no The Realm of Nature CHAP, vn 



air. 1 His numerous experiments show that in one cubic 

 centimetre of the air of great cities there are hundreds of 

 thousands of motes ; in the air of small villages there are 

 thousands, and there are hundreds even in the open 

 country far from towns or factories. The purest air 

 met with was on one occasion on the summit of Ben Nevis 

 where one cubic centimetre contained only one dust-mote, 

 the mean of ten observations. These minute motes catch- 

 ing and scattering the sunlight are the agents by which the 

 whole atmosphere is so illuminated that not even the bright- 

 est of the stars is visible by day. If the air were free from 

 dust we should probably see the Sun shining from a perfectly 

 black star-filled sky, and one side of a house would be dazz- 

 lingly illuminated, the other in a shadow of absolute darkness. 

 The blue colour of the clear sky ( 153) is largely due to 

 the scattering of sunlight by the dust-motes of the higher 

 layers. The red tints produced at sunrise and sunset 

 ( 297) and the lingering twilight of high latitudes have a 

 similar origin. Twilight is produced when light from the 

 Sun, while still below the horizon, strikes on the upper 

 atmosphere, too obliquely for refraction ( 150) to bend 

 the rays down to the surface ; then the illuminated dust- 

 motes of the upper air light up the sky for hours with a 

 soft shimmer. 



REFERENCE 



1 J. Aitken, "On the Number of Dust Particles in the Atmo- 

 sphere." Transactions Roy. Soc. Edin. xxxv. p. I (1888). 

 See also Nature, xxxvii. 428 (1888) and xli. 394 (1890). 



BOOKS OF REFERENCE 



R. Angus Smith. Air and Rain. Longmans. 

 See also lists at end of Chapters VIII. and IX. 



