While in one and the same substance the decolorizing effect is more or less directly proportional to the fineness of the particles, corresponding to increased surface, it is nevertheless true that in this case, as in that of the absorption of gases, there are specific differences between different powders; so that for example no other substance can replace charcoal in the decolorizing effect which it produces upon colored solutions. It must not, however, be supposed that there is any special reason why _coloring_ matters, as such, should be taken up by preference. Coloring matters are of all kinds of chemical composition, and have in common only the fact that a relatively small amount produces a very strong coloring effect; hence their name, and hence also the apparently extraordinarily strong effect produced upon them by charcoal. This effect is not, however, by any means greater than it is in the case of many other compounds which are colorless. _Complex Action of Soils._—The powdery ingredients of soils, of course, share this power with all other powders. In the case of soils, however, the action is almost always much more complex than in that of charcoal, because solutions that are passed through the soil are apt to act chemically upon one or the other of its ingredients, usually resulting in a partial exchange of ingredients between the soil and the solution; one or more of the constituents of the solution being retained by the soil, while one or more of the (basic) soil constituents pass into the solution, in combination with its acidic ingredients. Thus when a very dilute (½ or 1%) solution of potassic chlorid is filtered through almost any soil, the first portions passing through will be practically free from potash, but will contain the chlorids of calcium and magnesium. But as more of the solution is passed through, potash passes also ultimately without absorption. In addition to the zeolitic and clay portions of the soil, the humus is very effective in absorbing mineral ingredients from solution, and retaining them in such manner as to be readily available to plant growth. (See chap. 8, p. 124.) In view of the almost invariable conjunction of physical and chemical effects, it may be fairly said that no solution, at least of mineral salts, can pass through the soil without being changed in its concentration and chemical composition. It is sometimes difficult to decide to which of the two classes of effects the several changes may be due.