104 THE MOVEMENTS OF SOIL WATER [chap. 



surface is known as surface tension ; it is only another 

 way of looking at the property which is sometimes 

 called capillarity, the power of liquids to move upwards, 

 downwards, or sideways, along, or into any system of 

 fine tubes. By means of a few glass tubes drawn out 

 fine and some coloured water, one may easily see that 

 water will rise in such tubes above the level outside, 

 and will rise farther the finer the tube in which the 

 experiment is tried. When one corner of a sponge or a 

 towel is dipped into water, the whole soon becomes 

 wetted, and the behaviour of the water may either be 

 regarded as a creation of a stretched film from fibre to 

 fibre, or a motion of the water along the fine channels 

 formed by the closeness of the fibres. In the same way 

 water may be conceived to move in the soil along the 

 fine channels formed between particles which are touch- 

 ing at their salient points ; it is quite easy to show that 

 soil is never packed solid, there is always about 40 per 

 cent of free pore-space between the particles, like the 

 spaces which must exist between the spheres making 

 up a heap of shot, however closely they are packed 

 together. But just as we have seen that, for any motion 

 of water to take place in the soil, the film of water must 

 be continuous from particle to particle, so from this 

 other point of view there must be no gaps in the soil 

 destroying the continuity of the fine tubes, because 

 liquids can move but a small distance along a wide tube. 

 In interpreting the relations between water and the soil 

 particles, either way of looking at things — capillarity 

 or surface tension — may be employed ; perhaps the 

 stretched film is the more useful, because it fixes 

 attention on the fact that the surface exposed by the 

 soil particles is their active and effective part in all such 

 actions. It is easy to understand that the more 

 extensive the total surface possessed by the soil 



