ECONOMIC WOODS OF THE UNITED STATES 53 



imately as follows: in contents of living cells, 5 per cent; satu- 

 rating cell walls, 35 per cent; free water, 60 per cent. 



In a living tree the wood nearest the bark contains the most 

 water. If no heartwood is present the decrease toward the pith 

 is gradual; otherwise the change is quite abrupt at the sap wood 

 limit. In Pinus palustris, for example, the weight of the fresh 

 wood within an inch of the bark may be 50 per cent of water; 

 that between one and two inches, only 35 per cent; that of the 

 heartwood, only 20 per cent. The water content of any par- 

 ticular section of a tree depends upon the amount of sapwood, 

 and is therefore greater for the upper than for the lower portions 

 of the stem; greater for limbs than bole; greatest of all in the 

 roots. 



The water content of wood can readily be determined in the 

 following manner: saw off a thin section of wood; weigh careful- 

 ly on a delicate balance; dry in an oven at a temperature of 100 C. 

 until a constant weight is obtained; reweigh. The difference 

 between the fresh weight and the dry weight is the amount of 

 moisture contained. Computed on a basis of the fresh weight, 



fresh weight dry weight 



Per cent of moisture = - r-r - X 100. 



fresh weight 



Thus if the weight of the original block of wood was twice the 

 final weight, there was as much water as wood; in other words, 

 one-half, or 50 per cent, of the original weight was water. The 

 figures in the preceding paragraph are on this basis. 

 Computed on a basis of dry weight, 



. . , fresh weight dry weight 



Per cent of moisture = - X 100. 



dry weight 



In the problem cited above the loss of moisture was 100 per cent 

 of the dry weight. This method furnishes a constant basis for 

 comparison, while the other varies with every change in moisture 

 degree. Subsequent references to the per cent of moisture will 

 refer to computation on the basis of dry weight. 



It is impossible to remove absolutely all the water from wood 

 without destroying the wood. Wood is considered thoroughly 

 dried when it ceases to lose weight in a constant temperature of 

 100 C., though it still retains 2 to 3 per cent of moisture, and 

 if exposed to higher temperature will continue to give up water. 



Seasoning, which is essentially drying, adds appreciably to 

 the strength, and, in slightly less proportion, to the stiffness of 



