 INTRODUCTION TO THE OUTLINE OF SCIENCE Is it not the great philosopher and mathematician Leibniz, who said that the more knowledge advances, the more it becomes impossible to condense it into little books? Now this outline of science is certainly not a little book, and yet it illustrates part of the meaning of Leibniz's wise saying, for here within reasonable compass there is a library of little books, an outline of many sciences. It will be profitable to the student in proportion to the discrimination with which it is used. Lord is not in the least meant to be of the nature of an encyclopedia, giving condensed and comprehensive articles with a big full stop at the end of each, nor is it a collection of primers, beginning at the very beginning of each subject and working methodically onwards. That is not the idea. What then is the aim of this book? It is to give the intelligent student-citizen, otherwise called the man in the street, a bunch of intellectual keys by which to open doors which have been hitherto shut to him, partly because he got no glimpse of the treasures behind the doors and partly because the portals were made forbidding by an unnecessary display of technicalities. Laying aside conventional modes of treatment and seeking rather to open up the subject as one might on a walk with a friend, the work offers the student what might be called informal introductions to the various departments of knowledge. We put it another way. The articles are meant to be clues which the reader may follow till he has left his starting point very far behind. Perhaps when he has gone far on his own he will not be ungrateful to the simple book of Instructions to Travelers which this outline of science is intended to be. The simple bibliographies appended to the various articles will be enough to indicate first books. Each article is meant to be an invitation to an intellectual adventure, and the short lists of books are merely finger-posts for the beginning of the journey. We confess to being greatly encouraged by the reception that has been given to the English serial issue of the outline of science. It has been very hardy, we might almost say, enthusiastic, for we agree with Professor John Dewey that the future of our civilization depends upon the widening spread and deepening hold of the scientific habit of mind. And we hope that this is what the outline of science makes for. Information is all to the good. Interesting information is better still. But best of all is the education of the scientific habit of mind. Another modern philosopher, Professor L. T. Hobhouse, has declared that the evolutionist mundane goal is the mastery by the human mind of the conditions internal as well as external of its life and growth. Under the influence of this conviction the outline of science has been written. For life is not for science, but science for life. And even more than science, to our way of thinking, is the individual development of the scientific way of looking at things. Science is our legacy. We must use it if it is to be our very own. Introduction There is abundant evidence of a widened and deepened interest in modern science. How could it be otherwise when we think of the magnitude and the inventfulness of recent advances? But the interest of the general public would be even greater than it is if the makers of new knowledge were more willing to expound their discoveries in ways that could be understanding of the people. No one objects very much to technicalities in a game or on board a yacht, and the air clearly necessary for tourist and precise scientific description. It is certain, however, that they can be reduced to a minimum without sacrificing accuracy when the subject in view is to explain the gist of the matter. So this outline of science is meant for the general reader who lacks both time and opportunity for special study and yet would take an intelligent interest in the progress to science which is making the world always new. The story of the triumphs of modern science is one of which man may well be proud. Science reads the secret of the distant star and anonymizes the atom, foretells the date of the comet's return, and predicts the kinds of chickens that will hatch from a dozen eggs, discovers the laws of the wind that bloweth where it listeth and reduces to order the disorder of disease. Science is always setting forth on Columbus voyages discovering new worlds and conquering them by understanding. For knowledge means foresight, and foresight means power. The idea of evolution has influenced all the sciences, forcing us to think of everything as with the history behind it, for we have traveled far since Darwin's day. The solar system, the earth, the mountain ranges, and the great deeps, the rocks and crystals, the plants and animals, man himself and his social institutions, all must be seen as the outcome of a long process of becoming. There are some 80 odd chemical elements on the earth today, and it is now much more than a suggestion that these are the outcome of an inorganic evolution, element giving rise to element, going back and back to some primeval stuff from which they were all originally derived, infinitely long ago. No idea has been so powerful a tool in the fashioning of new knowledge as this simple but profound idea of evolution that the present is the child of the past and the parent of the future. And with the picture of a continuity of evolution from nebula to social systems comes a promise of an increasing control, a promise that man will become not only a more accurate student, but a more complete master of his world. It is characteristic of modern science that the whole world is seen to be more vital than before. Everywhere there has been a passage from the static to the dynamic, thus the new revelations of the constitution of matter which we owe to the discoveries of men like Professor Sir J. J. Thomson, Professor Sir Ernest Rutherford, and Professor Frederick Soddy have shown the very dust to have a complexity and an activity heretofore unimagined. Such phrases as dead matter and inert matter have gone by the board. The new theory of the atom amounts almost to a new conception of the universe. It bids fair to reveal to us many of nature's hidden secrets. The atom is no longer the indivisible particle of matter it was once understood to be. We now know that there is an atom within the atom that what we thought was elementary can be dissociated and broken up. The present-day theories of the atom and the constitution of matter are the outcome of the comparatively recent discovery of such things as radium, the x-rays, and the wonderful revelations of such instruments as the spectroscope and other highly perfected scientific instruments. The advent of the electron theory has thrown a flood of light on what before was hidden or only dimly guessed at. It has given us a new conception of the framework of the universe. We are beginning to know and realize of what matter is made and what electric phenomena mean. We can glimpse the vast stores of energy locked up in matter. New knowledge has much to tell us about the origin and phenomena, not only of our own planet, but other planets, of the stars and the sun. New light is thrown on the source of the sun's heat. We can make more than guesses as to its probable age. The great question today is, is there one primordial substance from which all the varying forms of matter have been evolved? But the discovery of electrons is only one of the revolutionary changes which give modern science an entrancing interest. As in chemistry and physics, so in the science of living creatures there have been recent advances that have changed the whole prospect. A good instance is afforded by the discovery of the hormones, or chemical messengers, which are produced by ductless glands, such as the thyroid, the suprarenal, and the pituitary, and are distributed throughout the body by the blood. The work of physiologists like Professor Starling and Professor Baylis has shown that these chemical messengers regulate what may be called the pace of the body, and bring about that regulated harmony and smoothness of working which we know is health. It is not too much to say that the discovery of hormones has changed the whole of physiology. Our knowledge of the human body far surpasses that of the past generation. The persistent patience of microscopists and in technical improvements like the ultramicroscope have greatly increased our knowledge of the invisible world of life. To the bacteria of a past generation have been added to multitude of microscopic animal microbes, such as that which causes sleeping sickness. The life histories and the weird ways of many important parasites have been unraveled, and here again knowledge means mastery. To a degree which has almost surpassed expectations there has been a revelation of the intricacy of the stones and mortar of the house of life, and the microscopic study of germ cells has wonderfully supplemented the epic-making experimental study of heredity which began with Mendel. It goes without saying that no one can call himself educated who does not understand the central and simple ideas of Mendelism and other new departures in biology. The procession of life through the ages and the factors in the sublime movement, the peopling of the earth by plants and animals, and the linking of life to life in subtle interrelations, such as those between flowers and their insect visitors, the life histories of individual types and the extraordinary results of the new inquiry called experimental embryology. These also are among the subjects with which this outline will deal. The behavior of animals is another fascinating study, leading to a provisional picture of the dawn of mind. Indeed, no branch of science surpasses an interest that which deals with the ways and habits, the truly wonderful devices, adaptations and instincts of insects, birds and mammals. We no longer deny a degree of intelligence to some members of the animal world, even the line between intelligence and reason is sometimes difficult to find. Fresh contacts between physiology and the study of man's mental life, precise studies of the ways of children and wild peoples, and new methods like those of the psychoanalyst, must also receive the attention they deserve, for they are giving us a new psychology and the claims of psychocal research must also be recognized by the open-minded. The general aim of the outline is to give the reader a clear and concise view of the essentials of present-day science, so that he may follow with intelligence the modern advance and share appreciatively in man's continued conquest of his kingdom. Signed J. Arthur Thompson. End of introduction. The first part of Chapter 1 of The Outline of Science. