viii TABLE OF CONTENTS 



CHAPTER PAGE 



24. Heat, a Form of Energy ; Temperature ... 47 



25. Measurement of Temperature ; Thermometers . . 49 



26. The Three States of Matter: Solids; Liquids; Gases 51 



27. The Transference of Heat ; Conduction ; Convection ; 



Radiation 52 



28. The Measurement of Heat; Calories; Specific Heat 56 



29. Light 57 



30. Direction of Light Movement ; Refraction ... 58 



31. The Intensity of Light; Its Variation; Candle Power 61 



32. Reflection of Heat and Light ; Angles of Incidence 



and of Reflection 62 



33. The Speed of Light ; How Determined ... 64 



34. Theories concerning Light ; Wave Motion . . 65 



35. Sound ; Wave Motion 65 



TV. THE EARTH'S CRUST. 



36. Land and Water Areas ; Their Proportions . . 68 



37. Interchange of Land and Water Areas . . .68 



38. Characteristics of Land Surfaces . .' . . 70 



39. Characteristics of Water; Hydrogen; Oxygen; In- 



compressibility . . . . . .71 



40. Materials composing the Land; Igneous, Sedimentary 



and Metamorphic Rocks ; Coal ; Oil . . .75 



4 1 . Structure of the Land Areas ; Strata .... 79 



42. Rock Weathering ; Its Causes 82 



43. Soil ; Local and Transported Soils ; Subsoil ; Loam . 85 



44. Fertile Soils ; Humus ; Worms ; Bacteria . . 90 



45. Agricultural Soils ; Sand ; Silt ; Clay . . . ' 93 



46. Soil Water ; Capillarity ; Seepage ; Springs ; Dry 



Farming; Irrigation ...... 96 



47. Alkali Soils; Drainage 103 



48. Soil and Man ; Proper Tillage ; Fertilizers . .105 



49. Value of Soils ; Varying Crops 106 



V. THE ATMOSPHERE OF THE EARTH. 



50. The Origin of the Atmosphere . . . ... 110 



51. The Composition of the Air; Oxygen; Carbon Di- 



oxide ; Nitrogen . . . . . . .111 



52. Weight of Air; How Found 114 



53. Expansion of Air when Heated . . . . ' . 115 



54. Weight of Air as Affected by Heat and Cold . .116 



55. Pressure of Air v . 1 1 7 



56. Decrease of Volume due to Pressure . . . .118 



