STUDY OF MOLECULAR ADHESION. 417 



evident when the suspension which has been in contact with these 

 various colloidal solutions is repeatedly washed. If this washing 

 is kept up until no detectable amount of the colloid remains, we find 

 that the treated suspension, when placed in distilled water, remains 

 suspended as well as if the colloidal solution were still present. 

 This fact is brought out very clearly on employing gum arabic, 

 agar, gelatin, gum tragacanth, glue or pseudoglobulin. It seems 

 reasonable to believe then, that barium sulphate, when treated with 

 solutions of these substances, absorbs something from them and 

 forms with the colloid a complex which is sufficiently stable to 

 resist numerous washings with distilled water.* Such is not the 

 case with the suspension that has been treated with sugar or glycer- 

 ine syrup. A suspension treated in this way after washing becomes 

 like untreated suspension; it takes nothing from these solutions 

 which hold it in suspension through their viscosity. 



Serum albumin, euglobulin and fibrinoglobulin also form com- 

 plexes with barium sulphate; these complexes, however, withstand 

 washing poorly. 



*** 



It is easily demonstrable that a complex between the insoluble 

 barium sulphate and the colloids is formed : a weak solution of gum 

 arabic treated with a sufficient quantity of barium sulphate and 

 then freed of it by centrifugalization is no longer able to dissem- 

 inate fresh barium sulphate. 



If, on the other hand, a substance such as calcium phosphate, 

 which is easily dissolved in acetic acid, is used instead of barium 

 sulphate, the adsorbed colloid may be liberated from the complex 

 after washing by dissolving the adsorbing substance; the colloid 

 liberated in this manner is demonstrable by the fact that it is able 

 to disseminate fresh barium sulphate. 



When a washed complex, such as barium sulphate plus agar, is 



* Substances other than barium sulphate, for example, animal charcoal, lead 

 iodide, calcium oxalate, mercurous sulphate, manganese carbonate and copper 

 oxide are held in suspension more or less well by gum arabic. Gum arabic, 

 however, does not succeed in dissociating fine particles of kaolin, chromate of 

 silver, sulphate of calcium and sulphate of zinc. It seems reasonable that such 

 should be the case, since we are dealing with various powdered substances, the 

 adsorbing power of which will evidently not be the same for the colloid under 

 consideration. 



