488 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS with teeth. The medium was dripped into a two part acrylic box which held the teeth in the lower compartment. A piece of muslin impregnated with plaque derived from saliva was in contact with the teeth while the upper part of the box maintained a pool of nutrient in contact with the plaque and teeth. In this paper Pigman used enamel hardness measurements to assess the effect of topical treatments but other workers, for example Francis and Meckel (28), have reported the production of lesions in this type of equipment. CONCLUSION The techniques mentioned have been used to assess products containing fluorides which became popular as a topical treatment because of the observed reduced caries experience of people living in areas with fluoride in the water supply. This effect was observed and exploited before the mechanism of its action was fully understood and it is possible that, based on the suggestions of Jenkins referred to previously, biochemical or bacterio- logical tests would have been at least as appropriate as the chemical ones described. Any new caries inhibiting agents which may be proposed are likely to be based on knowledge derived from a study of the carious process. The point at which they are intended to attack this process will be known and so specific tests will be defined to study their efficiency. The laboratory assessment procedures will consequently be realistic so that, in the clinical trial which will still be necessary, these expensive and scarce facilities will be used to test only those materials likely to have a marked beneficial effect on dental health. ACKNOWLEDGEMENTS I should like to thank Mr. A. Saxton for the use of the electron micro- graphs, some of which are to be published shortly in another journal. I also gratefully acknowledge permission by the following, to reproduce illustrations-- Figure 3- The editor, Journal of Dental Research Figure 12-The editor, Advances in fluorine research and dental caries prevention Figure 15 - The editor, British Dental Journal. (Received: 20th February 1967)

THE LABORATORY EVALUATION OF PROPHYLACTIC DENTIFRICES 489 REFERENCES (1) Fosdick, L. S., J. Arner. Dent. Ass., 40 133 (1950). (2) Mansbridge, J. N. Brit. Dent. J., 109 848 (1960). (3) Manly, R. S. and I-lodge, H. C. J. Dent. Res., 18 138 (1989). (4) Manly, R. S. and Bibby, B. G. J. Dent. Res., 9.8 160 (1949). (5) U.S. Pat: No. 3,105,798. (6) Gray, J. A. J. Dent. Res.. 41 633 (1962). (7) Gray, J. A. J. Dent. Res., 44 493 (1965). (8) Gray, J. A., Schweizer, H. C., Rosevear, F. B. and Broge, R. W. J. Dent. Res., 87 638 (19S8). (9) Jenkins, C. N. and Spelts, R. L. J. Physiol., 19.1 21 (1953). (10) Brudevold, F., Gardner, D. E. and Smith, F. A. J. Dent. Res., 815 420 (1956). (11) Isaac, S., Brudevold, F., Smith, F. A. and Gardner, D. E. J. Dent. Res., 87 318 (1958). (12) Spiers, R. L. Brit. Dent. J., 107 209 (1959). (13) Weatherell, J. A. and Hargreaves, J. A. J. Dent. Res., 48 958 (1964). (14) Weatherell, J. A. and Hargreaves, J. A. Advances in fluorine research and dental caries prevention, 4 181 (1966). (15) MiJhlemann, H. R., Schutt, A. and K6nig, B[. G. Helv. Odont. Acta., 8 147 (1964). (16) Wharton, H. W. Anal. Chem., 34 1296 (1962). (17) Brudevold, F., Chilton, N. W. and Wellock, W. D. J. Oral Therapeutics and Pharrnacol. 1 1 (1964). (18) Walter, H. G. Helv. Odont. Acta., 9. 40 (1958). (19) MiJhlemann, H. R. and Wolgensinger, F. Helv. Odont. Acta., Ii 35 (1959). (20) Holmes, A. W. and Middleton, J. D. Brit. Dent. J., 113 380 (1962). (21) Morley, C .W. and Holems, A. W. Brit. Dent. J., 115 71 (1965). (22) Herd, J. K. and Overell, B. G. Brit. Dent. J., 117 286 (1964•. (23) Brudevold, F., McCann, H. G. and Gordon, P. Caries-Resistant Teeth. Ciba Foundation Symposium (1965). Churchill, London. (24) Jenkins, G. N. Brit. Dent. J., 119 535 (1965). (25) Von der Fehr, F. Acta. Odont. Scan&, 19 431 (1961). (26) Pigman, W:., Elliott, H. C. and Laffre, R. O. J. Dent. Res., 31 627 (1952). (27) Pigman, W. and .Newbrun, E. J. Dent. Res., 41 1304' (1962). (28) Francis, M.D. and Meckel, A. H., Arch. Oral Biol., 8 1 (1963). DISCUSSION MR. N.J. VAN ABBI•: You mentioned that you use pH measurements rather than phosphate determinations to assess in vivo tooth solubility. I wonder how far the spread of results that you have, which seemed to be unaccountable, was due to trying to measure pH values accurately on small quantities of weak acids. THE LECTURER: We are fairly satisfied that, with attention to detail, we can achieve a good degree of reproducibility in determining pH values. We do, of course, rely on our statistician to show that the differences reported are statistically significant. MR. N.J. VAN ABBr.: One has seen an occasional statement that the case hardening effect due to fluoridation makes it more difficult to recognise a cavity with the probe. Is there any suggestion that this can be true in the case of a fluoride toothpaste? THE LECTURER: The recognition of what is a carious lesion is a vexed one in the context of clinical trials and has been the subject of much learned discussion. The phenomenon of 'reversals' in clinical trials shows that the development and recording of lesions is not a simple matter. If the suggestion were true then the apparent benefit derived from fluoride toothpastes would be inflated.