stance? Prout, an Englishman, had put forward the view in 1816 that the atom of hydrogen was the fundamental unit of the structure, but this did not accord with the experimental values of the relative weights of the atoms of different elements; the more careful the experimental work, indeed, the greater was the discord. The hypothesis was, however, interesting because it was simple and even courageous, for few would have adventured any theory on the meagre data then available to speculate upon.

Although Prout was wrong about hydrogen, we now know that he was just wrong enough not to be right. His mistake appears hardly more than that he said 'half a crown' when he ought to have said 'two shillings and sixpence,' though indeed it was actually much more fundamental than that. For the sole constituents of all atoms are believed, at the present time, to be but two. One is the electron, a mobile electrified unit of mass, minute compared with that of the lightest atom; the other is the proton, the entity which remains when the atom of hydrogen has been deprived of the single electron with which it is endowed by Nature. If Prout had said, not that hydrogen was the common material of matter, but that the only constituents of hydrogen, the electron and the proton, were, he would have been right. But, of course, he had not the slightest inkling of such a thing.

The discovery of the independent existence of the electron was made by Sir J. J. Thomson in 1897. This was a discovery of great importance, for it was soon realised that the electron might be the common unit in the structure of atoms indicated by the periodic system of classification. It was soon shown to be a constituent of all forms of matter, and its vibrations within the atom were regarded as the cause of the spectrum lines emitted by glowing bodies. But its real interest lay in the fact that it gave hope of a successful attack on one of the most fundamental of scientific problems-the detailed structure of the atom.

It turned out, however, that the electron is not the only constituent of the atom. The existence of the other constituent, the proton, was revealed by the study of radio-activity, but only slowly; and it was not till 1913

that the exact rôle played by it was revealed. But in the meantime a flood of light was thrown upon the structure of the atom by the work of Rutherford and his co-workers on the radio-activity of the heaviest chemical elements-the radio-elements.

A radio-element is one which emits spontaneously aor B-particles. The first of these is a particle which is equal in weight to four hydrogen atoms, and is expelled from the centre of the atom with a velocity of about 10,000 miles per second. This almost incredible velocity is surpassed by that of the B-particle, another name for the electron, a disembodied charge of electricity with a weight, if you can apply such a property to such a thing, of only 10 of that of hydrogen, the lightest atom. The B-particle's velocity may be from 50,000 miles to nearly 186,000 miles per second, which is the greatest velocity known to science. These atomic explosions involve energies which are gigantic compared with those to which we are accustomed in everyday life, and they have interesting consequences; but their importance is chiefly that they afford the investigator a glimpse of Nature's workings and allow him to observe though, as it happens, not to control in any way, changes occurring in the core of the atom.

A radio-element differs from one which is not radioactive only in that in a given interval of time, say a second, a definite fraction of the total number of its atoms happen to disintegrate. Each disintegrating atom expels either an a- or a 3-particle and becomes, in consequence, an atom different from what it was prior to the act of disintegration, or, what is the same thing, from an atom that has not disintegrated. A radio-element consequently contains always two kinds of atoms, those that have disintegrated, and those that have not. Those that are unchanged comprise the rare radio-element which is known as the parent; those that have disintegrated comprise a new element known as the product. The product is perfectly distinct from its parent in physical and chemical properties, and can be easily separated from it by the ordinary methods of analytical chemistry. If now the product happens, like its parent, to be radio-active, a certain fraction of it will disintegrate per second to form its product, a third substance, and

this body, if radio-active, will produce a fourth, the fourth a fifth, and so on, till a body is reached which has not the power of distintegrating, when the series of elements abruptly ends. Such a series is called a disintegration series, and three of these are known at the present time. In a disintegration series each element but the last is the parent of the one that follows, and, except the first, the product of the one that precedes. The first body, the head of the series, is called a primary radio-element, or sometimes the original parent. Uranium and thorium are two of the three primary radio-elements.

