192 ELEMENTS OF GENERAL SCIENCE 



Of course, if there is energy in the coal, it must have been 

 in the plants from which the coal was made. We can burn 

 wood to make steam in about the same way that we can burn 

 coal. Indeed, wood is still sometimes used as fuel under 

 boilers. Where, then, did the plants get their energy ? Where 

 do they get it now ? When we were studying about how 

 plants make their food, we found that they could do so 

 only when they had light. Light is the power that runs the 

 factory. By means of the energy that comes to it in the 

 form of light, the plant is able to separate the carbon 

 from the oxygen of the carbon dioxide and to build up 

 those complex compounds from which its body is constructed. 

 When we burn the wood, oxygen again unites with the other 

 elements to form carbon dioxide, water, and other com- 

 pounds, and, most important to urf, the energy that was put 

 into the compound is released in the form of heat. 



The sun, then, is the source of the energy which we find 

 stored up in wood and in coal. If we were to take any 

 other example and trace back the energy, we should in all 

 cases arrive at the sun as the last term in our series. The 

 sun is the great source of the world's energy. The sun is 

 the engine that runs the trolley car. 



208. Amount of energy from the sun. Some careful studies 

 have been made to find out how much energy the earth 

 receives from the sun. It is believed that the amount is 

 equivalent to more than 200,000,000 horse power for each 

 inhabitant of the earth. It is this continual supply of energy 

 from the sun that maintains the temperature of the earth's 

 surface, evaporates water, causes convectional air currents, 

 and provides the energy for photosynthesis. Since the earth 

 is continually giving off heat to the surrounding space, it 

 is continually losing energy. The part of the sun's energy 

 which is stored up in plants and animals, or used in evapo- 

 rating water, supplies most of the energy which is used by 

 man in accomplishing his work. This is probably not more 



