III. THE SUBSOIL: 1. Mechanical structure of soil 41 2. Permeability of soil 42 3. Water capacity of soil 42 4. Water-retentive power of soil 43 5. Capillary movements of water in soil 44 6. Temperature of soil 45 7. Subsoil air 47 8. Movements of water in soil 47 9. Micro-organic life in soil 50 10. Absorbing powers of soil 51 IV. SELF-PURIFYING POWERS OF SOIL. NATURAL SELF-PURIFICATION OF SEWAGE 52 V. Artificial Self-Purification of Sewage: 1. General observations 68 2. Artificial self-purification of sewage in intermittent contact beds: _a._ Name of process 70 _b._ Explanation of process 71 _c._ Water capacity of bed, and silting up 73 _d._ Absorbing powers of filling material 78 _e._ Consumption of oxygen by the filling material 79 _f._ Formation of carbonic acid 80 _g._ Nitrogen 80 _h._ Formation of nitric acid 80 3. Artificial self-purification of sewage in septic tanks: _a._ Name of septic tank 81 _b._ Covered or open septic tank 81 _c._ Explanation of process 83 _d._ Velocity of flow through tank 85 _e._ Destruction and liquefaction of sludge in septic tanks 87 _f._ Formation of gas in septic tank 88 _g._ Mixing action of septic tank 89 _h._ Micro-organisms in effluent from septic tank 89 4. Continuous contact beds 89 VI. MANAGEMENT OF PLANTS FOR THE ARTIFICIAL SELF-PURIFICATIONOF SEWAGE 90 VII. SOME OBSERVATIONS ON THE DEPOSITION OF SUSPENDED MATTERS IN TANKS 91 VIII. CONCLUDING REMARKS 93 POSTSCRIPT 95 NATURAL AND ARTIFICIAL SEWAGE TREATMENT. BY LIEUT.-COL. ALFRED S. JONES, V.C. ASSOC. M. INST. C.E., ETC. [Sidenote: Introductory remarks.]