SPECIFIC GRAVITY. 27 



rings ; but there are certain exceptions to this rule, as in the 

 case of sprucewood from high Alpine districts with annual zones 

 only 1 or 2 millimeters broad. Hartig considers that the 

 specific gravity of conifers increases and falls with the volume- 

 increment : this is due to the fact that, with an increasing 

 sectional area, the lumina of the water-conducting organs can be 

 reduced ; on the contrary, when the sectional-area increment is 

 reduced, the lumina must be larger. This, however, holds good 

 only for individual trees, and it must not be laid-down as a 

 general rule that larger sectional-area increment always implies 

 heavier. wood. {Vide Plate II.) 



3. Differences due to Substances contained in the Tissues. 



Among the materials present in woody tissue, water, resin and 

 reserve nutritive material are the most important, the amount 

 of these substances in the walls and lumina of woody tissue 

 necessarily influencing the specific gravity of the wood. 



(a) Water. — The weight of water in wood varies, according to 

 species, tree-part, season and locality, between 30 and 55 per 

 cent, of the total weight of the wood, the wood of felled trees 

 being of all degrees of moisture ; in practice a distinction is 

 made between green timber, with an average of 45 per cent, of 

 water (as is the case if recently-felled trees) ; wood dried in the 

 forest, after lying for some time in breezy forest depots ; and, 

 finally, air-dried wood, which has been for a long time kept under 

 cover in timber-yards, and retains only 10 to 11 per cent, of water. 



For scientific purposes, absolutely dry wood is obtained only 

 by placing wood in drying-chambers at a temperature of 105° C. 

 (221° Fahr.) until it no longer loses weight in a sensitive 

 balance. Such a state of dryness is retained only whilst the 

 wood is in the drying-chamber ; after removal, it speedily re- 

 absorbs moisture and becomes heavier. 



The greater or less volume of water contained by wood also 

 influences its specific gravity indirectly, by its eff'ects on the 

 mass of the wood. As wood dries it shrinks, and shrinkage 

 tends to increase its specific weight. 



It has often been asserted that the season of felling has 

 an influence on the specific gravity of wood. If the absolute 



