All these facts are important in relation to the use 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 brittleness. Hickory is elastic, white pine is brittle. Stiffness is the ability to resist bending, and hence is the opposite of pliability or flexibility. A wood may be both stiff and elastic; it may be even stiff and pliable, as ash, which may be made into splints for baskets and may also be used for oars. Willow sprouts are flexible when green, but quite brittle when dry. Elasticity is of great importance in some uses of wood, as in long tool handles used in agricultural implements, such as rakes, hoes, scythes, and in axes, in archery bows, in golf sticks, etc., in all of which, hickory, our most elastic wood, is used.[8] HARDNESS OF WOOD. Hardness is the ability of wood to resist indentations, and depends primarily upon the thickness of the cell walls and the smallness of the cell cavities, or, in general, upon the density of the wood structure. Summer wood, as we have seen, is much harder than spring wood, hence it is important in using such wood as yellow pine on floors to use comb-grain boards, so as to present the softer spring wood in as narrow surfaces as possible. See _Handwork in Wood_, p. 41, and Fig. 55. In slash-grain boards, broad surfaces of both spring and summer wood appear. Maple which is uniformly hard makes the best floors, even better than oak, parts of which are comparatively soft. The hardness of wood is of much consequence in gluing pieces together. Soft woods, like pine, can be glued easily, because the fibers can be forced close together. As a matter of fact, the joint when dry is stronger than the rest of the board. In gluing hard woods, however, it is necessary to scratch the surfaces to be glued in order to insure a strong joint. It is for the same reason that a joint made with liquid glue is safe on soft wood when it would be weak on hard wood.[9] TOUGHNESS OF WOOD. Toughness may be defined as the ability to resist sudden shocks and blows. This requires a combination of various qualities, strength, hardness, elasticity and pliability. The tough woods, _par excellence_, are hickory, rock elm and ash. They can be pounded, pulled, compressed and sheared. It is because of this quality that hickory is used for wheel spokes and for handles, elm for hubs, etc. In the selection of wood for particular purposes, it is sometimes one, sometimes another, and more often still, a combination of qualities that makes it fit for use.[10]