534 ILLUSTRATIVE EXPERIMENTS 



34. Cataphoresis. 



(a) Mac rosea pre uhscivalion of the titoveinciU of colloids in, (di. electric 

 Jldd. Material, etc., required : 



U-tiibe, 15-25 cm. high x 2-3 cm. inside diameter, fitted with rubber 

 corks pierced by two holes (]). 91, Fig. 19). In one hole in each cork put a 

 short length of glass tubing to act as a vent for gas and water. The other 

 hole carries the electrodes, preferably of platinum or silver, but copper will 

 answer quite well. These electrodes may be flat, but more rapid results will 

 be given if the metal foil is rolled into cylindrical form. (Diameter of cylinder 

 2 mm. less than that of the tube.) 



Eyg white sol (p. 562). Fill the lower third of the U-tube with the neutral 

 solution, and gently drop a small disc of smooth waiting paper on top of each 

 surface. With a pipette fill each limb with distilled water, so that there is a 

 clear demarcation at the water sol interface. Gently insert the corks carrying 

 the electrodes. Allow to stand undisturbed for 15 minutes. Now pass at 

 least 110 volts D.C. across the electrodes for half an hour or so and note the 

 alteration in the levels of the sol in each limb. With a D.C. voltage of about 

 200 the gradient in this tube would be about 5V/cm., so that in 15 minutes the 

 level of the sol should move quite distinctly. Reverse the direction of the 

 current for a similar time. Repeat the experiment with some egg white sol 

 which has been made (a) slightly acid with acetic acid, or (b) faintly alkaline 

 with dilute sodium hydrate. 



A rough demonstration of the movement of colloids in an electric field may 

 be given by fitting up a small cylindrical zinc water bath (or even a cocoa tin) 

 with a central cylindrical roll of copper foil suspended from a glass or wooden 

 cross-piece. Fill the jar with the colloid to be studied and leave overnight. 

 Positively charged colloids will adhere to the copper foil, while those carrying 

 a negative charge will be found in association with the zinc. 



Colloids to try. 1. Egg white (a) neutral to litmus, slightly electronegative, 

 (6) acidified with acetic acid, (c) made just alkaline with sodium hydrate. 



2. 1 per cent, gelatin, (a) isoelectric, (b) electropositive, (c) electronegative. 



3. 0-2 per cent, night blue. 4. 0-2 per cent, alkali blue. 5. Mixture of 100 parts 

 of 2 per cent, alizarin red and 2 c.c. saturated picric acid. 



(6) Microscopic observation of movement of colloids in an electric field 

 (Fig. 20, p. 91, and letterpress, p. 92). The electrodes (Fig. 20) 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 the centre of the 

 space between the electrodes, making contact with them and covered with 

 a I in. slip. After 10 minutes the microscope is focussed on the central 

 layer of liquid (particles not in contact with glass and free to move) -and a 

 current of 4-5 volts (keep amperage low) passed between the electrodes. 

 Determine the sign of the electric charge on dialysed iron sol, gold sol, night 

 blue, alkali blue, etc. Fit an eyepiece micrometer and with a stop-watch 

 determine the velocity of the particles in centimetres per volt per second. 



35. Electric Endosomose. 



(a) The passage of water through a membrane by electrical means may be 

 observed in the preparation of a semipermeable copper ferrocyanide mem- 

 brane when the solutions are forced into the pores of the earthenware pot 

 by an electrical current (Experiment 5). 



(6) A clean porous pot, fitted with a manojneter and a non-polarisable 



