activity was discovered, that atoms were indestructible and unchangeable. Substances which were not compounds were called "elements". The Russian chemist Mendeléev 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 so far none have been actually observed.
The discovery of radio-activity necessitated new views as to "atoms". It was found that an atom of one radio- active element can break up into an atom of another element and an atom of helium, and that there is also another way in which it can change. It was found also that there can be different elements having the same place in the series; these are called "isotopes". For example, when radium disintegrates it gives rise, in the end, to a kind of lead, but this is somewhat different from the lead found in lead-mines. A great many "elements" have been shown by Dr. F. W. Aston to be really mixtures of isotopes, which can be sorted out by ingenious methods. All this, but more especially the transmutation of elements in radio-activity, led to the conclusion that what had been called "atoms" were really complex structures, which could change into atoms of a different sort by losing a part. After various attempts to imagine the structure of an atom, physicists were led to accept the view of Sir Ernest Rutherford, which was further developed by Niels Bohr.
In this theory, which, in spite of recent developments, remains substantially correct, all matter is composed of two sorts of units, electrons and protons. All electrons are
