COLLOIDS AND COLLOIDAL SOLUTIONS 385 



(5) Behaviour towards Suspensoids. Suspensoids and emulsoids, if 

 they have opposite electrical charges, mutually precipitate one another. 

 This property is made use of in precipitating proteins from solution 

 (many, if not all, of the alkaloidal reagents act in this way) : 



If to some egg-albumin solution or dilute serum an equal volume 

 or more of colloidal ferric hydroxide be added, and then about *5 to 

 I gm. of sodium sulphate and the mixture be well shaken, a brownish 

 mass containing the protein and excess of ferric hydroxide is pre- 

 cipitated. The filtered solution will not contain protein as shown by 

 the biuret reaction, Millon's reaction, etc. 



(6) Electrical Charge. The electrical charges on the particles of 

 an emulsoid sol are chiefly due to the reaction of the medium, e.g. 

 albumin in neutral solution is not charged and does not travel in an 

 electric field. Albumin in faintly acid solution has a positive charge 

 and travels to the negative pole. Albumin in faintly alkaline solution 

 has a negative charge and travels to the positive pole. 



(7) Adsorption. If an emulsoid sol be precipitated by electrolytes 

 or by suspensoids, dissolved substances are taken out of solution in the 

 same way as with suspensoids. 



Special Properties of Emulsoids. 



The properties of emulsoids show many differences among them- 

 selves and many differences from the properties of suspensoids and 

 emulsions. 



(1) Silicic Acid. 



Silicic acid sol on treatment with an electrolyte oehaves like a 

 suspensoid ; a small quantity of electrolyte causes gel formation. The 

 gel takes the form of a jelly which gradually becomes more viscous 

 and sets to a hard mass with no separation of water. The change of 

 state is continuous, proceeding until the mass sets. 



Thus, if excess of sodium silicate solution of sp. gr. 1-16 be added 

 to 2N hydrochloric acid, an opaque gel containing the salt is formed 

 which gradually becomes more viscous and sets. The rigid gel cannot 

 be redissolved. The colloidal solution thus resembles a suspensoid 

 sol in that the transformation is irreversible. 



(2) Gelatin and Agar. 



Both gelatin and agar dissolve in hot water. The solution on 

 cooling sets to a jelly. A quite stiff gel is formed by 2 per cent 

 agar solution. These non-rigid or elastic gels can be dissolved again 

 on warming. The transformation is reversible ; they show the pheno- 

 menon of hysteresis. The setting-point is influenced by the presence 

 of salts : citrates raise the setting-point, thiocyanates lower it or may 

 prevent setting. 



