Page:Popular Science Monthly Volume 45.djvu/500

482 upon a given surface of the containing walls than when the volume of the gas was greater. Since the temperature remains the same, the same number of molecules will be-driven off from a given surface of these walls as before. There will, accordingly, be a condensation upon the walls, which will continue until enough gaseous molecules have been removed to make the exchange again even.

These are the three known methods of changing water vapor to the liquid form—viz., by lowering the temperature of the vapor and the other bodies in contact with it until the point of saturation has been passed, by compressing the vapor until there are enough molecules in unit volume to produce saturation, and by allowing the vapor molecules to strike upon some surface which will immediately take them into solution or into chemical combination. I know of no other method by which water vapor, or any other vapor, can be changed into the liquid form.

The conditions necessary for the precipitation of the aqueous vapor from the atmosphere are, then, as follows:

(1) The air must contain enough molecules of water vapor to more than saturate it, and must contain at the same time either solid or liquid bodies upon which these vapor molecules may be held fast by cohesion; or (2) the air which does not contain enough water vapor to saturate it may come in contact with solid or liquid substances, which combine with or dissolve the water molecules which strike upon them.

This latter condition can manifestly play no important part in atmospheric precipitation. The only condition under which such substances could cause condensation above the earth's surface would necessitate their distribution throughout the atmosphere, and if they were so distributed they would constantly absorb the atmospheric vapor until, loaded down with it, they would sink to the earth, and there would be a condition of perpetual rainfall.

For the general precipitation of atmospheric vapor we must accordingly depend upon the condensation due to cohesion. Of this form of condensation, dew is the simplest illustration. During the day the earth and the solid bodies upon its surface are raised by the sun's radiation to a temperature higher than that of the surrounding air. So long as this is the case the atmospheric vapor will not condense upon them, even if the air be cooled to the point of saturation. In the night the same substances which absorbed the sun's heat fastest now radiate it fastest and soon become colder than the surrounding air. As soon as they 4re cooled to the temperature of saturation of the surrounding air the vapor molecules begin to condense upon their surface.

Now, the condensation of water vapor in the air above the