At Muscle Shoals a mushroom city of 20,000 sprang up on an Alabama cotton field in six months.  The raw material, air, was as abundant there as anywhere and the power, water, could be obtained from the Government hydro-electric plant on the Tennessee River, but this was not available during the war, so steam was employed instead.  The heat of the coal was used to cool the air down to the liquefying point.  The principle of this process is simple.  Everybody knows that heat expands and cold contracts, but not everybody has realized the converse of this rule, that expansion cools and compression heats.  If air is forced into smaller space, as in a tire pump, it heats up and if allowed to expand to ordinary pressure it cools off again.  But if the air while compressed is cooled and then allowed to expand it must get still colder and the process can go on till it becomes cold enough to congeal.  That is, by expanding a great deal of air, a little of it can be reduced to the liquefying point.  At Muscle Shoals the plant for liquefying air, in order to get the nitrogen out of it, consisted of two dozen towers each capable of producing 1765 cubic feet of pure nitrogen per hour.  The air was drawn in through two pipes, a yard across, and passed through scrubbing towers to remove impurities.  The air was then compressed to 600 pounds per square inch.  Nine tenths of the air was permitted to expand to 50 pounds and this expansion cooled down the other tenth, still under high pressure, to the liquefying point.  Rectifying towers 24 feet high were stacked with trays of liquid air from which the nitrogen was continually bubbling off since its boiling point is twelve degrees centigrade lower than that of oxygen.  Pure nitrogen gas collected at the top of the tower and the residual liquid air, now about half oxygen, was allowed to escape at the bottom.

The nitrogen was then run through pipes into the lime-nitrogen ovens.  There were 1536 of these about four feet square and each holding 1600 pounds of pulverized calcium carbide.  This is at first heated by an electrical current to start the reaction which afterwards produces enough heat to keep it going.  As the stream of nitrogen gas passes over the finely divided carbide it is absorbed to form calcium cyanamid as described on a previous page.  This product is cooled, powdered and wet to destroy any quicklime or carbide left unchanged.  Then it is charged into autoclaves 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.