PROTOPLAS.MIC STRUCTURE 79 



Wohler and Plucldcmann' found that iron oxide adsorljcd 

 10 times as much benzoic acid as acetic acid; chromic 

 oxide adsorbed both about equally; while i)hitinum 

 sponge adsorbed more acetic than benzoic, but both 

 slightly. According to Freundlich, gelatine adsorbs 

 sugar only after having been treated with fomialdehyde.* 

 The influence of the specific molecular structure of the 

 adsorbent on its selective adsorption is well shown in 

 the investigations of Marc. A crystalline adsorbent, 

 BaC03 (rhombic) adsorbs KXO3 (rhombic) but not, 

 or slightly, NaN03 (hexagonal); CaC03 (hexagonal) 

 adsorbs NaN03 but not KN03.^ These observations 

 throw an interesting light on the phenomena of crystal- 

 lization; it is well known that the specific molecular 

 configuration of a substance determines the form in 

 which it cpy'stallizes, as originally shown by Pasteur's 

 observations on the separation of kevo- and dextro- 

 tartrate in separate crystals in the crystallization of the 

 optically inactive solution. Apparently the abstraction 

 of molecules from solution and their deposition to form 

 the regular solid structure or cr}'stal are determined by 

 conditions of the same kind as those detemiining selec- 

 tive adsorption. Adsorption of molecules at the surface 

 of the crystal is a preliminary to the growth of the latter; 

 this growth is evidently dependent on mutual apj)osition 

 of molecules similar in configuration and dimensions and 

 with their axes parallel.*' In organic growth— another 



MVdhler and Pliiddcmann, Z, physik. Chcni., LXII (1908), 664. 

 ' Freundlich, op. cU., p. 514. 



3 Marc, Z. physik. Clicm., LXXXI (1913), 641. 



4 Crystal growth, in fact, appears to afToni the clearest cases of 

 specificity in adsorption. 