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. This recording is by Mark Smith of Simpsonville, South Carolina. The Outline of Science, edited by J. Arthur Thompson. Chapter 1 The Romance of the Heavens. First part. The Scale of the Universe. The Solar System. The story of the triumphs of modern science naturally opens with astronomy. The structure of the universe which the astronomer offers to us is imperfect. The lines he traces are often faint and uncertain. There are many problems which have been solved. There are just as many about which there is doubt, and not with standing our great increase in knowledge, there remain just as many which are entirely unsolved. The problem of the structure and duration of the universe, said the great astronomer Simon Newcomb, is the most far reaching with which the mind has to deal. The solution may be regarded as the ultimate object of stellar astronomy, the possibility of reaching which has occupied the minds of thinkers since the beginning of civilization. Before our time the problem could be considered only from the imaginative or the speculative point of view. Although we can today attack it to a limited extent by scientific methods, it must be admitted that we have scarcely taken more than the first step towards the actual solution. Is the duration of the universe in time? Is it fitted to last for ever in its present form? Or does it contain within itself the seeds of dissolution? Must it, in the course of time, and we know not how many millions of ages, be transformed into something very different from what it now is? This question is intimately associated with the question whether the stars form a system. If they do, we may suppose that system to be permanent in its general features. If not, we must look further for our conclusions. THE HEAVENLY BODYS The heavenly bodies fall into two very distinct classes so far as their relation to our earth is concerned. The one-class, a very small one, comprises a sort of colony of which the earth is a member. These bodies are called planets, or wanderers. There are eight of them, including the earth, and they all circle round the sun. Their names, in the order of their distance from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. And of these Mercury, the nearest to the sun is rarely seen by the naked eye. Uranus is practically invisible, and Neptune quite so. These eight planets, together with the sun, constitute, as we have said, a sort of little colony. This colony is called the Solar System. The second class of heavenly bodies are those which lie outside the Solar System. Every one of those glittering points we see on a starlit night is at an immensely greater distance from us than is any member of the Solar System. Yet the members of this little colony of ours, judged by terrestrial standards, are at enormous distances from one another. If a shell were shot in a straight line from one side of Neptune's orbit to the other, it would take five hundred years to complete its journey. Yet this distance, the greatest in the Solar System as now known, accepting the far swing of some of the comets, is insignificant compared to the distances of the stars. One of the nearest stars to the earth that we know of is Alpha Centauri, estimated to be some twenty-five million millions of miles away. Serious, the brightest star in the firmament is double this distance from the earth. We must imagine the colony of planets to which we belong as a compact little family swimming in an immense void. At distances which would take our shell, not hundreds, but millions of years to traverse, we reached the stars. Or rather, a star, for the distances between stars are as great as the distances between the nearest of them and our sun. The earth, the planet on which we live, is a mighty globe bounded by a crust of rock many miles in thickness. The great volumes of water which we call our oceans lie in the deeper hollows of the crust. Above the surface an ocean of invisible gas, the atmosphere, rises to a height of about three hundred miles, getting thinner and thinner as it ascends. Except when the winds rise to a high speed, we seem to live in a very tranquil world. At night, when the glare of the sun passes out of our atmosphere, the stars and planets seem to move across the heavens with a stately and solemn slowness. It was one of the first discoveries of modern astronomy that this movement is only apparent. The apparent creeping of the stars across the heavens at night is accounted for by the fact that the earth turns upon its axis once in every twenty-four hours. When we remember the size of the earth, we see that this implies a prodigious speed. In addition to this, the earth revolves around the sun at a speed of more than a thousand miles a minute. Its path around the sun, year in, year out, measures about five hundred and eighty million miles. The earth is held closely to this path by the gravitational pull of the sun, which has a mass three hundred and thirty-three thousand four hundred thirty-two times that of the earth. If at any moment the sun ceased to exert this pull, the earth would instantly fly off into space straight in the direction in which it was moving at the time, that is to say, at a tangent. This tendency to fly off at a tangent is continuous. It is the balance between it and the sun's pull, which keeps the earth to our almost circular orbit. In the same way the seven other planets are held to their orbits. Circling round the earth in the same way as the earth circles round the sun is our moon. Sometimes the moon passes directly between us and the sun and cuts off the light from us. We then have a total or partial eclipse of the sun. At other times the earth passes directly between the sun and the moon and causes an eclipse of the moon. The great ball of the earth naturally trails a mighty shadow across space and the moon is eclipsed when it passes into this. The other seven planets, five of which have moons of their own, circle round the sun as the earth does. The sun's mass is immensely larger than that of all the planets put together, and all of them would be drawn into it and perish if they did not travel rapidly round it in gigantic orbits. So the eight planets, spinning round on their axes, followed their fixed paths round the sun. The planets are secondary bodies, but they are most important because they are the only globes in which there can be life as we know life. If we could be transported in some magical way to an immense distance in space above the sun, we should see our solar system as it is drawn in the accompanying diagram, except that the planets would be mere specks, faintly visible in the light which they receive from the sun. This diagram is drawn approximately to scale. If we move still farther away, trillions of miles away, the planets would fade entirely out of view and the sun would shrink into a point of fire, a star. And here you begin to realize the nature of the universe. The sun is a star. The stars are suns. Our sun looks big simply because of its comparative nearness to us. The universe is a stupendous collection of millions of stars or suns, many of which may have planetary families like ours. The scale of the universe. How many stars are there? A glance at a photograph of star clouds will tell it wants that it is quite impossible to count them. The fine photograph reproduced in Figure 2 represents a very small patch of that pale white belt, the Milky Way, which spans the sky at night. It is true that this is a particularly rich area of the Milky Way, but the entire belt of light has been resolved in this way into masses or clouds of stars. Astronomers have counted the stars in typical districts here and there, and from these partial counts we get some idea of the total number of stars. There are estimated to be between 2 and 3,000 million stars. Yet these stars are separated by inconceivable distances from each other, and it is one of the greatest triumphs of modern astronomy to have mastered so far the scale of the universe. For several centuries astronomers have known the relative distances from each other of the Sun and the planets. If they could discover the actual distance of any one planet from any other, they could at once tell all the distances within the solar system. The Sun is, on the latest measurements, at an average distance of 92,830,000 miles from the Earth, for as the orbit of the Earth is not a true circle, this distance varies. This means that in six months from now the Earth will be right at the opposite side of its path round the Sun, or 185 million miles away from where it is now. Viewed or photographed from two positions so wide apart, the nearest stars show a tiny shift against the background of the most distant stars, and that is enough for the mathematician. He can calculate the distance of any star near enough to show this shift. We have found that the nearest star to Earth, a recently discovered star, is 25 trillion miles away. Only 30 stars are known to be within 100 trillion miles of us. This way of measuring does not, however, take us very far away in the heavens. There are only a few hundred stars within 500 trillion miles of the Earth, and at that distance the shift of a star against the background, parallax, the astronomer calls it, is so minute that figures are very uncertain. At this point the astronomer takes up a new method. He learns the different types of stars, and then he is able to deduce more or less accurately the distance of a star of a known type from its faintness. He of course has instruments for gauging their light. As a result of 20 years' work in this field, it is now known that the more distant stars of the Milky Way are at least 100,000 trillion miles away from the Sun. Our Sun is in a more or less central region of the universe, or a few hundred trillion miles from the actual center. The remainder of the stars, which are all outside our solar system, are spread out, apparently, in an enormous, disc-like collection, so vast that even a ray of light, which travels at the rate of 186,000 miles a second, would take 50,000 years to travel from one end of it to the other. This, then, is what we call our universe. Are there other universes? Why do we say our universe? Why not THE universe? It is now believed by many of our most distinguished astronomers that our colossal family of stars is only one of many universes. By a universe an astronomer means any collection of stars which are close enough to control each other's movements by gravitation. And it is clear that there might be many universes in this sense, separated from each other by profound abysses of space. Probably there are. For a long time we have been familiar with certain strange objects in the heavens which are called spiral nebulae. We shall see at a later stage what a nebula is, and we shall see that some astronomers regard these spiral nebulae as worlds in the making. But some of the most eminent astronomers believe that they are separate universes, island universes, they call them, or great collections of millions of stars like our universe. There are certain peculiarities in the structure of the Milky Way which lead these astronomers to think that our universe may be a spiral nebula, and that the other spiral nebulae are other universes. Vast as is the solar system, then, it is excessively minute in comparison with the stellar system, the universe of the stars, which is on a scale far transcending anything the human mind can apprehend. The solar system. The sun. But now let us turn to the solar system and consider the members of our own little colony. Within the solar system there