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Rutherford and Curie readings

Rutherford and Curie readings - 318 THE DEATH OF CERTAINTYZ...

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Unformatted text preview: 318 THE DEATH OF CERTAINTYZ SCIENCE AND WAR of hydrogen gas. As for the molecule of sulphuric acid, it cannot be determined, for we do not know whether there is further division of the molecule on its formation, or not.4 ' . Dalton had supposed that sulphuric ac1d was composed of two molecules of oxygen to one of radical and sulphurous acid of one molecule of oxygen to one of sulphur. These two assumptions are incompatible, for according to Gay-Lussac s results the quantities of oxygen in these two ac1ds for a given quantity of radical, are represented by 1 and 1 1/ 2. Besides, in his determination of the molecule he set out from a wrong value for the composmon of sulphuric acid, and it is only by chance that the mass 15 which he assigns to it, bears to his value for the mass of the oxygen molecule a ratio which approaches that presented by these two substances on our hypothesis. Let us now see what conjecture we may form as to the mass of the molecule of a substance which plays in nature a far greater part than sulphur or phospho- rus, namely, that of carbon. As it is certain that the volume of carbonic acid is equal to that of the oxygen which enters into it, then, if we admit that the volume of carbon, supposed gaseous, which forms the other element, is doubled by the d1v1sion of its molecules into two, as in several combinations of that sort, it will be necessary to suppose that this volume is the half of that of the oxygen with which it combines, and that consequently carbonic aCid results from the union of one molecule of carbon and two of oxygen, and is therefore analogous to sul— phurous and phosphorous acids, according to the preceding suppositions. In this case we find from the proportion by weight between the oxygen and the carbon, that the density of carbon as gas would be 0.832 with respect to that of air as unity, and the mass of its molecule 11.36 with respect to hydrogen. There is, however, one difficulty in this supposition, for we give to the molecule of carbon a mass less than that of to nitrogen and oxygen, whereas one would be inclined to attribute the solidity of its aggregation at the highest temperatures to a higher molecular mass, as is observed in the case of the sulphuric and phos— phoric radicals. We might avoid this difficulty by assuming a division of the molecule into four, or even into eight, on the formation of carbonic ac1d, for in that way we should have the molecule of carbon twice or four times as great as that we have Just fixed But such a composition would not be analogous to that of the other acids; and, besides, according to other known examples, the assumption or not of the gaseous state does not appear to depend solely on the magnitude of the molecule, but also on some other unknown property of substances. Thus we see sulphurous acid in the form of a gas at the ordinary temperature and pressure of the atmosphere notwithstanding its large molecule, which is almost equal to that of the solid sulphuric radical. Oxy- genated muriatic that of the solid sulphuric radical. Oxygenated muriatic acid gas has a densrty, and con- sequently a molecular mass, still more con51derable. Mercury, which as we shall see further on, should have an extremely large molecule, is nevertheless gaseous at a temperature infinitely lower than would be necessary to vaporise iron, the molecule of which is smaller. Thus there is nothing to prevent us from regarding carbonic acid to be composed 1n the manner indicated above—and therefore analogous to nitric, sulphuric, and phosphoric acids—and the molecule of carbon to have a mass expressed by 11.36. - . Dalton has made the same supposition as we have done regarding the composition of carbonic ac1d, and has consequently been led to attribute to carbona molecule equal to 4.4, which is almost in the same ratio to his value for that of oxygen as 11.36 is to 15, the mass of the molecule of oxygen according to us. Assuming the values indicated for the mass of the molecule of carbon and the density of its gas, car— bonic oxide will be formed, according to the experi- ments of M. Gay-Lussac, of equal parts by volume of carbon gas and oxygen gas; and its volume Wlll be equal to the sum of the volumes of its constituents. it will accordingly be formed of carbon and oxygen united molecule to molecule, with subsequent halving—~all in perfect analogy to nitrous gas. . . The mass of the molecule of carbonic ac1d Will be— 11.36 + 2 x 15.74 _ _ 1.5196 2 " 2075 " 0.07321 ’ and that of carbonic oxide will be— 11.36 + 15.074 13 0.96782 ‘ 7 0.07321 ’ ERNEST RUTHERFORD, THE NEWER ALCHEMY According to this theory it is evident that if the oxy- genicity of two acids and two alkalies which combine respectively in pairs, is not extremely different, and if at the same time the mass of the molecule of one of the