Popular Science Monthly/Volume 49/May 1896/Recent Work on X Rays
THE general interest which the so-called Röntgen rays have excited among the unscientific as well as among the specialists seems to justify a more extended treatment than their actual value to humanity, so far as at present known, would warrant. The following extracts are taken from the published statements of some of the more prominent physicists. They are more or less tentative, as all such work at present must necessarily be; but they are of interest, as showing the lines along which experimentation is going on, and they perhaps offer some indication of the probable future of this curious form of energy.
In a paper read before the Paris Academy of Sciences M. Jean Perrin says: "Two hypotheses have been propounded to explain the properties of the cathode rays. Some physicists think with Goldstein, Hertz, and Lenard, that this phenomenon is, like light, due to vibrations of the ether, or even that it is light of short wave length. It is easily understood that such waves may have a rectilinear path, excite phosphorescence, and affect photographic plates. Others think, with Crookes and J. J. Thompson, that these rays are formed by matter which is negatively charged and moving with great velocity, and on this hypothesis their mechanical properties, as well as the manner in which they become curved in a magnetic field, are readily explicable." A series of experiments which the author made, and which are given in Nature for January 30th, lead him to the following conclusions. "These results as a whole do not appear capable of being easily reconciled with the theory which regards the cathode rays as being ultra-violet light. On the other hand, they agree well with the theory which regards them as a material radiation, and which, as it appears to me, might be thus enunciated. In the neighborhood of the cathode, the electric field is sufficiently intense to break into pieces (into ions) certain of the molecules of the residual gas. The negative ions move toward the region where the potential is increasing, acquire a considerable speed, and form the cathode rays; their electric charge, and consequently their mass (at the rate of one valence-gramme for a hundred thousand coulombs), is easily measurable. The positive ions move in the opposite direction; they form a diffused brush, sensitive to the magnet, and not a radiation in the correct sense of the word."
Nikola Tesla, in a recent Electrical Review, describes the following experiment, which seems to support the view that these rays are material radiations: When an exhausted bulb is attached to the terminal of a disruptive coil, small streamers are observed which often break through the side of the bulb, producing a fine hole. "Now, the extraordinary thing is that, in spite of the connection to the outer atmosphere, the air can not rush into the bulb as long as the hole is very small. The glass at the place where the rupture is may grow very hot—to such a degree as to soften; but it will not collapse, but rather bulge out, showing that a pressure from the inside greater than that of the atmosphere exists. On frequent occasions the hole becomes so large as to be perfectly distinguishable to the eye. As the matter is expelled from the bulb the rarefaction increases and the streamer becomes less and less intense, whereupon the glass closes again, hermetically sealing the opening." He reports producing strong pictures at a distance of forty feet by the use of a bulb with a single terminal, which permits the use of practically any desired potential.
An account of some important experiments by L. Benoist and D. Hurmuzeson appears in the Comptes Bendus. They caused the rays of a Crookes tube, incited by a powerful coil, to act upon the gold leaves of a Hurmuzeson electroscope at the distance of about twenty centimetres from the tube, and alternately charged with positive and negative electricity. The insulation obtained by a disk of dielectrine which closes the tube admits of the perfect preservation of its charge for several months. The X rays immediately and completely discharge the electroscope, more rapidly if the charge is negative than if it is positive. "We have thus," they say, "a new way of investigation applicable to the study of these rays, and enabling us to gain important information as to their real nature. The plate to be studied being put in its place, the electroscope charged to about a divergence of forty degrees, the keeping tube replaced, and the Crookes tube set in activity, we have observed:
"1. Black paper (sixteen leaves interposed). The collapse of the gold leaves is immediate and complete in a few seconds; they do not rise again. 2. Plate of brass (two tenths of a millimetre in thickness). No change in the divergence of the gold leaves. 3. Plate of aluminum (one tenth of a millimetre). Immediate fall, complete in a few seconds; same result with plates of aluminum up to one millimetre, and even upward, and the Crookes tube being removed to the distance of thirty centimetres. The substances easily traversed are silver in beaten leaves, leaves of paper steeped in metallic solutions, vulcanized fiber, gelatin, celluloid, ebonite, etc. The substances not traversed, at least in the thicknesses employed, are brass, zinc, glass, and unglazed porcelain (three millimetres). Similar results have since been announced from several other investigators."
A paper by G. Jaumann, under the title of Longitudinal Light, is described in Nature: "It is based upon the law of electric discharge enunciated by Jaumann in 1888, according to which electric rays impinging at right angles upon a cathode surface favor the dissipation of the charge upon it. Hence, the writer argues, light vibrations must have a component in the direction of propagation; they must, in fact, contain longitudinal as well as transverse waves. It then becomes a question of how Maxwell's electro-magnetic equations, which do not admit of any but purely transverse vibrations, can be made to agree with these conclusions. Jaumann gives a simple answer. Let it be admitted that the specific induction capacity of a medium and its magnetic permeability are affected by the oscillations themselves. These 'constants' will then be variable, and when introduced as such into the equations longitudinal vibrations are at once seen to be possible. Each pencil of light will then be vibrating transversely along its center line, and toward the outer edge the vibrations will become more and more longitudinal. The author claims that this theory affords a natural and simple explanation of a large number of discharge phenomena."
