A force in nature known by its effects in fusion and evaporation. Formerly it was. supposed to be a subtle fluid, which was known as caloric. It is now regarded as a kind of motion, being in general a form of vibration or disturbance of molecules. One of the most important effects of heat is to alter the temperature of bodies. A piece of iron put into burning coals becomes hot, because the heat passes from the coals into the iron, until both have reached the same temperature. Heat also alters the dimensions of bodies. For example, the tire of a wheel is made a little too small, and when heated it enlarges so as to slip on easily. It cools down to the same size as it was at first, and then fits so tightly that it binds all parts of the wheel firmly together. The ends of rails are always left a little way apart on railroads ; for if rails were laid close together the heat of the sun might expand them, and push them out of place. Heat is communicated to different bodies in at least three distinct ways. First, by convection, as when water is heated in a kettle (over the fire). Second, by conduction - that is heat traveling from one end of a substance to the other end. Hence we have good and bad conductors of heat. Metals are good conductors, glass is a bad conductor, and wood is a still worse one. This is the reason why iron tools for heating in fires have wooden handles fitted to them. A third way is called radiation. This may be best illustrated by placing some substance near a fire. The heat passes over to it or is radiated to it from the fire.