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The RbF-ZrF4 and LiF-ZrF4 Systems 1411 geneous nucleation show that the nucleation process in these experiments is not homogeneous.The plots of maximum waiting time against 1/log2 (X/Xo) (Fig. 2) are found to be straight lines showing that log is proportional to 1/log2 (X/Xa) in the ranges of concentrations in which the kinetics is conveniently measurable. II show that the change of container walls has considerable effect on waiting time, which in turn indicates that the precipitation has been occurring at the glass (Pyrex) walls of the' cell (may be on electrodes too) by heterogeneous nucleation, the rate of which depends upon the nature of the surface, solvent and solute together with the degree of supersaturation and temperature. Results of TableAnyhow it cannot be said unequivocally that the observations are conditioned solely fjy a nucleation process. It was not possible to give any experimental evidence by the present method that the results may not be the consequence of growth process which follows nucleation.Acknowledgment.-We express our sincere thanks to Prof. G. G. Brown of Michigan University and Prof. G. W. Preckshot of University of Minnesota for sending us valuable information about their nucleation apparatus. We are grateful at the same time to Dr.

The rate law for the oxidation of zirconium was found to be initially intermediate to linear and parabolic, then parabolic, and then nearly cubic over the first 10-rain interval at both 400 ~ and 500~ At 400~ the oxidation rate became parabolic for an oxide thickness of about 300A. At 500~ the oxidation rate became nearly cubic at about 3300A, the transition from one rate to another being gradual rather than sharp. The activation energy for the parabolic region was 29.3 kcal/mole and for the nearly cubic region approximately 42 kcal/mole. The results obtained were independent of the shape of the specimen and its surface preparation. An analysis shows that the high solubility of oxygen in zirconium is not a sufficient condition to account for a cubic rate law of oxidation for a concentration-independent diffusion constant.

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