=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. [Illustration: 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. Important Topics 1. Conduction in solids, liquids, gases. 2. Non-conductors; uses, best non-conductors. 3. Radiation, three characteristics. 4. The sun's radiation, amount. The radiometer. Exercises 1. Does clothing ever afford us heat in winter? How then does it keep us warm? 2. Why are plants often covered with paper on a night when frost is expected? 3. Will frost form in the fall of the year sooner on a wooden or a cement sidewalk? Why? On which does ice remain longer? Why? 4. Why in freezing ice-cream do we put the ice in a wooden pail and the cream in a tin one? 5. Is iron better than brick or porcelain as a material for stoves? Explain. 6. Which is better, a good or a poor conductor for keeping a body warm? for keeping a body cool? 7. Should the bottom of a teakettle be polished? Explain. 8. How are safes made fireproof?