COLLOIDS. 45 



Inorganic colloids can also serve as protective colloids. BILTZ 1 has 

 shown that zirconium hydroxide protects gold better than gelatin. 



By the addition of organic protective colloids the inorganic colloids 

 which on evaporation otherwise become irreversible, are made reversible, 

 in that the dry residue is soluble in water again. On this depends 

 the use of the protective action in the preparation of permanent inorganic 

 hydrosols, and this is of importance in many cases. 



According to BECHHOLD 2 the filterability of suspension colloids through 

 collodion filters is increased by the addition of organic colloids. It is also 

 well known that certain finely divided substances (carbon) pass more 

 easily through a filter in the presence of protein than without protein. 



The action of the protective colloids is ordinarily explained accord- 

 ing to the theory of QUINCKE 3 on the mutual surface tension of the 

 active bodies, and the process belongs accordingly to the adsorption 

 phenomenon which will be discussed later. According to this theory 

 the protective colloid under certain conditions spreads like an envelope 

 around the particles. In this wise the entire mass takes the properties 

 of the protective colloid and is therefore not precipitated by the elec- 

 trolyte any more .than the protective colloid itself. In filtration the pro- 

 tective colloid acts to a certain extent like a lubricant. This theory of 

 colloid envelope has recently received support by experiments of 

 MICHAELIS and PiNCUSSOHN. 4 They found that when suspensions of 

 indophenol and mastic were mixed together the number of particles visible 

 in the ultramicroscope diminished; after mixing, the physical properties 

 of the indophenol (pseudofluorescence, positive cataphoreis) were not 

 evident. 



Electrolyte Precipitation of Hydrophile Colloids. The salts of 

 the alkalies precipitate the suspension colloids even in low concentra- 

 tions. The alkali salts behave differently with the organic col- 

 loids. This may in part be due to the fact that hydrophile colloids 

 have much less of a certain electric charge than the suspension colloids. 

 Egg-white by dialysis gradually loses its power of being influenced by the 

 electric current. For this reason the hydrophile colloids are often pre- 

 cipitated from their solution by alkali salt. For this purpose, firstly, 

 certain concentrations are necessary; secondly, the precipitates of the 

 hydrophile colloids are again soluble in water (reversible) in opposition 

 to those of the suspension colloids. In regard to the ability of dif- 

 ferent alkali salts to act precipitatingly certain laws have been formu- 

 lated, but they cannot be arranged in a general rule. 



On comparing the concentration of various salts just sufficient for precipita- 

 tion, where at one time the same anion with different cations was tested and 



1 Ber. d. d. Chem. Gesellsch., 35, 4431, 1902. 3 Ann. Phys. (3), 35, 580, 1888. 



2 Zeitschr. f. physik. Chem., 60, 301, 1907. 4 Bioch. Zeitschr., 2, 251, 1907. 



