The results of this action are highly important; one being the formation of clay, so essential as a physical ingredient of soils; the other the setting-free of potash, one of the most essential nutrients of plants. Hornblende and the related minerals are similarly acted upon so far as they contain the same substances. In all cases, of course, the silica (silicic acid) set free by the carbonic acid remains partially or wholly in the resulting soils, as such. Lime also at first mostly remains behind in the form of the carbonate; but potash and especially soda compounds, being mostly readily soluble in water, are largely carried away by the latter. The effect of carbonated water upon silicate minerals is greatly increased by the presence of ammonia (ammonic carbonate), which always exists in atmospheric water to a greater or less extent. This effect may readily be noted on the windows of stables, or other places where animal offal decays, by the dimming of the glass surfaces; also in glass bottles containing solution of ammonic carbonate. _Action of Oxygen._—The effects of atmospheric _oxygen_ on rocks are of course confined to those containing substances capable of farther oxidation. Chief among these are ferrous (iron monoxid) and ferroso-ferric oxid the latter imparting bottle-green, bluish and black tints to so many minerals and rocks that these colors may usually be taken as indicating its presence. By taking up more oxygen the ferrous and ferroso-ferric oxids are converted into ferric oxid or its hydrate (rust), the tints mentioned passing thereby into brick-red or rust color, according as the former or the latter (or sometimes their intermixtures) is formed. In either case there is an increase in bulk; and this when taking place in the cracks or crevices of minerals or rocks, tends, like the freezing of water, to widen the cracks and thus to increase the surface exposed to attack. Since ferrous compounds, when soluble in water, are injurious to plant growth, this oxidation is of no little importance, and in soils must be carefully maintained against a possible reversal. It is hardly necessary to insist that the action of all these chemical agents continues in the soils themselves, and that owing to the fineness of the material, resulting in an enormously increased surface exposed to attack, such action acquires increased intensity. This is the more true as in soils bearing vegetation there are always superadded the effects of the humus-acids resulting from the decay of vegetable matter, as well as of the acid secretions of the living plants. _Action of Plants and their Remnants in Soil Formation._