Radium and the Transmutation of Metals

A.D. 1903

MME. MARIE CURIE SIR WILLIAM RAMSAV
SIR OLIVER LODGE

The dream of alchemy has come true. Our scientists of today have seen with their own eyes the thing which their predecessors had for centuries ridiculed as an absurdity. We have watched while one element, a metal supposedly unchangeable and eternal, disintegrated and turned into another. The ancient alchemists dreamed of turning lead or copper into gold. We have not accomplished this; probably we shall never accomplish it. But we have been compelled to reconstruct our most fundamental scientific theories as to the permanence of matter. Upon the basic idea that the elements were eternal and unchangeable, we had founded all our knowledge of the material universe. Now our scientific thinkers are reaching out in new directions; they have been set tossing on a boundless sea of wonderment and awe.

This change sprang not merely from the discovery of radium, which was first revealed in 1898 by the two able French chemists, M. Curie and his wife. Their discovery simply added a new element to the eighty or more already known. But the new element had strange properties that drew all chemists to investigating it; and in the year 1903 several experimenters reached the startling conclusion of radium’s change into other metals. The honor of this remarkable and far-reaching discovery must thus be apportioned among various scientists. M. Curie died in 1906, but his wife continued alone along their path of investigation. Professor Rutherford of Montreal and Professor Soddy of Glasgow are among those who have shared with her most prominently in the surprising series of discoveries. We give here Mme. Curie’s own explanation of her work, then the first hesitant acceptance of the new idea by a great leading scientist, Sir Oliver Lodge, and then the later full review of the meaning of the facts as recognized by Sir William Ramsay, President of the British Science Association.

More recent experiments seem to bring copper also within the list of transmutable metals. Indeed, as an American scientist recently summed up our present knowledge: "The elements may no longer be considered immutable. Matter is probably of but a single sort, of which our commonest elements represent the more stable forms, which have resulted from a process of natural evolution."

MME. MARIE CURIE

WHEN one reviews the progress made in the department of physics within the last ten years, he is struck by the change which has taken place in the fundamental ideas concerning the nature of electricity and matter. The change has been brought about in part by researches on the electric conductivity of gas, and in part by the discovery and study of the phenomena of radioactivity. It is, I believe, far from being finished, and we may well be sanguine of future developments. One point which appears today to be definitely settled is a view of atomic structure of electricity, which goes to conform and complete the idea that we have long held regarding the atomic structure of matter, which constitutes the basis of chemical theories.

At the same time that the existence of electric atoms, indivisible by our present means of research, appears to be established with certainty, the important properties of these atoms are also shown. The atoms of negative electricity, which we call electrons, are found to exist in a free state, independent of all material atoms, and not having any properties in common with them. In this state they possess certain dimensions in space, and are endowed with a certain inertia, which has suggested the idea of attributing to them a corresponding mass.

Experiments have shown that their dimensions are very small compared with those of material molecules, and that their mass is only a small fraction, not exceeding one one-thousandth of the mass of an atom of hydrogen. They show also that if these atoms can exist isolated, they may also exist in all ordinary matter, and may be in certain cases emitted by a substance such as a metal without its properties being changed in a manner appreciable by us.

If, then, we consider the electrons as a form of matter, we are led to put the division of them beyond atoms and to admit the existence of a kind of extremely small particles, able to enter into the composition of atoms, but not necessarily by their departure involving atomic destruction. Looking at it in this light, we are led to consider every atom as a complicated structure, and this supposition is rendered probable by the complexity of the emission spectra which characterize the different atoms. We have thus a conception sufficiently exact of the atoms of negative electricity.

It is not the same for positive electricity, for a great dissimilarity appears to exist between the two electricities. Positive electricity appears always to be found in connection with material atoms, and we have no reason, thus far, to believe that they can be separated. Our knowledge relative to matter is also increased by an important fact. A new property of matter has been discovered which has received the name of radioactivity. Radioactivity is the property which the atoms of certain substances possess of shooting off particles, some of which have a mass comparable to that of the atoms themselves, while the others are the electrons. This property, which uranium and thorium possess in a slight degree, has led to the discovery of a new chemical element, radium, whose radioactivity is very great. Among the particles expelled by radium are some which are ejected with great velocity, and their expulsion is accompanied with a considerable evolution of heat. A radioactive body constitutes then a source of energy.

