Fire and Heat 91 



the stars and planets. The highest temperature obtained is 

 about 7800 C. or about 1800 higher than the estimated 

 temperature of the sun. Such achievements illustrate man's 

 increasing control of the forces of the universe and his progres- 

 sive adaptation of them to the needs and purposes of mankind. 

 40. Familiar instances of changes due to heat and cold. 

 Common experience presents many examples of changes in 

 mineral substances under the influence of heat and cold. Boil- 

 ing water poured upon a cold heavy glass dish is likely to cause 

 it to break. Why? How are fruit jars prepared for hot 

 fruits or vegetables ? What would happen if fruit jam at nearly 

 boiling temperature were poured into a cold glass container? 

 It is sometimes .difficult to unscrew the metal top from a glass 

 jar ; but if the top be submerged for a moment in hot water, 

 it may be easily unscrewed. Explain what happens. Which 

 responds to the heat more quickly, glass or metal? You may 

 find in the home other instances of such changes caused by heat 

 and cold. 



Exercise. What causes a wheel, as of a wagon, to " freeze " 

 on the axle ? How does the plumber provide for changes in the length 

 of steam and hot-water pipes? Are such pipes fastened securely to 

 walls and floors of buildings? Why? Are the ends of the steel rails 

 on railroads close together at ordinary temperatures? When the 

 weather is very cold? When it is very hot? Explain the conditions 

 observed. Are steel bridges bolted securely to the piers? Explain. 

 Find other examples where provision is made for changes in materials 

 used in structures and in products of manufacturing. 



SUMMARY 



Thermometers are used because we need to determine tempera- 

 ture more accurately than we can with our unaided senses. 



The expansion of liquid mercury is uniform and continuous with 

 rise of temperature. 



Alcohol is used in place of mercury when low temperatures are to 

 be indicated. 



On the Fahrenheit scale the freezing temperature of water is 

 marked 32 and the boiling temperature 212. 



