Popular Science Monthly/Volume 57/July 1900/New Sources of Light and of Roentgen Rays
| NEW SOURCES OF LIGHT AND OF RÖNTGEN RAYS. |
By HENRY CARRINGTON BOLTON, Ph. D.
AMONG the general laws of physical science, none seems more firm!}' established than that of the conservation and correlation of energy; according to this the various forms of energy that constitute the domain of experimental physics, heat, light, electricity, magnetism and chemical action, have reciprocal dependence and "can not originate otherwise than by devolution from some preëxisting force," or rather energy. That motion is convertible into heat, heat into light and both the former into electricity are phenomena familiar to every one who uses incandescent bulbs or rides in a trolley, and we do not usually recognize any production of light unaccompanied by heat. True, the little fire-fly is possessed of a mysterious power that enables it to emit light without enough heat to affect Langley's most sensitive bolometer, but the eminent Secretary of the Smithsonian has to admit that the "cheapest form of light" is produced by "processes of nature of which we know nothing." This little understood property called phosphorescence is shared by many living organisms, both animal and vegetable, as well as by substances of the mineral kingdom; to the former belong coelenterates, mollusks, Crustacea, fishes and insects, and decaying wood, certain mushrooms, etc.; to the latter the Bologna stone, so-called, and the commercial article called 'Balmain's paint.'
In the case of the mineral substances, barium or calcium sulfids and the like, the light-giving power is not an innate property, but is set in operation by exposure to the energy of sunlight, the light of burning magnesium or to some other source of actinism; moreover, the power thus acquired by insolation is a fugitive one, the substances exercising it after three or four hours become 'dead' and lose their activity. Excepting then these living beings and these phosphorescent bodies, light as commonly known to us is always correlated with heat; within the last four years, however, discoveries have been made in France that seem to modify the position taken by philosophers and to necessitate new views concerning the manifestations of that energy with which the universe is endowed. A group of French savants have found mineral substances that apparently give out light perpetually without any exciting cause, realizing the dream of the alchemists—a perpetual lamp consuming no oil. These substances also emit rays having the penetrating properties of X-rays, other rays affecting a photographic plate, and fourthly, rays causing air to become a conductor of electricity. The history of these discoveries can be briefly given.
Röntgen's discovery of the rays that pass through metals and solids opaque to light was made in 1895, and in the following year, Becquerel, a distinguished French academician, discovered that salts of the metal uranium (substances that had long been used in coloring china and glass) emit invisible radiations capable of discharging electrified bodies and of producing skiagraphic images on sensitive plates; he found that potassio-uranic sulfate emits rays that pass through black paper and give photographic impressions in the same way as Röntgen rays. This property is not limited to the brilliantly fluorescent uranic salts, but is shared by the non-fluorescent uranous salts, and is exhibited by compounds whether phosphorescent or not, whether crystalline, melted or in solution, as well as by the metal itself. The permanence of this activity is amazing, substances kept in a double leaden box more than three years continuing to exert the power.
Shortly after the announcement by Becquerel, experimenters found that other substances have the power of emitting 'Becquerel Rays,' such as calcium and zinc sulfids and compounds of thorium. In 1898 Mine. Sklodowska Curie, working in the laboratory of the Municipal School of Industrial Physics and Chemistry in Paris, devised a special apparatus for measuring the electrical conductivity of the air when under the influence of 'radio-active bodies,' and by its means studied the behavior of pitchblende (uraninite), and of other uranium minerals; finding that some specimens of pitchblende had three times as much energy as uranium itself, she came to the conclusion that the peculiar property is due to some unknown body contained in the minerals and not to uranium. Examining the mineral with the aid of her husband, the two found a substance analogous to bismuth, four thousand times stronger than uranium, which was named 'Polonium,' in honor of the native land of Mme. Curie. In Pecehfber of the same year, the lady received the Gegner prize of 4,000 francs awarded by the Academy of Sciences, as a substantial appreciation of her discovery, and later in the same month Mme. and M. Curie announced that they had found a second body in pitchblende, which they named 'Radium.' More recently, M. Debierne, working under the auspices of Mme. Curie, has discovered a third body, which he calls 'Actinium,' an unfortunate appellation because 'actinium' has already been used for an element announced by Dr. Phipson and since discarded.
These three 'radio-active' substances do not possess identical properties; their rays are unequally absorbed and are differently affected in a magnetic field; moreover radium emits visible rays, while polonium does not. Nor have they the same chemical affinities; polonium belongs to the bismuth group, radium to the barium and actinium to the titanium series. They have not been separated perfectly from their analogues, and consequently their chemical properties and the actual intensity of their physical activities is very imperfectly known. The difficulties of securing even small quantities of crude materials are enormous; Fritz Giesel obtained from one thousand kilograms of raw material only fifteen grams of active compounds, and Mme. Curie, operating on half a ton of the residues of uranium from a chemical manufactory, got about two kilograms of barium chlorid rich in radium, but the percentage of active substances in these mixtures is unknown.
