PROPERTIKS OF WOOD il7 



this reason it is excellent for mine timber. Resistance to tension, like 

 resistance to crushing, varies according to species. 



Splitting. — The splitting quality is greater in straight fibred woods 

 free from knots. Temperature also affects this property ; below de- 

 grees, wood splits with difficulty. Heat and moisture both favor 

 splitting. Hard woods are more difficult to split than soft woods, and 

 trunk wood splits better than branch wood. 



Durability. — The durability of woods varies with species, with en- 

 vironment, and with antiseptic substances which they contain, either 

 naturally or by impregnation. Sapwood lasts a much shorter time than 

 heartwood. Coppice oaks last longer than those grown in a high forest. 

 Immersion in ^yater removes alterable substances from wood and in- 

 creases its durability. 



Combustibility. — Combustibility is the capacity of wood to catch fire 

 at 300° (Cent.) when in contact with the air. Resinous woods, be- 

 cause of their resin, and soft broadleaf woods, because of their porosity 

 and their oxygen content, burn more easily than hardwoods. The more 

 carbon and the less hydrogen wood contains, the more ash it gives. 

 This is true of black, Scotch and maritime pine. The available heat- 

 producing effect of dry wood is only 39 per cent of the total heat pro- 

 duced by combustion ; about two-thirds of this heat is lost. Soil and 

 exposure influence the heating quality of wood. Thus, oak growing on 

 limestone soil is of superior heating quality; the heat-producing ca- 

 pacity of woods grown on southerly or easterly exposures is greater 

 than that of woods exposed to the west or north. This quality is 

 greater in branch wood than in log wood, and its maximum is found 

 in stump wood. The heat-producing capacity of woods is nearly in 

 proportion to their density, and the heat-radiating capacity is greater 

 in hard woods than in soft and resinous woods. It is uniform for all 

 woods when broken up into small pieces. Bakeries, glass factories 

 and pottery factories require wood which burns quickly and at high 

 temperature, such as resinous wood, aspen, alder, birch, linden ; on the 

 other hand, slow-burning woods with high radiating power, such as 

 oak, hornbeam and beech, are preferred for domestic uses. 



Characteristics of Different Species. — Hardwoods contain much 

 autumn wood — that is, much fibrous tissue. They have a horny ap- 

 pearance and are fine grained, taking a high polish. Pedunculate oak 

 growing in coppice with standards on tertiary or quaternary alluvial 

 deposit produces hard, dense, elastic wood, resistant to crushing, but 



