72 



THE TEXTURE 01 THE SOIL 



[chap. 



stretched film on the surface of a drop of liquid shrinks 

 as far as it can until the liquid is packed into the 

 smallest possible compass, into the form of a sphere. 



When a liquid and a solid are in contact, the form of 

 the surface and the resulting pressure or tension depend 

 on whether the liquid "wets" the solid or not For 



Water 



FlG. 3. — Capillary Rise and Depression 

 of Liquids in Glass Tubes. 



example, if a series of very fine or "capillary" glass 

 tubes are dipped into water and mercury respectively, 

 the water will rise up the tubes in inverse proportion to 

 their diameters, the mercury, which does not wet the 

 glass, will be correspondingly depressed. 



The water surfaces a, b y c (Fig. 3), are convex to 

 the water, and become more convex the narrower the 

 tube is; the pressure below the convex surface must 



