PROPERTIES OF WOOD. 53 



cause moisture softens the wood and this reduces the adhesion of the 

 fibers to each other. 7 



CLEAVABILITY OF WOOD. 



Closely connected with shearing strength is cohesion, a property 

 usually considered under the name of its opposite, cleavability, i. e., 

 the ease of splitting. 



When an ax is stuck into the end of a piece of wood, the wood 

 splits in advance of the ax edge. See Handwork in Wood, Fig. 59, 

 p. 52. The wood is not cut but pulled across the grain just as truly 

 as if one edge were held and a weight were attached to the other 

 edge and it were torn apart by tension. The length of the cleft 

 ahead of the blade is determined by the elasticity of the wood. The 

 longer the cleft, the easier to split. Elasticity helps splitting, and 

 shearing strength and hardness hinder it. 



A normal piece of wood splits easily along two surf ices, (1) along 

 any radial plane, principally because of the presence of the pith rays, 

 and, in regular grained wood like pine, because the cells are radially 

 regular; and (2) along the annual rings, because the spring-wood sep- 

 arates easily from the next ring of summer-wood. Of the two, radial 

 cleavage is 50 to 100 per cent, easier. Straight-grained wood is much 

 easier to split than cross-grained wood in which the fibers are inter- 

 laced, and soft wood, provided it is elastic, splits easier than hard. 

 Woods with sharp contrast between spring and summer wood, like 

 yellow pine and chestnut, split very easily tangentially. 



All these facts are important in relation to the u?e of nails. For 

 instance, the reason why yellow pine is hard to nail and bass easy 

 is because of their difference in cleavability. 



ELASTICITY OF WOOD. 



Elasticity is the ability of a substance when forced out of shape, 

 bent, twisted, compressed or stretched, to regain its former shape. 

 When the elasticity of wood is spoken of, its ability to spring back 

 from bending is usually meant. The opposite of elasticity is brittle- 

 ness. Hickory is elastic, white pine is brittle. 



Tor table of strengths of different woods, see Sargent, Jesup Collection, 

 pp. 166 ff. 



