10. Explain how your school room is heated and ventilated. 11. Should a steam or hot-water radiator be placed near the floor or near the ceiling of a room? Why? 12. In a hot-water heating system an open tank connected with the pipes is placed in the attic or above the highest radiator. Explain its use. (6) THE MOISTURE IN THE ATMOSPHERE, HYGROMETRY =166. Water Vapor in the Air.=--The amount of water vapor present in the air has a marked effect upon the weather and the climate of a locality. The study of the moisture conditions of the atmosphere, or hygrometry, is therefore a matter of general interest and importance. The water vapor in the atmosphere is entirely due to evaporation from bodies of water, or snow, or ice. In the discussion of evaporation, it is described as due to the gradual escape of molecules into the air from the surface of a liquid. This description fits exactly the conditions found by all careful observers. Since the air molecules are continually striking the surface of the liquid, many of them penetrate it and become absorbed. In the same manner many vapor molecules reenter the liquid, and if enough vapor molecules are present in the air so that as many vapor molecules reenter the liquid each second as leave it, the space above the liquid is said to be _saturated_ as previously described. (See Art. 18.) =167. Conditions for Saturation.=--If a liquid is evaporating into a vacuum, the molecules on leaving find no opposition until they reach the limits of the vessel containing the vacuum. Evaporation under these conditions goes on with great rapidity and the space becomes saturated almost instantly. If, however, air be present at ordinary pressure, many of the ordinary water vapor molecules on leaving are struck and returned to the water by the air molecules directly above. Those escaping gradually work their way upward through the air. This explains why it is that our atmosphere is not often saturated even near large bodies of water, the retarding effect of the air upon the evaporation preventing more than the layers of air near the water surface becoming saturated. Just as the amount of salt that can be held in solution in a liquid is lessened by cooling the solution (Art. 26), so the amount of water vapor that can be held in the air is lessened by lowering its temperature. If air not moist enough to be saturated with water vapor is cooled, it will, as the cooling continues, finally reach a temperature at which it will be saturated or will contain all the water vapor it can hold at this temperature. If the air be still further cooled some of the water vapor will condense and may form fog, dew, rain, snow, etc., the form it takes depending upon where and how the cooling takes place.