LIESGANG PHENOMENON 413 



tube put 1 c.c. of triply distilled water. To the first tube of one row add 

 1 c.c. of N/50HC1. Mix. Remove 1 c.c. of the mixture and add this to 

 the second tube, and so on down the row. This will give a series of acid 

 solutions as follows : N/100, N/200, etc. The other series of tubes is treated 

 in exactly the same way with N/50NaOH. The final 1 c.c. of each series 

 is discarded. Two drops of alizarin red and 1 c.c. of the gold sol are added 

 to each tube and the contents mixed. The tube showing the neutral alizarin 

 colour is picked out and its value determined. The whole gold sol is now 

 neutralised by the addition of the exact amount of NaOH or HC1 as the 

 case may be. 



30. Protective Action of Emulsoids (p. 80). Two equal portions (9 c.c.) 

 of neutral gold sol are treated (1) with 1 c.c. of a 0-1 per cent, gelatine sol 

 and (2) with 1 c.c. of distilled water. To both are added 1 c.c. of N/lNaCl 

 solution. Examine by pure transmitted light, i.e. by looking through the 

 tubes at a uniformly illuminated screen of white paper. 



31. Diffusion of Electrolytes and Colloids into Gels. Two-thirds fill four 

 test tubes with 3 per cent, gelatine and allow to gel. Add 1 per cent, 

 solution of (1) CuS0 4 , (2) picric acid, (3) colloidal iron, (4) Congo red, one 

 to each tube, and put aside for some days. 



32. Electrical Diffusion (p. 79). Fit up a U-tube with an electrode of 

 platinum- or silver-foil rolled cylindrically at the top of each tube. Fill the 

 tube two-thirds full with 3 per cent, gelatine containing a trace of citric acid, 

 and allow to stand overnight to form a gel. Fill one limb with coloured 

 electrolyte (e.g. CuS0 4 ), the other with acidulated water. Determine 

 roughly the rate of diffusion (2 hours). Then pass a current through the 

 tube (lighting supply with a lamp in circuit) and note rate of diffusion 

 (2 hours). Reverse the direction of the current for 2 hours or more and 

 note changes. Try various electrolytes and find which are forced into the 

 gel at the cathode and which at the anode. 



33. Adsorptive Stratification. (Liesgang Phenomenon, p. 79). " 4 grams 

 of gelatine are dispersed in 100 c.c. of water and 2 c.c. sat. potassium 

 bichromate are added to the sol. The mixture is poured on clean glass 

 plates to form a thin layer, about 0-45 c.c. per sq. inch of surface being 

 allowed. The plate is supported on a horizontal surface and the sol allowed 

 to set ; 10-15 minutes will be required. A large drop of 20-30 per cent, 

 silver nitrate is placed in the centre of the plate, preferably by allowing five 

 successive drops of about 0-1 c.c. each to fall on the same spot from a 

 small pipette. 



The drop should have a clean circular outline. The plate is kept in the 

 dark for 24-48 hours. At the end of this period any traces of the original 

 drop may be removed with a pointed strip of filter paper, and the gel is 

 then allowed to dry. (1) Use commercial gelatine. (2) Do not disturb 

 the plate after adding AgN0 3 , till excess has been removed. (3) A trace 

 of citric acid (5-10 drops of 5 per cent, solution to 100 c.c. of sol.) gives 

 wider rings. 



34. Electrophoresis (Fig. 10, p. 77, and letterpress, p. 78). The electrodes 

 (Fig. 10) are two strips of platinum- or silver-foil fastened parallel to one 

 another about 16 mm. apart (Chatterton's compound is an excellent fixative). 

 The slide is placed on a microscope with a paraboloid condenser (or with 

 a small stop) and the lighting, etc., arranged to suit the particular type of 

 condenser used. A large drop of the sol under examination is placed in 



