IRRIGATION. 207 



tion of the laws of nature, they respond to the gen- 

 tle vibrations, caused by the descent of genial and 

 fertilizing showers. 



286. Rain is only natural irrigation ; the water is 

 found like that of rivers, rich in oxygen, and organic 

 matter. The fertilizing power of rain, is referred to 

 the same causes, which lead to irrigation, to the salts 

 and geine, which rain water contains. Several chem- 

 ists have proved the existence of saline matters and 

 organic substances in the air. The falling rain, car- 

 ries down with it salts of ammonia, of lime, and a 

 flocky organic matter. These all may be supposed 

 floating in the air. The dry soils, give to the winds 

 an impalpable dust, its silicates and geine. When 

 hailstones, which have been formed in the regions of 

 perpetual frost, exhibit almost the same substances, 

 which are contained in rain water, the height at 

 which these matters float, would almost compel the 

 supposition that they exist in a gaseous state. From 

 the examination of hailstones, by Girardin, a French 

 chemist, it appears, that no sensible trace of ammonia 

 was detected during the evaporation of their water, 

 but there was found a notable quantity of lime and 

 sulphuric acid ; and above all, a large proportion of 

 an organic substance containing nitrogen. Melted 

 hailstones have the appearance of water, containing a 



