24 TECHNICAL PROPERTIES OF WOOD. 



the specific gravity of wood is calculated by dividing its weight 

 in grams by its volume ill cubic centimeters. Conversely the 

 absolute weight of a piece of wood can be ascertained by multi- 

 plying its volume by its specific gravity. 



[As one cubic foot of water at its greatest density weighs 1000 ozs. 

 = 62^ lbs.; multiplying G2J lbs. by the specific gravity of a wood 

 gives the weight of a cubic foot, or dividing the weight of a cubic foot 

 by 62J lbs., its specific gravity. — Tr.1 



A correct knowledge of the specific gravity of woods does not 

 give much information regarding their economic value, but is 

 of importance where much weight tells on the strength of a 

 structure, as for roofs, machines, wood for carriages, t^c. ; also 

 as regards the cost of timber transport. 



[In India, the great weight of many of the hardest woods, such as 

 the different kinds of iron-wood {M esua ferrea,X ylia dolahriformis),&.c., 

 renders them unsuitable for floating, and where other transport is not 

 available may altogether prohibit their use. Comparative lightness, 

 irrespective of their inherent good qualities, is one reason for the 

 extensive use of teak and Cedrela Tuna ; also owing to their light 

 weight many extremely soft woods, such as Bombax vialaharictim, are 

 used for packing-cases, though their durability is very inferior. — Tr.] 



Hardness, durability, heating-} owcr and amount of warping 

 also depend more or less on the specific gravity of woods. 



The specific gravity of woody substance, i.e., of the cell-wall, is 

 greater than that of water for all species of trees. According to 

 independent and accordant investigations by Sachs and R. Hartig 

 there is no essential difference between the specific gravity of 

 the substance of the more important woods, as for instance 

 oak, beech, birch, spruce and Scotch pine ; the specific gravity 

 of the woody substance of which they are composed may he 

 placed at 1*5G. In this respect no diflerence has been observed 

 between heartwood and sapwood. It is therefore clear that 

 diff'erences in the specific gravity of different woods are due to 

 their anatomical structure, and to the substances contained in 

 the lumina of their fibres and vessels. 



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