EXERCISES 231 



4. For what reasons is the size of the windows of our houses an 

 important matter from the point of view of public health? 



5. What are the uses of plate glass? Find out what materials are 

 used in making optical glass; that is, glass for spectacles, lenses, etc. 



6. How does it happen that so many candles are used nowadays, 

 when there are so many other ways of producing light? 



7. If you "blow out" a candle or kerosene flame, you can relight it 

 by holding a lighted match some distance above the wick. Explain. 



8. When a carbon incandescent lamp is old, its globe becomes very 

 hot while in use. Are you economical if you continue using it longer? 

 Explain why. 



9. What kind of gas does your family use for lighting and heating? 

 How much does it cost per thousand cubic feet? How much did you 

 use last month? 



10. What kind of incandescent lamps do you use at home? How 

 much does each cost? How long is the "life" of each lamp? For 

 what reasons is one of your electric lamps discarded? 



11. What is the price of electricity per kilowatt-hour in your city? 

 How much did your family use last month? 



