The cover image was created by the transcriber and is placed in the public domain.

ROENTGEN RAYS AND PHENOMENA OF THE ANODE AND CATHODE.

DR. WILLIAM KONRAD ROENTGEN. pp. [69] to [85].
Born in Holland, 1845.
From a photograph by Hanfstaengl, Frankfort-on-the-Main.

ROENTGEN RAYS

AND

PHENOMENA

OF THE

ANODE AND CATHODE.

PRINCIPLES, APPLICATIONS AND THEORIES

BY

EDWARD P. THOMPSON, M.E., E.E.

Mem. Amer. Inst. Elec. Eng.

Mem. Amer. Soc. Mech. Eng.

Author of “Inventing as a Science and an Art.”

CONCLUDING CHAPTER

BY

Prof. WILLIAM A. ANTHONY,

Formerly of Cornell University.

Past President Amer. Inst. Elec. Eng.

Author, with Prof. Brackett of Princeton, of “Text-Book of Physics.”

60 Diagrams. 45 Half-Tones.

NEW YORK:

D. VAN NOSTRAND COMPANY,

23 Murray and 27 Warren Street.

Copyright, 1896,

BY

EDWARD P. THOMPSON,

Temple Court Building, New York.

PREFACE.

In addition to the illustrated feature for exhibiting the nature and practical application of X-rays, and for simplifying the descriptions, the book involves the disclosure of the facts and principles relating to the phenomena occurring between and around charged electrodes, separated by different gaseous media at various pressures. The specific aim is the treatment of the radiant energy developed within and from a discharge tube, the only source of X-rays.

Having always admired the plan adopted by German investigators in publishing accounts of their experiments by means of numbered paragraphs containing cross-references and sketches, the author has likewise treated the investigations of a large number of physicists. The cross-references are indicated by the section sign (§). By reference, the analogy, contrast, or suggestiveness may be meditated upon. All knowledge of modern physics is based upon experiments as the original source. Inasmuch as many years may be expected to elapse before the innumerable peculiarities of the electrical discharge will be reduced to a pure science, and also in order that the contents of the book may be of value in the future as well as at present, the characteristic experiments of electricians and scientists are described, in general, by reference to their object, the apparatus used, the result, the inferences of the experimenter, and the observations of cotemporaneous or later physicists, together with a presentation here and there of theoretical matters and allusion to practical applications.

The classes of reader to which the book is adapted may best be known, of course, after perusal, but some advance intimation of the kind that the author had in view may be desired. Let it be known that, first, the student and those generally interested in science ought to be able to comprehend the subject-matter, because experiments are described, which are always the simplest means (e.g., in a popular lecture) for explaining the wonders of any given scientific principles or facts. Thus did Crookes, Tyndall, Thomson (both Kelvin and J. J.), Hertz, etc., disseminate knowledge—by describing their researches and reasoning thereon.

In view of the tremendous amount of experimenting which has been carried on during the past few years in connection with the electric discharge, it was difficult to determine just how far back to begin (without starting at the very beginning), so that the student and general reader, whose object is to become acquainted especially with the properties of cathode and X-rays, might better understand them. The author realized that it was necessary to go back further and further in this department of science, and he could not easily stop until he had reached certain investigations of Faraday, Davy, Page, and others, which are briefly noticed in an introductory sense. Take, for example, the inaction of the magnet upon X-rays in open air. [§ 79]. Of course, it would be of interest for the student to know about Lenard’s investigations relating to the action of the magnet upon cathode rays inside of the observing tube. [§ 72a]. It would follow, further, that he would desire to know about Crookes’ experiment relating to the attraction of the magnet upon cathode rays within the tube. [§ 59]. In order that he might not infer that Crookes was the first to investigate the action of the magnet upon the discharge, it was evident that the book could be made of greater value by relating the experiments of Prof. J. J. Thomson as to the discharge across and along the lines of magnetic force, [§ 31], and Plücker’s experiment on the action of the magnet upon the cathode column of light. [§ 30]. The interest became increased, instead of diminished, by noting De la Rive’s experiment on the rotation of the luminous effect of the discharge by means of the magnet. [§ 29]. Being now quite impossible to stop, Davy’s electric arc and magnetic action upon the same had to be alluded to, at least briefly. [§ 28]. On the other hand, the very earliest experiments with the discharge in rarefied air are not described—occurring as remotely as the eighteenth century—so ably treated of in Park Benjamin’s work. Those facts that have some mutual bearing are brought forward to serve as stepping-stones to the investigation of cathode and X-rays.

Secondly, the author often imagined that he was writing in behalf of the surgeon and physician and those who intend to experiment, especially when he found in his investigations of recent publications descriptions in detail of the electrical apparatus employed in experimenting with X-rays. He improved the opportunity of repeating the statements of the difficulties, and how they were overcome; also, the precautions necessary to be taken, and, besides, the kind of discharge tubes and apparatus best adapted for particular kinds of experiments. The chapter on applications in diagnosis and anatomy, etc., is of especial interest to physicians.

Thirdly, as the discovery of the Roentgen rays has established a new department of photography, those who are interested in this art may be benefited by the results and suggestions disclosed in connection with photographic plates, time of exposure, adjuncts for best results, precautions for obtaining sharp shadows, and steps of the process, from beginning to end, for carrying on the operation.

Fourthly, expert physicists and electricians, professors, etc., need something that the above classes do not, and this is the reason why the author has not assumed the burden of carrying any line of thought or theory from the beginning to the end of the treatise, nor has he made the book in any way a personal matter by criticising experiments, nor even by favoring the views of one over the other, unless it is in an exceptional case here and there; but in each instance the investigator’s name is given, and that of the publication in which the account may be found, so that the scientist may refer thereto to test the correctness of the author’s version of the matter, or to learn the nature of the minute details and circumstances.

The author suggests that the study of the phenomena of the discharge tube would not be amiss in scientific schools and colleges. He argues that in view of all experimenters in this line having been made enthusiastic and fascinated by reason of (1) the beautiful effects, (2) the field being always open to new discoveries, (3) the direct practical and theoretical bearing of the peculiar actions upon other departments of electricity, light, heat, and magnetism, (4) the pleasure in attempting to obtain results reported by others, and especially the large amount of valuable theoretical and practical instruction resulting therefrom, by repeating the experiments or studying them, and (5) the possible applications of the discharge tube in connection with electric lighting and in the new department of sciagraphy by X-rays, and for other good and valuable considerations—it follows that students who have been through or who are studying a text-book of physics and electricity would be greatly benefited by a course in the discharge-tube phenomena.

In view of the large amount of dictation necessary in order to complete the work in such a short period, and in order that the subject-matter might involve the treatment of the latest work of the French and German as well as of the English and American, and inasmuch as the journals of the latter did not always contain complete translations and, for better service in behalf of the readers, the authorship was shared with others, and, therefore, much credit is due to Prof. Anthony for final chapter, to Mr. Louis M. Pignolet for assistance in connection with French periodicals and academy papers ([§ § 63a], [84], [99], [101a], [103a], [112a], [124a], [128], at end, [139a], [154], [155], [156], [157], [158], and [159]); to Mr. N. D. C. Hodges, formerly editor and proprietor of Science, who obtained some pertinent accounts, ([§ 97a], [97b], [99A], [B], [C], [D], to [99T], inclusive) by investigations of recent literature at the Astor Library, New York; and also to Mr. Ludwig Gutmann (Member American Institute of Electrical Engineers) for a few translations from the German.

Credit is given in each instance to all societies and publications by naming them in the respective paragraphs herein. In nearly every case the author prepared his material from original articles and papers contributed by the investigators to the societies or periodicals.

The author has prepared himself to withstand, with about half as much patience as he expects will be required, all criticisms based upon disappointments which may be experienced by the true, or the alleged true, first discoverer of any particular property of the electric discharge not duly credited. He has been particular in presenting knowledge as to physical facts and principles, but not equally, perhaps, as to the originator of the experiment, or as to the actual first discoverer, for the simple reason that the book is in no sense a history not a biography. Where the paragraph has been headed, for example, “Swinton’s Experiment,” it means that that party (according to the article purporting to be written by him) made that experiment. Some one else may have made exactly the same experiment previously, yet the instruction is equally as valuable as though the researches of the first discoverer had been related. On the other hand, the author has never had any intention of giving credit to the wrong party. The dates in the captions indicate the general chronological order in behalf of those thus interested. With this explanation, it is thought that the claimants will be much more lenient in their criticisms concerning priority of discovery. While the developments have generally followed each other historically, as well as appropriately for the purpose of instruction, yet now and then it was preferable to place the description of a comparatively recent experiment in conjunction with some description of an experiment made at a much earlier date. For this reason, also, the book is not of a chronological nature. The subject-matter, as usual, is divided into chapters, but the sections are to be considered as subordinate chapters, having different shades of meaning, and the one not necessarily bearing a direct relation to the contents of its neighbor, but as, in a novel or a treatise on geometry, having its important part to play in conjunction with some later or preceding section.

Edward P. Thompson.

Temple Court Building, New York,

August, 1896.

CONTENTS.

CHAPTER I.

CHAPTER II.

CHAPTER III.

CHAPTER IV.

CHAPTER V.

CHAPTER VI

CHAPTER VII.

CHAPTER VIII.

CHAPTER IX.

CHAPTER X.

CHAPTER XI.

CHAPTER XII.

Total number of sections to this place, 199.

CHAPTER XIII.

CHAPTER XIV.

INTRODUCTION.

The new form of energy, for which there are two names—to wit, the Roentgen ray and the X-ray—is radiated from a highly exhausted discharge tube, which may be energized by an induction coil or other suitable electrical apparatus, such as a Holtz or a Wimshurst electrical machine. [§ 106]. The principle underlying the construction of the usual induction (or Ruhmkorff) coil is disclosed in the subject-matter of [§ § 1], [2], and [3], and is represented in diagram in Figs. 1 and 2 on page [17]. It would be well for the amateur or general scientific reader to study these sections carefully, for then he will have all the knowledge that is necessary for understanding the apparatus by which the discharge tube is energized. Of course, he will not comprehend the various mechanical details, nor the many electrical and mathematical relations existing in connection with an induction coil, but he will gain sufficient knowledge to appreciate what is intended when such a device is referred to here and there throughout the book. Since the time of Faraday, Page, and Fizeau induction coils of very large dimensions have been constructed, but none of them probably ever exceeded that built by Spottiswoode, during or about 1875, which was so powerful as to produce between the two electric terminals, in open air, a spark of 42 in. in the secondary current with only 30 small galvanic cells of the Grove type in the primary circuit. The cells are seldom used in this connection at the present time, the same being replaced by the dynamo, and the current being conveniently obtained from the regular incandescent-lamp circuit which may be found in almost any city. Those, therefore, who intend to become better acquainted with the details of the electrical apparatus should study in conjunction with this book some elementary treatise relating particularly to dynamos and electric currents.

The essential element in connection with the generation of X-rays is not the coil nor the dynamo, but the electric discharge, especially when occurring within a rarefied atmosphere, provided within a glass bulb, called the discharge tube throughout the book, but which has usually been called by different names, for example, the receiver of an air pump, or a Geissler tube, when the air is not very highly exhausted, or a Crookes tube (see picture at [§ 123]) when the vacuum is definitely much higher by way of contrast. It has also been called a Hittorff tube, the Lenard tube, and by several other names, according to its peculiar characteristics.

Fig. 1.—Head.

Fig. 2.—Broken Arm, Overlapping.
(Due to defective setting.)

Fig. 3.—Ribs.

Fig. 4.—Knee, Knickerbocker Buttons, Bullet in Femur.

FROM SCIAGRAPHS BY PROF. DAYTON C. MILLER. [§ 204].

For those who are not acquainted with the nature of the electric charge and discharge, nor with the peculiar and exceedingly interesting phenomena which various investigators have discovered from time to time, nor with the variety of effects according to the nature and the pressure of the atmosphere within the glass bulb, it is exceedingly difficult to understand with any degree of satisfaction the properties, principles, laws, theories, and manner of application of cathode and X-rays. Consequently, the greater part of the book treats of the electric charge and discharge in conjunction with certain kindred phenomena. Primarily, the meaning of the electric discharge may be derived by referring to Fig. 2, page [17], where there is shown an electric spark, indicated by radial lines between the terminals of a fine wire forming the long and fine coil or secondary circuit. Imagine that the wires are at great distances apart. Let them be brought closer and closer together. By suitable tests it will be found, for example, that no current passes through the wire, but when the points are brought sufficiently close together a spark will occur between the two terminals. [§ 2]. Sometimes instead of what is understood as a spark, a brush or glow takes place ([§ § 10] and [11]), and in fact a numerous variety of effects occur, a general name for all being conveniently termed an electric discharge. Even if no sudden discharge takes place, yet, as when the terminals are far apart, there may be a charge or a tendency, or, as it is technically called, a difference of potential, between the two electrodes, one of which is the cathode and the other the anode. This is comparable to a weight upon one’s hand, tending continually to fall, and always exerting a pressure, and it will fall when the hand is suddenly removed. This is in the nature more of an analogy than of an exact correspondence. A discharge through open air, while adapted to produce a great many curious as well as useful effects, does not act as a generator of X-rays. [§ 136]. Another class of phenomena is obtainable by exhausting the air to a certain extent from a discharge tube, thereby obtaining what is usually called a low vacuum. Such bulbs have been called Geissler tubes. Neither can X-rays be generated therefrom to any practicable extent, but only feebly if at all. [§ 118]. Hittorff, Varley ([§ 61a]), Crookes ([§ § 53] to [61], inclusive), were the first to discover and study the different phenomena that are obtained by diminishing the pressure within the discharge tube to a decrement of several thousand millionths of an atmosphere. This will explain why so many allusions have been made to the Crookes tube, for when the electric discharge is caused to take place in such a high vacuum X-rays are propagated in full strength.

Upon the first announcement of the discovery, electricians, eminent and otherwise, were of one mind in assuming the possibility of obtaining Roentgen rays from other sources than that of the highly evacuated discharge tube. Instead of speculating and theorizing, hosts of crucial tests were instituted, resulting negatively, and it is now safe to conclude that the electric discharge is the only primary source, and it is reasonably safe to assert that the discharge must take place within a highly evacuated enclosure.

The next stage of exhaustion, of no advantage to be considered, is that at which no discharge takes place ([§ 25]), and neither are any Roentgen rays propagated therefrom.

CHAPTER I

1. Faraday’s Experiment, 1831. Secondary Current by Induction. Experimental Researches, Proc. Royal. So. 1841.—In brief, the experiment involved the elements illustrated in the accompanying diagram, Fig. 1, p. [17]; a ring made of iron; upon the ring, two coils of copper wire, suitably insulated from each other and from the iron; a galvanometer included in circuit with one coil, and an electric battery of ten cells placed in circuit with the other coil. He found that upon breaking or completing connection with the battery, the needle was powerfully deflected. Without entering into further detail, it is important, however, to notice that he did not perform any experiments tending to establish the principle of increase of E. M. F. by making the very slight change now known to be necessary. [§ 2].

2. Page’s Experiment, 1838. Electric Spark by Induced Current. Pynchon, p. 427. Dr. Page performed an experiment in which the primary coil was but a few feet in length, while the secondary coil was 320 ft. He included, in the primary circuit, only a few cells of battery. The manner in which he first caused rapid interruptions of the circuit of the primary coil was by the use of what may be called a coarse file, Fig. 2, p. [17]. He discovered that the E. M. F. during the rapid interruption was so much increased over that of the small battery, that an electric spark would pass between the secondary terminals without first bringing them into contact with each other. [§ 6]. The result of these experiments was not only the generation of a current of high E. M. F. from a generator of low E. M. F., but also a current of great quantity as compared with currents obtained from frictional and influence machines, whose complete history is found in Mascart’s work on Electricity.

3. Fizeau’s Experiment. Spark in Secondary Increased by Condenser in Primary, 1853. Pynchon, p. 456.—He connected the plates of a condenser respectively to the terminals of an automatic circuit breaker in the primary circuit, and noticed that the sparks between the two terminals of the interrupter produced by the self-induced current were greatly diminished, while those of the secondary coil were about double in length. Since that time it has been universally customary to equip induction coils with condensers in like manner.

4. Vincentini’s Experiment. Condition of a Gas Around a Live Wire. Nuovo Cimento, Vol. XXXVI., No. 3. Nature, Lon., March 28, ’95, p. 514. The Elect., Lon., Feb. 8, ’95, p. 433. G. Vincentini and M. Cinelli found that the molecules of a gas at and near the surface of a platinum wire, rendered incandescent by a current, are electrified, and that with hydrogen their potential is about .025 volt above the mean potential of the wire. With air and carbonic acid gas the increment is about 1 volt. The apparatus, Fig. [II]., consists essentially of means for passing a current along a platinum wire, a bulb for preventing draughts, and an electrometer having a platinum disc electrode that could be adjusted to different positions. It was noticeable that the electrification did not reach a maximum instantaneously upon closing the current through the wire, but the time was less at points below the wire than above.

II

5. Henry’s Experiment. Magnetizing Radiations from an Electric Spark. Proc. Inter. Elect. Cong., 1893, p. 119. Preece alluded to Prof. Henry’s original experiment illustrating the action of an electric discharge [§ 2] at a distance. He placed a needle in the cellar. Disruptive discharges of a Leyden jar at 30 ft. distant, in an upper room, produced a magnetic effect upon the needle.

6. Faraday’s Experiment. Arc Maintained by Certain Metallic Electrodes at Low Voltage. Experimental Researches. Phil. Trans., Se. IX., Dec., 1894. § 107. to 1078. The generator employed in this experiment consisted of a few cells of a chemical battery, and he obtained, what he called, a voltaic spark. He observed that when the two terminals touched each other, a burning took place and an appearance as if the spark were passing on making the contact, the terminals being pointed and formed of metal. When mercury was the terminal, the luminosity of the spark was much greater than with platinum or gold, although the same quantity of current passed in both cases. He attributed the difference to a greater amount of combustion in the case of mercury, than in those of gold and platinum. He obtained almost a continuous spark by bringing down a pointed copper wire to the surface of mercury and withdrawing it slightly. Wheatstone, in 1835, analysed the light of sparks, and found them to be so characteristic that by means of the prism and the spectra formed, the metal could be known.

