Recent Developments.—Although develop ments of major importance have been made in all fields of the science, yet certain lines of investigation have extended so far, and have established such profound unifying principles, as to justify their recognition as new sections of physical science. The first of these is Elec tro-magnetic Radiation. Maxwell's mathemati cal theories of electricity indicated the possi bility of the propagation of energy through space as electro-magnetic wave motion; a pos sibility that was experimentally verified by Hertz. The velocity of these aHertziarP waves, which have since been applied with such great success to °wireless* communication, was found to be the same as the velocity of light: and from this followed directly the recognition of the identity of light radiation, of heat radia tion, and of those regions of radiation of frequencies above and below those correspond ing to visible light, respectively called ultra violet and infra-red or radiation, as all one and the same type of radiation, differing only in frequency and in methods of detection; while the discovery of X-radiation (and gam ma-radiation of radio-active materials) has extended our knowledge of electro-magnetic radiation to what seems to be the extreme up per end of the frequency scale. The importance of this unifying principle, in connection with the others to be mentioned later, to the compre hension of light phenomena can only be sug gested here; the study of light is now the study of electro-magnetic radiation; and theories advanced to explain its phenomena may be tested, not only throughout the little more than an octave of frequencies comprised by the vis ible spectrum, or even the much greater range available when the ultra-violet and infra-red regions are included, but throughout the innu merable octaves included between the longest *wireless* rays and the shortest gamma rays. Big unexplored intermediate gaps still exist in the °spectrum* of radiant electrical energy, due to the lack of suitable detectors or of known radiators for those regions, which leave ample opportunity for future investigation. In this field of study belong also the phenomena of fluorescence and phosphorescence, which, to gether with those of the °characteristic* X-radi ation, form a fascinating field of complex radiation phenomena, which seem to hold many of the secrets of the structure of matter. On the theoretical side, mathematical theories to account for the phenomena of thermal radia tion have been developed by Stephen, Wien, Planck and others, and the *quantum theory,* first advanced by Planck, and later modified by himself and others,• has aroused widespread in terest both on account of its revolutionary sug gestions, and because of its wide application and remarkable agreement with many experi mental facts.
Another field of investigation is that which began with the study of electric dis charge through rarified gases, and which has reached its present culmination in the modern electron theory (q.v.) of electricity. On the practical side, the toy of the scientist, the °vacuum° tube, has proven the progenitor of the X-ray tube, the °vacuum° tube so essential to "wireless° apparatus, the thermionic rectifier, P id many other valuable devices. The work of J. J. Thomson, C. T. R. Wilson, Millilcan, and others has shown that the °electrons° or car riers of negative electricity in the "vacuum° tube discharge, are invariably associated with a fixed amount of charge; and that this charge, whose value has been determined with great accuracy, is the ((elementary') electric charge, of which all other charges are exact multiples. And this conception of electric charge, together with Maxwell's theory of electric fields, pro vides a rational basis for the study of all of the electrical phenomena of conduction, thermo electricity and the like, as well as of the phenomena whose existence has been revealed to us through this theory, such as thermionic currents, photo-electricity, ionization in gases, etc.
Still another field of both theoretical and experimental development is that in which at tempts have been made to develop theories of the structure of matter. The dynamic theory of gases, previously quite fully developed, has been further advanced by ideas derived from progress in other fields and extended to explain many more phenomena. The discoveries in
radio-activity, disclosing as they have the ex istence of atomic disintegration, reveal a new complexity of atomic structure, and at the same time suggest a simplicity of atomic constituents, which, together with the knowledge already gained by the observation of the behavior of electrons in "vacuum° tube discharge, has stimulated many serious attempts to build up an electric atom, consisting of some form of positively charged, nucleus about whose centre electrons circle or vibrate at very high fre quencies. As is to be expected, only very partial success has been obtained in devising such an atom that will be in accord with all of the known atomic phenomena, such as the phenomena of radiation and of electricity, and which will at the same time have all of the necessary chemical properties. But many iso lated phenomena, such as diamagnetism and the Zeeman effect, have already been given very satisfactory explanations in this way, while promising attempts have been made toward the explanation of many other phenomena. as for example, paramagnetism and thermal radia tion ("magneton" and "quantum') theories), and many chemical phenomena, such as valency and electrolysis, can now be seen clearly to be electro-atomic in their nature. No one can prophesy the future of this class of investiga tion. In another direction, the discovery that X-ray wave lengths are comparable with the distances between the atoms in a crystal has enabled Bragg and others to study the struc ture of crystals, and not only to verify the regular arrangement of the atoms, which had been guessed at previously, but as well to de termine the exact positions and spacing of the atoms in a great many simple crystals: a work which is gradually extending to the analysis of more and more complex crystals. Whither will it finally lead? As the science grows the range of its knowledge and of its usefulness ever widens, yet at the same time it centres closer and closer around one great problem. Has the problem of the science of energy become that of answering the question, "What is matter?* Quite probably. But experiment shows that the mass of a moving electron is partly, and may be wholly, electromagnetic, and thus there is more than a suggestion of the possibility that matter is energy.
Historical.— The history of science, prop erly written, is more interesting than the his tory of wars and empires: for it tells the story of the development of human ideas. Yet in spite of this, very few histories of science are to be found. Only a brief outline of the his tory of physics may be given here; and, aside from the few references given in the bibliogra phy below, further knowledge of the history of physics must be gleaned from the biogra phies and writings of the men whose names are famous for their scientific achievements. Physics, in common with the other sciences, may trace its origins to remotest antiquity; but its modern development, which began with its recognition as an experimental science, dates from about the 16th century. Although the knowledge of the ancients of civilization — the Greeks, Romans, Egyptians and Arabians comprised many isolated scientific discoveries and inventions, and while the writings of their philosophers often turned toward the explana tion of them, yet it cannot be said that the ancients made any appreciable contributions toward the advancement of science; certainly none comparable with their contributions to other fields of thought. While it can be said that the authority of their master philosophers, such as Aristotle, stood a formidable barrier to scientific advancement for centuries, the con fused and intimate relations between scientific, metaphysical and mystical ideas, combined with an inexplicable disregard of experiment as a means of interpretation of nature, opposed any real advancement. The achievements of Archi medes in the fields of statics and hydrostatics stand among the few solitary exceptions that foreshadow the modern scientific viewpoint.