According to the theory which best accounts for the phenomena of radioactivity, a certain proportion of the atoms of a radioactive body is transformed in a given time, with the production of atoms of less atomic weight, and in some cases with the expulsion of electrons. This is a theory of the transmutation of elements, but differs from the dreams of the alchemists in that we declare ourselves, for the present at least, unable to induce or influence the transmutation. Certain facts go to show that radioactivity appertains in a slight degree to all kinds of matter. It may be, therefore, that matter is far from being as unchangeable or inert as it was formerly thought; and is, on the contrary, in continual transformation, although this transformation escapes our notice by its relative slowness.

Let us now consider the essential facts revealed by the study of radioactive substances, and examine them from the point of view of the hypothesis of the atomic transformation of matter. Among the radioactive elements, some appear to be permanently active (uranium, thorium, radium, actinium), while others lose their radioactivity little by little (polonium). The most powerful representative of the permanently radioactive substances is radium. According to the theory of transformation this substance changes very slowly, so that a given mass of radium would lose half its weight only in several thousand years. Consequently the quantity of radium which disappears from a gram of this substance in an hour is absolutely inaccessible to experiments. However, a gram of radium disengages each hour about 100 calories of heat. To conceive how enormous this disengagement of heat is, we remark that during the life attributable to radium the complete transformation of a gram of this substance would produce as much heat as the combustion of a ton of coal. The transformation of radium, then, if transformation there be, is not to be regarded as an ordinary chemical reaction, for the quantity of heat involved is of a far higher order. One is led to conceive, rather, that the atoms themselves are transformed, for the quantities of energy put in play in the formation of atoms are probably considerable.

Indeed, the phenomenon of radioactivity has a palpably atomic character, which was brought to light in the beginning of researches on the subject. It was precisely the absolute conviction that we were dealing with an atomic phenomenon which led M. Curie and me to the discovery of radium. If the radioactivity can not be separated from the atom it is very difficult to conceive anything but the atom itself involved in the transformation.

The effects produced by radium are very powerful, considering how small is the quantity of this substance at disposal for experiments. There is a spontaneous and continuous emission of rays, analogous to those which we know are produced by means of an induction coil in a Crookes tube, and these rays produce ionization of gas in the same manner. They are able, for example, to produce the rapid discharge of an electroscope. The energy of the rays is so great that the discharge is produced even across a thick metallic screen, for the rays can traverse such a screen.

Some of the rays comprise electrified particles moving with very great velocity. Some are charged positively, and their dimensions are comparable with those of atoms; while others are negative electrons, whose electric charge may be shown by direct experiments. Admitting that all these projectiles come from the atoms of radium themselves, it is difficult to avoid the conclusion that the departure of a positive particle must necessarily cause a modification of the atom which expels it.

Among the electrons emitted there are some whose velocity is enormous, and is in fact no less than nine-tenths the velocity of light. It has been found that the mass of these projectiles (which are the most rapid that we know of) is greater than that of slower-moving electrons, and this result may be considered as a confirmation of the theory according to which the mass of an electron is regarded as the result of electromagnetic phenomena.

The energy of the rays of radium is also manifested by their capacity for exciting the luminosity of various phosphorescent substances. Radium salts are, indeed, themselves luminous and the light is readily visible in certain conditions.

Here are now a new series of facts which are interpreted by the theory of radioactive transformation. Radium disengages continuously a substance which behaves like a gaseous radioactive material and which has received the name of the emanation. Air which has been in contact with a solution of radium salts is charged with the emanation, and may be drawn away and studied. Air containing the emanation is strongly conducting. A sealed glass tube in which the emanation has been imprisoned acts on the outside like a radioactive substance, and is able, for example, to discharge an electroscope. When the emanation is drawn into a flash containing zinc sulphide, the latter becomes luminous. The emanation is an unstable gas and spontaneously disappears, even from a sealed glass tube, at a rate in accord with a strict law, by which a given quantity of emanation diminishes by half in about four days. The emanation possesses the property of imparting radioactivity to all the bodies in contact with it, and such bodies are said to possess induced radioactivity.

In the theory of atomic transformation the emanation of radium is the first product of disintegration and is transformed in its turn. The induced radioactivity to which it gives rise is considered as due to a solid radioactive material, which results from the transformation of the radium emanation. Three different radioactive materials are distinguished in the induced radioactivity, which constitute three successive terms of the transformation. Each transformation is also accompanied by the emission of rays, and the expelled particles are also counted among the resulting products.

Induced radioactivity does not disappear completely; but there remains after the lapse of a day a very feeble residue which persists in part for years, and which is believed to be adding new terms to the series of successive transformations.

A new fact of great interest has come to the support of the theory of the transmutation of radioactive substances, and has, indeed, made it almost indispensable. It has been proved that radium, a perfectly definite chemical element, produces continually another perfectly definite chemical element, helium (Ramsay and Soddy). It is admitted that helium is one of the products of the disintegration of the atom of radium, and it is noteworthy that helium occurs in all the radium-bearing minerals.

