The effectiveness of fluorine in preventing dental caries has been demonstrated both in man' and The finding of a higher fluorine content in caries-resistant teeth4 indicates that this effect is the result of the incorporation of the fluorine into the teeth. Support for this is found in our unpublished work which shows a reduced acid solubility of fluorine-containing enamel and dentin from incisors of rats fed a high fluorine diet.The fact that fluorine combines actively with bone,5 calcium phosphate,' and hydr~xyapatite~ suggests that the fluorine may react similarly with the mineral phase of fully formed erupted teeth and give a reduction of solubility like that produced in the calcium phosphates.'The following experiments were designed to test this possibility.Human enamel and dentin powdered to pass a 100 mesh screen were separated and purified by the centrifugal-flotation method.' Five hundred milligram samples of enamel were shaken for periods of one hour in 250 cc of solutions containing one part of sodium fluoride in 25, 100, lo00 and 10, OOO parts of water, and then thoroughly washed in distilled water. Samples of dentin were similarly treated with 1/1OOO sodium fluoride solutions. Solubilities of these and untreated control samples were compared by measuring the respective weight losses of 50 nig samples after one hour in 20 cc of a 0.2 M acetic acid/sodium acetate buffer at pH 4.0.'The results, summarized in Fig. 1, show reduced solubilities in all the treated samples. Of enamel samples washed with 1/25, 1/10,

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