58 THE BOOK OF FORESTRY i 



Regarding the rate of growth certain of the hard- 

 woods, like chestnuts for instance, are more lasting if 

 they have grown rapidly ; evergreens generally are more 

 durable if they have grown slowly as there is a greater 

 proportion of the heavy summer wood in the annual 

 jing. , 



Thus hardwoods are likely to produce their best and 

 most durable lumber when grown in the open on good 

 soil, while the reverse is claimed to be true for the ever- 

 greens. 



With the rapid increase in the use of preservatives 

 a tree nowadays need only be hard. Resistance to 

 decay can be acquired through chemical treatment. 

 (See Chapter XII.) 



Weight. In spite of the fact that the wood substance 

 produced by all tree species is the same weight a 

 little more than one and a half times that of water 

 the timber sawed from different trees varies consider- 

 ably in weight. This is due to the compactness of 

 arrangment of the wood fibers and the amount of air 

 contained in the wood. Most of the pines will float 

 when green, but after they have been in the water 

 some time they become heavier than water and sink, 

 owing to the fact that water has replaced the air. Both 

 on account of the compactness of their structure and 

 their moisture content few of the broadleafed trees will 

 float, although some will do so after being dried out. 

 The amount of heat that is obtained on burning wood 

 depends largely on its weight and such heavy species 

 as hickory, oak, and sugar maple yield the best fuel 

 wood ; in fact there is a direct ratio between the weight 

 of wood and the heat it liberates upon burning. In 

 a general way it may be said that the heavier woods 

 are more valuable, as they are likely to be stronger 



