hi.] SURFACE TENSION 71 



of the molecular forces to be described later ; it is thus 

 prevented from flowing at all, and only moves by 

 diffusion. If we assume for clay particles a mean 

 diameter of 00002 mm., and a structure similar to A in 

 Fig. 2, p. 62, it is easy to show that no molecule in the 

 space between the spheres can be further than about J 

 of the diameter of a sphere, or 000004 mm - from one or 

 other surface, while the range of molecular forces as 

 calculated by Quincke extends to about 000005 mm. 

 from the surface. Spring has indeed shown that infiltra- 

 tion of water is impossible through clays or loams unless 

 they are first allowed to expand by taking up water. 



Surface Tension and Capillarity. 



The existence of attraction between the molecules 

 causes the free surface of any liquid to become a sort 

 of stretched elastic film, in tension itself, and exerting 

 a certain pressure inwards when free. The molecules 

 within the liquid are equally attracted in all directions 

 by the surrounding molecules, and are therefore in equili- 

 brium ; the molecules on the surface, having nothing on 

 one side, are only attracted inwards, and so, as a whole, 

 exert a pressure on the liquid similar to that which 

 would be caused by a stretched elastic skin over the 

 liquid. 



The existence of this force of "surface tension," as 

 it is called, may be demonstrated by many simple ex- 

 periments, e.g. y by the familiar fact that a clean needle 

 will float when placed carefully on the surface of water ; 

 or, by the fact that any portion of a liquid which is so 

 small that the force of gravity on it is not large 

 compared to the molecular forces, immediately assumes 

 the spherical shape. Of all figures, a sphere has the 

 smallest surface in proportion to its contents, *>., the 



