CHAPTER III 

 STRUCTURE OF MATTER 



18. Matter and Energy. It makes no difference whether we 

 consider the farthest star or study the rocks and soil of which 

 our own planet is composed, we everywhere find manifestations 

 of two very different things which scientists call matter and 

 energy respectively. Matter may be thought of as anything 

 that occupies space animals, plants, air, minerals, water in 

 fact, any object, big or little, that is known to exist, consists of 

 matter. Energy, on the other hand, is the power to do work 

 and is usually seen in anything that moves or affects matter, 

 and of course does not occupy space. Without energy the 

 world we live in would be cold, still and lifeless; without matter, 

 it could not exist at all. Matter and energy are thus sharply 

 distinguished, matter being a substantial thing, while energy is 

 that which produces change in it. Matter, however, is not 

 continuous; that is, it does not occupy all space. Between 

 the earth, with its enveloping atmosphere, and other heavenly 

 bodies are vast stretches in which, so far as we know, no matter 

 of any kind exists. Space on the earth that does not contain 

 matter is called a vacuum. The space in a thermometer-tube 

 above the mercury is a nearly perfect vacuum. 



19. The Molecular Theory. All matter is regarded as being 

 made up of very small particles, called molecules, which are far 

 too small to be seen even with the highest powers of the micro- 

 scope. Some idea of how small molecules really are may be 

 gained from the fact that a thimbleful of gas at ordinary 

 temperatures will contain more than 75,000,000,000,000,000- 

 000 molecules. Although nobody has ever seen molecules, and 



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