The recently discovered product, protoactinium,—isolated by Hahn and Soddy,—is the hitherto missing link between uranium Y and actinium. “This substance emits Alpha rays and has an estimated period of 10,000 years. The actinium series is believed to have its origin in a dual transformation of uranium X. The first branch product, representing about 4% of the total, is believed to be uranium Y, a Beta-ray product of period one day. This is directly transformed into protoactinium.” This element has not yet been obtained in a pure state.
Many of the radioactive elements are isotropic with known chemical elements—i.e., alike in their chemical properties, but dissimilar in radioactive properties. Since they cannot be distinguished—or separated—from the ordinary elements with which they are isotropic, by any chemical methods, they must occupy the same place in the periodic classification of the elements. Radium and mesothorium, for example (as Soddy was first to show) do not have the same atomic weight, but they cannot be distinguished from each other by any chemical methods. Therefore they both have the atomic number 88, though the atomic weight of radium is 226 and of mesothorium 228. (See Shipley, “Origin and Development of the Atomic Theory,” p. 64, Little Blue Book, No. 608.) Radium D and lead, and thorium and ionium, are examples of radioactive isotropes.
The nature of the end-product was first suggested by Boltwood, who pointed out the invariable presence of lead in old radium minerals, and in amount to be expected from their uranium content and geologic age. “Thus,” says Prof. T. W. Richards, of Harvard University, “we must adopt a kind of limited transmutation of the elements,” although not of the immediately profitable type [gold] sought by the ancient alchemists.”
Sir Ernest Rutherford, who succeeded Sir J. J. Thomson as Cavendish Professor of Physics at Cambridge University, was first to recognize that the rays from uranium and radium were not all alike, but consisted of three distinct kinds. In order to distinguish them clearly, without committing himself in advance as to their exact nature, he christened them Alpha, Beta, and Gamma rays—the first three letters of the Greek alphabet. We know now that the Alpha rays are positively charged helium atoms, with two negative electrons missing; that the Beta rays are negatively charged electrons (disembodied “particles” of electricity, exactly like cathode rays); and that the Gamma rays are a type of X-rays, not material particles but merely extremely short magnetic waves or oscillations, akin to ordinary light waves or rays.
Dr. R. A. Millikan calls them “the wireless waves of the denizens of the sub-atomic world. They are ether waves, just like light or just like wireless waves, except that the vibration frequency ... amounts to 30 billion billions per second. These are the Gamma rays.” This means that this number of light waves would pass a given point in space each second. Since these rays do not consist of charged particles they are not deflected by electromagnetic or electrostatic fields, as are the Alpha and Beta rays. It has been found that one gram of radium ejects 136,000,000,000 particles a second!
The Gamma rays of radium have such penetrating power that a half-inch sheet of lead will reduce their original intensity by only one-half, and they are not absolutely stopped by 20 inches. These invisible light waves, thousands of times shorter than those of visible light, are produced whenever a cathode ray (negative electron) hits matter. Of the atoms forming the substance penetrated, perhaps only one in a billion is struck. It has been said that the Gamma rays (and X-rays) are the result of the back-kick of ejected electrons. Prof. Comstock says that the connection between the Beta rays and the Gamma rays “is probably similar to that between the bullet and the sound in the case of a gun.” However this may be, we know that the Gamma rays are, after all, in essence only excessively minute light waves. While the longest visible light waves are 0.00008 centimeter, the longest Gamma rays are 0.000000013 centimeter; and whereas the shortest visible light waves are 0.00004 centimeter, the shortest Gamma rays are but 0.0000000007 centimeter.
The Beta particles are ejected with a velocity of from 90,000 to 160,000 miles a second.
Prof. Gustave Le Bon calculated that it would require 340,000 barrels of powder to discharge one bullet at this inconceivable speed! These negatively charged electrons normally revolve around the positively charged nucleus. Under certain conditions, an electron will make 2200 billion revolutions within an atom in one second.
Radium is not only continually losing matter and energy as electricity, but it is also losing energy as heat. Professor and Mme. Curie discovered that any substance placed near radium becomes itself a false radium. This applies to all substances. The acquired radioactivity persists for many hours, or even days, after the removal of the radium. In the case of zinc, these secondary radiations were found to be four times as intense as ordinary uranium. It vanishes sooner or later upon the removal from the neighborhood of the potent radium.
The radioactive something which passes out of radium was not the already known group of Alpha, Beta and Gamma rays, but an emanation akin to gas. Rutherford, its original discoverer, was not sure that it was a gas, so he cautiously gave it the name emanation. When the radium was heated, or dissolved in water, the quantity of emanation was greatly increased, which seemed to show that it was a gas of some kind occluded (bound up) in the radium. The quantity obtained was insufficient to bring the emanation within the testing power of spectroscope or balance.