86 GENERAL SCIENCE 



level. Above this line comes the second layer of the atmosphere, the strato- 

 phere (also called the "isothermal layer," because a thermometer carried 

 up through it would show little change of temperature, with change of ele- 

 vation). This layer has been penetrated by sounding balloons, carrying 

 meteorological apparatus but no human aeronaut, as far as 20 miles above 

 the earth. At about 50 miles the upper limit of twilight begins a region 

 in which the atmosphere consists chiefly of hydrogen. Near the lower border 

 of this region clouds of fine dust have sometimes been observed, shining by 

 reflected sunlight on summer nights. These "noctilucent clouds" are com- 

 monly explained as the product of volcanic eruptions on the earth (they were 

 frequently seen after the eruption of Krakatoa), but may be of cosmical origin. 

 Concerning the uppermost regions of the atmosphere we have little pos- 

 itive knowledge. Above about 130 miles from the earth Dr. Alfred Wegener, 

 the author of this diagram, believes that a gas ("geocoronium"), much lighter 

 than hydrogen, prevails, to which he attributes the characteristic green line 

 in the spectrum of the higher auroras. This is hardly more than a guess at 

 present. 



QUESTIONS 



1. Why does water boil more easily on some days than on others? 



2. Why do people say "it is going to rain," when the " kettle 

 boils easy." 



3. How could one tell the height of a place by finding the tem- 

 perature of boiling water ? 



4. Why will a heavy cover over a dish cause vegetables to cook 

 more quickly? 



5. Why is it impossible to boil eggs 

 (hard) at high altitudes unless the cover 

 of the kettle is weighted down? 



6. If the atmosphere were all removed 

 from the earth, would the ocean boil? Why? 



7. Would ice water boil in a vacuum? 



8. Why does candy boil so vigorously 

 when flavoring is added? (Suggestion: 

 There is alcohol in the flavoring.) 



_, 65 9. What causes the water to rise in a 



coffee percolator? 



10. What part of each section of this Chapter did you like best? 



11. What part of the Chapter do you consider of greatest im- 

 portance? Why? 



