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Hydroxylapatite is the stable orthophosphate of calcium in neutral and alkaline media. Its preparation in pure form is difficult, since with the customary precipitation reaction the monohydrogen phosphate ion always forms according to the reaction: 5Ca++ + 4P043-+ HzO % Ca5(OH)(P04)3 + HP04--Therefore, tetracalcium monohydrogen triphosphate ("octaphosphate"), Ca4H(P04) 3-2H20, which displays a structure similar to apatite, readily forms, solely or a t least partially, in the place of hydroxy1apatite.l Such precipitates change to the P-tricalcium phosphate upon calcination. I n the preparation of a uniformly crystalline product one must perform a tedious boiling-out process2 or the hydrothermal synthesis;3 either of these methods, however, accommodates only a small lot (ca. 70 mg., crystal size 0.3 mm.) or else the preparation has a small sodium content4 (0.5% from 2 g. of material and up to 2-mm. crystal size). In either case, considerable apparatus and expenditure of time are required.These difficulties can be overcome, for example, by carrying out the precipitation a t normal temperature in a medium of sufficiently high pH so that the Po43concentration is not exceeded by that of HP04-and, moreover, by

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The application of laser rays for caries prevention as for restorative purposes in oral rehabilitation causes severe changes in the structural and textural assembly of the dental hard tissue. Electron microscopical and X-ray investigations of the new calcium phosphate phases which arise demonstrate a deterioration of the affected tooth regions.