PRINCIPLES OF CHEMISTRY 131 



pound. To illustrate: H^O is composed of two atoms of 

 hydrogen and one atom of oxygen. According to the table 

 of atomic weights (page 129), water has a molecular weight 

 of 18, i.e., (1X2 (H 2 ) + 16 X 1(O) = 18). This means 

 that 2 parts of hydrogen combined with 16 parts of oxygen 

 form 18 parts by weight of water. 



137. Law of Combined Weights. When elements com- 

 bine to form chemical compounds, they unite according to 

 fixed proportions. To illustrate: When water is formed 

 from the combination of hydrogen and oxygen there must 

 be 2 parts of hydrogen to 1 part of oxygen. These gases 

 must unite in this proportion or water will not be formed. 

 Thus 2 atoms of hydrogen and 1 atom of oxygen unite 

 to form 1 molecule of water. 



138. Valence. If we examine a number of symbols of 

 binary compounds (compounds made of two elements) of 

 hydrogen, such as HC1, H 2 O, NH 3 , CH 4 , we find that the 

 first compound contains one atom, the second two atoms, 

 the third three atoms, and the fourth four atoms of hydrogen. 

 This means that the different elements are combined with 

 the same element (hydrogen), or other elements equivalent to 

 it in combining power, in different amounts as expressed by 

 the symbols. This power of an element to combine with 

 different amounts of hydrogen or its equivalent is calted a 

 valence. The combining power of hydrogen, which is one, 

 is selected as the unit. 



139. Chemical Action. Chemical change is due to the 

 action of chemical force, which like other forces cannot be 

 described, but is known by its effects. It is quite different, 



