86 PHYSIOLOGICAL CHEMISTRY 



particles runs from 1-130/x^, one /X/A being the millionth part 

 of a millimeter. 



Tyndall's Phenomenon. Colloidal solutions show an inter- 

 esting behavior known as Tyndall's phenomenon. If a beam 

 of light is passed through a colloidal solution, the path of the 

 ray becomes visible, in much the same manner as the path of a 

 ray of sunlight in a dusty room. The light is dispersed or re- 

 flected from the particles of the colloid. 



Electrical Properties of Colloids. Colloidal particles carry 

 electrical changes just as ions are electrically charged. This 

 may be demonstrated by passing an electric current through a 

 colloidal solution. The particles of the colloid will move to the 

 positive or negative pole according to the nature of the charge 

 carried, a colloid with a negative charge travelling to the 

 positive pole, and vice versa. This phenomenon is known as 

 cataphoresis. Whereas some colloidal particles probably have 

 but one electrical charge, undoubtedly they often carry more 

 than one. A protein in colloidal solution will have a positive 

 charge if the solution is acid in reaction, but a negative charge 

 if the solution is alakaline. We may imagine that this is 

 brought about as follows : in acid solution the protein combines 

 with some of the acid, for example hydrochloric acid. From 

 this complex compound negatively charged chlorine ions are 

 given off into the water, and positive charges will remain on 

 the colloid particles. In alkaline solution the protein forms 

 salts, such as the sodium salt. Sodium ions are given off, carry- 

 ing positive charges, and negative charges will remain on the 

 colloid particles. These facts are of great importance in many 

 of the precipitation reactions of the colloids. 



Methods of Precipitating" Colloids. Some colloidal solutions 

 will precipitate, the colloid flocking out, merely on standing. 

 Some will precipitate if they are boiled. Some substances are 

 soluble in hot water, but their solutions will solidify on cooling. 

 Some colloids are thrown out of solution by the addition of an 

 electrolyte. The suspensoid colloids are precipitated by adding 

 a very small amount of an electrolyte such as a salt or an acid, 



