ii6 PROTOPLASMIC ACTION AND NERVOUS ACTION 



homologous series. The contrast between NH4OH 

 (lipoid-soluble) and KOH (lipoid-insoluble) in their 

 ability to enter living cells is a striking one;^ and most 

 investigators who have studied the penetration of sub- 

 stances (like acids and bases) whose behavior can be 

 determined with accuracy, have found a general parallel- 

 ism between the rate of penetration and the lipoid- 

 solubility of the various compounds. This parallelism, 

 however, is not exact, indicating the presence of other 

 factors.^ 



Impermeability to neutral salts in balanced solution 

 is an especially important property of the plasma mem- 

 brane, since it renders possible permanent differences 

 of salt-content between protoplasm and surroundings. 

 This means differences of ionic content, and is probably 

 a condition of the normal electrical polarization ("physio- 

 logical polarization") of the membrane, since any 

 partition separating two solutions of different ion-content 

 is typically the seat of an electrical potential difference. 

 The results of the mineral analysis of cells, as well as 

 those of the plasmolytic and partition methods just 

 described, show clearly the inability of neutral salts to 



^ Cf. Warburg, loc. cit.; Harvey, loc. cit.; Gray, Proceedings of the 

 Royal Society, B, XCIII (1922), 104 (cf. p. no); Jacobs, Jour. Gen. 

 Physiol., V (1922), 181. A similar contrast exists between organic 

 acids (lipoid-soluble) and strong acids; cf. Loeb, Biochem. Zeitschrift, 

 XV (1909), 254; Bethe, loc. cit.; Gray, loc. cit. Jacobs' results especially 

 show the extraordinary ease with which carbon dioxide and ammonia 

 penetrate living protoplasm. 



2 Harvey, Intern. Z. physik. chem. Biol., I (1914), 463; Crozier, 

 loc. cit. Miss CoUett's studies on the toxicity of acids to infusoria show 

 a similar general relationship between organic solubility and toxicity 

 {Journal Experimental Zoology, XXIX [1919], 443, and XXXIV [1921], 

 67, 75)- 



