192 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of the •lmerican Dental •lssociation (1) which recognizes the probable effectiveness of such procedure but warns of potential dangers. Armstrong was reported in a newspaper article in November, 1940, as believing that topical application of fluoride would prevent caries. Bibby (10) reports on a new technique by which dental prophy- laxis is conducted with 1 per cent NaF, H202, and pumice, and he claims that two prophylaxes will reduce caries by 25 per cent while three prophylaxes will reduce caries by 43 per cent. Cheyne (15, 16) found that topical applications of solutions of from 500 to 60,000 ppm. of fluorine were useful in reducing carious activity in existing lesions and d•- creased the formation of n•w lesions. Jordan (38) and Knutson (40, 41, 42) reported that a solution of 2 per cent NaF, applied topically, would prevent new carious areas, but would not stop the carious process if previously initiated. An editorial in the British Medical yourhal (29) and a review by Hodge (34) both acknowledge the effectiveness of topical application, while further work by several other workers verifies the effectiveness of local application. These results all lead to a con- clusion that the topical application of fluoride has some action in pre- venting caries. This conclusion cannot be recorded at this point, however, without adding that in the writer's opinion, all the positive evidence indicates that repeated applications of preparations of very high fluoride content are required, and that there is, as yet, no con- vincing evidence that any signifi- cant value can be expected from the addition of reasonably low fluoride concentrations to denti- frices. Before entering upon a further discussion of the mechanism of fluoride action it is necessary to outline, as a working hypothesis, a mechanism of caries itself. Caries is regarded as a condition attacking the tooth from an outside surf act. The original attack of caries upon tooth structure is regarded by many as a simple chemical attack by acids. In order for the acids to produce the typical localized attack it is assumed that they form, and are concentrated within a dental plaque•an adhering, discrete sur- face deposit or a food particle wedged into a narrow space such as a crevice or an interproximal space, which holds the acids in situ in spite of rinsing of the surface "by water or saliva. The acids are presumably formed by living organisms acting within the plaque or particle on carbo- hydrates originating in foods. The organisms are acidogenic in nature and comprise one or more of the group of Bacillus acidolphilus, acidogenic Streptococci, yeasts, and molds. The carbohydrate substrate is typified by sugar, but other sub- stances may also be involved.

FLUORINE AND DENTAL CARIES 193 The reactions leading to carious attack can be represented as re- quiring all of the following con- tributing factors: / 1. Fermentable carbohydrate. 2. Acidogenic organism. 3. Plaque or static particle. 4. Susceptible tooth surface. Thus, if the chain of reactions were completely broken at any one point, caries could be prevented. It is practically impossible, of course, completely to avoid any one of the four contributing factors. There is, however, tenable evidence that caries can be reduced by at least four methods: 1. Reducing fermentable carbo- hydrate in diet. 2. Reducing the organisms in the mouth, •s by the use of germicidal agents or other agents inhibiting the fer- menting reaction. 3. Cleaning the teeth thoroughly to remove all food particles or plaques. 4. Increasing the resistance of the tooth surface to acid attack. Atkins (6), Dean (24), Jay (36, 37) and Miller (49) have reported that fluorine action is associated with low oral bacterial counts. Lipman (44) has stated that low concentrations of fluorine do inter- fere with fermentation of carbo- hydrate and Bibby (7) has found that fluorine in concentration of less than 1 ppm. reduces acid pro- duction by acidogenic bacteria. Atkins (6), Hodge (33) and McClure (47) have suggested that fluorine may decrease caries by interfering with the acidogenic process. In so far as fluorine may act within the mouth rather than by becoming an integral part of the tooth structure, its action could have little claim to specificity or natural physiological significance. Many other substances have been shown to influence caries by such action in the oral cavity, including penicillin, vitamin K, oxalate, am- monia, iodoacetic acid, and various germicides. The deposition of fluorine in tooth structure by the pre-eruptive consumption of fluoride-containing water can be accepted as fully established. Evidence that this is the important mechanism of fluoride caries prevention has been offered by Armstrong (4), Cox (18), Dean (23) and Irving (35). Of course, many workers who have claimed results from the post- eruptive topical application visual- ize the mechanism as absorption of fluorine by the tooth. The effect of fluorine in increasing the caries resistance of teeth appears to most workers to be accomplished by means of increasing the re- sistance of such enamel to attack by acids. Thus it has been reported that the enamel of fluorosed teeth is less acid soluble than that of normal teeth (33, 56). The treatment of ground enamel by topical application of fluoride