44 PHYSICAL CHEMISTRY IN BIOLOGY. 



Certain above-mentioned suspensions (mastic), as well as other particles 

 suspended in water, act the same as suspension colloids. ScnuLZE 1 has found 

 that cloudiness due to clay particles on the addition of clarifying bodies (alum, 

 lime) give a voluminous deposition. SCHLOESSING 2 found that clay suspensions 

 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 role 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 already been stated, the colloids are considered as carriers 

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

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

 posed by LINDER and PiCTON, 3 and subsequently has been substantiated 

 by many investigators. BILTZ 4 has made especially systematic investiga- 

 tions on this subject and finds that equally charged colloids do not pre- 

 cipitate each other. For the mutual complete precipitation of opposed 

 electrically charged colloids, a certain quantity relation 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 especially 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 5 have found with silver hydrosol that the presence 

 of protein prevented the flocking out by electrolytes. ZSIGMONDY & 

 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 Ice. 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 (As2S 3 , Sb 2 S 3 , CdS) are also 

 protected in the same manner against the influence of electrolytes 

 (A. MULLER and ARTMANN). ? 



1 Ann. Phys. (2), 129, 366, 1866. 5 Journ. prakt. Chem. (2), 56, 241, 1897. 



2 Compt. rend., 70, 1345, 1870. 6 Zeitschr. analyt. Chem., 40, 697, 1901. 



3 Journ. chem. Soc., 71, 572, 1897. 7 Oester. Chem. Ztg., 7, 149, 1904. 



4 Ber. d. d. chem. Gesellsch., 37, 1095, 1904. 



