IMBIBITION AND ADSORPTION 411 



(c) To show that imbibed fluid is held under compression. Tie a short 

 piece of surgical laminaria tanga to the stem of a hydrometer (near the 

 foot). Float in water and note the level. After some hours, again read 

 the hydrometer scale. If the water imbibed is not under compression both 

 readings should be the same. 



((/) Effect of various electrolytes on imbibition. First of all, prepare a 

 number of standardised gelatine discs as follows. Make a concentrated 

 solution of gelatine, adding a trace of a colloidal dye (e.g. Congo red) to 

 render the gelatine easily visible. Pour the hot solution upon a glass plate 

 and allow to set. With a large cork-borer (diam. 10-15 cm.), cut into discs 

 which are dried, measured and weighed. Seven Petri dishes are required 

 and are almost filled with the following fluids : (i) water, (ii) N/10HC1 or 

 H 2 S0 4 , (iii) N/lONaOH, (iv) N/2KI, (v) N/2NH 4 CNS, (vi) N/5CaCl 2 , 

 (vii) Sat. MgS0 4 . 



Put a few gelatine discs (not touching one another) into each dish, 

 immersing them quickly and completely to avoid the adherence of air 

 bubbles. After an hour's immersion some of the discs will have visibly 

 swelled. Leave for 24 hours and examine against a black background. 

 Measure and weigh. Which disc has swelled most ? Arrange the electro- 

 lytes in a descending order as they have favoured imbibition. This is the 

 so-called lytropic series. Instead of gelatine, 1 per cent, agar may be used. 

 The lytropic series will be in the same order. 



(e) Effect of Acid on Imbibition. Stretch a piece of catgut vertically 

 between a weight and a weighted frog-heart lever so that weight and catgut 

 lie in a tall cylinder. The lever may be made to mark a smoked rotating 

 drum. Set the drum going very slowly and almost fill the cylinder with 

 water. Note the changes. Now add sufficient hydrochloric acid to make 

 the whole fluid 1-2 per cent, acid and note the result. 



26. Shell Formation (Rhumbler). (1) Mix a little powdered glass with 

 chloroform and set a drop of the mixture in water. The glass particles 

 gather rapidly round the surface of the drop. 



(2) Repeat the experiment, using fine silver sand dispersed through oil 

 and finally dropped into 70 per cent, alcohol. The movements take place 

 more slowly and the drop requires about three hours to attain equilibrium. 



27. Adsorption. (1) Adsorption of Colloids to a Surface. Into a number 

 of flasks place a spoonful or two (2-3 gm.) of bone charcoal and pour into 

 each, one of the following fluids. Shake vigorously and then pour into a 

 large filter paper in a funnel. 



Liquids to try : Berlin blue, congo red, acid fuchsin (2-3 mg. per 100 c.c.). 



To show that lowering S.T. sets free the colloid, add methylated spirit 

 to funnel mixture and collect filtrate. 



(2) Adsorption of Colloid to Colloid. Capillary Analysis. Cut a number 

 of strips 2 x 15 cm. from a good filter paper. (Do not take the slip from 

 too near the edge of the sheet.) Hang two or three of these strips so that 

 each one dips its edge into a narrow-necked vessel (Erlenmeyer flask) con- 

 taining a fluid to be tested, taking care that the papers are immersed to the 

 same and to a sufficient depth (about 2 cm.), and that glass and paper do 

 not come in contact. Filter paper becomes negatively charged in contact 

 with water, and therefore positively charged colloids will become ' l fixed ' 

 electrostatically at the liquid-paper interface, while negatively charged 

 colloids will ascend with their dispersion media, (i) Flask i. Water. 



