INORGANIC NITROGEN FERTILIZERS 91 



behind to subsequently react with steam, with the formation 

 of carbon monoxide, in accordance with the equation : 



C + H 2 = H 2 + CO 



The intermittent method may be modified by utilizing gas 

 which contains sufficient carbon dioxide to prevent the 

 separation of the carbon according to the above equation. 

 Hydrogen obtained in this way is almost completely free 

 from carbon monoxide, and contains as its chief impurity 

 a little nitrogen, which, for the purpose of manufacturing 

 ammonia, is no disadvantage. To synthesize ammonia 

 from the mixture of nitrogen and hydrogen it is necessary 

 to compress it to the very high pressure of about 180 atmo- 

 spheres. The catalysts which are available, apart from the 

 rare metals of the platinum group, consist of uranium and 

 iron. The mass of the catalyst is generally iron, which is 

 increased in activity by traces of some other substances; 

 for example, the iron potash catalyst is one frequently used. 

 The gas then passes to a refrigerator, where the ammonia 

 is almost solidified at a temperature of 77 C. The gas then 

 returns to the catalyst, which is maintained at a temperature 

 of about 650 C. The economy of working depends very 

 largely upon the efficiency of the heat and cold interchanges, 

 which utilize the heat and cold employed in different parts 

 of the cycle. In the economical efficiency of the process too 

 great care cannot be paid to these temperature interchanges. 

 The construction of the ammonia retorts presents con- 

 siderable difficulty, not only on account of the high pressure, 

 but also on account of the temperature, which approaches 

 that of red heat. Moreover, there is a rapid deterioration of 

 the wall of the vessel under the influence of ammonia at high 

 temperatures. It is better to design the plant so that the 

 retort is completely surrounded by a nitrogen chamber at 

 the same pressure as that to which the mixture of gases 

 in the retort is subject, the whole being surrounded by 

 a wall capable of resisting the pressure. Owing to the high 

 pressure a burst occasionally takes place, which usually 

 results in a clean split. The method of heating the retort 



