We have found that the various saltpetres of natural production, or those obtained in artificial arrangements, are converted by the use of potash salts into potash saltpetre, and among the products so changed is natural soda saltpetre. Now to us in this country, so near the sources of abundant supply of soda saltpetre, this substitution becomes a matter of great interest. We possess and can produce the alkaline salt of potash in almost unlimited quantity, and, excepting for some special purposes, it is consumed for its alkaline energy alone. When soda saltpetre in proper proportion is dissolved and thus mixed with potash salt, an exchange of bases takes place, and no loss of alkaline energy follows. The soda in a quite pure state is eliminated from the soda saltpetre, and will serve for the manufactures of glass and soap; while the potash, taking the oxygen compound of the soda saltpetre, produces, as a final result, a pure and beautiful prismatic saltpetre, most economically and abundantly. Instead of working on a hundred pounds of earth to obtain at most eight or nine pounds of saltpetre, a hundred pounds of soda saltpetre will afford more than one hundred and nine pounds of potash saltpetre, when skilfully treated. Here, then, we have, by simple chemical treatment of an imported, but very cheap salt, a result constituting a source of abundant supply of potash saltpetre, _without the loss of the agent_ concerned in the transformation. We have traced slightly in outline the formation of saltpetre to the action of ozonized oxygen on nitrogen compounds, in the atmosphere, or in the earth,--the conditions being the same in both cases. If we pursue the study of this action of ozonized oxygen farther, we shall not restrict its combining disposition to these compounds, but prove that it has the power of uniting directly with the nitrogen naturally forming part of the pure air. While nitrogenized bodies are present, however, in the atmosphere, or in the humid artificial heaps of saltpetre plantations, the action of ozonized oxygen is on these, and the nitrous compounds formed unite with the bases lime, soda, and potash, also present, to form saltpetre. Under all the conditions necessary, we see the permanent gases, oxygen and nitrogen, leaving the atmosphere and changing from their gaseous to a solid dry state, when they become chemically combined with potash, and there are 53-46/100 parts of the gaseous matter and 46-54/100 parts of the potash in 100 parts of the saltpetre by weight. Having now found what saltpetre is and how it is formed, let us advance to the consideration of it as a source of power.