INORGANIC NITROGEN FERTILIZERS 89 



materials. Where the sulphuric acid is not previously 

 purified, it is sometimes possible to skim the arsenic sulphide 

 from saturators of an open type, but this is not a very satis- 

 factory method of dealing with the difficulty. Some effort 

 has also been made to convert the waste hydrogen sulphide 

 into pure sulphur by the " Claus kiln " method. The 

 result is not very satisfactory, as the sulphur is nearly 

 always contaminated by some tarry materials. 



In the ammonia plants attached to some of the large coke 

 ovens, efforts are now being made to produce ammonium 

 sulphate by the direct action of sulphuric acid on the gas, 

 without the preliminary production of liquor. When this 

 process is adopted, the gas is cooled by passage through a 

 heat interchange^ to deposit tar and a small amount of 

 ammonia liquor. The gas is then heated up again, by passage 

 through the other side of the heat interchanger, and passed 

 through sulphuric acid. Thus the ammonium sulphate is 

 produced at a temperature considerably higher than that at 

 which the gas has deposited its tar. The same risk of 

 producing coloured salts occurs in this process as in the 

 distillation method. As a certain amount of ammonia 

 liquor is produced by this method, a small ammonia still 

 is worked in the usual way, but the major part of the 

 ammonia is absorbed directly. 



Synthetic Ammonia. In recent years attempts have 

 been made to synthesize ammonia from nitrogen and hydro- 

 gen in the presence of a catalyst. (See Haber in References 

 to this section.) At high temperatures appreciable quantities 

 of ammonia can be made. Very great difficulties occur in 

 this manufacture, owing to the engineering difficulties of 

 working under very high pressures. The demand for 

 ammonia is, however, a great one, and is likely to increase, 

 owing to the necessity for the use of increasing quantities 

 of sulphate of ammonia for fertilizers ; it is highly prob- 

 able that more use will be made of this method in the future. 

 Up to the present it is known that no less than about one 

 million tons synthetic ammonium sulphate has been manu- 

 factured in Germany, but the published information regarding 



