II.] CHEMICAL FIXATION OF NITROGEN 39 



nitrogen gas is obtained by passing a current of air 

 over red-hot copper, the copper oxide formed being 

 afterwards reduced to the metallic state again by sending 

 over it a current of coal-gas while it is still hot. More 

 recently a process of obtaining nitrogen by fractional 

 distillation from liquefied air has been employed. The 

 resulting calcium cyanamide is a very fine dark grey 

 powder, light and rather difficult to sow alone because 

 it floats so readily in the air. 



Since the product also contains about 20 per cent. 

 of free lime, it readily absorbs water from the atmo- 

 sphere, the first change that takes place being the 

 slaking of the quicklime. At the same time the cyan- 

 amide begins to decompose slowly into ammonia and 

 calcium carbonate in eventual accordance with the 

 equation 



CaCNg + sHp = 2NH3 4-CaCOa, 



so that some loss of ammonia may take place if the 

 manure is left lying about exposed in a loose condi- 

 tion to the atmosphere. In bags, however, it may be 

 stored without any sensible loss. 



With superheated steam the reaction takes place 

 more rapidly, and with acids or acid manures salts of cal- 

 cium and ammonium are formed, preceded of course by 

 an active interaction between the free lime and the 

 acid. The same reaction may be expected to take 

 place when calcium cyanamide is applied to the soil : 

 it should change slowly into ammonia, which will be 

 arrested by the soil, and calcium carbonate. It has 

 been shown, however, by Lohnis, that the reaction with 

 water alone is slow and not particularly effective, but 

 that in practice certain soil bacteria bring about the 

 change. 



Commercial cyanamide contains as much as 20 