The theory of the radioactive transformation has been extended to all the radioactive bodies, and investigations have been made to determine if the radioactive substances heretofore considered as elements are not to be derived from one another. The origin of radium itself has been sought in uranium. It is well known that radium is found in the uranium-bearing minerals, and it appears from recent researches that the proportion between the quantities of radium and uranium is the same in all these minerals. Uranium may, then, be thought of as a mother substance, which disintegrates with extreme slowness, giving place to the production of radium and the products which succeed it. It appears also to be probable that the last term of the radioactive series is polonium. It may be recalled that uranium was the substance in which the property of radioactivity was discovered by M. Becquerel, and polonium is the first new substance which was discovered by the aid of radioactivity.

A series of analogous considerations has been established for another radioactive substance thorium. In this case thorium as a primary substance generates radiothorium, a substance recently discovered, which gives rise to the gaseous radioactive emanation of thorium and various products of radioactivity induced by this emanation. Actinium also gives place to a series of transformations similar to those of thorium, and it, like radium, produces helium.

I have already stated that radioactivity is a general property of matter. If the theory of radioactive transformation continues to inspire a growing degree of confidence, it will result in an important consequence for geology, and will lead to a careful study of the proportions of the elements occurring in rocks, with a view to deduce their relative ages.

It is plain that the hypothesis of radioactive transformation is well adapted to the present state of the science of radioactivity. It was among those proposed by M. Curie and myself at the beginning of our researches on radioactivity; but it has received its precise development by Rutherford and Soddy, to whom it is for this reason generally attributed. It seems to me, however, better not to leave the domain of demonstrated facts, and not to lose sight of other explanations of radioactivity which have been proposed. The actual state of the science does not seem to me far enough advanced to warrant a positive conclusion.

In closing, the general importance of the phenomena of radioactivity may be recalled. For physics the radioactive substances constitute a new implement of research in consequence of the rays they emit, and they have actively contributed to the development of the theory of the conduction of gas and of the nature of the electron. By their numerous chemical and physiological effects, and their possible influence on meteorology, these substances extend their sphere of action in the domain of all the science of nature; and it is probable that their importance for the development of science will go on increasing. Finally, it has been shown that there is nothing absurd in supposing that the energy we receive from the sun may be in part, or even in total, due to the presence of radio-active bodies which it may contain.

SIR OLIVER LODGE

Radium, like other far less active substances previously discovered, is constantly emitting, without apparent diminution, three kinds of rays: rays called r, which appear to be chiefly of the same nature as the x-rays of Rontgen; rays called B, or cathodic, which are found to consist of extremely minute flying corpuscles or electrons negatively charged; and rays called a, which appear to be composed of projected and positively charged atoms of matter flying away at an immense speed measured by Professor Rutherford, of Montreal. The whole power of emission is designated radioactivity, or spontaneous radioactivity to distinguish it from the variety which can be artificially excited in several ways, and was discovered in the first instance as a bare experimental fact by M. Becquerel. The most prominent, the most usually and easily demonstrated kind, are the p rays; for these possess remarkable penetrating power and can excite phosphorescent substances or affect photographic plates and electroscopes after passing through a great length of air or even through an inch of solid iron. But although these are the most conspicuous, they are not the most important. The most important by far are the a rays, the flinging off of atoms of matter. It is probable that everything else is subordinate to this effect and can be regarded as a secondary and natural consequence of it.

For instance, undoubtedly radium or any salt of radium has the power of constantly generating heat: M. Curie has satisfactorily demonstrated this important fact. Not that it is to be supposed that a piece of radium is perceptibly warm, if exposed so that the heat can escape as fast as generated-it can then only be a trifle warmer than its surroundings; but when properly packed in a heat-insulating enclosure it can keep itself five degrees Fahrenheit above the temperature of any other substance enclosed in a similar manner; or when submerged in liquid air it can boil away that liquid faster than can a similar weight of anything else. Everything else, indeed, would rapidly get cooled down to the liquid-air temperature, and then cease to have any further effect; but radium, by reason of its heat-generating power, will go on evaporating the liquid continually, in spite of its surface having been reduced to the liquid-air temperature. But it is clear that this emission of heat is a necessary consequence of the vigorous atomic bombardment-at least, if it can be shown that the emission is due to some process occurring inside the atom itself, and not to any subsidiary or surrounding influences. Now that is just one of the features which are most conspicuous. Tested by any of the methods known, the radioactivity of radium appears to be constant and inalienable. Its power never deserts it. Whichever of its known chemical compounds be employed, the element itself in each is equally effective. At a red heat, or at the fearfully low temperature of liquid hydrogen, its activity continues; nothing that can be done to it destroys its radioactivity, nor even appears to diminish or increase it. It is a property of the atoms themselves, without regard, or without much regard, to their physical surroundings or to their chemical combination with the atoms of other substances. And this is one of the facts which elevate the whole phenomenon into a position of first-class importance.

