526 VARIOUS FERTILITY FACTORS 



yields an annual revenue of more than $20,000,000, or about 

 three fourths of the total income of the Chilian government. Most 

 of the sodium nitrate imported into the United States is used for 

 the manufacture of explosives. 



Calcium nitrate. The artificial fixation of atmospheric nitrogen 

 by an economic and practical method is a problem whose solution 

 has been given much attention for many years; in fact, it has been 

 the dream of many a chemist and a dream which has only recently 

 been realized. 



Calcium nitrate is now produced to a limited extent (chiefly 

 at Notodden, Norway, by the aid of cheap water power) by the 

 Birkeland-Eyde process, in which a current of air is subjected to 

 powerful electric action, which results in the formation of nitrogen 

 tetroxid (as observed by Priestly as early as 1775), which by 

 somewhat complex reaction with water and oxygen yields nitric 

 acid. This is treated with lime to form calcium nitrate, which is 

 obtained in crystallized form, Ca(NO 3 ) 2 4 H 2 O, containing about 

 12 per cent of nitrogen. Calcium nitrate is a highly deliquescent 

 substance, and must be shipped in air-tight containers. In an 

 experiment at Rothamsted 10 grams of calcium nitrate (produced 

 at Notodden in 1906) absorbed 20 per cent of water and became 

 liquid in 3 days, and in 10 days about 50 per cent of water had 

 been absorbed. 



Calcium cyanamid. Calcium cyanamid is also a product result- 

 ing from the artificial fixation of atmospheric nitrogen, by a pro- 

 cess recently developed by Frank and Caro of Germany. The 

 primary materials used in the process are limestone, coke, and 

 nitrogen gas. Calcium carbid is first produced by heating a 

 mixture of burned lime and coke to a very high temperature pro- 

 duced by an electric furnace: 



CaO + 3 C = CaC 2 + CO. 



The calcium carbid is finely ground and then placed in closed 

 retorts and heated to the requisite temperature in an atmosphere 

 of nitrogen, which reacts with the calcium carbid with the forma- 

 tion of calcium cyanamid and separation of carbon : 



CaC 2 + 2 N = CaCN 2 + C. 



