398 Rajinesqut on Atmospheric Dust. 



tains, on Mount Etna, in Sicily, on the Alps, on the Alleghany 

 and Catskill mountains in America, &c. and on the ocean. 



4. It deserves to be considered under many views : which 

 are its invisibility, its shape and size, its formation and origin, 

 its motion, its deposition and accumulation, its composition, 

 its uses, and its properties. 



6. This dust is invisible, owing to the tenuity of its parti- 

 cles, but they become visible in the following instances ; 

 when the sun shines on them, since they reflect the light, 

 when their size is increased, and when they are accumulated 

 any where. 



6. The size of the particles is very unequal, and their 

 shape dissimilar ; the greatest portion are exceedingly small, 

 similar to a whitish or grayish spark, without any determina- 

 ble or perceptible shape ; the larger particles are commonly 

 lamellar or flattened, but with an irregular margin, and the 

 largest appear to be lengthened or filiform ; the gray colour 

 prevails. Other shapes are now and then perceptible with 

 the microscope. 



7. Among the properties of atmospheric dust are those of 

 being soft, as light as atmospheric air, of reflecting the rays 

 received directly from the sun, of possessing a kind of pecu- 

 liar electricity, which gives it a tendency to accumulate on 

 some bodies more readily than on some others, and of forming 

 an earthy sediment, which does not become effervescent with 

 acids. 



8. This dust is either constantly or periodically formed, but 

 chemically in the atmosphere like snow, hail, meteoric stones, 

 honey-dew, earthy rains, &c. by the combination of gaseous 

 and elementary particles dissolved in the air. Its analysis 

 has never been attempted by chemists ; but the earthy sedi- 

 ment which is the result of its accumulated deposition, preves 

 that it is a compound of earthy particles in a peculiar state of 

 aggregation, and in which alumine appears to preponderate, 

 rather than calcareous or silicious earths or oxides. 



9. Its motion in calm weather, or in a quiet room, is very 

 slow ; the particles appear to float in the air in all directions, 

 some rising, some falling, and many swimming horizontally, or 



