342 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Liquefication of plastic bodies under shear and regain of plastic nature on subsequent rest is known as thixotropic change and the process as thixo- tropy. Thixotropy is a very important factor in the behaviour of plastic bodies, as will be seen later. Returning to the rheological curve of a plastic body and confining our attention to the upper linear portion, we might designate this line as a differential viscosity, its differential nature being caused by a sort of memory of its previous shear history. Unlike the true fluid wherein: shearing stress Viscosity -• rate of shear = a constant the plastic body in the higher rheological regime of its linear part is •, (shearing stress) Plastic or residual viscosity = /k (rate of shear) constant The plastic body can therefore be considered to possess a residual vis- cosity, or a coefficient of resistance to shear after deflocculation by shear has been accomplished. However, as we know, it exhibits a rigidity in addition. Rigidity defeats attempts to measure it by any dynamic means, since its accompanying property of thixotropic change by shear would vitiate its constancy. It must therefore be determined by a static method of measurement. The best method is that employing the parallel plate plastometer, based on the technical instrument used in the rubber industry, but very greatly simplified to yield, paradoxically enough, more accurate measurement. This consists merely of two glass plates 6 in. X 6 in. X 0-25 in., fine- ground on their inner faces to eliminate slippage effects. The lower plate being carefully levelled, four 0.2 c.c. portions of the material under test are delivered from a hypodermic syringe in uniformly piled mounds at equidistant points on to the upper surface of the lower plate. The second plate is then carefully laid on to the mounds and left in an undisturbed state until the diameters of the pressed mounds cease to enlarge. In general, 15 minutes suffices for materials of butter-like consistency. The final diameters are measured by laying a transparent scale bearing concentric 1 mm. circles over each disc seen through the upper plate, the mean diameter being taken. Materials of higher rigidities may require the placing of additional weights on the top plate to obtain discs of sufficient diameter to ensure accuracy of measurement. W.g.V. Then, threshold rigidity-- 2.n.r•.R5 (dynes per cm. 2)

INTRODUCTION TO THE RHEOLOGY OF DISPERSE SYSTEMS 343 where W = total weight in grams of plate (and loading), g = 981, V = volume of each mound (0.2 c.c.), n = number of mounds (4), and R -- radius of discs. Another instrument useful only for comparative evaluation is the cone penetrometer. The cone penetrometer only permits of obtaining degrees of rigidity, and does not furnish absolute values. These values are not directly comparable with those obtained from the parallel-plate instrument, but work out on an average about 7 times the true value. Nevertheless, the instrument serves as a very handy and useful piece of apparatus for standardisation of subjec- tive rigidity and enables one to follow shelf-life constancy of a material. The cone penetrometer as used by the writer consists of a light hollow aluminium cone having an apex angie of 60% fitted from the inside of the apex with a light steel stem terminating in a small loop for suspension on a cotton thread. The bulk specific gravity of the cone is so adjusted by turning down the wall thickness until it just floats in water. The lateral face of the cone is graduated in circles 1 mm. apart. Small weights for loading the cone are cast in lead or fusible metal in the form of truncated cones perforated to admit the central stem. The cone, suitably loaded to attain a sufficient depth of immersion and suspended on its thread, is progressively lowered by means of a rack and pinion on to the levelled surface of the material to be tested contained in a large beaker, the thread being continually maintained just short of taut to ensure vertical penetration of the cone. For a cone of apex angle of 60 ø: 2. W.g g.L.(D -- d) 625 W- 163 L3(D - d) Rigidity-- •'L" 6 or L2 where W = total weight in grams, g: 981, L = depth of immersion in cms., D: bulk density of cone and d = specific gravity of material tested. Finally, we come to the question of measurement of thixotropy, the most important aspect of which is the determination of the rate of re-solidification of sheared materiM. The Pryce-Jones thixotrometer (Fig. 10) comprises an outer cylinder fixed to the housing of the instrument and an inner cylinder suspended on a torsion wire fixed to the head. To the upper end of the wire is attached an indicating needle and a scale on which the given twist may be read. A second needle is fixed to the top of the inner cylinder, which bears a locking device holding it in position when torsion is first applied from the head of the wire. The inner cylinder also carries a needle which can sweep over a circular scale and is furnished with a similar locking device. The annular space between the cylinders having been filled with material, the lower cylinder is locked in position and a torque of a selected value is