432 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS is valid (4). G^: rigidity in absence of electrolyte. I =ionic strength and K=slope/G^. This type of relationship is similar to that reported by Cumper and Alexander (5) but the glycerin present produces much smaller numerical values for the constants. With cations and monovalent anions the sign is negative, but multivalent anions in general had a positive sign in the equation and therefore produce an increase in the rigidity with increasing ionic strength. Gels containing aluminium chloride did not obey this equation and showed both shrinkage of the gel and plastic behaviour. Changes in the rigidity modulus of Laponile gels produced by mono- valent electrolytes are small relative to those with gelatin and in general monovalent electrolytes produce a slight increase in rigidity. This is in marked contrast to the effect of electrolytes on yield value where the changes occurring are both dramatic and large. These are too complex to be discussed in this paper and will be reported elsewhere. It is interesting that 1500 IOOO 5oc 300 B 600 300 600 900 Sheor s•ress, Nm -• 10 Figure •t tqow curves of 3 • Laponite (A) and 8•o bentonite (B). ( els at rest for 0.5 rain in viscometer before curve plotted.

SOME APPLICATIONS OF RIGIDITY AND YIELD VALUES 433 sodium citrate, usually regarded as a peprising agent behaves in the same way as the flocculating electrolytes. Once gelatin/glycerin mixtures have gelled the measurement of stress/ strain curves is impossible, but with Laponite they give far more funda- mental information than measurement of rigidity and it is interesting to compare the artificial material Laponite with the natural clay bentonite. Figs. 4 and $ show the flow curve for 3% Laponite and 8% bentonire. The static yield value (6, 7) is the maximum stress recorded and depends on the rate of application of the shearing stress and on the inertia of the cone 500 50C Laporate 3 % 0-5 m•n mln Bentorate 8% rain 300 300 600 600 600 Stress, Nm -2 xlO in 300 Figure 5 Development of static yield value in Laponite and bentonitc gels. so that results can only be regarded as comparative. On leaving the gel to age in the viscometer the static yield value increases as the gel structure reforms and both materials develop a "spur" value. Spur values have been recorded as a frequent occurrence in rheograms (8-10). They are a good indication of the existence of a three dimensional gel structure whose resistance has to be broken down before deformation can occur. The res- ponse time of the viscometer was too long to measure the very rapid break- down of the Laponite gel structure, but as a first approximation the de-