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Dust is usually classed as “foreign matter of the air.” Such a view of the shell of wind-blown dust is permissible. It might also be considered logical as regards the finer dust particles which do not reach the ground except by means other than their own gravity. It is hardly logical to consider the dust of the stratosphere as foreign matter of the air; for, as a matter of fact, it is permanently and not temporarily there. Moreover, cosmic dust seems to pervade every part of the universe which the solar system traverses, and the earth is constantly gathering dust from space.

“Invisible” Dust; Characteristics.—Practically nothing is known of the dust of the stratosphere, except that its presence is revealed in various ways. The particles themselves are too small to be discerned by any mechanical method at present known. En masse they reflect enough sunlight to reveal their existence, but not their form nor their constitution. In part, and probably to a great extent, they constitute the overhead effect noted by observers for more than six thousand years—the sky. An estimate of the size of such dust particles cannot be made with any degree of accuracy. It is safe to say that they are much smaller than the smoke particles that escape from the burning end of a cigar. It safe also to say that they are smaller than the particles which constitute the “blue haze.” As a matter of fact, rapid changes in sky polarization indicate about the only thing that can be authoritatively asserted—namely, that the invisible particles behave much like the molecular constituents of the air.

Measurements.—Under the stratosphere, the dust particles of the air are of every possible size, from those of the blue haze to the coarse rock waste of the simoon. The micromillimeter, practically the twenty-five thousandth part of an inch, is a convenient unit of measurement. It is convenient because of