CALCIUM AND ITS COMPOUNDS 393 



the Great War. When treated with hot water, calcium cyana- 

 mide is decomposed as follows: 



CaCN 2 + 3H 2 -> CaC0 3 4- 2NH 3 . 



In practice, pulverized nitro-lime is fed into an autoclave charged 

 with water. Impurities such as free calcium carbide and cal- 

 cium phosphide are immediately decomposed, and the gases 

 evolved allowed to escape. Small amounts of alkali are then 

 added to facilitate the evolution of ammonia and to prevent the 

 formation of complex nitrogen compounds. The autoclave is 

 closed and steam admitted until the pressure rises to 3-4 atmo- 

 spheres, the heat evolved by the decomposition of the cyanamide 

 being sufficient to carry the reaction to rapid completion. The 

 ammonia given off is absorbed in water, or converted directly 

 to ammonium sulphate. When the gas ceases to be given off, 

 steam is blown through the liquor in the autoclave until all residual 

 dissolved ammonia is expelled. 



The productive capacity of cyanamide plants in 1920 exceeded 

 1,750,000 tons, calculated as nitro-lime. 



Nitro-lime, when fused with sodium carbonate, gives sodium 

 cyanide NaNC, used hi the extraction of gold: 



CaCN 2 + C + NaaCOa -> CaCO 3 + 2NaNC. 



Other Compounds of Calcium. Calcium fluoride CaF 2 

 occurs as a mineral (fluorite). It is our source of hydrofluoric 

 acid (p. 207), and is used in metallurgy to lower the melting-point 

 of slags. The phosphates (p. 411) and bisulphite (p. 398) of 

 calcium, and bleaching powder CaCl(OCl) (p. 224) are elsewhere 

 discussed. 



Strontium Sr and Barium Ba. The compounds of these 

 elements closely resemble those of calcium in physical properties 

 and chemical behavior. Strontium salts confer a carmine-red 

 color to the Bunsen flame, barium salts a green color. Both are 



