464 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS measuring Newtonian viscosity. As far as I know there is nothing available for non- Newtonian measurement. DR. A. W. M•DDL•TO•: In page 458 you refer to a correlation between palate assessment of gum solutions and their rheological properties. Can you explain how this correlation was derived ? Ts• L•CTUR•R: As far as I can remember people were asked to estimate the degree of sliminess of the gum solutions presented to them, and rheological studies were simultaneously carried out on these solutions. They did find by statistical correlation that there seemed to be a relationship between the degree of sliminess and the degree of non-Newtonian behaviour of the material. The number of materials examined at the time, however, was quite limited, so I do not know whether this reported opinion would have very wide application. DR. A. W. MXD•L•TO•: Would the errors be likely to occur in the assessment of the subjective relations ? THE LECtUReR: Yes. From the little experience I had with gum solutions and similar things, they are very difficult to reproduce in consistency, unless one very carefully follows a standardized procedure and allows the solutions to age for a certain given time before testing. This certainly applies with gelatine. DR. A. R. RoGeRs: What methods do you use for studying distribution of sizes in the 1/• and less range ? Ts• L•C•UR•R: At the moment we are able to see those which are just below 0.5/• under the microscope, but we have no accurate method of determination I understand that Dr. Otterwill developed a method based upon light scattering principles which enable him to do this for polystyrene latex particles. These, of course, are different, but we are hoping to examine something along these lines next year. One of the major difficulties, even when you are able to measure these sizes, is how to find the average size. In all methods of calculating average size, apart from doing a pure arithmetical mean, one gets involved in summing the powers of the particle size, and all these submicroscopic sized particles would just cancel out. I have been thinking about this subject of late, because it is very important to our rheological studies. Possibly in order to show the effect of these submicroscopic sized particles one should calculate not a mean, but a reciprocal mean diameter, and then the very small particles would play a much larger part than the larger ones, which is the right order. DR. J. J. MAus•R: What would be the best method for measuring the spread- ability of thixotropic gels ? THE LEC•URSR: Any method which will enable you to measure viscosity ag very high rates of shear. DR. J. J. MAUS•ER: Is that a matter of spreadability ? TnE L•CTUR•R: I imagine that one would get a drastic breakdown of any strucgure which is present in the material, and this would be the same as the conditions pre- vailing at high rate of shear. The problem with all the rheological methods of measurement is that one does not know what shearing conditions one is using in the practical process. As yet no one has been able to calculate these. One or two

TECHNIQUES FOR ASSESSING RHEOLOGICAL PROPERTIES years ago someone tried to calculate the shearing forces involved in certain elementary operations, such as pouring a liquid or a non-Newtonian fluid out of a bottle or extruding it from a syringe, just to get some idea of the shearing conditions which should be applied in viscosity measurements, but to date no one has gone any further than this.