I?. E. EBERLY, JR.
812
k b , as compared with ka and k,. Higher pressure induces a faster exchange between the vapor phase and the bridged hydroxyls. From both sets of experiments, either under constant pressure and increasing temperature or at constant temperature and increasing pressure, it appears clearly that exchange processes can be summarized as follows. Below 180' and under DzO vapor pressures ranging between 4 and 16 mm., the more probable model is
vapor phase
-+
j s o l a t e d hydroxyls bridged hydroxyls (a box) (b box) ladsorbed water (c box)
Above 180°, under a DzO vapor pressure of 4.5 mm., the a and b boxes are interverted. At 100'
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and under a DSO vapor pressure of 2 mm., the exchange initiates from the c compartment. It would have been expected that a t low temperature and high pressure, the physically adsorbed water (c) would be connected with the gas phase and the other two compartments would interact with it. The privileged situation of the bridged hydroxyls derives probably from the fact that they constitute the main absorption sites, according to the views expressed by Siderov14K i ~ e I e v , ~and J others. Acknowledgments.-We wish to thank Dr. W. I
