1911.] 



The Properties of Colloidal Systems. 



243 



the " molecular " may be due to the presence of complex positive ions which 

 are unable to pass the membrane. Thus, the undissociated molecules having 

 only a low solubility, if complexes are formed, an aggregate consisting of an 

 internal phase of particles, which ionise at the surface, e.g. : — 



(Na 2 ©)„ ~ (tfa^-^a®)' +^a' 5 

 might be produced owing to the solubility of the undissociated molecules 

 being exceeded. In this way a typical condenser system is formed : — 



At the membrane-face these particles would, by osmotic forces, be 

 orientated thus : — 



and so contribute to the electromotive force discussed in the following section. 



Although Congo red is aggregated by foreign electrolytes there is some 

 uncertainty as to whether there is any such association of its molecules in 

 pure solutions. I stated in my previous paper on the osmotic pressure of 

 Congo red that the ultra-microscope is unable to resolve such solutions. The 

 path of the beam of light merely appears hazy. This haze may be the 

 expression of aggregates beyond the limits of resolving power of the 

 instrument used or it may be due to the individual molecules, or to anions, 

 which must be of very considerable dimensions. 



The Electromotive Force at the Membrane. 



If, in the dissociation of Congo-red solutions, complex or associated ions 

 are produced, both anion and cation containing the organic acid grouping, it 

 seems to follow that the membrane would be impermeable to both ions, so 

 that no complication would arise as to the part played by the diffusible ion, 

 since there is none. 



But, whatever may be the precise nature of the ions in question, the 

 following facts show that the cation is in point of fact diffusible and kept 

 within the membrane by electrostatic constraint alone. 



Laccjueur and Sackur pointed out* that, if the Xa" ions in the case of 

 * ' Beitr. chem. Physiol, und Pathol. Hofineister,' 1903, vol. 3, p. 203. 



