A property of bodies to resume their form upon the removal of any force by which they may have been deflected from it. In this respect all bodies which come within our knowledge are comprehended under one of these three distinctions. If two bodies, when pressed together, suffer an alteration in their form, and if afterwards, on removing that pressure, they recover their original figures, they are called elastic; if, when pressed, their forms are not in the least altered, they are called hard; and if, when pressed as above, they alter their forms, and retain the same after the pressure is discontinued, they are called soft; and both these last kinds of bodies are termed non-elastics. It is doubtful, however, whether any bodies are either perfectly hard, or soft, or elastic, - the air not being perfectly elastic, and water (which was for a long time held to be perfectly incompressible and non-elastic,) being in the opinion of many persons shown to be compressible, by the experiments of Canton and of Perkins.