[Sidenote: Suspended matters retained on the surface, soil acts like a sieve.] [Sidenote: Coating of surface of the land.] [Sidenote: Removal of suspended matters generally an advantage.] Portion of the suspended matters will be retained on the surface and the rest will be strained out in a mechanical manner in the pores, the soil acting as a sieve more or less fine according to its character. If the suspended matters are present in very large quantities it may happen that they will gradually form a coat on the surface of the land and choke the pores to the exclusion of air, and as this is a thing to be avoided in sewage farming it is in most cases advisable to remove them out of the liquid before it is poured upon the land. [Sidenote: The more finely divided the suspended matters are, the lighter the work of the land.] Even where such a removal has taken place there will still be left a certain portion of the suspended matters, and if these are in a finely divided state, such as is probably the result of their passage through fine strainers or pump valves, the work of the land will be considerably lightened. [Sidenote: Micro-organisms screened out in a mechanical way.] The micro-organisms contained in the liquid will be to a large extent screened out in a mechanical way with the suspended matters and deposited on the surface and in the upper layers of the soil. [Sidenote: Retention of matters in solution after removal out of the liquid is due to physical and chemical agencies.] The matters in solution will partly, after removal out of the liquid, be retained by the absorbing powers of the soil in the pores, a process that is due to physical and chemical agencies. [Sidenote: Absorbing powers gradually ripen.] It is well known that land which is being treated with sewage for the first time does not purify sewage so well as land that has been under systematic treatment for some time, and this is probably due to the absorbing powers, which gradually ripen until they have reached their maximum of efficiency. This process of gradual improvement seems to be due to the formation of a slimy coating round each particle of soil, which growth does not only assist mechanical filtration, but also possesses high powers of absorbing oxygen. [Sidenote: Depths to which polluting substances may penetrate into soil.] The depth to which polluting substances may penetrate into soil will probably differ in each case, but the following factors may be said to influence it, viz. the velocity of the downward flow, the nature and degree of the polluting liquid, and the character of the soil. Where, therefore, the powers of the soil are over-taxed the polluting substances may reach the level of the underdrains and pass out through them, in which case the effluent will be but little better than the raw liquid. It must be the aim of careful management to avoid this.