NN  +  OO   —­>   NO + NO
nitrogen  oxygen    nitric oxide

The —­> represents Jove’s thunderbolt, a stroke of artificial lightning.  We see on the left the molecules of oxygen and nitrogen, before taking the electric treatment, as separate elemental pairs, and then to the right of the arrow we find them as compound molecules of nitric oxide.  This takes up another atom of oxygen from the air and becomes NOO, or using a subscript figure to indicate the number of atoms and so avoid repeating the letter, NO_{2} which is the familiar nitro group of nitric acid (HO—­NO_{2}) and of its salts, the nitrates, and of its organic compounds, the high explosives.  The NO_{2} is a brown and evil-smelling gas which when dissolved in water (HOH) and further oxidized is completely converted into nitric acid.

The apparatus which effects this transformation is essentially a gigantic arc light in a chimney through which a current of hot air is blown.  The more thoroughly the air comes under the action of the electric arc the more molecules of nitrogen and oxygen will be broken up and rearranged, but on the other hand if the mixture of gases remains in the path of the discharge the NO molecules are also broken up and go back into their original form of NN and OO.  So the object is to spread out the electric arc as widely as possible and then run the air through it rapidly.  In the Schoenherr process the electric arc is a spiral flame twenty-three feet long through which the air streams with a vortex motion.  In the Birkeland-Eyde furnace there is a series of semi-circular arcs spread out by the repellent force of a powerful electric magnet in a flaming disc seven feet in diameter with a temperature of 6300 deg.  F. In the Pauling furnace the electrodes between which the current strikes are two cast iron tubes curving upward and outward like the horns of a Texas steer and cooled by a stream of water passing through them.  These electric furnaces produce two or three ounces of nitric acid for each kilowatt-hour of current consumed.  Whether they can compete with the natural nitrates and the products of other processes depends upon how cheaply they can get their electricity.  Before the war there were several large installations in Norway and elsewhere where abundant water power was available and now the Norwegians are using half a million horse power continuously in the fixation of nitrogen and the rest of the world as much again.  The Germans had invested largely in these foreign oxidation plants, but shortly before the war they had sold out and turned their attention to other processes not requiring so much electrical energy, for their country is poorly provided with water power.  The Haber process, that they made most of, is based upon as simple a reaction as that we have been considering, for it consists in uniting two elemental gases to make a compound, but the elements in this case are not nitrogen and oxygen, but nitrogen and hydrogen.  This gives ammonia instead of nitric acid, but ammonia is useful for its own purposes and it can be converted into nitric acid if this is desired.  The reaction is: