CHAPTER IVAN EPOCH-MAKING DISCOVERY
When radium was discovered by Mme. Curie in 1898, the effect upon the scientific world was startling, not to say “catastrophic”—as one author wrote at the time—since its activities ran counter to every known principle of physical science. “Some of the most solid foundations of science were destroyed, some of its noblest edifices wrecked, and scientists had to nerve themselves to face and investigate a new form of energy.”
So soon as radium compounds (salts) became available, however, the amount of energy given out in radioactive processes—the emission of powerful radiations which can be transformed into light and heat—was measured; and it was found that radium, weight for weight, gives out as much heat as any known fuel every three days, and in the course of fifteen years releases a quantity of energy nearly 2,000 times as much as is obtained from the best fuel, with no signs of exhaustion (Soddy). In the combustion of coal, the heat evolved is sufficient to raise a weight of water some 80 to 100 times the weight of the fuel from the freezing-point to the boiling-point. The spontaneous heat from radium is sufficient to heat a quantity of water equal to the weight of radium from the freezing-point to the boiling-point every three-quarters of an hour. In other words, a pound ofradium contains and evolves in its changes the same amount of energy as 100 tons or more of coal evolve in their combustion.
In ordinary chemical changes it is themolecules(groups of atoms) which are altered or rearranged; in radioactive change the atoms themselves suffer disintegration and rearrangements. The energy of radioactivity, then, is—according to the accepted view—intra-atomic—stored-up energy within the atom itself. It was calculated by Prof. Curie that the energy of one gram of radium would suffice to lift a weight of 500 tons to a height of one mile. If it were possible to obtain one cubic centimeter (a thimbleful) of the “emanation” from radium in the form of a gas, we should find that it possessed the power, altogether, of emitting more than seven million calories of heat! A thimbleful of this invisible gas would be more than sufficient to raise 15,000 pounds of water 1°. But in every mass of radium, small or large, not more than 13 trillionths of it is undergoing change per second.
“The processes occurring in the radio-elements,” says Rutherford again, “are of a character quite distinct from any previously observed in chemistry. Although it has been shown that the radioactivity is due to the spontaneous and continuous production of new types of active matter, the laws which control this production are different from the laws of ordinary chemical reactions. It has not been found possible in any way to alter either the rate at which the matter is produced or its rate of change when produced. Temperature,which is such an important factor in altering the rate of chemical reactions, is, in these cases, entirely without influence. In addition, no ordinary chemical change is known which is accompanied by the expulsion of charged atoms with great velocity.... Besides their high atomic weights, [they] do not possess in common any special chemical characteristics which differentiate them from the other elements.”
It was early observed by Curie and Laborde that the temperature of a radium salt is always a degree or two above that of the atmosphere, and they estimated that a gram of pure radium would emit about 100 gram-calories per hour. Giesel later showed that radium was always at a temperature 5° higher than the surrounding air, regardless of what the temperature of the air might be. This continues unchanged whether the temperature of the surroundings be 250° below zero Centigrade, or in the intense heat of an electric furnace.
“Perhaps,” remarks a writer inThe Scientific American(February, 1922), “there will come a time when we shall use the energy in the atoms to drive our machines, cook our food and heat our rooms. Besides, already today we are actually using—even if only a very tiny part—the atomic energy. Thus, for instance, the rays emanating from radium are used for therapeutic purposes and the electrons emanating from a glowing filament can be directed so easily that they can be used in a large number of apparatus for wireless telegraphy and telephony. Most probably plants also make useof this energy in their growth because it has been demonstrated that the rays of the sun liberate electrons from the green leaves, and lastly it may also be mentioned that we humans use a little of this intra-atomic energy when seeing with our eyes, which we are enabled to do by the photoelectric action of light.”A
During the course of the process of disintegration, atoms of uranium and thorium and their products give rise to no fewer than 36 different substances (A. S. Russell), and of these at least a dozen are “new elements.”
All of the 36 radioactive elements are disintegration products of one or the other of the two parent elements, uranium and thorium. They are arranged by the chemist in three series: namely, Uranium 1, Uranium 2 (the Actinium Series), and Thorium. In the first series there are known to be 15 transmutations of matter; in the second, 11; and in the third, 10. The periods of “half change”—the period required for one-half of a given quantity of a radioactive element to decompose—of the different radioactive elements vary all the way from thousands of millions of years for the longest lived primary elements—2.6x1010years for thorium, 8x109for uranium 1—to .002 second for actinium A. In the case of radium itself, 1,670 years are demanded for the disintegration of half of any portion, according to the exact measurements of Profs. B. O. Boltwood and Ellen Gleditsch. The stable end product appears to be in each case anisotopeof lead—leadshaving similar chemical properties but of differentatomic weights(i.e., different atomic composition).
