123 



If Olschewsky's suggestion that the particles of clay arc porous 

 is correct, the phenomena ascribed to adsorption may really be due 

 to adsorption within the capillaries or pores. 



The " scum " observable on some bricks is due to the salts adsorbed 

 by the clay and carried to the surface during the drying of the bricks. 



The capillary j^henotnena sho\\Ti by many clays and soils may also 

 be explamed on the hypothesis that clays are colloidal in character. 



The porosity of clays varies with the amount of water present, 

 some stiff plastic pastes being quite impervious, though the same 

 materials are porous when dry. This porosity appears to be associated 

 with the capillary structure of many clays and whilst it is a property 

 possessed by non-colloidal substances, it is a characteristic property 

 of some colloids. 



The semi -permeability of clays, like that of colloids, is a characteristic 

 property, and although its nature is by no means well understood, 

 it appears to confirm the presence of colloidal matter in clays. 



When clays are made into semi-permeable " membranes," they 

 behave according to their plasticity. The plastic clays effect a perfect 

 separation between the colloid and crystalloid solutions and are truly 

 semi-permeable, but very lean clays such as china clay are very 

 irregular in their action. In some cases, the presence of a crystalloid 

 may cause a sol to pass through a membrane, as when silicic acid is 

 mixed with sodium chloride both wUl pass through. It is also stated 

 by W. Ostwald^- that fresh colloids (particularly silica) will pass through 

 a membrane, but after keeping a few days they will not pass through. 

 There is no connection between the rate of diffusion through the 

 membrane and the molecular weight. 



According to Rohland', plastic clays Avill allow ferric chloride and 

 sugar (crystalloids) to diffuse, but not tannin (colloid). In emulsions 

 of oil and water, plastic cla}^? permit the (crystalloid) water to pass, 

 but not the (colloid) oil. In alcoholic solutions of fat, such clays 

 permit the alcohol to pass but not the fat. In aqueous rubber solutions, 

 plastic clays prevent the rubber from diffusing, and in albumen, 

 solutions the albumen is retained, both rubber and albaaien b?ing 

 typical colloids. The diffusibility or speed at which the substinces 

 dialyse through the membrane depends upon their nature. Thus, 

 water* which is a crystalloid, and electrolytes, e.g., salts dissolved 

 in it, diffuse rapidly, but colloids, such as ferric hydrate, hydrated 

 silica, hydrated alumina, and most products of organic life such as 

 starch, vegetable oils, and gelatin are either indiffusible or pass 

 through with extreme slowness. Colours, on account of their complex 

 composition, play a special part; they are retained by plastic clays, 

 though these colours are crystalloid and not colloid. Berlin blue, 

 potassium ferricyanide, aniline blue, sulphated triphenyl rosaniline, 

 aniline red, carmme, malachite green, fluorescin, aurin, and other 

 animal, vegetable, and tar colours, cannot diffuse through clay, and 

 this, in spite of their crystalloid nature. 



* Zschokke^* suggests that plasticity is possessed by all substances composed 

 of extremely minute particles with sviflficient affinity foj' each otlicr and willi 

 a power for combining witli water. 



