510 INFECTION AND RESISTANCE 



valence of the ion, a relationship which seems to be quite generally 

 true. It has been quite commonly considered that the effect of the 

 anion in the flocculation of negative colloids is negligible. That this 

 point of view is not tenable has recently been strikingly shown by 

 Sven Oden 15 in his work on colloidal sulphur. He finds that the 

 effect of electrolytes on sulphur sols, which, like arsenic sulphid, are 

 negative colloids, is distinctly the resultant of two factors, one a floc- 

 culating effect on the part of the cathion, the other a dispersing effect 

 on the part of the anion. It is not improbable that our views in 

 regard to the phenomena of electrolyte flocculation will undergo con- 

 siderable modification in the near future, as they are based on rather 

 scanty experimental evidence. Since the properties of sols of the 

 same materials differ very considerably with the most minute details 

 of their method of preparation, it is naturally difficult for the same 

 investigator even to obtain uniform results. 



4. The actual concentration of a given electrolyte required to 

 flocculate a sol depends also very greatly on the nature of the sol 

 itself. Some sols are precipitated by very small concentrations of 

 electrolytes (three to four one-hundredths normal acid being usually 

 sufficient for arsenic sulphid), while gelatin, albumin, and protein 

 substances in general require far higher concentrations. So marked 

 is the difference in many cases that attempts have been made to clas- 

 sify colloids on the basis of their conduct in this respect. Thus col- 

 loids which are very sensitive to electrolytes are called suspension 

 colloids, while those that are not very sensitive are called emulsion 

 colloids. To the first class belong all the true, rather coarse-grained 

 suspensions, while the sols that yield soft gelatinous flocculates 

 usually fall into the second class. It is also very frequently true that 

 the latter type shows the phenomenon of reversible flocculation (see 

 ante). This classification is for many purposes quite useful, but 

 cannot be considered as very fundamental. For example, if the elec- 

 trolyte used be a salt of a heavy metal most of the so-called emulsion 

 colloids, such as albumin, are irreversibly flocculated. 



5. The flocculation of sols by electrolytes is usually explained 

 as due to the phenomenon of absorption. That is, the flocculating 

 ion is absorbed from the solution by the dispersed particles. Since 

 in general the ion which is absorbed is the one whose electrical charge 

 is opposite to that of the dispersed particle the absorption results in 

 a reduction of the charge on the particle, and allows the aggregating 

 forces of surface tension to become operative. The evidence of the 

 validity of this assumption is considerable. Thus the flocculated 

 colloids always contain appreciable amounts of the ion which caused 

 the precipitation, which is prima facie evidence of the absorption. 

 Furthermore, the electrical charges on the particles may be meas- 

 ured by determining their rates of migration, and the effect of elec- 



15 "Inaug. Diss.," Upsala, 1913, pp. 118 et seq. 



