144 Defects of Timber'. — Qualities desirable in Fuel. 



no known prevention, and in many cases there are no means of 

 knowing their presence until the tree is cut. So far as caused or 

 increased by rapid seasoning, they may be prevented by cutting 

 only in the season when vegetation is dormant, and by drying 

 slowly in the shade. 



581. Wherever the bark becomes loose on a tree, to that extent 

 the wood looses its vitality under it, and it can only be healed by 

 the growing over of the wood from the side. These accidents may 

 be caused by external injuries, scorching from fires, sun-scalds, or 



• frost when the wood is full of sap. When the wood from opposite 

 sides meets over such a place, it will in two or three years consoli- 

 date entirely, and form continuous layers over it, but the dead 

 wood within, if not previously decayed, remains unchanged. These 

 bare spots should be covered over with coal-tar or paint, if the tree 

 is valuable, but if there are decayed places these should first be 

 cleaned out, and the surface should be dry when the tar or paint is 

 applied. 



582. In some cases wood in the first stages of decay shows a 

 change of color, before its strength or texture is impaired, as in 

 some species of the pine. It will generally be stopped by drying, 

 and may not afiect its value. The tint of wood is in a great de- 

 gree influenced by the nature of the soil upon which it has grown. 

 The larch and the fir on a dry, level, and deep sandy soil, are often 

 of a slightly reddish tinge, while in other situations they are nearly 

 white. It is probably from some chemical differences in the soil 

 that mahogany and other tropical woods of the same species are of 

 deeper shades of color in some localities than in others. 



CHAPTEK XrV. 



FUEL — CHARCOAL — WOOD-GAS. 



The Qualities desirable in Wood for Fuel. 



583. The best qualities of wood for fuel are found when it bums 

 easily and uniformly — neither too fast nor too slow — and when in 

 a given volume, it gives the greatest amount of heat. These qual- 

 ities are found, in highest degree at full maturity, and before decline. 

 In coppice-woods, grown from the sprouts, this quality is reached 

 sooner than from seed, and in those from old stocks sooner than 



