hold of Riemann's idea of space as a manifold. As this is wholly extra-physical and not in my realm I shall do no more than mention it.
Thus the phenomenon of gravitation remains a mystery; for so far every hypothesis made seems to have insurmountable difficulties. I am not sure that any of them shed light enough to even convince us that we are on the right track. It seems to have little or nothing in common with various other things of which we have some knowledge. A few years ago when radio-active substances were discovered, we found phenomena which at first did not seem to agree with well-known chemical and physical laws; but as experimentation progressed confirmation became more general and to-day many, if not all, the discrepancies have disappeared. Not so with gravitation. It still remains one of the least understood properties of matter. Probably if we could learn something of the mechanism of gravitation, that attraction between particles which only manifests itself at very small distances (cohesion) might be better understood.
Nearly two hundred and fifty years ago one of the greatest intellects connected with science turned his attention to gravitation. In that two hundred and fifty years physical science has made rapid advances. A boy who has completed a year's work in elementary physics could entertain Newton in electricity were it possible for the great philosopher to return to earth. After learning of the great progress in electricity, I can imagine him in his eager desire for knowledge turning to the boy and expecting some light on gravitation. Alas, not only the high school boy, but not even the most learned can give any definite information on gravitation. The problem is about where Newton left it.