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:

Conducting Power of Various Substances.—Despritz.

Radiating Power of Various Substances.—Leslie

From the above tables, it will be seen that water, being an excellent radiator, and of great specific heat, and iron a good conductor, these qualities, together with the small cost of the materials, combine to render them efficient, economic and convenient for the transmission and distribution of artificial heat.

By adopting certain standards we are enabled to define, compare and calculate so as to arrive at definite results, hence the adoption of a standard unit of heat, unit of power, unit of work, etc.

The standard unit of heat is the amount necessary to raise the temperature of one pound of water at 32° Fahr. one degree, i.e., from 32° to 33°.

Specific heat is the amount of heat necessary to raise the temperature of a solid or liquid body a certain number of degrees; water is adopted as the unit or standard of comparison. The heat necessary to raise one pound of water one degree, will raise one pound of mercury about 30 degrees, and one pound of lead about 32 degrees.

Table of the Specific Heat of Equal Weights of Various Substances.