III

7. Wurts’s Experiment. Non-arcing metals at high voltage. Trans. Amer. Inst. Elect. Eng. March 15, 1892. Ann. Chem. Phar. Sup. VII, 354 and VIII, 133. Chem. News, VII, 70; X, 59, and XXXII, 21, 129.—Mendelejeff and Meyer discovered that chemical elements occur in natural groups by a principle which they termed the periodic law. One of these groups includes zinc, cadmium, mercury and magnesium; and another group, antimony, bismuth, phosphorus and arsenic. Alex. J. Wurts, of the Westinghouse Electric Co. found that the metals of these groups are non-arcing, by which he means that with an alternating current dynamo of a thousand or more volts, and with the said metals as electrodes in the air only just escaping each other, it is impossible to maintain an arc as in the case of an ordinary arc lamp having carbon electrodes or in a lightning arrester usually having copper electrodes. He suggested and theorized that certain chemical reactions served to explain the phenomena. With low voltage—as 500, the arc was maintained between all metals. [§ 6]. A two pole lightning arrester is shown in Fig. [III] The arc formed, ceased instantly. One of the best metals for practical use is an alloy of 1/2 zinc and 1/2 antimony, or any metal electroplated with a non-arcing metal. Freedman observed a critical point with electrodes of brass. The current was gradually reduced until the arc became like the discharge of a Holtz machine whose condensers have been disconnected. See Elect. Power, N.Y., Feb. 1896, p. 119.

8. Wheatstone’s Experiment. Duration of Spark. Phil. Tran. 1834.—The short duration of an electric spark produced by a single disruptive discharge is easily made apparent by a rapidly rotating disc, having radial sectional areas of different colors. With reflected sunlight, the colors seem to blend into one tint upon the principle of the persistence of vision; (See Swain’s experiment. Trans. R. So. Edin. ’49 and ’61.); but when viewed by the flash of a spark, the colors are seen as distinctly separated as if the disc were at rest. By calculation, based directly upon a series of experiments, he found the duration of the spark to be about .000042 sec. It was discovered also, by the rotating mirror, that the apparently single spark was composed of several following each other in quick succession, and he concluded that the current during the discharge was intermittent. He considered each of the divisions of the spark as an electric discharge. Prof. Nichols, of Cornell University, and McKittrick obtained curves indicating the variation of E. M. F. during the existence of a spark. Trans. Amer. Inst. Elect. Eng. May 20, ’96.

8a. Feddersen, who used a Leyden jar, modified the experiment by having high resistances in the circuit through which the charge was effected. The duration of the spark was found to be increased. In one experiment, he employed a slender column of water as the resistance, 9 mm. in length. The spark endured .0014 second. With a tube of water 180 mm. the duration was .0183 second. He noticed also that the duration increases directly with the striking distance and with the electrical dimensions of the electrical generator. By varying the resistance of the circuit, he found as it became less, the discharge was intermittent, when further reduced, continuous, (difficult to obtain) [§ 11] and when very small, oscillatory—i.e., alternately in opposite directions.

9. Faraday’s Experiment. Brush discharge sound. Phil. Trans. Jan. 1837. Se. XII.—The brush discharge was caused to occur, in his experiments, generally from a small ball about .7 of an inch in diameter, at the end of a long brass rod, acting as the anode. With smaller balls he noticed that the pitch of the sound produced was so much higher as to produce a distinct musical note, and he suggested that the note could be employed as a means of counting the number of intermissions per second. See Mayer’s book on “Sound” § 77, on measuring number of vibrations in a musical note.

9a. Upon bringing the hand toward the brush the pitch increased. [§ 49]. With still smaller balls and points, in which case the brush could hardly be distinguishable, the sound was not heard. He alluded to the rotating mirror of Wheatstone as becoming not only useful but necessary at this stage. He considered the brush as the form of discharge between the contact and the air or else some other non or semi-conductor, but generally between the conductor and the walls of the room or other objects which are nearest the electrodes, the air acting as the dielectric. One experiment, he performed with hydrochloric acid led him to believe that that particular gas permitted of a dark or invisible discharge. Sometimes the air was electrically charged [§ 4] to a less distance than the length of the brush or light.

10. Brush in Different Gases. Striae Cathode Brushes. In the air, at the ordinary pressure he found the color to be “purple;” when rarefied still more purple, and then approaching to rose; in oxygen, at the ordinary pressure, a dull white; when rarefied, “purple;” and with nitrogen, the color was particularly easily obtained at the anode, and when nitrogen was rarefied the effect was magnificent. The quantity of light was greater than with any other gas that he tried. Hydrogen, as to its effect, fell between nitrogen and oxygen. The color was greenish grey at the ordinary pressure and also at great rarity. The striae were very fine in form and distinctness, pale in color and velvety in appearance, but not as beautiful as those in hydrogen. With coal gas, the brushes were not easily produced. They were short and strong and generally green, and more like an ordinary spark. The light was poor and rather grey. Also in carbonic acid gas the brush was crudely formed at the ordinary pressure as to the size, light and color. The tendency of the discharge in this case was always towards the formation of the spark as distinguished from the brush. When rarefied, the light was weak, but the brush was better in form and greenish to purple, varying with the pressure and other circumstances. As to hydrochloric acid, it was difficult to obtain a brush at the ordinary pressure. He tried all kinds of rods, balls and points, and while carrying on all these experiments he kept two other electrodes out in the air for comparison, and while he could not obtain any satisfactory brush in the hydrochloric acid gas, there were simultaneously beautiful brushes in the air. In the rarefied gas, he obtained striae of a blue color.

He compared the appearances also of the anode and cathode brushes in different gases at different pressures. He noticed that in air, the superiority of the anode brush was not very marked ([§ 41] at end.) In nitrogen, this superiority was greater yet. A line of theory ran through Faraday’s mind in connection with all these experiments, whereby he held that there is “A direct relation of the electric forces with the molecules of the matter concerned in the action.” [§ 47]. He made a practical application of the principles in the explanation of lightning, because nitrogen gas forms 4/5 of the atmosphere, and as the discharge takes place therein so easily.

From Magnetographs by Prof. McKay. p. [25].
1. Platinum wire.
2. Copper gauze.
3. Iron gauze.
4. Tinfoil.
5. Gold-foil.
6. Brass protractor.
7. Silver coin.
8. Platinum-foil.
9. Brass.
10. Lead-foil.
11. Aluminum.
12. Magnesium ribbon.
13. Copper objects.

From Sciagraph of Various Objects. p. [130].
By Prof. Terry, U. S. Naval Academy.

11. Glow by Discharge. Glow Changed to Spark. Motion of Air. Continuous Discharge During Glow. The glow was most easily obtained in rarefied air. The electrodes were of metal rods about .2 of an inch in diameter. He also obtained a glow in the open air by means of one or both of the small rods. He noticed some peculiarities of the glow. In the first place, it occurred in all gases and slightly in oil of turpentine. It was accompanied by a motion of the gas, either directly from the light or towards it. He was unable to analyze the glow into visible elementary intermittent discharges, nor could he obtain any evidence of such an intermittent action, [§ 8a]. No sound was produced even in open air. [§ 9]. He was able to change the brush into a glow by aiding the formation of a current of air at the extremity of the rod. He also changed the glow into a brush by a current of air, or by influencing the inductive action near the glow. The presentation of a sharp point assisted in sustaining or sometimes even in producing the glow; so also did rarefaction of the air. The condensation of the air, or the approach of a large surface tended to change the glow into a brush, and sometimes into a spark. Greasing the end of the wire caused the glow to change into a brush.

12. Lullin’s Experiment. Spark. Penetrating Power. Passage Through Solids. Encyclo. Brit. Article Electricity. He placed a piece of cardboard between two electrodes and discovered that a spark penetrated the material and left a hole with burnt edges. When the electrodes were not exactly opposite each other, the perforation occurred in the neighborhood of the negative pole. Later experiments have shown that a glass plate, 5 or 6 cm. in thickness, can be punctured by the spark of a large induction coil. The plate should be large enough to prevent the spark from going around the edges. The spark is inclined, also, to spread over the surface of the glass instead of piercing it, [§ 24]. Glass has been cracked by the spark in some experiments.

13. Fage’s Experiment. Spark. Penetrating Glass. Holes Close Together. Practical Application. La Nature, 1879. Nature, Dec. 26, 1879, p. 189. The length of the spark from the secondary coil in air was 12 cm. One terminal of the secondary passed through an ebonite plate (18 cm. × 12) and touched the glass. Olive oil was spread around said terminal ([§ 11] at end), and served to insulate the same. Oil dielectric in this connection originally employed at least prior to 1870. Remembered by Prof. Anthony as far back as 1872, who often performed the experiment according to instructions contained in a publication. The other terminal of the secondary coil was brought against the glass opposite the first terminal. The spark was then passed and the glass perforated, [§ 12]. By pushing the glass along to successive positions and passing the spark at each movement, holes could be made very close together. In Nature, of 1896, the author noticed that certain manufacturers were introducing glass perforated with invisible holes to be used for windows as a means of ventilation without strong draughts. Perhaps the fine holes were made by means of the electric spark.

14. Knochenhaurer’s Experiments. Conducting Power of Gas. Spark. Penetrating Power. Relation of E. M. F. to Pressure of Gas. 1834. Pogg. Ann., Vol. LVII., and Gordon, Vol. II. Boltzmann’s experiment (Pogg. Ann., CLV., ’75), and calculation indicated that a gas at ordinary pressure and temperature must have a specific resistance at least 10²⁶ times that of copper. Pogg. Ann., CLV., ’75. Sir William Thomson (Kelvin) confirmed this limit for steam, and Maxwell the same for mercury and sodium vapor, steam and air. From Maxwell’s MSS. Herwig was not sure but that the Bunsen burner flame and mercury vapor conducted. He allowed for the conductivity of the walls of the glass container. Braun treated of the conductivity of flames. Pogg. Ann., ’75.

14a. Varley found that 323 Daniel cells only just initiated a current through a hydrogen Geissler tube, and only 308 cells continued the current after once started. Knochenhaurer found that Harris’ (Phil. Trans., 1834) law did not hold exactly true, and that the ratio between the E. M. F. and the air pressure becomes greater and greater as the pressure becomes less and less. Harris thought the ratio was constant. The limits of his pressures were from 3 to 27.04 inches of mercury. Stated in other words, his results were the same as those of Harris and Masson (Ann. de Chimie, XXX., 3rd Se.), except that a small constant quantity should be added. [§ 16].

15. Gordon’s Experiment. Dust Particles Hasten Discharge. Gordon, Vol. II. Other experimenters had investigated the phenomena of the electric spark with different densities of the dielectric by a spark produced by a frictional or an influence machine, or, in a few cases, by powerful batteries without coils, while Gordon claims to be the first to carry out these experiments with an induction coil. He observed that when the discharging limit was nearly reached, small circumstances, such as a grain of dust or a rusting of the terminal by a former discharge, would cause the discharge to take place at a lower E. M. F., which should be allowed for.

16. Kelvin’s Experiment. Proc. R. So., 1860. Encyclo. Brit., Art. Elect. He used as the terminals, two plates. One of them was perfectly plane, while the other had a curvature of a very long radius. The object of this arrangement was to obtain a definite length of spark for each discharge. The plates were gradually moved away until the spark would no longer pass, and the reading of the distance was noted. The law which he found cannot well be expressed in the form of a rule or principle, because it is of a rather intricate nature, but a discovery resulted, namely in the case where the distance was greater, the dielectric strength was smaller for respective distances of .00254 and .535 cm. Many theoretical considerations in reference to this matter have been presented, notably that of Maxwell in his treatise on Electricity and Magnetism, Vol. I.

17. Cailletet’s Experiment. Spark. Penetrating Power. High Pressures. Increased Dielectric Strength. Mascart, Vol. I. He experimented with dry gas up as high as pressures of 700 lbs. per sq. inch. He found that the dielectric strength continues to increase with increase of pressure. He used about 15 volts in the primary and a powerful induction coil. The dielectric strength was so great that at the maximum pressure named above, the spark would not pass between the electrodes when only .05 mm. apart. [§ 25] and [11], near end.

18. Faraday’s Experiment. Discharges in Different Chemical Gases Variably Resisted. Exper. Res. Phil. Trans., Se. XII., Jan. ’36. Faraday passed on from the consideration of the effect of pressure, temperature, etc., and wondered whether there would be any difference in the law according to what gas was used. He arranged apparatus so that he could know, with air as a standard, whether another gas had a greater or less dielectric power. (Cavendish before him had noticed a difference.) He tabulated the results. They exhibited the following facts, namely that gas, when employed as dielectrics, depend for their power upon their chemical nature. [§ 10]. Hydrochloric acid gas was found to have three times the dielectric strength of hydrogen, and more than twice that of oxygen, nitrogen or air; therefore the law did not follow that of specific gravities nor atomic weights. See also De la Rue, Proc. Royal So., XXVI., p. 227.

19. Thomson’s Experiments. Gas as a Conductor. Visible Indication by Discharge. Nature, Lon., Aug. 23, ’94, p. 409; Jan. 31, ’95, p. 332, and other references cited below. Lec. Royal Inst. Proc. Brit. Asso., Aug. 16, ’94. In making comparisons, things of like nature should be considered. Take, for example, gas at .01 m. The number of molecules in such a rarefied atmosphere is comparatively small, while in an electrolyte there are molecules sufficient in number to produce 15,000 lbs. of pressure, if imagined in the gaseous state within the same space. By an experiment and rough calculation, Prof. J. J. Thomson, F.R.S., calculated that the conductivity of a gas estimated per molecule is about 10 million times that of an electrolyte, for example, sulphuric acid. [§ 14]. This is greater than the molecular conductivity of the best conducting metals. The experiment which is illustrated in Fig. [IV.] was a second experiment which did not serve as a basis for calculation, but exhibited very strikingly to the eye that gases having different pressures have different conductivities. For this apparatus he had two concentric bulbs, as indicated, one being contained within the other. The inner one had air rarefied to the luminous point. The outer one had a vacuum as high as it was practical to make it, and contained in a projection a drop of mercury, which, when heated, would gradually increase the pressure. Two Leyden jars were employed, and their outer coatings were connected to the coil which is seen surrounding the outer bulb, and the inner coatings were connected to the coils of a Wimshurst machine. The operation was as follows: When the mercury was cold, that is, with a high vacuum in the outer compartment, a bright discharge passed through the inner bulb, while the outer bulb was dark. When the mercury was heated, the outer bulb was bright, and the inner one was almost dark. By well-known principles of conductors and non-conductors, the operation was explained by Prof. Thomson, who assumed that the gas in the outer bulb is a conductor; then, at each spark will the alternating current in the coil induce currents of an opposite direction in the gas, which will become luminous, as occurred when the mercury was heated. The currents circulating in the gas act as a shield to the induction of the currents in the inner bulb. However, with the vacuum exceedingly high in the outer bulb, the air therein being a non-conductor comparatively, or for the given E. M. F., does not prevent the discharge through the inner bulb, which becomes, therefore, luminous. He next compared the dielectric power of a gas, a liquid and a solid. He found that the E. M. F. had to be raised, in order to produce the discharge,—higher in the liquid than in the gas, and higher in the solid than in the fluid. [§ 12].

IV.

20. Boltzmann, Gibson, Barclay, Hopkinson and Gladstone’s Experiments. Square Root of the Dielectric Capacity Equal to the Refractive Index. Phil. Trans., 1871, p. 573. Maxwell, Vol. II., § 788. Maxwell has argued elaborately upon results of some of the above experimenters upon the theory that the luminiferous ether is the medium for transmission of electricity, light and magnetism; therefore he predicted that the relation stated in the title above should exist. He acknowledged that the relation is sufficiently near a constant to show in connection with other results, especially those obtained, that his theory is probably correct.

21. PLÜCKER’s Experiment. Hermetically Sealed Vacuum Tube. Encycl. Brit., vol. 8, p. 64. Pogg. Ann., 1858, and vol. CXXXVI, 1869.—He engaged Geissler (according to Hittorf) to make a glass tube in which the platinum wire electrodes were sealed in the glass by fusion, as in the modern incandescent lamp. After the air was exhausted by a mechanical air pump through a capillary tube, the same was sealed with the flame of a spirit lamp. He thus established means whereby a practically permanent vacuum could be maintained within a glass bulb. Platinum expands by heat at about the same rate as glass: hence there is no tendency to crack and admit air.

22. Geissler’s Experiment. Luminosity of Vacuum Tubes by Friction. Increased by low temperature. Science Record, 1873.—By rubbing the vacuum tubes with an insulator—cat skin, silk, etc.—he observed that light was generated and that its color depended upon the particular gas forming the residual atmosphere. At a low temperature, the colors were more luminous. [§ 135]. The best form of tube consisted of a spiral tube contained within another tube. A modified construction involved the introduction of mercury. By exhausting the air, and shaking the tube, the friction or motion of the mercury against the glass produced luminous effects according to the gas. Only chemically pure mercury would cause the light, which endured for an instant after the rubbing ceased. [§ 63].

23. Alvergniat’s Experiment. Luminosity of Vacuum Tubes by Friction and Discharges. Different Vacua Required. Sci. Rec., 1873, p. 111. Comptes Rendus, 1873.—To obtain luminosity by charging the tubes with the coil, it was necessary to increase the degree of the vacuum—but when this was done the rubbing of the tube would not cause light. The tube employed was 45 cm. in length, and contained a small quantity of silicic bromide. The atmospheric pressure within the tube for obtaining the glimmer by friction was 15 mm.

24. Steinmetz’s Experiment. Luminous Effects by Alternating Current and Solid Dielectrics. Trans. Amer. Inst. Elec. Eng., Feb. 21, ’93.—In carrying on experiments in the accurate measurement of dielectric strength, he noticed that upon placing mica between the electrodes, as is hereinafter set forth, a spark did not at first form, but that which he called a corona. He attributed the appearances to a condenser phenomenon, or at least he suggested this as an explanation. [§ 3]. As soon as the corona reached the edge of the plate, the disruptive discharge took place, by means of the sparks passing over the edge of the dielectric. [§ 38]. He employed an alternating current dynamo of about 50 volts and 1 h.p., frequency of 150 complete periods per second. The E. M. F. of the alternator was varied, by changing the exciting current, up to 90 volts. Step-up transformers were employed. With a difference of potential in the secondary of 830 volts, and a thickness of mica of 1.8 mm. and when the experiment was performed in a dark room a faint bluish glow appeared between the mica and the electrodes. At 970 volts the glow was brighter, while at 1560 volts the luminosity was visible in broad day-light, and kept on increasing with the increase of E. M. F. He modified the experiment by using mica of a thickness of 2.3 mcm. The difference of potential was 4.5 kilo-volts. In addition to the bluish glow, violet streams or creepers broke out and increased in number and length as the E. M. F. became greater, forming a kind of aurora around the electrodes and on both sides of the mica sheet. A loud hissing noise occurred. [§ 9]. As soon as the corona reached the edges of the mica, the disruptive discharge occurred in the form of intensely white sparks and it was noticeable that the length of these sparks was 10 fold greater than could be obtained in the air at 17 kilo-volts. These sparks were so hot as to oxidize the mica, as apparent from the white marks remaining. The electrodes also became very hot, and the mica was contorted and finally broke down.

