PROTOPLASMIC STRUCTURE 79 



Wohler and Plliddemann^ found that iron oxide adsorbed 

 10 times as much benzoic acid as acetic acid; chromic 

 oxide adsorbed both about equally; while platinum 

 sponge adsorbed more acetic than benzoic, but both 

 shghtly. According to Freundlich, gelatine adsorbs 

 sugar only after having been treated with formaldehyde.'' 

 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 KNO3 (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 crystallizes, as originally shown by Pasteur's 

 observations on the separation of laevo- 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 crystal are determined by 

 conditions of the same kind as those determining 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 apposition 

 of molecules similar in configuration ^and dimensions and 

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



^Wohler and Pliiddemann, Z. physik. Chem., LXII (1908), 664. 



2 Freundlich, op. cit., p. 514. 



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



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

 specificity in adsorption. 



