RECENT DEVELOPMENTS IN PHYSICS.

THE

MODERN PHYSICS.

members of the Physics department hope to give from time to time short accounts of the more important developments in theoretical and experimental Physics. The following group of short papers exhibits the tendencies in modern physical science and it will be seen from their perusal that much of this work centres around the question of the ultimate in matter, energy, electricity and the ether. Perhaps all might be grouped under the one title, The Ether, its functions and properties.

Sir Oliver J. Lodge remarks in his delightfully readable little book, "The Ether of Space," "It has been said, somewhat sarcastically, that the ether was made in England." In a very real sense this is true, for aside from the purely metaphysical conceptions of ethers for various purposes, that of Plato to fill space, of Descartes to separate bodies from each ether, for planets to swim in, for electric atmospheres and magnetic effluvia the aggregate of which produced a sort of ether nausea amongst scientific men during the 18th century, the first physical fact requiring an ether of the modern kind was the wave theory proposed by Sir Robert Hooke in 1665. This theory was given a much wider scope and clearer presentation by Huyghen's after whom it is usually called. But the great prestige of Newton was too strong for wave theories and the development of the medium idea was delayed for nearly 100 years. In the opening years of the 19th century Sir Thomas Young established the wave theory on a sound. experimental basis. By a masterly series of experiments he showed that two sources of light under proper circumstances may produce darkness at definite points, a fact incomprehensible on any theory other than an undulatory one. The Ether was the medium necessary for the propagation of the wave motion.

Then, in the early thirties, Michael Faraday, the in

comparable experimenter and Joseph Henry, a school teacher of Albany, N.Y., established the unexpected fact that electromagnetic effects are transmitted through space apparently empty. These new effects made a new medium or ether necessary for their propagation. It had been known for nearly 2000 years that amber when rubbed attracted light bodies in its vicinity and that the lodestone attracted bits of iron. With all of these apparently isolated facts, each one requiring a medium for its peculiar actions it is no wonder that the ether was viewed with suspicion. It is not easy to imagine space filled several times over with different continuous media.

The appearance of Maxwell's truly epoch making treatise on Electricity and Magnetism in 1873 was timely. For here for the first time all of these phenomena are shown to be different manifestations of the same thing, viz., electrical actions propagated in one medium the ether as it has been called since that time. In a sense Maxwell was much ahead of his time and his view that light was a form of electromagnetic wave motion in the ether did not find ready acceptance even in England. The verification came at the hands of Heinrich Hertz to whom the work was suggested by the greatest German scientist of his day, Helmholtz. Hertz was able to show that electro-magnetic waves were a reality, that they could be subjected to exactly the same kinds of experimental tests as ordinary light waves with the same results, and that their velocity of propagation is identical with that of light. About the same time Rowland, of Baltimore, showed that an electric charge in motion produced the same effects as a current of electricity, a necessary consequence of Maxwell's theory. With these experimental results, the ether was freed from the suspicion under which it had existed for so long and became a physical conception in good standing. From this time forward advance in all branches of ether investigation has been rapid, and of a nature to appeal to the mind of the layman in scientific matters. Wireless telegraphy, X-rays, electrons, radio-activity and the transformations accompanying it are but a few catch words connoting a very wide field of experimental activity and a far-reaching extension of knowledge.

The question of the ultimate has been pushed farther back with the division of the atom into electrons; and the linking of the identities of matter and electicity has attractive possibilities for the student fond of that kind of speculative thinking which can be subjected to experimental verification. Nearly all of the work of the past few years that has attracted much attention is closely related, indeed almost inseparably connected with the problem of the ether. If matter is ultimately electricity, what then is electricity and how is it related to the ether, the medium in which it moves and exerts its influences? How does it operate on the ether to set it in vibration; of what nature is this vibration? Is electricity an entity or is it a state of strain or place of singularity in the ether itself? These are some of the questions. before us.

Notwithstanding the fact that there are serious difficulties attending the acceptance of the ether, and that there is a tendency to reject the ether on the part of a few owing to the difficulties involved, the difficulties introduced with its rejection are very much greater. It is safe to say that the majority of the more experienced physicists regard it as a necessary part of the mechanism of the universe. Michelson says, "Little as we know about it, we may say that our ignorance of ordinary matter is still greater."

