NITROGEN FROM THE AIR
There is a curious suggestiveness about this finding of aluminum at our very door, so to speak, some scores of centuries after the relatively rare and inaccessible metals had been known and utilized by man. But there is another yet more striking instance of an abundant element which man needed, but knew not how to obtain until the science of our own day solved the problem of making it available. This is the case of the nitrogen of the air. As every one knows, this gas forms more than three-fourths of the bulk of the atmosphere. But, unlike the other chief constituent, oxygen, it is not directly available for the use of plants and animals. Yet nitrogen is an absolutely essential constituent of the tissues of every living organism, vegetable and animal. Any living thing from which it is withheld must die of starvation, though every other constituent of food be supplied without stint; and the fact that the starving organism is bathed perpetually in an inexhaustible sea of atmosphere chiefly composed of nitrogen would not abate by one jot the certainty of its doom.
To be made available as food for plants (and thus indirectly as food for animals) nitrogen must be combined with some other element, to form a soluble salt. But unfortunately the atoms of nitrogen are very little prone to enter into such combinations; under all ordinary conditions they prefer a celibate existence. In every thunder-storm, however, a certain quantity of nitrogen is, through the agency of lightning, made to combine with the hydrogen of dissociated water-vapor, to form ammonia; and this ammonia, washed to the earth dissolved in rain drops, will in due course combine with constituents of the soil and become available as plant food. Once made captive in this manner, the nitrogen atom may pass through many changes and vicissitudes before it is again freed and returned to the atmosphere. It may, for example, pass from the tissues of a plant to the tissues of a herbivorous animal and thence to help make up the substance of a carnivorous animal. As animal excreta or as residue of decaying flesh it may return to the soil, to form the chief constituent of a guano bed, or of a nitrate bed,—in which latter case it has combined with lime or sodium to form a rocky stratum of the earth's crust that may not be disturbed for untold ages.
A moment's reflection on the conditions that govern vegetable and animal life in a state of nature will make it clear that a soil once supplied with soluble nitrates is likely to be replenished almost perpetually through the decay of vegetation. But it is equally clear that when the same soil is tilled by man, the balance of nature is likely to be at once disturbed. Every pound of grain or of meat shipped to a distant market removes a portion of nitrogen; and unless the deficit is artificially supplied, the soil becomes presently impoverished.
But an artificial supply of nitrogen is not easily secured—though something like twenty-five million tons of pure nitrogen are weighing down impartially upon every square mile of the earth's surface. In the midst of this tantalizing sea of plenty, the farmer has been obliged to take his choice between seeing his land become yearly more and more sterile and sending to far-off nitrate beds for material to take the place of that removed by his successive crops. The most important of the nitrate beds are situated in Chili, and have been in operation since the year 1830. The draft upon these beds has increased enormously in recent years, with the increasing needs of the world's population. In the year 1870, for example, only 150,000 tons of nitrate were shipped from the Chili beds; but in 1890 the annual output had grown to 800,000 tons; and it now exceeds a million and a half. Conservative estimates predict that at the present rate of increased output the entire supply will be exhausted in less than twenty years. And for some years back scientists and economists have been asking themselves, What then?
But now electro-chemistry has found an answer—even while the alarmists were predicting dire disaster. Means have been found to extract the nitrogen from the atmosphere, in a form available as plant food, and at a cost that enables the new synthetic product to compete in the market with the Chili nitrate. So all danger of a nitrogen famine is now at an end,—and applied science has placed to its credit another triumph, second to none, perhaps, among all its conquests. The author of this truly remarkable feat is a Swedish scientist, Christian Birkeland by name, Professor of Physics in the University of Christiania. His experiments were begun only about the year 1903, and the practical machinery for commercializing the results—in which enterprise Professor Birkeland has had the co-operation of a practical engineer, Mr. S. Eyde—is still in a sense in the experimental stage,—albeit a large factory was put in successful operation in 1905 at Notodden, Norway.
