232 FIRST BOOK OF FORESTRY 



kill the fungi. Usually the wood is boiled in the solution, 

 or else this is injected into the wood. Railway ties are thus 

 made to last much longer than they otherwise would. 



The sapwood of all kinds of trees is very perishable ; 

 but the heartwood is usually more or less resistant. No 

 wood is entirely exempt from destruction by decay. 



All cedars, cypress, redwood, etc., also locust, walnut, 

 white oak, chestnut, mulberry, and catalpa, make a durable 

 timber ; maple, beech, birch, poplar, ash, red oak, hick- 

 ory, black gum, spruce, balsam, and hemlock are perish- 

 able; while white pine, yellow or hard pine, elm, tulip, 

 and red gum seem to have an intermediate position. 



In dry countries, like our arid West, even perishable 

 woods last a long time ; while in moist and hot districts 

 even durable timber decays fast. Warmth and moisture 

 help, dryness and cold hinder, and full seasoning and 

 complete immersion prevent decay. 



Wood as compared with Iron 



Much is often learned by comparing a substance with 

 its natural or most impoi-tant competitor. Examining 

 wood and iron in this way, Ave find : 



1. Wood is a natural product ; iron the product of a 

 costly, complicated manufacture. Wood may be grown 

 wherever man wishes to use it ; the manufacture of iron 

 is practically confined to particular localities. The mines 



