THE WOOD 227 



SOME PHYSICAL PROPERTIES 



Weight. Most of our wood is lighter than water, and 

 therefore it floats. But if we leave a piece of wood in 

 water a long time it sinks ; and thus we see that the solid 

 or wood substance of which the cell walls are made is 

 heavier than water. If, therefore, the walls of all cells 

 are very thick, the wood, like many of our tropical woods, 

 may never float. Most of our useful timbers float when 

 dry, and the majority weigh about four to seven tenths as 

 much as water, or about twenty-five to forty-three pounds 

 per cubic foot, and hence about two to three and a half 

 pounds per foot, board measure. 



Moisture. When first formed the cell is filled with 

 living substance resembling the white of an egg, so that 

 the freshly formed wood is full of water. In an ordinary 

 log of pine, more than half the cells are empty, but their 

 walls are still moist. 



Most of the water in such a log is in the sapwood, but 

 there is much difference in different kinds of trees. Com- 

 monly it is supposed that the water in the wood, usually 

 called " sap," goes out of the log in the fall and returns in 

 the spring. The reason for the belief is the fact that 

 many trees bleed in the spring but not in winter. This 

 belief is erroneous, for by actual trial we find that the 

 wood contains about as much water in winter as in 

 summer. 



