Nitrogen is free as air. That is the trouble; it is too free. It is fixed nitrogen that we want and that we are willing to pay for; nitrogen in combination with some other elements in the form of food or fertilizer so we can make use of it as we set it free. Fixed nitrogen in its cheapest form, Chile saltpeter, rose to $250 during the war. Free nitrogen costs nothing and is good for nothing. If a land-owner has a right to an expanding pyramid of air above him to the limits of the atmosphere--as, I believe, the courts have decided in the eaves-dropping cases--then for every square foot of his ground he owns as much nitrogen as he could buy for $2500. The air is four-fifths free nitrogen and if we could absorb it in our lungs as we do the oxygen of the other fifth a few minutes breathing would give us a full meal. But we let this free nitrogen all out again through our noses and then go and pay 35 cents a pound for steak or 60 cents a dozen for eggs in order to get enough combined nitrogen to live on. Though man is immersed in an ocean of nitrogen, yet he cannot make use of it. He is like Coleridge's "Ancient Mariner" with "water, water, everywhere, nor any drop to drink." Nitrogen is, as Hood said not so truly about gold, "hard to get and hard to hold." The bacteria that form the nodules on the roots of peas and beans have the power that man has not of utilizing free nitrogen. Instead of this quiet inconspicuous process man has to call upon the lightning when he wants to fix nitrogen. The air contains the oxygen and nitrogen which it is desired to combine to form nitrates but the atoms are paired, like to like. Passing an electric spark through the air breaks up some of these pairs and in the confusion of the shock the lonely atoms seize on their nearest neighbor and so may get partners of the other sort. I have seen this same thing happen in a square dance where somebody made a blunder. It is easy to understand the reaction if we represent the atoms of oxygen and nitrogen by the initials of their names in this fashion: 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.