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The adiabatic (London) theory of reaction rates has been applied, for the first time, to reactions involving four atoms. It shows, unambiguously, that the thermal reaction of hydrogen with iodine will involve only iodine molecules, while the other halogens will react with hydrogen by way of the atoms. It shows further that the conversion of para to ortho hydrogen will involve an atom and a molecule rather than two molecules. It explains the well-known greater reactivity of the heavy halogen compounds over the lighter ones. These checks with experiment are independent of the ratio of coulombic to total binding except in the case of bromine. If the known theoretical ratio of coulombic to total binding for hydrogen is assumed to hold for the other atoms, approximate agreement with experiment is obtained. To obtain exact agreement in the reaction EL + L = 2HI the coulombic binding must be assumed to be only three and a half per cent, of the total binding. The theoretical evaluation of the coulombic binding is possible, but difficult, using the approximate eigenfunctions of

comparison. In presenting what seems to us the more important results of both experiment and theory we will include data obtained by the chemist, the physicist, and the spectroscopist, since all these fields must eventually be unified by an acceptable theory. None of the concepts involved in the two latter fields is as abstruse as may be supposed; their meaning is

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