25. Morgan’s Experiment. No discharge in High Vacua. Wiedemann, vol. 2. Phil. Trans., 1875, vol. 75.—He was led to believe by an experiment, that when the vacuum is sufficiently perfect, no electromotive force could drive the spark from one terminal to the other, however close together they may be. [§ 18]. Details of Morgan’s Experiments were as follows, given roughly in his own words:—A mercurial gauge about fifteen inches long, carefully and accurately boiled till every particle of air was expelled from the inside, was coated with tinfoil five inches down from its sealed end, and being inverted into mercury through a perforation in the brass cap which covered the mouth of the cistern, the whole was cemented together and the air was exhausted from the inside of the cistern, through a valve in the brass cap, which, producing a perfect vacuum in the gauge, formed an instrument peculiarly well adapted for experiments of this kind. Things being thus adjusted (a small wire having been previously fixed on the inside of the cistern, to form a communication between the brass cap and the mercury, into which the gauge was inverted), the coated end was applied to the conductor of an electrical machine, and notwithstanding every effort, neither the smallest ray of light nor the slightest charge could ever be procured in this exhausted gauge.

26. De La Rue and Müller’s Experiment. Constant Potential at the Terminals of a Discharge Tube. Phil. Trans., part 1, vol. 169, p. 55 and 155.—The apparatus consisted of an exhausted bulb, a chloride battery of 2400 cells and a large resistance adapted to be varied between very wide limits. The result was a constant potential at the electrodes of the bulb, during all the variations of the resistance. They concluded, therefore, that the discharge in highly rarefied gases is disruptive, the same as in air at ordinary pressure.

26a. Klingenberg’s Calculations. Direction of Discharge Tube Current in Secondary of Ruhmkorff Coil. Translated from the German, by Ludwig Gutmann. Extract of paper read by G. Klingenberg before the Electro-technischer Verein. It would naturally be inferred that an induction coil, the primary current of which is intermitted, and of one direction, would produce an alternating current in the secondary coil. The fact of the matter is, however, that a good induction coil will produce the sparking only in but one direction. [§ 41]. The reason is the following: If the coil had no self-induction nor capacity, then the current impulses would be represented by a rectangle a, Fig. [1]. On closing, the current would suddenly reach its maximum, which is determined by the terminal pressure and circuit resistance, and this current strength would be maintained as long as the circuit remained closed. On the opening of the circuit, the current would decrease just as suddenly; if not, the arc on opening of the circuit would oppose such sudden fall, therefore the corner will be slightly rounded at a, Fig. [2]. The influence of self-induction, which we find in any coil, is the force that will tend to oppose any change in the current strength. Therefore, the self-induction will be the cause of a retardation of the minimum current. On the other hand, it increases the size of the spark on opening. Next a condenser is enclosed in the main circuit, so that the spool is closed through it at the moment the current is intercepted. If we assume, for simplicity sake, that the magnetization of the iron is proportional to the current strength, then the primary current curve represents at the same time, the curve of the rate of change of line of force in the magnetic field. The secondary E. M. F. is determined by e = n(dw/dt)t t; the rise then will have a smaller E. M. F. than at the fall, like Fig. [3], except that the curve representing the fall should be shown as more nearly perpendicular to the abscissa.

V

27. Kinnersley, Harris and Riess’s Experiments. Spark. Pressure Produced by. Ganot, § 790, et al. Encyclo. Brit. Art. Elect.—These experimenters passed a spark through air contained over mercury, so that if the pressure of the air were increased, the mercury would move along through a capillary tube, having a scale so that the amount could be represented to the eye, as in the cut. (Fig. [V.]) The experiments proved that when a spark passes through the air, the pressure is increased, and it was concluded in view of several experiments, that the spark being the source of an intense, but small amount of heat, expanded the air, thereby causing the pressure in a secondary manner, through the agency of heat. A spark as short as 2 mm. will produce a considerable pressure of the mercury. Riess performed an experiment also in causing the spark to pass through cardboard, and also through mica located within the air chamber. [§ 12]. Other things being equal, the increase of temperature was less by using the solid material like mica or cards, than without. This illustrated that a part of the energy of the spark was converted into heat and a part into mechanical force, and explained why sound, [§ 24], is produced by a spark and by lightning.

CHAPTER II

VI

28. Davy, Bancalari and Quet’s Experiments. Electric Arc, Magnetism and Flame. Sound Produced. Practical Application of Electric Arc. Phil. Mag., 1801.—When the electric arc, for example between two carbon electrodes, occurs, in a powerful magnetic field, it is violently drawn to one side as first shown by Sir Humphry Davy, as if the wind were blowing it and sometimes it is broken into two parts. Fig. [VI.] Again a loud noise is produced. [§ 9]. Without the magnet, the appearance is as at the left. With the energized magnet, the arc and light, as a whole, are as shown at the right.

29. De La Rive’s Experiment. Rotation of Luminous Effect by Magnet. Application to Explain Aurora Borealis. Phil Trans., vol. 137, 1847. Pynchon, p. 471. Ganot, Sect. 928.—An oval discharge tube was employed, having a highly exhausted atmosphere (for those days) of spirits of turpentine. A cylindrically shaped pole of a magnet extended into the bulb half way, Fig. 4, p. [17]. The inner end of the magnetic pole formed one electrode of the tube, and the other electrode was a ring within the vacuum at the foot of the magnetic pole. A fountain of light extended from one end of the magnet pole to the other, and remained stationary, while the magnet was not energized; but the light was condensed into an arc and travelled around the magnet pole when a current was passed through the coils of the magnet. For similar action of magnet on a flexible and movable wire carrying a current, see experiments of Spottiswoode and Stokes, Proc. R. So., 1875. The aurora borealis rotates around the pole of the earth, and therefore, De La Rive thought that the phenomenon in his laboratory and in nature were but one and the same thing and different only in degree. He also extinguished an arc in open air by means of a powerful magnet.

VII

30. Plücker and Hittorf’s Experiments. Action of Magnet on Cathode Column of Light. Pogg. Ann., 1858 and 1869. Plücker found that the magnet acts on the cathode light in a rarefied atmosphere in a different manner from that on the anode light. In the former the light follows the magnetic curves and strike the side of the bulb, according to position of the poles, see Fig. [VII.] “Where the discharge is perpendicular to the line of the poles, it is separated into two distinct parts, which can be referred to the different action exerted by the electro-magnet on the two extra currents produced in the discharge.” Ganot. § 925.

31. Thomson’s Experiment. A Discharge Retarded Across and Accelerated Along the Lines of Magnetic Force. Nature, Lon., Jan. 31, 1895, p. 333. Lect. Royal Inst.—Prof. J. J. Thomson, F. R. S., performed an experiment which illustrates that the electrical discharge is retarded in flowing across the lines of magnetic force and accelerated in flowing with or parallel to such lines. As illustrated in Fig. 20, p. [17], he employed a large electro-magnet adapted to be cut in and out of circuit. He had two air chambers, one a bulb, indicated by a circle, and the other a tube bent into a rectangle, indicated by the dotted square. Between these, was an adjustable coil having its terminals connected to the outside coatings of Leyden jars. When the discharge took place between the poles of the magnet, that is, in the direction of the lines of force, the discharge was helped along by the magnetic field, but when it took place across the bulb, that is, across the lines of force, the discharge was retarded. “The coil can be adjusted so that when the magnet is ‘off’ the discharge passes through the bulb, but not round the square tube; when, however, the magnet is ‘on,’ the discharge passes in the square tube but not in the bulb.”

Some Experiments prior to Lenard’s.

32. Thomson’s Experiment. Resistance Offered to Striae by a Thin Diaphragm. Lect. Royal Inst. Nature, Lon. Jan. 31, ’95, p. 333.—It has often been remarked that lightning always takes the easiest path. The same has been noticed with references to the artificial electric spark. Prof. J. J. Thomson, F.R.S. performed an experiment, which not only confirms this principle but does so in an emphatic manner, and proves it true in reference to the electric discharge in rarefied gases. He arranged a very thin platinum diaphragm so as to divide a Geissler tube into two compartments, Fig. 19, p. [17]. He then formed a passage way around the diaphragm, which could be opened and closed by mercury, by respectively lowering and raising the lower vessel of mercury along the barometer tube. When the passage way is opened around the diaphragm, the luminosity extends through the passage way in preference to going through the diaphragm. When the passage way is closed by mercury, the discharge goes through the thin metal plate. The same was found to occur when the platinum leaf was replaced by a mica scale.

33. Sir David Solomon’s Experiment in 1894. Proc. Royal So., June 21, ’94. Nature, Lon. Sept. 13, ’94, p. 490.—With a tube having a perforated diaphragm, he noticed a “forcing effect” at and near the hole. The striae had the appearance of being pushed through from the longer part of the tube—the diaphragm not being in the centre. There was no passage way around the diaphragm—only through the small puncture. [§ 19].

CHAPTER III

34. Riess’s Experiment. Electric Images. Riess’s Reibungs. vol. 2, § 739.—He laid a coin upon a plate of glass and charged the same electrically about one-half of an hour or more. Upon removing the coin and sprinkling the plate with dust, an engraving of the coin was visible upon the glass. [§ 13]. A suitable dust is licopodium powder.

35. Sanford and McKay’s Experiment. Electrographs. Original Contribution by Prof. McKay of Packer Inst., Brooklyn, May, ’96.—The picture of the coins in Fig. [IX], was produced by the apparatus shown in Fig. [VIII], t, t, tinfoil, p, photographic plate with coins on sensitive side, all wrapped in black paper. Fig. [VIII] represents the general arrangement for taking electrographs. This particular one was made by removing the upper tinfoil and touching each coin successively with wire from one of the poles, while the other wire was connected with tinfoil on the opposite side. The condenser thus formed is charged and discharged many times by a Holtz machine or induction coil. This is not a new discovery, it was first described by Prof. Sanford, I think, of Leland Stanford University, two or three years ago. Other claimants of earlier date probably exist.

36. Lichtenberg’s Experiment. Dust Figures. Pictures Drawn with Anode and Cathode. Göttingen, 1778-79. Motum Fluidi Electriciti.—He drew two independent superposed pictures upon a flat surface of an insulating material, for example, rosin. One picture was drawn with one terminal of a charged Leyden jar. Another picture was drawn with the other terminal of a charged Leyden jar. He sprinkled upon the surface over the two pictures, a dust made of a mixture of red lead and sulphur powder. The former became attracted to the picture drawn with the cathode, and the latter to that made with the anode, so that the two figures were clearly visible. Before sprinkling the powders upon the surface it is necessary to stir them together whereby they become oppositely electrified.

VIII
Arrangements for Taking Electrographs. [§ 35], p. [19].

From Electrographs of Coins. [§ 35], p. [19].
Taken by Prof. McKay.

X

The sulphur arranges itself in tufts with diverging branches and the red lead in small circular patches. The particular materials, namely, the sulphur and red lead were first used by Villarsy. In case only one powder is employed, for example, licopodium, it adheres to both the positively and negatively electrified portion of the insulating plate, but in larger quantities upon the latter portions. Fig. [X], shows rosin disc covered with licopodium powder after touching the disc with the knob of a Leyden jar.

36a. Hammer’s Photo-Electric Dust Figures. From personal interview.—According to experiments of Elster and Geitel, hereinafter noted, [§ 98], Hammer’s dust figures shown in the accompanying half-tone cut may possibly be accounted for on the principle of the discharge of negatively electrified bodies by light. Mr. William J. Hammer, Mem. Amer. Inst. Elect. Eng., has a historical collection of incandescent lamps (Elect. Eng., N.Y., April 29, ’96, p. 446.) which were arranged on shelves in a glass case standing obliquely in the sunlight about an hour a day. After the lapse of many months, the very fine dust within the case lodged upon the inner surface of the glass in such a manner as to produce oval dust figures corresponding somewhat to the shapes of the lamps and some of them, appear after reproduction by the half-tone process in the accompanying cut. When the figures are inspected closely and the circumstances are known, no one can doubt that the sun and lamps acted as agents in their formation. As to the correct explanation, the matter has not been sufficiently discussed by scientists (presented here for the first time) to enable the author to render the opinions of others, but it is of interest in connection with Roentgen rays and the discharge of electrified bodies by light. As a matter of course, the surfaces of the lamps would reflect the light in such a way as to make bright spots (movable, however, with the sun) upon the glass of the containing case, and if the latter were in any sense charged by negative atmospheric electricity, this light would cause a variable amount of dust to be attracted according to the intensity of the rays striking the glass. These remarks are in the nature merely of a suggestion of a hypothesis. The heavy curved black line in the cut is a part of the frame of the glass case. The incandescent lamps do not show, simply because the case was empty when the photograph was taken. That the figures were not due to chemical action was shown by rubbing off some of the dust with the fingers. Finger marks were pictured on the figures. Off hand, Mr. Hammer and Prof. Anthony intimate air convection by differentiation of temperature, as a possible cause.

Fig. 1.—Hammer’s Dust-figure on Glass. [§ 36]., p. [21].

Fig. 2.—Hammer’s Historical Collection of Incandescent Lamps, contained in case having the dust figures. [§ 36], p. [21].

36b. Independently of the above peculiar phenomenon, Mr. Hammer recently had on exhibition at the Electrical Exposition of the National Electric Light Association in New York, 1896, a portrait formed of fine dust upon a pane of glass. The circumstances were as follows, as remembered by the author. Mr. Hammer happened to be in some place where an artisan was removing a photograph from an old frame. The glass which protected the portrait exhibited a fac-simile in dust on the inner surface. The glass had not been in contact with the photograph, because of a thick passe-partout surrounding the picture. Neither was the glass an old negative photographic plate. Further test and inspection tended to prove that the dust picture was executed by some action of the heat or light of the sun. Prof. Benjamin F. Thomas, of the University of the State of Ohio, in an interview, scarcely thought that the result was due to convection, because the dust print was so sharply defined. The principle of the discharge of bodies by light may be applicable perhaps, but further experiment would be necessary as a more secure foundation. It is common to find the print of a picture in a book upon the opposite page, being due merely to the pressure of the inked surface, as in the art of printing. This explanation cannot be applied to the dust portrait, because there was no contact between the photograph and the glass.

37. Karsten’s Experiment. Electrical Images Developed by Condensed Moisture. Riess’s Reibungselect., vol. II., § 739.—He arranged the following articles in the following order: First, a metal plate suitably insulated; secondly, a piece of a glass plate on top of the metal plate, and, thirdly, a coin or small metal object on top of the glass. Sparks were then allowed to pass for several minutes from a Holtz or similar machine to the coin. The image of the latter appeared by removing the glass plate and breathing upon it. The bas-relief of the image on the coin also was visible in all its details, appearing as in Sanford’s Electrograph, [§ 35]. Theoretical considerations led others to believe that the figures of Riess and Karsten are due to a different cause from that involved in the figures of Lichtenberg, for the former are thought to be due to a molecular action of a permanent nature upon an insulating material. A slight change in the color often occurs, thereby outlining the object.

Dust-portrait on Glass, [§ 36]., p. [23], discovered by William J. Hammer.
Lighter portions, dust; darker portions, due to less or no dust. Finger-marks across the shoulder and at right. Exposure 8 years. Portrait as sharp and clear as a daguerreotype. During exposure in frame, distance of glass from photograph, 1/16 inch. Above half tone was made from a photograph of the dust-portrait only after several unsuccessful attempts by different photographers. The original dust-portrait is scarcely visible. Let every one examine closely glass plates when taken from old frames.

37a. McKay’s Experiment. Magnetographs. From Personal Notes by Request. April, 1896.—Although this experiment does not belong to that class connected with discharge tubes, yet the phenomenon has a theoretical interest in connection with X-rays. He obtained a photograph of different objects in the dark by means of radiations from the poles of an electro-magnet after two hours’ exposure, but it need not have been so long, as he obtained clear images in five minutes in one experiment with frequent variations of current by means of a rheostat, and by approach and recession of the armature. The elements involved in the experiment were arranged in the following order: First, a large inverted magnet for supporting 100 lbs., the poles hanging downward. Next in order was a wooden board pressing flatwise against the ends of the poles of the magnet. Next, the objects and the sensitive plates backed thereby and all enclosed in a completely opaque wrapping extending over the sides, face, back, etc., of these two elements. Next in order was an armature about as heavy as the magnet would support. The cut herein represents the photograph that was produced of the different objects named. By reading Prof. McKay’s very detailed description in the Scientific American, April 18, 1896, p. 249, the reader may feel certain that the photograph was not due to light for he tried the experiments in different ways and with various precautions. In a course of experiments carried on by student Austin, about Feb. 15, ’96, in the Dartmouth laboratory, a sciagraph of what appeared to be the lines of force was obtained by means of X-rays, but upon repeating the experiment the result was negative. See Elect. Engineer, Mar. 11, ’96, p. 257. Article by E. B. Frost.

XI

38. Piltchikoff’s Experiment. Liquid Bas-relief Facsimiles by Electric Discharge. Pro. Acad. Sci., Paris, March, ’94. The Electr., Lon., April 13, ’94, p. 656.—These shadow pictures were obtainable either with the anode or cathode, the particular machine employed being a large Voss. To either pole was electrically connected a pointed wire which was held just above the surface of castor oil, in a copper pan. A remarkable effect was obtained of the shadow of a piece of mica, Fig. [XI], of whatever shape, located between the point and the surface. [§ 24]. Let it be observed that this shadow was not one in the sense of light and darkness but it consisted of a plateau within a depression, the former being of the same shape as though it were a shadow of the mica triangle. To illustrate the experiment better, let the mica be supposed to be removed, then will there be a depression formed in the oil upon bringing the metallic point near to the surface. Now insert the insulating sheet between the point and the surface, then will there be an elevation within the depression of the same shape that the shadow would be.

39. Gernez’s Experiment. Distillation of Liquids by Discharge. Phys. So., Paris, 1879. Nature, Nov. 20, 1879, p. 72.—In order that the apparatus with which he experimented may be understood, imagine a tube standing vertically in another tube. The two concentric tubes communicate with each other at the top only. The Holtz machine is the generator. The liquids in the two tubes at the beginning stand at the same level. Sparks are passed through the adjacent air, which is in contact with both liquids. The liquid at the cathode rises and at the anode falls. [§ 38]. Such was the experiment performed by Gernez. He was inclined to conclude that the effect was due to “An electrical transport of liquids along the moistened surfaces of the tubes.” When the liquid was alcohol, it actually went over as by distillation, three times as fast as water. A soluble salt in water increased the rate of distillation; and so also did the addition of a small quantity of sulphuric acid or ammonia. No distillation of bi-sulphide of carbon, tetra chloride of carbon, nor turpentine occurred. Query: Can alcohol be concentrated or practically distilled upon this principle?

40. De La Rue and Müller’s Experiment. Striae. Black Prints on Walls of Tube. Phil. Trans., 59, ’78.—Particles of the metal of the electrodes were deposited upon the inside of the glass forming permanent black striae or bands [§ 44], at points corresponding to the spaces between the luminous striae. [§ 6]. near the end.

