DEVELOPMENT OF MACHINELESS PERMANENT WAVING 189 velope. d the first adoption of the conventional Bishinger croquignole clip to a self-pad holding clip. Evans U.S. Patent No. 1,927,544 was an even greater improvement which was followed by Koehler's U.S. 2,261,163. All of the above are merely the high spots in the development of the machineless art, but hand in hand with this development went the commercial acceptance of machine- less permanent waving. Before January, 1932, may be considered the zero figure commercially for machineless waving although a few pads were sold. From this zero start and battling the already firmly and highly developed elec- trical waving, the machineless pads had reached at the start of the war an estimated sale of four to five hundred million pads per year and represented about 50 to 60 per cent of the total waves given in the permanent waving industry. Even today as the intensity of scientific investigation leads away from thermochemistry to advanced organic chemistry, machineless waving is still holding its total gross volume of pads sold annually, here and abroad.

FLUORINE AND DENTAL CARIES* B.y T. H. /Issociate Director of Research, Lever Brothers Co., Cambridge, Mass. O•E Or T•E most fertile fields of dental research has been the study of the role played by fluorine in dental health. No brief summary can pretend to cover the field exhaustively. FLUORINE IN TOOTH ENAMEI. The presence of fluorine in human tooth enamel was reported as early as 1805 by Gay-Lussac and Ber- thoiler (31). Crichton-Browne (20) in 1892 thought "it well worthy of consideration whether the reintro- duction into our diet... of a supply of fluorine in some suitable natural form... might not do something to fortify the teeth .... " Such early references were, per- haps, prophetic, but cannot be credited as originating the current scientific consideration. The current knowledge of the action of fluorine stems from a study of mottled tooth enamel, a disfiguring dental defect. Recognition of fluorine as the agent in production of mottling of enamel sprang, in 1931, from the * Presented at the May 13, 1947, Meeting, New York City. work of Churchill (17) and of Smith, Lantz, and Smith (52). Later studies proved that mottled enamel is endemic in nature, and directly related to the concentration of fluorine in the drinking water consumed by children during tooth- formation years. Dean (22) has shown that fluorine at 1 ppm. in the drinking water produces mottled enamel of the mildest form in about 10 per cent of children, while 4 ppm. produces more definite mot- tling in 90 per cent of cases. Black and McKay (11) had re- ported, as early as 1916, that the incidence of caries was less in mottled teeth than in normal teeth. Cox (18) was apparently the first, specifically, to suggest (in 1937) a beneficial role for fluorine in pre- venting caries. Within limits, there appears to be a distinct relationship between fluorosis and caries, and in areas of edemic fluorosis there is an inverse relationship, caries de- creasing as fluorosis increases. This evidence is supported by reports 'that the fluorine content of sound teeth is higher than that of carious teeth (3). 100