Rain drops, in fact, grow as they fall, whether by continuance of condensation, or by union with other drops. They should, therefore, 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 condensation 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 temperature and moisture of the air. There is, however, another element, 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 fortuitously 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 distinguished, 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 demonstrate the existence of this dust. For that purpose he employed a very ingenious method, which permitted him to count all the particles, 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,
