ORIENTATION AT AN INTERFACE 



47 



one another and at right angles to the surface. The surface layer 

 or layers, because of their orderly arrangement, will, therefore, 

 have a larger niuuber of niolecvdes per unit area than the interior 

 of the liquid with its higgledy-piggledy arrangement of molecules 

 (Fig. 9). That is, the surface will tend to decrease under the 

 micompensated Newtonian attraction of under surface molecule 

 for surface molecule, and the tension so produced will cause the 

 orientation of the molecules at the surface, the effect on any 

 individual molecule being determined by the extent to which the 

 surfaces of that molecule (on which equal forces are acting) 



I'lG. 9. — To show how t)ie surface of water differs from the interior in tlie orientation of the 

 molecules. A similar orientation occurs at the fjlass-water interfaces. 



deviate in shape from spheres- drawn about the centre of mass. 

 Various experiments ha\'e been devised to determine to Avhat 

 extent this orientation is transmitted to molecules lying at a 

 distance from the surface. Hardy's proof that the oriented layer 

 may be several molecules thick is both simple and unequivocal. 

 He allowed the fluid under test to be drawn in by capillarity 

 between, say, a microscope slide and a weighted cover-slip. The 

 force exerted is sufficient to lift the cover-slip and its weight from 

 their bed. Every molecule of fluid drawn in nnist, by the fact 

 that it is drawn in, be under the influence of the glass surfaces. 

 The fluid is now frozen, and the cover-slip broken away. The 

 layer of solidified fluid left is quite visible and is capable of measure- 



