D. INTERNAL PRESSURE DUE TO CURVED FILMS. 



144. Blow a soap bubble on a glass tube; why is it spherical? 

 note decrease in size when left to itself, clue to expression of air 

 (let it blow against a flame). Why must a curved film press in- 

 ward? Does this internally directed pressure vary with the degree 

 of curvature? How? Determine this by blowing and balancing 

 two unequally sized bubbles on opposite ends of a Y-tube. Which 

 presses air into the other? Explain. Internally directed pressure 

 in inversely proportional to the radius of the spherical drop or 

 bubble. 



145. Try deforming with a glass rod olive oil drops of different 

 sizes in the alcohol. (Experiment 140.) Which show most resis- 

 tance to deformation, the large or the small ones? \\hich most 

 quickly regain their shapes ? Explain. 



146. Is the cylinder a stable form when the principle of 

 minimal surface is considered? The following experiment shows 

 how stability is attained when the form of a cylinder is imposed 

 upon a liquid. Make a cylinder of 70^ alcohol in paraffin oil, in 

 the following way : Have a layer of alcohol in the bottom of the 

 tumbler under the oil. Take a glass tube about l /j to !< inch 

 in diameter, and holding the finger over one end, put the other end 

 down to the bottom of the vessel, then remove the finger and allow 

 the alcohol to rise in the tube. Now rather gently lift the tube 

 straight out of the oil, when a cylinder of alcohol will be left in the 

 oil. Observe how it instantly breaks up into spheres. ( Why 

 does fluid from a spout usually break up into drops ? ) 



E. CHANGES OF SURFACE TENSION IN FLUIDS. 



147. Different fluids have different surface tensions. Try drop- 

 ping slowly water and toluol from a pipette. Which has the 

 greater surface tension? How indicated in this experiment? 



148. The surface tension of a fluid may be altered by the 

 presence of another substance. Float a clean thin rubber band on 

 the surface of a dish of clean water. Now touch the end cf a glass 

 rod to the surface within the band, the rod having been previously 

 dipped in oil. Result? Explanation? Now touch similarly the 

 water outside the band. Explain the result. Test the influence of 

 alcohol and chloroform on the surface tension of water. 



149. Why does a drop of alcohol break a soap bubble? Drop 

 a drop of alcohol on a thin layer of water on a clean glass plate. 

 Result? Explain. 



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