North of Coquimbo are the desert provinces of Atacama, Antofagasta, Tarapacá, and Tacna, where the rain so seldom falls that no useful vegetation can thrive except in a few places where irrigation is possible, yet which are the chief source of Chile’s revenue and wealth. These constitute the fourth, or almost exclusively mineral zone, and, aside from their gold and silver and copper, contain the famous nitrate of soda beds, the only known extensive deposit of the kind in the world, though here they are found thickly scattered over a strip four hundred and sixty miles long, averaging about three miles in width. Every year more than 2,000,000 tons (in 1910 it was 2,367,000 tons, worth $86,018,000) are exported to fertilize the fields and make the gunpowder of Europe and the United States, to say nothing of the iodine and other by-products extracted in the process of preparation. “Plants make use of nitrogen only when it is present in the soil in the form of nitrates,” says the _Pan American Bulletin_ (Review Number, August, 1911)— “And nitrate of soda is the only fertilizer that contains this food in a suitable and available form. The manner of using it, once it is applied, is the subject of technical, agricultural chemistry, but every year it is better understood and results are more satisfactory. On the first discovery of the value of nitrate, it was scattered promiscuously in the soil in its crude form, just as it was taken from the beds in Chile. As the industry advanced, it was found that it was more economical to export a purer mineral, and that, also, the purer the mineral, the more plant nourishment it offered, provided that the need of the plant was carefully investigated. The results have been a more highly developed agriculture and the saving of certain by-products, of which iodine is one, the profit from which aids the manufacture. Another use for nitrate is in the manufacture of nitric acid, and, ultimately, of many kinds of explosives....