The most striking test for radioactivity is the power of exciting phosphorescence in suitable substances: as, for instance, in diamond. Sir Wm. Crookes has shown that by bringing a scrap of radium, wrapped in any convenient opaque envelope, near a diamond in the dark, it glows brilliantly; whereas the "paste,’ variety remains dull. But although the excitation of phosphorescence is the most striking test and proof of the power of radioactivity, because it appeals so directly to the eye, it is by no means the most delicate test; and if that had been our only means of observation, the property would be still a long way from being discovered. It was the far weaker power of a few substances-substances found in Nature and not requiring special extraction and concentration, such as Madame Curie applied to tons of the oxide-of-uranium mineral called "pitchblende" in order to extract a minute amount of its concentrated active element-it was the far weaker power of naturally existing substances such as that of pitchblende itself, of thorium, and originally of uranium, which led to the discovery of radioactivity. And none of these substances is strong enough to excite visible phosphorescence. Their influence can be accumulated on a photographic plate for minutes, or hours, or days together, and then on developing the plate their radioactive record can be seen; but it is insufficient to appeal direct to the eye. In this photographic way the power of a number of minerals has been tested.

The emission of atoms does not seem, at first hearing, a very singular procedure on the part of matter. Many forms of matter can evaporate, and many others emit scent; wherein, then, lies the peculiarity of radioactive substances, if the power of flinging away of atoms at tremendous speed is their central feature? It all depends on what sort of atoms they are. If they are particles of the substance itself, there is nothing novel in it except the high speed; but if it should turn out that the atoms flung off belong to quite a different substance-if one elementary body can be proved to throw off another elementary body-then clearly there is something worthy of stringent inquiry. Now, Rutherford has measured the atomic weight of the atoms thrown off, and has shown that they constitute less than 1 per cent. of the atoms whence they are projected; and that they probably consist of the metal helium.

But the radioactivity of the substance itself-a substance like radium or thorium-is by no means the whole of what has to be described. When the emission has occurred, when the light atoms have been thrown off, it is clear that something must be left behind; and the properties of that substance must be examined too. It appears to be a kind of heavy gas, which remains in the pores of the radium salt and slowly diffuses away. It can be drawn off more rapidly by a wind or current of air, and when passed over suitable phosphorescent substances it causes them to glow. It is, in fact, itself radio-active, as the radium was; but its chemical nature is at present partly unknown. Its activity soon ceases, however, gradually fading away, so that in a few days or weeks it is practically gone. It leaves a radioactive deposit on surfaces over which it has passed; a deposit which is a different substance again, and whose chemical nature is likewise different and unknown. The amount of substance in these emanations and deposits is incredibly small, and yet by reason of their radioactivity, and the sensitiveness of our tests for that emission, they can be detected, and their properties to some extent examined. Thus, for instance, the solid deposit left behind by the radium emanation can be dissolved off by suitable reagents, and can then be precipitated or evaporated to dryness and treated in other chemical ways, although nothing is visible or weighable or detectable by any known means except the means of radioactivity. So that directly one of the chain of substances which emanate from a radioactive substance ceases to possess that particular kind of activity, it passes out of recognition; and what happens to it after that, or what further changes take place in it, remains at present absolutely unknown. So it is quite possible that these emanations and deposits and other products of spontaneous change may be emitted by many, perhaps all, kinds of matter, without our knowing anything whatever about it.

That being so, what is the meaning of the series of facts which have been here hastily summarized; and how are they to be accounted for? Here we come to the hypothetic and at present incompletely verified speculations and surmises, the possible truth of which is arousing the keenest interest. There are people who wish to warm their houses and cook their food and drive their engines and make some money by means of radium; it is possible that these are doomed to disappointment, though it is always rash to predict anything whatever in the negative direction, and I would not be understood as making any prediction or indicating any kind of opinion, on the subject of possible practical applications of the substance, except, as we may hope, to medicine. Applications have their place, and in due time may come within the range of practicability, though there is no appearance of them at present. Meanwhile the real points of interest are none of these, but of a quite other order. The easiest way to make them plain is to state them as if they were certain, and not confuse the statement by constant reference to hypothesis: guarding myself from the beginning by what I have already said as to the speculative character of some of the assertions now going to be made.

