CHAPTER III 



FIRE 



16. Sources of energy. If we wish to make a machine work, 

 we must apply energy to it, from any one of several sources. 

 We might use the heat of the sun or the movement of the 

 tides, the energy of the wind or of falling water. Another 

 source of energy commonly employed, especially in modern 

 industrial cities, is the chemical energy of fuel. The burning 

 fuel yields heat. This is then transformed into motion or into 

 electricity by means of suitable machinery, and is thus used for 

 doing much of our necessary work. The process of burning 

 is so common, is so much employed as a convenient source 

 of energy, and is so closely related to the liberation of energy 

 in living bodies, that it is necessary for us to know something 

 more about it. 



17. Burning. When something burns we may notice three 

 peculiar changes : 



1. The fire gives off heat, 



2. The fire gives off light. 



3. The fuel seems to be destroyed. 



From cold fuel we get heat and light — two kinds of energy. 

 How does this happen ? 



On closer study we find that the heat and light are not 

 created by the fire out of nothing ; they are transformed out of 

 other energy which has been present in the fuel all the time. 

 This latent, or resting, energy is chemical energy, and repre- 

 sents the power to produce chemical changes in matter. As to 

 the disappearance of the fuel, we can find, on making suitable 

 experiments, that the total amount of stuff is the same after 



II 



