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.

The absolute viscosity of highly enriched heavy-oxygen water, HZ1'0, has been measured at 5°C intervals between 15 and 35"C, using a semi-automatic closed viscometer of the suspended-level type.A linear extrapolation was performed to determine the viscosity of 100 % heavy-oxygen water, based upon the measured viscosity of natural water and that of a highly enriched heavy-oxygen water sample. The effect of isotope substitution on the viscosity of natural water has been analyzed for both HZ1'0 and DZ160. The viscosity ratio at 25°C for Hz180/Hz160 is 1.0536, whereas that for Dz160/Hz160 is 1.2316.The viscosity of the different isotopic species of water may be useful in the elucidation of the structure of the liquid state. The effect of isotope substitution on liquid viscosity has been investigated almost exclusively for the case of the replacement of hydrogen by deuterium,l e.g., deuteroalcohols, deuterium substituted benzene and nitromethane. The viscosity of D2160 has been measured several times,2 most recently by Millero, et aL3 To the best of our knowledge, however, there is no report on the measurement of the viscosity of heavy-oxygen water.*The isotope effect may be analyzed by considering the relation of the viscosity to the molecular weight, molar volume and energy required to break the intermolecular bonds (van der Waals and hydrogen bonds), i.e., the energy of activation for viscous flow. During the course of the discussion, we will attempt to analyze the results using this approach. EXPERIMENTAL MATERIALSThe heavy-oxygen water sample (94.41 % l80) used in the experiments was produced in our Isotope Separation Plant. The water, produced initially as D2180, was converted (normalized) to H2180 as described by Steckel and S ~a p i r o . ~The purification of the H2180 sample was performed under high vacuum in order to prevent isotope exchange. The sample was distilled into a vessel containing some sodium in a glass tube.5 After the sodium had reacted, the solution was degassed and bromine was distilled into the reaction vessel. The resulting solution was kept a few days at room temperature. The purification procedure was completed with three non-ebullioscopic vacuum distillations of the sample from a vessel at room temperature to a vessel cooled in ice.* Since submission of the manuscript we have discovered a paper, I. B. Rabinovich and V. G .

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.

The absolute viscosity of four oxygen-17 water samples is reported at 5°C intervals between 15 and 35°C. The absolute viscosity of 100 % H2170 is determined by extrapolation of the measured viscosity of a highly enriched oxygen-17 sample, i.e., 86.53 % 1 7 0 . The values calculated, based upon Eyring's theory of viscosity, for the other oxygen-17 samples are in excellent agreement with the experimental results. The isotope effect has been analyzed for H2"0 relative to natural water. The viscosity ratio at 25°C for H2170/H2'60 is 1.0226, whereas that for H2180/H2'60 and D2'"0/ H2l6O is 1.0536 and 1.23 16, respectively.

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