We shall begin the study of nitric acid by taking stock, as it were, of the natural sources of supply. The free acid is not found in Nature except for very small traces in the air after thunderstorms. We have, therefore, to rely entirely on that which can be obtained artificially. Until quite recently, it could be said that there was only one method of making the acid, namely, by the distillation of a mixture of potassium or sodium nitrates and concentrated sulphuric acid. Now, however, nitric acid is being made from the air, though as yet only in small quantity, notwithstanding the great development of this method owing to war requirements; hence, we are still mainly dependent on the naturally occurring nitrates just mentioned. Potassium Nitrate (nitre, saltpetre, sal prunella) is found in the soil of hot countries, especially in the neighbourhood of towns and villages where the sanitary arrangements are primitive. In very favourable circumstances, it may even appear as a whitish, mealy efflorescence on the surface of the ground. To obtain the salt, it is only necessary to agitate the surface soil with water and, after the insoluble matter has settled down, to evaporate the clear solution. Potassium nitrate is required for making gunpowder, which, until quite recent times, was the only explosive used in warfare. Continental countries that could not afford to rely entirely on sea-borne nitre had to make their own. The refuse of the farmyard, mixed with lime and ashes, was made up into a heap of loose texture, which was periodically moistened with the drainage from the stables. In the course of years, saltpetre and calcium nitrate were formed in the surface layers, from which they were extracted from time to time. The farmer was then allowed to pay part of his taxes in nitrates. Sodium Nitrate, also called caliche, Chili-saltpetre, or Chili-nitrate, comes mainly from South America. The beds extend for a distance of about 220 miles in Chili, Peru, and Bolivia, between the Andes mountains and the sea. The deposit is about 5 ft. thick, and its average breadth 5 miles. The crude material is treated with water in steam-heated wooden vats. The clear solution is evaporated, and the residue obtained is washed with the mother liquor and dried. This product may contain as much as 98 per cent. of the nitrate. [Illustration: Fig. 5. PREPARATION OF NITRIC ACID]