ENERGY 19 



in fact, in this sense, it may be said that heat is motion, for by 

 simply increasing the heat of a substance, its molecules may be 

 made to move faster and farther apart, while withdrawing the 

 heat reverses the action. The differences noted in the three 

 states in which matter is capable of existing are largely due to 

 the different amounts of heat they contain, for solids may be 

 made liquid by adding heat, and liquids may be turned to gases 

 by adding still more heat. Adding heat to a gas increases the 

 speed of its molecules and causes them to move outward with 

 greater force. It is these myriads of flying molecules, striking 

 against whatever confines them, that produces the pressure 

 noted in all gases when confined. In steam boilers, the pres- 

 sure of the confined steam sometimes becomes so great that it 

 causes disastrous explosions. When a gun is fired, it is the 

 pressure of the gas generated that gives the speed to the bullet 

 (53). Gases and liquids are often called fluids. The mole- 

 cules of fluids move easily and smoothly past one another, but 

 in solids the motion is more restricted. All are in motion, 

 however. 



25. Forms of Energy. Besides heat and the energy of 

 motion already mentioned, there are three other forms of 

 energy that are more or less familiar. These are light energy, 

 electric energy and chemical energy. All forms of energy are 

 closely related; in fact, are but different forms of the same 

 thing, as is shown by their changing from one form to another, 

 and by their producing changes in matter. Electricity, for 

 instance, may be produced by chemical energy and used to 

 give motion to cars, elevators and the like. The same force, 

 properly treated, will light the cars, and if desired, heat them 

 as well. Chemical energy is often liberated when two sub- 

 stances are united, and frequently appears in the form of 

 heat. Water put on quicklime may release enough chemical 

 energy to boil the water. Instances of this are often seen 

 when plaster is being mixed. Sulphuric acid poured into water 



