THE LESSONS OF RADIUM*
Tax discovery of the property of matter known as radio- activity is probably the most important achievement of physical science since Newton taught us to understand gravi- tation. It has advanced our knowledge of the physical con- stitution of the universe nearly as much as the work of Darwin advanced biological theories as to the origin of life. The twentieth century, indeed, promises to carry the story of evolution one stage farther back than did the nineteenth. Darwin has enabled us to see how the process of evolution, dimly guessed at by philosophers and poets for twenty centuries, may actually take place in that still mysterious entity which we call living matter ; how the structureless cells of protoplasm, which were almost certainly the first representatives of life on the earth, have, through the lapse of uncounted and immemorial ages, given birth to man himself and all his high hopes and wonderful works. The chemists and physicists who have been working at radio-activity for the last twenty years have carried the process of evolution farther back, and applied it to inanimate matter. Till within the last generation the atom as defined by Dalton was the ultimate concept of the student of Nature. Every material substance was found to be composed of the so-called atoms of some sixty or seventy elementary bodies, which no method known in our laboratories or discovered in Nature could split up into simpler forms. These atoms were thought to be indivisible, permanent, with no visible beginning and no apparent end. But we are now in the full glamour of the discovery that these atoms themselves are but highly complicated arrangements of still smaller entities, the "corpuscles" which Professor J. J. Thomson predicted many years ago from mathematical considerations, and which the recent researches into radio- activity have definitely established as existing under the name of electrons. It is now believed that every atom is a system of these tiny electrons, which are in constant motion like that of the planets in our solar system, and owe their• remarkable stability to that very motion. Some atoms, like that of hydrogen, consist only of a few hundreds of these electrons, whilst others, like that of radium itself, probably contain hundreds of thousands of them. These electrons are thought to be all precisely similar, and it is probable that they con- stitute that primitive zer-stof, or primordial matter, which many speculators have held to be the foundation of the material universe. Further, it is practically certain that they are closely related to electricity, consisting either of particles of this primitive matter, each of which bears a unit charge of electricity, or perhaps solely of such electrical charges, without any material basis at all in the usual sense of the word. This fat•-reaching hypothesis promises to revolu- tionise our views of the ultimate constitution of the universe, and forms the most fascinating addition to physical or • (I) The Electron Theory. By E. E. Fournier d'Albe. Loudon : Longmeas and Co. [5.. net.1--(2) The New rhymes and Chemistry. By V f. A. Shenstone. London Smith, Elder, and Ca [7s. ed. set.]—(3) Con- duction of Electricity through Gases. By J. J. Thomson. Second Edition. Cambridge : at the University Frees. rie. j—(4) Esdio-artios Transforms. limo. By E. Rutherford. London : A. Constable and Co. [les. net.]
chemical theories which has been put forward within living memory.
The four books now lying on our table contain a most interesting account of the facts and researches which have contributed to the general acceptance of this new theory. Mr. Fournier d'Albe has given us a lucid popular account of the main outlines of the electron theory as it exists at the present day. Mr. Shenstone, of Clifton College, has reprinted the very able and informing essays on various aspects of this theory which he has contributed during the last year or two to the Cornhill Magazine. These two books are excellent specimens of popular exposition, and may be safely procured by the layman who wishes to be introduced on an easy path to the delightful fields of the new knowledge. Professor J. J. Thomson's work on Conduction of Electricity through Gases is already a classic, and the appearance of a revised edition, brought up to date by many additions, will be welcomed by all students of radio-activity and its allied subjects. The Canadian Professor Rutherford, who is another of the chief leaders in this department of research, Las printed the Sfflli- man Lectures which he delivered in 1905 at Yale University —with certain additions incorporating the results of later investigations—and has in them given us the best treatise for the general reader which has yet been published on this subject. Whilst his writings are always authoritative, and therefore welcome to the student, they have been divested in this volume of most of the technical and mathematical subtleties which necessarily repel the general reader in such a book as that of Professor Thomson, and there is hardly a page which cannot be understood by the intelligent layman. This is no inconsiderable achievement, from the point of view which holds that one chief object of science is to enlighten and expand the views of mankind at large as to the wonders of Nature, and Professor Rutherford's admirable work should be in the hands of all who wish to keep in touch with the latest theories about the universe.
Though the researches of Professor Thomson date from 1:• 1, the actual starting-point of the branch of physics which concerns itself specially with radio-activity was the discovery of M. Becquerel in 1896 that certain compounds of the rare metal uranium were able to emit rays which would darken a photographic plate, in the entire absence of anything corre- sponding to ordinary light-rays. This led to the work of Madame Curie on the waste products of uranium ores, from which she finally succeeded in extracting some minute portions of a salt which was many thousand times more active than the original uranium salt. This is now believed to be a compound —usually the chloride or bromide—of a new element called radium. It proved to have a singularly important property, which has equipped investigators with a new method of research, surpassing in delicacy even that in which the spectroscope is used. The rays emitted by radio-active bodies ionise the air through which they pass, and thus make it a conductor of electricity. Consequently a charged electroscope loses its charge as soon as a minute portion of any radium salt is brought into its neighbourhood. This test can be made amazingly delicate, and has enabled us to learn a wonderful deal about the properties of radium compounds, although the largest quantity of them with which any investigator has yet been able to work is less than a grain. Professor Rutherford gives a very clear and interesting account of what has since been done in this way. We now know that a radium compound is continually emitting three kinds of rays which have a wonderful power of penetrating ordinary matter. One kind, known as the Alpha rays, consists of a stream of material particles which are on good grounds believed to be atoms of the rare gas helium, which the spectroscope long ago showed to exist in the sun's atmosphere, but which was only discovered on the earth by Ramsay in 1895. The Beta rays are also a stream of material particles, but far smaller than any known atom : they are probably detached electrons, or units of negative electricity. The Gamma rays are true ether-waves, akin to the Röntgen rays. In the act of emitting these radiations the atom of radium is of course losing a portion of its matter. This is the kernel of the great discovery that a true atom is able to break up into smaller particles, and therefore that it must consist of some arrangement of such particles, which—and not the atoms—are the true unit of matter. In losing its electrons an atom is converted- into an entirely different atom, which has quite different chemical
mid physical properties from those of the parent. A great part of Professor Rutherford's book consists of a description of these radio-active transformations. He is able to trace the successive degradation of the radium atom through at least-six diverse stages, each pranced' by a loss of a definite number of electrons, until at length, as he believes, it ends assn atom of lead,—one of the commonest and least active of metals. This is very different from the alchemist's dream of the transmutation of lead into gold, but it is at least as epoch-making a discovery. Only those who have studied the matter closely can appreciate the wonderful triumphs of manipulation and reasoning which have thus enabled our physicists to get inside the atom, and trace the motions of the infinitely small.electrons which. compose it. We are on the threshold of a complete revolution in our ideas as to the constitution of matter ; and it is on the. cards that we may equally-revolutiouise our industries by finding some way to avail ourselves of the incredible stores of dormant energy which these new researches have shown to lie within the atoms of which all tangible matter is built up.
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