SURFACES 25 



determined by weighing or measuring, but the surface which may 

 vary greatly with the selfsame weight. That in fact not the mass 

 but the surface of the dispersed phase is of importance in adsorp- 

 tion, is evident from the following. 



FREUNDLICH and SCHUCHT* permitted dyes to be adsorbed by 

 amorphous, i.e., ^colloidal, mercuric sulphid flocks. When the HgS 

 became crystallin the dye dissolved again. We have here a counter- 

 part of enzym action in which the adsorbed enzym (e.g., pepsin) is 

 liberated when the adsorbing substrate (fibrin) changes its surface 

 as it is split up. W. MECKLENBURG* succeeded in obtaining different 

 curves in the adsorption of phosphoric acid by colloidal stannic acid 

 and of arsenic by ferric hydroxid when, starting with solutions of 

 identical concentration, he precipitated stannic acid and ferric 

 hydroxid at different temperatures; all the other conditions were 

 identical. The lower the temperature at which stannic acid or ferric 

 hydroxid gel were formed the more phosphoric acid or arsenic was 

 adsorbed. The peculiarity of these curves was the similarity in their 

 shape which is described by mathematicians as "affinitive"; on this 

 account MECKLENBURG called them affinitive adsorption curves. 

 They can not be otherwise explained than that the same mass of 

 adsorbent may have a different surface depending on the tempera- 

 ture at which it was formed. 



Adsorption is a phenomenon which is conditioned by the decrease 

 of the surface tension of the solvent in respect to the dissolved sub- 

 stance, at the interface between the solvent and adsorbent. In 

 1888, G. QUINCKE showed that substances which decrease the surface 

 tension between the solvent and the dispersed phase must collect 

 about the dispersed phase with the formation of a film. H. FREUND- 

 LICH has elaborated and experimentally established the theory of 

 adsorption phenomena, basing his ideas on GIBBS' Theorem. 1 The 

 marked diminution of the surface tension of water by fats, fatty 

 acid, soaps, albumin and its cleavage products, and enzymes is 

 characteristic; and it is not surprising that these substances are 

 very easily adsorbed. 



From what has been already said, adsorption appears to be a 

 purely physical phenomenon in which the chemical relations between 

 adsorbent and adsorbed substance play no part whatever. [This 



1 GIBBS' Theorem states: A dissolved substance is positively adsorbed if it 

 depresses the surface tension, negatively adsorbed when it raises it. WILLARD 

 GIBBS deduced these relations for gaseous mixtures and not for fluid solutions. 

 The statement of W. GIBBS, that a small amount of a dissolved substance may 

 powerfully depress surface tension but cannot raise it much, is likewise important 

 (for further proof see H. FREUNDIICH'S " Kapillarchemie"). 



