VII. 



SPECIAL MANURES 



137 



Organic bases and other substances were also present, but were 

 not estimated. 



In order to prepare a marketable commodity from this liquid, it is 

 heated and lime is added. The ammonia volatilises, partly as car- 

 bonate and sulphide, partly as free ammonia, and is received in sul- 

 phuric acid, whereby sulphate of ammonia is formed, and carbon 

 dioxide, sulphuretted hydrogen and other gases are evolved; these 

 are led away and suitably disposed of. The liquid is then evaporated 

 in leaden pans until it crystallises. Formerly the ammoniacal liquor 

 was sometimes directly neutralised with sulphuric acid and the solu- 

 tion evaporated. In this case the resulting sulphate was impure and 

 contained the highly objectionable thiocyanate, NH 4 CNS, which is 

 very injurious to vegetable life. Its detection in a specimen of sul- 

 phate is easy, its presence being at once indicated by the production 

 of a blood-red colouration when the solution is mixed with a little 

 ferric chloride solution. In the modern product, this impurity is 

 rarely present. Another possible impurity of importance is arsenic, 

 which may be present in the sulphuric acid ; it, too, is objectionable, 

 being highly poisonous both to animals and plants. 



The sulphuric acid used in the manufacture of sulphate of am- 

 monia ought to be either the arsenic-free acid specially prepared for 

 the purpose from brimstone, or pyrites acid which has been freed from 

 arsenic. Certain forms of Spanish pyrites give acid containing as 

 much as 1 per cent or even 2 per cent of arsenious oxide and a few 

 years ago were brought into prominence from the numerous cases of 

 arsenic poisoning by beer, the arsenic being traced to the sulphuric 

 acid used in the preparation of the glucose added to the wort in the 

 brewing. Arsenic, if present in large quantities, usually imparts a 

 yellow colour (due to As 2 S 3 [?]) to the ammonium sulphate. 



Theoretically, coal containing 1-3 per cent of nitrogen ought to 

 yield about 149 Ib. of sulphate of ammonia per ton ; but in practice, 

 the coal used in gasworks and for coke-making only gives about 20 Ib. 

 of sulphate per ton of coal, owing to a large portion of the nitrogen 

 being retained in the coke, some being evolved as free nitrogen and 

 some as pyridine, pyrrol and other nitrogenous tarry products. 



In the manufacture of producer and water gas, especially by some 

 of the most recent methods (e.g., by the " Mond " process), where the 



