CHAP. ix. THE IMPORTANCE OF DUST. 83 



effects of rain on vegetation, as compared with any 

 amount of artificial watering, though, no doubt, largely 

 due to the minute quantity of ammonia which the rain 

 brings down with it from the air, must yet be partly 

 derived from the organic or mineral particles which 

 serve as the nuclei of every raindrop, and which, being 

 so minute, are more readily dissolved in the soil and ap- 

 propriated as nourishment by the roots of plants. 



It will be observed that all these beneficial effects of 

 dust are due to its presence in such quantities as are pro- 

 duced by natural causes, since both gentle showers as 

 well as ample rains and deep blue skies are present 

 throughout the vast equatorial forest districts, where 

 dust-forming agencies seem to be at a minimum. But 

 in all densely-populated countries there is an enormous 

 artificial production of dust from our ploughed fields, 

 from our roads and streets, where dust is continually 

 formed by the iron-shod hoofs of innumerable horses, 

 but chiefly from our enormous combustion of fuel pour- 

 ing into the air volumes of smoke charged with uncon- 

 sumed particles of carbon. This superabundance of 

 dust, probably many times greater than that Avhich would 

 be produced under the more natural conditions which 

 prevailed when our country was more thinly populated, 

 must almost certainly produce some effect on our 

 climate; and the particular effect it seems calculated to 

 produce is the increase of cloud and fog, but not neces- 

 sarily any increase of rain. Rain depends on the supply 

 of aqueous vapor by evaporation ; on temperature, which 

 determines the dew point; and on changes in barometric 

 pressure, which determine the winds. There is prob- 

 ably always and everywhere enough atmospheric dust 



