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. Aitken, 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, and saturate it with vapor, and then by cooling cause the vapor to condense, the resultant water is deposited directly. If the receiver is filled with air not cleared of its dust, the cooling of the mixture of air and vapor provokes first the formation of a fog that marks the presence of dust, because each particle of dust becomes a nucleus, a center of condensation, for the vapor. Finally, if the cooling is carried far enough, the water formed falls in very fine droplets, each one of which incloses a dust particle. Mr. Aitken has succeeded in counting these droplets, by introducing only a very small volume of dusty air into the receiver and finally filling it with absolutely pure air. He has thus found that the external air contains on the average 32,000 particles of dust per cubic centimetre after a rain of considerable duration, and 130,000 particles in fine weather. There are 1,860,000 particles in the same volume of air in the middle of a room, and 5,420,000 particles near the ceiling. The figures look fanciful, but they are exact, for they have been corroborated by numerous consistent experiments and agree with the determinations that have been made by other methods.