts, or railway-sleepers. The eventual product 

 is a rotton, moist, pale or dark brown substance, with a 

 fracture partly fibrous and partly crumbling, and is scented 

 like humus or fungi. Wherever the variations of moisture 

 are greatest, as just above, at, or just below, the surface of 

 the ground, rotting is most rapid and continuous. Stakes, 