Professor Birkeland has thus accomplished what many investigators in various parts of the world have been striving after for years. The significance of his accomplishment consists in the fact that he has demonstrated the possibility of making nitrogen combine with oxygen in large quantities and at a relatively low expense. The mere fact of the combination, as a laboratory possibility, had been demonstrated in an elder generation by Cavendish, and more recently by such workers as Sir William Crookes, and Lord Rayleigh in England and Professors W. Mutjmaan and H. Hofer in Germany. Moreover, the experiments of Messrs. Bradley and Lovejoy, conducted on a commercial scale at Niagara Falls, had seemed to give promise of a complete solution of the problem; had, indeed, produced a nitrogen compound from the air in commercial quantity, but not, unfortunately, at a cost that made competition with the Chili nitrate possible. Equally unsuccessful in solving this important part of the problem had been the experiments, conducted on a large scale, of Professors Kowalski and Moscicki, at Freiburg.
All these experimenters had adopted the same agent as the means of, so to say, forcing the transformation—namely, electricity. The American investigators employed a current of ten thousand volts; the German workers carried the current to fifty thousand volts. The flame of the electric arc thus produced ignited the nitrogen with which it came in contact readily enough; but the difficulty was that it came in contact with so little. Despite ingenious arrangements of multiple poles, the burning-surface of the multiple arc remained so small in proportion to the expenditure of energy that the cost of the operation far exceeded the commercial value of the product. Such, at least, must be the inference from the fact that the establishments in question did not attain commercial success.
The peculiarity of Professor Birkeland's method is based upon the curious fact that when the electric arc is made to pass through a magnetic field, its line of flame spreads out into a large disk—"like a flaming sun." The sheet of flame thus produced represents no greater expenditure of energy than the lightning flash of light that the same current would produce outside the magnetic field; but it obviously adds enormously to the arc-light surface that comes in contact with the air, and hence in like proportion to the amount of nitrogen that will be ignited. In point of fact, this burning of nitrogen takes place so rapidly in laboratory experiments as to vitiate the air of the room very quickly. In the commercial operation, with powerful electro-magnets and a current of five thousand volts, operating, of course, in closed chambers, the ratio between energy expended and result achieved is highly satisfactory from a business standpoint, and will doubtless become still more so as the apparatus is further perfected.
To the casual reader, unaccustomed to chemical methods, there may seem a puzzle in the explanation just outlined. He may be disposed to say, "You speak of the nitrogen as being ignited and burned; but if it is burned and thus consumed, how can it be of service?" Such a thought is natural enough to one who thinks of burning as applied to ordinary fuel, which seems to disappear when it is burned. But, of course, even the tyro in chemistry knows that the fuel has not really disappeared except in a very crude visual sense; it has merely changed its form. In the main its solid substance has become gaseous, but every atom of it is still just as real, if not quite so tangible, as before; and the chemist could, under proper conditions, collect and weigh and measure the transformed gases, and even retransform them into solids.
In the case of the atmospheric nitrogen, as in the case of ordinary fuel, a burning "consists essentially in the union of nitrogen atoms with atoms of oxygen." The province of the electric current is to produce the high temperature at which alone such union will take place. The portion of nitrogen that has been thus "burned" is still gaseous, but is no longer in the state of pure nitrogen; its atoms are united with oxygen atoms to form nitrous oxide gas. This gas, mixed with the atmosphere in which it has been generated, may now be passed through a reservoir of water, and the new gas combines with a portion of water to form nitric acid, each molecule of which is a compound of one atom of hydrogen, one atom of nitrogen, and three atoms of oxygen; and nitric acid, as everyone knows, is a very active substance, as marked in its eagerness to unite with other substances as pure nitrogen is in its aloofness.
In the commercial nitrogen-plant at Notodden, the transformed nitrogen compound is brought into contact with a solution of milk of lime, with the resulting formation of nitrate of lime (calcium nitrate), a substance identical in composition—except that it is of greater purity—with the product of the nitrate beds of Chili. Stored in closed cans as a milky fluid, the transformed atmosphere is now ready for the market. A certain amount of it will be used in other manufactories for the production of various nitrogenous chemicals; but the bulk of it will be shipped to agricultural districts to be spread over the soil as fertilizer, and in due course to be absorbed into the tissues of plants to form the food of animals and man.