CHAPTER IV

41. Gassiot’s Experiment. Striae. Tube in Primary Current. Current Vibratory. Phil. Trans., ’59, p. 137. Bakerian Lectures. Phil. Trans., ’58, p. 1. Proc. R. So., x., pp. 36, 393, 404; xii., p. 329; xxiii., p. 356.—The form of tube in which to obtain luminous striae to the best advantage was that of a dumbbell with the electrodes located respectively in the balls—afterwards confirmed by Sir David Solomons, Bart. Proc. Royal So., June 21, ’94. Nature, Lon., Sept. 13, ’94, p. 490. He obtained in the vacuum luminosity with 500 Daniell’s cells, which he found to be the least E. M. F. that could be employed. He omitted, and apparently overlooked, the introduction of an automatic interrupter in the circuit and the use of a very low E. M. F. [§ 52]. In conjunction with Spottiswoode, 1,080 cells of chloride of silver (about 2,000 volts) were employed, first without, and then with condensers. One of the condensers consisted of the usual tinfoil type, and the other of a self-induction kind, namely of about 1,000 feet of wire. The results were striae with the condensers, and no striae without the condensers. [§ 8a]. The results suggested to them that there was some relation in principle between the striae and vibration of the current. They therefore built an ingenious apparatus to test whether this was true or not, and they found such was the case by the following related means. If a current passing directly from the primary battery through the condenser and the discharge tube is undulatory or intermittent in any sense, then it would be able to induce a current in the secondary of the induction coil. [§ 8] at centre. They found that there was a current thus induced, and they detected it by means of a small discharge tube which became luminous. Fig. 3 p. [17]. This was an independent tube near the top of the figure, having nothing to do with the one containing striae, which were produced by the primary current and shown at the right. Dr. Oliver Lodge, F.R.S., in treating of the cathode and X-rays in The Elect., Lon., Jan. 31, ’96, p. 438, stated the following with reference to Gassiot’s experiments: “In the days of Gassiot and other early workers ([§ 43]) on the discharge in rarefied air, it was the stream from the anode that chiefly excited attention. It is this which developed the well-known gorgeous effects which used to be shown at nearly every scientific conversazione.”

42. Poggendorff’s Experiment. Effects of Interrupting a Current Within Discharge Tube. Phil. Mag., 4th Se., vol. x., 1855, p. 203-207.—Imagine an electric bell vibrator and magnet within the glass receiver upon an air pump. Upon connecting the magnet and vibrator in series with a small electric battery, it is evident that in the open air, as usual in electric bells, there will be a minute violet spark at the terminals of the circuit breaker. [§ 6]. Now, let the air be exhausted as far as possible by means of a mechanical pump as constructed in 1855. Poggendorff performed such an experiment, and he noticed that in the poor vacuum the ordinary violet spark became yellow, while blue light like a small enveloping tube surrounded the hammer of the vibrator and wire leading to the opposite contact and a little projection extending away from the hammer. His experiment was unique, because showing for the first time that a current from a battery, if interrupted in the vacuum, will not only produce the usual minute spark, but that a blue tube of light will be produced around the conductors within the vacuum.

43. De La Rue and Müller’s Experiment. Source of the Striae at the Anode. Number of Striae Varied by Change of Current. Phil. Trans., 1878.—By an arrangement of means for causing different pressures, they made a discovery, namely, that as far as the eye is concerned the striae begin to have their existence at the anode. [§ 46]. Imagine the internal gas pressure to become less and less. First, a violet luminosity occurs around the anode as in [§ 42]. As the pressure becomes less and less, luminous striae move toward the cathode accompanied by more and more striae, which increase either to form a column reaching a certain distance or else extending through the whole distance between the electrodes. [§ 46]. They observed that when the E. M. F. was constant and the current changed, the variation in the appearance of the striae was very regular. [§ 41]. With some tubes the number of striae increased with the increase of current, while with a decrease of current the number of striae became less and less. [§ 8a]. With some tubes the number of striae increased while the current decreased. [§ 8a]. With the use of a condenser, then as the E. M. F. decreased together with a diminution of current, the number of striae varied. The striae nearest the anode vanished first, as they diminished in number with the fall of the E. M. F. The striae on the other hand originated at the anode, when the charge of the condenser was gradually increased from a minimum, and then the striae continued to increase from the anode as the source. As to the color of the striae, the same was changed by an alteration of the current.

44. Solomons’ Experiment. Dark Bands by Small Discharges. Nature, Lon., Sept. 13, ’94. Proc. R. So., June 21, ’94.—Solomons found that in a very dark room, striae (alternate light and darkness) appeared with very minute discharges, and as the current was increased, they vanished, appearing again when the discharge was strong. He could not obtain them until the luminous column extended to the glass forming the large glass tube. [§ 40].

45. Spottiswoode’s Experiment. Governing the Motion of Striae. Effect Upon Motion by Diameter of Discharge Tube. Motion Stopped by Magnet. Proc. R. So., vol. 33, p. 455.—Spottiswoode found that he could obtain motion when he desired. He introduced some constant resistances and also a rheostat of fine adjustment. The least change of resistance caused some effect upon the striae. The general principle that he established was that letting it be assumed that the striae are stationary then; “An increase of resistance produces a forward flow, and a decrease of the resistance a backward flow,” differences of as little as 1 ohm in the primary current caused the effect. Sometimes the velocity of the flow is fast and sometimes slow, being so rapid in certain instances that the unaided eye cannot distinguish them, but they are known to exist by the use of the revolving mirror. [§ 46]. With tubes of small diameter, compared with their length, he noticed the fact that the striae in one portion of the tube moved faster than those in another portion. [§ 46]. Sometimes one group moved while the other one was stationary. Sometimes they moved in opposite directions. This last named phenomenon occurred also in very wide tubes. The points at which the charge took place he called nodes. He discovered external means for stopping this action. He did it by means of a magnet located opposite one end of the tube. [§ 31]. When the magnet was energized, all motion ceased. [§ 31].

From Sciagraph of Foot Deformed by Pointed Shoes. [§ 204].
By Prof. Miller.

From Hammer’s Molecular Sciagraph. [§ 117]., p. [114].

46. Thomson’s Experiment. Velocity of Striae Checked at the Cathode. Nature, Lon. Jan. 31, ’96, p. 330.—A tube 50 ft. long was exhausted, [§ 8a]., as to striking distance. In this particular experiment, he caused a single interruption in the primary of the induction coil, and observed the motion of the striae from the anode to the cathode by means of a rotating mirror. [§ 43]. The luminosity began at the anode and travelled toward the negative with a high velocity, but upon its arrival at the negative pole its velocity was checked. He said that the striae did not disappear at the cathode like a rabbit would in entering a hole, but they lingered around the electrode for some time. As a consequence of this delay, he found as expected, an accumulation of positive electricity, [§ 4], in the neighborhood of the cathode. It is a general principle, therefore, that when a discharge passes between a gas and metal, there is an accumulation, illustrating that the discharge experiences a difficulty or resistance. [§ 32] and [33]. The experimenter, Prof. J. J. Thomson, acknowledged that Profs. Liveing and Davy had noticed similar effects.

47. Thomson’s Experiment. Disruptive Discharge and Electrolysis. Nature, Lon. Jan. 31, ’95. Lect. S. Inst. The Electr., Lon. vol. 31, p. 291, 316, and vol. 35, p. 578. Trans. R. So., ’95.—The discharge of electricity through conducting liquids is, with scarcely an exception, (example, mercury) accompanied by a chemical action. Faraday and Davy both performed early experiments in this direction. Prof. J. J. Thomson has set forth some instructive facts and which act as evidence that there is a close relation between the disruptive discharge and chemical action between the dielectric and electrodes. [§ 6] and [7]. He made this experiment in connection with his investigations relating to the difficulty the positive electricity experiences in passing from a gas to the negative electrode. [§ 46]. He carried this experiment further, by testing gases of different chemical natures. The apparatus he employed consisted first of an alternating current generator, a high tension converter, a bulb for containing first one gas and then another, whose metal electrodes were connected with the secondary of the transformer, and an electrometer connected to a third electrode which could be moved about within the bulb. The operation was as follows: when the bulb contained oxygen which is an electro-negative gas, the third movable electrode received a positive charge in whatever part of the bulb it was moved to, but with hydrogen instead of oxygen at atmospheric pressure, the third electrode received a positive charge far away from the arc between the other electrodes, but very near the arc it received a negative charge. He then rarefied the atmosphere of hydrogen and he noticed that the space where the third electrode became negative, contracted, and at about 1/3 of an atmosphere became practically nothing, so that the said third electrode connected to the electrometer became slightly positive at all points within the hydrogen. [§ 4]. The next step consisted in using a bulb, having oxydized copper electrodes and a hydrogen atmosphere at the pressure where there was only positive electricity, that is about 1/3 of an atmosphere. This remarkable phenomenon occurred; there was no positive electricity, but only negative. When the copper oxide was reduced, the positive electricity only, existed in all parts of the bulb. In brief, bright copper electrodes left a positive charge in the gas, while oxydized electrodes left a negative charge. He argued upon the results of this experiment to account for the delay in the passage of the electricity from the gas to the metal, [§ 46]. In later experiments, he used the spectroscope to detect decomposition. [§ 6], at end.

48. De La Rue and Müller’s Experiment. Heat Striae. Phil. Trans., vol. 159, 1878—They arranged for the best conditions, that is, when a small number of striae occurred in conjunction with a wide, dark interval. [§ 44]. They found that the heat was greatest at the position of maximum luminosity, but they also found that heat was generated at the dark spaces. A novel feature was the discovery of the development of heat in the middle of the tube even when there was no luminosity, [§ 9a], near end, so that they thought it probable there may be what might be termed heat striae, independently of luminous striae.

49. Spottiswoode and Moulton’s Experiment. Sensitive State. Air-Gap in Circuit Forms Best Method of Obtaining. Branch Current to Earth Verified by a Telephone. Sensitive State by a Single Quick Discharge. Phil. Trans., 1879, p. 165, and April 8, 1880.—By sensitive state of luminous effects in a Geissler tube is meant the susceptibility of the light ([§ 28]) to an outside conductor connected to earth. Fig. 5, p. [17]. When one’s hand is brought near a Geissler tube the change near the hand sometimes occurs and sometimes it does not. [§ 8]. In the first place, the effect is more easily noticeable if the vacuum tube is comparatively wide or thick in diameter. With the electric egg, for example, the luminous effect, instead of extending more or less across the space between the electrodes, reaches from one of the poles to a conductor on the outside of the egg, provided said conductor has an earth connection or large capacity. Some of the light continues to exist nevertheless between the two poles. The general principle is that the division exists because of the re-distribution or branching of the disruptive discharge. It was not known why the luminosity should be affected by such an outside conductor sometimes, and remain the same at other times but the above named experimenters discovered causes which could be depended upon to produce the sensitive state. The apparatus will be described. They had the usual Geissler tube with the platinum wire electrodes, and a Holtz machine as the generator. They were led to believe that intermissions of the current had a great deal to do with the production of the sensitive state, and accordingly they arranged for an air-gap in circuit with the machine and with the vacuum tube. [§ 51]. They not only observed that such a gap caused the sensitive state, but that an increase in the length of the gap made the luminous column more sensitive. They increased the gap so much that the ramifications of the light could be seen. If an induction coil is employed as the secondary generator, a condenser should be coupled up in connection with it. The two in combination thereby produce the sensitive state, but upon cutting out the coil and charging the tubes from the condenser the sensitiveness can not be detected. Instead of the permanent air-gap, may be employed a rapid circuit interrupter, coupled up between a Holtz machine and a vacuum tube. The manner of coupling up is to place the interrupter in a shunt to the vacuum tube. Difficulty had been found in early experiments to obtain the sensitive state with those vacua which give striae. With a rapid circuit interrupter and an induction coil, the breaks occurring 240 per second, the luminous column was not only broken up into striae, but were acted upon by the approach of an outside conductor connected to earth. The sensitive state is not always made apparent by the appearance of attraction of the luminous light to the outside conductor. Sometimes the light seems to be repelled. These two phenomena may be caused in the same tube. This feature of the sensitive state constitutes the beginning of radiations of energy through the walls of a vacuum bulb, like X-rays. Some action or other in these cases takes place through the glass. They tried an experiment in which one of the electrodes of the vacuum tube was entirely on the outside. The electrical discharge was found to be sensitive, for the discharge was changed in its appearances by the presence of an outside conductor connected to earth. Another cause of the sensitive state was observed, namely, the brevity of the charge. This may be illustrated with a Leyden jar, which is known to give an almost practically instantaneous discharge. A single discharge from such a jar produced a flash of light which was in the sensitive state. The nomenclature by which the experimenters defined the cause of the phenomena is made up of the words: Re-distribution of electricity, and a relief of the external strain.

49a. No re-distribution took place unless the outside conductor was connected to earth or to a conductor of large capacity, nor would an outside conductor, which was already charged, serve to exhibit the sensitive state. The re-distribution effect was proved by means of a telephone connected in circuit between the outside conductor and the earth Fig. 5, p. [17]. When the state was sensitive, that is, during the use of the air-gap, the telephone produced a sound in unison with the intermissions occurring at the air-gap. [§ 9] and [9a].

50. Reitlinger and Urbanitzky’s Experiment. Sensitive State Illustrated by a Flexible Conductor Within the Discharge Tube. Proc. Vienna Acad., 1879. Nature, Nov. 20, 1879.—The discharge tube was 20 cm. long. It had the usual platinum electrodes, and it stood upright. From the upper electrode, was suspended a strip of tinfoil in the middle of the tube, which was connected to a pump so that the density of the gas could be varied. At atmospheric pressure, the secondary current of a Ruhmkorff coil connected to the electrodes caused the strip to be attracted to the glass tube. The attraction was less and less as the process of exhaustion was carried on, and when a pressure indicated by 7 mm. was reached, the strip was neither attracted nor repelled, but hung downward the same as without any electricity whatever, but it was attracted by a neighboring shell-lac rod which had been rubbed with cloth, and it was repelled by a glass rod which had been rubbed with amalgam, it being assumed that the strip was connected to the anode. § [36]. The opposite action took place when it was connected to the cathode. As the exhaustion continued and became greater and greater, these actions died away also up to a rarefaction of about 4 mm. Independently of the degree of rarefaction, the flexible strip of tinfoil was always deflected by an outside conductor connected to earth. [§ 49].

51. Tesla’s Experiment. Incandescent Electrode by High Potential and Enormous Frequency. System Referred to by Roentgen for Generating Powerful X-Rays. U. S. Letters Pat., No. 454, 622, June 23, ’91. Martin’s Researches of Tesla; Trans. Amer. Inst. Elec. Engineers, May 20, ’91; Elec. Review, N.Y., June 24, ’93, p. 226; Lect. Franklin Inst., Feb. 24, ’93, and Nat. Elec. Light Asso., Mar. 1, ’93; also Lect. in Europe. Later he again experimented in this direction, see Elec. Review, N.Y., May 20, ’96, p. 263.—By the U. S. Patent Office he was granted, among other claims, the following: “The improvement in the art of electric lighting herein described, which consists in generating and producing for the operation of lighting devices, currents of enormous frequency and excessively high potential, substantially as herein described.” A simple combination of circuits together with great skill in the construction of apparatus involving high powers of insulation, resulted in the production, within a vacuum, of an electrode radiating intensely white light. The circuit may be easily traced in the diagram Fig. 17 p. [17]. Briefly described, there may be noticed an alternating current generator of comparatively low E. M. F. The current from this generates a secondary current by means of an induction coil. This secondary current generates a tertiary current by a second induction coil. An air-gap for automatic and intermittent disruptive discharges, [§ 49] near end, is in the circuit of the secondary coil of the first named induction coil, which is directly charged by the alternating current generator. The gap may be noticed between the two balls. In shunt to the air-gap is a condenser (see Fizeau, chapter I.) represented by several parallel lines. The lamp consists merely of an evacuated bulb having an electrode of carbon or other refractory material, which is connected to one pole of the last secondary coil while the other pole may be outside, and may consist, for example, of the walls of a room, which in such a case should be of some electric conducting material. The higher the vacuum the more intense the light; he found no limit to this rule. Fig. 16a p. [17] illustrates his ideal method of lighting a room. He found that with two plates at a distance apart as indicated and connected to the poles of the coil, and with electrodeless vacuum bulbs, the latter became bright in space—no mechanical or electrical connection other than air and the assumed ether.

52. Moore’s Experiment. Luminosity in Discharge Tube by Self-induced Currents. Trans. Amer. Inst. Elect. Eng., Sept. 20, ’93 and April 22, ’96. Several U. S. Letters Patent. Invented 1892.—During or about 1831, Prof. Henry discovered that when the circuit of a primary battery was interrupted, a self-induced current, which he called an extra current, was produced. When the circuit was closed, there was also a self-induced current, but very much feebler than that obtained on interruption. The self-induced current occurred only at or about the instant of interruption or completion. He found also that the self-induced current produced by interruption was enormously increased in E. M. F. if the circuit included a helix of very long and fine wire. It was further increased by the presence of an iron core. With one or two cells, the spark upon interruption was scarcely visible, but with a fine wire 30 or 40 feet long, an appreciable spark was obtained during interruption. With but a comparatively few cells, and with a magnet for example like a telegraph relay, the E. M. F. arose to several thousand volts at the instant of interruption. D. McFarland Moore introduced into such a circuit a Geissler tube and provided a rapid automatic interrupter. Page, Ruhmkorff and others had, at an early date, noticed the desirability, in operating Geissler tubes by secondary currents, to obtain quick interruption in the primary circuit in order to produce the best effects in the Geissler tube. Moore caused the interruptions to take place in a vacuum, so high that a disruptive electrical discharge could not pass. The break was therefore, absolutely instantaneous and complete. By this system, illustrated in diagram in Fig. 18, p. [17], he obtained all the luminous effects, actions by magnets, the sensitive state, striae and all the other phenomena heretofore noticed in Geissler tubes and some of those obtained by Tesla with his apparatus as just described. In greater detail, it will be noticed that he had a dynamo of rather low E. M. F., generally 100 volts, and a high vacuum containing a circuit interrupter operated automatically by a magnet outside like a vibrator in an electric bell. The magnet served also as the self inductive device. The magnet and interrupter were in series with each other, therefore, while the Geissler tube was in series with the magnet, and the electrodes extended either inside of the Geissler tube or remained on the outside. He performed numerous experiments on similar lines and developed the system on a large scale, whereby rooms (e.g. the hall of the Amer. So. Mech. Eng., N.Y.) have been illuminated as if by other artificial illuminants, by employing long and numerous vacuum tubes. Among several discoveries was that of the production of a bright pencil of light along the axis of a long open helix, which formed one of the internal electrodes. The Patent Office made strenuous efforts to determine the degree of novelty, assuming that some one else must have conceived the idea of employing a self-induced current to operate Geissler tubes; but nothing nearer than Poggendorff’s experiment [§ 42] could be found, and therefore the following claim (in patent 548576, Oct. 22, ’95,) was granted among a hundred or so relating to developments and details and particularly covering the vacuum interrupter. “The method of producing luminous effects, consisting in converting a current of low potential into one of high potential, by rapidly and repeatedly interrupting the low potential current in its passage through a self-inductive resistance, and passing the former current through a Geissler tube, thereby producing light within the tube.”

