48 THE THEORY OF IONS 



without a negative. The simplest example of 

 polarity is seen in the magnet. It pervades the 

 material and immaterial universe. 



The elements exhibit this polarity in different 

 ways. It may be like that of a magnet, in which 

 there are two poles and two only. Thus, oxygen is 

 bipolar and capable of attracting to itself two ions 

 of hydrogen to form a molecule of water. Hydrogen 

 on the other hand is unipolar, and the ion only 

 attracts and holds a single ion of chlorine to form 

 a molecule of HC1. Chlorine is also unipolar. The 

 H ion is positive, the Cl ion carries a negative 

 electrical charge. An element having a single pole 

 must create the opposite pole by induction in 

 another body. The analogy between atomic or 

 electrical and magnetic polarity can be rendered 

 evident by pith-balls. If a pith-ball charged with 

 positive electricity is brought near to a negatively 

 charged one, they mutually attract one another, 

 and each becomes the pole of the other. Separated 

 to a distance each carries its own charge, and they 

 no longer influence each other ; but each draws an 

 opposite electrical charge from the nearest conductor, 

 and thus creates for itself the opposite pole. Simi- 

 larly, when a glass rod is rubbed with silk, it becomes 

 positively electrified, and induces the opposite kind 

 of electricity in the silk. Polarity involves the 

 opposition of two relations or poles which attract 

 each other ; this is an indispensable condition. 

 Electrical polarity differs from magnetic polarity in 

 the fact that in magnetic polarity both poles arejn 

 one body, in electric polarity they are in separate 

 bodies. 



