that losses or gains of mass or weight must occur in the formation of the heavier atoms. But we know that the ratio of mass to weight is the same for all substances, from hydrogen, the lightest, up to uranium, the heaviest, and even, as Southern's experiments on uranium and my own on radium have shown, for radio-active substances. Now in the formation of the heavier atoms alterations in mass must have occurred; in spite of this the ratio of weight to mass has not been altered. As enormous changes in energy are involved in changes of mass of the size we are considering—far greater than any we can produce by processes we can use in the laboratory—this is about the severest conceivable test to which we can put the constancy of the ratio of mass to weight; that it can stand it is a result of fundamental importance in the theory of gravitation.
We may ask, does this remarkable constancy in the ratio of mass to weight, which holds in the case of all known atoms, hold also for the very much smaller particles, the electrons? Have these minute negatively electrified bodies any weight at all, or is, as might be expected on one of the electrical theories of gravitation, their weight abnormally large in comparison with their mass? It is perhaps beyond our powers to weigh these particles, but it is not so hopelessly beyond but that, with the improvements in technique which we may reasonably expect as the result of experience, we may entertain hopes of being able to do so before very many years have elapsed.
In the case of the lighter elements, where the changes in mass accompanying the formation of the atom may reasonably be expected to be small, we may take the nearest integer to represent what the mass would have been if there had been no change on aggregation. Taking