28 BACTERIOLOGICAL CHEMISTRY 



to gelatinise. The lyophilic colloids comprise systems in 

 which the disperse phase has a considerable attraction 

 for the continuous phase. The outstanding examples of 

 this class are the pix)tein and polysaccharide colloids, 

 which are characterised by a tendency to gelatinise or 

 form gels under appropriate conditions. 



The lyophobic systems are much less stable than the 

 lyophilic systems ; in other words, lyophobic sols depend 

 almost entirely on their surface charge for stability. This 

 surface charge can be measured by observation of the 

 direction and speed of migration of the particles when 

 submitted to a known potential gradient. It has been 

 found for most lyophobe systems that if the charge or 

 the surface potential of the particles is greater than about 

 ±15 millivolts the sol is stable. If the charge is reduced 

 by any means below this critical value the particles tend 

 to aggregate into larger and larger masses until finally 

 they settle out completely. One of the easiest ways of 

 altering the surface charge on the particles is to add an 

 electrolyte to the sol. The electrolyte will dissociate 

 into positively and negatively charged ions ; a negatively 

 charged sol like gold or collodion will adsorb the positively 

 charged metallic ions with a progressive neutralisation 

 of the charge on the sol particles . If sufficient electroljrte 

 is added to reduce the surface charge below the critical 

 value the sol will coagulate. Since the ions of monovalent 

 metals, like sodium, carry a less charge than those of 

 divalent metals, like calcium, and these less than the 

 ions of trivalent metals, such as lanthanum, it is necessary 

 to add more of a sodium salt than of a calcium salt and 

 more of a calcium salt than of a lanthanum salt to have 

 the same coagulating effect on a given sol. For example, 

 a certain collodion sol was found to be equally effectively 

 coagulated by N/2-NaCl, by N/lG-CaCla and by N/680- 

 LaClg. In the case of positively charged sols, like ferric 

 hydroxide, it is the negatively charged anion which is 

 adsorbed on to the sol particles and is important in 



