140 PHYSICAL CHEMISTRY IN MEDICINE. 



are readily able to do so, especially electrostatic forces, 

 with which we are going to deal chiefly to-day. 



As is well known, many crystalloids, such as salts, acids, 

 and bases, give off their constituents at the electrodes 

 when a current is sent through them. We call such sub- 

 stances electrolytes, and a much-used theory assumes, as 

 is well known, that there exist in aqueous solutions of 

 electrolytes, besides the electrically neutral molecules, the 

 electrically charged dissociation products, the so-called 

 ions. Upon the migration of these ions toward the 

 electrodes is dependent the conduction of electricity. The 

 ions are said to be positive when they wander to the 

 negative pole to be discharged and deposited, and negative 

 when they wander to the positive pole. In this way the 

 H ion, which all acids have in common, is electropositive, 

 the remaining portion of the molecule electronegative, 

 In the same way the OH ion, which all alkalies have in 

 common, is electronegative. The strength of an acid 

 or a base is determined by the concentration of these 

 ions. 



Colloids behave in an entirely different way. If an 

 electric current is sent through a pure colloidal solution, 

 the colloidal particles move, in contrast to the electrolytes, 

 in only one direction. They accumulate at either the 

 positive or negative electrode, from which we conclude 

 that they have either a negative or a positive charge. A 

 connection has shown itself to exist between this electrical 

 charge and the process of precipitation. Several inves- 

 tigators, more especially BILTZ, have shown that only 

 oppositely charged colloids mutually precipitate each 

 other, and that the entirely precipitated colloidal material 

 no longer "has an electrical charge, that is to say, no 