acids is not in a different ratio to its alkali from that of the other acid with regard to its own alkali the ratio between the numbers of molecules which gives neutrality may be the same in both compounds; but in the contrary case, the ratio may vary in such a way that instead of the equality of volumes, or of combi- nation molecule to molecule which we see between carbonic and a few other acids on the one hand and ammonia on the other, there may be other simple ratios such as 1 to 2, &c., which give the neutral state. Nevertheless, the simplicity which will always exist amongst these ratios, in conjimction with the infor~ mation we may obtain from other sources as to the mass of the molecules and the degree of oxygenicity of the components, will sometimes put us in a posi- tion to determine, or at least conjecture, what are the simple ratios which may occur in a given case; but it is the task of experiment to confirm or correct these theoretical estimates. Notes 1 Thus, for example, the integral molecule of water will be composed of a half-molecule of oxygen with one molecule, or, what is the same thing, two half— molecules of hydrogen. 2 In what follows I shall make use of the exposition of Dalton’s ideas given in Thomson’s System of Chem— istry. (See Alembic Club Reprints, No. 2, p, 42.) 3 This was written before I had seen the Memoir of Davy on oxymuriatic acid, which also contains new experiments on sulphur and phosphorus. In it he determines the density of sulphurous acid gas, and finds it to be only 2.0967, which gives new force to the above considerations. If we adopt this density, we find that in sulphuric acid 167 instead of 138; but perhaps this density of sulphurous acid, according to Davy, is somewhat too low. 4 Davy, in the Memoir alluded to, has made the same suppositions as to the relative number of molecules of oxygen and radical in sulphurous and sulphuric acids. From his determination of the density of sul- phurous acid gas, the density of the sulphuric radical would be 1.9862, and its molecule 27.13, that of hydrogen being taken as unity. Davy, by a similar cal- culation, fixes it at about half viz., 13.7, because he supposes the molecule of oxygen to be equal to about half our molecule, using Dalton’s hypothesis with 319 respect to water. He finds nearly the same mass, viz., 13.4, by taking as his starting-point the density of sulphuretted hydrogen, which his experiments make equal to 1.0645, a result only slightly different from Kirwan’s, and by assuming that this gas (which contains, as we know, its own volume of hydrogen combined with sulphur) is composed of one molecule of sulphur and one of hydrogen. As we suppose the molecule of sulphur to be nearly twice as great, we must assume that this gas is the product of the iuiion of one mole— cule of the radical with two at least of hydrogen, and that its volume is twice that of the gaseous radical, as in so many other cases. I say at least with two mole— cules of hydrogen, for if there were hydrogen already in ordinary sulphur, as known experiments on this substance indicate, its quantity also must be added. If, for instance, ordinary sulphur were composed of one molecule of sulphuric radical and one of hydro— gen, sulphuretted hydrogen would be composed of three molecules of hydrogen and one of radical. This could be decided by the comparison of the specific gravity of sulphuretted hydrogen and sulphurous acid gas, if both were known exactly For example, supposing Davy’s determination for sulphuretted hydrogen to be exact, the molecule of the sulphuric radical, on the supposition of only two molecules of hydrogen, would be 27.08, that of hydrogen being taken as unity; but on the supposition of three mole- cules of hydrogen, 27.08 would still be the Stun of one molecule of radical and one of hydrogen, so that the former would be reduced to 26.08. If the exact density of sulphurous acid gas confirmed one or other of these results, it would confirm by that means one or other of these hypotheses: but we are not suf- ficiently agreed about these densities to be able to draw any conclusion in this respect from the determi- nations hitherto existing. 8.6 Ernest Rutherford, The Newer Alchemy Ernest Rutherford (1871—1937) was born in Nelson, New Zealand and began his career as a physicist at Canterbury College, Christchurch. He moved to Cambridge to study with I]. Thomson. With Frederick Soddy, in 1903, he published an article arguing that radioactive substances were in fact going through a series of transformations as pieces (appearing as alpha and beta rays) broke off. With H. Geiger and E. Marsden, he studied the deflection of alpha particles passing through foil, 320 THE DEATH OF CERTAINTYZ SCIENCE AND WAR and from these experiments determined the struc— ture of the atom, consisting mostly of empty space with a dense nucleus and orbited by electrons. With the help of Niels Bohr, who applied quantum theory to the results, the Bohr—Rutherford model of the atom was developed. Rutheiford won the Nobel Prize in 