It should be definitely understood at the beginning that cold signifies the absence of heat, just as darkness implies the absence of light, so when one speaks of cold getting into a house what is really meant is either the entrance of cold air by some opening or else the escape of the heat.
There are three distinct methods by which heat energy is transferred from one place to another, depending upon the medium or substance that transfers the heat.
a. A solid transmits heat by the method called conduction.
b. A fluid, either a liquid or a gas, transmits heat mainly by the method called convection.
c. Space transmits the energy of hot objects by the method called radiation.
Fig. 130.—Solids conduct heat.
151. Conduction.—To illustrate conduction, place in a gas flame the ends of same metal wires supported as in Fig. 130. In a short time the other ends of the wires become hot enough to burn one's hand. This may be explained as follows: The hot gas flame contains molecules in violent vibration and those striking the wire set its molecules rapidly vibrating. Since, in a solid, the molecules are held in the same relative positions, when one end of a wire is heated the rapidly vibrating molecules at the hot end set their neighbors vibrating and these the next in turn and so on until the whole wire is hot. It is a fortunate circumstance that different substances have different rates of conductivity for heat. To realize this, suppose that our clothing were as good a conductor as iron, clothing would then be very uncomfortable both in hot and in cold weather. The best conductors for heat are metals. It is interesting to note that, as a rule good conductors of heat are also good conductors of electricity, while poor conductors of heat are also poor electric conductors. Careful experiments in testing the rate that heat will be conducted through different substances show the following rates of conductivity.
Fig. 131.—Water is a poor conductor of heat.