are a large number of problems that interest us. What is the size, mass, and distance of each of the planets? What satellites, like our moon, do they possess? What are their temperatures? And those other sporadic members of our system, comets and meteors, what are they? What are their movements? How do they originate? And the sun itself, what is its composition? What is the source of its heat? How did it originate? Is it running down? These last questions introduce us to a branch of astronomy which is concerned with the physical constitution of the stars, a study which, not so very many years ago, may well have appeared inconceivable. But the spectroscope enables us to answer even these questions, and the answer opens up questions of yet greater interest. We find that the stars can be arranged in an order of development, that there are stars at all stages of their life history. The main lines of the evolution of the stellar universe can be worked out. In the sun and stars we have furnaces with temperatures enormously high. It is in such conditions that substances are resolved into their simplest forms, and it is thus we are enabled to obtain a knowledge of the most primitive forms of matter. It is in this direction that the spectroscope, which we shall refer to immediately, has helped us so much. It is to this wonderful instrument that we owe our knowledge of the composition of the sun and stars, as we shall see. That the spectroscope will detect the millionth of a milligram of matter, and on that account has discovered new elements, commands our admiration. But when we find in addition that it will detect the nature of forms of matter, trillions of miles away, and, moreover, that it will measure the velocities with which these forms of matter are moving with an absurdly small percent of possible error, we can easily acquiesce in the statement that it is the greatest instrument ever devised by the brain and hand of man. Such are some of the questions with which modern astronomy deals. To answer them requires the employment of instruments of almost incredible refinement and exactitude, and also the full resources of mathematical genius. Whether astronomy be judged from the point of view of the phenomenon of light, the vast masses, the immense distances, the eons of time, or whether it be judged as a monument of human ingenuity, patience, and the rarest type of genius, it is certainly one of the grandest, as it is also one of the oldest of the sciences. The Solar System In the Solar System we include all those bodies dependent on the sun which circulate round it at various distances, deriving their light and heat from the sun, the planets and their moons, certain comets, and a multitude of meteors. In other words, all bodies whose movements in space are determined by the gravitational pull of the sun. The Sun Thanks to our wonderful modern instruments and the ingenious methods used by astronomers we have today a remarkable knowledge of the sun. Look at the figure of the sun in the frontispiece. The picture represents an eclipse of the sun. The dark body of the moon has screened the sun's shining disc and taken the glare out of our eyes. We see a silvery halo surrounding the great orb on every side. It is the sun's atmosphere, or crown, corona, stretching for millions of miles into space in the form of a soft, silvery-looking light. Probably much of its light is sunlight, reflected from particles of dust, although the spectroscope shows an element in the corona that has not so far been detected anywhere else in the universe, and which in consequence has been named coronium. We next noticed in the illustration that at the base of the halo there are red flames peeping out from the edges of the hidden disc. When one remembers that the sun is 866,000 miles in diameter, one hardly needs to be told that these flames are really gigantic. We shall see what they are presently. Regions of the Sun The astronomer has divided the sun into definite concentric regions or layers. These layers envelop the nucleus or central body of the sun, somewhat as the atmosphere envelops our earth. It is through these vapor layers that the bright, white body of the sun is seen. Of the innermost region, the heart or nucleus of the sun, we know almost nothing. The central body or nucleus is surrounded by a brilliantly luminous envelope or layer of vaporous matter, which is what we see when we look at the sun, and which the astronomer calls the photosphere. Above, that is overlying, the photosphere there is a second layer of glowing gases, which is known as the reversing layer. This layer is cooler than the underlying photosphere. It forms a veil of smoke-like haze and is of from 500 to 1,000 miles in thickness. A third layer or envelope immediately lying over the last one is the region known as the chromosphere. The chromosphere extends from 5,000 to 10,000 miles in thickness, a sea of red tumultuous surging fire. Chief among the glowing gases is the vapor of hydrogen. The intense white heat of the photosphere beneath shines through this layer, overpowering its brilliant redness. From the uppermost portion of the chromosphere great fiery tongues of glowing hydrogen and calcium vapor shoot out for many thousands of miles, driven outward by some prodigious expulsive force. It is these red prominences which are such a notable feature in the picture of the eclipse of the sun already referred to. During the solar eclipse of 1919 one of these red flames rose in less than seven hours from a height of 130,000 miles to more than 500,000 miles above the sun's surface. This immense column of red-hot gas four or five times the thickness of the earth was soaring upward at the rate of 60,000 miles an hour. These flaming jets or prominences shooting out from the chromosphere are not to be seen every day by the naked eye. The dazzling light of the sun obscures them, gigantic as they are. They can be observed, however, by the spectroscope any day and they are invisible to us for a very short time during an eclipse of the sun. Some extraordinary outbursts have been witnessed. Thus the late Professor Young described one on September 7, 1871, when he had been examining a prominence by the spectroscope. It had remained unchanged since noon of the previous day, a long, low, quiet-looking cloud, not very dense or brilliant, or in any way remarkable except for its size. At 12.30 p.m. the Professor left the spectroscope for a short time and on returning half an hour later to his observations he was astonished to find the gigantic sun flame shattered to pieces. The solar atmosphere was filled with flying debris and some of these portions reached a height of 100,000 miles above the solar surface. Moving with a velocity of which, even at the distance of 93 million miles, was almost perceptible to the eye, these fragments doubled their height in ten minutes. On January 30, 1885, another distinguished solar observer, the late Professor Tachini of Rome, observed one of the greatest prominences ever seen by man. His height was no less than 142,000 miles, eighteen times the diameter of the earth. Another mighty flame was so vast that supposing the eight large planets of the solar system ranged one on top of the other, their prominence would still tower above them. The fourth and uppermost layer or region is that of the corona, of immense extent and fading away into the surrounding sky. This we have already referred to. The diagram shows the dispositions of these various layers of the sun. It is through these several transparent layers that we see the white light body of the sun, the surface of the sun. Here let us return to and see what more we know about the photosphere, the sun's surface. It is from the photosphere that we have gained most of our knowledge of the composition of the sun, which is believed not to be a solid body. One of the photosphere shows that the outer surface is never at rest. Small bright cloudlets come and go in rapid succession, giving the surface through contrast and luminosity a granular appearance. Of course to be visible at all at 92,830,000 miles the cloudlets cannot be small. They imply enormous activity in the photosphere. If we might speak picturesquely, the sun's surface resembles a boiling ocean of white-hot metal vapors. We have today a wonderful instrument, which will be scribed later, which dilutes, as it were, the general glare of the sun, and enables us to observe these fiery eruptions at any hour. The oceans of red-hot gas and white-hot metal vapor at the sun's surface are constantly driven by great storms. Some unimaginable energy streams out from the body, or muscles of the sun, and blows its outer layers into gigantic shreds, as it were. The actual temperature at the sun's surface, or what appears to us to be the surface, the photosphere, is of course unknown, but careful calculation suggests that it is from 5,000 to 7,000 degrees centigrade. The interior is vastly hotter. We can form no conception of such temperatures as must exist there. Not even the most obdurate solid could resist such temperatures, but would be converted almost instantaneously into gas. But it would not be gas as we know gases on the earth. The enormous pressures that exist on the sun must convert even gases into thick, treacle-y fluids. We can only infer this state of matter. It is beyond our power to reproduce it. SUN SPOTS It is in the brilliant photosphere that the dark area is known as sunspots up here. Some of these dark spots, they are dark only by contrast with the photosphere surrounding them, are of enormous size, covering many thousands of square miles of surface. What they are we cannot positively say. They look like great cavities in the sun's surface. Some think they are giant whirlpools. Certainly they seem to be great whirling streams of glowing gases with vapors above them, and immense upward and downward currents within them. Round the edges of the sunspots rise great tongues of flame. Perhaps the most popularly known fact about sunspots is that they are somehow connected with what we call magnetic storms on earth. These magnetic storms manifest themselves in interruptions of our telegraphic and telephonic communications, in violent disturbances of the Mariner's compass, and in exceptional or rural displays. The connection between the two sets of phenomena cannot be doubted. Even although at times there may be a great spot on the sun without any corresponding magnetic storm effects on the earth. A surprising fact about sunspots is that they show definite periodic variations in number. The best-defined period is one of about eleven years. During this period the spots increase to a maximum in number and then diminish to a minimum, the variation being more or less regular. This can only mean one thing. To be periodic the spots must have some deep-seated connection with the fundamental facts of the sun's structure and activities. Looked at from this point of view their importance becomes great. The aurora borealis is one of the most beautiful spectacles in the sky. The colors and shape change every instant, sometimes a fan-like cluster of rays, and other times long golden draperies gliding one over the other. Blue, green, yellow, red, and