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 rôle 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