PKOPEETIES OF WOOD. 49 



THE WEIGHT OF WOOD. 



Wood substance itself is Iieavier than water, as can readily be 

 proved by immersing a very thin cross-section of pine in water. Since 

 the cells are cut across, the water readily enters the cavities, and the 

 wood being heavier than the water, sinks. In fact, it is the air en- 

 closed in the cell cavities that ordinarily keeps wood afloat, just as 

 it does a corked empty bottle, altlio glass is heavier than water. A 

 longitudinal shaving of pine will float longer than a cross shaving 

 for the simple reason that it takes longer for the water to penetrate 

 the cells, and a good sized white pine log w'ould be years in getting 

 water-soaked enough to sink. As long as a majority of the cells are 

 filled with air it would float. 



In any given piece of wood, then, the weight is determined by 

 two factors, the amount of wood suljstancc and the amount of water 

 contained therein. The amount of wood substance is constant, but 

 the amount of water contained is variable, and hence the weight va- 

 ries accordingly. Moreover, considering the wood substance alone, 

 the weight of wood substance of different kinds of wood is about the 

 same; namely, l.G times as heavy as water, whether it is oak or pine, 

 ebony or pojilar. The reason why a given bulk of some woods is 

 lighter than an equal bulk of others, is because there are more thin- 

 walled and air-filled cells in the light woods. Many hard woods, as 

 lignum vitae, arc so heavy that they will not float at all. This is 

 because the wall of the wood cells is very thick, aiul the lumina are 

 small. 



In order, then, to find out the comparative weights of different 

 woods, that is, to see how much wood substance there is in a given 

 volume of any wood, it is necessary to test absolutely dry specimens. 



The W'-eight of wood is indicated either as the weight per cubic 

 foot or as sjjecific gravity. 



It is an interesting fact that different parts of the same tree have 

 different weights, the wood at the base of the tree weighing more 

 than that higher up, and the wood midway between the pith and bark 

 weighing more than either the center or the outside.^ 



■■How much different woods vary may be seen by the following table, 

 taken from Filibert Roth, Timber, Forest Service Bulletin No. 10, p. 28; 