Prof. Oliver Lodge, of University College, Liverpool, is reported as having said that he felt inclined to adopt the view that the new rays were longitudinal waves in the ether; and that if this were so the discovery would open up a department of physics as large as light, sound, or electricity. Later, in a letter to Nature, discussing the theory of the anodal origin of the X rays, he says: "The term 'anode rays' for the rays discovered with so much éclat by Prof. Röntgen, whether they be the same as those previously discovered by Dr. Lenard or not, is suggested by the remarks of Mr. A. W. Porter at a recent meeting of the Royal Society. They certainly do not start from the cathode, but from some opposed surface—a surface which may be an actual anode. and which always has some anodic properties. From each point of such a surface rays start in all directions; this is proved by the shadows they cast of slits, holes, and wires."
Prof. S. P. Thompson takes exception to the term anodic, as applied to the X rays. He says: "Is it quite correct, as Prof. Lodge puts it, to call the X rays anodic, because they start from a point opposite the cathode? It may be true that a surface upon which the cathodic discharges are being directed acquires thereby some properties common to the anode, but it is not an actual anode; . . . hence I submit that anti-cathodic would be a more correct term to use in describing them."
Prof. A. W. Porter, of University College, London, in a letter to Nature, says: "In your last issue, in the account of the work in the Comptes Rendus, you state that M. de Heen 'proves conclusively that the X rays proceed from the anode and not the cathode.' May I point out that I have proved that this is undoubtedly true for the bulb I have been using throughout my experiments on the X radiation? The bulb is one in which the negative electrode is concave, and the negative stream is thereby focused to a point on the anode, which is a platinic disk placed near the center of the bulb. By measuring the positions of different parts of a radiograph of a series of concentric zones of tin foil placed in a measured position, I have shown that the actinic rays diverge from the anode disk."
It was announced from Rome that Prof. Salvioni, of Perugia, had discovered a means by which these radiations could be made to so far assist the eye as to enable it to see through all objects which the rays could penetrate, so that the contents of a closed space were revealed.
From Prof. Salvioni's description of the apparatus, which follows, it will be seen that he has made no new discovery, and that it is quite incorrect to say that the observer actually sees the objects. He simply sees the shadows on the phosphorescent screen. The fluorescent light affects the retina like ordinary light, and is quite distinct from the X rays. What is really seen is a shadow picture of the object.
The apparatus is very simple, and is thus described by Prof. Salvioni: "This cryptoscope consists of a small cardboard tube about eight centimetres high. One end is closed by a sheet of black paper, on which is spread a layer of fish glue and calcium sulphide; this substance I have found to be very phosphorescent under the action of Rontgen rays. Within the cardboard tube, at the other end, at which the eye is placed, is fixed a lens, giving a clear image of the phosphorescent paper. On looking through this cryptoscope one can see, even in a light room, the shape and position of metallic bodies inclosed in boxes of cardboard, wood. aluminum, and within the flesh." These experiments have since been repeated by many other investigators with perfect success; the observer in one case examining the bones of his own hand.
Gustave le Bon, in the Revue Scientifique, in summing up the results of his experiments on the X rays, says: "These experiments, which have been varied in all ways, are fundamental. They show us that the degree of thickness of the opaque plates is absolutely without importance in the passage of the 'lumière noire.' They also indicate that the 'lumière noire' is propagated under other laws than those which govern ordinary light. . . . This light can be transformed into radiations which propagate themselves as electric currents. They are not, however, electric radiations, because they produce effects which ordinary electric currents will not produce. We find ourselves, then, in the presence of a form of energy which is not light, as it only has part of light's properties and does not obey the laws of the propagation of light, and which is not electricity, since electricity in all known forms does not produce the same effects. The 'lumière noire' must be considered as a new force added to the few of these which we already know."
In a letter to Nature, Lord Blytheswood describes the following experiment with a Wimshurst electrical machine of one hundred and twenty-eight three-foot plates, the machine being driven by a motor of about one horse power and a half. "A thick sheet of lead was placed upright between the poles of the electric machine, as a screen, and was connected to the ground, the two poles being insulated. A sensitive dry plate was put into the camera dark slide, with a metallic object to be photographed (a steel washer with holes in it), and this was connected by a wire which passed out of the dark slide to the ground. The whole was wrapped up in four folds of a black velvet focusing cloth, and was put in some cases between the negative pole and the lead screen, and in other cases between the positive pole and the lead screen, the plane of the slide being perpendicular to the line of discharge. In all cases good strong negatives were obtained with exposures of about twenty minutes. The machine was arranged to give a silent brush discharge during the experiments." Several other physicists have reported obtaining shadow pictures without the aid of a Crookes tube, by using an electric current or simple sunlight, and a fluorescent screen, after very long exposures. Henri Becquerel recounts the following interesting experiment: "I inclose a photographic plate in two folds of very thick paper, so that the plate does not become shaded on exposure to the sun for a day. On the outside of this paper a plate of phosphorescent material is placed, and the whole is exposed to the direct rays of the sun for several hours. When the plate is developed we find that the silhouette of the phosphorescent substance appears in black on the proof. If a coin is interposed between the phosphorescent substance and the paper, its image appears on the proof. A thin sheet of glass may be interposed to preclude the possibility of chemical action."
