It is not proposed here to describe these methods in detail, but it may be mentioned that one of them, known as the “arc process,” imitates the action of lightning in combining the nitrogen and oxygen that occur naturally in the air, while the others utilize nitrogen that has been previously separated from the air by the liquid air process. The arc process requires, for commercial success, a large supply of cheap electrical power, and it is at present almost confined to Norway and Sweden, where electricity is obtained from waterfalls. In this process air is blown through a huge electric flame, spread out by a powerful electromagnet. The air yields nitric oxide, which is combined with water to form nitric and nitrous acids, and these substances are combined with others to form marketable products. The most widely used fixation process, and the one which the United States Government proposed to employ in the large plants that were in course of construction in this country at the close of the war, is known as the “cyanamide process.” This process requires, as a part of its raw materials, large supplies of limestone and coke, from which calcium carbide is made in an electrical furnace. The calcium carbide, at red heat, absorbs nitrogen, forming an intermediate product from which, by further processes, are made ammonia and nitric acid. A third method of fixing atmospheric nitrogen, which has been applied on a vast scale in Germany and is now coming into use in other countries, is commonly called the “Haber process.” In this process nitrogen is combined with hydrogen, obtained from water, to form ammonia, the combination being facilitated by the presence of what chemists call a “catalyzer,” i. e., a substance that enables other substances to combine without itself undergoing any change. Several different catalyzers have been used in the Haber process. Two or three other methods of nitrogen fixation are beginning to assume commercial importance. While the power of the wind holds an important place among the resources of the atmosphere, it cannot be said that the utilization of this resource has undergone developments in modern times at all comparable with the striking inventions and discoveries we have just been recording, if we except the use of the wind in aeronautics. Atmospheric resources used by aeronauts will be discussed in subsequent chapters.