368 PROTOPLASM 



endosmotic flow of water through capillaries, the ions are appar- 

 (Mitly the H+ and 0H~ ions of the water. On colloidal metal 

 particles, the ions could conceivably also be those of water, 

 but the evidence is strongly against this possibility; it indicates 

 that the ions are those of a salt of the metal. Pauli has described 

 the situation for solid colloidal suspensions. The ionic environ- 

 ment of colloidal gold particles consists of a compact inner layer of 

 ions of gold chloride, AuCl2~ (or AuCU"), and a diffuse outer 

 layer of H+ ions. 



The thickness of the ionic atmosphere around colloidal particles 

 {i.e., that part of it which determines the charge on the particle) 

 varies from the unimolecular dimension of the compact Helm- 

 holtz double layer (from 1 to 20 A. U., or about 1 |JL^l) to the hun- 

 dred or more molecules in the depth of an ionic cloud. The 

 depth of the latter has been calculated to be 0.96 m/x for a N /1() 

 solution of salt, 9.6 m.^x for a A^/100 solution of salt, and 1,010 ran 

 for pure (conductivity) water. 



The potential at the surface of a colloidal particle — the so-called 

 electrokinetic potential — represents the difference between the 

 electric pressure on the colloidal particle and that in the surround- 

 ing medium. This potential difference is of the order of 40 to 

 50 mv. (0.050 volt). The potential at the surface of colloidal 

 gold has been found to be 45 mv. (under the conditions of the 

 experiment). The value depends upon the thickness of the ionic 

 cloud, which, in turn, is dependent upon the concentration of the 

 surrounding electrolyte. The electrokinetic potential f can be 

 calculated in terms of the rate of movement U . 



_ 47rr?C/ 



As the Helmholtz potential f is the difference in potential 

 between the adhering layer of ions and that of the immediately 

 surrounding ionic atmosphere, it is evident that this potential 

 difference will depend upon the amount of the surrounding 

 medium which is to be considered a part of the immediate 

 environment of the particle. In other words, if we are measuring 

 the drop in potential between two electrodes immersed in a com- 

 mon liquid, we must determine just where one electrode with its 

 potential ends, and the other with its potential begins. 



