To Lavoisier this double principle seemed self-evident. It was not a hypothesis that required testing, but a touchstone by which other hypotheses might safely be tested. If, in the course of some chemical operation, weight increased, then no further proof was required to show that mass had increased also, and that matter had been added. If, on the other hand, weight diminished, then no further proof was required to show that mass had diminished also, and that matter had been subtracted. Whatever other qualities matter might gain or lose, mass and gravity were indestructible and unchanging. Men of science seemed, on the whole, content silently to assume these principles of conservation without inconveniently raising the question of evidence. Philosophers have not always been so cautious. Kant supposed himself to have demonstrated them _a priori_. Schopenhauer followed suit. Spencer declared their contraries to be inconceivable. Mill said they were proved by experience. In short, all these eminent thinkers vied with each other in conferring upon this doctrine the highest honours permitted by their respective philosophies. But apparently they were hasty. Recent discoveries have changed our point of view. Mass (it seems) is no longer to be regarded as unchanging. When bodies move at speeds approaching the velocity of light their mass rapidly increases; so that this quality, which is peculiarly characteristic of matter, must be removed from the category of those which persist unchanged, and placed in the category of those which change but can always be restored. Are we so to class gravitation? Would the weight of a body moving nearly at the speed of light increase as, in like circumstances, its inertia increases? If the answer is “no,” then the link is broken which has for long been thought to connect gravity and mass. If the answer is “yes,” then what Kant regarded as certain _a priori_ is false; what Spencer regarded as “inconceivable” is true; another carrier of “persistence” is lost, and some fresh characteristic must be found which will remain unchanged through all time, and under all conditions. If this characteristic should turn out to be electric charge, what a curious light it will throw upon our tendency to “beliefs of conservation”[15]. After long seeking for some indestructible attribute of matter; after taking up and rejecting size, shape, weight, mass, and (perhaps) impenetrability, we shall at last find the object of our quest in a conception which has (I suppose) been clearly realised only within the last hundred years, about which our senses tell us nothing, and of which the general run of educated mankind are still completely ignorant! III