Woods and Their Properties 47 



Woods differ also in their grain; some are straight 

 grained, in others the fibers are interlaced. Which 

 ones are easier to split ? Name some kinds of wood that 

 are hard to split, and others that are easy. Is there 

 much difference in the cleavability of pieces from the 

 same species of tree? Some woods can be split very 

 easily along the lines between the annual rings. 



In yellow pine the summerwood is much harder than 

 the springwood, as you will see if you push the point of 

 a pin or a knife blade into it. Is the summerwood lighter 

 or darker than the springwood? Can you see any ob- 

 jection to using yellow pine for lead pencils, instead of 

 a wood of more even texture ? 



Density of wood. The hardness and strength of a 

 wood depend on the amount of cell-wall substance in it, 

 which varies with the weight of the dry wood. Foresters 

 and builders are therefore much interested in the densities 

 of different woods ; for if they know the density, they 

 can judge to some extent whether or not a wood will be 

 valuable for certain purposes. They know that if it 

 is heavy, it contains many thick-walled fibers and will 

 prove to be hard and strong. If it is light, the walls 

 of the fibers are thin, and it will, therefore, lack hard- 

 ness and strength. 



Finding the density of wood. If a piece of wood 

 floats with half its volume out of water, it does so be- 

 cause it is half as dense as water. Arbor vitae is so 

 light that it will float with two thirds of its bulk above 

 water. A cubic foot of it weighs only one third as 

 much as a cubic foot of water, or a block of arbor vitae 

 of any size weighs only one third as much as an equal 