Atoms of matter are not simple, but complex; each is composed of an aggregate of smaller bodies in a state of rapid interlocked motion, restrained and coerced into orbits by electrical forces. An atom so constituted is fairly stable and perennial, but not infinitely stable or eternal. Every now and then one atom in a million, or rather in a million millions, gets into an unstable state, and is then liable to break up. A very minute fraction of the whole number of the atoms of a substance do thus actually break up, probably by reason of an excessive velocity in some of their moving parts; an approach to the speed of light in some of their internal motions-perhaps the maximum speed which matter can ever attain being presumably the cause of the instability. When the breakup occurs, the rapidly moving fragment flies away tangentially, with enormous speed-twenty thousand miles a second-and constitutes the a ray of main emission.

If the flying fragment strikes a phosphorescent obstacle, it makes a flash of light; if it strikes (as many must) other atoms of the substance itself, it gets stopped likewise, and its energy subsides into the familiar molecular motion we call "heat"; so the substance becomes slightly warmed. Energy has been transmuted from the unknown internal atomic kind to the known thermal kind: it has been degraded from regular orbital astronomical motion of parts of an atom into the irregular quivering of molecules; and the form of energy which we call heat has therefore been generated, making its appearance, as usual, by the disappearance of some other form, but, in this particular instance, of a form previously unrecognized.

Hitherto a classification of the various forms of energy¹

has been complete when we enumerated rotation, translation, vibration, and strain, of matter in the form of planetary masses, ordinary masses, molecules, and atoms, and of the universal omnipresent medium "ether," which is to "matter" as the ocean is to the shells and other conglomerates built out of its dissolved contents. But now we must add another category and take into consideration the parts or electrons of which the atoms of matter are themselves hypothetically composed.

The emission of the fragment is accompanied by a convulsion of the atom, minuter portions or electrons being pitched off too; and these, being so extraordinarily small, can proceed a long way through the interstices of ordinary obstacles, seeing, as it were, a clear passage every now and then even through an inch of solid lead, and constituting the x-rays; while the atoms themselves are easily stopped even by paper. But the recoil of the main residue is accompanied by a kind of shiver or rearrangement of the particles, with a suddenness which results in an x-ray emission such as always accompanies anything in the nature of a shock or collision among minute charged bodies; and this true ethereal radiation is the third or x-ray of the whole process, and, like the heat-production, is a simple consequence of the main phenomenon, which is the break-up of the atom.

The emission over, and the fragment of the atom gone, the residue is no longer radium, but is something else. What it is we do not yet know; but since it is produced in isolated atoms here and there, with crowds of foreign substance between, there is no cohesion or any continuity between its particles; they are separated like the atoms of a gas, or like the molecules of a salt in a very dilute solution in which there are millions or billions of times as many atoms of the solvent as there are of the dissolved salt. So they are easily carried away by any motion of the medium in which they are mechanically embedded; but they retain their individuality, and their radioactive power persists, because the breaking-up process is by no means finished, stability is far from attained: indeed, the instability is more marked that it was in the original substance; for whereas in the original substance only one single atom here and there out of a million of millions was affected by it, here in the diffusing emanation or first product of incipient atomic dissociation every atom seems unstable or at least to be in a very critical condition. So that in a time to be reckoned in minutes or days or months (according to the nature of the emanation, whether it be from thorium or radium, or uranium), a further breakdown has occurred in every atom; and so its accompaniment of radioactivity ceases. The radio-active power has disappeared from the emanation, but it has not wholly ceased: it has been transferred this time to a solid deposit which has been the residual outcome of the second break-up. For the atoms of this deposit also are unstable and break up, in a time which can be reckoned in months, days, or minutes, apparently in roughly inverse order to the duration of the parent emanation. Another and another substance has also been suspected, by Rutherford and Soddy, as the outcome of this third break-up; while gradually the radio-active power of the resulting emanations becomes imperceptible, and further investigation by present methods becomes impossible for lack of means of detection of sufficient delicacy.

Here, then, we appear to have, in embryo, a transmutation of the elements, the possibility of which has for so long been the guess and the desire of alchemists. Whether the progress of research will confirm this hypothesis, and whether any of the series of substances so produced are already familiarly known to us in ordinary chemistry, remains to be seen. It is not in the least likely that any one radioactive substance can furnish in its stages of collapse the whole series of elements; most likely one substance will give one series, and another substance will give another; and it may be that these emanations are new and unstable elements or compounds such as are not already known, or it may be that they approximate in properties to some of the known elements without any exact coincidence. The recognized elements which we know so well must clearly be comparatively stable and persistent forms, but it does not follow that they are infinitely stable and perpetual; the probability is that every now and then, whether by the shock of collision or otherwise, the rapidity of motion necessary for instability will be attained by some one atom, and then that particular atom will fling off the fragment and emit the rays of which we have spoken, and begin a series of evolutionary changes of which the details may have to be worked out separately for each chemical element.