Edison’s Beneficent X-ray Exhibit, [§ 82], p. [71], and [§ 132], p. [126].
Calcic tungstate screen at center, sciascope near right.

CHAPTER V

53. Crookes’ Experiment. Dark Space Around the Cathode. Lect. Brit. Asso., Shef., Eng., Aug. 22, ’79.—According to Lenard (The Electr., Lond., Mar. 23, ’94) Hittorf discovered the cathode rays, and Varley, [§ 61a]., and Crookes studied them. The pressure of the residual gas was 1 M. of an atmosphere. Prof. Crookes, F.R.S., maintained the evacuated space in communication with the air pump and with an absorbent material. Before his time most experimenters worked with a vacuum not much less than 30,000 M. The first experiment is illustrated in diagram, at Fig. 6 p. [17], but the vacuum was not the highest in this type. The tube was cylindrical and was provided with electrodes at the ends. Another electrode was located at the centre and was made the cathode, while the two terminal electrodes were made the same pole; namely, the anode. Upon connecting the tube in circuit with the secondary of a large induction coil, the luminosity did not extend either continuously or in striae throughout the length of the tube. Former investigators had likewise noticed the dark space. The space and glass on each side of the central cathode were dark. The dark space extended for about one inch on each side of the negative pole. It is not intended here, any more than in former cases, to present theories in explanation further than to briefly allude to any conclusion at which the experimenter himself arrived. Crookes’ explanation of the phenomena has not been universally accepted, nor has it been proved otherwise. The knowledge of the existence of rays, now known as Roentgen rays, will assist in formulating theories upon the Crookes’ phenomena and may either confirm some of his views or overthrow them. Crookes considered that the residual atmosphere was in such a state as to be as different in its properties from gas, as gas is from liquid and liquid from solid, and therefore he named the attenuated atmosphere radiant matter, or fourth state of matter. He concluded that the remaining particles of the gas forming the radiant matter moved in straight lines over a great distance as compared with that moved through by molecules at the ordinary pressure. He called this distance the “mean free path.” If his theory is correct, this dark space is due to the fact that the molecules in motion at and near the cathode do not bombard each other and therefore do not produce the effect of light. When the motion is arrested by particles of gas themselves, within the bulb, then is light generated. The force propelling the particles from the positive pole was supposed to be less. In order to let the experiments speak for themselves, as much as possible, without being too much influenced by the opinion of the experimenter; the theory is only briefly alluded to as above, and will not be further applied in the presentation of his other experiments. In view of the radical discoveries of Lenard and Roentgen, after the installation of the Crookes phenomena, it has been the policy of the author to present all the experiments as facts for evidence in behalf of the general theories, which may be hereafter formulated independently of old theories. Therefore, the reader should bear in mind the teachings of the various experiments with the view of arriving at general principles and hypotheses.

54. Relation of Vacuum to Phosphorescence.—He started with such a high vacuum that he could not obtain any electrical discharge. [§ 25]. There was, therefore, no phosphorescence in the glass tube, whatever. The caustic potash, which had been employed to absorb the last trace of moisture and carbonic acid gas, was slightly heated, and very gradually. Then it was noticed that a current began to pass and that the glass became green, and apparently on the inner surface. As the heat continued, the green passed gradually away and was replaced by striae, which first appeared to extend across the whole diameter of the glass tube ([§ 40]) which was a long cylindrical tube, and then became concentrated toward the axial line of the tube. Finally, the light consisted of a pencil of purple. [§ 10]. When the source of heat was removed so that the moisture and carbonic acid gas could be absorbed again by the potash, the striae appeared, and then the other effects just named, only in the reversed order, until the tube acted like an infinite resistance. Phosphorescence is the correct word, because the light existed for a few seconds after cutting off the current.

55. Phosphorescence of Objects Within the Vacuum Tube.—The construction in Fig. 7, p. [17], shows how a diamond was caused to phosphoresce within a Crookes’ tube, being supported in a convenient manner in the centre of one of the tubes, while electrodes were located near the ends and were formed of disks facing the diamond. Upon connecting the disks to the respective poles of the secondary conductor, and by performing the experiment in a rather dark room, the diamond became brilliantly phosphorescent, radiating light in all directions. He experimented with many substances in this way, but found that the diamond was the best—almost equal to one candle power. In order to exhibit the phosphorescence of glass in a striking manner, he charged three small tubes simultaneously. One was made of uranium glass which radiated a green light. Another was an English glass which appeared blue, and the remaining one was German glass which phosphoresced a bright green. Notice difference with respect to light which does not perceptibly cause phosphorescence of glass. The uranium glass was the most luminous. Luminous paint, as prepared by Becquerel, and later by Balmain, which has the property of storing up light and afterwards radiating it in a dark room for several hours, became more phosphorescent in the Crookes tube than when subject to day-light. Phosphorescence of the mineral phenakite, the chemical name of which is glucinic aluminate, was blue, the emerald, crimson, and spodumene, which is a double silicate, were yellow. The ruby phosphoresced red, whatever its tint by day-light. In one tube he had rubies of all the usual tints by day-light, but they were all of one shade of red by the action of the disruptive discharge in the tube.

56. Darkness and Luminosity in Arms of V Tube. See Fig. 8, p. [17]. It will be noticed that in Fig. 6, p. [17], the tube was straight. Crookes desired to see what effect would take place in a bent tube. He therefore employed a V shaped tube, having electrodes in the ends—one in each arm. Upon causing the electrical discharge to take place through the tube, one arm was luminous and the other was dark. Whatever the E. M. F. was, the appearances remained the same. No luminosity would bend from one arm of the V shaped tube to the other. The cathode arm was always luminous and the anode dark. With a less degree of vacuum, both arms were luminous, according to early experimenters who thus brilliantly lighted tubes of the most fantastic shapes.

57. Cathode Rays Rectilinear. Radiate Normally From the Surface of the Cathode. In his lecture he had, side by side, two bulbs, one, in which the vacuum was of such a degree, that a blue stream of light existed between the negative pole and positive pole, [§ 54], at centre. It is evident that the vacuum in this bulb was not very high. Fig. 9, p. [17], shows a stream extending from the negative to the positive pole, Fig. 10, p. [17], is the same kind of a tube only the vacuum is about 1 to 2 m. In other words, the vacuum in the latter was just so high that a discharge took place, and instead of the luminous effect being like that with a low vacuum, there was a patch of green light directly opposite the concave negative pole. The radiations from this pole were rectilinear, crossing each other at a focus within the bulb and producing upon the glass a phosphorescent spot. It should be remembered that the word radiations is used as a mere matter of convenience. Directly opposite the concave cathode, there was a green patch of light on the inner surface of the glass. It was shown that it made no difference where the anode was. This fact becomes useful in carrying on experiments in connection with Roentgen rays, and it may have a great deal to do with the solution of the theoretical problems in connection with electrical discharges in vacuum tubes. In regard to the three streams shown in Fig. 9, p. [17], it may be stated that only one occurred at a time in the experiment, for, first one anode was connected in circuit, and then the next by itself, and then the third one by itself, while the concave pole was always negative. Each time the anode was changed, the stream changed, and connected that pole which was in circuit, [§ 43], but similar changes made upon the tube with a high vacuum, did not alter the position of the phosphorescent spot. This and other experiments show that the radiations took place perpendicularly from the surface of the cathode.

58. Shadow Cast Within the Discharge Tube. This is illustrated in Fig. 11, p. [17], where there is a negative polar disk at the small end of the egg shaped tube, and a cross near the large end, the same forming the positive pole. The cross is made of aluminum. There was a novel action, however, discovered in addition to the mere casting of a shadow. The glass which had become phosphorescent except within the shadow, became after a while, less phosphorescent. Its property to phosphoresce became less as proved by removing the cross, which was arranged to fall down upon tipping the bulb. Immediately, the part which was within the shadow became brighter than the rest of the glass, thereby reversing the appearances, by making a luminous picture of the cross upon only partially phosphorescent glass. A remarkable feature is that the glass never recovered its first exhibited power of phosphorescence, neither did this power entirely become nothing, however many times the tube was employed. Was the deposit of metal from the cathode the cause?

58a. Mechanical Motion Produced by Radiations from the Negative Pole. It occured to Crookes that the radiations from the cathode might perhaps cause a wheel to turn around. He therefore had a minute wheel made by Mr. Gimingham, like an undershot water wheel, and its axle rested on two rails of glass, so that it might roll along from one end of the tube to the other. The vanes were exactly opposite to the plane surface of the cathode. The molecular stream or radiations, or whatever they may be, possibly vibrations, from the cathode, were so powerful mechanically that the wheel was caused to run up hill, the tube being inclined very slightly. On the principle that action and reaction are equal, he built another device in which the negative electrode was movable, and he observed that when the current was on, the negative electrode moved slightly. Upon these principles he built the well-known Crookes radiometer in which the vanes rotated by reaction of the radiations. The vanes in this form of radiometer were made of aluminum, and a cup of hard steel served as the bearing, Fig. 12, p. [17]. One side of each disk was coated with a thin scale of mica. The aluminum disks formed the cathode, while the anode was located at the top. The operation consisted in connecting the terminals as stated, so that the vanes were the negative poles and it was observed that the little wheel rotated. The vacuum was not as high as that for obtaining phosphorescence. With a low vacuum, an envelope of violet light existed near the surface of the aluminum vanes. Effects were carefully studied by maintaining connection with the pump. At the pressure of .5 mm. there was a dark cylinder opposite the aluminum extending to the glass, and this was the pressure at which the vanes began to rotate. The dark spaces opposite each vane became larger and larger in width, until they appeared to be opposed or resisted by the inner surface of the glass, and then the rotation became very rapid. He modified this experiment by having vanes entirely of mica, and by having the cathode disconnected electrically from the vanes, Fig. 13, p. [17]. A coil of metal near the vanes served as the cathode. The anode was at a distance in the top of the tube as in Fig. 12, p. [17]. During the electrical discharge, the wheels rotated by radiations from the coil which formed the cathode. He made the discovery that when this coil was heated red hot conveniently by a current from a primary battery, the vanes also rotated, showing that there is probably some relation between the radiations from the cathode and heat rays. The fact remains however, that both kinds of rays produced rotation, directly or indirectly.

59. Action of Magnet Upon Cathode Rays.—He had two tubes, one of which is shown in Fig. 14 and the other in Fig. 15, on page [17]. In the former, the vacuum was so low that a violet stream of light existed between the electrodes. In the other, the rays were invisible, but were converted into luminosity by projection at an exceedingly slight angle, upon a phosphorescent screen arranged along the length of the tube and inside thereof. Inasmuch as the whole surface of the cathode in the latter case radiated parallel and invisible rays, he cut off some of them by a mica screen having a hole in the centre and located near the negative pole, so that only a pencil of invisible rays could go through the mica screen and act upon the phosphorescent screen. In both cases, there was visible a straight pencil of light. Now notice the effect which took place upon locating a magnet as indicated in the figures. With the low vacuum, the pencil was bent out of its course but returned again to the line of its original path. [§ 28]. With the high vacuum, the rays were bent but did not return to their original direction nor parallel thereto. In the former case, the magnet acted as upon a very delicate flexible conductor, while in the latter, it acted, as Crookes said, like the earth upon projectiles. He modified the latter experiment in order to determine if the similarity between this phenomenon and gravitation existed in other respects. He anticipated that if the molecular resistance to the rays were increased they would be bent more out of their course like a horizontally projected bullet. He therefore heated the caustic potash sticks slightly, and in view of the liberation of molecules of water within the vacuum tube, the rays, he thought, would be resisted; and such was the case to all appearances, for then the pencil of light was bent out of its course to a greater extent, although the magnetic power remained the same as well as the E. M. F. producing the electric discharge. He therefore established, apparently, the principle that the magnetic actions upon cathode rays vary somewhat in their nature according to the degree of vacuum. In either case, it may be stated incidentally, that when the magnet was moved to and fro, the pencils of light waved back and forth.

In the modified form of construction over that shown in Fig. 15, p. [17], he caused a wheel to rotate that was located in the high vacuum. The vanes of the wheel were so located that the faces of the same were perpendicular to the direction of the pencil of the rays radiating from the cathode. When the magnet deflected the rays, the wheel ceased rotation.

60. Mutual Repulsion of Cathode Rays.—If the little mica screen, as shown in Fig. 16, p. [17] has two holes, and if there are two cathodes instead of one, there will also be two pencils of light. He performed an experiment involving the latter modification, and the result was something that could not have been predicted. The two pencils, as displayed by the long fluorescent screen, repelled each other like molecules similarly electrified. The white pencils, it will be noticed, were repelled from each other and showed their condition when both of the negative poles were in circuit. The black pencils show the location of both of the pencils when only one pole is in circuit at a time, the direction being perpendicular to the plane of the cathode disc ([§ 57]) at end.

61. Heating and Lighting Power of Cathode Rays. Heat of Phosphorescent Spot.—By making the cathode concave as in Fig. 10, p. [17], and so locating it that the focus of the cathode rays falls upon some substance, the latter becomes very hot. In this way Crookes melted wax on the outside of the bulb at the phosphorescent spot. Further than this, the heat was so great that it cracked the glass without at first injuring the vacuum; next the glass at this point softened, and the air, by its pressure, rushed into the bulb, forcing a hole through the soft part. He performed an experiment also which illustrated the intensity of the heat when the rays were brought to a focus. He used an unusually large electrode like a concave mirror, and in the focus, which was near the centre of the bulb, he supported a small piece of iridio-platinum. At first, with a moderately low E. M. F., the metal was made white hot. When a magnet was caused to approach, the rays were drawn to one side, [§ 59], and the little piece of metal cooled. He then put in all the coils of an inductorium, and allowed the metal not only to become white hot, but to become so heated that it melted. How little did Prof. Crookes know about the most important phenomena associated with his experiment. Although he was so exceedingly enthusiastic and ingenious in planning his experiments, and in reasoning, yet it seems almost mysterious that he should have been subjected to what have become known as X-rays, which passed into his body, and would have photographed portions of his skeleton, and which would have performed outside of the tube many of the acts that were noticed within. Seventeen years elapsed between the time of Crookes on the one hand, and Lenard and Roentgen’s discoveries on the other. Dr. Lodge, F.R.S., (The Elect., Lon., Jan. 31, ’96, p. 438,) and Lenard, in his first paper, attributed to Hittorf the discovery of the mere existence of cathode rays, but credited to Crookes the full establishment of their properties, deduction of their principles and formulation of an ingenious theory.

61a As an appropriate conclusion to Crookes’ work, I cannot do better than to let Lord Kelvin repeat what he said in his Pres. Addr., Ro. So., Nov. ’93, see also The Elect., Lon. Feb. 14, ’96, p. 522, showing that a small portion of the credit is due not only to Hittorf, [§ 53], but to Varley. “His short paper of 1871, which, strange to say has lain almost or quite unperceived in the Proceedings during the 22 years since its publication, contains an important first instalment of discovery in a new field, the molecular torrent [§ 53], at centre, from the ‘negative pole,’ the control of its course by a magnet, [§ 59], its pressure against either end of a pivoted vane of mica, [§ 59], at end, and the shadow produced by its interception by a mica screen, [§ 58]. Quite independently of Varley, and not knowing what he had done, Crookes (Roy. Inst. Proc., April 4, ’79, vol. LX, p. 138. Ro. So. Trans., ’74, “On attractions and repulsions resulting from radiation” Part II, ’76, parts III and IV, ’76, part V, ’78, part VI, ’79) was led to the same primary discovery, not by accident and not merely by experimental skill and acuteness of observation.” * * * * “He brought all his work more and more into touch with the kinetic theory of gases; so much so, that when he discovered the molecular torrent he immediately gave it its true explanation—molecules of residual air, or gas or vapor projected at great velocities (probably, I believe not greater in any case than 2 or 3 kilometers per second, [§ 61b]), by electric repulsion from the negative electrode. This explanation has been repeatedly and strenuously attacked by many other able investigators, but Crookes has defended (Presidential address to the Inst. Elect. Eng., 1891.) it, and thoroughly established it by what I believe is irrefragable evidence of experiment. Skillful investigations perseveringly continued brought out more and more wonderful and valuable results; the non-importance of the position of the positive electrode, [§ 57], near end, the projection of the torrent perpendicularly from the surface of the negative electrode, [§ 57], at end; its convergence into a focus and divergence thenceforward when the surface is slightly concave, [§ 47], near beginning; the slight but perceptible repulsion, [§ 60], between two parallel torrents due, according to Crookes, to negative electrifications of their constituent molecules; the change of the direction of the molecular torrent by a neighboring magnet, [§ 59]. the tremendous heating effect of the torrent from a concave electrode when glass, metal or any ponderable substance is placed in the focus, [§ 61]. the phosphorescence procured on a plate coated with sensitive paint by a molecular torrent skirting along it, Fig. 15, p. [17]; the brilliant colors—turquoise blue, emerald, orange, ruby-red—with which grey, colorless objects, and clear, colorless crystals glow on their struck faces when lying separately or piled up in a heap in the course of a molecular torrent, [§ 55]. “electrical evaporation” of negatively electrified liquids and solids, [§ 59]. (Ro. So. Proc., June 11, ’91.) the seemingly red-hot glow, but with no heat conducted inwards from the surface, of cool solid silver kept negatively electrified in a vacuum 1/1,000,000 of an atmosphere, and thereby caused to rapidly evaporate, [§ 40] and [139a]. This last named result is almost more surprising than the phosphorescent glow excited by molecular impacts on bodies not rendered perceptibly phosphorescent by light, [§ 55], at centre. Both phenomena will usually be found very telling in respect to the molecular constitution of matter and origination of thermal radiation, whether visible as light or not. In the whole train of Crookes investigations on the radiometer, the viscosity of gases at high exhaustion, and the electro-phenomena of high vacuums, ether seems to have nothing to do except the humble function of showing to our eye something of what the molecules and atoms are doing. The same confession of ignorance must be made with reference to the subject dealt with in the important researches of Schuster and J. J. Thomson on the passage of electricity through gases. Even in Thomson’s beautiful experiments, showing currents produced by circuital electro-magnetic induction in complete poleless circuits, the presence of molecules of residual gas or vapor seems to be the essential. It seems certainly true that without the molecules, electricity has no meaning. But in obedience to logic, I must withdraw one expression I have used. We must not imagine the “presence of molecules is the essential.” It is certainly an essential. Ether is certainly also an essential, and certainly has more to do than merely to telegraph to our eyes to tell us what the molecules and atoms are about. If the first step towards understanding the relations between ether and ponderable matter is to be made it seems to me that the most hopeful foundation for it is knowledge derived from experiment on electricity in high vacuum; and if, as I believe is true there is good reason for hoping to see this step made, we owe a debt of gratitude to the able and persevering workers of the last 40 years who have given us the knowledge we have; and we may hope for more and more from some of themselves and from others encouraged by the fruitfulness of their labors to persevere in the work.”

