Physics

The Sub-divisions of Physics.

Meanwhile the science of physics has made gigantic strides, throwing off such branches as sound, light, heat, elasticity, hydrodynamics and the kinetic theory of gases. It was found that, although the atoms of matter were too small to be dealt with individually, the results of their com bined action could be formulated statistically, in accordance with the laws of probability; and that, although the atoms of a gas were flying about in all directions at random, yet their num ber was so great that the pressure on any surface exposed to their random bombardment could be treated as perfectly uniform, ex cept indeed when the exposed surface was extremely small. In the latter case individual atoms might exert a noticeable effect, giving occasionally a one-sided bombardment, and exerting an otherwise inexplicable propelling action on the body exposed to it. Thus ' originated a discrimination, now universally recognized, between the peculiar activities displayed by very small things and the cus tomary behaviour of things on so large a scale that we can handle and directly apprehend them by our senses.

So far as matter was concerned there seemed little difference between physics and chemistry, except that physics sought to probe into the deeper-seated motions underlying molecular combination, and to investigate not only the anatomical groupings but the actual working of the agencies which rendered those groupings possible. It sought to understand the hidden meaning of chemical affinity and the nature of cohesion generally, and indeed of gravitation too. It was not satisfied with the idea of forces acting at a dis tance, or of the mere clinging together of polarized particles; it sought to dive down into the hidden meaning of attraction, and to explain if possible the very atoms themselves.

In all this it was mightily helped by the development—since Newton's time, in fact mainly in the 19th century—of two great and important new branches, the sciences of electricity and mag netism. These are agents which, like light, operate through what we call empty space or vacuum, and therefore seem primarily as sociated not with matter as such, but with the more fundamental ether. The fact that mankind has no sense organ for their direct appreciation involved the consequence that, unlike sound, light and heat, they had to be discovered as well as critically examined and studied. Nevertheless in our own time these new powers have become dominant; they have been taken up by engineers, and applied on an extensive and beneficent scale, so that they are now more or less familiar to everybody. The applications of electricity and magnetism are of great and growing importance, but that is not what constitutes their vivid interest to a physicist. Their importance is that they give a clue and greatly extend our knowledge of the properties of the ether, which had only very partially been displayed in the familiar form of light. Indeed,

Clerk Maxwell, perhaps the greatest man of science in the i9th century, made the far reaching discovery that light itself was an electromagnetic vibration, a fact which has now blossomed into the engineering application of radio telegraphy.

The phenomenon called radiation, of which visible light is one small portion, contains the key to the interaction between ether and matter, and is the link between the physics of the 19th and that of the loth century. All matter is always radiating, and if it does not cool or otherwise lose energy it is only because it absorbs exactly the quantity and quality which it emits. Stag nation of any kind is non-existent in the material universe. Every particle quivers, and the ether quivers in sympathetic unison. The problem of their interaction is being attacked and partially solved by the mathematical physicists of the loth century.

It may be asked, indeed it often is asked, how discoveries can be made by means of calculation. The answer is quite definite. The mathematical physicist probes down into the fundamental activities, so as to be able to formulate in his mind what is really going on, and to express the actions which can be observed in a definite and quantitative form called an equation, especially in that form called a differential equation, which expresses inter actions among the smallest parts. The discovery of the relation between electricity and magnetism on the one hand, and light on the other, will serve as an illustration of the process. Clerk Maxwell took the known phenomena of electricity and the known phenomena of magnetism, together with their interaction so brilliantly discovered and examined by the early physicists of all countries at the beginning of the 19th century culminating in the work of Faraday, and threw them into the form of equations. These, when combined and manipulated in accordance with the laws of pure mathematics, resulted in a differential equation in time and space, containing nothing but electric and magnetic quantities, which nevertheless corresponded with the known equa tion for waves, such as those of sound or of light or on the surface of water. He perceived therefore that electromagnetic experi ments could in the future probably give rise to such waves, and that they would travel at a speed which could be calculated in terms of the electric and magnetic constants of a vacuum, i.e., of the ether. He inaugurated a set of experiments which would determine, not indeed those constants separately, for they are still unknown, but their product, viz., the product which appeared in his differential equation as determining the speed of the waves. He found that the velocity with which his waves ought to travel was identical with that of light, and thus came to the conclusion, and was ultimately held to prove for all time, that light was an electromagnetic phenomenon. On this basis a whole new develop ment began, and continues to this day.