COLLOIDS. 23 



which do not settle after months are precipitated in 24-48 hours by a minimum 

 quantity of lime or magnesia. He also calls attention to the essential r61e which 

 the salts of sea water must play in the sedimentation of the cloudy fresh water 

 flowing into the sea (delta formation). 



In consideration of the conditions just mentioned, under which the 

 suspension colloids are precipitated by electrolytes, the mutual precipita- 

 tion ability of suspension colloids is of considerable interest. Accord- 

 ing to what has been stated previously, the colloids are considered as carriers 

 of electricity, and it has been proved that the oppositely charged col- 

 loids can act precipitatingly upon each other. This rule was first pro- 

 posed by LINDER and PiCTON, 1 and has subsequently been substantiated 

 by many investigators. BILTZ 2 has made especially systematic investiga- 

 tions on this subject and finds that colloids carrying the same kind of 

 charge do not precipitate each other. For the mutual complete precipita- 

 tion of opposed electrically charged colloids, a certain quantitative rela- 

 tion is necessary. On the action of two colloids with opposite charges in 

 variable quantities an optimum of the precipitation action is noticed; while 

 on overstepping the desirable precipitation conditions in both directions 

 no precipitation occurs at all. 



In analogy with the mutual precipitation ability of the colloids, BILTZ believes 

 that the especial great ability of most salts of the heavy metals to precipitate 

 colloids lies in the hydrolytically split and colloid-dissolving metallic hydroxides. 



Protective Colloids. Certain hydrophile colloids, which are precip- 

 itated with difficulty by electrolytes, have the power of protecting 

 suspension colloids against the precipitating action of electrolytes. MEYER 

 and LOTTERMOSSER 3 have found with silver hydrosol that the presence 

 of protein prevented the flocking out by electrolytes. ZsiGMONDY 4 

 has investigated the relative action of the protective colloids and has 

 found considerable differences. The figure in milligrams of colloid which 

 is just insufficient to protect 10 cc. of gold solution (0.0053-0.0058 

 per cent) against the action of 1 cc. 10 per cent NaCl solution is called the 

 gold equivalent for the respective colloid. Gelatin offers the best pro- 

 tection, then comes isinglass, casein, ovalbumin, gum arabic, Irish moss, 

 dextrin, starch. The colloidal sulphides (As2Sa, Sb2S3, CdS) are also 

 protected in the same manner against the influence of electrolytes (A. 

 MULLER and ARTMANN 5 ) . Inorganic colloids may also act as protective 



'Journ. chem. Soc., 71, 572 (1897). 



2 Ber. d. d. chem. Gesellsch., 37, 1095 (1904). 



3 Journ. prakt. Chem. (2), 56, 241 (1897). 



4 Zeitschr. analyt. Chem., 40, 697 (1901). 

 5 Oester. Chem. Ztg., 7, 149 (1904). 



