In this country there are numerous caves of great extent in Kentucky, Tennessee, and Missouri, from which saltpetre has been manufactured. Under the most favorable conditions of abundance of labor, obtainable at a low price, potash saltpetre can be made at a cost about one-fourth greater than the average price of India saltpetre, and those sources of supply are the best natural deposits known on this side of the Rocky Mountains. Where there is an insufficient supply of manure in a country, resort to the artificial production of saltpetre is simply a robbery committed on the resources of the agriculturists, and it is only during the pressure of a great struggle like that of the wars of Napoleon, that the conversion into saltpetre of materials which can become food for the community would be permitted. Hitherto, in peaceful times, our supply of saltpetre has come from India through commercial channels; but twice within a few years this course of trade has been interrupted by the British Government, and the price of a necessary article has been greatly enhanced,--leading reflecting minds to the inquiry after other sources whence to draw the quantity required for an increasing consumption. On the boundary between Peru and Chili, in South Peru, about forty miles from the ports of Conception and Iquique, is a depression in the general surface of a saline desert, where a bed of soda saltpetre, about two and a half feet thick and one hundred and fifty miles long, exists. The salt is massive, and, occurring in a rainless climate, it is dry, and contains about sixty per cent. of pure soda saltpetre. In Brazil, on the San Francisco, the same salt is found extending sixty or seventy miles,--and again near the town of Pilao Arcado, the beds being about two hundred and forty miles from Bahia, but at present inaccessible for want of roads. The Peruvian native saltpetre is rudely refined in the desert, and then transported on the backs of mules to the shipping-port. As found in commerce, it is less impure than India saltpetre; and it might be usefully substituted for the latter in the manufacture of gunpowder, were it less deliquescent in damp atmospheres. For chemical purposes it now replaces India saltpetre, but the larger consumption is perhaps as a fertilizer of land, in the cool and humid climate of England, the low price it bears in the market permitting this consumption.