OPTICAL AND ELECTRICAL PROPERTIES OF COLLOIDS 77 



Chlorids and nitrates diminish the double refraction; sulphates are 

 without effect. Phenols change the direction of the refraction. 



The phenomenon is important for the understanding of the double 

 refraction of organized structures (plant fibers, muscle, horn, etc.). 



Electrical Properties. 



If two electrodes are placed in a solution of a hydrosol as free as 

 possible of electrolytes, and a current is allowed to pass through, we 

 immediately notice the movement of the colloid to one of the elec- 

 trodes. Various suspensions (suspensions of clay, rosin, etc.), as 

 well as most hydrophobe hydrosols migrate to the anode, whereas 

 the colloidal metal hydroxids (iron or aluminium oxid hydrosol, etc.) 

 move to the cathode. Hydrophile colloids (albumin, etc.) exhibit, 

 if almost free from electrolytes, no definite recognizable directive 

 tendency. The zone of H ion concentration in which there is no 

 migration has been named by L. MICHAELIS the isoelectric zone. 

 The addition of acids causes migration of these colloids to the 

 cathode, alkalies to the anode; they then behave as if they were 

 salts of the acids or alkalies involved, and this may actually corre- 

 spond with the facts (see p. 149 et seq.). 



This movement of suspensions and hydrosols against the water, 

 under the influence of the electric current, is called cataphoresis. 



The colloid particles behave like ions and their speed of migration 

 is similar in rate. ZSIGMONDY calculated from the speed of migration 

 (0.002 mm.) and the diameter (50 MM) of the particles of a colloidal 

 silver solution, that the particles were charged with 297 X 10~ 10 

 electrostatic units which is the equivalent of 62 elemental units. 

 Such a particle is in a certain sense an ion of 62 valencies. 



If a protective colloid (albumin, gelatin, etc.) is added to a sus- 

 pension, the latter acts as if its entire mass was composed of the 

 protective colloid. The commercial inorganic colloids (collargol, 

 lysargin, etc.) do not behave in an electric current as does pure 

 colloidal silver, but as albumins or albumoses. Their direction may 

 be changed at will by the addition of acids or alkalis. 



The process may be reversed, that is, the water may be moved 

 under the influence of electrical difference in potential, provided 

 the suspension is held fixed. The experimental procedure is as 

 follows: Instead of a clay suspension we choose a porous clay wall 

 D (Fig. 8), permeable for water, by means of which a U-shaped 

 tube is divided into two parts. If the tube is filled with water and 

 into each branch an electrode is introduced, the water moves under 

 the influence of the electric current and, in fact, it will rise on the 

 cathode side until it exerts a certain pressure against the anode 