1908 and zoos created Lord Rutherford of Nelson in 1931. In this lecture I shall give a brief account of modern work on the transmutation of the elements. The title is intended to suggest a contrast to that ancient form of alchemy which had such an extraordinary fasci- nation for the human mind for nearly two thousand years. The belief in the possibility of the transmuta— tion of matter arose early in the Christian Era. The search for the Philosopher’s Stone to transmute one element into another, and particularly to produce gold and silver from the common metals, was unremittingly pursued in the Middle Ages. The exis- tence of this idea through the centuries was in no small part due to a philosophic conception of the nature of matter which was based on the authority of Aristotle. On this View, all bodies were supposed to be formed of the same primordial substance, and the four elements, earth, air, fire and water, differed from one another only in possessing to different degrees the qualities of cold, wet, warm and dry. By adding or subtracting the degree of one or more of these qualities, the properties of the matter should be changed. To the alchemists, imbued with these con- ceptions, it appeared obvious that one substance could be transmuted into another if only the right method could be found. In the early days of Chem~ istry, when the nature of chemical combination was little understood, the marked alteration of the appearance and properties of substances by chemi- cal action gave support to these Views. From time to time there arose a succession of men who claimed to have discovered the great secret, but we have the best reasons for believing that not a scintilla of gold was ever produced. When we look back from the standpoint of our knowledge today, we see that transmutation was a hopeless quest with the very limited facilities then at the disposal of the experi- menters. With the development of experimental science and the steady growth of chemical knowl- edge, the ideas of transmutation were gradually dis— carded and ceased to influence the main advance of knowledge. At the same time these old alchemistic ideas have persisted in the public mind, and even to this day impostors or deluded men appear who claim to have a recipe for making gold in quantity by transmutation. These Charlatans are often so con- vincing in their scientific jargon that they disturb for a time the sleep of even our most hard—headed fin— anciers. We shall see that it is now possible by modern methods to produce exceedingly minute quantities of gold, but only by the transmutation of an even more costly element, platinum. As the knowledge of Chemistry grew the old idea of transmutation was seen to be untenable. It was found that matter could be resolved into eighty or more distinct elements, the atoms of which appeared to be permanent and indestructible. The ordinary physical and chemical forces then at our command appeared to be unable to alter in any way the atoms of the elements. This idea of the perma— nency of the atoms received a rude shock when it was found in 1902 that the atoms of two well—known elements, uranium and thorium, were undergoing a veritable process of spontaneous transformation, although at a very slow rate. This conclusion fol« lowed from the discovery of the radioactivity of these two heavy elements which spontaneously emit penetrating types of radiation capable of blackening photographic plates and discharging an electrified body. This radioactivity is a sign of the instability of the atoms concerned. Occasionally an atom breaks up spontaneously with explosive violence hurling from it either a fast a— or y—particle. The a-particle is a charged atom of helium of mass 4 which is shot out at a speed of about 10,000 miles per second. The B- particle, which is another name for the light negative electron, is generally expelled with a much higher speed. Sometimes a penetrating radiation of the X- ray type, known as the 'y—rays, accompanies the transformation. RADIOACTIVE TRANSFORMATIONS If we take a gram of the element uranium, about 24,000 atoms break up per second with the emission of an ale-particle. Yet the number of atoms in a gram is so great, that it would take about 4500 million years before half of the atoms are transformed. As a ERNEST RUTHERFORD, THE NEWER ALCHEMY result of the emission of an a~particle of mass 4 from the uranium atom of weight 238, a new atom is formed of atomic mass 234. The atoms of this new element are very tuistable and break up rapidly with the emission of a swift B—particle from each atom. This process of transformation, once started, contin— ues through a succession of stages, each unstable atom giving rise to another. The well-known element radium has its origin from the transformation of uranium, and is the fifth product in the series. The activity of a radioactive body, measured by the specific radiation it emits, diminishes with time according to a geometrical progression. If the activ- ity falls to 1/ 2 in a time T, known as the