2. Films become viscous as a result of molecular attraction, the more strongly attracted film being the more viscous.
3. Organic colloids increase plasticity by rendering the water film viscous.
4. The tendency for tensile strength to vary with plasticity is explained by molecular attraction between grains.
5. Change in viscosity or in thickness of film is beyond the region of ex periment.
Granting that these arguments may be valid and may be substan tiated by facts, it will be shown later that they may be considered as establishing the existence of an effect rather than the existence of a cause.
Size of Grain Theory of Plasticity—It has been shown earlier in this discussion that the size of the grains as determined in the mechanical analysis does not agree with the normal fineness of grain in the clay as it issues from the pug-mill; there are bundles of grains that success fully withstand the disintegrating effect of water in the pugging pro cess, but which are to a large extent disintegrated in the process of mechanical analysis. It is obvious therefore that conclusions based wholly on the results obtained in the mechanical analysis cannot be considered as necessarily agreeing with the facts observed in the actual behavior of the clay under factory conditions. In many clays, however, these bundles are broken down to such an extent that the analytical results indicate quite accurately their actual working properties.
Because in the mechanical analysis the coarser grains have been re ported as sand and the finer particles as silt and clay, not a few have been led to conclude that clay particles, or at least particles in which clay substances constitute a large proportion, cannot be present in a clay as large grains after thorough disintegration in water. Grout has shown, however, that this conception is entirely erroneous.' In Table XVII is given the amount of clay substance that he obtained first from the analytical analysis, second, calculated from ultimate analysis, and third, obtained from mechanical analysis.
Mr. Grout has also given' results of the chemical analysis of a com plete mixture of the several grades of fineness obtained from 16 samples of clay as follows: In this he has proved conclusively that the "clay substance" is pres ent in every grade of fineness. His own conclusions from these analyses
are, however, rather startling. He says: "The silica percentage is higher in the coarser portions, where it probably is present in the form of sand dr quartz. Alumina is higher in the finer material, but total fluxes are also higher, so that the finest particles are not the purest kaolin." In order better to show the validity of his conclusions his data has been calculated into molecular equivalents as given in the following table: A review of Grout's mechanical analysis of the West Virginia clays discloses the fact that he made 26 determinations: 6 plastic fire clays, pp. 160, 162, 163. 233 and 251, 1 flint fire clay, p. 218, 7 shales, pp. 249, 251, 242 and 262, • 10 river clays, pp. 263, 265, 270, 272, 274, and 276.
1 glacial clay, p. 265, 1 residual surface clay, p. 200.
It is assumed, therefore, that the samples, the analyses of which are given in Table XVI, are composites of the several grades of grains from the above clays. Being in most cases very impure clays, it is con sidered that although a study of the possible mineral make-up of each grade is at the best largely based on hypothetical assumptions, such a study would aid in our attempt to understand the constitutional make up of our clays.
On the assumption that all the alkali is present as a RO in orthoclase feldspar, the molecular ratio and ratio by weight of kaolin, feldspar and quartz present in each grade would be as follows : This data checks the fact developed in Table XV, i. e., that clay sub stance is to be found in all of the grades of fineness, in the coarsest as well as the finest. It also shows that more than 50 per cent of the coarsest group, or as it is customarily called, "coarse sand," may be kaolin, or is at least kaolinitic in composition.