346 THE POPULAR SCIENCE MONTHLY. 



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 whirl- 

 winds 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 aerial 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 atmos- 

 pheric 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. 



Fig. 2. Corpuscles extracted by the Magnet from the Sediment op Rain-water at 

 Sainte-Marie-du-Mont, Manche. (500 Diameters.) 



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 aerolites. 

 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 micro- 

 scope, 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 Daubree observed in the case of a meteorite at Orgueil. Showers 

 of fire have been mentioned, or of sparks which seem to be formed of 



