146 MANURES. VII. 



of. The liquid is then evaporated in leaden pans until it 

 crystallizes. Formerly the ammoniacal liquor was sometimes 

 directly neutralised with sulphuric acid and the solution evapo- 

 rated. 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 sulphate is easy, its presence being at once indi- 

 cated 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 

 ammonia 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 % or even 2 % of arsenious 

 oxide and have just recently been brought into prominence 

 from the numerous cases of arsenic poisoning by beer, the 

 arsenic being traced to the sulphuric acid used in the prepa- 

 ration of the glucose added to the wort in the brewing. 

 Arsenic, if present in large quantities, usually imparts a 

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



Theoretically, coal containing 1*3% of nitrogen ought to 

 yield about 149ft). of sulphate of ammonia per ton ; but in 

 practice the coal used in gas works and for coke-making only 

 gives about 20tt). 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 temperature is kept low, the yield of 

 sulphate may amount to three or four times that just given. 



The magnitude of the ammonium sulphate manufacture 

 may be judged from the following table, which gives the 

 production in the United Kingdom during the years 1896, 

 1897, 1893, and 1899 : 



