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The density p of two silicon crystals has been determined by precision hydrostatic weighing, based on very accurate solid density standards (SDS's). Densities of additional crystals were gained from density differences measured by the temperature-of-f1oatation method. The molar mass M of the crystals was determined by comparing the isotopic abundances against the known abundances of the National Bureau of Standards (NBS) reference material SRM 990 using activation analysis and mass spectroscopy. Combining the measured values for M and p leads to M / p = (12.058 822 ± 0.000 013) cnr' / mol. With the known volume of the unit cell the following value for the Avogadro constant was calculated: N A = (6.022 137 ± 0.000 007) x 10 23 mol-I.

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We present an ultra-precise flotation method (±0.1 ppm at 99% level-of-confidence) for density-of-water comparisons close to its temperature of maximum density. The method consists of observing two equilibrium temperatures of the given silica float: below and above the maximum density point. Discussion of measurement sensitivity and results for various water samples are given.