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Rafinesque on Atmospheric Dust. . $99 
forming a variety of curved lines; what is most singular, is — 
that no two particles appear to have exactly the same direc- 
tion; yet after a while the greatest proportion fall down ob- 
calm day. When a current of air is created naturally or 
artificially in the open air or in a room, you perceive at 
once an increased velocity in their motion; they move with 
rapidity in all directions; but when a strong current or wind 
prevails, they are carried with it in a stream, preserving how- 
ever, as yet, their irregular up and down motion. 
10. Its formation is sometimes very rapid, and its accumu- 
lation very thick in the lower strata of our atmosphere, but 
the intensity is variable. Whenever rain or snow falls, this 
dust is precipitated on the ground by it, whence arises the 
purity of the air after rain and snow; but asmall share is still 
left, or soon after formed. In common weather it deposits 
itself on the ground by slow degrees, and the same in closed 
rooms. It forms then the dust of our floors, the mould of our 
roofs, and ultimately the surface of our soil, unless driven by 
winds from one place to another. 
11. I have measured its accumulation in a quiet room, and 
have found it variable from one fourth of an inch to one inch 
in the course of one year; but it was then in a pulverulent, 
fleecy state, and might be reduced by compression to one- 
third of its height, making the average of yearly deposit 
about one sixth of an inch. In the open air this quantity must 
be still more variable, owing to the quantities carried by 
the winds and waters to the plains, valleys, rivers, the sea, 
&c. or accumulated in closed places or against walls, houses, 
&e. I calculate, however, that upon an average, from six to 
twelve inches are accumulated over the ground in one hun- 
dred years, where it mixes with the soil and organic exuvie, 
to form the common mould. 
12. The uses of this chronic meteor are many and obvious. 
It serves to create mould over rocks, to increase their de- 
composition, to add to our cultivable soil, to amalgamate we 
alluvial and organic deposits, to fertilize sandy and unfruitful 
tracts in the course of time, to administer to vegetable life, &c. 
