84 



DISCOVERY 



attracted to the vast quantities of nitrogen in the air 

 as a fKassiblc source of supply, if only a means could be 

 found of converting the nitrogen into nitrates at a 

 reasonable cost. In solving this problem, perhaps 

 the greatest problem of the day, the scientists first 

 of all turned to nature to find out how she accom- 

 plished the process, and to see whether they could not 

 take a leaf out of nature's book. Now, if we glance 

 at the nitrogen-cycle diagram above, we at once see 

 two methods suggested, both of which have since 

 been imitated on a commercial scale with success ; 

 and in addition two new methods have been discovered 

 which are capable of competing with the Chili salt- 

 petre on a commercial basis. Of these four methods 

 only one is independent of cheap electrical power; 

 this is based on the culture of nitrogen-fixing bacteria, 

 the symbiotic bacteria to which reference has already 

 been made. This was done by Nobbe and Hiltner 

 in 1896, who sold these cultures for soil inoculation 

 under the trade name of " Nitragen." 



The electrical methods are, however, of much greater 

 interest, and appear also to be sounder commercial 

 propositions as well. 



Of these the Norwegian Process, as it is often called, 

 was based on an experiment of Cavendish (1785), 

 though really it only reproduces on a small scale 

 exactly what happens during a thunderstorm. Caven- 

 dish's experiment was in itself only a refinement of 

 one performed by Priestly (1779) ; it merely consisted 

 in passing sparks in a small quantity of air in an on- 

 closed space ; he thus caused a little of the nitrogen to 

 combine with the oxygen to form oxides of nitrogen, 

 which dissolved in water, with the production of nitric 

 acid. The miniature thunderstorms of Cavendish 

 and Priestly were improved uf>on by Siemens and 

 Halske (1902). They arranged a powerful arc light in 

 an enclosed space through which air could be drawn ; 

 then they spread the flame of the arc with a strong 

 electro-magnet, thus getting the maximum volume of 

 flame from the minimum of current. As the air 

 was drawn through this immensely hot flame, com- 

 bination took place, and oxides of nitrogen were formed. 

 From these beginnings the process of Birkeland and 

 Eyde (1905) was developed, until to-day in Norway, 

 where electric power is cheap owing to the abun- 

 dance of waterfalls, huge generators are producing 

 nitric acid directly from the air. In its broadest 

 outlines the process is not difficult to describe. A 

 current of many thousand volts produces an enormous 

 flaming arc between terminals of copper tubing, 

 through which water continually flows ; the whole is 

 enclosed in a furnace-like box of brick with metal 

 casing. An alternating current is employed so that 

 the arc passes from pole to pole every one-fiftieth of 

 a second, and in addition a powerful electro-magnet 



spreads the flame. Through this arc, resembling a 

 great ball of flame some 6 feet across, a current of 

 air is allowed to pass, and in its passage the nitrogen 

 in it partially combines with the oxygen, its partner, 

 to form oxides of nitrogen. The gases are then drawn 

 off up a series of towers, where they come into contact 

 with dilute nitric acid and milk of lime. This latter 

 combines with the nitric acid, and the salt (calcium 

 nitrate) resulting from this combination fixes the 

 atmospheric nitrogen in a form suitable for use as a 

 fertiliser. This is sold as Norwegian saltpetre. Such a 

 process, however, is only possible where very cheap 

 electric power, obtained from a natural source, is 

 available ; two other processes have for this reason 

 been developed, which are capable of being run on 

 commercial lines from power derived from coal. 

 Both of them depend on the production of ammonia 

 from nitrogen, separated from the air, by making it 

 combine with hydrogen, a matter of no small difficulty, 

 for nitrogen is, with the exception of the rare gases of 

 the atmosphere helium, argon, xenon, neon, and kryp- 

 ton, perhaps the most inert of all the elements. This 

 inertness of nitrogen is very curious, when one con- 

 siders how e.xtremely active this element becomes, 

 for good and evil, when properly combined. Not only 

 are the fertiliser and the explosives industries at the 

 mercy of the nitrogen atom, but the dyeing industry 

 as well owes its being to this remarkable element. 



The first of these two processes is usually referred 

 to as the Cyanamide Process, because this compound 

 is the characteristic feature of the method. As yet 

 Norway is the home of this process, but it is a British 

 company that is responsible for the operations in- 

 volved. Calcium carbide, the familiar materijd used 

 for the production of acetylene gas, is first of all manu- 

 factured on a gigantic scale. Lime and coal are heated 

 together in an electric furnace the temperature of 

 which reaches something like 3,000° C. The carbide, 

 after cooling, is ground to a fine powder in an atmo- 

 sphere of nitrogen, a precaution which is necessary so 

 as to avoid explosions, which might otherwise take place 

 from the accidental production of acetylene. Inci- 

 dentally use is here made of the inertia of nitrogen, a 

 property referred to above. 



The powdered carbide is then placed in large cylindri- 

 cal furnaces, through the sohd lids of which a carbon 

 electrode passes down the centre of the furnace. 

 Nitrogen is kept percolating through the powder, and 

 the temperature is gradually raised to about 1,100° C. 

 by the current till combination takes place and cyan- 

 amide is formed. Tliis substance, a compound of 

 calcium, carbon and nitrogen with some free carbon, 

 which is liberated during the reaction, is known com- 

 mercially as " nitrolim." The nitrogen used in the 

 manufacture of cyanamide from calcium carbide is 



