86 HEAT AND ENERGY 



Very low temperatures are reached by similar means ; 

 even air and other gases have been liquefied when put 

 under great pressure and cooled by other gases expand- 

 ing around them. The temperature of liquefied air is 

 about 191 C. below zero. It has been computed that 

 absolute cold (i.e. a condition of no heat at all) would be 

 reached at 273 C. below zero, or - 459.4 F. The low- 

 est degree that has been reached is about 250 C. 



QUESTIONS 



1. Can cold be put into a body? How may a substance be 

 cooled ? In what way is this commonly done ? 



2. Do any substances naturally have very low temperatures? 

 Why? What must be done in order to get very low degrees? 

 What two processes are commonly used? 



3. Explain how cold is produced by melting. Show how this 

 method is used in freezing cream. 



4. State examples of cold produced by vaporization. How, in 

 genera], is artificial ice made? 



5. How are gases liquefied at low degrees? What is the 

 condition of absolute cold ? At what degree would it be reached ? 



SECTION V 

 ENERGY 



98. Transformation of Energy. We have learned that 

 energy is the ability to cause motion (5), and we know 

 that this ability may be given from one body to another. 

 For example, a coiled spring may lie at full length on a 

 table with no ability to cause motion ; but press its coils 

 together (Fig. 64) and it is then able to exert force to 

 get back to its former length. Energy is put into the 



