ARTICLES 581 



merits is reserved for the end of this article, while plasticity 

 is dealt with immediately below. 



The plastic properties of clay are of great importance in 

 ceramics as well as in agriculture, and the possible causes of 

 the phenomenon have given rise to much speculation. They 

 have been summarised by Hancock under the following four 

 headings. 



(a) Chemically Combined Water. — ^This is removed by heating 

 to 500° C, and clay so treated is no longer plastic when worked 

 up with water. But against the view that the combined water 

 determines the plasticity is the fact that clays with the highest 

 percentage of such water are not necessarily the most plastic 

 (e.g. China Clay). 



{b) Size and Shape of the Grains. — Over fifty years ago it was 

 suggested that a plate-like or laminated structure would permit 

 adjacent grains to slide over each other, and that this would 

 allow a mass of clay to be deformed without rupture. Some 

 confirmation was obtained by the plastic behaviour of normally 

 non-plastic, but scaly substances, such as gypsum, talc, and 

 calcite, when finely ground and mixed with water. 



An alternative supposition was that the grains were approxi- 

 mately spherical, and thus could slide over each other without 

 the mass cracking. But some clays, deficient in plasticity, 

 are known to contain a large percentage of ovoid particles. 



(c) Molecular Forces. — Le Chatelier and others have 

 elaborated a theory of molecular and capillary attraction, 

 consideration of which is outside the scope of this article. 

 For our present purpose it is sufficient to bear in mind that any 

 theory of plasticity is eventually susceptible to mathematical 

 analysis and exposition. 



(d) Colloids. — The idea that the colloidal material in the 

 soil is the cause of its plastic properties is the most promising. 

 If we assume a gel skeleton throughout the mass, produced 

 by the imbibition of moisture by the colloidal material when 

 the soil is worked up with water, the soil would be plastic, 

 and, on drying, the gel-structure would contract and harden, 

 so that the soil would then exhibit increased cohesive properties. 



In the present stage of development of soil physics there 

 is little more that can be said on the subject of plasticity, 

 because the investigations have so far been of an empirical 

 nature which cannot readily be summarised. 



(2) The Soil Moisture 



The amount of moisture present in the soil at any time 

 is the balance of gains over losses. Moisture may be obtained 

 from the atmosphere in the form of rain and mist and from the 

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