therefore, expose the air to a very high temperature and then very suddenly cool it to a temperature so. low that the nitrogen oxide already formed does not decompose at an appreciable rate.
These conditions have been practically realized in only one way—by causing an electric discharge, similar to that in an ordinary arc lamp, to take place in air. The temperature of the arc is enormously high, but the air just outside of it is comparatively cool; so that any nitrogen oxide formed at the boundaries of the arc mixes at once with the colder air and thus escapes decomposition. The excess of air containing the oxides of nitrogen is then passed into towers filled with quartz over which water is trickling, whereby nitric acid is formed.
It is not necessary to enter further into details; for these are the essential features of the commercial process for the manufacture of nitric acid which is now being carried out on a large scale at Notodden in Norway. Aside from the cost of installing and maintaining the electrical and absorbing apparatus, the only large expense involved in the process is the cost of power used in producing the electric discharge. The works must therefore be located where water-power is obtainable at the lowest possible cost; and Norway was naturally chosen as the seat of the industry in Europe. The saltpeter factories there are already utilizing 200,000 horse-power; and thousands of tons of their product have been shipped to this country, for use in fertilizing the fruit orchards of California and the sugar plantations of Hawaii.
Almost simultaneously with this process for the manufacture of nitrate there is being developed a process for the artificial production of ammonia, its competitor in the fertilizer field. The aim is to produce this compound also from its elementary constitutents, nitrogen and hydrogen. Nearly pure nitrogen can now be obtained cheaply from the air by a commercial process which up to twenty years ago had been carried out only on the smallest laboratory scale; namely, by liquefying air with the aid of a liquid-air machine, and then distilling the mixture of nitrogen and oxygen, much as a mixture of alcohol and water is distilled in the rectification of spirit. The nitrogen, having a much lower boiling-point, passes off first, yielding a gas containing less than half a per cent, of oxygen, which can readily be removed from it by chemical means. Pure hydrogen can be obtained cheaply by the decomposition of water in two or three different ways. The raw materials needed for the production of ammonia, although not costless like the air and water used in making nitric acid, are therefore obtainable at low cost; and the main problem again consists in finding a practical way of causing them to combine.
It is a curious fact that difficulties are met with here which are just the reverse of those encountered in the synthesis of nitric acid. Ammonia is a compound on which temperature has the opposite effect:
