=25. Capillary Action in Soils.=--The distribution of moisture in the soil depends largely upon capillary action. When the soil is compact the minute spaces between the soil particles act as capillary tubes, thus aiding the water to rise to the surface. As the water evaporates from the surface more of it rises by capillary action from the damper soil below. Keeping the soil loose by cultivation, makes the spaces between the particles too large for much capillary action, thus the moisture is largely prevented from rising to the surface. In the semi-arid regions of the West "_dry farming_" is successfully practised. This consists in keeping the surface covered with a "dust mulch" produced by frequent cultivation. In this way the moisture is kept below the surface, where it can be utilized during the hot dry summer by the roots of growing plants. Important Topics 1. Attractive forces between liquid molecules. 2. Cohesion (like molecules); adhesion (unlike molecules). 3. Special effects of this force are classified as (a) capillary action, and (b) surface tension. Exercises 1. What evidence of capillary action have you seen outside of the laboratory? 2. What is the explanation for capillary action? 3. Where are surface films found? 4. What are three common effects of surface films? 5. Explain why cravenette cloth sheds water. 6. If a circular glass disc 10 cm. in diameter requires 50 grams of force to draw it from the water, what is the cohesion of water per square centimeter? 7. What is the weight in grams of 1 ccm. of water? of a liter of water? 8. Name five examples of adhesion to be found in your home. 9. Under what conditions will a liquid wet a solid and spread over it? 10. When will it form in drops on the surface? 11. Explain the proper procedure for removing a grease spot with benzine. 12. What difference is there between a liquid and a fluid? 13. Why cannot a "soap bubble" be blown from pure water? 14. Which are larger, the molecules of steam or those of water? Why? 15. Why is the ground likely to be damp under a stone or board when it is dry all around? 16. Why does any liquid in falling through the air assume the globule form? 17. Give three examples of capillary attraction found in the home. Three out of doors. 18. Why does cultivation of the soil prevent rapid evaporation of water from the ground? (4) EVIDENCES OF MOLECULAR FORCES IN LIQUIDS AND SOLIDS =26. Solutions.=--A crystal of potassium permanganate is placed in a liter of water. It soon dissolves and on shaking the flask each portion of the liquid is seen to be colored red. The dissolving of the permanganate is an illustration of the attraction of the molecules of water for the molecules of the permanganate. We are familiar with this action in the seasoning of food with salt and sweetening with sugar.