COLLOIDS 409 



(3) Dialyse some blood serum. What is the precipitate ? How do 

 you explain this ? 



21. Faraday-Tyndall Phenomenon (p. 70). Arrangement of apparatus. 

 The fluid to be examined is placed in a prismatic cell (small flat-sided 

 specimen jar, say, 5" x 5" x 2J"). One large face of the cell is covered with 

 a black velvet curtain. Light from any optical projector is passed through 

 the cell in a plane parallel to the long side. A lens is interposed so that the 

 focus falls about the middle of the cell forming a cone. A darkened room 

 is essential for any but individual demonstrations. 



(1) Fill the cell with water and show that the beam is hardly shown. 

 (With conductivity water the beam is not seen.) 



(2) Dust lycopodium or puff smoke into the beam outside the cell to 

 show how small particles in air affect the visibility of the light. 



(3) Add about 1 c.c. of an egg albumin solution to the water in the cell. 

 Why is the cone bluish in tinge ? 



(4) Eeplace the solution with another containing a red gold sol. Why 

 is the cone green ? 



22. Brownian Movement (p. 75 and Fig. 8). (1) Clean slides and cover 

 glasses in hot potassium bichromate-sulphuric acid mixture, rinse in 

 distilled water and then in two changes of alcohol. Keep till required 

 in alcohol. 



(2) When ready to examine a sol, withdraw a slide from alcohol (using 

 forceps) and evaporate dry over a clean flame. Cool. A large drop of 

 sol free from air bubbles is placed on the centre of the slide. The cover 

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

 electric light, such as the Ediswan " Point o' light " tungsten arc, and focus 

 on the central portion of the fluid. A 1 per cent, suspension of gamboge 

 shows the Brownian movement well. 



23. Gelation. A. Heat a small quantity of 1 per cent, solution of 

 (1) gelatine, (2) serum, (3) dextrine. Cool. What is the result ? Has 

 reheating any effect ? 



B. Effects of Solutes on Gelation. Into four boiling tubes put the same 

 quantity of 2| per cent, gelatine. In one tube dissolve about 5-7 per cent, 

 magnesium sulphate crystals. Potassium iodide crystals are added to the 

 second tube, while a few drops of 40 per cent, formalin are mixed with the 

 gelatine in the third tube. The fourth tube is left as a control. Allow all 

 tubes to stand overnight and examine by tilting and shaking. How do 

 you explain the varied viscosity ? 



Sulphates, citrates and phosphates increase the viscosity of aqueous 

 emulsoid gels. Iodides, bromides cyanides and some chlorides similarly 

 decrease viscosity. Alcohol, formaldehyde, etc., in small amounts increase 

 viscosity. 



24. Determination of the Relative Viscosity of a Liquid. Principle. 

 When a liquid flows through a narrow tube the velocity of flow depends 

 mainly (a) on the force producing the flow and (ft) on the resistance to flow 

 produced by the viscosity or internal friction of the liquid. In Chap. 

 XXIV. p. 278 we considered the shearing of the different layers of the 

 blood stream. The liquid, we saw, could be regarded as made up of a 

 number of concentric tubes sliding past one another. When the liquid 

 is moving through the narrow tube there will be, under constant conditions, 

 a constant difference in velocity between the different tubular layers. The 