white combine to give a glorious display of color. The theory of its origin is still in part obscure, but there can be no doubt that the aurora is related to the magnetic phenomena of the earth and therefore is connected with the electrical influence of the sun. It is from the study of sunspots that we have learned that the sun's surface does not appear to rotate all at the same speed. The equatorial regions are rotating quicker than regions farther north or south. A .45 degrees from the equator seems to take about two and a half days longer to complete one rotation than a point on the equator. This, of course, confirms our belief that the sun cannot be a solid body. What is its composition? We know that there are present in a gaseous state such well-known elements as sodium, iron, copper, zinc, and magnesium. Indeed, we know that there is practically every element in the sun that we know to be in the earth. How do we know? It is from this photosphere, as has been said, that we have won most of our knowledge of the sun. The instrument used for this purpose is the spectroscope, and before proceeding to deal further with the sun and the source of its energy it will be better to describe this instrument. A wonderful instrument and what it reveals. The spectroscope is an instrument for analyzing light. So important is it in the revelations it has given us that it will be best to describe it fully. Every substance to be examined must first be made to glow, made luminous, and as nearly everything in the heavens is luminous, the instrument has a great range in astronomy. And when we speak of analyzing light we mean that the light may be broken up into waves of different lengths. What we call light is a series of minute waves in ether, and these waves are, measuring them from crest to crest, so to say, of various lengths. Each wavelength corresponds to a color of the rainbow. The shortest waves give us a sensation of violet color, and the largest waves cause a sensation of red. The rainbow, in fact, is a sort of natural spectrum. The meaning of the rainbow is that the moisture laid in air has sorted out these waves in the sun's light according to their length. Now the simplest form of spectroscope is a glass prism, a triangular-shaped piece of glass. If white light, sunlight, for example, passes through a glass prism, we see a series of rainbow-tinted colors. Anyone can notice this effect when sunlight is shining through any kind of cut glass, the stopper of a wine decanter, for instance. If, instead of catching with the eye the colored lights as they emerge from the glass prism, we allow them to fall on a screen, we shall find that they pass by continuous gradations from red at the one end of the screen, through orange, yellow, green, blue, and indigo, to violet at the other end. In other words, what we call white light is composed of arrays of these several colors. They go to make up the effect which we call white, and now just as water can be split up into its two elements, oxygen and hydrogen, so sunlight can be broken up into its primary colors, which are those we have just mentioned. This range of colors produced by the spectroscope we call the solar spectrum, and these are, from the spectroscopic point of view, primary colors. Each shade of color has its definite position in the spectrum. That is to say, the light of each shade of color corresponding to its wavelength is reflected through a certain fixed angle on passing through the glass prism. Every possible kind of light has its definite position, and is denoted by a number which gives the wavelength of the vibrations constituting that particular kind of light. Now other kinds of light, besides sunlight, can be analyzed. Light from any substance which has been made incandescent may be observed with the spectroscope in the same way, and each element can be thus separated. It is found that each substance, in the same conditions of pressure, etc., gives a constant spectrum of its own. Each metal displays its own distinctive color. It is obvious, therefore, that the spectrum provides the means for identifying a particular substance. It was by this method that we discovered in the sun the presence of such well-known elements as sodium, iron, copper, zinc, and magnesium. Every chemical element known, then, has a distinctive spectrum of its own when it is raised to incandescence, and this distinctive spectrum is as reliable a means of identification for the element as a human face is for its owner. Whether it is a substance glowing in the laboratory or in a remote star makes no difference to the spectroscope. If the light of any substance reaches it, that substance will be recognized and identified by the characteristic set of waves. The spectrum of a glowing mass of gas will consist in a number of bright lines of various colors and at various intervals. Corresponding to each kind of gas, there will be a peculiar and distinctive arrangement of bright lines. But if the light from such a mass of glowing gas be made to pass through a cool mass of the same gas, it will be found that dark lines replace the bright lines in the spectrum. The reason for this being that the cool gas absorbs the ray of light emitted by the hot gas. Experiments of this kind enable us to reach the important general statement that every gas, when cold, absorbs the same rays of light which it emits when hot. Crossing the solar spectrum are hundreds and hundreds of dark lines. These could not at first be explained because this fact of discriminative absorption was not known. We understand now. The sun's white light comes from the photosphere, but between us and the photosphere there is, as we have seen, another solar envelope of relatively cooler vapors, the reversing layer. Each constituent element in this outer envelope stopped its own kind of light, that is, the kind of light made by incandescent atoms of the same element in the photosphere. The stoppages registered themselves in the solar spectrum as dark lines, placed exactly where the corresponding bright lines would have been. The explanation once attained, dark lines became as significant as bright lines. The secret of the sun's composition was out. We have found practically every element in the sun that we know to be in the earth. We have identified an element in the sun before we were able to isolate it on the earth. We have been able even to point to the clueless places on the sun, the centers of sunspots, where alone the temperature seems to have fallen sufficiently low to allow chemical compounds to form. It is thus we have been able to determine what the stars, comets, or nebulae are made of. A unique discovery. In 1868 Sir Norman Lockyer detected a light coming from the prominences of the sun which was not given by any substance known on earth, and attributed this to an unknown gas which he called Helium, from the Greek Helios, the sun. In 1895 Sir William Ramsey discovered in certain minerals the same gas identified by the spectroscope. We can say therefore that this gas was discovered in the sun nearly thirty years before it was found on earth. This discovery of the long lost air is as thrilling a chapter in the detective story of science as any in the sensational stories of the day, and makes us feel quite certain that our methods really tell us of what elements sun and stars are built up. The light from the corona of the sun, as we have mentioned, indicates a gas still unknown on earth, which has been christened Coronium, measuring the speed of light. But this is not all. Soon a new use was found for the spectroscope. We found that we could measure with it the most difficult of all speeds to measure, speed in the line of sight. Movement at right angles to the direction in which one is looking is, if there is sufficient of it, easy to detect, and if the distance of the moving body is known, easy to measure. But movement in the line of vision is both difficult to detect and difficult to measure. Yet even at the enormous distances with which astronomers have to deal, the spectroscope can detect such movement and furnish data for its measurement. If a luminous body containing, say, sodium is moving rapidly towards the spectroscope, it will be found that the sodium lines in the spectrum have moved slightly from their usual definite positions towards the violet end of the spectrum, the amount of the change of position increasing with the speed of the luminous body. If the body is moving away from the spectroscope, the shifting of the spectral lines will be in the opposite direction towards the red end of the spectrum. In this way we have discovered and measured movements that otherwise would probably not have revealed themselves unmistakably to us for thousands of years. In the same way we have watched and measured the speed of tremendous movements on the sun, and so gained proof that the vast disturbances we should expect there actually do occur. Is the sun dying? Now let us return to our consideration of the sun. To us on the earth the most patent and most astonishing fact about the sun is its tremendous energy. Heat and light in amazing quantities pour from it without seizing. Where does this energy come from? Enormous jets of red glowing gases can be seen shooting outwards from the sun, like flames from a fire, for thousands of miles. Does this argue fire as we know fire on the earth? On this point the scientist is sure. The sun is not burning, and combustion is not the source of its heat. Combustion is a chemical reaction between atoms. The conditions that make it possible are known, and the results are predictable and measurable. But no chemical reaction of the nature of combustion, as we know it, will explain the sun's energy, nor indeed will any ordinary chemical reaction of any kind. If the sun were composed of combustible material throughout, and the conditions of combustion as we understand them were always present, the sun would burn itself out in some thousands of years, with marked changes in its light and heat production as the process advanced. There is no evidence of such changes. There is instead strong evidence that the sun has been emitting light and heat in prodigious quantities, not for thousands, but for millions of years. Every addition to our knowledge that throws light on the sun's age seems to make for increase rather than decrease of its years. This makes the wonder of its energy greater. And we cannot avoid the issue of the source of the energy by saying merely that the sun is gradually radiating away an energy that originated in some unknown manner, a way back at the beginning of things. Reliable calculations show that the years required for the mere cooling of a globe like the sun could not possibly run to millions. In other words, the sun's energy must be subject to continuous and more or less steady renewal. However it may have acquired its enormous energy in the past, it must have some source of energy in the present. The best explanation that we have today of this continuous accretion of energy is that it is due to shrinkage of the sun's bulk under the force of gravity. Gravity is one of the most mysterious forces of nature, but it is an obvious fact that bodies behave as