The combination of optical and elastic studies on sols of Wyoming bentonite containing the smallest particles sizes (15 to 20 μ equivalent spherical diameter) have led to the conclusion that the gelation of this system is due to the formation of a fibrillar network in which the plates are aggregated end-to-end in the form of flat, ribbon-like, filaments. The investigation of the effect of the addition of uni- and divalent electrolytes upon the rigidity is found to be consistent with the hypothesis that the particles are not in close contact, held by chemical bonds, but are situated at equilibrium distances which are determined by the combination of repulsive forces between their electrical double layers and attractive van der Waals forces.

The theory of the light scattering by a system of thin rectangular plates has been developed, and applied, to establish the existence of ribbon-like aggregates in undialysed sols(pH 8.9) at concentrations as low as 0.05 per cent.