SURFACE TENSION 403 



drop is about 1 cm. from the open end of the tube and this end is sealed 

 in a small bunsen flame. The other end of the tube may then be similarly 

 sealed. The upper diagram, in Fig. 75, shows the appearance (actual size) 

 of a filled and sealed tube. The dark drops are A. The first and last 

 drops, 1 and 9, are large and are not taken into account. The drops are 

 numbered in the order in which they were put into the tube, i.e. the open 

 end of the tube is to the right of the diagram. The tubes are cemented 

 to a microscope slide and a coverslip fixed with Canada balsam. The 

 slide is placed in a Petri dish with enough water to cover the tubes. 

 (Constant temperature.) Under the microscope the tubes present an 

 appearance like that shown in Fig. 75. lower diagram. With an eyepiece 

 micrometer, measure the drops. After an interval whose length depends 

 on the solvent, the drops are remeasured. 



In what direction, if any, does the alteration in size take place ? That 

 is, do the drops of B become larger or smaller ? If B drops increase in 

 size it shows that the vapour pressure of B is less than that of A, and, 

 consequently, the osmotic pressure of B is greater than that of A, and 

 vice versa. A = 2-5 per cent, glucose in water, B = 2-5 per cent. NaCl in 

 water. Time about 20 hours. 



9. Camel-hair Brush Expts. Many illuminating experiments may be 

 made with a small camel-hair paint pencil. Under water the hairs diverge, 

 but when the surface tension of the water-hair surface is increased, 

 e.g. by removing the brush from the water, the hairs form a compact pencil. 



10. Boy s Leather Sucker (p. 303). To show that surface tension is the 

 causative factor suspend a microscope slide horizontally in the receiver of 

 an air pump. By means of a drop of water between, cause a second slide 

 to adhere to the lower surface of the first slide in such a way that the second 

 slide may be loaded. Load with the maximum weight and exhaust the 

 receiver. Repeat the expt. under various conditions, e.g. trace of oil, 

 ester, bile salts, etc. 



11. Work done by altering Surface Tension. In performing the experi- 

 ment mentioned in p. 137 (Fig. 24), it is necessary to have a good soap 

 solution. It may be made as follows from pure sodium oleate and glycerol. 

 To 500 c.c. of distilled water in a stoppered bottle of 1 litre capacity, add 

 200 c.c. of pure glycerol, shake and allow to stand in the dark for a week. 

 Siphon off the clear underlying fluid and add four drops of concentrated 

 ammonia. Keep well stoppered and away from light. 



12. Experiments with Soap Bubbles. Make a film on the wide end of a 

 conical tube (filter funnel), closing the other end with the finger. What 

 happens when the finger is removed ? Where does the film come to rest 

 and why ? 



13. Camphor " Water-beetle. Prepare a rectangular piece of camphor. 

 To one short side affix a short piece of stick and place the whole thing on 

 the surface of water in a large dish. How do you explain the direction of 

 the movements ? Remove the stick and replace the camphor in the water. 



14. Camphor-Benzene " Amoeba." (Brailsford Robertson.) The amoeba 

 is made of a saturated solution of camphor in benzene to which a dye 

 (e.g. carmine) has been added to make the solution easily visible when 

 placed in water. Place a drop of the solution on the surface of clean water 

 in a clean Petri dish. The movements may be slowed down by the addition 

 of the faintest trace of oil. Generally the first '" amoeba " disintegrates 



