Rajinesque on Atmospheric Dust. 399 



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 awhile the greatest proportion fall down ob- 

 liquely, somewhat in the same manner as a light snow in a 

 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 a small 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 p-ilverulent 

 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, 

 &c. 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 exuviae, 

 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 the 

 alluvial and organic deposits, to fertilize sapdy and unfruitful 

 tracts in the course of time, to administer to vegetable life, kc, 



31 * 



