41 () JOURNAL OF FORESTRY 



shrinkage of woods ; thus species containing a large amount of resin 

 warp less than the others. In order to avoid deformation of pieces 

 of wood due to shrinkage, the bark must be left on. or the trees cut 

 up after they are dried. 



Heat Conductivity. — Expansion of woods under the action of heat is 

 considerably less than that of metals and even of glass. Wood is a poor 

 conductor of heat, but its conductivity is greater lengthwise than 

 radially. Heavy woods, being less porous, are better conductors than 

 light woods. 



Conductivity to Electricity. — Wood is an insulator of electricity, and 

 this quality is more pronounced in proportion to the lightness of the 

 wood. The more moisture wood contains, the more readily it conducts 

 electricity ; hence the more moisture trees contain the more likely they 

 are to be struck by lightning. 



Sound is conducted "readily by normal wood ; on the other hand, 

 the presence of fungi or worms can be detected by the failure of the 

 wood to conduct sound. 



Wood is readily penetrated by X-rays. 



Wood Elasticity.— The elasticity of wood is its capacity for being 

 bent without breaking. The maximum of elasticity is found in green, 

 sound wood, its minimum in completely dried wood. Dry woods 

 regain their elasticity when dipped in hot water or subjected to the 

 action. of steam; the process of curving or arching pieces of wood is 

 based on this fact. Among broadleaf species, rapidly growing trees 

 are more elastic than slow-growing trees ; therefore broadleaf trees 

 from a coppice with standards are more elastic than those from high 

 forests. The opposite is true of resinous species. Trees growing in 

 fresh soil with northern exposure are usually more elastic than those 

 on other exposures. In sawed timbers, resistance is greater in the 

 radial direction than in a direction tangential to the ligneous layers ; 

 hence the superiority of timber sawn perpendicular to the medullary 

 grain (debit sur mailles). The elasticity of wood is shown by its 

 resistance to bending. This resistance is measured mechanically by 

 the load of rupture. -For a single species, elasticity varies with con- 

 ditions of growth, and is not in proportion to the density of the 

 wood. Resistance to crushing is also measured by the load of rupture, 

 and varies with conditions of growth. It increases in proportion to 

 drying, an important consideration for mine timbers. Aleppo pine, 

 among others, is remarkable for its resistance to crushing, and for 