61b. Thomson’s Experiment. Velocity of Cathode Rays. The Elect., Lon., Oct. 5, ’94, p. 762; Phil. Mag., ’94.—The object of the experiment of J. J. Thomson was to determine whether the velocity approached that of light or that of molecules. The apparatus he employed involved the rotating mirror, which was fully described in Proc. Royal So., ’90, slightly modified. The rays were caused to produce phosphorescence, while the mirror was so adjusted that when at rest, the two images on the phosphorescent strips appeared in the same rectilinear line. Many other elements comprised the apparatus. All the steps were performed carefully and according to the best methods, but the results are those which in this experiment are of particular interest, for by knowing the velocity of the rays, their nature is better appreciated and that of the X-rays can be better deduced. The velocity bore a close relation to that of the mean square of the molecules of gases at temperatures zero ° C. or in the case of hydrogen, 1.8 × 10⁵ cm. per second. As compared with such a velocity, that of the cathode rays was found to be in the neighborhood of 100 times as great, and this agrees very nearly with the velocity of a negatively electrified atom of hydrogen acquired under the influence of the potential fall, which occurred at the cathode. In further evidence of the verity of this statement, he made a rough calculation upon the curve or displacement produced by a magnet upon the rays. [§ 59]. He stated: “The action of a magnetic force in deflecting the rays shows, assuming that the deflection is due to the action of a magnet on a moving electrified body, that the velocity of the atom must be at least of the order we have found.”

Fig. 1.

61b. Perrin’s Experiment. Cathode Rays Charged With Negative Electricity. Corresponding Positive Charges Propagated in the Reverse Direction and Precipitated upon the Cathode. Comptes Rendus, CXXI., No. 20, p. 1130; The Elect., Lon., Feb. 14, ’96, p. 523.—Jean Perrin’s object was to discover whether or not internal “Cathode rays were charged with negative electricity.” That they were had often been assumed by others, namely, Prof. J. J. Thomson, who considered cathode rays as due to negatively charged matter moving at high speed. [§ 61b]. Again, Prof. Crookes, principally, and others, showed that they were possessed of mechanical properties and that they were deflected by a magnet. [§ 59]. Perrin called attention to the above investigations and also alluded to the theoretical considerations of Goldstein, Hertz and Lenard, who favored the analogy of cathode rays to light—whose phenomena are well answered by the accepted theory concerning assumed etherial vibrations, which, in both cases, have rectilinear propagation, [§ 57], excite phosphorescence, [§ 54] and [55], and produce chemical action upon photographic plates. Great ingenuity was displayed, as might be expected, in the manner in which Jean Perrin proved the proposition named in the title of this section, at the Laboratory of the École Normale and also in M. Pallet’s Laboratory. First, therefore, let the elements of the discharge tube be thoroughly understood. As usual, the disk N is the cathode, referring to accompanying Fig. [1]. A, B, C, D, is a metal cylinder having a small opening at the right hand end toward the cathode. Concentrically, is a similar cylinder, acting as an electrical screen and having a like opening similarly located as indicated. It corresponds to and plays the part of the Faraday cylinder, being connected to earth. The principle involved in this apparatus was based upon the laws of influence, which permitted him to ascertain the introduction of electric charges within a conducting envelope, and to measure such charges. During the discharge, the cathode rays were propagated from the cathode to and within the cylinder A, B, C, D, which immediately and invariably became charged with negative electricity. To prove that the charge was due to the cathode rays, he deflected them away from the opening in the protecting cylinder E, F, G, H. The cylinder was not under these circumstances charged, the rays being outside. He went further and made some quantitative analysis in a rough way to begin with. He related: “I may give an idea of the amount of the charges obtained when I state that with one of my tubes, at a pressure of .001 m. of mercury, and for a single interruption of the primary coil, the cylinder A, B, C, D, received sufficient electricity to bring a capacity of 600 C. G. S. units to a potential of 300 volts.” Upon the principle of the conservation of energy, he was induced, he said, to search for corresponding positive charges. “I believe I have found them in the very region where the cathode rays are generated, and that they travel in the reverse direction and precipitate themselves on to the cathode.” He verified this corollary by means of a modified feature of the apparatus shown in Fig. [2]. The construction was the same except that there was a diaphragm having a perforation β´ within the protecting cylinder and opposite the smaller cylinder exactly as indicated, so that the positive electricity which had entered through β could only act on the cylinder A, B, C, D, by traversing also the hole β´. “When N was the cathode, the rays emitted traversed the two apertures at β and β´ without any difficulty, and caused the gold leaves of the electroscope to diverge widely. But when the protecting cylinder was the cathode, the positive flux, which, as was shown by a previous experiment, enters by the aperture β, did not succeed in separating the gold leaves, except at very low pressures. If we substitute an electrometer for the electroscope we shall see that the action of the positive flux is real, but that it is very small and increases as the pressure decreases.”

Fig. 2.

He inferred that: “These results, taken as a whole, do not appear to be easily reconcilable with the theory that the cathode rays are ultra-violet light. On the contrary, they support the theory that attributes these rays to radiant matter, [§ 54], near centre, a theory, which may at present, it seems to me, be enunciated as follows: In the vicinity of the cathode the electric field is sufficiently strong to tear asunder into ions some of the molecules of the residual gas. The negative ions start off toward the region where the potential increases, acquire a considerable velocity, and form cathode rays; their electric charge, and consequently their mass (at the rate of one gramme equivalent per 100,000 coulombs) is easily measured. The positive ions move in the reverse direction; they form a diffused tuft, susceptible to magnetism, but are not a regular radiation.”

61c. Zeugen. Comptes Rendus, Jan. 27, 1896.—In a note regarding the experiments of Roentgen, called attention to his own communications to the Academie des Sciences in February and August 1886, describing his photographs of Mt. Blanc taken in the night by the invisible ultra-violet rays. This note is entered as many newspapers reported the photograph to be due to cathode rays, imagine the intense phosphorescence upon a screen at the top of the mountain, if such were the case.

62. Goldstein’s Experiment. Phosphorescence of Particular Chemicals by Cathode Rays. Nature, Lon. Feb. 21, ’95, p. 406. Weid. Ann., No. II, ’95.—Lithium chloride when acted upon by cathode rays, phosphoresced to a dark violet color or heliotrope, which it retained for some time in a sealed tube. Chlorides generally and other haloid salts of potassium and sodium showed similar effects. The colors were superficial and could be driven away rapidly either by heating or the action of moisture.

63. Kirn’s Experiment. Spectrum of Post Phosphorescence of Discharge Tubes. Wied. Ann., May, ’94. Nature, Lon. June 7, ’94, p. 131.—Carl Kirn compared the spectra of the phosphorescence of a vacuum bulb, during and immediately after the discharge. The details are as follows: The spectrum of the after-glow, [§ 54], at end and 22, was found to be continuous. In this connection, see a plate showing different kinds of spectra, for example, Ganot’s Physics, frontispiece. The spectrum shortened from both directions to a band between the wave lengths of 555 and 495µµ. The spectrum then continued to grow shorter and shorter until it disappeared at the line E, which is the position of the greatest luminosity of the solar spectrum. For experiments on spectrum, see Fraunhofer in Gilbert’s Ann., LVI. During the discharge, the spectroscope showed a line spectrum corresponding very closely to those of carbonic acid gas and nitrogen. Some authorities had suggested that perhaps the after phosphorescence and the beginning of the incandescence of a solid body, were the same kind of light, but this experiment shows that such is not the case, unless some relation exists on the ground that the two phenomena are exactly opposite to each other, and it confirms similar results obtained by Morrin and Riess. The result indicates that the nature of the phenomenon is not identical in all respects with light produced at a high temperature.

63a. De Metz’s Experiment. Chemical Action in the Interior of the Discharge Tube. Internal Cathode Rays. L’Ind. Eler., May 10, ’96, and Comptes Rendus, about April, ’96. Translated by Louis M. Pignolet. He used a cylindrical discharge tube divided into two halves which fitted together by an air-tight ground joint. In one-half were the anode and the cathode; in the other half was the holder containing the sensitive paper or films. The holder was exposed to the direct action of the cathode rays and was closed by a cover of cardboard or sheet aluminum. The objects to be photographed were placed between the cover and the sensitive film or paper. The tube was connected to a Sprengel pump which maintained its vacuum during the experiments. In this way, twelve photographs were taken from which it appeared that cathode rays, like X-rays, penetrate cardboard and aluminum, but are stopped by copper (1.26 mm.) and platinum (0.32 mm.). Poincaré, in a note in the same publications as the foregoing, criticised the results of the experiments of De Metz, claiming they did not prove irrefutably that cathode rays possessed the essential properties of X-rays, for the cathode rays in impinging on the cover of the holder would generate X-rays, [§ 91], which would give the results obtained. Poincaré did not deny the fact.

63b. Hertz’s Experiment. The Passage of Cathode Rays Through Thin Metal Plates Within the Discharge Tube. Diffusion. Wied. Ann., N. F. 45; 28, 1892. Contributed by request, by Mr. N. D. C. Hodges of the Hodges Scientific News Agency, N.Y. Found in records at Astor Library.—A piece of uranium glass was covered partly on one side (which he calls the front side) with gold-leaf, and on the gold leaf were attached several pieces of mica. This front side was then exposed to cathode rays. So long as the exhaustion had not proceeded far, and the cathode rays filled the whole tube with a blue cone of light, only the portion of the uranium glass outside the gold-leaf screen showed any phosphorescence. But as soon as the exhaustion had progressed far enough, and the light began to disappear, the genuine cathode rays struck the covered glass, and the phosphorescence manifested itself behind the gold-leaf. When the cathode rays were fully developed, the gold-leaf hardly had any effect, while the mica cast deep black shadows. The same experiment was tried with silver-leaf, aluminum and alloys of tin, zinc and copper. Aluminum showed the best results; sheets which allowed no light to pass, allowing the cathode rays free passage. The rays after their passage through the metal screens did not continue their straight course, but seemed to be diffused much as light is diffused by passing through a cloudy medium. In this connection reference is made to the work of Goldstein, who had noticed also the reflection of “electric” rays. Wied. Ann., N. F. 15; 246, 1882. In 1893, Goldstein published further accounts concerning actions in discharge tube. Wied. Ann., vol. 48, p. 785.

Diagram of Lenard’s Apparatus. pp. [53] to [69].

CHAPTER VI

65. Lenard’s Experiments. Cathode Rays Outside of the Discharge Tube. Wied. Ann., Jan., ’94, Vol. LVI., p. 225; The Elect., Lon., Mar. 23 and 30, ’94, Apr. 6, ’94; and Elect. Rev., Lon., Jan. 24, ’96, p. 99.—Of more importance in connection with X-rays is the consideration of Lenard’s experiments than any others. The reader must bear in mind that his exhaustive investigations resulted from his discovery (founded upon a hint from Hertz) that the cathode rays might be transmitted to the outside of the generating discharge tube. His interest, therefore, in the discovery was so great that his researches extended to the minutest details. Passing from these introductory remarks, the characteristics of the tube that he employed will be explained first. Reference may now be made to the accompanying Fig. [A]. He employed several different kinds of tubes, but finally settled upon one of which the essential elements are shown in the said figures. It was permanently connected to the pump, [§ 53], so that the pressure within could be varied. Opposite the cathode, which consisted of a thin disk of aluminum, the end of the tube was provided with a thick metal cap, having a perforation, which in turn was closed by a thin aluminum sheet secured by marine glue in an air-tight manner, and called a window. The anode was a heavy brass cylinder, shown in section, within the discharge tube and surrounding the leading-in wire of the cathode. The anode and the aluminum window were connected to each other, electrically, and to earth, as well as two a secondary terminal of an induction coil, whose electrodes were in shunt to those of the discharge tube, in order that the operator might adjust the sparking distance which rapidly increased with the exhaustion. The induction coil had a mercury interrupter.

65. Properties of Cathode Rays in Open Air.—In all directions around the window upon the outside and in the open air, a faint bluish glow ([§ 11] and [140]) extended and vanished at a distance of 5 cm., as indicated by dotted lines in Fig. [B] at beginning of this chapter. The degree of luminosity may be judged by saying that it was not sufficient to admit of investigation by the ordinary pocket spectroscope. A new window was void of luminosity; but with use, bluish gray and green and yellow spots occurred thereon.

66. Phosphorescence by Cathode Rays.—Substances which generally phosphoresced by light and cathode rays in the generating bulb, [§ 55], also phosphoresced under the influence of the rays in open air, excepting eosin, gelatin, both phosphorescent in light, were not so in cathode rays; so also with solutions of fluorescein, magdala red, sulphate of quinine and chlorophyll. Phosphorescence was less if the rays first passed through a tube of glass or tinfoil lengthwise. The phosphorescent light of the phosphides of the alkaline group, uranium glass, calcspar and some other substances, was so great that the luminosity of the air was invisible by contrast. The maximum distance at which phosphorescense was discernable in open air was about 8 cm. The best phosphorescent screen consisted of paper saturated with pentadecylparatatolylketone. In order to prepare it, he laid a sheet of paper upon glass and applied the fused chemical with a brush. As to the color of the phosphorescence and fluorescence of different substances, and as to the degree of luminosity outside of the vacuum tube, they were about the same as reported by Crookes when located within the discharge tube. §55. Baric and potassic and other double cyanides of platinum, common flint, glass, chalk and asaron all exhibited the same property as when exposed to ultra-violet light, that is, fluoresced or phosphoresced. Sulphide of quinine in the solid state fluoresced, but not in solution. Petroleum spread on a piece of wood fluoresced, and also fluorescent-hydrocarbons generally.

66a. The cathode rays were not easily transmitted by tinfoil or glass, because the degree of phosphorescence on the screen was greatly reduced by interposing such sheets. The phosphorescense ceased also by deflecting internal cathode rays from the window by a magnet. For full treatment of the phenomena of phosphorescence, see Stokes’ experiments, described in Phil. Trans., 1852, Art. “Change of Refrangibility of Light.” In brief, Stokes’ theory assumes that such substances have the power of reducing the refrangibility. Example: Ultra-violet light, highly refractive, is changed to yellowish green, less refrangible, by reflection from uranium glass.

67. The Aluminum Window, a Diffuser of Cathode Rays. [§ 63b]. The conclusion arrived at by mounting the phosphorescent screen in different positions and at different angles as well as by observance of the gaseous luminosity, was that the aluminum window scattered the rectilinear parallel cathode rays in all directions, [§ 57].

68. Transmission of External Cathode Rays Through Metals.—The phosphorescence was not diminished apparently by an intervening gold-leaf or silver or aluminum foil, while it was extinguished by quartz .5 mm. thick which also cut off the atmospheric glow beyond itself. The leaves and foil did not so act. The difference of thickness should be borne in mind, as metal, as thick as the quartz did not transmit. As to other substances, tissue paper cast a slight shadow, which was darker with an additional sheet; but the shadow was independent of color and blackness, [§ 154]. Ordinary writing paper was roughly, proportionally opaque, while the shadow was black with cardboard .3 mm. thick. Glass films as made by blowing glass, cast faint shadows when .01 mm. thick. He proved that there was little difference as to the transmitting power of conductors and dielectrics when thin. Mica and collodion sheets .01 mm. thick cast scarcely any shadow. The reader may bear in mind the striking differences between these properties of cathode rays, and X-rays, [§ 135], it being assumed always that the generating devices are the same; for example, water permitted the cathode rays (were these simply feeble X-rays?) to be transmitted only when in very thin layers. Even soap water films which were only .0012 mm. thick cast shadows, although very faintly. The shadows of drops of water were black, while water several feet thick has been traversed by X-rays from a small set of apparatus. By careful measurements he found that the law of transmission must be different from that of light, for in the latter, many substances are opaque although exceedingly thin, while with cathode rays, the same will traverse all films. Goldstein and Crookes reported that thin mica, glass and collodion films made very dark shadows, [§ 58], within the discharge tube, whereas Lenard found that outside of the vacuum tube, in open air, the transparency was greater than according to the earlier experimenters, but he acknowledged that Crookes and Goldstein were inconvenienced and limited in the number of observations because it is so difficult to carry on such experiments within an hermetically sealed tube. Again, he acknowledged that perhaps the cathode rays of those experimenters were of a different kind. The construction shown in the above figures was modified by using a very thin glass window instead of aluminum, and the results were the same allowing for the different opacity, to ordinary light, of aluminum and glass.

The cathode rays acted upon the sense of smell and taste as the nose and mouth could detect ozone, [§ 84], at end.

69. Propagation. Turbidity of Air. Upon studying the shadows on the phosphorescent screen, it was noticed that the rays were bent around the edges of the object. Again, when the object had a slit, diffusion could be noticed by the shape (as in Crookes Ex., Fig. 15, p. [17],) of the luminous portion of the phosphorescent screen. In Fig. [B], at beginning of this chapter, the spatter work represents the shape of the luminous portion, the darker part representing the most luminous surface of the screen, the latter being held at right angles to the thick plate, having the slit and opposite the aluminum window. By varying these experiments, especially by changing the angle of the screen he found that not the all rays were diffused, but as in the passage of light through milk, some were transmitted in rectilinear lines.

70. Photographic Action.—He performed with sensitive silver compound papers, an experiment somewhat similar to those with phosphorescent bodies and also others. Behind a rather thick opaque plate the chemical film was not acted upon, but the rate of blackening near the aluminum window without obstruction of intermediate bodies was about the same as that with befogged sunlight. The former, moreover, was acted upon at a much greater distance than that at which phosphorescence was exhibited and beyond the atmospheric luminosity. By means of shadow pictures or sciagraphs, he compared the shadows produced by the external cathode rays with those which would have been obtained by light. Referring to Fig. [C], beginning of this chapter, the sensitive plate was half covered with a plate of quartz, Q, and half with a plate of aluminum, A´ overlapping the quartz. With light, the shadows would have appeared as in said figure, that is, one-half black as produced by aluminum, a quarter rather light as produced by quartz, and the other quarter bright, or a similar arrangement, according to whether the negative or the positive photograph is considered; but with the cathode rays, the appearance of the developed plate was as in Fig. [D]., beginning of this chapter. The quartz cast the black shadow, while the aluminum, the lighter one. Furthermore, the luminosity of the air produced a variable light on the other quarters. A similar appearance was produced by casting shadows of such plates upon the phosphorescent screen; but, of course, the picture was not a permanent one. The photographic plate served to accumulate the power, for the cardboard which cast a faint shadow upon the phosphorescent screen, showed a black shadow upon the photographic paper by sufficiently long exposure. At the same time, strips of thin metal were placed side by side between the chemical paper and the cardboard, and they showed different degrees of shading. The cardboard was quite thick, being .3 mm. Prof. Slaby (see Elect. Rev., Lon., Feb. 7, ’96), after Röntgen’s discovery, produced sciagraphs of the bones of the hand at the window of the Lenard tube. Lenard doubted whether the cathode rays produced direct chemical action. Iodine paper became bluish, but he could not obtain other chemical effects usually produced by light, and other agencies, for example, oxygen and hydrogen mixed together in the proportion to form water, and which were in their nascent state, and which were located in a soap-bubble, did not explode or ignite. No effect was produced upon carbon bi-sulphide nor hydrogen-sulphide, although the exposure was very long. Ammonia was not formed when the rays acted upon a mixture of three parts hydrogen and one part nitrogen, as to volume. He thought that he noticed a small expansion of air, hydrogen and carbonic acid separately located in a vessel having a capillary tube and water to indicate the expansion. He attributed the slight expansion to an indirect action, although very slight, caused by heat produced by the cathode rays, [§ 27], and yet neither the thermopile nor the thermometer showed any calorific effects although the thermopile responded to the flame of a candle 50 cm. distant.

