THE COMPOSITION OF THE SOIL 105 



highly plastic, and can be moulded into shapes which remain 

 permanent on drying and baking. It shrinks very much on 

 drying and absorbs heat ; on moistening again, however, there 

 is a considerable swelling and evolution of heat. The re- 

 versibility of the process has been studied by van Bemmelen 

 (20, 25), who has also shown that the rate at which water is 

 lost on drying over sulphuric acid is not essentially different 

 from the rate at which evaporation takes place from a pure 

 water surface under the same conditions. The separate 

 particles of clay are so small that, when placed in water, they 

 assume a state of Brownian movement and sink only very 

 slowly in spite of their high specific gravity. Traces of 

 electrolytes have a profound effect on these properties ; small 

 quantities of acids or salts cause the temporary loss of 

 plasticity, impermeability, and the property of remaining long 

 suspended in water without settling ; the clay is now said to 

 be flocculated. The change can be watched if a small quantity 

 of any flocculating substance is added to the turbid liquid 

 obtained by shaking clay with water ; the minute particles 

 are then seen to unite to larger aggregates which settle, 

 leaving the liquid clear. There is, however, no permanent 

 change, deflocculation takes place and the original properties 

 return as soon as the flocculating agent is washed away. 

 Alkalis (caustic soda, caustic potash, ammonia and their car- 

 bonates) produce the reverse effect : they deflocculate clay, 

 intensifying its stickiness and impermeability and causing it 

 to remain suspended in water for long periods^ (see p. 161). 



Clay is thus an electro-negative colloid, its reaction prob- 

 ably being conditioned by a trace of potash liberated by 

 hydrolysis. 



Further, it appears to act as a semipermeable membrane 

 in relation to the movement of water : ^ this property might 



^ Leoncini and Masoni (172) were unable to find that the modification in 

 permeability in soil caused by saline solutions had any relation to their powers 

 of flocculating clay. 



' Lynde, J. Phys. Chetn., 1912, 16, 759-778, and y. Amer. Soc. Agron., 1913, 

 5, 102-106. ^ 



