Liquid is a substance that possesses no rigidity, and that offers great resistance to compression - i.e. it changes its shape under the action of very small forces, but not its bulk. The rapidity with which a small force can distort a liquid depends upon its viscosity (q.v.), there being much difference in this respect between a mobile liquid like water and a viscous liquid like glycerine. Many solid substances assume the liquid form under the application of sufficient heat, the actual transition being more or less abrupt and being accompanied with the apparent disappearance of heat. This is lost as heat, but appears as an increased kinetic energy of the particles of the substance as a liquid, which enables them to move past each other with a freedom impossible when the substance was solid.