71. Cathode Rays and Electric Forces Distinguished. The earth connection heretofore mentioned with the aluminum window was for the purpose of dispensing with sparking, but even then the approach of another conductor connected to earth would cause some sparking. Sparks could be drawn when the cathode rays were deflected from the aluminum window by a magnet. Fig. [E], at beginning of chapter. He argued that the rays and the electric forces of the spark are non-identical. He was not satisfied with this as an absolute proof, and he instituted others. He enclosed the whole generator in a large metal box. In the observation space, that is, around and near the window, he located another box, having an aluminum front facing the window. See Fig. [E], at beginning of chapter. It was within this second box that he took the sciagraph shown in Fig. [D], at beginning of chapter. It is important to notice that sparks could not be drawn at points within the said second box, shown at the left, even by a metallic point shown projecting thereinto. No spark occurred whatever, not even from the aluminum front. Sparking occurred when the pointed wire was extended to a considerable distance outside of the back of the small box, but it was remarked that the electric force did not enter through the front wall but was introduced “from behind into the box, by the insulation of the wire.” No one can, therefore, enter the objection that the cathode rays experimented with, were generated from the aluminum window as a cathode. They came from the cathode referred to entirely within the vacuum tube. Prof. J. J. Thomson, F. R. S., had at an early date conjectured that cathode rays did not pass through thin films of metal, but that these films acted as intermediate cathodes themselves. See his book on “Recent Researches,” p. 26, also The Elect., Lon. March 23, ’94, p. 573, in an article by Prof. Fitzgerald, who names that citation.

72. Cathode Rays Propagated, but not Generated in a High Vacuum.—The proposition was proved by having two tubes, one called the generating tube and one the observation tube, the former being like that shown in Fig. [A], at beginning of chapter, which is partly repeated in Fig. [F], at beginning of chapter, combined with the observation tube, which contains the two electrodes for casual use; but the one on the right is a disk extending nearly throughout the cross sectional area, and having a small central opening. Although both tubes were connected to the air pump, yet, by means of stop-cocks, the vacuum in one tube could be maintained at a maximum degree for hours, while the other was at a minimum. The first experiment was performed with a vacuum, about as high as that employed in Crookes’ phosphorescent experiments, [§ 53]. There was a patch of green light, [§ 57], at the extreme left end of the observation tube and the glass was green at the right, [§ 54], and a little to the left of the perforated disk electrode a. The other electrode of this tube was located at the upper left and lettered k.

72a. The magnet deflected the rays in the observing tube as indicated by the partial extinction of the phosphorescent patch. He noticed that with the rarefied atmosphere the amount of turbidity was enormously reduced, or in other words, that the rays were propagated more nearly in rectilinear lines. All the experiments on the cathode rays, in this observing tube, were of about the same nature as those which could be produced in the discharge tube.

From Sciagraph of Cat’s Leg, by Prof. William F. Magie.
Copyright, 1896, by William Beverly Harison, pub. of X-ray pictures, 59 Fifth Ave., New York City.

72b. The principal experiment consisted in exhausting the observing tube to such a degree that cathode rays could not be generated therein. The vacuum was so perfect that when used as a discharge tube all phosphorescence gradually died away until it disappeared, and no current passed ([§ 25]) except on the outside surface of the glass. The coil was so large, electrically, that the length of the spark between spheres was 15 cm. Upon charging the right hand tube and generating cathode rays, it was determined by means of magnetic deflection, phosphorescence and other effects, that the cathode rays traversed the highest possible vacuum ([§ 19], near end, where energy must have passed through the high vacuum to produce luminosity in the inner bulb). The external and internal rays were certainly different forms of energy. Inasmuch as he noticed that rarefied air was less turbid and less absorptive than air at ordinary pressures, it occurred to him to make a very long tube, namely, 1 m, or a little over 3 feet. He employed very severe steps for obtaining an exceedingly high vacuum, the operation occupying several days. The pump used was a Toepler-Hagen, while a Geissler pump was employed separately for the discharge tube. The pencil of cathode rays traversed the whole length of the long tube. See a portion of the apparatus in Fig. [G], at beginning of this chapter. One disk was of metal and perforated with a pin hole and the other was a phosphorescent screen, so that when the cathode pencil passed through the hole in the plate a patch was seen upon the phosphorescent screen. The phosphorescent spot was always, no matter what the relative distances of the disks were from each other, and from the end of the tube, substantially the same as it would have been by calculation assuming that there was no turbidity effect. The patches, in each instance, were a little smaller in diameter than the calculated ones. For example with one measurement, at certain distances, the actual diameter of the patch was 2.5 mm., while the calculated diameter was 2.9 mm. In his experiments with light under the same conditions, the luminous spots were also a little smaller than the calculated or geometrical. The disks had iron shoes and were moved to different positions by a magnet. He concluded, therefore, that in what may be called a perfect vacuum, light and cathode rays have a common medium of propagation, namely, the assumed ether. Prof. Fitzgerald, in The Elect., Lon. Mar. 23, ’94, does not agree broadly with him in this; neither does he contradict him. He argues rather on the point that the cathode rays and light rays are not identical, but Lenard does not affirm this, because the magnet will attract the former and not the other. Prof. Fitzgerald admits this and calls to mind that even in a vacuum, as obtained by Lenard, there were still ten thousand million molecules per cu. mm. and therefore he thinks it is better to look to matter rather than ether as the medium of propagation of cathode rays. [§ 61b]. On the other hand, Lenard agrees with certain other predecessors, Wiedemann, Hertz and Goldstein, in favor of cathode rays being etheric phenomena. See Wied. Ann., IX., p. 159, ’80; X., p. 251, ’80, XII., p. 264, ’81; XIX., p. 816, ’83; XX., p. 781, ’83. The vacuum with which Lenard operated, was .00002 mm. pressure, obtained by cooling down the mercury to minus 21° C. This vacuum was so high that all attempts to prove the presence of matter failed. Neither did the exceedingly high vacuum deaden the cathode rays. On the other hand, as noted, they were assisted rather than hindered. [§ 135].

73. Cathode Rays. Phenomena in Different Gases.—The apparatus consisted of an observing tube having a tubular gas inlet and outlet both in one end and arranged in line with the cathode of the discharge tube. See construction in Fig. [H], at beginning of this chapter, the tube being about 40 cm. long and 3 cm. in diameter. He was very careful in every case to chemically purify and dry the particular gas. He omitted the perforated disk and provided an opaque strip of the phosphorescent screen on the side toward the window and made his observations from the other side, the object of the experiment being particularly to test the transmission of cathode rays in different gases. With any particular gas, he moved the phosphorescent screen along by means of a magnet until the shadow on the screen became invisible. It is evident that the distances of the screen from the window for different gases would indicate the relative transmitting powers. He also modified the experiment by varying the density of the gases, hydrogen being taken as 1 as usual, nitrogen 14, and so on. The transmitting power of hydrogen was nearly five times as great as that of nitrogen, air, oxygen and carbonic acid gas, which did not much differ. [§ 10] and [18]. Sulphurous acid was a very weak transmitter. All the gases became luminous near the window as in air. [§ 65]. The colors were all about the same as far as distinguishable, [§ 11], which was difficult in view of the brightness of the phosphorescence on the glass. It was a universal rule, that when the density decreased, the transmitting power increased. In high vacua, in all gases, the rays went through the space in rectilinear lines in all directions from the window, and generally it made no difference what gas was employed provided the vacuum was as high as hundredths of a millimetre. At this pressure all gases acted the same. To be sure, the phosphorescence did not occur at this high vacuum at a great distance as might be expected, but it should be remembered that the intensity of the rays varied as the square of the distance, and, therefore, at very great distances, the action was very weak.

74. Cause of Luminosity of Gas Outside the Discharge Tube.—At ordinary pressures, in the cases of hydrogen and air, as has been noted, the gas became luminous in the observing tube, the effect being, of course, the same as entering open air, represented in Fig. [A], beginning of this chapter. In order to determine the luminosity at less pressures, the gas, of whichever kind, was enclosed in a rather long observing tube and only at rather high vacua did the bluish and sometimes reddish gaseous luminosity disappear. Upon grasping the tube with the hand or approaching any conductor connected to earth, of large capacity, the column stopped at that point so that the remainder of the tube, beyond the hand, measured from the discharge, was dark. The phosphorescence on the glass wall of the tube produced by the cathode rays was not influenced in any way by outside conductors, such as the hand. Cathode rays themselves were not stopped apparently by the hand, because the phosphorescent screen and glass, located beyond the hand, became luminous. He concluded, therefore, that the glowing of the gas had no close connection with the cathode rays. He proved this also by deflecting the cathode rays in the discharge tube from a certain space, and yet the gaseous luminosity remained. As an exception, the cathode rays sometimes appeared to be closely associated with the light column. He attributed the luminosity of the gas in general, at low pressures, not to the cathode rays, but directly to the electric current or some kind of electric force, [§ 11] and [14], which, as already remarked, permitted sparks to be drawn from the aluminum window and surrounding points.

The negative glow light in Geissler tubes, [§ 30], is also to be regarded as gas illuminated by cathode rays. (Compare Hertz, Wied. Ann., XIX., p. 807, ’83.) Between that phenomenon and the glow observed here and attributed to irradiation, there exists a correspondence, inasmuch as in both cases the light disappears at high exhaustions, [§ 53], appears fainter and larger when the pressure increases, [§ 54], and then becomes brighter and smaller, [§ 54]. But, whereas, the glow in the Geissler tube has become very bright and small at 0.5 mm. pressure, the gas in our experiment remains much darker up to 760 mm. pressure, and yet the illuminated spot is much larger. This difference cannot, therefore, be attributed to an inferior intensity of the rays here used. But it will be explained, [§ 76], as soon as we can show that at higher pressures cathode rays of a different kind are produced, which are much more strongly absorbed by gases than the rays investigated hitherto and produced at very low pressures.

Use of Stops in Sciagraphy. (Perch.) [§ 107]., p. [101].
By Leeds and Stokes.

Fig. I, p. [52], illustrates the apparatus by which he studied the rectilinear propagation and whereby he found that it was rectilinear only in a very high vacuum. In the figure, the gas is at ordinary pressure, and it will be noticed that the turbidity of the same is indicated by the curved lines while the dotted lines show the volume that would be occupied by light or other rectilinear rays, unaccompanied by any kind of diffusion. In the observing tube, there was a disc having a central hole at a. Beyond this disc, measured from the aluminum window, was a fluorescent screen which, as well as the perforated disc, could be moved to different distances by means of a magnet acting on a little iron base. It is evident that upon moving the fluorescent screen to different distances, the diameter of the luminous patch would be a measure of the amount of turbidity. The curved lines intersecting the peripheries of the luminous spots indicate, therefore, the field of the cathode rays, so that said field would appear like a kind of curved cone if the same were visible. Although hydrogen is the least turbid gas, yet the phosphorescent patches were all larger except with a high vacuum than they could have been with rectilinear propagation. An additional characteristic of the phosphorescent spot, was its being made up of a central bright spot and a halo less luminous, appearing like some of the pictures of a nebula, see Fig. I´, p. [52], the darker or centre indicating the brighter portion. In a perfect vacuum the halo did not exist. He performed a similar experiment with ordinary light. No halo occurred on a paper screen which was used instead of the phosphorescent screen, but upon introducing a glass trough of dilute milk between the window and the perforated disc, or between the disc and the paper screen, nuclei and halos were obtained, illustrating a case of the effect of a turbid fluid upon light, and assisting in proving that gases act as a turbid medium to cathode rays as milk and similar substances do to light; also in other gases than hydrogen, and by the use of cathode rays, nuclei and halos were not obtained at high exhaustion, all the gases becoming limpid. Taking into account pressure and density, all gases behaved the same as to the power of transmission when they were of the same density, without any regard whatever to their chemical nature. Density alone determined the matter, according to Lenard.

75. Cathode Rays of Different Kinds are Variably Diffused.—He discovered the remarkable property, contrary to his expectation, that if the rays are generated at high pressures, they are capable of more diffusion than when generated at lower pressures. This can be easily proved by any one, for it will be noticed that upon increasing the pressure in the discharge tubes the spots on the phosphorescent screen will not only grow darker but larger and more indefinite as to the nucleus and halo. He called attention to the agreement with Hertz, who also found that there were two different kinds of rays, see Wied. Ann., XIX, p. 816, ’83, also see Hertz’s experiment. Lenard also pointed out the analogue in respect to light, which, when of short wave length, is more diffused in certain turbid media than that of greater wave length. Although Lenard held that his experiment proved that cathode rays were phenomena in some way connected with the ether, yet he pointed out an important difference in connection with the property of deflection of the rays by the molecules even of elementary gases like hydrogen, producing diffusion of the rays, which accordingly may be considered as behaving like light in passing through, not gases, but vapors, liquids and dust. In the case of the cathode rays the molecules of a gas acted as a turbid medium, but in the case of light, turbidity is only exhibited by vapors or certain liquids, as so eloquently explained by Tyndall, in “Fragments of Science,” 1871, where it is shown that aggregation of molecules, like vapors or dust in the presence of light, make themselves known by color and diffusion, whereas the substances in a molecular or atomic state do not serve to show the presence of rays of light.

76. Law of Propagation.—Lenard recognized continually that there were two kinds of cathode rays. One of them may have been X-rays without his knowing it. In the latter part of ’95, he made some experiments especially of a quantitative nature as to the principle of absorption of the rays by gases. By mathematical analysis, based upon experiments, he arrived at the principle that the absorptivity of a gas is proportional to its pressure, or what is the same thing, to its density, or as to another way of stating the law, “the same mass of gas absorbs at all pressures the same quantity of cathode rays.” See Elect. Rev., Lon., as cited, p. 100.

77. Charged Bodies Discharged by Cathode Rays.—An insulated metallic plate was charged first with positive electricity and in another experiment with negative electricity. In each instance, the plate was discharged rapidly by the cathode rays as indicated by the electroscope, and the same held true when a wire cage in contact with the aluminum window, surrounded the electroscope and the metallic plate. The effect was stopped by cutting off the cathode rays by quartz .5 mm. thick. The discharge took place, however, through aluminum foil. A magnet was made to deflect the internal cathode rays, whereupon the discharge did not take place, all showing that the discharge of the insulated plate was directly due to those rays. A remarkable occurrence was the accomplishment of the discharge at a much greater distance than that at which phosphorescence was exhibited. See also Roentgen’s experiment—who suggested that Lenard had to do with X-rays in this experiment, but thought they were cathode rays. The maximum distance for the discharge was about 30 cm. measured normally to the aluminum window. He caused a discharge of a plate also in rarefied air. He admitted that the experiments were not carried far enough to know whether the effect was due to the action of the cathode rays upon the surface of the window, or upon the surrounding air, or upon the plate. The author could not find in Lenard’s paper any positive or negative proof that he had actually deflected the external cathode rays by a magnet while passing through air or gas at ordinary pressure. He had deflected them while passing through a very high vacuum in the observing tube. Dr. Lodge, who briefly reviewed Lenard’s experiments, expressed the same opinion. See The Elect., Lon., Jan. 31, ’96, p. 439. For theoretical considerations of the electric nature of light, the discharge law in the photo-electric phenomena, the simple validity of the discharge law, the occurrence of interference surfaces in the blue cathode light, the cathode rays in the axis of symmetry, the necessary degrees of longitudinal electric waves, the frequency of the cathode rays, and proof of longitudinal character of cathode rays, see Jaumann in The Elect., Lon., Mar. 6, ’96; translated from Wied. Ann., 571, pp. 147 to 184, ’96, and succeeding numbers of The Elect., Lon., which were freely discussed in foreign literature contemporaneously.

78. De Kowalskie’s Experiment. Source, Propagation and Direction of Cathode Rays. Acad. Sci., Paris, Jan. 14, ’95; So. Fran. Phys. Jan. ’95; Nature, Lon. Jan. 24, ’95; Feb. 21, ’95.—The conclusions he arrived at are, 1. The production of the cathode rays does not depend on the discharge from metallic electrodes across a rarefied gas, nor is their production connected with the disintegration of metallic electrodes. 2. They are produced chiefly where the primary illumination attains suitable intensity, that is, where the density of the current lines is very considerable. 3. Their direction of propagation is that of the current lines at the place where the rays are produced, from the negative to the positive poles. They are propagated in the opposite direction to that in which the positive luminosity is supposed to flow. [§ 43]. He employed a Goldstein tube reduced at the centre. [§ 41]. It was found that the cathode rays are formed not only at the negative electrode, but also at the constriction, directly opposite the cathode. De Kowalskie carried on further experiments in this line in order to be satisfied with the principles named above, which he formulated. In one tube, he was able to produce cathode rays at either end of the capillary tube forming the constricted part of a long vacuum tube. No electrodes were employed. The tube was merely placed near a discharger through which “Tesla currents” were passed? He seems to have been working with X-rays without knowing it; for his results agree with those of Roentgen and later experimenters that the source of X-rays is the surface of a substance where it is struck by cathode rays. The statements were about as definite as could be expected at that date.

Hand, by Oliver B. Shallenberger, taken with Focus tube.
[§ 137], p. [136].

CHAPTER VII

79. Roentgen’s experiments. X-Rays, and A New Art. Wurz. Physik. Med. Gesell. Jan. ’95; Nature, Lon., Jan. ’96; The Elect., Lon. April 24, 96; Sitz. Wurz. Physik. Inst. D. Uni. Mar. 9, 96.—Uninfluenced By A Magnet In Open Air.—Although Lenard recognized several kinds of cathode rays, which differed as to penetrating and phosphorescing power, yet he always held, or inferred at least that they were deflected by a magnet, outside, as well as inside, (proved [§ 72a].) of the discharge tube. [§ 59]. Prof. Wilhelm Konrad Roentgen subjected his newly discovered rays to the action of very strong magnetic fields in the open air, but no deviation was detected. This is the characteristic which more than anything else has served to distinguish X-rays from cathode rays. This property has been confirmed by others. He employed the principle of magnetic attraction of internal cathode § [59], rays to shift the phosphorescent spot, for then he noticed that the source of X-rays fluctuated also.

