The flocculation of soils. III

Abstract

In support and extension of the conclusions drawn in the earlier papers the following facts and deductions are submitted:1. The only sparingly soluble substance, from a random collection examined, whose suspensions showed the same abnormal flocculation by calcium hydroxide that is shown by clay, were certain phosphates of iron aluminium and calcium. The abnormal flocculation of these phosphates is open to an explanation quite analogous to that already advanced for the flocculation of clay.2. Until the amount of calcium hydroxide added to a suspension of clay or phosphate reaches a certain amount its abnormal flocculating power is not manifested. The amount required to produce the abnormal flocculation is greater for a fat clay than for a lean one. This is in agreement with the view that the abnormal flocculation is caused by a coagulation of emulsoid matter, for obviously such coagulation will not become dominant until a sufficient amount of the precipitant has been added.3. The lime absorbed by a soil can be wholly and completely removed by a dilute acid treatment which cannot very appreciably decompose the unweathered minerals. It is therefore concluded that the absorption of lime by a soil is an absorption by the soil colloids and not by the unweathered minerals.4. The ignition of a soil for a few minutes over a bunsen flame increases the amounts of iron and aluminium dissolved by acid. Evidence is brought to show that this is due to a destruction of the colloids which bind the particles together, and a consequent exposure of a larger surface.5. The effect of a partial ignition on the base absorbing power of soils and subsoils is described and the results are claimed to be in agreement with the view that the particles in the aggregates are bound together by gelatinous colloidal matter.