This extraordinary principle exists in all bodies, and may be pressed
out of them. The blacksmith hammers a nail until it becomes red hot,
and from it he lights the match with which he kindles the fire of his
forge. The iron has by this process become more dense, and percussion
will not again produce incandescence until the bar has been exposed in
fire to a red heat, when it absorbs heat, the particles are restored to
their former state, and we can again by hammering develop both heat and
light.--_R. Hunt, F.R.S._