INORGANIC NITROGEN FERTILIZERS 87 



would have been objectionable as a manure, since arsenic 

 applied in manure to crops is liable to be taken up by the 

 crops themselves. When arsenic is applied to the soil as an 

 impurity in a manure, it passes into the barley grain, and 

 thence into the beer brewed from malted barley. Although 

 the amounts likely to pass in this way are undoubtedly 

 small, the consumer naturally objects. The hydrogen 

 cyanide may also be a cause of some slight difficulty owing 

 to the colour which it produces in the ammonium sulphate. 

 Should local alkalinity occur, ammonium cyanide would 

 be formed, and as small amounts of iron are sure to be 

 present, and would be reduced to the ferrous state by 

 hydrogen sulphide, the conditions are suitable for the for- 

 mation of ammonium ferro-cyanide : 6NH 4 CN -f Fe(OH) 2 

 = (NH 4 ) 4 FeC 6 N 6 +2NH 4 OH. On the local alkalinity being 

 replaced by acidity, ammonium ferro-cyanide and ferrous 

 salts in solution will react, and, after exposure to air, 

 produce Prussian blue, resulting in the formation of a blue 

 ammonium sulphate. Hofmann, Arnoldi and Hiendlmaier 

 found that when the reacting substances were equivalent to 

 FeSO 4 + 6HCN + 4NH 3 the precipitate after oxidation had 

 the formula Fe 2 C 6 N 6 NH 4 ,iJH 2 O, but when the proportions 

 reacting were FeSO 4 + 6HCN + 2NH 3 , Fe 2 C 6 N 6 H.2H 2 O 

 was formed after oxidation. 



In the presence of air, local alkaline conditions would 

 also produce ammonium cyanide which, acting on the free 

 sulphur, produced from hydrogen sulphide by oxidation, 

 would form ammonium sulpho-cyanide, NH 4 SCN. Subse- 

 quently a ferric sulpho-cyanide, Fe(SCN) 3 , which possesses 

 a red colour will be formed, and under these conditions the 

 ammonium sulphate will be coloured red. Both the blue 

 and red colours are avoided by preventing local alkalinity. 

 In some old forms of apparatus the gases passing into the 

 saturator enter by a wide funnel-shaped pipe. This arrange- 

 ment is liable to produce coloured products, as this system 

 favours local alkalinity. It is now more common to use a 

 coil of lead pipe, perforated with small holes, through which 

 the ammonia gas is driven with some small amount of force, 



