8. Sand allows air to pass through but clay does not . . 15 9. A brick allows both air and water to pass through it 17 10. Lime added to turbid clay water soon makes the clay settle . . . . . . . . . . . . . . . . . . . . . . . 20 11. Sand dunes, Penhale, Cornwall. _Phot._ Geological Survey . . . . . . . . . . . . . 23 12. Blowing sand covering up meadows and ruining them. _Phot._ Geological Survey . . . . . . . . . . . . . 25 13. Model of a spring . . . . . . . . . . . . . . . . . . 26 14. Foot of chalk hill at Harpenden where a spring breaks out. _Phot._ Lionel Armstrong . . . . . . . . . . . 27 15. The little pool and the spring. _Phot._ Lionel Armstrong . . . . . . . . . . . . . . 28 16. Water spouting up from a bore hole, Old Cateriag Quarry, Dunbar. _Phot._ Geological Survey . . . . . 29 17. Sandy soils in wet and in dry positions . . . . . . . 31 18. Map of the roads round Wye . . . . . . . . . . . . . . 32 19. Peat bog in Hoy, Orkney: peat is being cut for fuel. _Phot._ Valentine & Son . . . . . . . . . . . . . . 39 20. Rye growing in surface soil, subsoil, and sand . . . . 42 21. Mustard growing in surface soil, subsoil, and sand . . 43 22. Mustard growing in soil previously cropped with rye, and in soil previously uncropped . . . . . . . . . . 45 23. Pieces of grass, leaves, etc. change to plant food in the surface soil lint not in the subsoil . . . . . 50 24. Soil in which earthworms have been living and making burrows . . . . . . . . . . . . . . . . . . . . . . 55 25. Fresh soil turns milk bad, but baked soil does not . . 57 26. Soil contains tiny living things that grow on gelatine 58 27. Our breath makes lime water turn milky . . . . . . . . 59 28. Something in the soil uses up air and makes lime water turn milky . . . . . . . . . . . . . . . . . . . . . 61 29. Soils are able to stick to water: clay or loam soils do this better than sands . . . . . . . . . . . . . . . 65 30. Water can pass from wet to dry places in the soil, it can even travel upwards . . . . . . . . . . . . . . 66 31. Plants growing in soils supplied from below with water. All the water the plants get has to travel upwards 67 32. Mustard growing in soils supplied with varying quantities of water . . . . . . . . . . . . . . . . 69 33. Wheat growing in moist and in dry soils . . . . . . . 71 34 _a_ and _b_. Plants found on a dry soil had narrow leaves, those on a moist soil had wider leaves. _Phot._ S. T. Parkinson . . . . . . . . . 72, 73 35. Plants give out water through their leaves . . . . . . 74 36. Stephen Hales's experiment in 1727 . . . . . . . . . . 75