"I want you to think over our lesson on evaporation," said Mr. Wilson, "and the part that heat plays in regulating the quantity of vapor the air absorbs. It taught us that the air, at every temperature, absorbs vapor up to a certain point, but beyond that it will take no more. We say that the air is saturated and that, for each degree of temperature, it has its point of saturation. Air, at the ordinary temperature of about 60°F., may contain a certain amount of moisture, and yet not be saturated. But suppose that air becomes cooled, what will happen then?""The cooler air cannot hold so much vapor as it did at 60°F., sir," said Fred.
"Quite right, Fred, and the same quantity of moisture would be sufficient to quite saturate it at the lower temperature.
"Now, let us suppose the air to be cooled still further. It was already saturated; its capacity for holding moisture is still less under the new conditions; some of the moisture must go. It does go. It is condensed, and falls in little round drops of water which we call dew. The temperatureat which the air begins to deposit dew is known as the dew-point.
"Before our lesson began I stood a tumbler of coldwater on the table. Come and look at the tumbler, Fred, and tell me whether you see anything unusual about it.""The outside of the tumbler is covered with little drops of water, sir," said Fred, "and some of them are running together and trickling down the sides of the glass.""It is so," replied Mr. Wilson. "Now, the first point to settle is where these drops of water came from. They did not come from the glass, either by oozing through its sides or by overflowing from the top. They came from the moisture or vapor in the air of the room. Let me explain.
"The air of the room is warm, and is able, at its present temperature, to hold a certain amount of water-vapor. The glass is cold, and it reduces the temperature of the air all round it, first to its point of saturation, and afterwards to its dew-point, and then it is that the drops of dew begin to form on the cold glass. Dew is always formed in this way-not in the air, like fog and cloud, but on the cold surface of solid bodies. There is always some solid body colder than the dew-point of the air around. The air comes into contact with this colder body, and is robbed of some of its heat. The loss of heat compels it to give up some of its vapor, and this is deposited as drops of dew on the surface of the cold body itself.
"All day long the earth and all the objects on its surfaceare absorbing more heat from the sun than they can give out by radiation. When night comes on, the absorbing process is over, and the radiation only is going on. Consequently, the surface of the earth (and of most solid bodies at night) is continually losing heat, and is, as a rule, colder than the air near it. Hence the formation of dew when this warm air comes into contact with it.
"When the temperature of the earth and the solid bodies on its surface sinks below 32°F., the moisture is deposited in the form of tiny particles of ice. We call it then hoar-frost.
"Dew is usually formed on a fine, clear, still night,succeeding a warm day. Very little dew is formed on a cloudy night. The clouds act like a curtain, and prevent the radiation of heat from the earth.
"Dew is formed abundantly on grass and foliage generally, on trees, and on wooden palings, but very little is formed on the gravel paths or the stone pavements. Gravel and stone are bad absorbers, and bad radiators of heat."