If there be any truth in this speculation, matter is an evanescent and transient phenomenon, subject to gradual decay and decomposition by the action of its own internal forces and motions, somewhat as has been suspected and to some extent ascertained to be the case for energy. If it be asked, "How comes it, then, that matter is still in existence? Why has it not already all broken down, especially in these very radioactive and therefore presumably rapidly decadent forms of radium and the like?" the question naturally directs us to seek some mode of origin for atoms, to conjecture some falling together of their pristine material, some agglomeration of the separate electrons of which they are hypothetically composed, such as is a familiar idea when applied to the gravitational aggregates of astronomy which we call nebula and suns and planets.

We may also ask whether many other phenomena, known but not understood, are not now going to receive their explanation. The light of the glowworm and firefly and other forms of life is one thing which deserves study; the Brownian movements of microscopic particles is another. Are we witnessing in the Brownian movements any external evidence, exhibited by a small aggregate of an immense number of atoms, of the effects of internal rearrangement and emission of the parts of the atoms, going on from the free surface of the particle? And can it be that the light emitted by the glowworm-which is true light and not technical radioactivity, and yet which is accompanied by a trace of something which can penetrate black paper and affect a light-screened photographic plated is emitted because the insect has learned how to control the breaking-down of atoms, so as to enable their internal energy in the act of transmutation to take the form of useful light instead of the useless form of an insignificant amount of heat or other kind of radiation effect; the faint residual penetrating emission being a secondary but elucidatory and instructive appendage to the main luminosity?

Many more questions may be asked; and if the conjectures now rife are to any great extent confirmed, it is clear that many important avenues for fruitful experimental inquiry will be opened up. Among them an easy and hopeful line of investigation, lying in the path of persons favorably situated for physically examining the luminous emission of live animals, may perhaps usefully be here suggested.

And let me conclude by asking readers to give no ear to the absurd claim of paradoxers and others ignorant of the principles of physics, who, with misplaced ingenuity, will be sure to urge that the foundations of science are being uprooted and long-cherished laws shaken. Nothing of the kind is happening. The new information now being gained in so many laboratories is supplementary and stimulating, not really revolutionary, nor in the least perturbing to mathematical physicists, whatever it may be to chemists; for on the electric theory of matter it is the kind of thing that ought to occur. And one outstanding difficulty about this theory, often previously felt and expressed by Professor Larmor-that matter ought to be radioactive and unstable if the electric theory of its constitution were true-this theoretical difficulty is being removed in the most brilliant possible way.

SIR WILLIAM RAMSAY

In the days of the early Greeks the word "element" was applied rather to denote a property of matter than one of its constituents. Thus, when a substance was said to contain fire, air, water, and earth (of which terms a childish game doubtless once played by all of us is a relic), it probably meant that they partook of the nature of the so-called elements. Inflammability showed the presence of concealed fire; the escape of "airs" when some substances are heated or when vegetable or animal matter is distilled no doubt led to the idea that those airs were imprisoned in the matters from which they escaped; hardness and permanence were ascribed to the presence of earth, while liquidity and fusibility were properties conveyed by the presence of concealed water. At a later date the "Spagyrics" added three "hypostatical principles" to the quadrilateral; these were "salt," "sulphur," and "mercury." The first conveyed solubility and fixedness in fire; the second, inflammability; and the third, the power which some substances manifest of producing a liquid, generally termed "phlegm," on application of heat, or of themselves being converted into the liquid state by fusion.

It was Robert Boyle, in his "Skeptical Chymist," who first controverted these ancient and medieval notions, and who gave to the word "element" the meaning that it now possesses-the constituent of a compound. But in the middle of the seventeenth century chemistry had not advanced far enough to make his definition useful, for he was unable to suggest any particular substance as elementary.

