EARLY in January the world at large was startled by the announcement of a discovery which, so far as we can yet tell, seems destined to prove an epoch-making incident in the development of physical science-the discovery of a hitherto unknown form of radiant energy, possessing a marvellous power of penetrating bodies opaque to ordinary light, and actinic enough to produce a shadow picture upon a sensitized plate. No one has yet secured sufficient data to be able to estimate fully either the theoretical significance or the practical utility of the discovery. None of the innumerable instances in which, both in Europe and in America, the original experiments of the discoverer have been repeated, and his observations confirmed, has yet served to throw absolutely conclusive light upon the nature of the new rays; nor have any afforded more than a mere hint of the bearing of the new phenomena upon accepted scientific hypotheses. What is already known, however, has brought scientists face to face with the possibility that their long-established theories regarding cosmic questions of the utmost magnitude-radiant energy, the luminiferous ether, gravitation, the constitution and forces of matter-may ere long have to undergo great modification. But whatever may be the theoretical import of the discovery, its practical value is already assured; and the benefits which will accrue from its application to the uses, especially of surgery and of some of the technical arts, can hardly be overestimated.

The latter years of this century have formed a period of unprecedented scientific activity. The developments of electrical science under Edison, Tesla, Bell, and otherswhich have made the present an age of realized impossibilities are familiar to every one. But, to go no farther

Vol. 6.-1.

Copyright, 1896, by Garretson, Cox & Co.

back than a couple of years ago, we note a succession of scientific triumphs which tend to overthrow all our ideas of the limitations of human power, and to render us credulors of almost anything within the bounds of metaphysical possibility Only last year chemists were astonished to learn from Lord Rayleigh and Professor Ramsay that the familiar air had all along been keeping from earnest search

α

b

TYPICAL FORM OF VACUUM TUBE.

At a we have one electrode and at the other. They consist of platinum disks attached to plati

num wires which are sealed in the glass.

Let the

electrode a be connected to the negative, b to the positive pole of the induction coil A. As the air pressure in the tube is reduced, the color and the

general appearance of the discharge continually

change character. When the pressure reaches a

small fraction of a millimetre of mercury the intensity of the discharge in the gas itself becomes very much reduced, but in its place appears a strong fluorescence of the glass. This fluorescence

is produced by faint streamers which proceed in straight lines from the negative electrode, as indicated by the straight lines in the figure, from the disk at a toward the terminal of the tube. These streamers are called the cathode rays. The X rays are supposed to emanate from the luminescent spot in the wall of the tube where the cathode rays terminate.

ers the secret of the existence within itself, in considerable proportions, of an unknown and strangely inert element, argon, and to learn also that helium, so long known only as a constituent of the sun and of some other heavenly bodies, was also a body terrestrial. Again, by the liquefaction of air and hydrogen, Professor Dewar has created a new chemistry of cold; while the corresponding field of exceedingly high temperature has been thrown open to exploration by M. de Moissan's development of the electric furnace. Moreover,

among the products of the latter-the gift of physics to its sister science, chemistry-is found calcic carbide, whose use in the manufacture of acetylene gas, the first hydrocarbon to be manufactured artificially on a large scale, constitutes a great triumph of chemical synthesis, just as the discovery of argon was a triumph of analysis.

To these almost revolutionary achievements we have now to add another-the discovery of X rays (provisionally so called by their discoverer because their nature is still problematical), and the development of a new art of "shadowgraphy" or "radiography," whereby pictures are

taken of objects inaccessible to ordinary rays of light, such as the skeleton within the body, metallic objects encased in wood or leather, etc.*

It was on November 8, 1895 (if any date may be assigned for a discovery made by researches along lines: expressly indicated by predecessors), that Dr. Wilhelm Konrad Röntgen, professor of physics in the University of Würzburg, Bavaria, made the discovery which has now rendered his name a household term the world over. He announced it at the December meeting of the Würzburg PhysicoMedical Society; and on January 4, 1896, described it at the celebration of the semi-centennial of the founding of the Berlin Physical Society. Since that time, not only have the scientific laboratories of the world been almost wholly given up to the study of the rays, but the sensational and superficial aspects of the new phenomena have taken hold of the popular imagination and monopolized the attention of the unscientific world in a way that finds no parallel since the time when Edison became known as the "Wizard" through his development of the phonograph, the electric light, and other wonders associated with his name.

