_Mechanical action_ is a common source of heat. The rubbing of a match producing heat enough to occasion flame is a familiar example. The spark produced in what is called striking fire is the burning of a particle of steel set on fire by the blow. The Indian was accustomed to light his fire by the rubbing together of two dry sticks till he learned an easier way from civilized neighbors; and the blacksmith, previous to the invention of phosphorus matches, often lighted his fire by touching a sulphur match to a nail made red-hot by rapid and continued hammering. Machinery has sometimes been set on fire by friction, and the water around a mass of metal has been so heated by boring as even to boil. If you stretch a piece of India rubber several times in quick succession, and then apply it to your lips, you will perceive that the motion has warmed it. 273. =Relations of Heat and Light.=--Heat is sometimes alone, and is sometimes in intimate union with light. All substances have some amount of heat, and it passes from them to other bodies in their neighborhood that happen to have less heat in them. In doing this it may or may not have the company of light. In the radiation of heat from a stove, unless it be heated to redness, there is no light with the heat; but from an open, burning fire the light and heat come together. But the rays of the sun give us the best example of the union of light and heat. Traveling together at an equal pace they are most curiously mingled, as you will see when I come to speak particularly of light. I will now proceed to notice the principal effects of heat; viz., expansion, liquefaction, and vaporization. [Illustration: Fig. 192.]