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.
| Material. | Conductivity. |
|---|---|
| Gold | 100 |
| Silver | 97 |
| Copper | 89 |
| Brass | 75 |
| Cast iron | 56 |
| Wrought iron | 37 |
| Zinc | 36 |
| Tin | 30 |
| Lead | 18 |
| Marble | 2.4 |
| Fire clay | 1.1 |
| Water | 0.9 |
Radiating Power of Various Substances.—Leslie
| Material. | Radiating Power. |
|---|---|
| Lampblack | 100 |
| Water | 100 |
| Writing paper | 98 |
| Glass | 90 |
| Tissue paper | 88 |
| Ice | 85 |
| Wrought lead | 45 |
| Mercury | 20 |
| Polished lead | 19 |
| Polished iron | 15 |
| Gold, silver | 12 |
| Copper, tin | 12 |
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°.