It was while following up the researches of Hertz and Lenard on the problem of the cathode rays from a vacuum tube, that Röntgen discovered the X rays. He had encased a Crookes tube in a covering of black paper imper

* Various names have been suggested for the pictures taken by means of the X rays: Shadow-prints, shadowgraphs, cathodographs, skiagraphs (from the Greek word for shadow), photo-skiagraphs, skotographs (from the Greek word for darkness), radiographs, radiotypes. They are not properly termed photographs, for, though the X rays may possibly be of a nature akin to ordinary light, yet these rays differ in important particulars from the visible rays of the spectrum, and from the infra-red and the ultra-violet rays, the cathode rays, or any other hitherto observed form of manifestation of the radiant energy familiarly known as light. Ordinary photographic sensitive plates are used, which are also developed in the customary way; but the pictures are in reality shadow-prints or silhouettes, due merely to interception of the X rays by objects more or less opaque to them. They reveal, within their outlines, no variation of detail save that of fainter or deeper shadow seemingly dependent upon the varying thickness and also (so far as yet known) upon the varying density, or (as Professor Dewar and others think) the varying atomic weight, of objects interposed in the path of the rays. Thus, organic substances, as a rule, are perme ated more readily than inorganic; and the least permeable, so far as yet observed, are such dense substances as glass and platinum. Iodine is very opaque; sulphur and in general other inorganic substances, more or less so; and the introduction into the molecule of an organic compound, of one or more atoms of sulphur, iodine, or other inorganic substance, produces opacity.

It has been suggested that the "photographic" effect is not due to any direct action of the X rays upon the sensitive film, but that the rays operate by setting up some sort of phosphorescence in the glass at the back of the sensitive film. In corroboration of this, Professor Dewar and others have shown the X rays to be transmutable into light rays affecting the eye.

+ Pronounced Rent-gen, the g being hard.

A Crookes tube is simply a modification of a Geissler tube. It consists of a bulb of glass, usually egg-shaped. from which the air has been almost exhausted. At one end the positive current is brought into the tube by means of a fused platinum wire; and an electrode consisting of a small disk-shaped peace

vious to ordinary light; but noticed that a sheet of paper sensitized with barium platino-cyanide, which was lying near by, was rendered luminescent. Investigation showed that the effect was caused by invisible rays or waves emahating from the tube and having unusual penetrative power. It merely remained for him then to study the properties of the newly found rays, and to announce the results of his researches to the world.

To be more particular. All students of physics are more or less familiar with the appearance of a high-vacuum tube through which a powerful electric discharge is passing. It emits a beautiful phosphorescent light varied by brushes of intenser luminosity at the electrodes. The rays from the anode vary in color under various conditions, but are far less brilliant and less peculiar in their properties than the cathode rays, which shoot from the negative electrode. The cathode rays were observed and studied as long ago as 1891 by the late Professor Heinrich Hertz of the University of Bonn, who showed that they would permeate thin metal; and in 1894, Hertz's assistant, Dr. Philip Lenard, supplemented this by showing that the cathode rays would not only penetrate thin films of aluminium, wood, and other substances, but produce photographic results beyond. Mr. Tesla, too, several years ago, made public the following statement:

"Certain kinds of waves which I called 'sound waves of electrified air' are propagated from conductors when a strong rapidly vibrating current passes through them, such as sudden discharges from condensers. These propagate in straight lines like sound. They are longitudinal waves penetrating bodies, and they cannot be stopped by interposing metal plates."

Moreover, as long ago as 1893, Professor Fernando Sanford of the Leland Stanford, Jr., University, in California, succeeded in obtaining, by the use of electricity, impressions of coins on photographic plates under conditions that excluded the operation of ordinary rays of light (Vol. 4, p. 234). And last year one Hans Schmidt of Munich maintained in a contribution to the Photograph Review, that the invisible ultra-violet rays in electric light pierced through blackened paper, thin wood, india rubber, and other materials, while thin layers of metal kept them back. And long before the development of photography or of the modern theory of light, the mysterious fact was noted, of pictures of objects being found imprinted on the bodies of persons struck with lightning.

Thus, it must be admitted that, the mere production of radiotypes by means of invisible rays, is no new achievement. The way to Röntgen's discovery was paved by the researches of all those who have made

of platinum or other suitable metal, is placed at the end of the wire, within the tube. At the other end of the tube is the spot where a similar electrode receives the current which has been transmitted through the vacuum. Where the current enters is called the anode, and where it leaves is called the cathode. These are otherwise known as the positive and negative poles, and are often indicated by a plus and minus sign respectively (see illustration). The use to which these tubes have been put is mainly to study the behavior of electricity when passing through gases of various densities. The systematic study of vacuum discharges dates from the time of Faraday, and is most prominently associated with the names of Plucker, Geissler, Hittorf, Goldstein, Hertz, and Lenard in Germany, and Spottiswoode and Crookes in England.

Vacuum tubes of various shapes, even ordinary incandescent light bulbs, it is claimed, have been successfully used; and the X rays have also been produced when one electrode, and even both, have been external to the tube employed.