78 



THE TEXTURE OF THE SOIL 



[chap. 



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, c (Fig. 4) are convex to 



WaXer 



FrG. 4. Capillary Rise and Depression 

 of Liquids in Glass Tubes. 



the water, and become more convex the narrower the 

 tube is ; the pressure below the convex surface must 

 be less than atmospheric, or the water would not 

 stand higher within than without the tube ; further, 

 the pressure beneath a f the most convex and therefore 

 most stretched surface film, is lower than the pressure 

 beneath b, and still lower than that beneath c. Per 

 contra, the mercury surfaces are convex outwards, and 



