518 ILLUSTRATIVE EXPERIMENTS 



TvpirAL Resi-lts at 17° C. 



Liquid to be tested ..... Water. Soap Boln. 



Number of drops . . . . . .60 112 



Total volume of dro})s ..... 3 c.c. 3 c.c. 



Time taken for fluid to fall from upper to lower 



mark ....... 43 sees. 50 sees. 



Surface tension relative to water . . . — — = 0-5 approx. 



(ii.) Capillary-Tube Method. A fluid wliicli wets the tube will rise in it 



2ct 

 to a height /(, which may be calculated from the formula h =^^ where a = 



surface tension, r = radius of the capillary, and D = specific gravity of the 

 liquid. For approximate determinations, the height h may be measured by 

 calipers against a millimetre scale. Short lengths of thermometer tubing 

 may be used, having an internal diameter of from 0-3 to 2 mm. The tubes 

 should be prepared and cleaned as directed in Experiment 17 (i) above. 



(a) Select a tube, thoroughly rinse it in distilled water and clamp it verti- 

 cally, dipping into distilled water. Be certain that the meniscus moves freely 

 in the tube, and that no drops of liquid adhere to the internal walls above the 

 meniscus. If this is not so, the tube is dirty and a fresh clean one should be 

 selected. 



{b) Measure the height to w^hich distilled water (at room temperature) rises 

 in the tube. Repeat, using the soap solution above. See also Experiment 48. 

 18. Adsorption to a Surface. 



(1) Adsorfition by Charcoal. («) Put 10 c.c. of a 0-1 per cent, solution of 

 crystal violet -\- 1 gm. of bone charcoal into a tube. Shake. Filter. The 

 filtrate is colourless. Return the precipitate to the cleaned tube. Add 

 10 c.c. of distilled water. Shake. Practically no colour leaves the charcoal 

 for the water. Now add a few cubic centimetres of methylated spirit and 

 shake. The dye leaves the charcoal. 



(6) Repeat the experiment, using N'lOOHCl in place of the dye. Te.st the 

 filtrate with inetJiyl red. The hydrochloric acid has been adsorbed completely. 



(2) Hay's Test for Bile Salts. The surface tension of water is sufficient to 

 permit of placing "' flowers of sulphur "' on it without rupturing the surface. 



Take three test tubes and place in (i.) about 10 c.c. of w^ater : in (ii.) about 

 5 c.c. of water -)- 5 c.c. of methylated spirit well mixed together ; and in 

 (iii.) 10 c.c. of water containing bile salts (or ox bile). On to the surface of 

 each tube, sprinkle a small pinch of powdered sulphur. In tubes (ii.) and (iii.) 

 the sulphur will rupture the surface and sink to the bottom of the tube. 



(3) (a) Flotation. Prepare a finely ground mixture of 9 parts of animal 

 charcoal and 1 part of aluminium silicate (clay). Shake 3 or 4 gms. of this 

 mixture with about 100 c.c. of water, so as to get a suspension; Allow" to 

 stand and decant off the supernatant liquid. Repeat this process so as to 

 get rid of the coarser particles of the clay. 



Now shake the greyish black siispension with any capillary active sub- 

 stance iq.v.), and pour the mixture into a clean boiling-tube. The mixture 

 will separate into two sharply defined layers. The charcoal will adhere to 

 the glass-, air- and water-interfaces of the upper liquid {e.y. benzol), while the 

 lower aqueous layer will contain practically all the clay. 



(b) Flotation. (Ostwald.) Cut a few pieces from a sized paper heavily 

 printed on one side, and the same number of similar size from a similar un- 



