SCIENCE AND INDUSTRY. 
means of replacing from the atmosphere the combined nitrogen con- 
tinuously liberated during combustion, &e. Once combined with others 
the element lends itself to many chemical changes. The difficulty lies 
in causing it to enter into combination. 
It may be said that every known reaction for converting free 
nitrogen into some one or other of its compounds has been critically 
examined with a view to its commercial utilization. Much of this 
work was carried out before the war, but the desperate need for high 
explosives during the last five years resulted in the expenditure of 
millions of pounds on the industry in Germany, England, France, the 
United States of America, and other countries. As a result, its future 
trend can be forecasted with some degree of confidence. 
The processes which have been used, and are still in operation on 
the large scale, are— 
1. The Are process, in which air alone, or mixed with excess of 
oxygen, is blown through a high tension electric are. Several modifi- 
cations of the original Birkeland-Eyde system have been introduced, 
but the principle is the same in all. It has been established that the 
action is almost purely thermal, and that there is only one primary 
product, namely, nitric oxide. The action N, + O, = 2 NO is strongly 
reversible at the temperature employed, so that the mixture coming 
from the are is cooled as rapidly as possible. Cooling, however efficient, 
is not instantaneous, and dissociation of part of the nitric acid always 
occurs; the rest is converted into nitrogen peroxide, and, through the 
reaction of that substance with water, into weak nitric acid. The 
power requirements of this process are very large, and there is no 
prospect of its establishment except where very cheap water power is 
available. Further, nitric acid is difficult to transport, and to convert 
it into a salt for transport with subsequent reconversion into nitric 
acid would cost too much. 
2. The Cyanamide Process.—This depends on the ability of calcium 
carbide to re-act with nitrogen according to the equation— 
Ca, + N, = CaCN, + C. Carbide prepared in the known manner is 
finely ground in an atmosphere of nitrogen, charged into special 
furnaces, and heated to about 800° C. in nitrogen obtained by the frac- 
tional distillation of liquid air. On completion of the process the 
product is re-ground, and may be treated with steam to give ammonia. 
The main reaction is not reversible, and the power requirements are 
low compared with those of the are process, but the cyanamide process 
consists of a number of difficult operations. 
3. The Haber Process—This remarkable process, due to a member 
of that race from which has come the bulk of Germany’s intelligentia, 
is a triumph of physical chemistry and of engineering. The velocity of 
the action 2N, + 3H, — 2NH, increases with increasing pressure in 
the direction from left to right more rapidly than does the opposed 
action, in accordance with the law of mass action. For that reason 
operations are conducted at pressures of 180-200 atmospheres. At the 
highest attainable pressures the rate of reaction is still neglig ble at 
ordinary temperatures, so that an elevated temperature must be 
employed (700°). Rise of temperature increases the velocity of the 
back action also, but the difficulty so caused was overcome by the use 
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