STEAM AND VAPORISATION. 



that its molecules vibrate still more energetic- 

 ally, hit one another harder blows, and drive 

 one another still further apart. The time 

 comes when their vibrations are so energetic 

 and their distances from one another so much 

 increased that they are able to take that definite 

 step in overcoming their mutual attraction 

 which places them in a position of comparatively 

 independent movement. They can move over 

 and past one another with easy friction, almost 

 with no friction. Their comparative rigidity 

 in fixed positions is entirely gone. The sub- 

 stance is a liquid ; the solid has melted under 

 the influence of heat. 



Next take a liquid, and make it hotter. Sup- 

 pose it to be water in a closed boiler or cylinder 

 such as we are taking for our experiment. 

 The increasing heat means increasingly vigorous 

 vibration of the molecules, until the blows they 

 deal one another are so violent that some of 

 those on the surface of the liquid are thrown 

 off into the space above, through which they 

 bound and rebound with great speed and 

 energy, striking against the inside walls of the 

 cylinder and frequently against one another. 

 This is the state of a vapour, which very much 

 resembles that of a gas. If a locomotive boiler 

 were magnified to be of the size of the world, 

 and all the molecules of steam within it in 

 the same proportion, they would seem like 

 india-rubber balls flying freely in all directions, 



141 



