WORLDS NOT REALISED.*
Miss CLERKE has given us a remarkable book in this lucid and invaluable summary of our actual knowledge concerning the vast sidereal universe upon whose edge we seem to stand. At the moment scientific thought is concentrating itself upon problems that deal with the nature of the infinitely little, and this treatise upon things infinitely great is a good illus- tration of the fruitfulness of modern methods. We live between two worlds not realised,—the remote and majestic world of starry systems, and the not less wonderful and scarcely more intimate world of atom and electron. Vast groupings of matter under unimaginable conditions and at inconceivably great distances are the one world, while minute groupings of matter under conditions that seem equally unimaginable and at distances inconceivably small are the other world. That such worlds exist at all is in itself a modern idea; that they are intimately related and are each capable of yielding material which will, in part at least, explain the other is a conception that belongs to the last few years. To practically all ancient and mediaeval, and to most modern, minds the structure of the physical universe presented and presents nothing that can be called a problem. It is true that the Greeks speculated with extreme ingenuity on the motion of the planets. It is remarkable that Democritus named and asserted the indivisible atom, while Tertullian in his Latin rendering, in atomo, of the phrase it gi7-6,46q, (1 Cor. vi. 51-52), with reference to the resurrection of the body appears instinctively to have conceived of an electronic reconstitution of matter that must appeal to Sir Oliver Lodge. Moreover, the mediaeval believers in the Philosophers Stone and the transmutation of metals asserted the oneness of all matter with no doubtful voice. There was to them one element only, which took to itself many shapes. The ages have, indeed, again and again stumbled upon truth, and found the experience wonderful, unbelievable, and painful. But to the thinking mind of those ages the structure of the physical universe was not a problem to be solved, but a mystery to be wondered at, and perhaps worshipped. To the unthinking mind then, as now (and the unthinking mind on such subjects comprehends the vast multitude of men), the universe was, and is, neither a mystery nor a problem, but a fact that neither calls for nor needs explanation.
Among the charmed company of thinkers, however, the • Problems in aleirophyeice. By Agnes M. Clerks. Containing 81 Illua. trations. London: A. and C. Black. [20s. net.]
mystery slowly became a problem. To that sage of the Middle Ages, Roger Bacon, the whole question was certainly a problem, for he was a scientist of the modern type. As Miss Clerke points out, Kepler introduced the term "physical astronomy," while Francis Bacon demanded inquiries into the nature of the substance of the heavenly bodies, inquiries into " their qualities, properties, and influences, as well as into the source of the motive power acting upon them." In fact, "he broadly laid down the lines of a new astronomy, indistinguish- able from modern astrophysics." But much, if not most, that the two Bacons foresaw was destined to wait for its development for long centuries. The gravitational property of matter as affecting the entire universe was not finally asserted until the opening of the nineteenth century, and the knowledge of such a property was an antecedent condition to progress as opposed to conjecture. The formulation of the atomic theory was perhaps almost as necessary, but the next direct link, as opposed to the indirect relationship implied in structural affinities, was the identification in 1852 of the sun- spot and terrestrial magnetic periods, "showing the reality of solar influences exercised in a manner not easy to apprehend, but capable of being brought to the test of experimental investigation." Then came the astounding discovery of the spectroscope, "the discovery," as Professor Keeler says, "that the light which reveals to us the existence of the heavenly bodies also bears the secret of their constitu- tion and physical condition." Miss Clerke puts the use of the spectroscope clearly when she states that " observations of the heavenly bodies by means of their analysed light came to the aid of observations through their integrated light. Their radiations, visible and invisible, were brought within the range of detailed study." The application of the camera as recently as 1876 by Sir William Huggins to spectroscopic work started a new era, the importance of which was placed beyond all doubt by his discovery in 1879 of the ultra-violet series of hydrogen lines in stellar spectra. This was followed by the application of the chemical method to the motion of the stars. " The determination of radial velocities through changes in the refrangibility of light emanating from the bodies actuated by them" has given us accurate means of dealing with stellar mass and motion that could never have been obtained from direct telescopic observation. The method is of use, more- over, in cases where it will always be impossible to observe directly, as in the case of the excessively remote and numerous double stars. The science of astrophysics without the aid of photography could never have advanced as it has ; but, as Miss Clerke points out, the science itself is capable of propagating error. " Its concern is with phenomena falling partly within, partly without the range of ordinary ex- perience. It has to do with matter in transcendental states." It has to apply to absolutely unknown regions " rules gathered from observations over a comparatively narrow area. The indefinite continuity of natural laws is assumed by it, but certainly on no sufficient warrant." But the value of the science—the practical value for our earth here— cannot be overrated. Groping in the unknown, it may bring, it has already brought, facts down to earth of what may prove infinite importance. "The discovery of unknown laws, as well as the explanation of phenomena by laws already known," says Professor Keeler, "is one of its most important objects." That importance grows more and more as days go by, and men of science realise the part played in the structure of the universe by certain earths and metals of rare occurrence on this planet. For instance, helium until March, 1895, was only known as a sun-element. A certain substance in the sun gives under the spectroscope a particular bright yellow ray. The spectrum had apparently no earthly equivalent. The Scandinavian mineral, clevite—" a kind of pitch- blende, composed of uranate of lead mixed with rare earths "—was tested. " The gas evolved from it at Uni- versity College gave a brilliant spectrum, in which the pro- minence line D3 shone conspicuous. Helium was indeed captured." The identity was subsequently confirmed by a very wonderful experiment by Professor Hale in America and Sir William Huggins in England. This substance, with its remarkable structure and properties, is only one of a series of strange substances that are throwing an entirely new light on the intimate structure of matter. The strangest of these is radium, but new secrets perhaps remain to be revealed by
the capture of coronium, a diaphanous form of matter (in capable of absorbing light) abounding in the corona of the sun, and detected through its bright green ray by the spectroscope.
