REACTIONS IN HETEROGENEOUS SYSTEMS 49 



because the charge on the adsorbed substance will either 

 increase or decrease the adsorption, depending on whether 

 the charge on the adsorbing surface is of opposite or of the 

 same sign as the adsorbed material. 



The less the surface tension the easier it is to produce an 

 increase in the extent of surface. For instance solutions with 

 a low surface tension, such as soap solution, can easily be made 

 to froth. Hamburger * finds that substances which decrease 

 the surface tension increase the phagocytic power of leucocytes. 



When the surface tension is reduced to zero the solutions 

 mix and the surface between them becomes irregular as the 

 liquids diffuse into each other. 



Two IMMISCIBLE SOLUTIONS 



In speaking about osmotic pressure we mentioned a semi- 

 permeable membrane, that is one which permits the passage 

 through it of the solvent but not of the solute. As cells are 

 frequently said to be surrounded by a semi-permeable mem- 

 brane we must examine some of the phenomena of membranes. 



Before dealing with membranes, however, we must consider 

 the case in which we have two immiscible liquids with a 

 surface of separation. Water and phenol below 68-4 C. will 

 not mix in all proportions but form two solutions, one of 

 phenol in water and the other of water in phenol. As these 

 are in equilibrium the vapour pressure of water must be the 

 same in each. On adding some substance which dissolves in 

 water and not in phenol the vapour pressure of the watery 

 phase is decreased and water passes out of the phenolic phase 

 until equilibrium is once more established ; therefore the 

 volume of the phenolic phase is decreased. Further equi- 

 molecular solutions will cause equal changes in vapour 

 pressure and therefore equal degrees of shrinking. 



A salt the ions of which differ in solubility in the two phases 

 will be distributed so that the phase in which the positive ion 

 is more soluble will be positively charged and the other phase 

 in which the negative ion is more soluble will be negatively 

 charged, therefore there will be an electrical potential at the 

 surface of separation. 



MEMBRANES 



When two liquids or two portions of the same liquid 

 are demarcated by a solid material we say that the phases are 



* H. J. Hamburger, Physikalisch-Chemische Untersuchungen tiber 

 Phagoxyten, Wiesbaden, 1912, pp. 135-171. 

 4 



