Heat is a word freely used, yet difficult to define. The word “heat” is
commonly used in two senses: (1) to express the sensation of warmth;
(2) the state of things in bodies which causes that sensation. The
expression herein must be taken in the latter sense.
Heat is transmitted in three ways—by _conduction_, as when the end of
a short rod of iron is placed in a fire, and the opposite end becomes
warmed—this is conducted heat; by _convection_ (means of currents) such
as the warming of a mass of water in a boiler, furnace, or saucepan;
and by _radiation_, as that diffused from a piece of hot metal or
an open fire. Radiant heat is transmitted, like sound or light, in
straight lines in every direction, and its intensity diminishes
inversely as the square of the distance from its center or point of
radiation. Suppose the distance from the center of radiation to be 1,
2, 3 and 4 yards, the surface covered by heat rays will increase 1,
4, 9 and 16 square feet; the intensity of heat will diminish 1, 1/4,
1/9, and 1/16. and so on in like proportions, until the heat becomes
absorbed, or its source of supply stopped.
Whenever a difference in temperature exists, either in solids or
liquids that come in contact with or in close proximity to each other,
there is a tendency for the temperature to become equalized; if water
at 100° be poured into a vessel containing an equal quantity of water
at 50°, the tendency will be for the whole to assume a temperature of
75°; and suppose the temperature of the surrounding air be 30°, the
cooling process will continue until the water and the surrounding air
become nearly equal, the temperature of the air being increased in
proportion as that of the water is decreased.
The heat generated by a fire under the boiler is transmitted to the
water inside the boiler, when the difference in the specific gravities,
or, in other words, the cold water in the pipes being heavier than that
in the boiler sinks and forces the lighter hot water upward. This heat
is radiated from the pipes, which are good conductors of heat to the
air in the room, and raises it to the required temperature. That which
absorbs heat rapidly, and parts with it rapidly, is called a good
conductor, and that which is slow to receive heat, and parts with it
slowly, is termed a bad conductor.
The following tables of conductivity, and of the radiating properties
of various materials, may be of service: