Vol. 7, No. 2 AUS this magazine, place a 1 cent stamp on this notice, mail the magazine, and it will be placed in the hands of our soldiers or sailors destined to proceed overseas. No wrapping-No Address. A. S. Burleson, Postmaster-General. AUGUST, 1918 THE SCIENTIFIC MONTHLY CONTENTS THE MECHANISM OF LIGHT EMISSION. PROFESSOR E. P. LEWIS THE STATUS OF SEALING IN THE SUBANTARCTIC ATLANTIC. ROBERT THE ENGINEERING PROFESSION FIFTY YEARS HENCE. DR. J. A. L. WADDELL 130 149 158 BIRD MIGRATION IN ITS INTERNATIONAL BEARING. JOSEPH GRINNELL 166 One Hundred Years of the American Journal of Science; Hours, Fatigue and Health in British Munition Factories; Scientific Items . Barber's First Course in General Science By FREDERICK D. BARBER, Professor of Physics in the Illinois State Nor- A recent notable endorsement of this book occurred in Minneapolis. A Committee on General Science, representing each High School in the city, was asked to outline a course in Science for first year High School. After making the outline they considered the textbook situation. In this regard, the Committee reports as follows: "We feel that, in Science, a book for first year High School use should be simple in language, should begin without presupposing too much knowledge on the part of the student, should have an abundance of good pictures and pler. ~of material to choose from. Barber's First Course in General Science seems to us to bes. meet these requirements and in addition it suggests materials for home experiments requiring no unusual apparatus, and requires no scientific measurements during the course. We recommend its adoption." Other Interesting Opinions on the Book Follow: SCHOOL SCIENCE AND MATHEMATICS:-It is one of the very best books on general science that have ever been published. The biological as well as the physical side of the subject is treated with great fairness. There is more material in the text than can be well used in one year's work on the subject. This is, however, a good fault, as it gives the instructor a wide range of subjects. The book is written in a style which will at once command not only the attention of the teacher, but that of the pupil as well. It is interesting from cover to cover. Many new and ingenious features are presented. The drawings and halftones have been selected for the purpose of illustrating points in the text, as well as for the purpose of attracting the pupil and holding his attention. There are 375 of these illustrations. There is no end to the good things which might be said concerning this volume, and the advice of the writer to any school board about to adopt a text in general science is to become thoroughly familiar with this book before making a final decision. WALTER BARR, Keokuk, Iowa:-Today when I showed Barber's Science to the manager and department heads of the Mississippi River Power Co., including probably the best engineers of America possible to assemble accidentally as a group, the exclamation around the table was: "If we only could have had a book like this when we were in school." Something similar in my own mind caused me to determine to give the book to my own son altho he is in only the eighth grade. G. M. WILSON, Iowa State College:—I have not been particularly favorable to the general science idea, but I am satisfied now that this was due to the kind of texts which came to my attention and the way it happened to be handled in places where I had knowledge of its teaching. I am satisfied that Professor Barber, in this volume, has the work started on the right idea. It is meant to be useful, practical material closely connected with explanation of every day affairs. It seems to me an unusual contribution along this line. It will mean, of course, that others will follow, and that we may hope to have general science work put on such a practical basis that it will win a permanent place in the schools. Henry Holt and Company NEW YORK BOSTON CHICAGO LAUG @ P9 LIBRAR THE SCIENTIFIC MONTHLY AUGUST, 1918 THE MECHANISM OF LIGHT EMISSION By Professor E. P. LEWIS UNIVERSITY OF CALIFORNIA N THE SCIENTIFIC MONTHLY for February, 1917, Professor Guthrie gave an interesting account of the development of the electromagnetic theory of light. He explained how it had been demonstrated that light waves are very short electric waves similar in all respects except size to the electric waves used in wireless telegraphy. The latter are emitted from conductors of finite size in which electric charges oscillate, and may be several miles in length; the former are radiated from small negatively charged particles called electrons vibrating in molecules or atoms, and are measured in millionths of a millimeter. So far as the theory of light transmission is concerned, there is reason to believe that our knowledge has approached finality. There seems to be no acceptable alternative to the conclusion that light is due to wave motion in the hypothetical medium called the ether, concerning which we may never know more than we do now, but which it seems necessary to postulate as the seat of electrical and magnetic phenomena. We may, however, hope to learn much more than we now know concerning the processes in matter which cause the radiation and absorption of light. Under the term light, we must include the invisible radiations which lie on both sides of the narrow range of frequencies or wave-lengths which are included in the visible spectrum-the short ultra-violet and X-ray radiations on one side and the longer infra-red waves, often mistakenly called heat waves, on the other. Electromagnetic theory and the effect of a magnetic field on radiating sources (the Zeeman effect) make it certain that the shorter light waves, at least, are set up by the periodic displacements of VOL. VII.-7. electrons in the atom. The frequencies of vibration must be determined by the forces in the atom due to the number and arrangement of the positive and negative charges in it, hence the problem of radiation is intimately connected with that of atomic structure, and this in turn with all the properties of matter; and it is also dependent upon the relationship between matter and ether which makes possible the interchange of energy between the two. Hence the mechanism of radiation is a subject of great importance-in fact, probably the most important and the most interesting of the problems which confront the physicist to-day. Some general facts concerning radiation are familiar to all. We know that most luminous sources are very hot-red-hot at a moderate temperature, white-hot, that is to say emitting all colors, at very high temperatures. From this we may infer that heat is the cause of radiation in such cases, and that the colors emitted depend upon the temperature. Since heat is energy of molecular motion, we might jump to the conclusion that the agitation of the molecules sends out waves in the ether just as the jumping of a trout sends out waves in water. But unfortunately such a simple explanation seems insufficient, for a high temperature is not in all cases necessary to produce luminosity. The reader may recall some familiar illustrations of light emission by sources which are not hot. Many substances phosphoresce brightly at ordinary temperatures or even at such low temperatures as that of liquid air. The glowworm emits light of colors which are not radiated by carbon or a metal until it reaches white heat. The aurora glows brightly in the atmosphere at elevations where intense cold prevails. On the other hand, air and many other gases and vapors do not emit visible radiation even when heated to the highest degree. It is evident that other causes than energetic molecular motion may cause radiation. Our next inference might be that light is due to the vibrations of atoms within molecules which may not themselves possess much translatory energy, but this hypothesis proves insufficient in the case of monatomic gases, such as helium and mercury vapor. There seemed to be no explanation possible so long as it was assumed (without any rational basis, as we now see) that the atom is indivisible and unchangeable. No progress was possible until the discovery of the electron and of the atomic disintegration characteristic of radioactive processes proved the complexity of atoms. In general luminous sources emit waves of many different lengths and frequencies of vibration, each frequency corresponding to a different color. In order to analyze the light into its components, which is the first step toward obtaining a definite knowledge of what takes place in the source, the use of some form of spectroscope is necessary. What follows will be made clearer by the description of a simple form of spectroscope, to recall to the reader how the light is analyzed and what is meant by the "lines" of a spectrum. The light from the source is focused on a narrow slit, through which it passes in a divergent beam. A lens placed in this beam forms an image of the slit on a screen placed at the proper distance. If a prism is introduced into the path of the light, the beam will be refracted toward the base of the prism, and the deviation will be different for each color. If only one color (frequency) is present in the light, a single refracted image of the slit, of that color, may be thrown on a screen or a photographic plate. If two or more colors are present, there will be two or more images of the slit in different positions. These slit images are known as spectral lines. If the light is white, there will be an infinite number of slit images, corresponding to the infinite number of shades of color in white light, forming a continuous spectrum. If certain colors are removed by placing color screens in the path of the light there will be gaps in the spectrum, called absorption lines, corresponding to the absent slit images. Incandescent solids all give continuous spectra, with radiations extending beyond the red, and also beyond the violet at very high temperatures. Luminous gases and vapors, however, do not usually emit all colors, but only a finite number, giving rise to a corresponding number of bright lines. A series of observations by many investigators, and finally the work of Kirchhoff and Bunsen, about 1859, resulted in the recognition of the capital fact that no two elements have the same spectrum, that is, lines corresponding to each other in number and position. This makes the spectroscope an important instrument for the identification and discovery of elements in terrestrial and celestial sources, and serves also the important purpose of giving us significant data for the study of atomic structure and the relation between matter and ether which causes the emission and absorption of radiant energy. In 1814 Fraunhofer, an optician of Munich, observed that there are many dark lines in the spectrum of the sun. The explanation was found, but not fully grasped, by Foucault in 1849, who discovered that a pair of very close dark lines in the |