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The explanation of semi-permeable membranes most widely 

 accepted at the present time is that of selective solubility, suggested 

 by L'Hermite^^. The membrane is permeable to those substances 

 which dissolve in it but not to others. 



As the semi-permeability of clays appears to be connected with the 

 plasticity, any treatment which will increase the latter should increase 

 the former. Rohland^ has found this to be the case with some lean 

 clays he has examined. Some of the phenomena occur whenever 

 plastic clay is mixed with solutions, as the particles allow the 

 crystalloids in the latter to pass through them, but retain the colloids 

 on their surface. In this way, the adsorption of crystallised matter 

 as well as colloidal matter occurs ; but as the particles of clay are so 

 minute the effects are scarcely distinguishable, and clays aj)pear to 

 be capable of absorbmg both colloidal and crystalloidal substances. 



The permeability of raw clays has been studied by Spring, who 

 found that when such clays are confined so that they cannot expand, 

 they will only absorb enough water to fill the .pores. The amount 

 absorbed varies from 3 per cent, with some fireclays to 25 per cent, 

 with some sandy loams. When not confined in this manner, the 

 extent to which the water can permeate a clay is dependent on the 

 amount of non-plastic material it contains, and increases when sand 

 or grog is added. The permeability of a fired clay is an important 

 characteristic, and is described later. 



The more permeable a clay, the more easily can it be dried and 

 heated without damage, large pores being prefereable to small ones. 



Wet clay in the form of a stiflf-plastic paste is generally considered 

 to be extremely impermeable, but, as already mentioned, this is only 

 a relative property, as such a mass of clay, if left in water, will, in 

 time, fall to pieces. Clay which has been suspended in water and 

 allowed to settle is usually quite jjermeable, as are many natural 

 clay deposits. It is only when the material has been " worked " or 

 " pugged " that it becomes impermeable. 



The plasticity'^ of clays is one of their most important properties. 

 Plasticity may be defined as that property of a material which enables 

 it to change its form without rupture, the new shape being retained 

 when the deformatory force is removed. In other words, a material 

 is said to be jJastic when it can be kneaded or pressed into any desired 

 shape, and remains in that shape when the kneading ceases or the 

 pressure is removed; this alteration of shape being capable of being 

 re^jeated indefinitely. It' is a characteristic of many substances 

 besides clays,* though clays possess it to the most marked degree. 

 Ashley' has pointed out that very few people agree exactly with the 

 conception of plasticity. Thus, a brickmaker terms a clay plastic 

 when it works well in his machine, and is capable of being kneaded 

 into a " good " paste, but a potter usually places more emphasis on 

 the binding power of the clay, though he terms this its plasticity. 



Although these definitions are sufficient for practical purposes, 

 they are not entirely satisfactory, nor is there any explanation of the 



* The " possible plasticity " is that which can be developed under the best 

 kno«ni conditions. For many purposes, it is not necessary to develop the 

 plasticity of a clay to the utmost. 