half-period, it falls to 1/4 in a time 2T, to 1/8 in a time 3T, and so on. It can readily be calculated that at a time ZOT the activity has decreased to less than one-millionth of the initial value. This law of decay holds universally for all radioactive bodies; but the half—period T, which has a characteristic value for each active body, varies enormously for different substances. For example, the half—period of uranium is 4500 million years and for radium 1600 years, but is only one-mil- lionth of a second for one of the products of radium known as radium C’. This law of decay is an expres- sion of the fact that number of atoms breaking up in unit time is on average always proportional to the munber of unchanged atoms present. Such a result is to be expected if the individual atoms break up according to the laws of chance. The wonderful sequence of transformations which occur in uraniiun is shown in Fig. 1, Where the circles represent the nuclei of the successive atoms which are formed. The half~period of transformation . D: U1 CALM . U I Hmnnrmu+5x109vm DAYS mam ~~ 2+?!» HALF PERIOD) 305mm 266 MINUIES 197mm IHMCONDS AMI)?!- “ 1"” 5‘ T” Fig. 1.. Uranium series of elements. The u atomlc mass, the lower the atomic numb the broad arrow shows the relative dista H4 MINUTE; 105 was 321 is added below, while the nature of the particle expelled, whether a or B, is indicated. It would take too long to discuss the methods by which this sequence of changes has been definitely established, but attention should be draw to the extraordinary simplicity of the relations which connect together the whole series of transformations. We now know that the chemical properties of an element are defined by its atomic number, which also represents the number of natural units of charge on the atomic nucleus. Since electricity is atomic in character, the nuclear charge is always given by a whole number, which varies from 1 for the lightest nucleus, hydrogen, to 92 for the heaviest element, uranium. The atomic number of each nucleus and also its atomic mass in terms of O = 16 are shown within the circles. The a- or ,B—particle which is liberated has its origin in the nucleus itself. The expulsion of an a-particle, which carries two positive units of charge and has a mass 4, thus lowers the atomic number of the resid- ual nucleus by two units and its mass by four units. On the other hand, the expulsion from the nucleus of a B-particle which carries a unit charge of negative electricity raises the nuclear charge by one unit. On account of the very light mass of the ,B-particle, the mass of the resulting atom is to a first approximation unchanged. These simple considerations based on the nature of the radiation emitted serve to explain in a satisfactory way the atomic number and mass of all the elements in the long sequence. It is now well established that mass and energy are equivalent. Knowing the precise mass of the a-particle (helium Rn D Ra E Ra F Ra G 75 YEARS 5-0 DAYS ’36 DAYS 0" Pb El Te Pb pper number in each circle gives the er and nuclear charge. The length of nce of travel of the oz—particles. 322 THE DEATH OF CERTAINTY: SCIENCE AND WAR nucleus) and the maximum kinetic energy of the expelled a— or fi-particle, it is possible to calculate exactly the atomic masses of all the atoms in the series provided the atomic mass of uranium is known. The end product of the series, which shows no trace of activity, has the same atomic number as lead, but its atomic mass 206 differs from that of ordinary lead 207 ' 2. ELEMENTARY PARTICLES The transformation of the radioactive bodies brought to our knowledge the swift a- or B—particles as probable constituents of a heavy atomic nucleus. Subsequent research on the transmutation of the ordinary elements has disclosed the existence of several other types of elementary particles which are liberated as the result of explosions of atomic nuclei. The most important of these new particles are the proton, neutron, deuteron and positive electron. The proton is another name for the hydrogen nucleus of charge 1 which has a nuclear mass of 1 - 0076. The neutron is an uncharged particle of mass slightly greater than the proton, namely 1 ' 0090. It is now believed that these two particles, the proton and neutron, are closely related to each other. Under the intense forces which exist in atomic nuclei, it is believed that the neutron may be converted into a proton by the removal from it of a negative electron. Conversely, the proton may be converted into a neutron by the addition of a negative electron. While we have no definite proof so far of such mutual con- versions, the general evidence certainly supports the idea that there exists a definite connection between these two particles. It is natural to suppose that the neutron is a very close combination of a proton and a negative electron, although at the moment the explanation of the differences in masses between these two particles presents certain difficulties. Th...
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