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 credulous 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

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

A

TYPICAL FORM OF VACUUM TUBE.

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

num wires which are sealed in the glass. Let the hydrogen, Professor

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.

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 develop

ment 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 tubet 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 emanating 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 obtain ing, 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 Plücker, 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.

any contribution to the problem of electric discharges through rarefied gases-Lenard, Hertz, Crookes, and the rest. What lends peculiar significance to Röntgen's work, and entitles him to all the honor of a great discovery, is that he has revealed the existence of a sort of rays, which, whatever their real nature may prove to be, have an incomparably greater range and penetrative power, besides other unique and important properties, that markedly distinguish them from all other known forms of radiant energy, invisible as well as visible.

The original experiments and conclusions of Professor Röntgen are described by him in the following article "On a New Kind of Rays,"* which, owing to its historical importance, is here given entire:

1. A discharge from a large induction coil is passed through a Hittorf vacuum tube, or through a well-exhausted Crookes or Lenard tube. The tube is surrounded by a fairly close-fitting shield of black paper; it is then possible to see, in a completely darkened room, that paper covered on one side with barium platino-cyanide lights up with brilliant fluorescence when brought into the neighborhood of the tube, whether the painted side or the other be turned toward the tube. The fluorescence is still visible at two metres' distance. It is easy to show that the origin of the fluorescence lies within the vacuum tube.

2. It is seen, therefore, that some agent is capable of penetrating black cardboard, which is quite opaque to ultra-violet light, sunlight, or are light. It is therefore of interest to investigate how far other bodies can be penetrated by the same agent. It is readily shown that all bodies possess this same transparency, but in very varying degrees. For example, paper is very transparent; the fluorescent screen will light up when placed behind a book of a thousand pages; printers' ink offers no marked resistance. Similarly the fluorescence shows bebind two packs of cards; a single card does not visibly diminish the brilliancy of the light. So, again, a single thickness of tin foil hardly casts a shadow on the screen; several have to be superposed to produce a marked effect. Thick blocks of wood are still transparent. Boards of pine two or three centimetres thick absorb only very little. A piece of sheet aluminium 15 mm. thick, still allowed the X rays (as I will call the rays, for the sake of brevity) to pass, but greatly reduced the fluorescence. Glass plates of similar thickness behave similarly; lead glass is, however, much more opaque than glass free from lead. Ebonite several centimetres thick is transparent. If the hand be held before the fluorescent screen, the shadow shows the bones darkly, with only faint outlines of the surrounding tissues. Water and several other fluids are very transparent. Hydrogen is not markedly more permeable than air. Plates of copper, silver, lead, gold, and platinum also allow the rays to pass, but only when the metal is thin. Platinum 2 mm. thick allows some rays to pass; silver and copper are more transparent. Lead 1.5 mm. thick is practically opaque. If a square rod of wood 20 mm. on the side be painted on one face with white lead, it casts little shadow when it is so turned that the painted face is parallel to the X rays, but a strong shadow if the rays have to pass through the painted side. The salts of the metals, either solid or in solution, behave generally as the metals themselves.

3. The preceding experiments lead to the conclusion that the

* Translated by A. Stanton, from the Sitzungsberichte des Würzburger Physik medic Gesellschaft. 1895, and reprinted from Nature, January 23, 1896.

density of the bodies is the property whose variation mainly affects their permeability. At least no other property seems so marked in this connection. But that the density alone does not determine the transparency, is shown by an experiment wherein plates of similar thickness of Iceland spar, glass, aluminium, and quartz were em ployed as screens. Then the Iceland spar showed itself much less transparent than the other bodies, though of approximately the same density. I have not remarked any strong fluorescence of Iceland spar compared with glass (see below).

4. Increasing thickness increases the hindrance offered to the rays by all bodies. A picture has been impressed on a photographic plate of a number of superposed layers of tin foil, like steps, presenting thus a regularly increasing thickness. This is to be submitted to photometric processes when a suitable instrument is available.

5. Pieces of platinum, lead, zinc, and aluminium foil were so arranged as to produce the same weakening of the effect. The an nexed table shows the relative thickness and density of the equivalent sheets of metal:

From these values it is clear that in no case can we obtain the transparency of a body from the product of its density and thickness. The transparency increases much more rapidly than the product de

creases.

6. The fluorescence of barium platino-cyanide is not the only noticeable action of the X rays. It is to be observed that other bodies exhibit fluorescence-e. g., calcium sulphide, uranium glass, Iceland spar, rock salt, etc.

Of special interest in this connection is the fact that photographic dry plates are sensitive to the X rays. It is thus possible to exhibit the phenomena so as to exclude the danger of error. I have thus confirmed many observations originally made by eye observation with the fluorescent screen. Here the power of the X rays to pass through wood or cardboard becomes useful. The photographic plate can be exposed to the action without removal of the shutter of the dark slide or other protecting case, so that the experiment need not be conducted in darkness. Manifestly, unexposed plates must not be left in their box near the vacuum tube.

It seems now questionable whether the impression on the plate is a direct effect of the X rays, or a secondary result induced by the fluorescence of the material of the plate. Films can receive the im pression as well as ordinary dry plates.

I have not been able to show experimentally that the X rays give rise to any calorific effects. These, however, may be assumed, for the phenomena of fluorescence show that the X rays are capable of transformation. It is also certain that all the X rays falling on a body do not leave it as such.

The retina of the eye is quite insensitive to these rays; the eye placed close to the apparatus sees nothing. It is clear from the experiments that this is not due to want of permeability on the part of the structures of the eye.