COLLOIDS 513 



precipitation of gold suspensions by means of sodium chlorid. The 

 method was to find the amount of the protective colloid that was just 

 necessary to prevent the flocculation of a fixed amount of a given gold 

 sol by a fixed amount of the salt. In this way it was observed that 

 the protective action of different colloids is very different. Thus if 

 the amount of starch in solution which is necessary for protection 

 be taken as 2,500 the amounts of various other colloids required are 

 as shown in the following table : 



Protective colloid Starch Dextrin Gum Arabic Albumin Gelatin Glue 



Amount required 2,500 1,200 40 25 1 1 



5. Very simple theories are devisable to explain the interactions 

 between different colloidal solutions. Thus two oppositely electrical 

 colloids may be considered to precipitate one another mutually be- 

 cause of the electrical attraction existing between all oppositely 

 charged particles. This results in bringing together the oppositely 

 charged particles with the formation of relatively neutral aggregates, 

 which, as shown in the discussion of the flocculation by electrolytes, 

 is a condition favorable to precipitation. For very obvious reasons, 

 then, no flocculation would be expected when two like charged col- 

 loids are mixed. Many objections may be made to the unqualified 

 acceptance of this explanation. It offers, nevertheless, a valuable 

 leading idea when not accepted too dogmatically. 



A number of factors probably contribute to the protective action 

 of many colloids. To some extent the effect may be purely mechani- 

 cal, since increased viscosity imparted by the presence of the protec- 

 tive emulsion colloid will shorten the mean free path of the floc- 

 culable particles, and thus materially lessen the probability of im- 

 pacts between them. Consequently flocculation will not occur as 

 readily. Further, the ultramicroscope gives considerable evidence 

 of the existence of another and very important factor. It seems 

 certain, at least in many cases, that the protective colloid arranges 

 itself in a film or coating around the flocculable particles, and in this 

 way prevents the aggregation of the particles. These factors are not, 

 however, sufficient to explain all cases of protective action. It must 

 be considered that in some cases, at least, the protective colloid 

 exerts a truly dispersive effect, such as would result from a nullifica- 

 tion of surface tension forces. The ease with which a small amount 

 of soap will emulsify a large amount of oil is difficult to explain on 

 any other hypothesis. When an oil is shaken with pure water little 

 or no emulsification results, w r hile in the presence of the soap as a 

 protective colloid the same amount of work in shaking accomplishes 

 enormously greater results. In fact, the oil will spontaneously 

 emulsify by merely standing in contact with the soap solution. The 

 equilibrium condition of oil in contact with pure water is reached 