if they attracted one another, and Newton worked out the law of this attraction. We men say, without trying to go too deeply into things, that every particle of matter attracts every other throughout the universe. If the diameter of the sun were to shrink by one mile all round, this would mean that all the millions of tons in the outer one mile thickness would have a straight drop of one mile towards the center. And that is not all, because obviously the layers below this outer mile would also drop inwards, each to a less degree than the one above it. What a tremendous movement of matter, however slowly it might take place, and what a tremendous energy would be involved. Astronomers calculate that the above shrinkage of one mile all round would require fifty years for its completion, assuming, reasonably, that there is close and continuous relationship between loss of heat by radiation and shrinkage. Even if this were true we need not feel over-anxious on this theory, before the sun became too cold to support life many millions of years would be required. It was suggested at one time that falls of meteoric matter into the sun would account for the sun's heat. This position is hardly tenable now. The mere bulk of the meteoric matter required by the hypothesis, apart from other reasons, is against it. There is undoubtedly an enormous amount of meteoric matter moving about within the bounds of the solar system, but most of it seems to be following definite routes round the sun like the planets. The stray erratic quantities destined to meet their doom by collision with the sun can hardly be sufficient to account for the sun's heat. Recent study of radioactive bodies has suggested another factor that may be working powerfully along with the force of gravitation to maintain the sun's store of heat. In radioactive bodies certain atoms seem to be undergoing disintegration. These atoms appear to be splitting up into very minute and primitive constituents. But since matter may be split up into such constituents, may it not be built up from them? The question is whether these radioactive elements are undergoing disintegration or formation in the sun. If they are undergoing disintegration and the sun itself is undoubtedly radioactive, then we have another source of heat for the sun that will last indefinitely. Next of this part of Chapter 1. The Outline of Science, Chapter 1, Part 2. This recording is by Mark Smith of Simpsonville, South Carolina. The Outline of Science, edited by J. Arthur Thompson. Chapter 1, Part 2. The Planets, Life and Other Worlds It is quite clear that there cannot be life on the stars. Nothing solid or even liquid can exist in such furnaces as they are. Life exists only on planets, and even on these its possibilities are limited. Whether all the stars, or how many of them, have planetary families like our sun, we cannot positively say. If they have, such planets would be too faint and small to be visible tens of trillions of miles away. Some astronomers think that our sun may be exceptional in having planets, but their reasons are speculative and unconvincing. Probably a large proportion at least of the stars have planets, and we may therefore survey the globes of our own solar system and in a general way extend the results to the rest of the universe. In considering the possibility of life as we know it, we may at once rule out the most distant planets from the sun, Uranus and Neptune. They are probably intrinsically too hot. We may also pass over the nearest planet to the sun, Mercury. We have reason to believe that it turns on its axis in the same period as it revolves around the sun, and it must therefore always present the same side to the sun. This means that the heat on the sunlit side of Mercury is above boiling point, while the cold on the other side must be between 2 and 300 degrees below freezing point. THE PLANET VENUS The planet Venus, the bright globe which is known to all as the morning and evening star, seems at first sight more promising as regards the possibility of life. It is of nearly the same size as the earth, and it has a good atmosphere, but there are many astronomers who believe that, like Mercury, it always presents the same face to the sun, and would therefore have the same disadvantage, a broiling heat on the sunny side and the cold of space on the opposite side. We are not sure. The surface of Venus is so bright, the light of the sun is reflected to us by such dense masses of cloud and dust, that it is difficult to trace any permanent markings on it, and thus ascertain how long it takes to rotate on its axis. Many astronomers believe that they have succeeded, and that the planet always turns the same face to the sun. If it does, we can hardly conceive of life on its surface in spite of the cloud screen. We turn to Mars, and we must first make it clear why there is so much speculation about life on Mars, and why it is supposed that, if there is life on Mars, it must be more advanced than life on the earth. IS THERE LIFE ON MARS? The basis of this belief is that if, as we saw, all the globes in our solar system are masses of metal that are cooling down, the smaller will have cooled down before the larger, and will be further ahead in their development. Now Mars is very much smaller than the earth, and must have cooled at its surface millions of years before the earth did. Hence, if a story of life began on Mars at all, it began long before the story of life on the earth. We cannot guess what sort of life-forms would be evolved in a different world, but we can confidently say that they would tend toward increasing intelligence, and thus we are disposed to look for highly intelligent beings on Mars. But this argument supposes that the conditions of life, namely air and water, are found on Mars, and it is disputed whether they are found there in sufficient quantity. The late Professor Percival Lowell, who made a lifelong study of Mars, maintained that there are hundreds of straight lines drawn across the surface of the planet, and he claimed that there are beds of vegetation marking the sites of great channels or pipes, by means of which the Martians draw water from their polar ocean. Professor W. H. Pickering, another high authority, thinks that the lines are long, narrow marshes fed by moist winds from the poles. There are certainly white polar caps on Mars, they seem to melt in the spring, and the dark fringe around them grows broader. Other astronomers, however, say that they find no trace of water vapor in the atmosphere of Mars, and they think that the polar caps may be simply thin sheets of whorefrost or frozen gas. They point out that, as the atmosphere of Mars is certainly scanty, and the distance from the sun is so great, it may be too cold for the fluid water to exist on the planet. If one asks why our wonderful instruments cannot settle these points, one must be reminded that Mars is never nearer than 34 million miles from the Earth, and only approaches to this distance once in 15 or 17 years. The image of Mars on the photographic negative taken in a big telescope is very small. Astronomers rely to a great extent on the eye, which is more sensitive than the photographic plate. But it is easy to have differences of opinion as to what the eye sees, and so there is a good deal of controversy. In August 1924 the planet will again be well placed for observation, and we may learn more about it. Already a few of the much disputed lines, which people wrongly call canals, have been traced on photographs. Astronomers who are skeptical about life on Mars are often not fully aware of the extraordinary adaptability of life. There was a time when the climate of the whole Earth from pole to pole was semi-tropical for millions of years. No animal could then endure the least cold, yet now we have plenty of arctic plants and animals. If the cold came slowly on Mars, as we have reason to suppose, the population could be gradually adapted to it. On the whole it is possible that there is advanced life on Mars, and it is not impossible, in spite of the very great difficulties of a code of communication, that our elder brothers may yet flash across space the solution of many of our problems. Jupiter and Saturn Next to Mars, going outward from the Sun, is Jupiter. Between Mars and Jupiter, however, there are more than 300 million miles of space, and the older astronomers wondered why this was not occupied by a planet. We now know that it contains about 900 planetoids, or small globes of from five to five hundred miles in diameter. It was at one time thought that a planet might have burst into these fragments, a theory which is not mathematically satisfactory. Or it may be that the material which is scattered in them was prevented by the nearness of the great bulk of Jupiter from uniting into one globe. For Jupiter is a giant planet, and its gravitational influence must extend far over space. It is 1,300 times as large as the Earth, and has nine moons, four of which are large, in attendance on it. It is interesting to note that the outermost moons of Jupiter and Saturn revolve round these planets in a direction contrary to the usual direction taken by moons round planets, and by planets round the Sun. But there is no life on Jupiter. The surface which we see in photographs is a mass of cloud or steam which always envelops the body of the planet. It is apparently red-hot. A red tinge is seen sometimes at the edges of its cloud-belts, and a large red region, the red spot, 23,000 miles in length, has been visible on it for half a century. There may be a liquid or solid core to the planet, but as a whole it is a mass of seething vapors whirling around on its axis once in every 10 hours. As in the case of the Sun, however, different latitudes appear to rotate at different rates. The interior of Jupiter is very hot, but the planet is not self-luminous. The planet's Venus and Jupiter shine very brightly, but they have no light of their own. They reflect the sunlight. Saturn is in the same interesting condition. The surface in the photograph is steam, and Saturn is so far away from the Sun that the vaporization of its oceans must necessarily be due to its own internal heat. It is too hot for water to settle on its surface. Like Jupiter, the great globe turns on its axis once in 10 hours, a prodigious speed, and must be a swirling, seething mass of metallic vapors and gases. It is instructive to compare Jupiter and Saturn in this respect with the Sun. They are smaller globes, and have cooled down more than the central fire. Saturn is a beautiful object in the telescope because it is 10 moons to include one which is disputed, and a wonderful system of rings around it. The so-called rings are a mighty swarm of meteorites, pieces of iron and stone of all sorts and sizes, which reflect the light of the Sun to us. This ocean of matter is some miles deep and stretches from a few thousand miles from the surface of the planet to 172,000 miles out in space. Some astronomers think that this is volcanic material which has been shot out of the planet. Others regard it as stuff which would have combined to form an 11th moon, but was prevented by the nearness of Saturn itself. There is no