OPTICAL AND ELECTRICAL PROPERTIES OF COLLOIDS 85 



The discoverers explain the phenomenon by saying that the salts 

 mentioned undergo strong hydrolytic dissociation so that the col- 

 loidal iron and aluminium hydroxid present in the solution act as 

 " protective colloids" somewhat like gelatin or albumin. 



The flocculation hitherto described occurs only with true hydro- 

 phobe colloids. If the individual particles possess a film of native 

 albumin, gelatin, an albumose (such as Na lysalbinate), dextrin, etc., 

 the particles act as if they were the "protective colloids" involved 

 (see p. 77); in other words they are salted out only by very large 

 amounts of electrolytes. The colloid precipitate can be redissolved 

 in water providing the protective colloid is not denatured by the 

 electrolytes. On this account all the colloidal metal sols used thera- 

 peutically such as collargol, bismon, etc., are " stabilized" by hy- 

 drophile colloids. Without this protection it would be impossible 

 to preserve them for any length of time and they would be floccu- 

 lated when they were prepared for intravenous use by dilution with 

 physiological salt solution. 



R. ZSIGMONDY considers as characteristic of hydrophile colloids, 

 the protection they give to his gold hydrosol against flocculation by 

 electrolytes. He designates as the gold figure, the amount (in mg.) 



of the colloid in question, which was just sufficient to prevent the 



2 



flocculation of 10 c.c. of a gold sol. by 1 c.c. - NaCl sol. The floccu- 

 lation is accompanied by a change in color from red to blue, and it is 

 sufficient to observe this change. 

 The following data from R. ZSIGMONDY illustrate this point: 



Colloid. 



Gold figure in mg. 



Gelatin 



Casein 



Egg albumen 



Gum arabic 



Tragacanth 



Dextrin 



Potato starch 



Sodium stearate . . 

 Sodium stearate . . 

 Sodium oleate . . 



0.005-0.01 



0.01 



0.06-0.3 (depending upon its origin and ash content) 



0.15-0.25 



2 



10-20 



25 



10 (added at 60) 

 0.001 (added boiling hot) 

 0.4-1 