We may, therefore, well believe that astrophysics will give an immense impetus to terrestrial physics,—will, indeed, give to chemistry and physics much more than it has received from them. That is its great practical, though indirect, value. Its direct value lies in the marvellous and ever- increasing revelations of wonders in, and the orderliness of, the heavens. Miss Clerke, after dealing elaborately with the problems in solar physics—she regards the existence of a " true reversing layer" of the sun outside the photosphere as estab- lished, and as the cause of the dark lines in the solar spectrum —passes to the vast question of sidereal physics. This sub- ject is limited by the fact that, of course, surface-phenomena do not arise, and thus the observer is unable to consider the particular class of problems that are connected with the sun. On the other hand, the pencils of star-light with which the spectroscope deals differ not only in glory, but have special characteristics which "are found to be indexes to conditions of temperature, density, magnetic strain, or electrical excitation, in part imitable in the laboratory, in part transcending, and hence contributing to enlarge terrestrial experience." There appears to be a relationship between the spectra given by the stars and the manner of their distribution ; and if this is so, we may anticipate extraordinary revelations as to the relation- ship of space itself to the scattered stars. " We are all waves," says Professor Osborne Reynolds. Space is the real thing, and matter is merely a wave-motion in a real substance. If this is so, we may believe that the distribution of the stars in space is definitely determined by inherent spatial character- istics of which we catch glimpses in the spectra. Not the least remarkable fact in the spectroscopic analysis of stellar light is the frequent detection of helium. Though it rarely occurs on the earth, it literally abounds in the heavens. There is a whole class of " helium " stars apparently composed entirely of this substance. We suppose it would be rash to suggest that these stars are composed of disintegrating forms of matter such as radium, and that the exclusive presence of helium gas betrays an in- stability of physical structure in the star. The fact that helium stars are for the most part surrounded by what Miss Clerke calls " cosmic fog," and are centres of nebulous mist, would seem to suggest that these stars are the scene of physical processes of atomic integration or disintegration of the most astonishing kind. Certainly the experiments now being made with respect to physical structure, combined with the information derived from spectroscopic observation of the stars, will vastly enlarge our knowledge as to the structure and mechanics of the universe.
Helium is but one of many stellar ingredients recognised beyond all doubt. It is interesting to note that the nebular spectrum has not as yet revealed the presence of any metals that we know (p. 175). This fact in itself would seem to sug- gest that cosmic processes deal with one ultimate material which yields in later stages varieties of forms. Oxygen, nitrogen, and hydrogen have been detected as stellar com- ponents. " In the spectrum of Rigel some iron lines faintly emerge." Silicon is a frequent stellar constituent. Sodium, calcium, magnesium, iron, silicon, titanium, vana- dium, barium, possibly chromium and nickel, and numerous unrecognised metals all seem to occur in the "hydrogen" stars, which form a later (though overlapping) stage of development than is observed in the helium class. The ripest class of stars—the class known as solar stars—give a crowded and pronounced metallic spectrum. All this would appear to suggest that the universe we look upon actually shows us the evolution of innumerable substances from one ultimate material or electric condition. The reversal, in the case of temporary stars, of the normal process of develop- ment is really a confirmation of this view. It would be strange, but perfectly possible, for our laboratory experiments on the structure of matter to receive conclusive confirmation from spectroscopic observations of nebulous mist and the successive order of stars.
It is impossible to deal further here with Miss Clerke'a able ana fascinating work. She has gathered together the
material tor her book from all available quarters. .In bringing into narrow compass the numerous scattered monographs on parts of her subject she has done good work for science, while her genius for exposition has enabled her to state in clear English, marred only by a too frequent excess of circumlocu- tion, an extremely technical ease. The making of such a book is, we believe, its own, and a very genuine, reward. But Miss Clerke has also the satisfaction of knowing that the value of her work is entirely recognised by the few who are in fact able to express an opinion, and that recognition has fitly been crowned by her election as a member of the Royal Astro- nomical Society.
Previous page
