328 THE CHEMISTRY OF CREATION. 



every object upon which the sun shines, every 

 tree, rock, and hill before us,- that feels the 

 genial influence of its rays, is an instrument 

 for communicating motion to the air, and, in a 

 degree by no means to be despised, assisting to 

 preserve the intermixture and purity of the 

 atmosphere. 



The effect of these imperceptible movements 

 is perhaps slight for a given time, but in their 

 constant operation, at times when no breath of 

 air stirs around us, they contribute largely to 

 preserve the air of our apartments and cities in 

 a condition fit for respiration, or at least more 

 fit by far than if they did not exist. When we 

 reflect how soon by their agency and that of 

 diffusion a cloud of smoke is dispersed, until 

 not a trace remains ; how soon an offensive gas 

 is wafted away, or a volume of dust dispersed, 

 we shall be better able to appreciate this unseen 

 instrumentality. 



Proportionately to its increased force is the 

 purifying influence of wind. The last-named 

 movements are able to dissipate impurities only 

 to a small extent, and within a very limited area. 

 Wind, on the contrary, is a body of air in move- 

 ment sufficiently powerful to sweep away every 

 accumulation of foreign ingredients in the atmo- 

 sphere generated by accident, or by the influ- 

 ence of congregated masses of mankind. How 



