The probability that one element might be changed into another was increased by the demonstration of the connexions between uranium and radium. The metal uranium has been classed with the elements since it was isolated in 1840. In 1896, Becquerel found that compounds of uranium, and also the metal itself, are radio-active. In the light of what is now known about radio-activity, it is necessary to suppose that some of the minute particles of uranium emit particles lighter than themselves, and change into some substance, or substances, different from uranium; in other words, it is necessary to suppose that some particles of uranium are spontaneously disintegrating. This supposition is confirmed by the fact, experimentally proved, that uranium emits α-rays, that is, atoms of helium, and produces a substance known as uranium X. Uranium X is itself radio-active; it emits β-rays, that is, it gives off electrons. Inasmuch as all minerals which contain compounds of uranium contain compounds of radium also, it is probable that radium is one of the disintegration-products of uranium. The rate of decay of radium may be roughly expressed by saying that, if a quantity of radium were kept for ten thousand years, only about one per cent. of the original quantity would then remain unchanged. Even if it were assumed that at a remote time the earth's crust contained considerable quantities of radium compounds, it is certain that they would have completely disappeared long ago, had not compounds of radium been reproduced from other materials. Again, the most likely hypothesis is that compounds of radium are being produced from compounds of uranium.
Uranium is a substance which, after being rightly classed with the elements for more than half a century, because it had not been separated into unlike parts, must now be classed with the radium-like substances which disintegrate spontaneously, although it differs from other radio-active substances in that its rate of change is almost infinitively slower than that of any of them, except thorium.[12] Thorium, a very rare metal, is the second of the seventy-five or eighty elements known when radio-activity was discovered, which has been found to undergo spontaneous disintegration with the emission of rays. The rate of change of thorium is considerably slower than that of uranium.[13] None of the other substances placed in the class of elements is radio-active.
On [p. 192] I said, that when the radio-active substances had been labelled elements, the facts of radio-activity led some chemists to the conclusion that the other bodies which had for long been called by this class-name, or at any rate some of these bodies, are perhaps not true elements, but are merely more stable collocations of particles than the substances called compounds. It seems to me that this reasoning rests on an unscientific use of the term element; it rests on giving to that class-name the meaning, substances asserted to be undecomposable. A line of demarcation is drawn between elements, meaning thereby forms of matter said to be undecomposable but probably capable of separation into unlike parts, and true elements, meaning thereby groups of identical undecomposable particles. If one names the radio-active substances elements, one is placing in this class substances which are specially characterised by a property the direct opposite of that the possession of which by other substances was the reason for the formation of the class. To do this may be ingenious; it is certainly not scientific.
Since the time of Lavoisier, since the last decade of the eighteenth century, careful chemists have meant by an element a substance which has not been separated into unlike parts, and they have not meant more than that. The term element has been used by accurate thinkers as a useful class-mark which connotes a property—the property of not having been decomposed—common to all substances placed in the class, and differentiating them from all other substances. Whenever chemists have thought of elements as the ultimate kinds of matter with which the physical world is constructed—and they have occasionally so thought and written—they have fallen into quagmires of confusion.
Of course, the elements may, some day, be separated into unlike parts. The facts of radio-activity certainly suggest some kind of inorganic evolution. Whether the elements are decomposed is to be determined by experimental inquiry, remembering always that no number of failures to simplify them will justify the assertion that they cannot be simplified. Chemistry neither asserts or denies the decomposability of the elements. At present, we have to recognise the existence of extremely small quantities, widely distributed in rocks and waters, of some thirty substances, the minute particles of which are constantly emitting streams of more minute, identical particles that carry with them very large quantities of energy, all of which thirty substances are characterised, and are differentiated from all other classes of substances wherewith chemistry is concerned, by their spontaneous mutability, and each is characterised by its special rate of change and by the nature of the products of its mutations. We have now to think of the minute particles of two of the seventy-five or eighty substances which until the other day had not been decomposed, and were therefore justly called elements, as very slowly emitting streams of minuter particles and producing characteristic products of their disintegration. And we have to think of some eighty substances as particular kinds of matter, at present properly called elements, because they are characterised, and differentiated from all other substances, by the fact that none of them has been separated into unlike parts.
