The nature of the characteristic elasticity of rubber and other rubber-like substances has been elucidated by Kuhn and Mark, who showed that it is due to the ability of the thread-like molecules of these substances to fold or curl by free rotation of the separate monomeric elements about the single bonds linking them together into a long hydrocarbon chain. A curled form of these thread-like molecules being more probable than an extended one, they must display a tendency to curl simply because of their heat motion, without any actual forces tending to produce such an effect. As a result, an extension of a rubber-like body (without change of volume), entailing a similar average extension of all the molecular chains constituting it in a definite direction, is resisted by an elastic force proportional to the energy of heat agitation, that is, to the absolute temperature. This situation is quite similar to that which is found in the case of an ideal gas. The pressure exerted by the latter on the walls of the vessel containing it is due, not to a mutual repulsion of the molecules, as was believed long ago, but simply to their heat agitation. The main difference consists in the fact that, instead of a pressure, the heat agitation (free rotation) of the thread-like molecules produces a tension, increasing with the average degree of extension in the given direction. This difference is associated with a difference in the sign of the heating effect produced by a mechanical deformation: whereas a gas is heated on compression and cooled on expansion. In the case of rubber we find an exactly opposite behavior.