NITRATE DEPOSITS 135 



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 dis- 

 solves 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 Kothamsted, 

 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 



