ABSTRACTThe addition of an acid to a clay suspension usually causes rapid sedimentation, while the addition of a base reverses the process. This is generally explained by saying that clay is a negative colloid and hence flocculated or precipitated by the hydrogen ions of the acid and for the same reason deflocculated by the base. These experiments showed that the effect of hydrogen‐ion concentration upon the rate of sedimentation2was relatively great and explains why changes in thephvalues produce such noticeable changes in plasticity and viscogity They show also that elutriation is an uncertain process withoutphcontro