DUST IN THE AIR. 7 



ness it is to the earth only as the thin film of dust which 

 settles on our furniture. 



But then, again, some of the mineral-dust which finds 

 its way into our streets, and thence into our houses, has 

 been made altogether outside the world, and has floated 



Fig. 2. MICROSCOPIC SECTION OF METEORIC STONE. 



down through the air from regions of which we know 

 nothing more than the little we can learn from this dust 

 and by means of the telescope. 



Cosmic or meteoric dust (Fig. i) is of the same nature as 

 those larger fragments of matter which, when they take fire on 

 coming into contact with the earth's atmosphere, and blaze 

 for a few instants in the sky, we call "shooting" or " falling" 

 stars. The number of these meteorites (Fig. 2) is simply in- 

 conceivable, since they form, we are told, a ring one million 



