PROPICRTIKS or WOOD 415 



produces mainly the elements of summer wood, which are less thick 

 and less lignified. 



Effect of Site Upon the Mechanical Properties of Wood. — The site 

 most favorable to the growth of a species furnishes the maximum 

 hardness and density, hence the maximum utility. The same is true 

 of the resistance of wood to compression and rupture, which varies 

 according to the specific gravity. This has been proved by tests of 

 eucalyptus, whether grown in Australia or in France ; of fir from both 

 plain and mountain ; of eastern white pine, both in America and France, 

 etc. Specific gravity is a function of soil quality. Good soil generally 

 produces hard, heavy wood. The specific gravity varies, of course, 

 with the species. It also varies with temperature, hot climates pro- 

 ducing the densest and heaviest woods. Resins and coloring matters 

 increase specific gravity. Sapwood is usually lighter than heartwood, 

 the average difference being 6 per cent. An' open stand (light) in- 

 creases the density and hardness of woods, and these qualities are also 

 affected by the time of felling. Woods are harder and heavier if cut 

 when the sap is not circulating. The greatest density of a tree is at the 

 root collar ; hence wood from this part of the tree is most in demand 

 for fuel. 



Effect of Management. — Management — that is, the method of grow- 

 ing trees — has a certain influence on the density of the wood. Thus, 

 in resinous trees, the proportion of spring wood is in direct ratio to 

 growth activity. Since turpentine is found especially in autumn 

 wood, it is the slow-growing trees that are richest in resin, and densest. 

 In broadleaf woods, on the other hand, growth of the annual layer 

 takes place in the autumn wood, thus favoring the density of the wood. 

 For instance, isolated oak is denser than oak grown in a dense stand ; 

 here the qualitv of the wood is in direct proj^ortion to the thickness 

 of the annual rings. 



S-cCclling and Shrinkage of JVood.— Wood swells in air saturated 

 with moisture, and shrinks in dry air; that is, the walls of the cells 

 swell and shrink. If evaporation is rapid, cracks appear; the wood 

 warps. In passing from the green to the dry stage, wood shrinks at 

 the rate of to 1 per cent lengthwise, 3 to 5 per cent on a radius 

 of the cross section, and 6 to 10 per cent in circumference. Thus 

 shrinkage lengthwise is very slight, and tangential shrinkage great. 

 This causes small cracks or large centripetal cracks. Heartwood con- 

 tains less water than sapwood, hence warps less. Resin decreases 



