What physics has to say about matter, and the physical world generally, from the standpoint of the philosopher, comes under two main heads: first, the structure of the atom; secondly, the theory of relativity. The former was, until recently, the less revolutionary philosophically, though the more revolutionary in physics. Until 1925, theories of the structure of the atom were based upon the old conception of matter as indestructible substance, although this was already regarded as no more than a convenience. Now, owing chiefly to two German physicists, Heisenberg and Schrödinger, the last vestiges of the old solid atom have melted away, and matter has become as ghostly as anything in a spiritualist seance. But before tackling these newer views, it is necessary to understand the much simpler theory which they have displaced. This theory does not, except here and there, take account of the new doctrines on fundamentals that have been introduced by Einstein, and it is much easier to understand than relativity. It explains so much of the facts that, whatever may happen, it must remain a stepping-stone to a complete theory of the structure of the atom; indeed, the newer theories have grown directly out of it, and could hardly have arisen in any other way. We must therefore spend a little time in giving a bare outline, which is the less to be regretted as the theory is in itself fascinating. The theory that matter consists of “atoms”, _i.e._ of little bits that cannot be divided, is due to the Greeks, but with them it was only a speculation. The evidence for what is called the atomic theory was derived from chemistry, and the theory itself, in its nineteenth-century form, was mainly due to Dalton. It was found that there were a number of “elements”, and that other substances were compounds of these elements. Compound substances were found to be composed of “molecules”, each molecule being composed of “atoms” of one substance combined with “atoms” of another or of the same. A molecule of water consists of two atoms of hydrogen and one atom of oxygen; they can be separated by electrolysis. It was supposed, until radio-activity was discovered, that atoms were indestructible and unchangeable. Substances which were not compounds were called “elements”. The Russian chemist Mendeleev discovered that the elements can be arranged in a series by means of progressive changes in their properties; in his time, there were gaps in this series, but most of them have since been filled by the discovery of new elements. If all the gaps were filled, there would be 92 elements; actually the number known is 87, or, including three about which there is still some doubt, 90. The place of an element in this series is called its “atomic number”. Hydrogen is the first, and has the atomic number 1; helium is the second, and has the atomic number 2; uranium is the last, and has the atomic number 92. Perhaps in the stars there are elements with higher atomic numbers, but