94 Common Science 



125. Telegraph wires and wire fences put up in the summer must 



not be strung too tightly. 



126. Candy usually draws in somewhat from the edge of the pan 



as it hardens. 



127. A meat chopper can be screwed to a table more tightly than 



you can possibly push it on. 



128. A floor covered with linoleum is more easily kept clean than a 



plain wood floor. 



129. Rough seams on the inside of clothes chafe your skin. 



130. You can take the top off a bottle of soda pop with an opener 



that will pry it up, but you cannot pull it off with your 

 ringers. 



SECTION 16. Cooling from expansion. 



We get our heat from the sun ; then why is it so cold up 

 on the mountain tops ? 

 What is coldness ? 



Here is an interesting and rather strange thing about 

 heat and expansion. Although heat expands things, 

 yet expansion does not heat them. On the contrary, 

 if a thing expands without being heated from an outside 

 source, it actually gets cold ! You see, in order to ex- 

 pand, it has to push the air or something else aside, and 

 it actually uses up the energy of its own heat to do this. 

 You will understand this better after you do the next 

 experiment. 



Experiment 31. Wet the inside of a test tube. Hold the 

 mouth of the test tube against the opening of a carbon 

 dioxid tank. Open the valve of the tank with the wrench 

 and let the compressed gas rush out into the test tube until 

 the mouth of the test tube is white. Shut off the valve. 

 Feel your test tube. 



What has happened is this: The gas was tightly 

 compressed in the tank. It was not cold; that is, it 



