CLINICAL tests have shown that topical application of fluorides to erupted teeth will reduce dental caries.1 Stannous fluoride has been found to be more effective than sodium fluoride in reducing decay when applied topically.2, 3,4 In an attempt to elucidate the mechanisms by which the fluoride exerts its protective action, extensive studies5' 6, 7 have been made of the effects of inorganic salts on rate of solution of enamel in acid solutions. Since many salts, and particularly the fluorides, reduce the solution rate, studies of the nature of the chemical and physical changes which occur in the enamel have been made using electron microscopy and x-ray and electron diffraction.8-13 It has been shown that treatment of enamel with sodium fluoride results in the formation of calcium fluoride. The diffraction studies have shown no such changes after treatment with stannous fluoride, but have suggested the presence on the enamel surface of an amorphous layer of unknown composition. The prior work does not include a comparative study of the visible effects of the reagents on the structure and properties of intact enamel surfaces.The present work shows the effects on whole enamel surfaces of treatments with aqueous solutions of stannous fluoride, sodium fluoride, and nonfluoride stannous salts, over a range of pH values. In addition, the effects resulting from the exposure of these treated surfaces to acid buffer solutions are described. EXPERIMENTAL METHODThe teeth used in this investigation were human maxillary first incisors which were stored in a 0.5 per cent formaldehyde solution. No prior history of the teeth was available. Rectangular pieces of whole enamel from these teeth were cut from the labial surface. The area of the enamel surface varied between 4 and 16 sq. mm., and generally it was possible to cut 9 such specimens from a single tooth. The cutting was done with a circular jeweler's slotting high-speed steel saw operated at low speeds under water to prevent heating and drying of the tooth.

Fifteen abutment teeth with pathologic mobility were splinted with removable telescoped bridges. Mobility was measured at the time the splints were placed and at 1‐ and 12‐month intervals. Average mobility of the abutment and control teeth did not change significantly during the experimental period.