Every time a flash of lightning passes between a cloud and the earth, oxygen and nitrogen combine in the path of the spark, producing oxides of nitrogen. These dissolve in water, and are washed into the earth as a very dilute solution of nitric acid. As long ago as 1785, H. Cavendish imitated this natural phenomenon. A reference to the diagram (Fig. 7) will show how nitric acid can be made from the air on a small scale. The globe contains air under slightly increased pressure. The platinum wires or carbon rods are connected with the terminals of an induction coil, which in its turn is connected to accumulators supplying the current required. When the coil is put into action, a spark passes across the gap between the ends of the carbon rods. With a larger coil and a more powerful battery, there is an arching flame which can be blown out and re-lighted. This is actually nitrogen burning in oxygen. The result in either case is the same; the air in the globe sooner or later acquires a reddish-brown colour due to oxides of nitrogen, which, when shaken with water, form a very dilute solution of nitric acid. The same process is now carried out on a large scale. Air is driven by fans through a very powerful electric arc, whereby 1·5 to 2 per cent. is converted into nitric oxide. This combines spontaneously with more oxygen to form nitrogen peroxide, which, when dissolved in water, gives a very dilute solution of nitrous and nitric acids. [Illustration: Fig. 7. NITRIC ACID FROM AIR] The absorption of the oxides of nitrogen is carried out systematically. The mixed gases, after passing through the arc, are passed through a series of towers filled with acid-resisting material over which a stream of water is flowing. The solution of nitric acid so obtained is very dilute, but by using the liquid over and over again, a moderately strong solution is ultimately produced. This is collected in granite tanks and neutralized with lime, forming calcium nitrate or Norwegian saltpetre, as it is now called. This is a new industry and a rapidly-growing one; in the course of five years (1905-1909) the annual output of Norwegian or “air” saltpetre increased from 115 to 9,422 tons. Mountainous countries like Norway and Switzerland are perhaps in a specially favoured position with respect to this industry. Rapid streams and waterfalls, in conjunction with turbines, are used for driving the dynamos, and in this way electricity is produced at very low cost. It is interesting, however, to note that a plant for the manufacture of nitric acid from air has now been established in Manchester. CHAPTER IV THE HALOGEN ACIDS