The mole fraction scale gives simpler (i.e., more nearly linear) relations between the variables determining kinetics on changing solvents for the bromacetatehyphen;thiosulphate reaction than does the usual volume molar scale. The generality and basis of this finding is discussed from the viewpoint of Rabinowitch's cage model theory as well as from that of equilibrium theory. The quenching of fluorescence, which involves diffusion as a rate determining step, is a more nearly linear function of the fluidity when the data are expressed in mole fraction than in the volume molar scale. Experiments are in progress to determine, if possible, whether the cage model or a modified Smoluchowski diffusion treatment fits the facts better for low activation energy reactions like the quenching of fluorescence in solution.
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