DIALYSIS 87 



experiment, also due to Liesegang, is to fill a glass tube with a 

 4-5 per cent, gelatin sol containing about 10 per cent, sodium 

 chloride. When the gel has set, the tube is immersed in a silver- 

 nitrate solution which will diffuse steadily into the gel from both 

 ends, leaving continuous bands of silver chloride. These bands 

 approach one another, but they do not meet. A clear space is left 

 in the middle of the tube. 



The explanation of the phenomenon seems to be that as the silver 

 ions (of the original experiment) diffuse into the gel they meet the 

 bichromate ions, and some of the silver forms silver bichromate and 

 is precipitated. Now, as the diffusing ions move into the gel in 

 straight lines (Rcigel and Widjoff), and as conditions are similar 

 for the whole first line of advancing ions, they will undergo 

 precipitation practically sinudtaneously and form a layer of silver 

 bichromate in one plane. By the formation of the layer a certain 

 amount of gelatin has lost its bichromate. Diffusion of bichromate 

 ions will, therefore, occur to fill this gap. At the same time, silv^er 

 ions are advancing outwards and being precipitated. These 

 newly formed precipitates are at first attracted to the first ring and 

 adsorbed, so increasing the thickness of the ring. On account of 

 the greater concentration of the invading ions, they advance more 

 rapidly than the diffused solute and so pass beyond the first ring, 

 and after crossing a space with too low a concentration of opposing 

 ions to cause combination to take place, again form a ring of 

 precipitate, and so on. By the time that the silver has reached 

 the periphery of the plate, its concentration and, therefore, its 

 diffusion rate have become very low. The rings will, therefore, 

 have a longer time, i.e. greater opportunity to adsorb the solute, 

 and so they will be heavier and more widely separated. 



Dialysis. — If a tube open at both ends and filled with a gel is 

 placed so that one end is immersed in water and the other end in a 

 solution, it will be found after a time that a considerable quantity 

 of the solute w'ill have passed through the intervening colloid and 

 be distributed in the water. If sufficient time were given, the 

 concentration of the solute in the water at both ends of the tube 

 would become equal. On the other hand, a colloid sol placed in 

 the solution would not have diffused appreciably into the gel. 

 This gives us a method of separating colloids from salts. The 

 thickness of the intervening gel is reduced to that of a thick film, 

 and instead of using for this purpose a gel soluble in water like 

 gelatin, a w^ater-holding substance like collodion is employed. 

 Instructions are given in Part II. for the preparation of various 

 types of dialysers, i.e. pieces of apparatus, principally consisting of 

 a film of collodion or a piece of parchment which can be used to 



