Page:Popular Science Monthly Volume 45.djvu/670

650 that by reason of the continuous motions in the atmosphere the equality of temperature, which would exist if all the strata were equally warmed, never can come to pass.

When the rising columns of air contain a sufficient quantity of vapor, it will at a certain height be condensed into drops and form clouds. We say that the cooling is the cause of the condensation. But it is now maintained and proved by experiment that cooling alone is not adequate to do this, and that condensation takes place only on the surface of some solid or liquid body; not in the free, pure air, but on the surface of the dust particles scattered through it. Every drop of a steam-jet or a cloud is a particle of dust covered with water. The experimental proof of this is easily made. We fill a large flask with dustless air by pressing ordinary air through wadding and conducting it into the flask till all the air originally therein has been replaced with filtered air. The wadding holds back all the dust particles. We then let a jet of steam from a boiler into the dustless air of the flask. It remains invisible. Not a sign of the usual cloudy appearance is perceptible. All that we observe is that the inner walls of the flask begin to trickle; the steam is condensed only on them, for there is no other fixed surface. If, now, some ordinary dusty air is blown into the flask, it at once appears to be filled with a thick, rolling cloud. The cloud is composed of as many drops as dust particles have been admitted. If only a little dust is admitted, all the vapor is precipitated upon it, and so loads it with water in a short time that it sinks in heavy drops to the ground. It is raining in our flask. It will soon become clear, and the vapor will be invisible as before.

Without dust there would be no condensation of water in the air—no fog, no clouds, no rain, no snow, no showers. The only condensing surface would be the surface of the earth itself. Thus the trees and plants, and the walls of houses, would begin to trickle whenever cooling began in the air. In winter all would be covered with a thick, icy crust. All the water which we are accustomed to see falling in rain-pours or in snow would become visible in this way. We should at once feel on going out of doors that our clothes were becoming wet through. Umbrellas would be useless. The air, saturated with vapor, would penetrate the interior of houses and deposit its water on everything in them. In short, it is hard to conceive how different everything would be, if dust did not oft'er its immeasurable extent of surface everywhere to the air. To this we owe it that the condensation of water is diverted from the surface of the earth to the higher, cooler atmospheric strata.

Since the importance of dust in meteorological phenomena has been recognized, experiments have been made in counting the