84 THE TEXTURE OF THE SOIL [chap. 



condition; nor have they been drawn so closely 

 together as to cohere without the aid of any water. 

 It is not quite intelligible why a piece of dried clay 

 becomes soft and swells again when wetted, nor 

 why the particles should once more move apart. 



Hygroscopic Moisture, 



If the withdrawal of water from a soil by evapora- 

 tion be continued, a point is at last reached when the 

 soil becomes air-dry : it still retains some water, which 

 will vary in amount with the degree of humidity of the 

 atmosphere and the temperature. This last film of 

 water is held very closely and in a somewhat different 

 manner from the ordinary film held by surface tension, 

 though the two shade off into one another. For example, 

 the film of hygroscopic moisture can be produced by 

 condensation alone, when perfectly dry soil is placed in 

 an atmosphere containing water vapour : the surface 

 of materials like glass, sand, etc., has sufficient attrac- 

 tion for water to condense it from a state of vapour. 

 The amount of hygroscopic water retained by different 

 types of soil when air-dried and then allowed to 

 stand in a saturated atmosphere at ordinary tempera- 

 ture, is given in the table below ; it will be seen to 

 be more or less proportional to the surface possessed 

 by the soil particles, clay and humus retaining the 

 most. 



This hygroscopic moisture cannot be of any service 

 to plants : Sachs has shown by experiments in pots that 

 tobacco plants will begin to wilt before the soil has 

 parted with all its moisture. When wilting began with 

 a sandy soil the sand in the pot still contained 1-5 per 

 cent, of water, with a clay soil 8 per cent, of water ; with 

 a mixture of sand and humus there was as much as 



