92 THE TEXTURE OF THE SOIL [chap. 



point if the brush be dipped in water and withdrawn. 



Or again, a flat sandy beach from which a smooth sea 



is receding will often show above tide-mark one stretch 



of sand quite dry and loose, in which the feet sink 



deeply, and another very soft stretch immediately left 



by the tide where the sand grains are completely 



surrounded by water. Between the two is a stretch of 



sand of the same character, but firm to walk upon ; 



this is partly wet, and there is enough water to form a 



film round the grains and hold them in position with a 



certain amount of force. That this sand is really just 



as loosely arranged as the softer tracts that are either 



wetter or drier, may be seen by the fact that it will 



easily pack more closely together under repeated gentle 



pressure with the foot. The shrinkage of soils, especially 



of clays, as they dry, may be attributed in part to the 



surface tension of the films surrounding the groups of 



soil particles ; as the water content is lessened the films 



exert more force in their effort to contract, and drag 



some of the particles closer together, especially the very 



small particles whose weight is trivial compared to the 



forces exerted by the film. In part also we must reckon 



the swelling of the " gel " films as they imbibe water. 



Clay shrinks more than other soils because of the larger 



quantity of " gel " it contains and the greater number 



of particles, their small size, and the higher proportion 



of pore space into which motion can take place. The 



tenacity of wet clay is due to the number of water 



films that have to be ruptured ; the vastly greater 



cohesion of dry clay probably to the fact that many 



of the particles have been dragged within the range of 



one another's molecular forces. There is a stage in the 



drying of clay when it will fall to pieces when worked ; 



probably this represents the stage analogous to the 



partly wet sand, when cohesion is due to the surface 



