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 Mme. 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 December 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 chloride 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 platino-cyanid 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 radio-active 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?