ELASTICITY. The word elastic, derived originally from the Greek verb EX avvECV to drive, has acquired its present mean ing largely from the writings of Robert Boyle. In 166o, in a description of his experiments relating to the spring of the air he says, "There is a spring or elastical power in the air in which we live." This spring he attributed to an effort made by each particle of air to drive its neighbours away. For the sake of illustration, a particle of air was at that time pictured with little springs attached to it, these springs being compressed by those of neighbouring particles.
The chief characteristic of a spring is its power of recovering a former durable shape and size when the load causing a temporary deformation is removed. The spring possesses this power whether it is stretched or compressed. A similar power of recovery, when deforming forces are removed, is possessed to some extent by any solid body. This power is described in a general way by saying that the body is elastic. If recovery is prevented by some other body or bodies, the effort which the body makes to recover its original durable form may be represented by an elastic force or system of elastic forces. This application of the word elastic to the forces exerted by deformed solid bodies was made by Boyle. In an essay on effects of motion, written in 1685, he says, "By the elastical force of the bent bow the string is brought into a violent state of tension."