402 CHEMICAL DISCOVERY AND INVENTION 



The first step is to reduce the carbide to powder, and to avoid 

 access of moisture, the grinding is performed in air-tight apparatus. 

 This grinding is associated with a system of screening, and 

 separation of the material into several different grades of fine- 

 ness according to the size of the perforations through which it 

 passes. The powder exposed to contact with nitrogen at a 

 temperature of 800 to 900 C. absorbs the gas, and the process 

 is attended by the evolution of much heat. The heat liberated 

 is, however, not sufficient to make the process automatically 

 continuous. At Odda the furnaces in which the combination is 

 effected are drum-shaped portable vessels and are brought into 

 position, when charged with carbide, by means of an overhead 

 electric traveller, by which also they are removed when absorp- 

 tion of nitrogen is complete. 



After being placed in position, seven in a row, the necessary 

 connections are made for admitting the nitrogen under pressure, 

 and the supply of electric current which is passed through a 

 carbon rod placed in the centre of each. In the view of the 

 furnace house shown there are 196 furnaces, each producing 

 about 1 ton of nitrolime per week, the total output being, there- 

 fore, about 10,000 tons per annum. 



The nitrolime of commerce consists of nearly two-thirds of its 

 weight of calcium cyanamide, CaN-CN, corresponding to a total 

 content of nitrogen equal to 20 to 22 per cent. 



The use of nitrolime as a manure is explained by the ultimate 

 conversion of the whole of this nitrogen when in contact with 

 water into ammonia. Nitrolime also contains about 20 per cent 

 of free lime, 7 to 8 per cent of silica, alumina, and iron as im- 

 purities, and 14 per cent of carbon, which is in the form of graphite. 



By the slow action of atmospheric air, which contains carbon 

 dioxide and moisture, cyanamide is converted into urea, and 

 hence old specimens of nitrolime may contain appreciable quan- 

 tities of this substance. 



It is probable that in the soil a change of this kind precedes 

 the final elimination of the nitrogen in the form of ammonia. 

 First carbon dioxide and water vapour would produce calcium 

 carbonate and cyanamide : 



CaNCN-hC0 2 +H 2 0=CaC0 3 +H 2 NCN. 



The cyanamide by uniting with water forms urea : * 



