Q. When a man has been almost drowned, why is suspended animation restored by rubbing?
A. The vital heat of the body (which had become latent by the action of the water) is again developed by friction: and, as soon as this animal heat can be excited, the vital powers of the body are restored.
Q. Why do two pieces of ice (rubbed together) melt?
A. Ice contains 140 degrees of latent heat, and (when two pieces are rubbed together) their particles are compressed, and this latent heat rolls out and melts the ice.
Q. Are not forests sometimes set on fire by friction?
A. Yes; when two branches or trunks of trees (blown about by the wind) rub violently against each other, their latent heat is developed, and sets fire to the forest.
Q. What is meant by compression?
A. The act of bringing parts nearer together; as a sponge is compressed by being squeezed in the hand.
Q. Cannot heat be evolved from common air merely by compression?
A. Yes; if a piece of German tinder be placed at the bottom of a glass tube, and the air in the tube compressed by a piston,[11] the tinder will catch fire.