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It is generally agreed that fluorine is a constant component of enamel and dentin and that of all structures in the body, the teeth probably contain the largest amount of this element. In spite of efforts extending over the last 130 years no agreement has been reached as to the actual amount of fluorine in enamel and dentin. McClure (9) reviewed the literature of the period 1803-1933 and found the reported fluorine content of enamel, dentin and whole teeth to vary from zero to 1.4 per cent. Jacob and Reynolds (7), whose work was not reviewed by McClure, found whole teeth to contain 1.8 to 3.5 per cent fluorine. Of the older reports most credence has been placed in the results of Gautier and Clausmann (6) who found the fluorine content of enamel to vary between 0.118 and 0.172 per cent. It was McClure's opinion that the creditable literature indicated the presence of 0.1 to 0.2 per cent fluorine in enamel.The analyses made since McClure's review have indicated a lower fluorine content of teeth and teeth structures than had been found by the older investigators. The recent improvements in fluorine analytical methods have resulted in a downward trend of the determined amounts of fluorine in biological materials. Klemet (8) found enamel and dentin to possess an identical fluorine content, namely 0.05 per cent. Bowes and Murry (3) reported enamel to contain 0.02 per cent fluorine. They (4) later found mildly mottled teeth to contain 0.035 per cent fluorine in the enamel and 0.07 per cent in the dentin.

of Minnesota, Minneapolis That the susceptibility and immunity of teeth to caries is to be accounted for, at least in part, by differences in composition is a hypothesis that has long been attractive. The evidence for the claims (7, 8) that such differences exist have failed of confirmation. We (3) found enamel and dentin fractions prepared from sound and carious teeth to possess compositions, with respect to the calcium, phosphorus, magnesium, and carbonate contents, which were not significantly different. LeFevre and Hodge (9) found whole tooth inorganic substance prepared from carious specimens to differ from that obtained from sound teeth only with respect to a slightly higher moisture content of carious tooth substance.2 Without regard to the data to be presented below, there is evidence that marked changes in some of the properties of enamel accompany changes in its fluorine content. The amount of fluorine in mottled enamel is relatively slight (4) in comparison to the total inorganic material; yet it produces, or is accompanied by, profound changes in the appearance and properties of the enamel. In this connection it is worthy of mention that it has not been demonstrated that mottling of enamel is due entirely to the effect of fluorine combined in the inorganic phase and is possible that the condition is produced, in part, by the operation of the fluoride ion on enzymes during the calcification of the enamel. The effect of fluorine added to the diet on the quality of the teeth of rats first observed by Schulz and Lamp (13) and independently (10) has since been noted by numerous workers. The hardness of the incisor teeth, which in the rat grow continuously throughout life, is augmented by increasing, within certain limits, the amount of fluorine in the diet so that they are not worn at the usual rate and therefore become much longer than those of normally fed animals. Such teeth may be regarded as possessing some superior qualities even though they may not be adapted to an efficient use by the rat and in spite of the fact that the calcification of the enamel and dentin may be abnormal. Armstrong (1) fed an adequate diet to rats to which was added during alternate three week periods 0.1 per cent sodium fluoride. Alternating transverse bands of mottled and normal enamel were thus produced across the incisor teeth. These teeth after attaining a considerably increased length frequently fractured, but the fracture line was usually transversely situated through a band of normal enamel. It thus appears that the hardness and strength of the incisor teeth of rats is improved by a moderately increased fluorine content of the mineral phase or by some change produced in the structure of the tooth which occurs during the formation of the enamel and dentin under the influence of a higher than normal concentration of fluorine.The literature (11) contains occasional statements to the effect that moderately mottled human teeth are apparently more resistant to caries than normal teeth. Every dental clinician interviewed by u...

Routine analysis of enamel and dentin requires a simple method for the separation of these substances from each other and from cementum. Procedures commonly employed for the preparation of enamel, most of which utilize mechanical forces-grinding or desiccation, or combinations of these-usually give low yields, and the purity of the product is questionable. Rosebury, in connection with his study of the protein of enamel, indicated difficulties with available methods, and pointed out extreme precautions necessary to obtain a pure product.2 Our method takes advantage of the differences in the specific gravity of enamel and dentin. Finely pulverized whole-tooth is suspended in a liquid having a specific gravity intermediate between the two materials, causing the heavier enamel to sink and the lighter dentin (and cementum) to float on the surface. The specific gravity of dentin is 2.2-2.3; that of enamel, 2.7-2.8. In order to make the conditions optimal for the preparation of a pure-enamel fraction, even at the sacrifice of a small amount of this material in the dentin fraction, we employ a liquid having a density nearer that of enamel; namely, 2.53 at 250C. Technique. U.S.P. IX bromoform is washed four times, or until the washings are free from acid and bromide, and dried by triple filtration through dry paper. The product, stored in the dark, is not used without repurification after it is one week old. Of the purified bromoform, 50 cc. (144 gm.) mixed with 10 cc. of absolute alcohol give a liquid with the theoretical density of 2.53 at 250C. This solution, freshly prepared each day, is protected from light. To separate the enamel and dentin fractions, we use a specially designed funnel ( fig. 1, a), consisting of three parts-1 Presented at the twelfth general meeting