ELEMENTS AND COMPOUNDS 15 



19. Law of Definite Proportion. A study of the com- 

 bination of elements has shown that always when elements 

 unite to form compounds, a definite weight of each ele- 

 ment enters into the composition. This is known as the 

 law of definite proportion. Chemical combination always 

 takes place between definite weights of the elements, and 

 a chemical compound always contains the same elements 

 in exactly the same proportion by weight. The law of 

 definite proportion is one of the foundation principles of 

 modern chemistry, and has enabled the chemist to deter- 

 mine the composition of bodies. This law is founded 

 upon facts independent of any hypothesis, and the accu- 

 racy of the law has been demonstrated by many investi- 

 gators. 



The theories relating to the composition of matter, par- 

 ticularly to atoms and molecules, are in harmony with 

 this law of definite proportion. It is believed, since 

 chemical combination takes place between definite masses 

 of elements, it must also take place between the smallest 

 particles of the substances. Since the smallest particles 

 which enter into chemical composition are the atoms, 

 chemical combination must take place between the atoms. 

 The atoms all possess definite weights. Hence it can 

 readily be understood why chemical combination takes 

 place between definite weights of the elements. The 

 next step in the study of the composition of matter is the 

 way in which the elements combine, or the power of com- 

 bination ; this is known as valence. 



20. Valence. The valence of elements is the power 

 which an atom of one element has of holding in chem- 