The study of radio-activity has introduced into chemistry and physics a new order of minute particles. Dalton made the atom a beacon-light which revealed to chemists paths that led them to wider and more accurate knowledge. Avogadro illuminated chemical, and also physical, ways by his conception of the molecule as a stable, although separable, group of atoms with particular properties different from those of the atoms which constituted it. The work of many investigators has made the old paths clearer, and has shown to chemists and physicists ways they had not seen before, by forcing them to think of, and to make use of, a third kind of material particles that are endowed with the extraordinary property of radio-activity. Dalton often said: "Thou knowest thou canst not cut an atom"; but the fact that he applied the term atom to the small particles of compounds proves that he had escaped the danger of logically defining the atom, the danger of thinking of it as a particle which never can be cut. The molecule of Avogadro has always been a decomposable particle. The peculiarity of the new kind of particles, the particles of radio-active bodies, is, not that they can be separated into unlike parts by the action of external forces, but that they are constantly separating of their own accord into unlike parts, and that their spontaneous disintegration is accompanied by the production of energy, the quantity of which is enormous in comparison with the minuteness of the material specks which are the carriers of it.
The continued study of the properties of the minute particles of radio-active substances—a new name is needed for those most mutable of material grains—must lead to discoveries of great moment for chemistry and physics. That study has already thrown much light on the phenomena of electric conductivity; it has given us the electron, a particle at least a thousand times lighter than an atom of hydrogen; it has shown us that identical electrons are given off by, or are separated from, different kinds of elementary atoms, under definable conditions; it has revealed unlooked-for sources of energy; it has opened, and begun the elucidation of, a new department of physical science; it has suggested a new way of attacking the old problem of the alchemists, the problem of the transmutation of the elements.
The minute particles of two of the substances for many years classed as elements give off electrons; uranium and thorium are radio-active. Electrons are produced by sending an electric discharge through very small traces of different gases, using electrodes of different metals. Electrons are also produced by exposing various metals to the action of ultra-violet light, and by raising the temperature of various metals to incandescence. Electrons are always identical, whatever be their source. Three questions suggest themselves. Can the atoms of all the elements be caused to give off electrons? Are electrons normal constituents of all elementary atoms? Are elementary atoms collocations of electrons? These questions are included in the demand—Is it possible "to imagine a model which has in it the potentiality of explaining" radio-activity and other allied phenomena, as well as all other chemical and physical properties of elements and compounds? These questions are answerable by experimental investigation, and only by experimental investigation. If experimental inquiry leads to affirmative answers to the questions, we shall have to think of atoms as structures of particles much lighter than themselves; we shall have to think of the atoms of all kinds of substances, however much the substances differ chemically and physically, as collocations of identical particles; we shall have to think of the properties of atoms as conditioned, in our final analysis, by the number and the arrangement of their constitutive electrons. Now, if a large probability were established in favour of the view that different atoms are collocations of different numbers of identical particles, or of equal numbers of differently arranged identical particles, we should have a guide which might lead to methods whereby one collocation of particles could be formed from another collocation of the same particles, a guide which might lead to methods whereby one element could be transformed into another element.
To attempt "to imagine a model which has in it the potentiality of explaining" radio-activity, the production of kathode rays, and the other chemical and physical properties of elements and compounds, might indeed seem to be a hopeless undertaking. A beginning has been made in the mental construction of such a model by Professor Sir J.J. Thomson. To attempt a description of his reasoning and his results is beyond the scope of this book.[14]
The facts that the emanation from radium compounds spontaneously gives off very large quantities of energy, and that the emanation can easily be brought into contact with substances on which it is desired to do work, suggested to Sir William Ramsay that the transformation of compounds of one element into compounds of another element might possibly be effected by enclosing a solution of a compound along with radium emanation in a sealed tube, and leaving the arrangement to itself. Under these conditions, the molecules of the compound would be constantly bombarded by a vast number of electrons shot forth at enormous velocities from the emanation. The notion was that the molecules of the compound would break down under the bombardment, and that the atoms so produced might be knocked into simpler groups of particles—in other words, changed into other atoms—by the terrific, silent shocks of the electrons fired at them incessantly by the disintegrating emanation. Sir William Ramsay regards his experimental results as establishing a large probability in favour of the assertion that compounds of copper were transformed into compounds of lithium and sodium, and compounds of thorium, of cerium, and of certain other rare metals, into compounds of carbon. The experimental evidence in favour of this statement has not been accepted by chemists as conclusive. A way has, however, been opened which may lead to discoveries of great moment.