atoms a and a' on the one hand, and b and b' on the other, attract each other at atomic distance, and that so intimate a union would not be easily broken down by external attractions acting at the comparatively great distances which separate the molecules. It is, therefore, probable that the status quo would be maintained inde- finitely if it were only necessary to take into account the action of the affinities. But the kinetic theory teaches us that the material particles are not in a state of rest, but that the molecules move about, and that the atoms them- selves vibrate around a position of equilibrium ; and from this we may assume that the meeting of two heterogeneous molecules may be accompanied by certain circumstances which are favourable to the establishment of new unions. This would be so if the contact * of these molecules took place at the two atoms which are predisposed to unite, and at the moment when the oscillation phase of the atoms was greatest and the atoms were thus thrown towards the exterior of the molecules. Under these circumstances the extra-molecular attraction (of b for a, for example) would become preponderating and would bring about the double exchange. But if the possibility of a chemical reaction depends on the atomic and molecular movements, it must also depend on the temperature, for those movements are functions of this.
These explanations will suffice to show that the coeffi- cient & is a constant which refers to the nature of the sub- stances brought together, to the temperature, and to the other physical conditions of the reaction.
We have now to interpret the rdle of the active masses. If one gram-equivalent of each of the substances aa' and bb' be dissolved in 1 litre of solution, there will be a certain number of molecular collisions per second. Only collisions
��1 The word contact must not be taken in a too literal sense. A more or less close approach of the two heterogeneous molecules is rather to be understood.
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