178 OUTLINE OF CHEMICAL STUDY 



contain three atoms ; two of these atoms must be those 

 of the element hydrogen, and one atom must be of 

 oxygen. If the two elements combined in any other 

 proportion (say one atom of each), or if they combined 

 with any other element, the molecule formed would not 

 be that of water. Thus we may say that chemical com- 

 bination takes place only between definite proportions of 

 certain elements. 



Every element does not by any means combine with 

 every other element. Some elements may combine with 

 several different ones, while others can unite directly 

 with only two or three. The study of what elements 

 combine with certain others is of course an important 

 part of the chemist's work. 



Sometimes the same elements may combine in more 

 than one proportion. In such cases the resulting com- 

 pounds would of course be different. For example, 

 hydrogen and oxygen combine to form water (two atoms 

 of hydrogen and one of oxygen), while if two atoms of 

 oxygen unite with the two of hydrogen, a very different 

 substance is formed. The three elements, carbon, hydro- 

 gen, and oxygen, combine in a great many different 

 proportions, forming as many different compounds. 



204. Decomposition. When a compound is broken 

 up into its elements it is said to be decomposed. 



Naturally each element has a stronger affinity for 

 some of the elements with which it may combine, than 

 for others. Thus in some compounds the elements will be 

 more strongly united than in others. Some compounds 

 are so weak that they slowly decompose if simply left 



