So far as clays are concerned, their plasticity appears to be connected with the presence of combined water as well as of mechanically mixed water, for if either of these are removed, plasticity--both actual and potential--is destroyed. The part played by water is not, however, completely known, for the many theories which have been advanced only cover some of the conditions and facts. A number of observers agree that the molecular constitution of clay is peculiar and that it is to this that plasticity is due. Yet the curious fact that the purest clays--the kaolins--are remarkably deficient in plasticity shows that molecular constitution is not, alone, sufficient. Others hold that the remarkably small size of clay particles enables them to pack together more closely than do particles of other materials and to retain around them a film of water which acts partly as a lubricant, facilitating the change of shape of the mass when under pressure, and partly as an adhesive, causing the particles to adhere to each other when the pressure is removed. Zschokke has laid much emphasis on the importance of molecular attraction between clay and water as a cause of plasticity, and has suggested that the absorption of the water effects a change in the surfaces of the clay particles, giving them a gelatinous nature and enabling them to change their form and yet keep in close contact. The fact that mica, fluorspar and quartz, when in a sufficiently finely divided state, are also slightly plastic, appears to be opposed to the molecular constitution theory. Smallness of grain undoubtedly has an influence on the plasticity of clay, coarse-grained clays being notably less plastic than others. Daubrée pointed out that felspar, when ground with water, develops plasticity to a small extent, and Olschewsky carried this observation further and has suggested that clays owe their plasticity to prolonged contact with water during their removal from their place of formation and previous to or during their deposition. A further confirmation of this theory is due to Mellor (3) who showed that on heating china clay with water under very considerable pressure its plasticity was increased and that felspar and some other non-plastic materials developed plasticity under these conditions.