26 . 



the organic nutrient is, however, the more will potassium show its 

 superiority over rubidium. An observation which appears to throw 

 some light on one physiological difference between potassium and 

 rubidium salts may be mentioned here: The cultures of Cladothrix 

 and of Penicillium formed floating masses in the solution containing 

 potassium salts, ^ while they gradually sank to the bottom in those con- 

 taining rubidium salts. As the swimming is probably caused by the 

 presence of fatty matter, it seems that i)otassium salts are more favor- 

 able for producing fat than are rubidium salts. Further, spore forma- 

 tion is almost entirely prevented in Penicillium when rubidium is 

 offered in place of potassium. Only after an increase of the magne- 

 sium sulphate was a scanty formation of spores noticed. 



The secondary and primary phosphates are the most favorable forms 

 in which to offer potassium salts to fungi. In case phosphoric acid is 

 applied in combination with ammonia, potassium may be added as 

 lactate or tartrate or as other assimilable organic salts. 



Finally it may be mentioned that of all the alkali salts potassium 

 salts exert the most powerful positive chemotaxis upon bacteria, and 

 that next to them come rubidium salts (Pfeffer). 



PHYSIOLOGICAL SUPERIORITY OF POTASSIUM SALTS. 



The question as to what i3eculiar property of potassium its physio- 

 logical capacity must be ascribed, implies also the questions: Why 

 can the physiological functions of potassium salts not be performed by 

 the related sodium salts'? In reviewing all the properties of the two 

 metals and of their compounds can not such chemical differences be 

 discovered as would also explain the great physiological difference? 

 Can there not be found in potassium chemical properties that give it 

 a certain superiority over sodium"? Long ago the writer searched for 

 striking and characteristic differences and believes he is justified in 

 calling attention to the following facts, which i^rove that potassium 

 and its oxid can bring on in certain cases a so-called chemical con- 

 densation which sodium and its oxid can not. For instance, carbonic 

 oxid can be condensed by potassium to triquinoyl, a benzene derivative, 

 but not by sodium (Lerch, Metzki). Phenol added to fusing i)otas- 

 sium hydro xid, will, under condensation, yield diphenol among other 

 things, but with sodium hydroxid, oxidation, but little condensation, 

 is observed, resorcin and phloroglucin resulting (Barth). 



Among other noticeable differences may be mentioned, (1) that cer- 

 tain potassium salts condense ethyl aldehyde to aldol, while sodium 

 salts change it to croton aldehyde (Kopp and Michael) 5 (2) that potas- 



1 The writer prepared the culture solutions with the purest materials, consisting 



in this case of— 



Per cent. 



Sodium acetate 0.5 



Glucose 1.0 



Di-ammonium phosphate 1 



Magnesium sulphate 02 



Potassium tartrate 10 