Radium is spontaneously luminous, and all the bodies emit rays that excite phosphorescence in gems, fluorite and other minerals; they communicate radiant energy to inactive substances, and they exert chemical action, transforming oxygen into ozone and producing changes in the color of glass and of barium platino-cyanid.
Through the enterprise and liberality of the Smithsonian Institution, and by the courtesy of Professor Langley, I have enjoyed the opportunity of studying small specimens of these rare and costly substances; they comprised ten grams of 'radio-active substance' prepared by a manufacturing chemist of Germany and smaller quantities of 'radium' and of 'polonium' from Paris. On removing the wrappings of the German specimens in a dark room, they were seen to emit greenish-white light that gave to the enveloping papers a peculiar glow, similar to the fluorescence produced by Röntgen rays. Simple tests of the radium showed that it gave the usual reactions of barium; on boiling it with water it lost its luminosity, but on heating to dull redness this property returned in the dark. It also caused a barium platinocyanid screen to fluoresce.
Experiments to test the actinic power of these bodies gave interesting results; on exposing sections of photographic plates, at distances of five inches, from two to twelve minutes, bands were obtained varying in intensity with the duration of action. By exposing sensitive plates behind negatives to the radiant materials from two to three hours, excellent transparencies were secured; on substituting Eastman's bromide paper good prints were obtained.
The penetrating power of the rays emitted permits the production of skiagraphs; the plates were enveloped in Carbutt's black paper (impermeable to light), and on them were laid pieces of tinfoil cut in openwork pattern; after one hour's exposure negatives were secured plainly showing the pattern. Analogous experiments were carried on with the specimens from Paris, but they were only one fifth as strong in effects; that labelled 'polonium sub-nitrate' had positively no action on the plates used.
The primary source of the energy manifested by these extraordinary substances has greatly puzzled physicists, and as yet remains a mystery: Mme. Curie, speculating on the matter, conjectured that all space is continually traversed by rays analogous to Röntgen rays, but far more penetrating, and not capable of being absorbed by certain elements of high atomic weight, such as uranium and thorium. Becquerel, reflecting on the marvellous spontaneous emission of light, said: "If it can be proved that the luminosity causes no loss of energy, the state of the uranium is like that of a magnet which has been produced by an expenditure of energy and retains it indefinitely, maintaining around it a field in which transformation of energy can be effected; but the photographic reductions and the excitation of phosphorescence require an expenditure of energy, of which the source can only be in the radioactive substances." Somewhat later, Becquerel hazarded the opinion that the radiation is composed at least in part of cathodic rays; but these have been proved to be material, hence the induced activity must be caused by material particles impinging upon the substances excited. This materialistic theory seems to be confirmed by the results of ingenious experiments made by Mme. and M. Curie; they placed a sensitive plate beneath a salt of radium supported on a slab of lead, in the vicinity of an electro-magnet. Under these conditions, when the current was passing, the rays emitted were bent in curved lines upon the sensitive plate, making impressions.
It may be objected, says a French writer, that the materialistic theory requires us to admit actual loss of particles of matter, nevertheless the charges are so feeble that the most intense radiation yet observed would require millions of years for the removal of one milligram of substance.
While writing these lines, we have news of experiments that seem to throw doubt on the elementary character of these radio-active bodies; Bela von Lengyel, of Budapest, claims to have prepared the so-called 'radium' synthetically. By fusing with the heat of electricity uranium nitrate mixed with a small percentage of barium nitrate, and treating the mass with acids, he obtained a substance that gives out actinic rays, Röntgen rays, excites platino-cyanid screens and causes air to conduct electricity; in short, the Hungarian chemist gets material possessing all the properties characteristic of the 'element' announced by Mme. Curie.
Admitting that radio-active bodies can be manufactured to order, are we any nearer explaining their mysterious powers?
Speculations as to the future history and applications of these wonder-working bodies press upon even the dullest imagination; if a few grams of earth-born material, containing only a small percentage of the active body, emit light enough to affect the human eye and a photographic plate, as well as rays that penetrate with X-ray power, what degree of luminosity, of actinism and of Röntgenism (if the term may be allowed), is to be expected from an hundred weight of the quintessence of energy purified from interfering matter? And to what uses is this light-generating material to be applied? Are our bicycles to be lighted with discs of radium in tiny lanterns? Are these substances to give us the 'cheapest form of light?' Are we about to realize the chimerical dream of the alchemists?
Seriously, in what direction is profound study of these substances going to lead us? Will it not greatly extend our knowledge of physical manifestations of energy and their correlation? In what corner of the globe will be found the cheap and convenient supply of the raw material yielding the radio-active bodies? Will not chemists be obliged to reexamine much known material by laboratory methods conducted in the dark? Many of us have worked up pounds of pitchblende to extract the uranium oxids, and in so doing have poured down the waste-pipe or thrown into the dust-bin the more interesting and precious bodies.
Whatever the future may bring, scientists are deeply indebted to Becquerel and to Mme. and M. Curie for placing in their hands new methods of research and for furnishing a novel basis for speculation destined to yield abundant fruits.