STUDY OF MOLECULAR ADHESION. 425 



A complex is formed between the suspension and the citrate 

 similar to that which occurs when barium sulphate is added to a 

 colloidal solution (Section I). The dissemination of barium sul- 

 phate by sodium citrate corresponds with Spires' * idea. This 

 author thinks that the presence of electrolytes in small amount 

 is necessary for the conservation of colloidal solutions. The 

 salts capable of producing this effect would vary with the colloid 

 under consideration ; small amounts of alkali will sustain silicic 

 acid and stannic acid in a colloidal condition; small amounts of 

 acids act in the same manner on colloid ferric hydrate and on 

 gelatin. Malfitano t has shown, also, that colloidal ferric hydrate 

 filtered through collodium becomes more and more unstable as it 

 loses its electrolyte (HC1) ; this writer believes that colloidal ferric 

 hydrate formed by hydrolysis of a ferric salt is unable to remain 

 alone in colloidal condition, and acquires such a condition only 

 through adsorption of the ions, Fe or H. 



My own data may be compared with the facts that were mentioned 

 by Arthus a few years ago.$ This writer found that if sodium 

 citrate (0.5 to 1 per cent) is added to an emulsion of clay in water, 

 the precipitating dose of the salts of alkalis or of alkali earths is 

 considerably increased (20 to 28 times). "Although in the instance 

 of calcium salts we may advance the hypothesis of a double inter- 

 change of salts leading to the production of calcium citrate and 

 sodium chloride, no such hypothesis could be offered in the case of 

 NaCl. In this instance sodium citrate evidences an antagonism 

 to sodium chloride, the precipitating salt; the citrate, then, may be 

 regarded as endowed with a direct antiprecipitating property." 

 This phenomenon is interesting when we consider the inhibiting 

 action of the citrate on the coagulation of blood and milk, which is 

 not accompanied by precipitation of calcium. Our researches 

 show that the antiflocculating property of sodium citrate, observed 

 by Arthus, is not clue to some unknown obstacle to flocculation by 

 sodium salts in general which this substance offers; we have indeed 

 met with certain examples of flocculation by sodium chloride which 

 are not inhibited by the citrate (in particular, flocculation of the 



* Hoffmeister'sBeitriigezurchem. Phys. u. Pathologie, Vol. 5. 

 f C. R. Acad, de Sciences de Paris, 1905. 

 t C. R. de Soc. de Biol., 1902. 



