Solutions causing flocculation—Having discussed the deflocculating solutions in detail, it will not be necessary to dwell at length on the flocculating solutions, for the effect on clay of each class of solution is the converse of the other. It is important to note that solutions which have surface tensions higher than that of water tend to cause floccula tion.
The nature of solids affects flocculation in several ways. First, if the clay or soil under examination contains a large quantity of calcium or magnesium carbonate', it has been found that solutions having a surface tension as low as that of ammonia will cause flocculation. Data are not available concerning clay mixtures high in other minerals, but as is about to be shown, clays that have comparatively low content of clay substance probably have as an average for the several mineral grains a low poten tial in comparison with the potential of water. Clays high in products of decomposition of organic matter may be flocculated by ammonia. In fact the "potential" of the impure clays may be so low as to permit am monia solutions to flocculate their grains. Pure clays, i. e., kaolins, re quire for their flocculation solutions having a "potential" that is higher than that of pure water.
Second, near' the surface of any soil there is a concentration of solu tions. This is adsorption. If' the solid is exceedingly porous, this ten dency to concentration near the surface is heightened. It is well known that salts, which are concentrated near the surface of solids are precipi tated or at least are left clinging to the solids when the water is with drawn. Soils', even sand, possess the property of attracting and fully absorbing salts which cannot be wholly washed out by new quantities of water. Solutions of many of the salts are materially weakened when brought in contact with solids, because of the adsorption of the salts, but if the surface of the solid be relatively small no weakening of the solu tion may be perceptible.
Summary—The well-known facts concerning a plastic clay when wetted with water are, first, that its finer portions are composed of a countless number of minute grains, the composition of which has been shown to agree closely with that of pure clay substance; second, that even the coarser grains are composed largely of kaolinite and other minerals cemented into clusters or bundles; third, that clays having a high con tent of minerals other than kaolin, are flocculated by solutions having a surface tension lower than that of water, while the clays which are practically pure kaolinite in composition require for their flocculation solutions that have a surface higher than water; fourth, that clay particles extract salts from solutions and hold them near and on their surface at a high degree of concentration; fifth, that clay substance ex hibits this property of adsorbing salts to a much higher degree than any of the common anhydrous minerals, a fact that makes the extreme fine ness of the "clay substance in clays" of considerable significance.
The known facts concerning solutions are : first, that all solutions have a surface tension which is increased with increased concentration; sec ond, that those solutions which have a surface tension higher than that of pure water, tend to cause flocculation of kaolin grains.
On putting together the known facts concerning clay and water, it is evident that the film of water surrounding the grains of clay, (when the mass is in a plastic condition) has a very high potential, owing to the high degree of concentration of the salts that are held to the kaolin grains by adsorption.