Fig. 134.—Cross-section of the vacuum flask in a Thermos bottle.
153. Radiation is the method by which heat comes to us from the sun across space containing no tangible matter. It is also the method by which heat gets to us when we stand near a fire. Everyone has noticed that this heat is cut off by holding an object between the person and the fire. This fact indicates that radiant heat travels in straight lines.
The radiation of heat is believed to be accomplished by means of waves in a medium called ether, which is invisible and yet pervades everything. Three of the most important characteristics of radiation are first, heat is transferred by radiation with the speed of light, or 186,000 miles per second. This fact is shown by the cutting off of both the sun's heat and light at the same instant during an eclipse of the sun. Second, radiant heat[I] travels in straight lines, while other modes of transferring heat may follow irregular paths. The straight line motion of radiant heat is shown by its being cut off where a screen is placed between the source of heat and the object sheltered. Third, radiant heat may pass through an object without heating it. This is shown by the coldness of the upper layers of the atmosphere and also by the fact that a pane of glass may not be heated appreciably by the heat and light from the sun which passes through it.
When radiant energy falls upon any object it may be (a) reflected at the surface of the object, (b) transmitted through the substance, (c), absorbed. All three of these effects occur in different degrees with different portions of the radiation. Well-polished surfaces are good reflectors. Rough and blackened surfaces are good absorbers. Transparent objects are those which transmit light well, but even they absorb some of the energy.
154. The Radiometer.—Radiant heat may be detected by means of the radiometer (Fig. 135). This consists of a glass bulb from which the air has been nearly exhausted. Within it is a wheel with four vanes of mica or of aluminum mounted on a vertical axis. One side of each vane is covered with lampblack, the other being highly polished. when exposed to radiant heat from any source the vanes revolve with the bright side in advance.
The bulb is so nearly exhausted of air that a single molecule remaining may travel from the walls of the bulb to the vanes without coming in contact with another molecule.
The blackened sides absorb more heat than the highly polished sides. The air molecules striking these blackened sides receive more heat and so rebound with greater velocity than from the other side, thus exerting greater pressure. The blackened sides therefore are driven backward. If the air were not so rarified the air molecules would hit each other so frequently as to equalize the pressure and there would be no motion.
Fig. 135.—A radiometer.
Sun's Radiation.—Accurate tests of the amount of the sun's radiation received upon a square centimeter of the earth's surface perpendicular to the sun's rays were made at Mt. Wilson in 1913. The average of 690 observations gave a value of 1.933 calories per minute. These results indicate that the sun's radiation per square centimeter is sufficient to warm 1 g. of water 1.933°C. each minute. Although the nature of radiation is not discussed until Art. 408-411 in light, it should be said here that all bodies are radiating heat waves at all temperatures, the heat waves from cool bodies being much longer than those from hot bodies. Glass allows the short luminous waves to pass through freely but the longer heat waves from objects at the room temperature pass through with difficulty. This is the reason why glass is used in the covering of greenhouses and hot beds. Water also absorbs many of the longer heat waves. It is therefore used in stereopticons to prevent delicate lantern slides from being injured by overheating.