The optical microscope revealed that when air-saturated dilute aqueous suspensions of coarse kaolinite and dickite crystals were evacuated, negatively charged bubbles formed on and between the particles, which showed characteristic spatial relations to the bubbles and to each other.

Crystals leached with HCl developed edge-to-face flocculation over a wide acid pH range and presented their edges to the bubbles, indicating positive edge-charges. The addition of Na-montmorillonite reduced the number of particles adhering to the bubbles and impeded the edge-to-face flocculation.

In slightly acid suspensions, the particles became adlineated to form chains linking bubbles together or dependent from them.

At pH <3 or >7 the bubbles retained relatively few particles probably because the negative charge density on bubbles decreased at pH <3, while the crystals lost their positive edge charges at pH>7. At pH>7 crystals were often retained by their basal surfaces.

Crystals saturated with Ca2+ or Pb2+ showed face-to-face flocculation and tended to present their basal surfaces to the bubbles. The relatively large amounts of Ca- and particularly of Pb-clay secured by the bubbles may be associated with the lessened wettability caused by these cations.

The mantles of adherent particles affected the stability of the bubbles, both by preventing the coalescence of neighbors, and by lengthening the diffusion paths for dissolved air passing from smaller to larger bubbles.