To sum up our knowledge of the ether, it is regarded as a continuous medium filling all space as far as we can judge. It has very great rigidity or is perfectly elastic and with low density, of solid rather than of fluid nature. It offers little or no resistance to the motion of matter through it, and is not carried along by the passage of material bodies through it. Some of these apparently contradictory properties with which it is necessary to endow it are difficult to understand because our ideas are obtained from observation of ordinary matter, which must be very, very different from the ether. But it must be confessed that the difficulties are great and one attempt to reconcile theory with experiment is the theory of Relativity which cuts the Gordian Knot by rejecting the ether altogether as will be seen by a following paper.

A. L. C.

THE NATURE OF X-RAYS AND THE STRUCTURE OF CRYSTALS. It will at first seem strange to place under one heading two apparently unrelated subjects, yet one of the most unexpected of the recent developments of Physics links them inseparably together.

In the past few years the scientific world has given attention to three distinct views as to the nature of that important class of radiations known as Roentgen or X rays. One of the characteristic features of these rays is that they do not exhibit the phenomenon of refraction, i.e., they do not, like ordinary light waves, alter their direction in passing from one substance to another and thus casting shadows of 'opaque' objects in their path. This is one of the properties of the rays that have given them such value in the medical world.

The chief points to be noticed in the production of the rays are as follows: When a current of electricity is forced through an X-ray bulb, the cathode, as the negative plate in the tube is called, emits a stream of very small bodies known as electrons. They shoot across the vacuum with velocities of from ten to twenty thousand miles per second and as they are negatively charged produce all the effects of an ordinary electric current. When these are stopped at impact with the walls of the tube (or with a plate of platinum set in the tube for that purpose) their energy is in some way transformed into that special type of radiation called X-rays. It is well known that when such charged bodies are suddenly stopped a pulse or half wave spreads out in all directions from the point struck a sort of electrical splash. It is known also that such a pulse would not alter its course in passing from one substance to another, for refraction depends on the regular reactions of a train of waves. Consideration of these facts led Sir George Stokes and Sir J. J. Thomson in England, and Dr. Weichert, in Germany, to suppose that these pulses might be the X-rays themselves. Though this idea in the hands of Thomson, Barkla, Chapman, Sadler and others has led to many important discoveries, the more we know of it the less does it promise to bring the solution for our problem.

Again, it is well-known that Radium and other radioactive substances emit three distinct types of rays. The first two, the alpha and the beta rays, are known to be flights of

material particles; the former bearing positive and the latter negative charges. The third group, the gamma rays, exhibit all of the characteristics of very "hard" X-rays and are almost universally regarded as being identical with them. Dr. Bragg has suggested that these gamma rays ( and incidentally the X-rays) might be streams of oppositely charged material particles in couples. This view also has led to the discovery of new facts in regard to the behaviour of the rays, but has met with bitter opposition chiefly from Barkla (of King's College, London) and his students.

While these two theories and the vigorous battle of their supporters has held the attention of the scientific world for the past five years, there have always been men, chiefly in Germany, who maintained that the Roentgen rays were really light of very short wave length. The crucial test for waves of any kind is that they show the phenomena of interference, i.e., two or more beams emanating from the same source and moving along different paths to the same point will reinforce one another wherever the two wave trains vibrate together and will destroy one another wherever they vibrate oppositely. One case of the phenomena is found where the waves comes through a set of small openings and the resulting illumination takes the form of a series of brilliant patches of light at points where the wave trains vibrate together and dark spots. or lines wherever the wave trains are vibrating oppositely to one another. Any one can easily observe a diffraction pattern of this kind for himself if he will but look at a distant electric light (arc) through his pocket handkerchief held stretched between his hands. A distinct pattern of light spots and dark bands is seen. This pattern, moreover, depends on the geometrical form of the opening or on the geometrical relation of the set of openings, and this one can see for himself by simply pulling the handkerchief cornerwise so as to change. the square openings between the threads into diamond-shaped ones. The resulting pattern changes. Of course one does not see these patterns in looking at the light through a lattice of laths although the geometrical form of the openings is the same as in the case of the handkerchief, for the size of the holes relatively to the length of the wave is also a factor. Thus for shorter waves finer structures are required.