Page:Popular Science Monthly Volume 17.djvu/360

346 of the formation of beds by the settlement of the dust, covering the largest mountains, as Popocatepetl and Orizaba, rising to the height of about 12,000 feet on the slopes and reaching a thickness in the valleys of from 250 to 325 feet. These formations are occasioned by whirlwinds of dust which are frequent on the Mexican plain, and are stopped by the elevated chain of the mountains, as the mud in a river is stopped by a sand-bar. The action of the prevailing winds in other parts of the globe promotes the formation of similar deposits, which may be called aërial lands.

These are the dusts of the lower regions of the atmosphere. But the air contains other particles in the most minute degree of division. The waters of the sea, impinging on the coast in waves, are broken into thousands of particles which are taken up and evaporated by the winds. The saline residue of these particles adds a new element to the atmospheric dust; the vapors furnished by the sea go to the formation of clouds and fogs. Rising beyond the regions of ordinary clouds, these vapors ascend to the colder strata, where they are converted into a dust consisting of minute crystals of ice which form the cirrhus clouds and the ice-fields of the upper regions. These masses of frost, which can only be distinguished when they are approached by a balloon, but the existence of which is well established, are agents in the formation of halos and parhelia, and descend in cold winds to the surface.



If we pass these heights, into the extreme limits of the atmosphere, we shall find ourselves in the presence of dust from a new source—of that which is furnished by the combustion of incandescent aërolites. The fruits of the study of meteoric astronomy prove that the surface of the earth is continually receiving cosmic materials either in the form of meteorites or shooting-stars, or of an impalpable dust. A ship passing to the south of Java in January, 1859, was assailed by a very fine ferruginous dust. Ehrenberg examined some of it with the microscope, and found that it was formed of melted globules of oxide of iron, and did not hesitate to regard it as consisting of particles of a mass of meteoric iron, which had been melted off by the operations of atmospheric friction. In other cases the particles may originate in the disintegration of the substance of the meteors, when the soluble salts with which they are cemented are dissolved by atmospheric moisture, as Daubrée observed in the case of a meteorite at Orgueil. Showers of fire have been mentioned, or of sparks which seem to be formed of