It is announced by Mr. Edison that calcium tungstate (properly crystallized) gives a splendid fluorescence with the Röntgen rays, far exceeding that of platino-cyanide.
A rather ingenious explanation of the X rays is offered by Mr. J. W. Gifford. He likens the Crookes tube to a vibrating tuning fork, which, if sounding simple A, would set an A violin string vibrating not only A but its octave and the fifth to its octave, and quite a host of other overtones or harmonics of rapidly decreasing wave length which would seem to have no theoretical limit. The waves of long period from a Crookes tube would pass through wood, paper, or the human body, without much resistance, but would be absorbed or reflected by the denser metals. But if objects capable of taking up their vibrations lay in the path of these long rays they would set them vibrating like the violin string, and might in the same way produce overtones which did not before exist. These overtones may include waves of such short lengths as to cause the objects themselves to become luminous. If so, the light waves in question, although they are distinctly instrumental in darkening a photographic plate exposed to them, have nevertheless not passed through, and could never pass through the obstacles easily traversed by the electric waves which gave them origin.
Prof. Ogden N. Rood, of Columbia College, has quite recently published in Science an account of some important work on the reflection of these "rays." The mirror used was a new sheet of ordinary platinum foil. Great care was taken to prevent any rectilinear emanations from the discharge tube reaching the sensitive plate, which was contained in an ordinary plate holder, being covered with two sheets of aluminum, each 0·17 millimetre in thickness, and the draw slide, and over the whole was fastened a netting of iron wire. "After an exposure of ten hours it was found that a good image of the netting had been produced on the vertical strip of the plate exposed to the reflected rays. This image had various deformations, the vertical lines representing the netting being, as a general thing, most distinct; in some places, however, the horizontal lines had the upper hand, and there were a few spots where both were equally distinct. These facts and the character of the deformations point very strongly to the conclusion that in the act of reflection from a metallic surface the Röntgen rays behave like ordinary light." Further experiments were made to ascertain the percentage of the rays reflected. The result arrived at, which Prof. Rood says is only to be regarded as a first approximation, was that platinum foil reflects the one two-hundred-and-sixtieth part of the X rays incident on it at an angle of forty-five degrees. Prof. G. Vicentini and Dr. G. Pacher, in a paper read before the Reale Istituto Veneto di Scienze, report having found distinct evidence of an irregular refraction from a parabolic brass mirror.
Regarding the value of these rays in surgery there are at present hardly sufficient data to warrant a positive conclusion. However, a most thorough and profusely illustrated article appeared in the American Journal of the Medical Sciences for March, 1896, entitled The Clinical Application of the Röntgen Rays. It is much the best exposition which this branch of the subject has yet received, and if one can depend on the pictures, the new agent promises to be, if nothing more, at least a great aid to diagnosis.
An interesting commercial application of the rays is announced by Bugnet and Goscard in the Comptes Rendus. "The proofs which we have the honor to submit to the Academy show in juxtaposition silhouettes of genuine diamonds and of imitations, both loose and set. Prolonged exposure soon succeeds in causing the silhouettes of genuine diamonds to disappear, while false diamonds continue to behave like opaque substances. The same procedure has also enabled us to distinguish natural jet from its mineral imitation."
At a meeting of the Royal Society, on February 13th papers on the Röntgen rays were read by Lord Kelvin and Prof. J. J. Thompson. A discussion followed, the general tone of which showed that, although many interesting points have been cleared up, there is still considerable difference of opinion among the authorities regarding fundamentals, and, while extremely valuable experimental work has been done, we are yet far from a final explanation of the origin and properties of this new (?) form of energy.
The following, from an unbeliever, may perhaps be of interest: Ch. V. Zenger says, in speaking of some pictures obtained by Domalip, Professor of Electrotechnics at the Polytechnicum of Prague: "The interesting point is that Domalip has obtained electric images on a plate by means of plates of copper, brass, zinc, lead, and steel. This is, in my opinion, the proof that there is here merely a phenomenon of electric induction producing phosphorescence of the gelatin, and at the same time an electric discharge in the gelatin, and, lastly, the fluorescence of the ambient air, and as in case of the dark discharge of electricity. In my opinion, these are the three agents which determine the decomposition of the silver salts in the sensitive layer. There are no special radiations, no X rays, and no dark light."