MANUFACTURED COMMERCIAL FERTILIZERS 527 



The nitrogen gas is obtained from the air, either by passing 

 air through a hot tube containing copper turnings, which remove 

 the oxygen by forming copper oxid (the oxid being again reduced 

 by substituting coal gas for air), or by the Linde liquid-air process, 

 in which advantage is taken of the difference between the boiling 

 points of nitrogen ( 194 C.) and oxygen ( 184 C.), the nitro- 

 gen being evaporated at the lower temperature. 



Potassium cyanid is a well-known substance with the formula 

 KCN, or N=C K, and the group or radicle, N = C , is called 

 cyanogen, somewhat as the group NH 4 is called ammonium, and 

 the group NH 2 is called the amido group. Cyanamid contains 

 the two groups, thus N = C N=H 2 , and by replacing the two 

 hydrogen atoms by one bivalent calcium atom, calcium cyanamid 

 (N=C N=Ca) is produced. Calcium cyanamid itself con- 

 tains 35 per cent of nitrogen; and, if the product could be made 

 with the one atom of free carbon as the only impurity, the nitro- 

 gen would still reach 30 per cent, but about one third of the 

 commercial article consists of other impurities (coal ash, lime, cal- 

 cium carbid, sulfid, phosphid, etc.), the nitrogen being thus re- 

 duced to about 20 per cent. An analysis of a commercial sample 

 gave the following results: 



Calcium cyanamid (CaCN 2 ) 57.0 per cent 



Carbon 14.0 per cent 



Lime (CaO) 21.0 per cent 



Silicon dioxid 2.5 per cent 



Iron oxid 4.0 per cent 



Calcium sulfid, phosphid, and carbonate . . . 1.5 per cent 



When first added to the soil the commercial calcium cyanamid 

 with its impurities produces an injurious effect upon young plants, 

 and to avoid this it is applied a week or two before seeding. For 

 the same reason it cannot safely be used as a top-dressing. It has 

 a tendency, because of its lime content, to absorb moisture and 

 carbon dioxid from the air, and for protection is usually treated 

 with a small amount of heavy petroleum. A ten-gram sample 

 of calcium cyanamid exposed for 12 days at Rothamsted increased 

 30 per cent in weight, and some loss of ammonia occurred. 



