“It will help us to picture the world of atoms if we have, to begin with, some idea of the size of these units. Let us begin with a gramme of hydrogen (1/453 of a pound), which is not a very large quantity. How many atoms will it contain? If the atoms were made up into bundles of a million-million, and then into a million-million of these bundles, we should have about a gramme and a half of hydrogen. That is to say, the weight of one atom of hydrogen is about a million-millionth of a million-millionth of a gramme and a half. Other atoms weigh more than the atom of hydrogen, but not enormously more; an atom of oxygen weighs sixteen times as much, an atom of lead rather more than 200 times as much. _Per contra_, an electron weighs very much less than a hydrogen atom; it takes about 1,850 electrons to weigh as much as one hydrogen atom.” ELECTRONS The inner rings of electrons give rise to X-rays when they are disturbed, and it is chiefly by means of X-rays that their constitution is studied. The nucleus itself is the source of radio-activity.... The most complex atom known is that of uranium, which has, in its normal state, 92 electrons revolving round the nucleus, while the nucleus itself probably consists of 238 hydrogen nuclei and 146 electrons.... Under normal conditions, when the hydrogen atom is unelectrified, the electron simply continues to go round and round the nucleus, just as the earth continues to go round and round the sun. The electron may move in any one of a certain set of orbits, some larger, some smaller, some circular, some elliptical. But when the atom is undisturbed, it has a preference for the smallest of the circular orbits, in which the distance between the nucleus and the electron is about half a hundred-millionth of a centimetre. It goes round in this tiny orbit with very great rapidity; in fact its velocity is about a hundred-and-thirty-fourth of the velocity of light, which is 186,000 miles a second. Thus the electron manages to cover about 1,400 miles in every second. To do this, it has to go round its tiny orbit about seven thousand million times in a millionth of a second; that is to say, in a millionth of a second it has to live through about seven thousand million of its “years”!