8(j DISPERSE SYSTEMS 



and which causes a sokite to exert osmotic pressure, viz., the 

 kinetic energy of the particles (ions, atoms, molecules, or larger 

 aggregates). 



Electrical Diffusion. — The rate at which electrolytes diffuse 

 into gels may be increased by the passage of an electric 

 current. This method is sometimes employed in the adminis- 

 tration of drugs, — so called ionic medica- 

 tion. " Metal "-ions (cations) are carried 

 i into the tissues from the positive electrode 



I of any current-supply device, while " acid "- 



ions (anions) are driven in from the nega- 

 ' five electrode (see Chap. XI. and Part II., 



p. 529). 



LiESEGANG Phenomenon. — If a gel con- 

 tains a substance in solution and a second 

 substance capable of reacting with the 

 first is allowed to diffuse into the gel, 

 the product of the reaction is deposited 

 in strata separated by clear intervals 

 (Part II.). These banded precipitates were 

 first prepared by Liesegang in a slightly 

 different form. A quantity of 4 per cent, 

 gelatin sol to which had been added 2 c.c. 

 of a concentrated solution of potassium 

 bichromate was poured on a clean glass 

 plate and allowed to set to a gel in a very 

 thin film. When firm, a large drop of 25 

 per cent, silver nitrate was placed in the 

 centre of the film, the plate being kept 

 horizontal. After remaining undisturbed 

 (in the dark) for two days or so, concentric 

 rings of silver bichromate were found round 

 the original drop, separated by clear zones 

 free from the precipitate, the distances 



Fig. 17. — Adsorptive stratifl- i , , i • • i • ±. 



cation of silver bichromate in betwecii the succcssivc Tiugs Dcmg greater 



an agar gel. (Bradford, Bio- ,, i?j_i i? j_i j_ j_i 



chemicaiJourmi.) the further from the centre they are 



formed. Since the publication of the 

 details of the original experiment many different gels and 

 mutually precipitating salts have been tried. For example, 

 water-glass may be used as medium, or with certain precautions 

 agar-agar, or even a test tube full of a fine powder (flowers 

 of sulphur) or packed with vertically placed capillary tubes. 

 The illustration (Fig. 17) shows beautiful rings of silver-bichro- 

 mate in an agar gel. A very instructive modification of the 



