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the reverse process deflocculation. Charcoal is an example of a gel that has been permanently set by heat. Schloesing, in 1876, actually isolated the colloids from clay. The attribution of plasticity to the presence of colloids appears to have been arrived at in 1903 independently by three observers P. Eohland in Germany, and Cushman and Ries in the States. In the same year Acheson found that the addition of 2 per cent, of tannin to a lean clay materially augments its plasticity. Clays transported by CERAMIC CHEMISTRY. 15 water and deposited in some new place, reasoned Acheson, have greater plasticity than those found near their parent rock, and as it is unlikely that the water itself has im- parted this property to the clay, it follows that the washings from the forests have been the agent that increases plas- ticity. Accordingly he tried the effect of several vegetable extracts on lean clays, with the result mentioned above. The discovery was immediately claimed to support the colloid theory, and it was extended by Acheson and others, such as Weber in Germany, to the use of sodium silicate, alkalies, carbonates, and some other salts for promoting plasticity, as a proof that the colloids were being deflocculated. In 1908, Harrison Ashley advanced a method for estimating the colloids in a clay by taking advantage of a property well developed in colloids, the power of absorption that is, of taking other substances out of solution on suspension. Ashley found that colloids