Abstract
Recent speculation and experiment on the mechanism of isothermal chemical change has led to the realization that we are aware of no truly unimolecular process in the gaseous state. Where a gas has been found to decompose in accordance with the first-order law, further experiment has shown that the action is dependent on the intervention of a second species of molecule, which may be a trace of a foreign substance or the material of the vessel-walls or—as in the case of nitrogen pentoxide—the molecules formed by the dissociation. From this it is but a short step to the conviction—which is being forced on us by other considerations—that the conjunction of two molecules is the first step in all chemical reactions. The facts in regard to reactions in liquids are less certain, for numerous apparently unimolecular changes in solution are known. Now we know that any molecule in a liquid is much subject to the attractions of its neighbours. It is not, therefore, surprising to find that the velocity of these first-order changes (as of others) is, as a rule, largely influenced by the solvent in which they proceed. Little, however, is known of unimolecular liquid reactions where no solvent medium is used. The present research was an attempt, in the first place, to see how far a reaction which proceeded in a pure liquid could be regarded as unimolecular in the strict sense. It was then hoped to use such a reaction to investigate the energy exchanges in chemical reaction in the light of recent speculation, for in a first-order process conditions are likely to be simplest.

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