oclaves and steam at high temperature and pressure is admitted.
The steam acting on the cyanamid sets free ammonia gas which is carried
to towers down which cold water is sprayed, giving the ammonia water,
familiar to the kitchen and the bathroom.

But since nitric acid rather than ammonia was needed for munitions, the
oxygen of the air had to be called into play. This process, as already
explained, is carried on by aid of a catalyzer, in this case platinum
wire. At Muscle Shoals there were 696 of these catalyzer boxes. The
ammonia gas, mixed with air to provide the necessary oxygen, was
admitted at the top and passed down through a sheet of platinum gauze of
80 mesh to the inch, heated to incandescence by electricity. In contact
with this the ammonia is converted into gaseous oxides of nitrogen (the
familiar red fumes of the laboratory) which, carried off in pipes,
cooled and dissolved in water, form nitric acid.

But since none of the national plants could be got into action during
the war, the United States was compelled to draw upon South America for
its supply. The imports of Chilean saltpeter rose from half a million
tons in 1914 to a million and a half in 1917. After peace was made the
Department of War turned over to the Department of Agriculture its
surplus of saltpeter, 150,000 tons, and it was sold to American farmers
at cost, $81 a ton.

For nitrogen plays a double role in human economy. It appears like
Brahma in two aspects, Vishnu the Preserver and Siva the Destroyer. Here
I have been considering nitrogen in its maleficent aspect, its use in
war. We now turn to its beneficent aspect, its use in peace.

III

FEEDING THE SOIL

The Great War not only starved people: it starved the land. Enough
nitrogen was thrown away in some indecisive battle on the Aisne to save
India from a famine. The population of Europe as a whole has not been
lessened by the war, but the soil has been robbed of its power to
support the population. A plant requires certain chemical elements for
its growth and all of these must be within reach of its rootlets, for it
will accept no substitutes. A wheat stalk in France before the war had
placed at its feet nitrates from Chile, phosphates from Florida and
potash from Germany. All these were shut off by the firing line and the
shortage of shipping.

Out of the eighty elements only thirteen are necessary for crops. Four
of these are gases: hydrogen, oxygen, nitrogen and chlor