TX WORK AND ENERGY 129 



does to the chemical combination of the coal with a part of 

 the atmosphere. Or putting the case in another way, we have 

 seen that there exists a certain amount of potential energy in the 

 coal and atmosphere which is capable of becoming kinetic as soon 

 as the temperature at which the coal can combine with oxygen has 

 been reached. Whence comes the potential energy of the coal ? 

 Coal results from the compression of vegetable material which 

 lived and nourished on the earth ages and ages ago. This vege- 

 table material formed the tissues of mosses and other similar 

 plants, which in the presence of sunlight have the power, by 

 virtue of the green colouring matter they contain, of decomposing 

 one of the gases of the atmosphere, carbon dioxide, splitting it 

 up into its elements, carbon and oxygen, reserving the former 

 for themselves and returning the latter to the air. l 



This carbon unites with the elements of water contained in the 

 plants forming compounds which build up the tissues of which 

 the plant is constructed. The tissues of the plant represent 

 from our point of view the work done in splitting up the carbon 

 dioxide by absorbing the energy of radiation. They still represent 

 this energy when they have assumed the condition of the coal, 

 and it is in this sense that coal is poetically referred to as 

 "bottled sunshine." 



If the earth receives so great an amount of energy from the 

 sun, it is easy to understand that the total quantity of energy 

 which is given out by the sun must be enormously greater. But 

 this radiation from the sun's surface is continually going on ; 

 that is, the sun is constantly losing energy, and this cannot go 

 on indefinitely without the loss being made good. How is the 

 energy of the sun maintained ? It has been suggested that the 

 heat generated by the impact of the meteorites which fall upon 

 the sun in great numbers is capable of accounting for this energy ; 

 and that in addition to this a slight shrinkage of the sun's mass 

 in cooling evolves a large amount of energy. But interesting as 

 this subject is we cannot pursue it further here. 



1 The student must remember to re-read this after studying Chapter XV. 



