STUDY OF MOLECULAR ADHESION. 423 



in colloidal substance, the flocculation of which by salts should 

 diminish in accordance with the amount of colloid present. This 

 fact would seem to us to demonstrate that it is the stable colloid 

 adhering to the powder which opposes in great part the flocking 

 action of the electrolytes. The same fact also probably holds 

 in the protection by a stable colloid against precipitation of an 

 unstable colloid by salts. 



*** 



Apart from the flocculation of complexes by electrolytes, all the 

 facts described to the present are due to the same cause, namely, 

 a substitution of one form of adhesion for another. When a stable 

 colloid transforms the unstable mixture of barium sulphate and 

 water into a much more delicate and stable suspension, it is due to 

 the fact that one adhesion (barium sulphate plus colloid) is sub- 

 stituted for another adhesion (barium sulphate plus barium 

 sulphate). In order for this to occur the attraction of the suspen- 

 sion for the colloid must naturally be stronger than the attraction 

 of the suspension for itself. 



Likewise the fact that a stable colloid such as gum or serum 

 disseminates barium sulphate by separating the mutual adhesion 

 of the particles of barium sulphate resembles the fact that a 

 stable colloid may inhibit the adhesion of barium sulphate to 

 another colloid. Thus, in the presence of serum, starch solution 

 and peritoneal fluid fail to agglutinate barium sulphate; the sus- 

 pension adheres to the colloids of the serum and remains dis- 

 seminated in spite of the presence of the agglutinating colloids. 

 In the same way a stable colloid (gum) prevents the mutual 

 flocculation of fresh barium sulphate with a washed complex of 

 barium sulphate plus gum. In this instance the fresh suspension 

 adheres to the free stable colloid and not to the washed complex. 



II. 



We have stated that barium sulphate may be held in fine suspen- 

 sion by substances other than stable colloids. Citrate of sodium, 

 the inhibiting action of which on coagulation of the blood and milk 

 is well known, will do this. The statements we have made about 

 suspensions of powders by stable colloids render unnecessary any 



