THE INTERPRETATION OF RADIUM.*
Tuonan radioactivity is by far the youngest of the sciences— having barely got into its teens—the time has now come when we can safely take stock of its results, especially when we have Professor Rutherford and Mr. Soddy, two of its most distinguished founders, to act as guides. These results are extraordinarily interesting and important, alike because they have carried us at one stride further• into the intimate secrets of the material universe than man has ever previously pene- trated, and because they indicate some possibility of practical consequences which may revolutionize the whole system of society by abolishing the "struggle for existence," in so far as it is conditioned by the costliness of power. In centuries to come the first decade of the twentieth century may be taken as the epoch of the true birth of material civilisation, alike by the speculative philosopher and the historian of mankind.
Radioactivity, till quite lately, has been in the position which some thinkers predicate of the Fourth Dimension; it was all about us, but our senses were not attuned to perceive it. It is only seventeen years since Professor Henri Becquerel, in the course of an inquiry whether ordinary bodies emitted any rays akin to the newly discovered X or Röntgen rays, was fortunately led to experiment with some fine crystals of the double sulphate of uranium and potassium, which he had had lying by him in his laboratory at Paris for some fifteen years. These crystals were wrapped in two thicknesses of black paper and placed in the dark under a photographic plate, with
' (1) Radioactive Substances and 'their Radiations. By E. Rutherford. Cambriday : At the University Press. [15s. net.]--(2) The Interpretation or-Radium. By Frederick Soddy. London John Murray. [6s. net.]—
(3) Rad.um, and Radioactivity. By A. T. Cameran. London : [2s. 6d.]
an aluminium medallion interposed. After a time it was found that a well-marked photograph of the medallion had been secured, under conditions which quite precluded the possi- bility of ordinary light having been the agent of the picture. " The results of these experiments, which were communicated to the Academy of Sciences at Paris on February 24th, 18964. indicated that the salt of uranium emitted a type of radiation capable of penetrating through a considerable thickness of matter opaque to ordinary light. These simple experiments mark the discovery of a new property of matter—radioactivity —the further investigation of which was to lead to such remarkable consequences " as are fully explained in the three. admirable books now before us. A host of eager investigators, stimulated by the éclat of Dr. Rikagen's wonderful discovery, set to work on this hint. Foremost among them were the world-famous M. and Mme. Curie—he French, she Polish—who, in 1898, discovered the new element to which they gave the appropriate name of radium, since it is the most active of all radioactive substances. Since then the boundaries of science have been extended by the discovery of a whole unsuspected. world of energy existing within the atoms which were pre- viously supposed to be the ultinta ratio of the chemist and the physicist. It is this feature which gives the new science of radioactivity its vast importance, and which sets it on a footing of its own as an independent science—not a mere department of chemistry or physics. It deals with the ultimate constitution of matter, and reveals the existence of new sources of energy as far surpassing those furnished by coal or falling water as the steam-engine and the dynamo surpass the windlass or the horse-gin.
We may now take it as fairly established that radioactivity is a newly discovered property of matter, possibly shown by all kinds of matter under certain conditions, but normally associated with certain of the rarer metals, and especially with the new elements, radium and actinium, and the older ones,. uranium and thorium. It consists in the continual emission of rays of one or more of three well-marked types—the con- tinual emission of energy, that is to say, without any apparent loss of power or fresh supply of fuel. Of these types the Alpha and Beta rays consist of streams of material particles, • projected with so high a velocity that they can penetrate material substances which are quite opaque to light; in the case of the Beta rays this velocity approaches that of light itself—nearly 200,000 miles a second—a proposition which would have been thought grossly incredible only a few years ago, but is now established beyond the reach of reasonable doubt. The Gamma rays are probably a true wave-motion,. akin to the X rays. All these rays can be detected by their power of impressing a photographic plate, and their power of ionizing a gas—of making it conduct electricity, that is—sn that a gold-leaf electroscope brought under their influence is. promptly discharged. The latter test is so delicate that it has. made it possible for investigators to work with quantities of matter which no other physical method could even detect.. Radioactivity is, first and foremost, a science of the infinitely little. There is not yet an ounce of radium in existence in all the hundreds of laboratories where its properties are being studied and all the hospitals where its curative powers are being tried. That well-known toy, the spinthariscope, is charged by dipping a fine needle-point into an invisible- deposit of radium. From that amazingly minute particle of matter the electronic bombardment goes on for years without ceasing, each explosion lighting up the phosphorescent screen with a score or more of well-marked scintillations. The bulb- of a thermometer full of gas serves the investigator to carry out all the ordinary operations of the chemist dealing watt. gallons of oxygen or hydrogen. This, we may say in passing, is no field for the amateur or the sucking Davy. The price of pure radium bromide is at present about £16 per milli- gramme—say half-a-million sterling per ounce—and although this is an artificial price which has no relation to the difficulty of producing radium, there is no immediate likelihood of its being much diminished. And the appliances needed to deal with these minute specks of matter are so delicate and costly that only the well-equipped laboratory can provide them.
