DIFFUSION INTO OKLS 529 



glass, prepared like the slide, is gently placed on the drop. Use a good 

 electric light, such as the Ediswan " Point-o-lite," and focus on the central 

 portion of the fluid. A 1 per cent, suspension of gamboge shows the Brownian 

 movement well. 



2'). Diffusion. 



[a) Colloid into colloid . Two-thirds fill four te.st tubes with sterile 3 per 

 cent, gelatin. Plug tubes with cotton-wool. When the gel has set firmly, 

 place 0-1 per cent, sols of (1) Congo red, (2) Prussian blue, (3) colloidal iron, 

 and (4) black India ink, one to each tube. Keep tubes plugged with cotton- 

 wool and in a cool place. Examine after two days and again after two 

 weeks. 



(h) Crystalloid into colloid. Prepare a number o^ tubes of gelatin as above. 

 When the gelatin has set, pour various coloured solutions over the jelly and 

 examine as above. Try copper sulphate, methylene blue, methyl violet, 

 picric acid. 



(c) Diffusion of acid into fidatin. Mix a 4-5 per cent, gelatin sol with just 

 alkaline phenolphthalein. When set cover with acidified night blue. The 

 acid diffuses rapidly from the dye. Two coloured layers separated by a 

 colourless layer are ])roduced in a very short time. 



{d) Electrical diffusion (p. 86). Fit up a U-tube with an electrorle of 

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

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

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

 electrolyte {e.g. CuSOj), 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. 



30. Adsorptive Stratification. (Liesegang Phenomenon, p. 86.) 



(a) Four grams of gelatin 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. in. 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 suc- 

 cessive 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 gelatin. (2) Do not disturb 

 the plate after adding AgNOg, 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. 



(6) (i.) Fill a test tube to about two-thirds with the bichromate-gelatin 

 mixture mentioned above. When the gelatin has set fill the remaining 

 one-third with (approximately) 10 per cent, silver nitrate. Keep in the 

 dark and reasonably cool. 



(ii.) Prepare a gelatin sol containing 3 gm. of gelatin and 80 c.c. of water. 

 When solution is complete add 20 c.c. of a 50 per cent, solution of MgCL . 6H.,0 

 and mix. Allow to set and then cover with concentrated ammonia. Cork 

 well. Examine after a week. The rings are scarce and well separated. 



(c) Saturate the gelatin in Experiment (6) (ii.) above with a ca[)illary 

 active substance like quinine. Whv are no rings formed ? 



