energy, and so of greater diameter, the indeterminateness gradually assumes a different form, and finally becomes of but little importance. Whatever form the electron may assume while it is describing a little orbit near the nucleus, by the time it is describing a very big orbit far out it has become a plain material particle charged with electricity. Thus, whatever the reason may be, electrons which are describing orbits in the same atom must all be in different orbits. The electrons in their orbits are like men on a ladder; just as no two men can stand on the same rung, so no two electrons can ever follow one another round in the same orbit. The neon atom, for instance, with 10 electrons, is in its normal state of lowest energy when its 10 electrons each occupy one of the 10 orbits whose energy is lowest. For reasons which the quantum theory has at last succeeded in elucidating, there are, in every atom, two orbits in which the energy is equal and lower than in any other orbit. After this come eight orbits of equal but substantially higher energy, then 18 orbits of equal but still higher energy, and so on. As the electrons in each of these various groups of orbits all have equal energy, they are commonly spoken of, in a graphic but misleading phraseology, as rings of electrons. They are designated the _K_-ring, the _L_-ring, the _M_-ring and so on. The _K_-ring, which is nearest to the nucleus, has room for two electrons only. Any further electrons are pushed out into the _L_-ring, which has room for eight electrons, all describing orbits which are different but of equal energy. If still more electrons remain to be accommodated they must go into the _M_-ring and so on. In their normal states, the hydrogen atom has one electron in its _K_-ring, while the helium atom has two, the _L_, _M_, and higher rings being unoccupied. The atom of next higher complexity, the lithium atom, has three electrons, and as only two can be accommodated in its _K_-ring, one has to wander round in the outer spaces of the _L_-ring. In beryllium with four electrons, two are driven out into the _L_-ring. And so it goes on, until we reach neon with 10 electrons, by which time the _L_-ring as well as the inner _K_-ring is full up. In the next atom, sodium, one of the 11 electrons is driven out into the still more remote _M_-ring, and so on. Provided the electrons are not being excited by radiation or other stimulus, each atom sinks in time to a state in which its electrons are occupying its orbits of lowest energy, one in each.