706 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In most cases it is desirable that the evaluation is carried out on in vivo skin and this requires care in establishing sufficiently stable and reproducible conditions. An example of the problems involved is given for smoothness and a mathematical and measurable model has been derived in terms of scratch resistance, friction and surface topography. All three of these properties can be affected by the water content of the stratum corneum, using emol- lients, soap or water alone. Application of the smoothness equation derived in this paper could prove useful where evaluation of cosmetic 'smoothing' treatments require confirmation and physical understanding. ACKNOWLEDGMENT Thanks are given to Unilever staff who helped in carrying out some of the original experimental work. (Received: 31st January 1973) REFERENCES (18) (19) (20) (21) (22) (23) (24) (1) Rothman, S. J. Amer. Med. Ass. 178 33 (1961). (2) Harry, R. G. Modern cosmetology (1962) (Leonard Hill). (3) Powers, D. H. J. Soc. Cosmet. Chem. 9 103 (1958). (4) Prall, J. K. VIL F.S.C.C. Congress Barcelona (1970). (5) Mali, J. W. J. Soc. Cosmet. Chem. 15 367 (1964). (6) Schneider, W. J. Soc. Cosmet. Chem. 12 22 (1961). (7) Edwards, E. and Duntley, S. Amer. J. Anat. 65 4 (1939). (8) Wright, V. Biophysicalproperties of skin 437 (1971) (Wiley-Interscience) ed. H. R. Elden. (9) Kenedi, R. M. Advances in biomedical engineering 1 229 (1971) (Academic Press). (10) Schade, H. Z. Exp. Pathol. Therapie, 11 369 (1912). (11) Dick, J. G. J. Physiology, 112 102 (1951). (12) Tregear, R. T. and Dirnhuber, P. J. Invest. Dermatol. 45 119 (1965). (13) Kirk, E. and Kvorning, S. A. J. Gerontol. 4 273 (1949). (14) Remington, J. W. Tissue elasticity (1957) (American Physiological Society). (15) See Instructions for use of the Harpenden skin-fold calliper, made by British Instruments Ltd. (16) Peck, S. M. and Glick, A. W. J. $oc. Cosmet. Chem. 7 530 (1956). (17) Gibson, I. M. VI I.F.S.C.C. Congress Barcelona (1970) also in J. Soc. Cosmet. Chem. 22 635 (1971). Prall, J. K. IV I. F.$.C.C. Congress Paris (1966). Jenkins, H. L. and Tresise, J. A. J. $oc. Cosmet. Chem. 20 451 (1969). Bakiewicz, T. A. J. $oc. Cosmet. Chem. 24 245 (1973). McOsker, P. E. and Beck, L. W. J. Invest. Dermatol. 48 372 (1967). Zigler, M. J. Amer. J. Psychol. 31 73 (1920). Scott-Blair, G. W. J. $oc. Cosmet. Chem. 11 181 (1960) 17 45 (1966). Powers, D. H. and Fox, C. J. $oc. Cosmet. Chem. 10 109 (1959).

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