In the procedure the progress of diffusion is determined by measuring the change in conductivity over time.

Vol. 60 the hydrogen ion in SiOH groups can be replaced by calcium ions in solutions of pH greater than five and less than thirteen (Fig. 7).4. The similarity between the surface areas determined by nitrogen adsorption and those found by sorption measurements assuming a /3-cristobalite structure indicates that the local order of amorphous silicas is that of 0-cristobalite. The lack of correlation between the surface areas of dried Ludox (measured by nitrogen adsorption) and the sol (determined by the sorption method) may be due to the change in orientation of the silica structure on dehydration.Additional study is necessary to establish the cause of the difference between the "bound" water content of opaline silica found in the sorption experiment and the content measured by the ignition methods (Table II).It might be suggested as an alternate mechanism that the calcium hydroxide in solution reacts with soluble monomeric Si(OH)4 (silicic acid), which goes into solution at a rate proportional to the sur-face areas and water contents. Two pieces of evidence refute this mechanism. First, the similarity of (x/m)max values at 30 and 82°is inconsistent with the difference in the solubility of silica at the two temperatures. Second, it frequently has been reported that silicic acid goes into solution slowly.10•12 Alexander, et al.,10 found that it was necessary to wait 20 days to reach the equilibrium concentration at room temperature.Acknowledgments.-The author wishes to acknowledge the assistance of Mr. J. Pellicane of this Laboratory in making the measurements reported in this study. Thanks are due to Dr. F. Pundsack for the well-characterized opaline silica •and the stimulating discussions on the nature of the surface of silica. The nitrogen adsorption measurements were made by Mr. G. Reimschussel and Miss M. Cronin of this Laboratory. The author also wishes to thank Dr. Stephen Brunauer (Portland Cement Association, Skokie, 111.) for his helpful comments on the manuscript.

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The flash photolysis of the charge-transfer band of 1-methylpyridinium and l-methylcollidinium iodide in degassed dichloromethane produced transient absorptions which were attributed to 12'-. Both systems showed permanent loss of parent compound and growth of 13-absorptions. The rate of decay of Iz-from l-methylcollidinium iodide fit second-order kinetics. A M solution of l-methylquinolinium iodide in dichloromethane gave the same second-order transient but appeared to be completely reversible. In deoxygenated acetone the k / e was observed to be ca. lo5 at 6800 A and deoreased with increasing iodide concentration.Solutions of 1-methylquinolinium iodide in l-propanol gave an additional transient absorption at ca. 5300 A

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