The modern conception of the elements was much strengthened by Dalton’s revival of the Greek hypothesis of the atomic constitution of matter, and the assigning to each atom a definite weight. This momentous step for the progress of chemistry was taken in 1803; the first account of the theory was given to the public, with Dalton’s consent, in the third edition of Thomas Thomson’s "System of Chemistry" in 1807; it was subsequently elaborated in the first volume of Dalton’s own "System of Chemical Philosophy," published in 1808. The notion that compounds consisted of aggregations of atoms of elements united in definite or multiple proportions familiarized the world with the conception of elements as the bricks of which the universe is built. Yet the more daring spirits of that day were not without hope that the elements themselves might prove decomposable. Davy, indeed, went so far as to write in 1811: "It is the duty of the chemist to be bold in pursuit; he must recollect how contrary knowledge is to what appears to be experience. . . . To inquire whether the elements be capable of being composed and decomposed is a grand object of true philosophy." And Faraday, his great pupil and successor, at a later date, 1815, was not behind Davy in his aspirations when he wrote: "To decompose the metals, to reform them, and to realize the once absurd notion of transformation-these are the problems now given to the chemist for solution."

Indeed, the ancient idea of the unitary nature of matter was in those days held to be highly probable. For attempts were soon made to demonstrate that the atomic weights were themselves multiples of that of one of the elements. At first the suggestion was that oxygen was the common basis; and later, when this supposition turned out to be untenable, the claims of hydrogen were brought forward by Prout. The hypothesis was revived in 1842, when Liebig and Redtenbacher, and subsequently Dumas, carried out a revision of the atomic weights so some of the commoner elements, and shoed that Berzelius was in error in attributing to carbon the atomic weight 12.25 instead of 12.00. Of recent years a great advance in the accuracy of the determinations of atomic weights has been made, chiefly owing to the work of Richards and his pupils, of Gray, and of Guye and his collaborators, and every year an international committee publishes a table in which the most probable numbers are given on the basis of the atomic weight of oxygen being taken as sixteen. In the table for 1911, of eighty-one elements, no fewer than forty-three have recorded atomic weights within one-tenth of a unit above or below an integral number. My mathematical colleague, Karl Pearson, assures me that the probability against such a condition being fortuitous is 20,000 millions to one.

The relation between the elements has, however, been approached from another point of view. After preliminary suggestions by Dobereiner, Dumas, and others, John Newlands in 1862 and the following years arranged the elements in the numerical order of their atomic weights, and published in The Chemical News of 1863 what he termed his law of octaves-that every eighth element, like the octave of a musical note, is in some measure a repetition of its forerunner. Thus, just as C on the third space is the octave of C below the line, so potassium, in 1863 the eighth known element numerically above sodium, repeats the characters of sodium, not only in its physical properties-color, softness, ductility, malleability, etc.-but also in the properties of its compounds, which, indeed, resemble each other very closely. The same fundamental notion was reproduced at a later date, and independently, by Lothar Meyer and Dmitri Mendeleeff; and to accentuate the recurrence of such similar elements in periods, the expression "the periodic system of arranging the elements" was applied to Newlands’ arrangement in octaves. As every one knows, by help of this arrangement Mendeleeff predicted the existence of then unknown elements, under the names of eka-boron, eka-aluminum, and eka-silicon, since named scandium, gallium, and germanium, by their discoverers, Cleve, Lecoq de Boisbaudran, and Winckler. It might have been supposed that our knowledge of the elements was practically complete; that perhaps a few more might be discovered to fill the outstanding gaps in the periodic table.

But we are confronted by an embarras de richesse. The discovery of radioactivity by Henri Becquerel, of radium by the Curies, and the theory of the disintegration of the radioactive elements, which we owe to Rutherford and Soddy, have indicated the existence of no fewer than twenty-six elements hitherto unknown. To what places in the periodic table can they be assigned?

Beginning with radium, its salts were first studied by Madame Curie; they closely resemble those of barium. The atomic weight, too, falls into its place; as determined by Madame Curie and by Thorpe, it is 89.5 units higher than that of barium; in short, there can be no doubt that radium fits the periodic table, with an atomic weight of about 226.5. It is an undoubted element.

But it is a very curious one. For it is unstable. Now, stability was believed to be the essential characteristic of an element. Radium, however, disintegrates-that is, changes into other bodies, and at a constant rate. If one gram of radium is kept for 1,760 years, only hall a gram will be left at the end of that time; hall of it will have given other products. What are they? We can answer that question. Rutherford and Soddy found that it gives a condensable gas, which they named "radium emanation,’; and Soddy and I, in 1903, discovered that, in addition, it evolves helium, one of the inactive series of gases, like argon. Helium is an undoubted element, with a well-defined spectrum; it belongs to a well-defined series. And radium emanation, which was shown by Rutherford and Soddy to be incapable of chemical union, has been liquefied and solidified in the laboratory of University College, London; its spectrum has been measured, and its density determined. From the density the atomic weight can be calculated, and it corresponds with that of a congener of argon, the whole series being: helium, 4; neon, 20; argon, 40; krypton, 80; xenon, 130; unknown, about 178; and niton (the name proposed for the emanation to recall its connection with its congeners and its phosphorescent properties), about 222.4. The formation of niton from radium would therefore be represented by the equation: radium (226.4) = helium (4) + niton (222.4).

