This ozonized oxygen, when brought into mixture with many nitrogenized bodies, forms with them nitrous acids, completely destroying their former condition and composition; hence, in the atmosphere, this part of the oxygen becomes a purifier of the whole mass, from which it removes putrescent exhalations, miasmatic vapors, and the effluvia from every source of sea or land.  Very curious are the effects of this active oxygen, which is ever present in some portion of the atmosphere.  Moved by the wind, mixed with the impure upward currents rising from cities, it seizes on and changes rapidly all foulness, and if the currents are not too voluminous, the impure air becomes changed to pure.  As ozonized oxygen can be easily detected, we may pass from the city, where (overpowered by the exhalations) it does not exist, and find it in the air of the vicinity; and moving away several miles, ascertain that a normal amount there prevails, and that step by step, on our return to abodes of a dense population, the quantity diminishes and finally all disappears.

We are now prepared to answer the second part of the question which was suggested, and to find that nitrous acids formed in the atmosphere by direct oxidation of nitrogenous matter may unite with the ammonia present to produce one kind of saltpetre; and when the rains or the dews carry this to the earth, the salts of lime, potash, and soda there found will decompose this ammoniacal saltpetre, and set the ammonia free, to act over again its part.  So in regard to decomposing organic matters in the soil:  ozonized oxygen changes them in the same way.  The earth and calcareous rocks of caves, penetrated by the air, slowly produce saltpetre, and before the theory of the action was understood, artificial imitation of natural conditions enabled us to manufacture saltpetre.  Animal remains, stratified with porous earth or the sweepings of cities, and disposed in long heaps or walls, protected from rain, but exposed to the prevailing winds, soon form nitrous salts, and a large space covered with these deposits carefully tended forms a saltpetre plantation.  France, Prussia, Sweden, Switzerland, and other countries, have been supplied with saltpetre from similar artificial arrangements.

But the atmosphere is washed most thoroughly by the rains falling in and near tropical countries, and the changes there are most rapid, so that the production of saltpetre, favored by moisture and hot winds, attains its highest limit in parts of India and the bordering countries.

During the prevalence of dry winds, the earth in many districts of India becomes frosted over with nitrous efflorescences, and the great quantity shipped from the commercial ports, and that consumed in China, is thus a natural production of that region.  The increased amount due to tropical influences will be seen in the instances here given of the produce from the rich earths of different countries:—­

Natural.