316 Introduction to the Study of Science 



closed boiler when the water boils? 23. What change takes place in 

 the volume of steam when the pressure is increased ? 24. What use is 

 made of this fact in explaining a boiler explosion? 25. What changes 

 take place in the volume of steam when the pressure is decreased? 

 26. What change takes place in the temperature of steam in ex- 

 panding? 27. How may the heat energy of expanding steam be 

 utilized? 28. A steam engine is called a heat engine. Explain. 



29. What is superheated steam ? How is it superheated (Fig. 23 H) ? 



30. Why does a simple steam engine not make use of all the heat 

 energy of steam? 31. What is the advantage of compound and 

 multiple expansion cylinders used in steam engines? 32. How high 

 are the temperatures and pressures used in modern steam engineering ? 



33. Mention two liquids which may be separated from a mixture 

 because of their differences in boiling points. 34. Mention four sub- 

 stances that have their boiling points below the freezing point of 

 water. 35. Mention four substances having their boiling points 

 between the freezing and boiling points of water. 36. Mention some 

 having their boiling points above that of water. 37. How is sugar 

 obtained from its solution? salt? 38. What may be inferred about 

 the strength of a salt solution from its boiling point? 39. What is 

 fractional distillation ? Illustrate in the making of petroleum products. 

 40. Mention a case where distillation is carried on for the sake of the 

 residue ; for the sake of the distillate ; for both. 



SUGGESTIVE QUESTIONS 



1. Is a thunderstorm the cause or result of " cooling the air "? 



2. Establish the volume of a pound of water from the weight of 

 one cubic foot of water. 



3. Is air like steam in changing its volume with changes in pres- 

 sure? 



4. What changes in temperature take place when the volume of 

 air is changed by compression or expansion? 



5. What changes in air take place as it blows from an inflated tire ? 



6. Explain why a cloud sometimes forms in the neck of a soda 

 water bottle when the cork is removed. 



VIII. ELECTRICITY AS A MOTIVE POWER 



140. Electrification of steam railroads. Wonderful as has 

 been the increase in the use of electricity as motive force in trans- 

 portation in cities and suburban districts, its installation for rail- 



