120 



RELATION OF HEAT TO COMFORT 



and form . 



and 



Explain your answer. 



vapor. Is anything lost in burning? 



Experiment 82. Does friction produce heat? 



Rub the palms of your hands together vigorously. Do 

 they become warm ? 



Rub the surfaces of two pieces of iron over each other 

 for about a minute. Is there a change in temperature? 



Why do we have to use oil in automobiles ? 



Why are ball bearings used in machinery? 



Is the heat produced by friction in machines an advantage 

 or disadvantage ? 



Do you know of any practical uses for heat produced by 

 friction? 



Experiment 83. Can heat be produced from chemical 

 energy other than by burning? 



Put some dilute hydrochloric acid in a test tube, and meas- 

 ure the temperature of the solution with a thermometer. 

 Put some granulated zinc in the solution, and after the 

 chemical action has gone on for about two minutes, measure 

 the temperature of the solution again. Did a temperature 

 change take place? 



Pour several drops of strong sulphuric acid into a beaker 

 of cold water. Feel the bottom and sides of the beaker. Has 

 the water changed in temperature? Was there a trans- 

 formation of chemical energy into heat energy in these ex- 

 periments? 



Activity 84. What do you know about fuels? 



Make a chart showing the information you can find about 

 various fuels. Divide your chart into three groups : solid 

 fuels, liquid fuels, and gaseous fuels. Under each group 

 heading write the names of fuels, where they are obtained, 

 their use, and their cost. 



Experiment 85. How may gas and coke be obtained 

 from coal ? 



Secure a test tube and fit it with a one-hole rubber stop- 

 per carrying a glass tube as shown in Figure 185. Place a 

 few lumps of soft coal in the test tube, place the stopper 



FIG. 185 



in the mouth of the tube, and heat in a Bunsen or alcohol 

 flame. What comes from the tube? Will it burn? Can you 

 find any evidence of tar in the tube? Examine the solid sub- 

 stance left in the tube. How does it differ from the original 

 coal? This is coke. Record your results. 



READINGS WHICH WILL HELP ANSWER 

 THE PROBLEM QUESTIONS 



What are the sources of heat energy for the earth? 



Heat energy is obtained from the sun. For hun- 

 dreds of millions of years the sun has been sending 

 vast quantities of heat through space to the earth, and 

 scientists believe that it will continue to do so for 

 many millions of years in the future. Scientists are 

 trying to discover how this tremendous quantity of 

 energy in the sun is produced and maintained, but 

 as yet it is one of the unsolved mysteries of the uni- 

 verse. Without the sun's rays our earth would soon 

 become a barren place, for plants and animals could 

 not live long without the heat. 



Heat energy is obtained from chemical action. Al- 

 though the sun is our most important source of heat, 

 we have not yet learned how to use the sun's rays 

 directly to heat our homes and run our engines. A few 

 solar engines have been invented, but none have 

 proved economical to operate. We depend very large- 

 ly on the heat obtained by burning coal, wood, oil, or 

 gas. This heat comes indirectly from the sun because 

 at some time in the past it was stored up by plants as 

 chemical energy. Materials that burn are really reser- 

 voirs of energy that entered them from the sun, oft- 

 times thousands of years ago. 



Burning is called combustion. What is combustion? 

 You learned in your study of air that oxygen com- 

 bines readily with many elements. This process, called 

 oxidation, may go on slowly or rapidly. If oxidation is 

 rapid enough to produce light and heat, it is called 

 combustion. 



During combustion chemical energy is converted 

 into heat energy. If we make this combustion take 

 place under a boiler of a steam engine, the steam 

 formed can be harnessed and made to do work. In 

 other words, we can convert heat energy into mechan- 

 ical energy. One of the great principles of science is 

 that energy can be neither created nor destroyed. This is 

 called the law of conservation of energy. All man is 

 able to do is to arrange conditions whereby he can 

 control the conversion of one form of energy into an- 

 other. 



There are other ways of transforming chemical 

 energy into heat energy besides combustion, but none 

 are of practical importance, so far as heating the home 

 is concerned. Several examples are given in the ex- 

 periments at the beginning of the topic. 



Electrical energy can be converted into heat. When 



