? Figure l. Apparatus used to determine the solubility of intact tooth enamel. Facing page 476

THE LABORATORY EVALUATION OF PROPHYLACTIC DENTIFRICES 477 immersed. The pieces of enamel are attached to the ends of the stirrers with blue inlay wax, only the required area of sound enamel being exposed. The results obtained with the equipment are shown in Fig. 2. The upper curve is that for untreated enamel while the lower one, indicating a lower solubility rate, is for enamel treated with a 0.1% solution of stannous fluoride, The third line represents the effect of a stannous fluoride tooth- paste. A slightly different technique, also mentioned by Gray, is that of immersion of a tooth or piece of tooth in successive aliquots of acid buffer. Fig. $ is reproduced from Gray's paper and it is apparent that the protection provided by SnF 2 survived only the first two exposures to acid. 2.0 1.6 • o.8 ra 0.4 SOUND ENAMEL UNTREATED SnF 2 NoF Figure 3. 0 4 8 12 16 TIME (MINUTES) Solubility of untreated and treated tooth enamel (successive immersion in acid). Thereafter the slopes of this and the control sample become parallel, indicating that the rates of solution are the same and that the protective effect of SnF 2 is confined to the surface of the enamel.