12 ATOMS. 



and carbon or charcoal are elements. Iron rust, on the 

 other hand, is a compound. There are, of course, as 

 many different kinds of atoms as there are of elements. 



5. COHESION. The force which binds 



What is Coke- r . , . -. . i, j 



sion? lllus- together atoms of the same kind is called 

 trate the sub- fa e attraction of cohesion, or simply co- 

 hesion. In the more tenacious substances ? 

 such as iron or copper, the force of cohesion is im- 

 mense. The strength of a horse is insufficient, for ex- 

 ample, to break an iron wire one-fourth of an inch in 

 thickness. It is because in every section of the wire 

 the atoms attract each other with a superior force. And, 

 as we may easily imagine a thousand sections in every 

 inch of the wire, we may see that there is in every inch 

 a force of attraction constantly exerted superior to the 

 strength of a thousand horses. Attraction between un- 

 like atoms in contact with each other, as between glue 

 and the wood to which it is applied, is called adhesion. 



6. GRAVITATION. Unlike the force of 

 Station' attraction mentioned in the preceding para- 

 graph, gravitation acts at all distances. It 



is the reason of the weight of bodies, one body weigh- 

 ing twice or three times as much as another, because it 

 has twice or three times the quantity of matter to at- 

 tract and be attracted by the earth. 



7. CHEMICAL ATTRACTION OR AFFINITY. 



What ^s Che- 



mical Attrac- The force which unites unlike atoms into 

 twn or Affim- com p OUn( is possessing new properties is 

 called chemical attraction or affinity. 

 Thus iron and oxygen unite by chemical attraction to 

 form iron rust, a substance different from either. So 



