Experiments have been made for the purpose of estimating these additions, and the results of those most carefully conducted show how very slightly the combined causes affect the general composition of our atmosphere; and although the present refined methods of chemists enable them to detect the presence of an abnormal amount of some substances, no research has yet been successful in determining how far this varies from the natural quantity at all times necessarily present in the atmosphere. It is, however, the comparatively minute portions of nitrogenous matter in the atmosphere that we are to consider as the source of the nitrous acids formed there, and of part of that found in the earth. From some experiments made during the day and night it has been found, that, under the most favorable circumstances, six millions six hundred and seventy thousand parts of air afford one part of nitrogenous bodies, if the whole quantity be abstracted! A portion only of this quantity can be withdrawn in natural operations, such as the falling of rain and the deposition of dew,--the larger part always remaining behind. When the oxygen of our atmosphere is exposed, while in its usual hygrometric state, to the influence of bodies attracting a portion of it, such as decomposing substances, or when it forms the medium of electrical discharges, it suddenly assumes new powers, acquires a greatly increased activity, affects our organs of smell, dissolves in fluids, and has been mistaken for a new substance, and even named "ozone." Among the new characters thus conferred on it is the power of uniting with or burning many substances. 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.