3© ACTIONS OF DRUGS ON SURFACES 



permit loo sodium ions to enter into a directly balanced 

 relation with one calcium ion. But if the site of action 

 of the ions is not in a bulk phase, but at surfaces such 

 as the cell surface and the surface of protein molecules, 

 then the position is radically changed. For example, if 

 one calculates the ratio of sodium : calcium ions at the 

 surfaces of cells in their normal environments, one finds 

 that whereas the composition of the bulk phase surround- 

 ing the cells has a ratio of the order of loo sodiums 

 to I calcium, the ratio at the surface is of the order of 

 I : I (see Table i). 



This then is a plausible theory for the explanation of 

 the balanced action of sodium and calcium in physiolo- 

 gical systems. 



The Oligodynamic Effect of Heavy Metals. Let us sup- 

 pose that the toxicity of a heavy metal is caused by the 

 formation of unionised complexes between a surface such 

 as that of a protein molecule and the metal ion. Then in 

 the surface we have the following reaction: 



Pr- + M+ ^ MPr (2) 



[Pr-j[M+] 



From the conditions we have just stated it follows that 

 equitoxic concentrations of different metals must give 

 the same value for [MPr]. In equation (3) the terms 



