78 HEAT AND ENERGY 



given out again whenever the mass changes back to 

 its solid state, or from a gas to a liquid. Farmers often 

 protect their supplies by taking tubs of water into their 

 cellars ; when it is cold enough to freeze the water, the 

 heat given off by the freezing liquid keeps the cellar 

 warm enough so that the vegetables do not freeze. 



Experiment 70. Into a glass dish put several pieces of ice ; 

 carefully note the temperature of the ice. Now apply heat slowly, 

 testing the temperature of the mixture from time to time as the 

 ice melts. Just before the last bit melts, remove the source of 

 heat, and when all is liquid test the temperature again. Was heat 

 given to the mass in the dish ? How do you know this ? Did this 

 heat raise the temperature of the whole ? What did it do 



QUESTIONS 



1. What are the common effects of heat? Does heating 

 always produce all of these effects in a body at the same time ? 



2. Define expansion and contraction. How in general is the 

 volume of a body affected by heat ? Do liquids and gases expand 

 and contract like solids ? Name any uses of expansion and con- 

 traction that you have seen. 



3. Why does ice float ? Show how water freezing in a crevice 

 may break a rock. What is the rule for expansion in water ? 



4. Give the general rule for changes of state due to heat 

 alone. What is meant by the words liquefying, solidifying, and 

 vaporization? What effect has pressure upon changes of state? 

 Compare the temperature of water boiling in a locomotive with 

 that of water boiling in an open dish. 



5. Define evaporation. What conditions assist evaporation? 

 Why put your clothing in a warm place to dry? 



6. When is air said to be saturated ? If it is then cooled, what 

 happens? How are clouds formed? What is a cloud made up 

 of ? Show how rain is formed in the cloud masses. 



7. Explain distillation. To what important uses is it put ? 



