238 THE CHEMISTRY OF CREATION. 



tively pure atmosphere is charged with im- 

 purities of various kinds. A pale bluish haze 

 rests upon it, and slightly tinges the air for 

 some distance ; and when a breeze blows along 

 the valley, it may be seen wafted for a mile or 

 two from the village. The larger the city, the 

 more dense this cloud of impurities, which is by 

 no means all made up of smoke, although its 

 opacity is chiefly due to that ingredient in its 

 composition. In London it is extremely rare 

 that even in the length of a street the air is 

 perfectly transparent; objects distant only a 

 few hundred yards are perceptibly enveloped in 

 a mantle of bluish haze. Some highly interest- 

 ing observations upon this subject have been 

 made by Dr. Smith, and read by him before the 

 British Association in 1848. Of these we shall 

 present an abstract. The town has always been 

 found to differ from the country. This general 

 feeling is more conclusive than any experiment 

 that can be made in a laboratory. The various 

 manufactures of large towns, the necessary con- 

 'ditions to which the inhabitants are subjected, 

 and the deteriorating influences of man himself, 

 all exert a powerful effect upon the state of 

 purity, or otherwise, of the surrounding atmo- 

 sphere. Dr. Smith caused a portion of air to be 

 passed continually through a certain quantity 

 of water for three months. He was thus able 



