PHYSICAL CHEMISTRY 31 



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. 



An instrument devised on the above principle is known as an 

 ultramicroscope. While the observer looks through a high 

 power microscope at a drop of the solution, a powerful beam of 

 light from an arc is passed through the solution from the side. 

 The observer sees tiny flashes of light reflected up from the col- 

 loidal particles. Only those of larger size will show this 

 phenomenon, as colloidal gold particles. Colloidal albumin is 

 invisible in the ultramicroscope. The limit of microscopic visi- 

 bility is about 0.1 /*,. Particles of this size or larger are called 

 microns. The limit of ultramicroscopic visibility is about 15 juyx 

 in electric light or 5 ///* in bright sunlight. Particles ranging 

 from 1-100 fjifj. are called submicrons, and those smaller than 

 1 /x/x, amicrons. Molecules and ions are much too small to be 

 visible in the ultramicroscope. As seen in the ultramicroscope, 

 colloidal particles appear to be jumping about rapidly. This is 

 known as Brownian movement, and is supposed to be due to 

 the bombardment of the particles by the molecules of the solvent. 



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 



