COLLOIDS. 49 



sion was found to correspond to the above formula for adsorption. 

 Recently FREUNDLICH l has carefully tested the adsorption of crys- 

 talloids by carbon. From these experiments it was shown that the 

 equilibrium could be quickly attained from both sides, i.e., that the 

 process was readily reversible. The above-given formula was found 

 sufficiently accurate for the case where only the total quantity of the 

 dissolved (to adsorb) substance varied. The series in which the organic 

 acids were adsorbed by silk, as found by APPLEYARD and WALKER, 

 were practically the same as with carbon. The influence of tem- 

 perature was slight. 



According to KusTER, 2 the combination between starch and iodine 

 is to be considered as an adsorption compound, and BILTZ 3 finds for the 

 division of As 2 C>3 between iron hydroxide (1) and water (2) the for- 

 mula -0.631. 



C2 



The 'theoretical foundations for the adsorption phenomenon are 

 not especially clear. Generally the adsorption is considered as con- 

 nected with segregation and surface tension phenomenon. At the con- 

 tact surface between a solid body and solution a surface tension exists 

 which is considered as positive, i.e., the same attempts to diminish 

 the contact surface. The surface energy used thereby tends to be a 

 minimum potential energy. As the product from size of surface and 

 surface tension are the same, and as the first cannot change, the sur- 

 face energy can only be diminished by a reduction of the tension. If, 

 therefore, the tension is diminished by increasing the concentration 

 of a substance dissolved in a fluid, then this substance tries to collect 

 itself at the surface in greater concentration than in other parts of the 

 fluid (OsTWALD, 4 FREUNDLICH 5 ). In regard to the surface tension 

 of solid-fluid we only know that it is positive, but can otherwise show 

 great differences (OsTWALD, 6 HULETT ? ). According to this theory 

 the facts are that certain solid substances possess the ability of adsorb- 

 ing . dissolved bodies, and for this reason the adsorbed substance 

 lowers the surface tension of the solid-fluid, and indeed, the more 

 the greater concentration in which it occurs. That especially carbon 

 and colloid substances are adsorption bodies lies in the fact that they 

 have an especially large surface due to their finely divided state or 

 porosity, which therefore, cet. par., must give then a great surface energy. 



1 Ueber die Adsorption in Losungen, Leipzig, 1906. 



2 Ann. d. Chem. u. Pharm., 283, 360, 1894. 



3 Ber. d. d. chem. Gesellsch., 37, 3138, 1904. 



4 Lehrb. d. allg. Chem., 2. Aufl., 2. Bd., 3. Teil, 237, 1906. 



5 Ueber Adsorption in Losungen, 50-51. 



Zeitschr. f. physik. Chem., 34, 495, 1900. 

 7 Ibid., 37, 385, 1901. 