It must be pointed out that a radio-active body never disintegrates all at once.' The process may go quickly or slowly according to the properties of the element disintegrating, but it always proceeds according to one settled plan. In a given interval of time there is always a definite fraction of the atoms of each radio-element which disintegrate, and this fraction is invariable. It is the same whether there be a million-million atoms present or a million only; no chemical combination with other atoms-or physical agency such as change of pressure or of temperature-seems able to affect the value of this fraction in the least degree. The fraction for the best known radio-element, radium, is per year. This means that if we were to weigh out 2,309 pounds (or any other unit of weight) of radium to-day, in a year's time 2,308 would remain absolutely unchanged and one would represent the weight of products into which the radium had disintegrated. Expressed otherwise, 99.95 per cent. of the radium fails to show any sign of radio-activity in the course of a year, so that the radio-activity of radium is due to a very small percentage of itself. Most radio-elements disintegrate more rapidly than radium; five disintegrate more slowly.

Radio-activity has been of the greatest service in throwing light on such vexed connexions of the past as the transmutation of the elements, the meaning of the periodic system of classification of the elements, and other problems connected with elements themselves; and it alone has revealed enough about the atom to make a study of its structure possible. One of its first successes was that it left no doubt in the investigator's

mind that atoms are no mere figments of the brain postulated as a convenient hypothesis, but real, separate entities like the sun and the moon and the planet we occupy.

Rutherford and his co-workers showed that the aparticle is merely an atom of the gaseous element helium electrically charged, and an important constituent certainly of radio-active atoms and probably also of the atoms of common elements. He was able to show in 1908 that it was possible to detect and register the effects of a single a-particle. There are now four different ways of doing this. Two methods, electrical and photographic, need not be described. The third is a method originally due to Sir William Crookes by which a flash of light visible in a dark room is produced by the impact of a single a-particle on a screen coated with crystals of the sulphide of zinc. The fourth is a method due to C. T. R. Wilson, which detects both single electrons and single a-particles. A gas saturated with moisture is suddenly cooled; the portions of the gas that have been struck by the a-particle act as centres of visible drops of water; and the water-drops along the track are visible to the eye or can be photographed. All these methods afford convincing evidence of the discrete nature of these particles and, at the same time, have been of great use as weapons in studying the structure of the atom.

Rutherford's experiments with a-particles led him next to his theory of the structure of the atom, the only one now accepted. On this theory the atoms of all elements are supposed to be of one simple type, each being conceived as a miniature solar system, and built up of two things only, positive and negative electricity.

The positive electricity is supposed to be at the centre of the atom, forming a core there; or, as it is called, the nucleus. Surrounding this nucleus are separate particles of negative electricity (electrons), which are spaced out to occupy the rest of the atom. The important things about the nucleus are: its size, which is very minute compared with the size of the atom; the fact that practically the whole of the mass (or weight) of the atom is concentrated on this tiny nucleus, paradoxical as this may appear to be; and the fact that the nucleus may have in it both negative and positive electricity

'slumped' together; but it appears to be composed of positive electricity only, because there is always more of the latter than of the former. The negative electricity, on the other hand, is not 'slumped' together, but exists as separate little entities (electrons), which may revolve round the nucleus like the movement of the planets round the sun. The size of each of these entities of negative electricity is very small compared with that of the atom, but it is somewhat larger than the whole of the positive nucleus. As there is but one nucleus, and as the number of electrons in an atom has been shown experimentally to be never greater than 92, and as all are very minute compared with the size of an atom, it follows that most of an atom-more than 99.99 per cent. of it is empty space. Consider an atom so magnified as to appear superficially like Ireland. On that scale the nucleus, situated at Athlone (the nearest town to the centre of Ireland) would be a ball of five or six feet in diameter only. Round that at different distances from Athlone to the sea, would be some forty or fifty (on the average) spheres of about the same radius, and these and the nucleus are the whole of the atom.

The amount of positive electricity on the nucleus of the atom, and the number of surrounding electrons in the atom, are invariably the same numerically. Thus, an atom whose nucleus has a charge of 22 positive units has 22 electrons. The lead atom, for example, has 82 positive charges on its nucleus and 82 surrounding electrons; the oxygen atom 8 positive charges on its nucleus and 8 surrounding electrons; the helium atom 2 and 2; the hydrogen atom 1 and 1. This number, which is both the number of electrons in the atom and the number of positive charges of electricity on the nucleus, is an important one. It is called the Atomic Number, and its fundamental importance in our knowledge of the atom's structure is due to the experiments of H. G. J. Moseley, Rutherford's most distinguished pupil. Each element has one atomic number and one only; its chemical and physical properties are known to depend upon the arrangement of electrons which surround the nucleus, but this arrangement in turn depends solely on the amount of the charge on the nucleus. Change the atomic number (if you can), and in consequence you