PEUMKABILITY 187 



is a function ol' the size (oi* speed) <>i" a particle in solution. Hy 

 a careful selection of membranes a mixed solution may be separated 

 into its constituent solutes. In general, a membrane acts like 

 a filter-paper made infinitely fine — so that ultramicroscopic 

 particles may be retained on the filter. Indeed, the process of 

 separating substances in solution from one another has been 

 termed " ultra-filtration " (p. 84). 



Alterations in Permeability. 



A living membrane, however, alters in its permeability. It may 

 at one time allow a solute to pass through and at another prevent 

 its passage : or at times allow a comparatively large particle to 

 pass through while retaining smaller particles. It may also 

 appear to " select " certain constituents of the surrounding fluid 

 to pass in, seemingly quite irrespective of their size compared 

 with their fellows. 



Consider for the moment the passage of material across the 

 membrane of the erythrocyte. On the one side we have the 

 corpuscular contents, viz. haemoglobin, potassium, phosphorus and 

 small quantities of calcium, glucose, etc. ; and, on the other side, the 

 blood-plasma richer in water, proteins and salts than the corpuscle. 

 Haemoglobin is freely solulile in plasma, but is held in greater 

 concentration than would readily dissolve in the volume of fluid 

 within the corpuscle (Chap. XXII.). The corpuscle also retains 

 certain salts, organic and inorganic, in very different concentra- 

 tions from those in which they occur in the plasma. Now glucose, 

 chlorides and phosphates under certain conditions may pass easily 

 in or out of the corpuscle, while sodium and potassium cannot 

 permeate. Even when the plasma is diluted with water, provided 

 the haemolytic concentration is not reached, the contents of the 

 corpuscle are retained. If various samples of the plasma in which 

 erythrocytes are suspended are diluted with different solutions 

 isotonic with the plasma, e.g. (a) sodimn chloride, (6) glucose, 

 (c) urea, (d) ammonium chloride, it will be seen that the cell is not 

 impermeable to all alike. The urea and the salts of ammonia with 

 sufficient water to keep them in solution will pass into the cell, 

 while the sodium salts will be kept out. 



M. H. Fischer has demonstrated with characteristic clearness 

 that the interface between a water-in-c^ phase and an ir-in-water 

 phase shows dilTerential permeability, where a; is a hydrophilic 

 colloid, or even a hydrophilic substance like phenol, quinoline, or 

 ether. Some coloured substances (such as neutral red, nile blue, 

 methyl violet, methyl green) all leave the iT-in-water phase, and 

 are concentrated in the water-in-a; phase so much that after a few 



