THE CONQUEST OF NATURE 



But unfortunately the atoms of nitrogen are very 

 little prone to enter into such combinations ; under all 

 ordinary conditions they prefer a celibate existence. 

 In every thunder-storm, however, a certain quantity 

 of nitrogen is, through the agency of lightning, made 

 to combine with the hydrogen of dissociated water- 

 vapor, to form ammonia; and this ammonia, washed 

 to the earth dissolved in rain drops, will in due course 

 combine with constituents of the soil and become avail- 

 able as plant food. Once made captive in this manner, 

 the nitrogen atom may pass through many changes and 

 vicissitudes before it is again freed and returned to 

 the atmosphere. It may, for example, pass from the 

 tissues of a plant to the tissues of a herbivorous animal 

 and thence to help make up the substance of a car- 

 nivorous animal. As animal excreta or as residue of 

 decaying flesh it may return to the soil, to form the 

 chief constituent of a guano bed, or of a nitrate bed, 

 in which latter case it has combined with lime or so- 

 dium to form a rocky stratum! of the earth's crust that 

 may not be disturbed for untold ages. 



A moment's reflection on the conditions that govern 

 vegetable and animal life in a state of nature will make 

 it clear that a soil once supplied with soluble nitrates 

 is likely to be replenished almost perpetually through 

 the decay of vegetation. But it is equally clear that 

 when the same soil is tilled by man, the balance of 

 nature is likely to be at once disturbed. Every pound of 

 grain or of meat shipped to a distant market removes a 

 portion of nitrogen ; and unless the deficit is artificially 

 supplied, the soil becomes presently impoverished. 



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