650 THE POPULAR SCIENCE MONTHLY. 



plained that by reason of the continuous motions in tlie atmos- 

 phere 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 con- 

 densation. But it is now maintained and proved by experiment 

 that cooling alone is not adequate to do this, and that condensa- 

 tion 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 ap- 

 pearance 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 



