III. 



FOKCE. 



In this chapter I will consider briefly some of the 

 forces of nature. Force may be defined as that which 

 can put matter in motion. 



Among the forces of nature may be mentioned 

 light, heat, electricity, magnetism, chemical affinity, 

 cohesion, adhesion, and gravitation. 



Forces act upon bodies in two ways — they cause 

 bodies to approach or to recede from each other; to 

 attract or to repel. 



As examples of the former, chemical affinity binds 

 atoms together to form molecules; cohesion causes 

 like atoms or molecules to cling together to form 

 masses of matter; gravitation causes each atom in the 

 universe to attract every other atom in the universe ; 

 and magnetism causes the magnet to attract iron or 

 steel. 



Heat, which causes bodies to expand, thereby sepa- 

 rating more widely their molecules, generally acts as a 

 repellant force. The like poles of magnets, bodies 

 electrified alike, and currents of electricity in unlike 

 directions repel each other. 



Some forces act only at insensible distances. 

 Chemical affinity, adhesion, and cohesion are exam- 

 ples. Gravitation acts at all distances. No limit can 

 imagined at which two bodies cease to attract each 

 other. 



Our knowledge of force is derived from its effects 



on matter. Matter in motion shows force at work. 



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