40. =Elasticity Shown in Other Ways.=--If a stick be bent, as in Fig. 2, as soon as the bending force is withdrawn the stick becomes straight again from its elasticity. It is this elastic force of the bow, straightening it, that speeds the arrow. Observe in this case that while the particles on the concave side of the bent bow are brought nearer together or compressed, those on the convex side are moved apart. This moving apart of the particles is often shown in India-rubber. You can see how very far apart particles that are in near neighborhood may be carried, if you will stick two pins close together in a strip of India-rubber before you stretch it. 41. =Degrees of Elasticity in Different Substances.=--Some substances have so very little elasticity that they are practically considered as having none. Lead is one of these. A rod of lead when bent remains so, and a leaden ball does not rebound. While aeriform substances are the most compressible of all, they are also the most elastic. Air compressed returns to its usual condition the moment that it is relieved from the pressure, and with a force proportioned to the amount of the pressure. So it is with steam and the gases. The varied results of this quality of aeriform substances will claim our attention more particularly in some other parts of this book. 42. =Definition of Elasticity.=--You see from the illustrations that have been given that elasticity is _that property of matter by which its particles, when brought nearer together or carried farther apart by any force, return to their usual condition when the force is withdrawn_.