398 - Rafinesque on Atmospheric Dust. 
tains, on Mount Etna, in Sicily, on the Alps, on the Alleghany 
and Cattskill mountains in America, &c. and on the ocean. 
5. 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. 
5. 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 color 
prevails. Other shapes are now and then perceptible with 
the microscope. ; 
7. Among the properties of atmospheric dust are those of 
ing 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. lis analysis 
has never been attempted by chemists; but the earthy sedi- 
ment which is the result of its accumulated deposition, prove 
that it is a compound of earthy particles in a peculiar state of 
ation, and in which alumine appears to perponderate, 
rather than calcareous or siliceous 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, oF 
