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(24.) Elasticity.—Some bodies, when compressed by mechanical agency, will resume their former dimensions with a certain energy when relieved from the operation of the force which has compressed them. This property is called elasticity; and it follows, from this definition, that all elastic bodies must be compressible, although the converse is not true, compressibility not necessarily implying elasticity.

(25.) Dilatability.—This quality is the opposite of compressibility. It is the capability observed in bodies to have their volume enlarged without increasing their mass. This effect may be produced in several ways. In ordinary circumstances, a body may exist under the constant action of a pressure by which its volume and density are determined. It may happen, that on the occasional removal of that pressure, the body will dilate by a quality inherent in its constitution. This is the case with common air. Dilatation may also be the effect of heat, as will presently appear.

The several qualities of bodies which we have noticed in this chapter, when viewed in relation to each other, present many circumstances worthy of attention.