SURFACE TENSION 515 



enough liquid is admitted into the tube to make a column of about 5 mm. 

 long. The finger is replaced on the end of the tube, which is then lifted 

 from the fluid and inverted so that the open end is uppermost. It is held 

 in a slanting position, and, by diminishing the pressure of the index finger 

 on the lower end, the globule of licpiid is allowed to slide down the tube, 

 its progress being regulated by the slant of the tube. The process is repeated, 

 using solution B and so on, using solution A and B alternately and finishing 

 with A. When all the drops are in, the collection is moved so that the last 

 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. 105, shows the appearance (actual size) 

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

 dro{)s, 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. 105, 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 5 drops increase in 

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

 quently, 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 w^ater. Time 

 about 20 hours. 



6. Turgor. 



(See Expts. 26 and 79 for precautions). Take a length of sausage- 

 skin parchment. Close one end tightly round a glass stopper. Fill with 

 treacle or a strong solution of sugar and then similarly close the other end. 

 Suspend horizontally in water from a loop round the middle. The ends, 

 which droop at first, giving the whole the appearance of an arch, soon begin 

 to assume a horizontal position. In a day or so the sausage skin will be 

 rigid and straight (Fig. 45). 



7-21. Experiments on Surface Tension. 



Soap Solution. In performing these experiments it is necessary to have 

 a good soap solution. It may be made as follows from pure sodium oleate 

 and glycerol. To 600 c.c. of distilled water in a stoppered bottle of 1 litre 

 capacity add 15 grams of pure sodium oleate (in flakes), and by occasional 

 shaking in the cold get it into solution. This may take a day or two. Then 

 add 200 c.c. of pure glycerol, shake and allow to stand, undisturbed in the 

 dark, for a week. Siphon off the clear undeiiyiiig fluid and add 4 drops of 

 concentrated ammonia. Kee]) well stoppered and away from light. 



7. Experiment with Soap Films. 



Make a film on the wade 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 rost and why ? 



8. Camel-hair Brush Experiments. 



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 



33—2 



