CHEMICAL ACTION 177 



of a substance that can exist alone. Clearly, a com- 

 pound will no longer exist if its molecules are divided 

 again, for each molecule is made of atoms of different 

 elements. The molecules of an element are also the 

 smallest bits that can exist alone, each molecule being 

 made of atoms that cannot be separated unless by unit- 

 ing with other different atoms. Between elements and 

 compounds, however, there is this difference : the atoms 

 of an element are just alike, and the same in substance 

 as the molecule, whereas the molecule of a compound 

 is made of different kinds of atoms. 



In any molecule, the force that binds the atoms together 

 is called chemical affinity. Its action in holding atoms 

 together in molecules is somewhat similar to that of 

 cohesion, which binds molecules together in masses. 

 Without cohesion we should have no masses of definite 

 form ; without chemical affinity, no substances of definite 

 composition. 



203. Chemical Combination. When two or more ele- 

 ments unite to form a compound they are said to com- 

 bine ; the process is called chemical combination. The 

 number of atoms which may combine to form a mole- 

 cule varies according to the substance formed ; some 

 molecules contain only two atoms, while others contain 

 nearly one hundred. In any one substance, however, 

 the molecule must always contain the same number of 

 atoms; moreover, these atoms must always be those of 

 the same elements, and each element must always be 

 present with the same number of atoms. For example : 

 water is a compound ; a molecule of water must always 