80. Source of X-Rays may Be At Points Within The Vacuum Space.—In one case, he employed a Lenard tube, and found that the X-rays were generated from the window which was in the path of the cathode rays. [§ 67]. Different bodies within the discharge tube were found to have different quantitative powers of radiating X-rays when struck by the cathode rays. He stated “If for example, we let the cathode rays fall on a plate, one half consisting of a 0.3 mm. sheet of platinum and the other half a 1 mm. sheet of aluminum, the pin-hole photograph of this double plate will show that the sheet of platinum emits a far greater number of X-rays than does the aluminum, this remark applying in every case to the side upon which the cathode rays impinge.” On the reverse side, however, of the platinum, no rays were emitted, but a large amount was radiated from the reverse side of the aluminum. [§ 67]. He admitted that the explanation was simple; but, at the same time, he pointed out that this, together with other experiments, showed that platinum is the best for generating the most powerful X-rays. One form with which he experimented is illustrated in Fig. [J], in principle, being described as a bulb in which a concave cathode was opposite a sheet of platinum, placed at an angle of 45° to the axis of the curved cathode, and at the focus thereof.

J

81. Reflection of X-Rays.—He emphasized the knowledge that there is a certain kind and a certain amount of reflection, such as that produced upon light and, as pointed out by Lenard, upon cathode rays, by certain turbid media. The following quotation sets forth the exact experiment to show slight reflection at metal surfaces. “I exposed a plate, protected by a black paper sheet 1 to the X-rays (e.g. from bulb J) so that the glass side 2 lay next to the discharge tube. The sensitive film was partly covered with star-shaped pieces (4 slightly displaced in the Fig.) of platinum, lead, zinc and aluminum. On the developed negative the star-shaped impressions showed dark (comparatively) under platinum, lead and more markedly, under zinc; the aluminum gave no image. It seems, therefore, that the former three metals can reflect the X-rays; as, however, another explanation is possible, I repeated the experiment with only this difference, that a film of thin aluminum foil was interposed between the sensitive film and the metal stars. Such an aluminum plate is opaque to the ultra-violet rays, but transparent to X-rays. In the result the images appeared as before, this pointing still to the existence of reflection at metal surfaces.”

82. Penetrating Power. The transmitted energy was tested both by a fluorescent screen and by a sensitive photographic plate. Either one was acted upon by the rays after transmission through what have ordinarily been called opaque objects. [§ 68]. for example, 1000 pages of a book. As in Lenard’s results, so in Roentgen’s, the color of the object had no effect, even when the material was black. [§ 68], near beginning. A single thickness of tinfoil scarcely cast a shadow on the screen. [§ 66a]. The same was true with reference to a pine board 2 or 3 cm. thick. They passed also through aluminum 15 mm. thick. [63b]. Glass was comparatively opaque, [§ 66a], as compared with its power of transmitting light, but nevertheless it must be remembered that the rays passed through considerable thickness of glass. The tissues of the body, water [§ 68], near centre, and certain other liquids and gases were found exceedingly permeable [§ 67]. Fluorescence could be detected through platinum 2 mm. thick and lead 1.5 mm. thick. Through air the screen was illuminated at a maximum distance of 1 m. A rod of wood painted with white lead cast a great deal more shadow than without the paint, and in general, bones, salts of the metals, whether solid or in solution, metals themselves and minerals generally were among the most resisting materials. [§ 155]. The experiments were performed in a dark room by excluding the luminosity of the tube by a thick cloth or card board entirely surrounding the tube. He performed the wonderful experiment, so often since repeated, of holding the hand between the screen of barium platino cyanide and the discharge tube, and beholding the shadow picture of the bones. This was the accidental step which initiated the new department of photography, and which gave to the whole science of electric discharge, a new interest among scientists and electricians and which thoroughly awakened popular interest. The whole world concedes to him the honor of being the originator of the new art. In view of sciagraphs of the bones of the hand upon the screen, it occurred to him in view also of Lenard’s experiments, on the photographic plate, to produce a permanent picture of the skeleton of the hand with the flesh faintly outlined. [§ 84]. The accompanying half tone illustration, page [37], was made by the Elect. Eng. N.Y. (June 3, ’96) by permission, and it represents the Edison X-ray exhibit at the New York Electrical Exposition of the Electric Light Association, 1896. Thousands of people, through the beneficence of Dr. Edison, were permitted to see the shadows of their bones surrounded by living flesh. The screen was made of calcic tungstate. The hand and arm were placed behind and viewed from the front. [§ 132], near beginning.

83. Penetrating Power and Density of Substances.—Although he found that there was some general relation between the thickness of materials and the penetrating power, yet he was satisfied that the variation of the power did not bear a direct relation to the density, (referring to solids) especially as he noticed a peculiar result when shadows were cast by Iceland spar, glass, aluminum and quartz of equal thickness. The Iceland spar cast the least shadow upon suitable fluorescent or photographic plate. The increased thickness of any one substance increased the darkness of the shadow, as exhibited by tinfoil in layers forming steps. Other metals, namely platinum, lead, zinc and aluminum foil were similarly arranged and a table of the results recorded. [§ 63b].

He concluded from these data that the permeability increased much more rapidly than the thickness decreased.

84. Fluorescence and Chemical Action. [§ 70] and [63a].—Among the substances that fluoresced were barium platino cyanide, calcium sulphide, uranium glass, Iceland spar and rock salt. In producing sciagraphs on the photographic plates, he found it entirely unnecessary to remove the usual ebonite cover, which, although black, and so opaque to light, produced scarcely any resistance to the rays. The sensitive plate, even when protected in a box, could not be kept near a discharge tube, for he noticed that it became clouded. He was not sure whether the effect upon the sensitive plate was directly due to the X-rays or to a secondary action, namely, the fluorescent light which must have been produced upon the glass plate having the film, it being well known that light of fluorescence possesses chemical power. He called attention to the fact that inasmuch as fluorescent light which can be reflected, refracted, polarized, etc., was produced by the rays; therefore, all the X-rays which fell upon a body did not leave it as such. [§ 67]. No effect was produced upon the retina of the eye although he temporarily concluded that the rays must have struck the retina in view of the great permeability of animal tissue and liquids. [§ 68], at end. Conclusions of this kind not based on experiment, are never reliable, even when offered by very high authorities. Again the rays were weak. Roentgen himself admitted that the salts of metals in solution ([§ 82], near centre) rendered the latter rather opaque. The eye ball is continually moistened with the solution of common salt. Further than this, Mr. Pignolet noticed in Comptes Rendus, Feb. 24, ’96, an account of an experiment of Darien and de Rochas. In anatomy it is common to experiment on fresh pig’s eyes in order to make comparisons with human eyes. The above named Frenchmen submitted the former to X-rays. The eyes were but slightly permeable thereto.

The Physical Institute, University of Würzburg,
WHERE PROF. ROENTGEN HAS HIS RESIDENCE, DELIVERS HIS LECTURES, AND PERFORMS HIS EXPERIMENTS.
From photograph by G. Glock, Würzburg. (Not referred to in book.)

85. Non-refraction and But Little Reflection of X-rays.—He employed a very powerful refracting prism made of mica and containing carbon bi-sulphide and water. The same prism refracted light but did not refract X-rays. No one would think of making prisms for examining light, of ebonite or aluminum, but he made such a prism for testing X-rays. But if there were any refraction he concluded that the refractive index could not have been more than 1.05, which may be considered as a proof that the rays cannot be refracted. He tried heavier metals, but the difficulty of arriving at any satisfactory results was due to the resistance of such metals to the transmission of the rays. Among other tests was one consisting in passing the rays through layers of powdered materials through which the rays were transmitted in the same quantity as through the same substances not powdered. It is well known that light passed into powdered transparent materials, is enormously cut off, deviated, diffused, refracted etc., in view of the innumerable small surfaces of the particles. Hence he concluded that there was little if anything in the nature of refraction or reflection of X-rays. [§ 146]. The powdered materials employed were rock salt, and fine electrolytic and zinc dust. The shadows, both on the screen and as recorded on the photographic plate were of substantially the same shade as given by the same materials of the same thickness in the coherent state. One of the most usual ways of testing refraction of light is by means of a lens. X-rays could not be brought to a focus with the lens of what ever material it was made. Among the substances tried were ebonite and glass. As expected, therefore, the sciagraph of a round rod was darker in the middle than at the edges; and a hollow cylinder filled with a more transparent liquid showed the centre portion brighter than its edges. If one considers this observation in connection with others, namely the transparency of powders, and the state of the surface not being effective in altering the passage of the X-rays through a body, it leads to the probable conclusion that regular reflection does not exist, but that bodies behave to the X-rays as turbid media to light, [§ 69].

86. Velocity of X-Rays In Different Bodies. p. 46.—Although he performed no direct experiment in this direction yet he inferred in view of the absence of refraction at the surfaces of different media, that the rays travel with equal velocities in all bodies.

87. Double Refraction and Polarization.—Neither could he detect any action upon the rays by way of refraction by Iceland spar at whatever angle the crystal was placed. As to this property of light see Huygen’s Works of 1690 and Malus’ Works of 1810. quartz also gave negative results. Prof. Mayer of Stevens Institute submitted to Sci., Mar. 27, ’96, the report of a crucial test for showing the non-polarization of X-rays. On six discs of glass, 0.15 mm. thick and 25 mm. in diameter, were placed very thin plates of Herapath’s iodo-sulphate of quinine. The axes of these crystals crossed one another at various angles. When the axes of two plates were crossed at right angles no light was transmitted; the overlapping surfaces of the plates appearing black. If the Roentgen rays be polarizable, the Herapath crystals, crossed at right angles, should act as lead and not allow any of the Roentgen rays to be transmitted. Prof. Mayer is well known as exceedingly expert in connection with minute measurements and in the manipulation of scientific experiments. Dr. Morton, Pres. Stevens Inst., attested the results as an absolute demonstration that X-rays are incapable of polarization. Stevens Indicator, Jan., ’96.

88. The Propagation of X-Rays Rectilinear.—There would be no difficulty in producing photographs of the bones of the hand with the rays of light, if it were not for the tremendous amount of reflection and refraction causing so much diffusion that no sharply defined shadow of the bones would be produced. By means of a powerful lens and a funnel pointed into a dark room, the author noticed that the condensed light thereby obtained when passed through the hand, and when the incident rays were parallel, came out so diffused that one would think that the light went through bones as easily as any part of the hand. An experiment of this kind serves to emphasize that the success of sciagraphy by X-rays is due not only to the great penetrating power, but to practically no refraction nor reflection. In view of the sharp shadows cast of objects even when located in vegetable or animal media, Roentgen was justified in giving the name of ray to the energy. He tested the sharpness of the shadow by making sciagraphs and fluorescent pictures not only of the bones of the hand, but of a wire wound upon a bobbin, of a set of weights in a box, of a compass, card and needle, conveniently closed in a metal case, and of the elements of a non-homogeneous metal. To prove the rectilinear propagation further, he received the image of the discharge tube upon a photographic plate by means of a pinhole camera. The picture was faint but unmistakable.

89. Interference. The rays of light may be caused to interfere with each other. See Newton’s Principia, Vol. III.; Young’s Works, Vol. I.—Theory points out that waves of ether of two pencils of light, when caused to be propagated at certain relative phases partially or wholly neutralize or strengthen each other. Roentgen could obtain no interference effects of the X-rays, but did not conclude that the interference property was absent. He was not satisfied with the intensity of the rays and therefore could not test the matter severely.

Fig. L.

90. Electrified Bodies Discharged by X-Rays. p. [47].—After Roentgen’s first announcement, others, and probably J. J. Thomson as the first, found that the X-rays would discharge both negatively and positively electrified bodies. Roentgen, in his second announcement, stated that he had already made such a discovery, but had not carried the investigation far enough to report satisfactorily on the details. At last he put forth an account of the whole phenomena and stated that the discharge varied somewhat with the intensity of the rays, which was tested in each instance by the relative luminosity of the fluorescent screen, and by the relative darkness produced upon the photographic plate in several instances. Electrified bodies, whether conductors or insulators, were discharged when placed in the path of the rays. All bodies whatsoever behaved in the same manner when charged. They were all discharged equally by the X-rays. He noticed that “If an electrical conductor is surrounded by a solid insulator such as paraffin instead of by air, the radiation acts as if the insulating envelope were swept by a flame connected to earth.” Upon surrounding said paraffin by a conductor connected to earth, the radiation no longer acted on the inner electrified conductor. The above observations led him to believe that the action was indirect and had something to do with the air through which the X-rays passed. In order to prove this, it was necessary for him to show that air ought to be able to discharge the bodies if first subjected to the rays, and then passed over the bodies. The apparatus for performing an experiment to test this prediction is shown in Fig. [L], which serves to illustrate also the manner in which he prevented electro-static influences of the discharge tube, leading in wires and induction coil. [§ 71], near centre. For this purpose he built a large room in which the walls were of zinc covered with lead. The door for his entrance and exit was arranged to be closed in an air-tight manner. In the side wall opposite the door there was a slit 4 cm. wide, covered hermetically with a thin sheet of aluminum for the entrance of X-rays from the vacuum tube outside of the room. All the electrical apparatus connected with the generation of the X-rays was outside of the room. No force whatever came into the room, therefore, except the X-rays through the aluminum. [§ 71]. In order to show that air which had been subjected to the X-rays would discharge a body immediately afterwards upon coming in contact therewith, he arranged matters so that the air was propelled by an aspirator. He passed air along a tube made of thick metal so that the rays could enter only through a small aluminum window near the open end. At over a distance of 20 cm. from the window was an insulated ball charged with electricity, and connected to any electroscope or electrometer. The professor used a Hankel electroscope. No published sketch was made by Roentgen; therefore, that shown in the figure was produced by inference from the description. The operation was as follows: The X-rays passed into the room through the aluminum window, and then into the metal tube through its aluminum window. When the air was at rest, the ball was not discharged. When the aspirator was at work, however, so that the air moved past the aluminum window and past the ball, the latter was discharged whether electrified positively or negatively. He modified the operation by maintaining the ball at a constant potential by means of accumulators, while the air which had been treated by X-rays was passed by the ball. “An electric current was started as if the ball had been connected with the wall of the tube by a bad conductor.” He was not sure whether the air would retain its power to discharge bodies as long as it remained out of contact with any bodies. He determined, however, that any slight “disturbance” of the air by a body having a large surface and not electrified, rendered the air inoperative. He illustrated this by saying that “If one pushes, for example, a sufficiently thick plug of cotton-wool so far into the tube that the air which has been traversed by the rays must stream through the cotton-wool before it reaches the ball, the charge of the ball remains unchanged when suction is commenced.” With the cotton-wool immediately in front of the window, it had no effect, showing, therefore, that dust particles in the air are not the cause of the communication of the force of the discharge from the X-rays to the electrified body. Very fine wire gauze in several thicknesses also prevented the air from discharging the body when placed between the aluminum window and the ball within the thick metal tube, as in the case of the cotton plug. Similar experiments were instituted with dry hydrogen instead of air, and, as far as he could discern, the bodies were equally well discharged, except possibly a little slower in hydrogen. He experienced difficulty in obtaining equally powerful X-rays at different times. All experimenters are acquainted with this difficulty. Further, he called attention also to the thin layer of air which clings to the surface of the bodies, and which, therefore, plays an appreciable part in connection with the discharge. [§ 16], near end. In order to test the matter further as to discharge of electrified bodies, he placed the same in a highly exhausted bulb and found that the discharge was in one case, for example, only 1/70 as rapid as in air and hydrogen at ordinary pressure, thereby serving as another proof that gas was the intermediate agency. Allowance should be made in all experiments in connection with the discharging quality of X-rays. The surrounding gas should be taken into account.

90a. Application of Principle of Discharge by X-Rays.—Professor Robb, of Trinity College, (Science, Apr. 10, ’96), proposed and explained and practically tested the principle of the discharge of X-rays to determine the relative transparencies of substances to X-rays. He plotted a curve in which the co-ordinate represented the charge of the condenser in micro-coulombs, and the abscissæ the time between charging and discharging the condenser. The same plan could be adopted, he suggested, for making quantitative measurements of the intensity of X-rays from different tubes or the same discharge tube at different times. J. J. Borgmann, of St. Petersburg, probably was the first to show that X-rays charged as well as discharged bodies. See The Elect., Lon., Feb. 14, ’96, p. 501. Soon, a similar announcement was made by Prof. Righi, of Bologna. [§ 90].

90A. Borgmann and Gerchun’s Experiments. Action of the X-Rays on Electro-static Charges and (La Distance Explosive.) Comptes Rendus, Feb. 17, ’96; from Trans., by Louis M. Pignolet.—A positively charged zinc disk connected to an electroscope lost its charge almost instantly and acquired a negative charge. When the charge on the zinc disk was negative, the loss was much slower and was not complete, a certain charge remaining. When the rays fell upon two small platinum balls connected to the terminals of an induction coil but separated beyond its sparking distance, sparking took place between them, showing that X-rays, like ultra-violet rays, increase the sparking distance of static charges.

90b. Righi’s Experiments. Bodies In The Neutral or Negative State, Positively Electrified By X-Rays. Comptes Rendus, Feb. 17, 1896. From Trans. by Louis M. Pignolet.—The measurements were made by this eminent Italian physicist, with a Mascart electrometer connected with the bodies upon which the X-rays impinged and enclosed in a grounded metallic case (Faraday cylinder) provided with an aluminum window for the entrance of the rays. A metallic disk connected with the electrometer lost its charge rapidly whether positive or negative.

[§ 99S]. Initial positive charges were not completely dissipated; negative charges were not only completely dissipated but the bodies acquired positive charges. Disks in the neutral state were charged positively by the X-rays the same as takes place with ultra-violet rays. The final positive potential was greater for copper than for zinc and still greater for retort carbon (“le carbon de cornue”) [90c]. at end. The various results are not conflicting if the particular materials are taken into accounts. [90c] at end.

90c. The experiments of Prof. Minchin, an expert in such measurements, are properly described here, in that they seem to clear up the superficial ambiguity. He formulated the conclusion (The Elect., Lon., Mar. 27, ’96, p. 736) thus:—“The X-rays charge some bodies positively and some negatively, and whatever charge a body may receive by other means, the X-rays change it, both in magnitude and sign, to the charge which they independently give to the body.” Thus, in the case of magnesium, if the same is first positively charged by any suitable means, then will the X-rays not only discharge it, but electrify it negatively, while if this metal is first negatively charged, the X-rays either diminish or increase the discharge. It must be remembered, however, that this is not true with all metals, for he found that gold, silver, copper, platinum, iron, aluminum, bismuth, steel and antimony, are all positively electrified.