Despite the present impossibility of producing a plastic material from artificially prepared colloidal hydro-alumino-silicates of the same ultimate composition as clay, and the fact that the addition of colloidal substances does not necessarily increase the true plasticity of clay, it cannot be denied that the presence of colloids has an important influence on it. The addition of starches, glue, gums and similar substances whilst apparently increasing the plasticity of clay does not do so in reality. The addition of 1 per cent. of tannin, on the contrary, has been found by Ries (6) to increase both plasticity and binding power. Plasticity appears to be composed of a number of characteristics so that it is scarcely likely that any single cause can be assigned to it. On the contrary, a study of the binding power, tensile strength, extensibility, adsorption, texture and molecular constitution of clays suggests very strongly that all these properties are involved in the production of plasticity and that it is due to the chemical as well as the physical nature of clay. No clay is entirely colloidal--or it would be elastic and not plastic--but all appear to contain both colloidal and non-colloidal (including plate-like) particles, and it is not improbable that materials in both these states are required, the colloidal matter acting as a cement. Ries (6) has, in fact, pointed out that colloids alone lack cohesiveness and solidity, and a fine mineral aggregate is necessary to change them into a plastic mass resembling clay. The relative proportions of the colloidal material and the sizes of the non-plastic grains will exercise an important influence on all the physical characteristics mentioned above, and therefore on the plasticity. The manner in which slightly plastic clays become highly plastic in nature is by no means certainly known. It has long been understood that the increase of plasticity is due to changes undergone by the clay during transportation. The most illuminating suggestion is that made by Acheson in 1902, who concluded that it is due to impurities in the water used in transporting the clay or remaining in contact with it during and after its deposition. These impurities may be considered as derived from the washings of forests, and after many experiments with plant extracts Acheson believed the most important substance in this connection to be tannin or gallo-tannic acid, a dilute solution of which he found increased the plasticity of china clay by 300 per cent. From this he further argued that the use of chopped straw by the Israelites in Egypt in the manufacture of bricks was unconsciously based on the tannin content of the straw increasing the plasticity of the material. [Illustration: Fig. 5. Chart showing rates of drying. (_After Bleininger._)] Beadle has stated that 2 per cent. of dissolved cellulose will increase the plasticity of china clay and make it equal to that of ordinary clay.