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not correspond to close packing. So little is f°_oWn about the effective (hydrated) sizc jg the ^knrticles that it would be morę rational to calculate the mechanically effective volume of the particles from the dilatancy concentration. On account of the uncertainty as to the real cause of it, this phenomenon is generally called " inverted ” or 1 inverse plasticity,” or, sometimes, " pseudo-dilatancy.”

Dilatancy is a property which causes much perplexity to the practical rheologist. No measure of the flow properties of materials which show this phenomenon is really reliable.¹ Dilatancy may be regarded as the opposite of thixotropy, sińce the consistency increases with shear and falls on standing.

A tremendous amount of work has been done on thixotropy. There is no method of getting flow cuiyes for thixotropic materials which is entirely satisfactory from the theoretical point of view. Many methods will give a flow curve for the com-pletely stirred state. One of the best methods is that of the bali viscometer, which measures and compares the ratę of fali of a bali through the liąuid, before and after stirring. Scott Blair uses this for honey, and so do Fulmer and Williams, who propose a special wali correction for the inter-pretation of the data. Goodeve and Whitfield have recently described experiments with a Couette type of viscometer. A coefficient of thixotropy is defined in dynes/cm.² as the limiting slope of the viscosity/reciprocal-shear-rate curves for high rates

1

Ą type of molecular dilatancy has already been discussed connection with Eyrings theory of the natureofyiscosity

2

reundlich and Edder (Trans. Farad. Sos., 1938, XXXIV., 308) iscribe a techniąue for dilatant matenals