66 NITROGEN FIXATION 



quired in the construction of the living substance of the plant. 

 Strangely enough, though the atmosphere contains over 70 per 

 cent, of this gas, the plant has no way of utilizing it directly but 

 only in compounds, such as ammonia and nitrates, etc. Com- 

 pounds of this kind exist in very meager quantities upon the 

 earth. The only sources of supply of any consequence are the 

 decaying animal and vegetable life; manures; small amounts 

 that are formed in the air and that are then carried to the earth 

 in rains; the guano deposits, now practically consumed; and the 

 saltpeter beds of Peru and Chile, which will be exhausted by 1925. 

 Consequently widespread alarm has arisen lest famines ultimately 

 result through lack of these nitrogen compounds, and the situ- 

 ation becomes the more serious since from one half to two 

 thirds of the nitrogen compounds placed on the soils are lost an- 

 nually in various ways, especially by the leaching out of these 

 substances by rains and drainage waters. Naturally of late 

 years much attention has been directed to devices for uniting 

 the nitrogen of the air with other elements in order to find a sub- 

 stitute for the rapidly disappearing nitrogenous compounds. 

 Working upon the fact that nitrogen compounds are formed in 

 the atmosphere by electrical discharges, several countries are 

 now manufacturing by electrical processes large quantities of 

 nitric compounds. One of the valuable substances thus formed 

 is calcium nitrate. Nitrogen is first combined with oxygen by 

 utilizing the high temperature of the electric arc (2500 to 3000 

 C.) and the nitric oxide gas thus formed is passed through milk 

 of lime, thus forming calcium nitrate. Another nitrogen com- 

 pound, valuable for certain soils, is calcium cyanimid. It is 

 formed by passing nitrogen into closed retorts containing pow- 

 dered calcium carbide heated to a temperature of 1100 C. 

 Under this temperature the calcium carbide (this substance 

 forms acetylene gas when placed in water) unites with the 

 nitrogen, forming calcium cyanimid and carbon. The cyani- 

 mid slowly decomposes in the soil, yielding ammonia which can 

 be either directly or indirectly utilized by the plant. While 

 the value and general utility of these artificially formed nitrogen 

 compounds has not been thoroughly tested, it is not to be ques- 