With all these obstacles in the way, it is remarkable how far knowledge has been advanced by the new science. In the first place, the "disintegration hypothesis" put forward in 1903 by Professor Rutherford and Mr. Soddy may now be said to ha. accepted as the only theory whioh eo-ordinates and explains-
all the observed facts. This theory has revolutionized our conceptions of matter and enlarged our ideas of the universe. It holds that the radioactive substances are elements in course of disintegration—that their atoms are not ultimate unchange- alle units, but are complex systems which break up while we watch them, giving birth to two or more totally different atoms. In the long series of radioactive elements which begins with uranium and almost certainly ends with lead— radium itself being a middle term in the series—the change from each element to the next succeeding one is due to the loss of an atom of helium, the rare gas which was first dis- covered by the spectroscope in the sun's atmosphere more than forty years ago, and was recognized on the earth by Sir William Ramsay in 1895. This must not be taken to mean that radium is a compound—in the chemical sense— of lead and atoms of helium. It seems rather to imply that the radioactive atom is a system, something like the solar system, in which a number of planetary electrons perform their orbits about a central sun. Probably the atoms of all substances are systems of this nature. The radioactive sub- stances are distinguished by the fact that their atoms are &instable, and every second some of them break up, throwing off a certain number of electrons which rearrange themselves into an atom of helium, while the remaining ones rearrange themselves into another kind of atom. When the last term in the series is reached, it seems that a permanently stable arrangement has been arrived at; lead, for instance, which seems to be the ultimate product of the uranium-radium series, shows no radioactivity which our present methods can detect. The whole of this beautiful generalization, which aevives the old speculations about some primitive Urstoff of the universe, is of course still hypothesis. What is practically certain is that the Alpha rays, from whatever substance they .are given off, are atoms of helium ; and so far nothing has been discovered inconsistent with Rutherford and Soddy's disintegration hypothesis, or Sir J. J. Thomson's still wider electronic theory of matter, which converts ultimate physics into the astronomy of the incredibly minute.
We can only glance at one other aspect of this fascinating
4' fairy tale of science." One of the most singular facts about radium salts is their steady emission of heat, by which they keep themselves a degree or two above the temperature of the
surrounding objects. Radium emits heat continuously at a rate sufficient to melt more than its own weight of ice per hour. When this fact was first announced in 1903 it excited much attention. We now know that this is only a secondary consequence of the steady emission of energy in the form of the various rays, the heating effect being due to the absorption of these rays in the active matter itself and the envelope con- taining it. The practical value of this discovery lies in the fact that radium and its allied elements are constantly emitting energy without any apparent diminution in the rate, or any change in their own substance which becomes sensibly apparent throughout a long term of years. It must not, of course, be supposed that the old law of the conservation of energy is in any way contravened by the behaviour of these new elements. The source of the emitted energy is intra-atomic. In other words, every atom—probably in every kind of substance, not merely in radioactive ones—is a tremendous storehouse or accumulator of energy, due to the motion of the electronic system which constitutes the atom. In the ordinary substance this energy never makes itself apparent. But when the atom of a radioactive substance breaks up, with the emission of a helium atom, there is a sudden outburst of energy, comparable in kind to, though infinitely greater in degree than, that which occurs when a light is put to gunpowder. The degree is so much greater than in cases of mere chemical combination that the intra-atomic energy of a pound of coal, if we could liberate and apply it, would probably be sufficient to drive the ' Mauretania' all the way to New York and back again. Is it possible that the final outcome of the new science of radioactivity will be to discover some means of tapping this vast store of energy which lies all about us inside the atoms ? Mr. Soddy indicates his belief in this possibility.
" By its conclusion," he says, "that there is imprisoned in ordinary common matter vast stores of energy which ignorance alone at the present time prevents us from using for the purposes of life, radioactivity has raised an issue which it is safe to say will mark an epoch in the progress of thought. With all our mastery over the powers of Nature we have adhered to the view that the struggle for existence is a permanent and necessary con-
dition of life. To-day it appears as though it may be but a passing phase, to be altogether abolished in the future as it has to some extent been mitigated in the past by the unceasing and, as it now appears, unlimited ascent of man to knowledge, and through knowledge to physical power and dominion over Nature."
Who would have thought, seventeen years ago, that such possi- bilities lay hid in the dark cupboard where Professor Becquerel shut up his uranium crystals with a photographic plate ?
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