evidence of life on Saturn. The moon. Mars and Venus are therefore the only planets beside the Earth on which we may look for life. And in the case of Venus, the possibility is very faint. But what about the moons which attend the planets? They range in size from the little 10 miles wide moons of Mars to Titan, a moon of Saturn, and Ganymede, a satellite of Jupiter, which are about 3000 miles in diameter. Might there not be life on some of the larger of these moons? We will take our own moon as a type of the class. A dead world. The moon is so very much nearer to us than any other heavenly body that we have a remarkable knowledge of it. In figure 14, you have a photograph taken in one of our largest telescopes of part of its surface. In a sense, such a telescope brings the moon to within about 50 miles of us. We should see a city like London as a dark, sprawling blotch on the globe. We could just detect a zeppelin or a deploticus as a moving speck against the surface. But we find none of these things. It is true that a few astronomers believe that they see signs of some sort of feeble life or movement on the moon. Professor Pickering thinks that he can trace some volcanic activity. He believes that there are areas of vegetation, probably of a low order, and that the soil of the moon may retain a certain amount of water in it. He speaks of a very thin atmosphere and of occasional light falls of snow. He has succeeded in persuading some careful observers that there probably are slight changes of some kind taking place on the moon. But there are many things that point to absence of air on the moon. Even the photographs we reproduce tell the same story. The edges of the shadow are all hard and black. If there had been an appreciable atmosphere, it would have scattered the sun's light onto the edges and produced a gradual shading off such as we see on the earth. This relative absence of air must give rise to some surprising effects. There will be no sounds on the moon because sounds are merely airwaves. Even a meteor shattering itself to a violent end against the surface of the moon would make no noise. Nor would it herald its coming by glowing into a shooting star as it would on entering the earth's atmosphere. There will be no floating dust, no scent, no twilight, no blue sky, no twinkling of the stars. The sky will be always black and the stars will be clearly visible by day as by night. The sun's wonderful corona, which no man on earth, even by seizing every opportunity during eclipses, can hope to see for more than two hours in all of a long lifetime, will be visible all day. So will the great red flames of the sun. Of course, there will be no life and no landscape effects and scenery effects due to vegetation. The moon takes approximately twenty-seven of our days to turn once on its axis. So for fourteen days there is a continuous night when the temperature must sink away down towards the absolute cold of space. This will be followed without an instant of twilight by full daylight. For another fourteen days the sun's rays will bear straight down with no diffusion or absorption of their heat or light on the way. It does not follow, however, that the temperature of the moon's surface must rise enormously. It may not even rise to the temperature of melting ice. Seeing there is no air, there could be no check on radiation. The heat that the moon gets would radiate away immediately. We know that amongst the coldest places on the earth are the tops of very high mountains. The points that have reared themselves nearest to the sun but farthest out of the sheltering blanket of the earth's atmosphere. The actual temperature of the moon's surface by day is a moot point. It may be below the freezing point or above the boiling point of water. The Mountains of the Moon The lack of air is considered by many astronomers to furnish the explanation of the enormous number of craters which pit the moon's surface. There are about a hundred thousand of these strange rings and it is now believed by many that they are spots where very large meteorites or even planetoids splashed into the moon when its surface was still soft. Other astronomers think that they are the remains of gigantic bubbles which were raised in the moon's skin when the globe was still molten by volcanic gases from below. A few astronomers think that they are, as is popularly supposed, the craters of extinct volcanoes. Our craters on the earth are generally deep cups whereas these ring formations on the moon are more like very shallow and broad saucers. Clavius, the largest of them, is 123 miles across the interior, yet its encircling rampart is not a mile high. The mountains on the moon rise to a great height and are extraordinarily gaunt and rugged. They are like fountains of lava rising in places to 26,000 and 27,000 feet. The lunar apanines have 3,000 steep and weird peaks. Our terrestrial mountains are continually worn down by frost acting on moisture and by ice and water but there are none of these agencies operating on the moon. Its mountains are comparatively everlasting hills. The moon is interesting to us precisely because it is a dead world. It seems to show how the earth or any cooling metal globe will evolve in the remote future. We do not know if there was ever life on the moon but in any case it cannot have proceeded far in development. At the most we can imagine some strange, lowly forms of vegetation lingering here and there in pools of heavy gas, expanding during the blaze of the sun's long day and frozen rigid during the long night. Meteors and Comets We may conclude our survey of the solar system with a word about shooting stars or meteors and comets. There are few now who do not know that the streak of fire which suddenly lights the sky overhead at night means that a piece of stone or iron has entered our atmosphere from outer space and has been burned up by friction. It was travelling at perhaps 20 or 30 miles a second. At 70 or 80 miles above our heads it began to glow as at that height the air is thick enough to offer serious friction and raise it to a white heat. By the time the meteor reached about 20 miles or so from the earth's surface it was entirely dissipated as a rule in fiery vapor. Millions of Meteorites It is estimated that between 10 and 100 million meteorites enter our atmosphere and are cremated every day. Most of them weigh only an ounce or two and are invisible. Some of them weigh a ton or more but even against these large masses the air acts as a kind of torpedo net. They gently burst into fragments and fall without doing damage. It is clear that empty space is, at least within the limits of our solar system, full of these things. They swarm like fishes in the seas. Like the fishes, moreover, they may be either solitary or gregarious. The solitary bit of cosmic rubbish is the meteorite which we have just examined. A social group of meteorites is the essential part of a comet. The nucleus or bright central part of the head of a comet consists of a swarm sometimes thousands of miles wide of these pieces of iron or stone. This swarm has come under the sun's gravitational influence and is forced to travel round it. From some dark region of space it has moved slowly into our system. It is not then a comet for it has no tail. But as the crowded meteors approach the sun the speed increases. They give off fine vapor-like matter and the fierce flood of light from the sun sweeps this vapor out in an ever-lengthening tail. Whatever way the comet is traveling the tail always points away from the sun. A great comet. The vapor-y tail often grows to an enormous length as the comet approaches the sun. The great comet of 1843 had a tail 200 billion miles long. It is, however, composed of the thinnest vapors imaginable. Twice during the 19th century the earth passed through the tail of a comet and nothing was felt. The vapors of the tail are, in fact, so attenuated that we can hardly imagine them to be white-hot. They may be lit by some electrical force. However that may be the comet dashes round the sun often at three or 400 miles a second that may pass gradually out of our system once more. It may be a thousand years or it may be 50 years before the monarch of the system will summon it again to make its fiery journey round his throne. The stellar universe. The immensity of the stellar universe as we have seen is beyond our apprehension. The sun is nothing more than a very ordinary star, perhaps an insignificant one. There are stars enormously greater than the sun. One such, Betelgeuse, has recently been measured and its diameter is more than 300 times that of the sun. The evolution of the stars. The proof of the similarity between our sun and the stars come to us through the spectroscope. The elements that we find by its means in the sun are also found in the same way in the stars. Matter, says the spectroscope, is essentially the same everywhere, in the earth and the sun, in the comet that visits us once in a thousand years, in the star whose distance is incalculable, and in the great clouds of fire mist that we call nebulae. In considering the evolution of the stars, let us keep two points clearly in mind. The starting point, the nebula, is no figment of the scientific imagination. Hundreds of thousands of nebulae, besides even faster irregular stretches of nebulus matter, exist in the heavens. But the stages of the evolution of this stuff into stars are very largely a matter of speculation. Possibly there is more than one line of evolution and the various theories may be reconciled. And this applies also to the theories of the various stages through which the stars themselves pass on their way to extinction. The light of about a quarter of a million stars has been analyzed in the spectroscope, and it is found that they fall into about a dozen classes which generally correspond to stages in the revolution. The age of stars. In its main lines, the spectrum of a star corresponds to its color, and we may roughly group the stars into red, yellow, and white. This is also the order of increasing temperature, the red stars being the coolest and the white stars the hottest. We might therefore imagine that the white stars are the youngest and that as they grow older and cooler, they become yellowish, then red, and finally become invisible, just as a cooling white hot iron would do. But a very interesting recent research shows that there are two kinds of red stars. Some of them are amongst the oldest stars and some are amongst the youngest. The facts appear to be that when a star is first formed, it is not very hot. It is an immense mass of diffuse gas glowing with a dull red heat. It contracts under the mutual gravitation of its particles, and as it does so, it grows hotter. It acquires a yellowish tinge. As it continues to contract, it grows hotter and hotter until its temperature reaches a maximum as a white star. At this point, the contraction process does not stop, but the heating process does. Further contraction is now accompanied by cooling, and the star goes through the color changes again, but this time in the inverse order. It contracts and cools to yellow and finally to red, but when it again becomes a red star, it is