The densities of H2018 and D2018 were measured and the following values obtained : H2018, $O = 1.107845f0400033, and D2018, djo = 1.21393f040008. The melting points were found by measuring the time-temperatureauve of warming. The following values were obtained : HzOl8, tmp = 0.28 -l002"C and D 2 0 1 8 , tmp = 4.02 f0-02"C. The thermal expansions were measured in small dilatometers from the melting points to -80°C. The following values for the maximum densities were obtained: H2018, t m a = 4.305&0.02"C, dmm = 1*11255f0*00003 glml, and D 2 0 1 8 , tmax = 11-46-lO.O3"C, dmax = 1.21691 50.oooO8 g/ml. An empirical equation covered by all data for the thermal expansion of all water species from the melting points to 80°C, is the following : where T = t-tmax, F = 1 for H20 and H2018, F = 1-0555 for D20 and D2018. 331 * a, b and Vdi have been obtained from the calibration with mercury.

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A method has been developed for measuring small differences between the vapour pressures of two samples. V.p. differences were determined between water samples with a high concentration of oxygen-18 ( H 2 0 1 8 and D 2 0 1 8 ) and samples with a low concentration in the temperature range 15-90°C. Differences between the vapour pressures of water containing -55 % 0 1 7 and samples containing high concentrations of 0 1 6 and 0 1 8 respectively, in the range 40-90°C, have also been measured. Using these results, vapour pressure ratios between the pure isotopic species were calculated.* i.e. the ratio of vapour pressure of water containing 100 % oxygen-16 to that of labelled water.

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