372 PROTOPLASM 



order of magnitude as that of ordinary electrolytic ions; but this 

 is to be explained on the basis of high valency. 



In the case of proteins, the analogy between colloidal particles 

 and ion is perfect in regard to the sign of the charge, as we shall 

 see in a moment. 



Charge and Potential. — It is necessary to distinguish clearly 

 between charge and potential; the former is often carelessly 

 used to indicate the latter. Potential is expressed in volts; 

 charge, in coulombs, or electrostatic units; potential is pressure; 

 charge is quantity. Bikerman showed that the coagulation of a 

 number of different solutions takes place at the same potential. 

 Were charge responsible, diverse values would result because of 

 the different dielectric constants of the solutions. There is a 

 definite relationship and interdependence between charge and 

 potential, as shown in the following formula for a plane 

 condenser (such as are colloidal particles) : 



47ro-5 



where f is the potential; a, the surface charge per unit area; 6, the 

 distance between the two layers of charges; and D, the dielectric 

 constant. 



The distinction between charge and potential finds application 

 in biology in some calculations by Abramson from data of 

 Northrop and deKruif and Mudd and Joffe for the f potential 

 of bacteria. The results show that the charge on the bacteria 

 does not decrease as potential drops and may even increase 

 (with increase in salt concentration) ; rate of agglutination 

 increases with fall in potential. 



Reversal in Charge. — It was difficult to explain why one 

 metal is positively and another negatively charged; selective 

 adsorption and relative solubility were the suggestions. Now 

 comes the task of explaining why a metallic colloidal particle 

 reverses its charge in a changed environment. Again use is 

 made of adsorption. A negative particle adsorbs positive ions 

 and becomes itself positive. Most substances when colloidally 

 dispersed can be of one sign or the other, depending upon their 

 environment (salt concentration, acidity, etc.). Examples of 

 this are numerous. Burton has reversed the sign of the charge 

 on a number of metals. The following table indicates the effect 



