10 COLLOIDS IN BIOLOGY AND MEDICINE 



and even crystalline particles, e.g., in soap jellies, unite to form 

 spongelike structures. 



J. M. VAN BEMMELEN compares the process of separation of 

 colloids with the condition , deduced from the phase rule govern- 

 ing the separation of two fluids not miscible in all proportions, e.g., 

 water and phenol. In jellies also, we have a phase containing much 

 colloid and little water, and another phase containing little colloid 

 and much water. This conception of the structures of jellies and the 

 process of separation is due to J. M. VAN BEMMELEN, 0. BUTSCHLI, 

 W. B. HARDY, G. QUINCKE, R. ZSIGMONDY and W. BACHMANN. 

 The views of O. BUTSCHLI are not accepted nowadays. He main- 

 tained that jellies are, broadly speaking, foamy structures having 

 microscopic cavities with firm net-like walls filled with fluid. Such 

 a structure can occur only exceptionally. 



The conception of jellies as spongy structures gives us a satis- 

 factory explanation of their properties. It explains their solidity 

 and their plasticity, their elasticity, and in short their various physi- 

 cal properties. 



The above assumption finds corroboration in another observation, 

 through the fact that jellies also act as ultrafilters and consequently 

 must be penetrated by fine capillaries whose diameter has been 

 determined by BECHHOLD (see p. 99). The penetration of jellies 

 by fine capillaries filled with fluid was demonstrated by another 

 observation. H. BECHHOLD and J. ZIEGLER* allowed salts which 

 would form precipitates with various properties to diffuse towards 

 each other in gelatin, e.g., potassium ferrocyanid and copper sulphate 

 which form a copper ferrocyanid membrane entirely impervious to 

 electrolytes; silver nitrate and sodium chlorid which form a silver 

 chlorid membrane which is permeable for electrolytes if the osmotic 

 pressure is higher on one side than on the other. Microscopic sections 

 through the membranes formed by the precipitates prove that the 

 gelatin is not deformed. Accordingly, when diffusion ceases, it is 

 because the diffusion paths have been obstructed, i.e., a precipitate 

 has been formed in the fluid phase so that the paths are closed and 

 of course the gelatin walls which contain little water are impassable 

 for electrolytes. Remelting is sufficient to reopen the diffusion 

 paths. ANDERSON determined a diameter of 5.2/z/z for the largest 

 pores of silicic acid jelly from the vapor pressure reduction which a 

 fluid undergoes in cylindrical capillaries. 



Until now we have assumed that sols exist only as aqueous solu- 

 tions and that there are only aqueous gels. This is by no means 

 true. Even THOMAS GRAHAM * showed that water could be replaced 

 by alcohol and glycerin. TH. SVEDBERG pulverized numerous metals 



