Johnson and Blake (21) supposed that plasticity is due to the clay being composed of extremely minute plates 'bunched together,' a view which was also held by Biedermann and Herzfield, Le Chatelier and others. Olschewsky enlarged this theory by suggesting that the plasticity of certain clays is dependent on the large surface and the interlocking of irregular particles with the plates just mentioned. These theories of interlocking are, however, incomplete, because the tensile strength of clays should accurately represent the plasticity if interlocking were the sole cause. Zschokke has shown that tensile strength is only one factor which must be determined in any attempt to measure plasticity. E. H. L. Schwarz (35) has suggested that many clays are composed of small globular masses of plates so arranged as to form an open network (fig. 4) which is sufficiently strong not to be destroyed by pressure. In the presence of water and much rubbing the plates are separated and are made to lie flat on each other, thereby giving a plastic and impermeable mass. If this is really the case it would explain the porosity and large surface of some clays and might account for their adsorptive power. [Illustration: Fig. 4. Illustrating the structure of a 'clay crumb.' (_After Schwarz._)] A theory which was first promulgated in 1850 by Way (4), but which has only received detailed attention during the last few years, attributes plasticity to the presence of colloid substances in clay or to the fact that clay particles possess physical characters analogous to those of glue and other colloids. These colloid substances have a submicroscopic or micellian structure; they are web-like, porous and absorb water eagerly. This water may be removed by drying, only to be re-absorbed on cooling, but if the heating temperature is excessive the structure of the colloids is destroyed. This colloid theory explains many of the facts noted by earlier investigators such as Aron, Bischof, Seger, Olschewsky, etc., but it is not entirely satisfactory, though Rohland (5)--to whom the present prominence of this theory in Europe is largely due--persistently maintains the contrary. One great objection is the fact that no characteristic _inorganic_ colloid substance has been isolated from pure clay. It is possible that some of the so-called 'colloidal' properties of clay may be due to the smallness of its particles and to their great porosity, as suggested by Olschewsky.