276 THE UTILISATION OF WOODLAND PKODUCE. 



Older wood is harder than younger wood in trees, and dry 

 wood generally harder than green wood, though heavy, hard 

 woods like Oak, Ash, Beech, Elm, Maple, and Sycamore are 

 easier to work when still moist than when dry. The tougher 

 the wood, the harder it is to saw, owing to the resistance offered 

 to the teeth of the saw in tearing the fibres apart. As regards 

 sawing, our woods may be classified thus : 



Hard to saw : Lime, Willow, Poplar. 



Medium : Birch, Oak, Alder, and resinous Larch and Pine. 



Easy to saw : Conifers. 



As a rule, woods with a specific gravity below 0*65 when 

 seasoned are classed as Softwoods, these including Aspen and 

 other Poplars, Willows, Lime, Horse-Chestnut, Alder, and Birch 

 (seasoned sp. gr. 0'64, though hard to cut, and often classed 

 as a hardwood) ; while the others are called Hardwoods Oak, 

 Ash, Elm, Sycamore, Beech, &c. 



5. DURABILITY means the length of time timber continues 

 sound and serviceable ; and this, of course, varies greatly 

 according to how and where it is used. But it depends mainly 

 on the extent to which the wood is exposed to alternating 

 dampness and dryness, especially during the warmer months 

 of the year ; for it is then that timber-boring insects (Anobium, 

 Ptilinus, Lymexylon, &c.) and saprophytic fungi (Polyporus, 

 Agaricus, Merulius, &c.), which are chiefly instrumental in 

 decomposing the woody tissues and the ligneous substances, 

 find the most favourable conditions for feeding and breeding. 

 The mere weight of wood gives no true indication of its dura- 

 bility ; because the larger the quantity of albuminoid substances 

 contained in any kind of wood, the more likely it is to be 

 attacked by fungi and insects which feed on these ; but, for one 

 and the same kind of wood (whether Oak, Beech, Larch, Pine, 

 &c.), the heavier it is the more durable, owing to the higher 

 proportion of lignin and preservative substances per unit of 

 volume. Oak and other hardwoods (except Beech) are usually 

 much more durable than softwoods, and Larch lasts longer 



