Pectoidal Theory of to indirect or circumstantial evidence, there are many facts observed by a good many careful scientists that seem to point to one thing that is more characteristic of kaolin grains than of any other of the inorganic substances or minerals of which a clay is composed, i. e., adsorptive power. Some investigators have even gone so far as to attribute the plasticity of kaolin grains to an adsorptive power or actual taking into the grains themselves of foreign salts from solution. They advance the theory that these minute grains have a micellian structure. To such substance they apply the name "Pectoid," and to the theory the name, "Pectoid theory." To many, the absorptive and adsorptive properties of a clay are one and the same thing, and so far as can be judged, the most radical believe in either the adsorptive or the pectoidal theory, and oscillate from one to the other in a manner that induces skepticism. The fact remains, how ever, that both use the same arguments, the only difference being in the conception. It is safe to warrant, that when the pectoid theorist real izes than in one gram of clay, the disintegration of which has been effected only by shaking in distilled water, there can exist from 400 to 1500 trillion free and independent sub-microscopic particles, to say nothing about the larger particles, they will find interstices between • these grains sufficient to satisfy even the most exaggerated conception of a micellian structure.
Dr. Cushman' in a brief review of the observations that point toward a colloidal substance as being the prime cause of plasticity, has given the following citations: "Daubree found that wet ground feldspar as sumed a plastic condition, whereas dry ground feldspar did not." Ost wald, the eminent German physical chemist, Arons, Bischol, Seger, Rokland, and Van der Bellen, accepted and advanced in substance the colloid theory. T. Way' stated that while particles of sand and chalk absorbed water, owing to surface attraction and capillarity, clays and soils with a clay base behaved in a quite extraordinary manner. The more clayey the soil the more water it seemed capable of absorbing. But this was not all; besides water this clay substance exhibited a greater facility for absorbing the bases contained in certain salts which 'were dissolved in the water." E. says that if clay is mixed with a solution of calcium car bonate, the clay will retain some of the carbonate. "Baolins do not retain more than 2 per cent of carbonate of lime in solution, while plastic clays can absorb from 10 to 20 per cent of it." It is common knowledge among chemists that clay can extract solu ble salts from solution and retain them very persistently against all attacks by dissolving mediums. Mr. found that catechu and
extract of sumac leaves, spruce bark, tea leaves, oak bark or straw would be absorbed by clays from solutions.
Further, advanced the theory that the action of hydro-carbons in solution was to deflocculate the particles. This he claims was proved by the fact that in the untreated clays the grains were bunched to gether while in the - treated clays the particles were separated.
In the foregoing citations there is evidence sufficient to formulate a conception of what changes have taken place in a clay from the time it was first formed vn situ by decomposition of the parent rock and left practically devoid of plasticity, until it was deposited elsewhere as a plastic clay. Organic matter would have deflocculated the particles, and the soluble salts, which are very naturally attracted to the kaolin grains, would soften when wetted, but the water could not extract them owing to the greater force exerted by the kaolin particles. Defloccula tion by organic matter, recementation by salts of various kinds, may have formed a cycle of events that in the end would cause a condition of affairs that makes possible the property described as plasticity.
Molecular attraction for foreign substances which is peculiar to kaolin particles, and not to a very- large degree to other common constituents of clay, may and does have its influence on plasticity affected by fine ness of grain. Plate structure, a natural form in which kaolin grains arrange themselves, is a possibility under the right conditions, but there is also a possibility that organic matter and adsorbed salts may operate in the destruction and formation of the plate grains. A clay that is of secondary origin, like our common clays, could not have passed through the many geological changes with which they are credited without being more or less deflocculated and saturated with these foreign substances. Micellian structure is not a necessary condition. Minuteness of grain and consequently large surface or adsorbing area is sufficient.
• Adsorption theory of plasticity—Existing data, accumulated for years by scientists, all point to the fact—which is almost beyond the theoretical state, lying wholly within the realm of experimentation— that the plasticity, tensile strength and general working properties of the clay can be traced back to the adsorptive property of kaolin. Fur ther, all the facts that have been cited in support of any and all of the theories are identifiable as conditions that allow of the fullest exhibition of the plasticity that seems to follow as a direct consequence of the adsorption of soluble organic and inorganic substances by the kaolin grains.