Phase transitions involving water play an important role in many natural and technological processes. Accurately predicting the nucleation rate of this important substance remains a difficult task. One way to improve the reliability of theoretical predictions is to develop an accurate base of experimental measurements. We have measured quantitative nucleation rates for both H2O and D2O in supersonic nozzles where characteristic nucleation rates ranging from 10~(14) < J / cm~(-3)s~(-1) < 10~(17). By applying the first and second nucleation theorems to the data we can directly estimate the number of molecules in the critical cluster n~* and the excess internal energy E_x(n~*), respectively. The agreement between these values and the classical values predicted assuming the critical cluster is a compact spherical object are really quite good even though under our conditions n~* is less than 10.
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