ASee Shipley, Maynard, “Electricity and Life,” ch. vi., Little Blue Book No. 722.
ASee Shipley, Maynard, “Electricity and Life,” ch. vi., Little Blue Book No. 722.
Isotopes are groups of elements which cannot be distinguished (or separated from) one another by any known chemical methods, and which differ only in the atomic weights of the members of the group. In the radioactive groups, the various elements differ also in degree of stability of their atoms.
Chemists cannot actually weigh the mass of an atom of an element on a pair of scales, or by any other method. But if we put down 16 as the “atomic weight” of oxygen, and ascertain the “combining weight” (ratio) of hydrogen to oxygen, we can determine the “atomic weight” of hydrogen (1.008). (See Shipley, “The A B C of the Electron Theory of Matter,” p. 14, Little Blue Book, No. 603.) The ratio of the masses ofanytwo elements in a chemical compound can be very accurately determined. Without going into the details here, it may be said that therelativeweights of the atoms of any element can be determined to 0.01% in many cases (by chemical analysis and synthesis); while theactualweight of any atom has not yet been determined to better than 0.1%.
Lead is produced from uranium by a successive series of losses of Alpha particles—or helium atoms. Omitting the less essential outcomes, or transition stages, we find that each atom of uranium spontaneously ejects three atoms of another element, helium, and therebyis converted into still another element, radium. By losing one atom of helium, radium, in turn, is converted into the so-called emanation, orniton. The latter quickly loses four more atoms of helium and is converted into lead, “uranium lead,” having an atomic weight of 206.08. Ordinary (common) lead, constituting the vast bulk of the lead of the world, has a much higher atomic weight, namely, 207 (Prof. Theodore Richards). Lead from thorium has an atomic weight of 208; from actinium, 206. So we have, in fact, four kinds of lead.
Omitting the less stable transition products, we may say, then, that an atom of uranium is converted into lead by the loss of eight atoms of helium—losing three to become radium, then one to become the emanation, and finally four to become lead. No known human agency can either retard or hasten this breaking down of the uranium atom into radium, or of the radium into emanation, with the final production of lead.
This statement has been universally accepted as true. Nevertheless, Dr. A. Glaschler stated (Nature[London], September 12, 1925) that he had succeeded in accelerating the change of uranium to uranium X (the first product of uranium 1) by submitting uranium oxide to “strong rushes of momentary high-tension currents.” As early as 1923, A. Nodon (Comp. rend., 176, 1705 [1923]) brought forward strong evidence of an increase of the activity of radioactive substances when outdoors and enclosed by envelopes of small absorbing power for Gamma rays as contrasted to the smallerradioactivity of the same substances in cellars and when heavily enveloped by lead. For a tentative explanation of this phenomenon, seeScience, January 8, 1926 (Vol. LXIII, No. 1619), pp. 44–45.
Both uranium and thorium, as we have just stated, break down and become radium, then change to helium and lead.
Says Rutherford: “Although thorium is nearly always present in old uranium minerals and uranium in thorium minerals, there does not appear to be any radioactive connection between these two elements. Uranium and thorium are to be regarded as two distinct radioactive elements.
“With regard to actinium, there is still no definite information of its place in the scheme of transformations. Boltwood has shown that the amount of actinium in uranium minerals is proportional to the amount of uranium. This indicates that actinium, like radium, is in genetic connection with uranium....”
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 atomicnumber88, though the atomicweightof 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 distinctkinds. 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 visiblelight, 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 afalseradium. This applies to all substances. The acquired radioactivity persistsfor 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 anemanationakin to gas. Rutherford, its original discoverer, was not sure that it was a gas, so he cautiously gave it the nameemanation. 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.
Nevertheless, the emanation has been detected, and investigated by the electroscope, which measures the radium rays by the power to discharge its electrified gold leaves. “The electroscope is about a million times more sensitive than the most sensitive spectroscope and yet the spectroscope is capable of detecting easily the millionth part of a milligram of matter” (Duncan).
