in this way seems to differ in different parts of the world. In British Guiana during a period of twenty years the quantity so formed amounted each year on an average to 1.88 pounds per acre, while in Utah only 0.356 pounds was obtained annually for a period of three years. As a result of the decay of organic matter on the earth there is usually present in the air a sufficient quantity of ammonia to combine with all the nitric acid formed, although in the tropics the latter may sometimes be in excess. The ammonium nitrate which results from the combination dissolves in the snow and rain and in this way is carried to the earth along with other ammonium salts. Except in some parts of the tropics the total nitrogen recovered in this way is usually several times greater than the nitric nitrogen formed by the electric discharge. At Rothamsted, England, it amounted on an average during a period of eight years to 3.37 pounds per acre annually. At Ottawa, Canada, the average for the past five years amounted to 6.18 pounds per acre.
The quantity of nitrates which is thus formed in the air is small when considered locally, but in the aggregate the amount of combined nitrogen which is thus restored by nature to the surface of the earth is very great and is estimated at about 100,000,000 tons.
Other examples of nitrogen compounds which are not of organic origin are the metallic nitrides and ammonium salts which are found in the vicinity of volcanoes at the time of an eruption. It is probable that they are not carried as such in the fumes of the volcano, but are formed near the surface of the earth through exposure of molten rock to an atmosphere of nitrogen in a way analogous to the manner in which nitrogen is fixed artificially by the Serpek process, which consists in heating an ore of aluminium with carbon in an atmosphere of nitrogen. According to this view, nitrids would be formed first, and then ammonia when the former would come in contact with water. Many analyses have been made of the gases given off by volcanoes in different parts of the world, and free nitrogen has always been an important constituent, but so far as known combined nitrogen has never been reported.
A nitrogen compound of inorganic origin is also to be found in the case of a rare mineral which occurs in some parts of Arizona, and which has the distinction of being the only insoluble nitrate occurring in nature. This mineral to which the name Gerhardtite has been given is a basic nitrate of copper and is supposed to have been formed in the earth by water charged with air percolating over copper ore. It affords an illustration of an unusual way in which the fixation of nitrogen may be brought about.
The organic processes, however, are by far the most important in bringing about the fixation of nitrogen and the formation of nitrates. That nitrogen is one of the principal constituents of plants has been known since the beginning of the last century, but the source of the
