More interesting still, and also more important, is the absorption of dissolved substances by soil. In this way is to be explained the decolorising effect and the retention of dissolved polluting substances such as are contained in sewage. In the same way soil has the power of destroying such poisons as strychnine, nicotine, coniine, etc., and the experiments of Falk and others go to show that ptomaines and toxines are likewise retained and rendered harmless by it. This absorbing power of soil is of the utmost importance in agriculture, and without it soil could not possess purifying powers for polluting liquids. It is quite true that in this process of purification other factors play an important part, but they could not come into play if this absorption did not exist. The absorbing powers of soil are in some way dependent on the presence of micro-organisms and air, and in the absence of these they will soon come to a standstill. IV. SELF-PURIFYING POWERS OF SOIL. NATURAL SELF-PURIFICATION OF SEWAGE. [Sidenote: Self-purifying powers of soil.] After these preliminary remarks it becomes necessary now to examine into the self-purifying powers of soil with special reference to sewage farms. Generally speaking, the term “self-purifying powers powers of soil” comprises all those processes which go on on the surface and in the pores of the soil of sewage farms, and by which polluting liquids such as sewage become purified as these take place under natural conditions and in a natural medium, the process of land treatment of sewage is called—see previous observations—“the natural self-purification of sewage.” [Sidenote: Self-purifying powers vary with local conditions.] [Sidenote: Soil best suited for sewage farms.] It should be stated at the outset that the self-purifying powers of soil will depend largely on the soil itself and the local conditions under which they come into play, so that observations made in one locality will not be immediately applicable to others without making full allowance for the differences; this will be clear from the preliminary remarks as to the character and properties of soils made in the previous pages. As will be pointed out more in detail later on, a subsoil that combines great permeability for air with high retaining and absorbing powers, is best suited for sewage farms. Let us now consider what becomes of water, sewage or any other polluting liquid containing organic substances after it has been poured out upon the surface of the ground, and for this purpose we will assume a subsoil of a suitable character and in fair condition for work with proper under-drainage. [Sidenote: Retention of liquid by pores of soil.] The liquid thus poured out upon the surface will sooner or later disappear in the soil, and will at first be retained in the pores of the zone of evaporation, which may be said to extend to the level of the under-drains. This retention is due to the retentive powers of soil.