Heat is distributed by radiation, conduction, and convection. By the first we mean that heated bodies have the power of projecting from themselves, by means of the ether, their own vibrations. Thus the sun is constantly distributing its light and heat in all directions. Conduction takes place where the molecules of a substance nearest a fire first become heated and then impart their motion to the remainder of the mass, somewhat as in a row of suspended ivory balls, the first of which, when struck, transmits its motion from ball to ball, the last one flying off.
Convection takes place in liquids and gases. Here the particles in contact with the heated body becoming lighter by expansion, rise, and are followed by others, thus forming a current.
WATER A POOR CONDUCTOR.
Ex.—Fill a tube nearly full of water, applying a flame to the upper part of the tube. The water at this point will readily boil, while that in the lower part of the tube remains cool, showing that water is a poor conductor, and that liquids must be heated by convection.
The process of warming a room illustrates the three methods of heat distribution. The heat passes through the stove by conduction, away from it by radiation, and to the remote parts of the room by convection.
EFFECTS OF HEAT.
They are four in number. Rise of temperature, expansion, liquefaction, evaporation. The first indication of the presence of heat is discovered by an elevation in temperature. Though man is not a reliable thermometer, he would be able, ordinarily, even if blind, to chronicle the progress of the sun, from horizon to horizon, by the increasing and decreasing warmth. The little thermometer placed beneath the tongue of the invalid gives reliable report of the combustion going on within his system. We see a thousand illustrations of the expansive effects of heat, many of which are familiar to all. The exceptions are more interesting than the rule, and less known, the ordinary rule being that heat expands and cold (absence of heat) contracts. Water contracts by cold until it reaches the temperature of 39°, and then expands with great violence until congelation is completed, at 32°. A British officer in Quebec filled a twelve inch shell with water, and closed the fuse hole with a wooden plug securely driven in with a mallet. Upon being exposed to intense cold the plug was projected a distance of several hundred feet, and a long tongue of ice was found protruding from the opening.
It is supposed that sufficient heat would convert all solids first into liquids, and then into gases. In the process of distillation, if we wish to retain its products, we combine both heating and cooling.
The knowledge of the melting and vaporizing point of substances is of immense value. We are enabled thus to drive off and secure the various ingredients entering into many complex substances. A notable instance is seen in the means used to secure the rich and varied products of petroleum.