DENSITY AND WEIGHT. 27 1 



expanding, and (4) its freedom from defects and unsoundness. 

 All these physical properties continually act and react on each 

 other; for the dryness or moistness of the wood affects its 

 density or specific weight as well as its total volume, and also 

 the direction in which shrinkage and expansion occur under 

 dry and moist conditions of the atmosphere. 



1. As KEGARDS DENSITY AND WEIGHT, the Specific Weight of 



the pure woody substance (exclusive of lumina and other hollow 

 spaces) in our woodland trees averages about 1*5 both for heart- 

 wood and sapwoocl ; but the specific gravity per cubic foot of 

 wood varies greatly for different kinds of trees, and for any 

 given kind it also varies greatly in the green and the seasoned 

 conditions. For technical purposes it is only the seasoned 

 weight that is important, as green timber is not used. As to 

 average weight when seasoned, British timber may be classified 

 thus : 



Heavy * (sp. gr. 07 to 075 ; 1 cb. ft. =44 to 47 Ibs.) : Oak (47 Ibs.), Ash, 



Beech, Hornbeam, Maple, Robinia, Elm (44 Ibs.). 

 Medium weight * (sp. gr. 0*6 to 07 ; 1 cb. ft. = 37 to 44 Ibs.) : Sycamore, 



Chestnut, Birch, Larch, Colorado Douglas Fir, Red Cedar (Thuja). 

 Light* (sp. gr. 0'45 to 0'6 ; 1 cb. ft = 28 to 37 Ibs.): Horse-Chestnut, 



Alder, Pacific Douglas Fir, Pine, Spruce, Silver Fir, Willow, Aspen, 



Poplar, Lime, Cypress, and most other Conifers. 



* While green and sappy, timber is usually more than one-third heavier 

 than it will afterwards be when saivn and seasoned. 



2. As REGARDS MOISTNESS OR DRYNESS, on the average green 

 timber contains sap or water to about one-half of its total weight 

 (42 per cent in hardwoods, 52 in softwoods, and 57 in Conifers) ; 

 and it still retains water to about 10 per cent of its weight 

 when thoroughly seasoned (8 to 10 in broad-leaved, and 10 to 12 

 in Conifer wood, or more if very resinous). To save time and 

 money, the more valuable kinds of furniture woods are now 

 usually dried in hot-air chambers, as this does not affect their 

 strength ; otherwise seasoning takes two to four years for barked 



