12 LABORATORY MANUAL OF GENERAL PHYSIOLOGY 



Lloyd, p. 20;. Lawrence, "Soap Films," 1929). Sketch shapes of 

 bubbles in each case. Text p. 50. 



7. Retreating Film. — Make a film of soap solution over the wide 

 end of a funnel; close the end of the stem with finger; remove finger. 

 Result? Explain. Why does the film move downward? 



8. Limnological Example of Surface Tension. — Place a living w^ater- 

 strider (Gerris) on a water surface. Add a drop of oil and note change of 

 posture (side view). This experiment was suggested by Prof. G. Evelyn 

 Hutchinson. For the ecology of the surface film cf. Needham and Lloyd, 

 "Life of Inland Waters," p. 327, 1916. 



9. Rapid Changes in Surface Tension. — Drop a few grains of camphor 

 on w^ater. Explain movement (cf. Scarth and Lloyd, p. 201). 



10. Internal Pressure in Liquid Drops. — Water is drawn up into a 

 vertical capillary tube, then allowed to drop until equilibrium. Raise a 

 dish of water until it touches the pendant drop. The distance through 

 which the meniscus falls measures the internal pressure due to surface 

 tension of suspended drop (cf. Sentis, Journ, de Phys., 3e. Ser. 6: 183, 

 1897). 



11. Crossing the Red Sea. — Draw a brush dipped in alcohol across a 

 layer of aqueous eosin on a glass plate. Explain the tangential movement 

 of film (cf. Exodus, 14: 21). 



12. Adhesion and Cohesion. — The molecular forces causing surface 

 tension are concerned with cohesion, adhesion and solution. Dip glass 

 into w^ater, and into Hg. Is the glass wetted? Is the fluid in each case 

 lifted against gravity? Test with a capillary tube. Place two glass 

 slides together with a film of water between and try to separate. Explain. 

 Experiment with the adhesion disc provided. 



13. "Negative" Surface Tension. — Place a speck of lecithin on a 

 microscope slide. Cover with a drop of N/100 HCl and a cover glass 

 supported on glass splinters. Under microscope observe films of maximum 

 area (at edge of lecithin). If possible observe in polarized light. In rare 

 cases packing in film may give special molecules their position of greatest 

 equilibrium (cf. Virchow's Archiv., 6: 571, 1854; Scarth and Lloyd, 

 p. 26; Bums, "Biophysics," p. 109; Leathers, Lancet, 1925, 803; Needham, 

 "Order and Life," p. 162, 1936). Text p. 167. 



14. Surface Tension and Electrical Charge. — Dip wires from two dry 

 cells (with rocking key in circuit) into opposite sides of a Petri dish con- 

 taining a Hg drop between the electrodes. Close and open the key. Try 

 reversing the current. Note movement of Hg relative to the poles. 

 Explain (Christiansen, Ann. d. Physik., 4th ser., 12: 1072, 1903). Text 

 p. 65. 



15. Dead Space. — Mix approximately equal parts of molar sodium 

 carbonate solution and half molar choral hydrate (about 3 cc. of each) 

 in a test tube. Let the tube stand undisturbed. Within ten minutes 