enormously denser and smaller than when it began as a red star. Consequently, the red stars are divided into two classes called, appropriately, giants and dwarfs. This theory, which we owe to an American astronomer, H.N. Russell, has been successful in explaining a variety of phenomena, and there is consequently good reason to suppose it to be true. But the question as to how the red giant stars were formed has received less satisfactory and precise answers. The most commonly accepted theory is the nebular theory. Nebulae are dim, luminous, cloud-like patches in the heavens, more like wisps of smoke in some cases than anything else. Both photography and the telescope show that they are very numerous, hundreds of thousands being already known, and the number being continually added to. They are not small. Most of them are immensely large. Actual dimensions cannot be given, because to estimate these we must first know definitely the distance of the nebulae from the earth. The distances of some nebulae are known approximately, and we can therefore form some idea of size in these cases. The results are staggering. The mere visible surface of some nebulae is so large that the whole stretch of the solar system would be too small to form a convenient unit for measuring it. A ray of light would require to travel for years to cross from side to side, its immensity is inconceivable to the human mind. There appear to be two types of nebulae, and there is evidence suggesting that the one type is only an earlier form of the other, but this again we do not know. The more primitive nebulae would seem to be composed of gas in an extremely rarefied form. It is difficult to convey an adequate idea of the rarity of nebular gases. The resistance of the nebulae The residual gases in a vacuum tube are dense by comparison. A cubic inch of air at ordinary pressure would contain more matter than is contained in billions of cubic inches of the gases of nebulae. The light of even the faintest stars does not seem to be dimmed by passing through a gaseous nebula, although we cannot be sure on this point. The most remarkable physical fact about these gases is that they are luminous. Once they derive their luminosity, we do not know. It hardly seems possible to believe that extremely thin gases exposed to the terrific coldest space can be so hot as to be luminous and can retain their heat and their luminosity indefinitely. A cold luminosity due to electrification, like that of the aurora borealis, would seem to fit the case better. Now the nebular theory is that out of great fire mists, such as we have described, stars are born. We did not know whether gravitation is the only or even the main force at work in a nebula, but it is supposed that under the action of gravity the far-flung fire mists would begin to condense round centers of greatest density, heat being evolved in the process. Of course the condensation would be enormously slow, although the sudden eruption of a swarm of meteors or some solid body might hasten matters greatly by providing large, ready-made centers of condensation. Spiral nebulae. It is then supposed that the contracting mass of gas would begin to rotate and to throw off gigantic streamers which would in their turn form centers of condensation. The whole structure would thus form a spiral, having a dense region at its center of condense matter along its spiral arms. Besides the formless gaseous nebulae there are hundreds of thousands of spiral nebulae such as we have just mentioned in the heavens. They are at all stages of development and they are visible to us at all angles. That is to say, some of them face directly towards us, others are edge-on and some are in intermediate positions. It appears therefore that we have here a striking confirmation of the nebular hypothesis. But we must not go so fast. There is much controversy as to the nature of the spiral nebulae. Some eminent astronomers think they are other stellar universes comparable in size with our own. In any case they are vast structures and if they represent stars in process of condensation they must be giving birth to huge agglomerations of stars to start clusters at least. These vast and enigmatic objects do not throw much light on the origin of our own solar system. The nebular hypothesis which was invented by Laplace to explain the origin of our solar system has not yet met with universal acceptance. The explanation offers grave difficulties and it is best while the subject is still being closely investigated to hold all opinions with reserve. It may be taken as probable however that the universe has developed for masses of incandescent gas. The birth and death of stars. Variable, new and dark stars dying suns. Many astronomers believe that in variable stars we have another star following that of the dullest red star in the dying of suns. The light of these stars varies periodically in so many days, weeks or years. It is interesting to speculate that they are slowly dying suns in which the molten interior periodically burst through the shell of thick vapors that is gathering round them. What we saw about our sun seems to point to some such stage in the future. That is however not the received opinion about variable stars. It may be that they are stars which periodically pass through a great swarm of meteors or a region of space that is rich in cosmic dust or a resort when of course a great illumination would take place. One class of these variable stars which takes its name from the star Algol is of special interest. Every third night Algol has its light reduced for several hours. Modern astronomy has discovered that in this case there are really two stars circulating round a common center and that every third night the fainter of the two comes and its companion and causes an eclipse. This was until recently regarded as a most interesting case in which a dead star revealed itself to us by passing before the light of another star. But astronomers have in recent years invented something the selenium cell which is even more sensitive than the photographic plate and on this the supposed dead star registers itself Algol is however interesting in another way. The pair of stars which we have discovered in it are hundreds of trillions of miles away from the earth yet we know their masses and their distances from each other. The Death and Birth of Stars We have no positive knowledge of dead stars which is not surprising when we reflect that a dead star means an invisible star standing toward death when we behold a vast population of all conceivable ages we presume that there are many already dead. On the other hand there is no reason to suppose that the universe as a whole is running down. Some writers have maintained this but their argument implies that we know a great deal more about the universe than we actually do. The scientific man does not know how infinite, temporal or eternal and he declines to speculate when there are no facts to guide him. He knows only that the great gaseous nebulae promise myriads of worlds in the future and he concedes the possibility that new nebulae may be forming in the ether of space. The last and not the least interesting subject we have to notice is the birth of a new star. This is an event which astronomers have seen every few years and it is a far more pretentious event than the reader imagines when it is reported in his daily paper. The story is much the same in all cases. We say that the star appeared in 1901 but you begin to realize the magnitude of the event when you learn that the distant blaze had really occurred about the time of the death of Luther. The light of the conflagration was 186,000 miles a second yet it is taken nearly three centuries to reach us. To be visible at all to us at that distance the fiery outbreak must have been stupendous. If a mass of petroleum ten times the size of the earth were suddenly fired it would not be seen at such a distance. The new star had increased its light many hundred fold in a few days. There is a considerable fascination to the speculation that in such cases we see the resurrection of a dead world, a means of renewing the population of the universe. What happens is that in some region of the sky where no star or only a very faint star had been registered on our charts we almost suddenly perceive a bright star. In a few days it may rise to the highest brilliancy. By the spectroscope we learn that this distant blaze reaches outpour of white-hot hydrogen at hundreds of miles a second. But the star sinks again after a few months and we then find a nebula around it on every side. It is natural to suppose that a dead or dying sun has somehow been reconverted in whole or in part into a nebula. A few astronomers think that it may have partially collided with another star or approached too closely to another with the result described on an earlier page. The general opinion now is that a faint or dead star had rushed into one of those regions of space in which there are immense stretches of nebulous matter and been, at least in part, vaporized by the friction. But the difficulties are considerable and some astronomers prefer to think that the blazing star may merely have lit up a dark nebula which already existed. It is one of those problems on which speculation is most tempting but positive knowledge is still very incomplete. We may be content, even proud, that already we can take a conflagration that has occurred more than a thousand trillion miles away and analyze it positively into an outflame of glowing hydrogen gas at so many miles a second. The shape of our universe Our universe, a spiral nebula. What is the shape of our universe and what are its dimensions? This is a tremendous question to ask. It is like asking an intelligent insect living on a single leaf in the midst of a great bazillion forest to say what is the shape and size of the forest? Yet man's ingenuity has proved equal to giving an answer even to this question and by a method exactly similar to that which would be adopted by the insect. Suppose, for instance, the forest was shaped as an elongated oval and the insect lived on a tree near the center of the oval. If the trees were approximately equally spaced from one another they would appear much denser along the length of the oval than across its width. This is the simple consideration that is guided astronomers in determining the shape of our stellar universe. There is one direction in the heavens along which the stars appear denser than in the directions at right angles to it. That direction is the direction in which we look towards the Milky Way. If we count the number of stars visible all over the heavens we find that they become more and more numerous as we approach the Milky Way. As we go farther and farther from the Milky Way the stars thin out until they reach a maximum sparseness in directions at right angles to the plane of the Milky Way. We may consider the Milky Way to form as it were the equator of our system and the line at right angles to point to the north and south poles. Our system in fact is shaped something like a lens and our sun is situated near the center of this lens. In the remotor part of this lens near its edge or