Niton, in its turn, disintegrates, or decomposes, and at a rate much more rapid than the rate of radium; half of it has changed in about four days. Its investigation, therefore, had to be carried out very rapidly, in order that its decomposition might not be appreciable while its properties were being determined. Its product of change was named by Rutherford "radium A," and it is undoubtedly deposited from niton as a metal, with simultaneous evolution of helium: the equation would therefore be:

niton (222.4) = helium (4) + radium A (218.4).

But it is impossible to investigate radium A chemically, for in three minutes it has half changed into another solid substance, radium B, again giving off helium. This change would be represented by the equation:

radium A (218.4) = helium (4) + radium B (214.4).

Radium B, again, can hardly be examined chemically, for in twenty-seven minutes it has half changed into radium C1. In this case, however, no helium is evolved; only atoms of negative electricity, to which the name "electrons" has been given by Dr. Stoney, and these have minute weight which, although approximately ascertainable, at present has defied direct measurement. Radium C1 has a half-life of 19.5 minutes, too short, again, for chemical investigation; but it changes into radium C2, and in doing so each atom parts with a helium atom, hence the equation:

radium C1 (214.4) = helium (4) + radium C2 (210.4).

In 2.5 minutes radium C2 is half gone, parting with electrons, forming radium D. Radium D gives the chemist a chance, for its half-life is no less than sixteen and a half years. Without parting with anything detectable, radium D passes into radium E, of which the half-life period is five days; and, lastly, radium E changes spontaneously into radium F, the substance to which Madame Curie gave the name "polonium," in allusion to her native country, Poland. Polonium, in its turn, is half changed in 140 days, with loss of an atom of helium, into an unknown metal, supposed to be possibly lead. If that be the case, the equation would run:

polonium (210.4) = helium (4) + lead (206.4).

But the atomic weight of lead is 207.1, and not 206.4; however, it is possible that the atomic weight of radium is 227.1, and not 226.4.

Attention has repeatedly been drawn to the enormous amount of energy stored up in radium and its descendants. That, in its emanation, niton is such that if what it parts with as heat during its disintegration were available, it would be equal to three and a half million times the energy available by the explosion of an equal volume of detonating gas-a mixture of one volume of oxygen with two volumes of hydrogen. The major part of this energy comes, apparently, from the expulsion of particles (that is, of atoms of helium) with enormous velocity. It is easy to convey an idea of this magnitude in a form more realizable by giving it a somewhat mechanical turn. Suppose that the energy in a ton of radium could be utilized in thirty years, instead of being evolved at its variable slow rate of 1,760 years for half-disintegration, it would suffice to propel a ship of 15,000 tons, with engines of 15,000 horse-power, at the rate of 15 knots an hour for thirty years-practically the lifetime of the ship. To do this actually requires a million and a half tons of coal.

It is easily seen that the virtue of the energy of the radium consists in the small weight in which it is contained; in other words, the radium-energy is in an enormously concentrated form. I have attempted to apply the energy contained in niton to various purposes; it decomposes water, ammonia, hydrogen, chlorid, and carbon dioxid each into its constituents; further experiments on its action on salts of copper appeared to show that the metal copper was converted partially into lithium, a metal of the sodium column; and similar experiments of which there is not time to speak indicate that thorium, zirconium, titanium, and silicon are degraded into carbon; for solutions of compounds of these, mixed with niton, invariably generated carbon dioxid, while cerium, silver, mercury, and some other metals gave none. One can imagine the very atoms themselves, exposed to bombardment by enormously quickly moving helium atoms, failing to withstand the impacts. Indeed, the argument a priori is a strong one; if we know for certain that radium and its descendants decompose spontaneously, evolving energy, why should not other more stable elements decompose when subjected to enormous strains?

This leads to the speculation whether, if elements are capable of disintegration, the world may not have at its disposal a hitherto unsuspected source of energy. If radium were to evolve its stored-up energy at the same rate that gun-cotton does, we should have an undreamed-of explosive; could we control the rate we should have a useful and potent source of energy, provided always that a sufficient supply of radium were forthcoming. If, however, the elements which we have been used to consider as permanent are capable of changing with evolution of energy, if some form of catalyzer could be discovered which would usefully increase their almost inconceivably slow rate of change, then it is not too much to say that the whole future of our race would be altered.

¹See the Philosophical Magazine for October, 1879.