94 POPULAR SCIENCE MONTHLY. 



Rain drops, in fact, grow as they fall, whether by continuance 

 of condensation, or by union with other drops. They should, there- 

 fore, be larger when they issue from the cloud in proportion as the 

 region where drizzle is formed is higher above the base of the cloud. 

 There is, however, a limit to the size they can attain, for the velocity 

 of their fall increases with their mass, and they are divided by the 

 resistance of the air. 



The five aspects under which we have regarded the formation of 

 rain are evidently five phases distinguished by our senses in the 

 progressive transformation which the vapor of water undergoes in 

 passing to the liquid state. It also sometimes happens that the con- 

 densation of the vapor in a cloud can only reach the first or second 

 stage of the transformation without extending to the other stages. 

 At other times it stops at the third phase, that of drizzling, which 

 may then, as rain does, cross atmospheric regions below the cloud, and 

 reach the ground, provided the base of the cloud is not too high and 

 the air passed through is not too dry. In short, we may conclude 

 that the formation of rain is due simply to variations in the tem- 

 perature and moisture of the air. There is, however, another ele- 

 ment, the intervention of which is indispensable, if not to reduce the 

 vapor to water, at least to cause that water to fall in rain, or under 

 the form of drops. This element is the atmospheric dust. 



"We designate generally as atmospheric dust all the corpuscles 

 which the atmospheric envelope of the earth holds in suspension; 

 but distinctions should be made. Some dust occurs in the air for- 

 tuitously and for the moment, such as troubles us in dry weather 

 when the wind is blowing. This is coarse, and so evident that we say 

 " It is dusty," and soon falls by its weight to the ground. There is 

 other dust which remains in the air almost permanently. It becomes 

 visible to the eye when illuminated against a dark background, as 

 when a sunbeam comes into a dark room. Other dust may be studied 

 under a microscope of low power; and still other, and the largest 

 proportion of that in the atmosphere, is so fine that it can not be dis- 

 tinguished, even with the most powerful instruments. 



This extremely fine and light dust is disseminated to heights 

 that may exceed fifteen or twenty or more miles. Cyclones, volcanic 

 eruptions, and immense prairie fires are the principal causes of its 

 production and expansion in the atmosphere. Mr. Aitkin, a Scotch 

 meteorologist, has made some remarkable experiments to demon- 

 strate the existence of this dust. For that purpose he employed a 

 very ingenious method, which permitted him to count all the par- 

 ticles, even those which could not be seen with a microscope. The 

 principle of his method is as follows: If we fill a receiver with air 

 that has been deprived of all its dust by passing it through a liquid, 



