24 GENERAL AND PHYSICO-CHEMICAL. 



colloids. Thus according to BILTZ 1 zirconium hydroxide protects gold 

 better than does 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 cataphoresis) were not 

 evident. 



Electrolyte Precipitation of Hydrophile Colloids. The salts of the 

 alkalies precipitate the suspension colloids even in low concentrations. 

 The alkali salts behave differently toward the hydrophile colloids. This 

 may in part be due to the fact that hydrophile colloids have much less 

 of a certain electric charge than the suspension colloids. For this reason 

 the hydrophile colloids are often precipitated from their solution by alkali 

 salts. 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 different alkali salts to act precipitatingly 

 certain laws have been formulated, but they cannot be arranged in a 

 general rule. 



1 Ber. d. d. chem. Gesellsch., 35, 4431 (1902). 



2 Zeitschr. f. physik. Chem., 60, 301 (1907). 



3 Ann. Phys. (3), 35, 580 (1888). 



4 Bioch. Zeitschr., 2, 251 (1907). 



