ENERGY AND FORCE 



45 



be released quickly. Many of the most violent explosives 

 possess less stored-up energy than the same weight of 

 other materials, but in the case of the explosive the energy 

 is all released in an instant, while with a substance such 

 as coal considerable time is required to release its energy. 

 When powder explodes, a gas is formed which would 

 occupy at ordinary pressure several hundred times as 



Eccentric 



FIG. 37. Diagram Showing Action of Steam in a Common Steam Engine of 

 the Reciprocating Type. 



much space as the original powder. In the gun the gas 

 is under great pressure on account of the limited space, 

 and as it expands it drives the bullet or projectile rapidly 

 before it. 



Molecular Forces. In the case of gases, the mole- 

 cules seem to have little attraction for each other. With 

 liquids, however, the molecules resist being pulled apart, 

 and with solids this force is very great, as is shown by 

 the weight required to break a small steel wire. 



Elasticity. - - The tendency of a substance to return to 

 its original form and volume after having been stretched 

 or distorted is called elasticity. Gases have perfect 

 elasticity of volume, but they have no shape. The elas- 



