COAGULATION 79 



Liesegang Phenomenon. 



If a gel contains a substance in solution and a second substance 

 capable of reacting with the first is allowed to diffuse into the gel, 

 the product of the reaction is deposited in strata separated by 

 clear intervals (Part II.). In Fig. 11 is illustrated a test tube 

 filled with 1 per cent, agar gel containing potassium bichromate. 

 On top of this was placed a solution of silver nitrate. It is 

 obvious that the silver bichromate formed is deposited in strata 

 separated by clear agar. No satisfactory explanation of this 

 phenomenon has been offered (Part II. p. 413). 



Electrical Diffusion. 



The rate at which electrolytes diffuse into gels may be increased 

 by the passage of an electric current. This method is sometimes 

 employed in the administration of drugs, so called ionic medica- 

 tion. " Metal "-ions (cations) are carried into the tissues from the 

 positive electrode of any current-supply device, while " acid "-ions 

 (anions) are driven in from the negative electrode (see Chap. X. 

 and Part II. p. 413). 



Coagulation of Gels and Precipitation of Sols. 



If a colloid has a positive charge when in a liquid of high H + 

 concentration and has a negative charge when the H + is low, an 

 intermediate point (isoelectric point] can be reached when the 

 colloidal particles has either no charge or, which comes to the 

 same thing, half of them with + and the rest . In such a case 

 the colloid would lose its electrical stability. Positive and negative 

 particles would coalesce to form aggregates too large to exhibit 

 Brownian movement and the colloid would separate into discrete 

 phases. 



This coagulation may be brought about by adding (a) acids or 

 alkalies, (b) suitable electrolytes or (c) colloids of opposite sign. 

 The coagulation of suspensoids (Class IV. colloids) by the above 

 means is easily carried out and is a reversible process. On washing 

 out the adsorbed precipitant the dispersoid is re-established. 

 Emulsoids (Class III.), on the other hand, are more stable than 

 suspensoids. They usually need the addition of a large quantity 

 of the coagulating substance and the resulting coagulum is 

 frequently irreversible. This may be due to alteration in the 

 continuous phase. We have seen that an emulsoid is a diphasic 

 system where the continuous phase is more or less a continuation 

 of the disperse phase. If a substance A is dispersed in water to 