possibly outside it altogether lies the great series of star clouds which make up the Milky Way. All the stars are in motion within the system but the very remarkable discovery has been made that these motions are not entirely random. The great majority of the stars whose motions can be measured fall into two groups drifting past one another in opposite directions. The velocity of one stream relative to the other is about 25 miles per second. The stars forming these two groups are thoroughly well mixed. It is not a case of an inner stream going one way and an outer stream the other. For every two stars in one stream there are three in the other. Now as we have said some eminent astronomers hold that the spiral nebulae are universes like our own and if we look at the two photographs we see that these spirals present features which in the light of what we have just said about our system are very remarkable. The nebulae and coma Baranissis is a spiral edge on to us and we see that it has precisely the lens-shape middle and the general flattened shape that we have found in our own system. The nebulae in Canis Venatisi is a spiral facing toward us and its shape irresistibly suggests motions along the spiral arms. This motion whether it is towards or away from the central lens-shape portion would cause a double streaming motion in that central portion of the kind that we have found in our system. Again, and altogether apart from these considerations there are good reasons for supposing our Milky Way to possess a double-armed spiral structure and the great patches of dark absorbing matter which are known to exist in the Milky Way would give very much the modeled appearance we notice in the arms which we see edge on of the nebulae in coma Baranissis. The hypothesis therefore that our universe is a spiral nebula has much to be said for it. If it be accepted it greatly increases our estimate of the size of the material universe. For our central lens-shape system is calculated to extend towards the Milky Way for more than 20,000 times a million million miles and about a third of this distance towards what we have called the poles. If, as we suppose each spiral nebula is an independent stellar universe comparable in size with our own then, since there are hundreds of thousands of spiral nebulae we see that the size of the whole material universe is indeed beyond our comprehension. In this simple outline we have not touched on some of the more debatable questions that engage the attention of modern astronomers. Many of these questions have not yet passed the controversial stage. Out of these will emerge the astronomy of the future. But we have seen enough to convince us that whatever advances the future holds in store the science of the heavens constitutes one of the most important stones in the wonderful fabric of human knowledge. Astronomical Instruments The Telescope The instruments used in modern astronomy are amongst the finest triumphs of mechanical skill in the world. With modern observatory the different instruments are to be counted by the score but there are two which stand out preeminent as the fundamental instruments of modern astronomy. These instruments are the telescope and the spectroscope and without them astronomy as we know it could not exist. There is still some dispute as to where and when the first telescope was constructed as an astronomical instrument however it dates from the time of the great Italian scientist Galileo who with a very small and imperfect telescope of his own invention first observed the spots on the sun the mountains of the moon and the chief force satellites of Jupiter. A good pair of modern binoculars is superior to this early instrument of Galileo's and the history of telescope construction from that primitive instrument to the modern giant recently erected on Mount Wilson, California is an exciting chapter of human progress but the early instruments gave only an historic interest the era of modern telescopes begins in the 19th century. During the last century telescope construction underwent an unprecedented development an immense amount of interest was taken in the construction of large telescopes and the different countries of the world entered on an exciting race to produce the most powerful possible instruments. Besides this rivalry of different countries there was a rivalry of methods. The telescope developed along two different lines and each of these two types has its partisans at the present day. These types are known as refractors and reflectors and it is necessary to mention briefly the principles employed in each. The refractor is the ordinary familiar type of telescope. It consists essentially of a large lens at one end of a tube and a small lens called the eye piece at the other. The function of the large lens is to act as a sort of gigantic eye. It collects a large amount of light and amount proportional to its size and brings this light to a focus within the tube of the telescope. It thus produces a small but bright image and the eye piece magnifies this image. In the reflector instead of a large lens at the top of the tube a large mirror is placed at the bottom. This mirror is so shaped as to reflect the light that falls on it to a focus once the light is again led to an eye piece. Thus the refractor and the reflector differ chiefly in their manner of gathering light. The powerfulness of the telescope depends on the size of the light gatherer. A telescope with a lens four inches in diameter is four times as powerful as the one with a lens two inches in diameter. For the amount of light gathered obviously depends on the area of the lens and the area varies as the square of the diameter. The largest telescopes at present in existence are reflectors. It is much easier to construct a very large mirror than to construct a very large lens. It is also cheaper. A mirror is more likely to get out of order than is a lens however and any irregularity in the shape reduces a greater distorting effect than in a lens. A refractor is also more convenient to handle than is a reflector. For these reasons great refractors are still made but the largest of them the great Yerks refractor is much smaller than the greatest reflector the one on Mount Wilson, California. The lens of the Yerks refractor measures three feet four inches in diameter whereas the Mount Wilson reflector measures a diameter of no less than eight feet four inches. But there is a device whereby the power of these giant instruments great as it is can be still further heightened. That device is the simple one of allowing the photographic plate to take the place of the human eye. Nowadays an astronomer seldom spends the night with his eye glued to the great telescope. He puts a photographic plate there. The photographic plate has this advantage over the eye that it builds up impressions. However long we stare at an object too faint to be seen we shall never see it. With the photographic plate however faint impressions go on accumulating. As hour after hour passes the star which was too faint to make a perceptible impression on the plate goes on affecting it until finally it makes an impression which can be made visible. In this way the photographic plate reveals to us phenomena in the heavens which cannot be seen even through the most powerful telescope. Telescopes of the kind we have been discussing telescopes for exploring the heavens are mounted equatorially. That is to say they are mounted on an inclined pillar parallel to the axis of the earth so that by rotating round this pillar the telescope is enabled to follow the apparent motion of a star to the rotation of the earth. This motion is affected by clockwork so that once adjusted on a star and the clockwork started the telescope remains adjusted on that star for any length of time that is desired. But a great official observatory such as Greenwich Observatory or the Observatory at Paris also has transit instruments or telescopes smaller than the equatorials in the same facility of movement but which by a number of exquisite refinements are more adapted to accurate measurements. It is these instruments which are chiefly used in the compilation of the nautical almanac. They do not follow the apparent motions of the stars. Stars are allowed to drift across the field of vision and as each star crosses a small group of parallel wires in the eyepiece that are recorded. Owing to their relative fixity of position these instruments can be constructed to record the positions of stars with much greater accuracy than is possible to the more general and flexible mounting of equatorials. The recording of transit is comparatively dry work. The spectacular element is entirely absent. Stars are treated merely as mathematical points. But these observations furnish the very basis of modern mathematical astronomy and without them such publications as the nautical almanac and the connoissals to tall would be robbed of the greater part of their importance. The spectroscope We have already learnt something of the principles of the spectroscope. The instrument which by making it possible to learn the actual constitution of the stars has added a vast new domain to astronomy. The simplest form of this instrument the analyzing portion consists of a single prism. Unless the prism is very large however only a small degree of dispersion is obtained. It is obviously desirable for accurate analytical work that the dispersion that is the separation of the different parts of the spectrum should be as great as possible. The dispersion can be increased by using a large number of prisms entering the second and so on. In this way each prism produces its own dispersive effect and when a number of prisms are employed the final dispersion is considerable. A considerable amount of light is absorbed in this way however so that unless our primary source of light is very strong the final spectrum will be very feeble and hard to decipher. Another way of obtaining considerable dispersion is by using diffraction grating instead of a prism. This consists entirely of a piece of glass on which lines are ruled by a diamond point. When the lines are sufficiently close together they split up light falling on them into its constituents and produce a spectrum. The modern diffraction grating is a truly wonderful piece of work. It contains several thousands of lines to the inch and these lines have to be spaced with the greatest amount of light. But in this instrument again there is a considerable loss of light. We have said that every substance has its own distinctive spectrum and it might be thought that when a list of the spectrum of different substances has been prepared spectrum analysis would become perfectly straightforward. In practice however things are not quite so simple. The spectrum emitted by a substance is influenced by a variety of conditions whether the temperature, the state of motion of the object we are observing all make a difference and one of the most laborious tasks of the modern spectroscopist is to disentangle these effects from one another. Simple as it is in its broad outlines spectroscopy is in reality one of the most intricate branches of modern science. End of chapter 1