140 



NITRATE OF LIME 



and, under the best of conditions, the cost of production probably equals 

 that of the same amount of nitrogen as nitrate of soda or sulphate of 

 ammonia. 1 



Nitrate of Lime. The formation of nitric acid when electric 

 sparks are passed through moist air was noticed by Cavendish in 

 1785. In recent years, successful attempts to practically utilise this 

 reaction in the manufacture of a nitrogenous manure have been made. 

 In 1905, the first factory designed for this purpose was started at 

 Notodden in Norway, working according to a method devised by 

 Birkeland and Eyde. Air is passed through a specially constructed 

 electric furnace in which the arc, produced by an alternating current, 

 is spread out into a flat flame by means of powerful electro-magnets. 



The effect of the high temperature is first, to dissociate the mole- 

 cules of gaseous oxygen and nitrogen into atoms : 



N, = 2N L , = 20 



and then to cause combination between the atomic oxygen and 

 nitrogen : 



O + N = NO. 



This last reaction is reversible and the condition of equilibrium is de- 

 termined by temperature. 



According to Nernst, the following figures give the observed and 

 calculated (on the basis of the law of mass action) percentages of 

 nitric oxide in air after being raised to the various temperatures. 



Not only is the amount of nitric oxide greater at high tempera- 

 tures, but the reaction occurs more rapidly. It is important that the 

 cooling of the gases after the transformation be as rapid as possible, 

 otherwise the reversed reaction leads to a great destruction of the 

 nitric oxide first formed. When, however, the temperature has sunk 

 to about 600, the nitric oxide unites with additional oxygen to form 

 nitrous fumes which escape reversal. 



The gases obtained, in practice, only contain about one or two per 

 cent by volume of nitric oxide as they come out of the furnace. The 

 nitrous fumes are absorbed in water or in alkaline solutions and yield 

 either nitric acid or nitrates or mixtures of nitrates and nitrites. Usu- 

 ally an excess of lime is employed and basic calcium nitrate is thus 

 obtained, containing about 75 per cent of Ca(NO 3 ) 2 and some free 

 lime. 



1 Vide Paper by Guye, Jour. Soc. Chem. Ind., 1906, 567. 



