SUMMARY 93 



moved. Heat, therefore, cannot pass from the inner bottle 

 by conduction or convection. To retard the passage of 

 radiant heat, the inner walls of the vacuum space are finished 

 with bright reflecting surfaces. 



Note to Students. Both the Centigrade and the Fahrenheit 

 scale are used in later discussions in this book. The student has 

 been accustomed to the English or Fahrenheit scale in everyday 

 life, and so occasionally the use of this scale prevents unnecessary 

 confusion. On the other hand, the Centigrade scale is preferred in 

 scientific work, and, like all the metric scales, is the rational system. 

 It is, therefore, used frequently hereafter in order to familiarize 

 students with it. In occasional discussions where one scale is used, 

 approximate equivalents in the other scale are added in parentheses. 



The following rules will be found useful in changing readings 

 from one scale to the other : 



To change Fahrenheit to Centigrade, subtract 32 from the number 

 of degrees and multiply the remainder by f . 



70 F. = (70 - 32) X * = 211 C. 



To change Centigrade to Fahrenheit, divide the number of degrees 

 by f and add 32. 



_ 10 C. = (- 10 ^ $) + 32 = 14 F. 



SUMMARY 



The sun is the source of the heat and light of the earth. 

 Heat has the capacity to do work, and is therefore a form of 

 energy. The sun is the source of the energy on the earth. 

 If a body has the ability to do work without actually being 

 at work, it is said to have potential energy ; the energy of a 

 body at work is called kinetic. There are different forms 

 of energy, such as heat, light, electricity, gravitation, chemi- 

 cal energy, and mechanical energy. Energy can neither be 

 created nor destroyed, but one form of energy may readily 

 be changed into another. Heat causes most substances to 



