II. THE SEWAGE QUESTION DURING THE LAST CENTURY:
A SHORT RETROSPECT 29
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