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 from the ordinary advection fogs. The distinction is a practical one. It is pertinent to add also that a city fog forms usually under a lid. But while the city fog condenses on particles that are hygroscopic, the fogs of swamp lands, rivers, and ponds condense on particles that are materially different. Condensation does not take place so readily—in other words, the dust particles are indifferent nuclei. A thick fog condensed on nuclei of an indifferent sort may be “eaten up” by a slight rise in temperature; it may rain itself to pieces by a drop in temperature.

Sources of Atmospheric Dust.—Aside from the dust picked up and carried by the winds, there are well-defined sources of floating dust that must be considered. Cosmic, or meteoric dust, is not born of the earth; it is gathered by the earth from space. Large particles fall to the earth; but those materially less than a micromillimeter constitute the floating dust of the air. The character of this dust can be recognized only when the particles fall to the earth or are trapped while floating near to its surface.

Many of the particles thus caught are tiny meteorites. These, in many instances, are metallic globules or floating metal bubbles. They are essentially different from the metallic particles of smeltery dust, emery-wheel dust, and brake-shoe dust, which also are metallic. The cosmic dust of non-metal character cannot be recognized with any degree of certainty; indeed, recognition of any sort of dust whose particles are less than a micromillimeter is difficult. The gathering of cosmic dust seems to be constant rather than sporadic.

Additions of volcanic dust to the floating dust of the air are made irregularly, but they come in enormous quantities. Much of the ash falls to the ground, but a very large part consists of particles fine enough to constitute floating matter. The eruption of Krakatoa, in 1883, projected so much floating