512 INFECTION AND RESISTANCE 



neither individual nor mutual flocculation occurs. Whether the 

 presence side hy side of two sorts of similarly charged disperse par- 

 ticles results in any change in the dispersion of either is not known. 

 Except to show that no mutual precipitation occurs, these cases 

 have been but little studied. 



2. When two oppositely electrical colloids are mixed mutual 

 precipitation may or may not occur. The factor which in the main 

 determines the outcome is the relationship between the amounts of 

 the two colloids used. If neither is present in too great excess com- 

 plete mutual precipitation in general occurs. If either is present 

 in great excess precipitation does not in general occur. 



3. The effect of a great excess of one colloid in preventing 

 mutual precipitation is very marked when the colloid in excess is 

 one of the emulsion-colloid type. Thus gelatin, a typical emulsion 

 colloid, when present in excess over another colloid very frequently 

 prevents all flocculation, even in fairly coarse-grained suspensions. 

 Advantage has been taken of this action in preventing scaling in 

 boilers. This scaling is due largely to the fact that lime salts held 

 in solution as bicarbonates are decomposed by heat, with the separa- 

 tion of calcium carbonate, at first in the highly dispersed state. This 

 gradually aggregates together and deposits on the interior of the 

 boiler as an amorphous flocculated colloid, which in time becomes 

 very compact, and in many cases crystalline. If, however, a small 

 amount of glue (impure gelatin) be added to the boiler water the 

 colloidal constituents of the water do not flocculate and compact, 

 but remain suspended, and may from time to time be blown off. 

 Another illustration is found in the preparations of photographic 

 emulsions. The silver halides flocculate very readily in pure water, 

 but in gelatin solution remain in a highly dispersed state, which is 

 necessary to the preparation of the plate. In this case, not only is 

 the suspension protected from flocculation, but also a degree of dis- 

 persion is reached which is far beyond anything attainable in pure 

 water. The same is true when lysalbinic acid is used to prevent 

 flocculation of colloidal silver. In pure water only very dilute sus- 

 pensions of metallic silver are obtainable, but in the presence of lysal- 

 binic acid suspensions containing as high as ninety per cent, of silver 

 are obtainable, the product being used medicinally under the name of 

 "argyrol." These are the phenomena known as "protective actions'' 

 and the gelatin, albumin, or other colloid which exerts the protective 

 action is spoken* of as a "protective colloid." 



4. The protective action of certain colloids is not only exerted 

 against the tendency of the protected colloid to spontaneously floc- 

 culate, but also a certain and very great protection is offered against 

 flocculation by electrolytes. Thus Zsigmondy 16 was able to find a 

 definite measure of the protective action of certain colloids on the 



16 Ztschr. f. analyt. Ch., 40, 697, 1902. 



