CHAPTER II 

 AIR AND WATER 



20. Questions for Discussion. 1. Why do clothes usually dry bettor 

 on warm days than on cold days? 2. Which can endure the higher 

 temperature, the man who perspires a great deal or the one who per- 

 spires but little? 3. Why do drops of water appear on cold-water 

 pipes on some summer days and not on others? 4. Why do these 

 drops form most abundantly when cold water is running through the 

 pipes? 5. Why are grass and other plants often moist in the morning 

 although no recent rain has fallen ? 6. Why does frost form on window- 

 panes in winter? 7. What are the conditions necessary to the deposit 

 of dew ? 8. How does the practice of flooding cranberry bogs help to 

 protect the cranberries against frost? 9. Why does the air feel more 

 invigorating on a clear day than on a damp day even though it is 

 cool? 10. Why does hot weather in Kansas produce less discomfort 

 than weather of the same temperature ordinarily would in New Jersey ? 

 11. Why are "muggy" days so oppressive? 12. A student of school 

 hygiene asserts that the air in the ordinary American schoolroom is 

 like the air of the desert. What causes this condition, and how can 

 it be prevented? 13. In what ways is the air of your schoolroom 

 unlike the air of the desert? 14. What improvements, if any, are needed 

 in the air of your schoolroom? 15. Do snow and ice evaporate during 

 cold winter weather? 



21. Evaporation. It is a matter of common observation 

 that if water is left exposed to the air, it soon disappears. 

 We say that it has evaporated. This does not mean that it 

 has gone out of existence. It has only changed into the form 

 in which it is invisible and can pass into the ah". It has be- 

 come a gas. The same thing is true of the surface of every 

 pond, lake, river, sea, and ocean indeed, of any water that 

 is exposed to the air. There is, therefore, always considerable 

 water in the atmosphere, and so long as it remains in the form 



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