STUDY OF MOLECULAR ADHESION. 421 



Some writers have concluded from Quincke's experiments that 

 any substance which diminishes the surface tension of substance 

 A for fluid B must be absorbed by A, and, on the other hand, that 

 any substance that has been adsorbed must diminish the surface 

 tension of the adsorbing substance. It is to be remembered, 

 however, that in certain instances the mutual adhesion of two 

 substances in fine suspension in a fluid results, not in a dis- 

 semination, but in an increase of surface tension. Thus the 

 mutual flocculation of two unstable colloids of different electric 

 charges indicates an increase of superficial tension resulting from 

 the adhesion of particles of the two colloids, which fact Biltz* 

 correctly compares with adsorption phenomena. 



* * 



We have endeavored to determine the reaction of our complexes 

 of barium sulphate plus colloid to electrolytes. Such complexes 

 as we have already mentioned may be removed from the colloidal 

 solution in which they have formed, and resuspended in distilled 

 water. We have stated that complexes washed in this manner 

 react to electrolytes and flock out as unstable colloids do when 

 these electrolytes are present. Alkalis, and particularly acids, 

 have the same effect. It is evident that in this case we are dealing 

 with an electrolytic effect and not with a phenomenon of plas- 

 molysis due to concentration of the salt, inasmuch as such washed 

 complexes do not flocculate when the salt is replaced by isotonic 

 sugar solution; such a sugar solution may be employed in any 

 concentration without flocculating the complex. 



This flocculation is reversible, inasmuch as when the salt solution 

 which flocculates is removed and replaced by distilled water, the 

 complex recovers its original appearance and is again dissemi- 

 nated. Although a reversibility occurs in the agglutination of 

 certain colloids and certain fine suspensions by means of salts of 

 alkalis and of alkali earth, the fact should be mentioned in the 

 present instance. We might suppose, for example, that in pre- 

 cipitating a complex of barium sulphate plus gum or the like, the 

 electrolyte simply detaches the fixed colloids from the suspension, 

 in other words, breaks up the complex and restores barium sul- 



* Berichte der deutsche chem. Gesellschaft, 1904, XXXVII. 