Calculations made by Rutherford show that if a thimbleful of this active gas could be collected, the bombardment of its powerful rays would heat to a red heat, or even might melt down, the walls of the glass containing it. The emanation emits only Alpha rays (or particles) forming helium.
The radium from which the emanation has been abstracted, after the lapse of an hour or so, loses 75% of its activity. During the course of a single month, radium will be found to have restored all its lost emanation. In thirty days it will have regained all its original activity. It was soon discovered that the emanation abstracted from the radium loses its radioactivity at the same rate and according to the same laws as the de-emanated radium regains it. The radium is therefore said to be “in equilibrium with its products.”
Since these processes are wholly outside the sphere of known controllable forces, and cannot be created, altered or destroyed—“since the process is independent of the chemical form of the radium, whether bromide, chloride, sulphate, etc., we are absolutely shut up to the conviction that it is a function of the atom. We are in the presence of an actual decay of the atom. The atom of radium breaks down into atoms of emanation and the atoms of emanation in their turn break down into something else. The activity of emanation decays and falls to half value in about 3.7 days.”
Although the amount of emanation produced from a gram of radium does not amount to more than a needle-point of the gas (= 1.3 cubic millimeter), this is sufficient to raise the temperature of 75 grams of water 1° per hour, which is enough heat to meltmore than its own weight of icein an hour, and to raise it to the boiling-point in the next hour, which is equivalent to 60,000 horse-power days! In other words, the heat evolved by the radium emanationis more than 3,500,000 times greater than that produced in any known chemical reaction: such as, for example, the union of oxygen and hydrogen to form water.
It was soon discovered that if the spectrum of this mysterious gas—or radium emanation—be examined again after an interval of about four weeks, it has changed into a familiar spectrum easily recognized as that of the gaseous element known as helium. Here the chemist comes face to face with the astounding fact that the element radium is decomposed and produces another element, helium—a discovery made by Ramsay and Soddy in the summer of 1903.
In the successive radioactive changes, one Alpha particle (sometimes called “ray”) is ejected from each atom disintegrated by the change—in some cases, at least, accompanied by Beta particles (negative electrons). The Alpha particle, as already stated, is really an atom of helium carrying two atomic charges of positive electricity—twice that of an atom of hydrogen. Strictly speaking, the Alpha particle is only thenucleusof a helium atom, since it has lost two of its negatively charged electrons, which are combined in the ordinary helium atom. The exact velocity of the expelled Alpha particle “varies in the different radioactive elements” (Joly)—say from 10,000 to 18,000 miles each second—a velocity sufficient to carry the particle around the earth in less than two seconds, if unchecked.
But these relatively heavy particles (of atomic size) are actually soon checked, evenby seven centimeters (about a third of a foot) of air. The Beta particle (1,845 the mass of a hydrogen atom) “shoots a hundred times as far [as the Alpha particle] and the Gamma rays are a hundred times more penetrating still” (Millikan). But the Alpha particle is sometimes ejected with a velocity nearly 40,000 times that of a rifle bullet,—the velocity of the latter being about half a mile a second. Even the super-guns which bombarded Paris could not eject a projectile with a speed of more than about a mile a second. Rutherford observes that if it were possible to give an equal velocity to an iron cannon ball, the heat generated on a target would be many thousand times more than sufficient to melt the cannon ball and dissipate it into vapor.
The flashes of light seen when the Alpha rays bombard a screen of zinc sulphide, as in Crookes’ spinthariscope, are due to cleavages produced in the zinc sulphide crystals by the impact of the Alpha rays (positive ions). Each impact on a crystal produces a splash of light big enough to be seen by a microscope.
In the phosphorescence caused by the approach of an emanation of radium to zinc sulphate, the atoms throw off the Alpha (helium) particles to the number of five billion each second, with velocities of 10,000 miles or more a second. If the helium projectile should chance to “crash” into an atom of nitrogen or of oxygen, an atom of hydrogen can be knocked out of it, as was discovered by Sir Ernest Rutherford, perhaps the most distinguished of Mme. Curie’s pupils. (Strictly speaking, thedisintegration particles are isotropes of helium, of atomic weight 3, the atomic weight of helium being 4.) Despite its large size as compared with an electron (or Beta particle), the Alpha particle passes through a glass wall without leaving a hole behind, and without in any way interfering with the molecules of the glass. It shoots through hundreds of thousands of atoms without ever going near enough to them to be deflected from its course.