THE ENERGY OF RADIUM

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combustion of the same weight of coal, the source of energy on which the world, in so far as it is modern, subsists.

Whence arises such a stream still flowing in this world of ancient lineage, from a material extracted from minerals found in rocks, many of them coeval with the beginning of geological time? Tracked to earth, the clue to the great secret, for which a thousand telescopes might have swept the sky for ever and in vain, lay in a scrap of matter, dowered with something of the same inexhaustible radiance that hitherto has been the sole prerogative of the distant stars and sun.

The solution of these problems followed the proof that the energy of radioactive substances was evolved in new kinds of change, which are distinguished from those studied in chemistry in two ways. In the first place they are more fundamental, and concern a plane in the complexity of matter hitherto not penetrated. It is the unit of matter, indivisible in chemical changes, the atom of the radioactive element, which in radioactive changes subdivides or disintegrates. Secondly, per unit weight of matter changing, energy of the order of a million times greater than in any previously known change is given out.

Just as chemical changes, the disruption and formation of molecules and the rearrangement of the component atoms out of which they are built -such changes as the explosion of dynamite or gunpowder-occur with far greater corresponding changes of energy than physical changes, like the change of state in the vaporisation of water or condensation of steam, so with these new changes, which are concerned with the inner architecture of atoms. All material processes studied hitherto have been concerned solely with the external relationships of atoms.

With the discovery of radioactivity the Rubicon was crossed, and physical science found itself in a new world, in the presence of giant-like primary manifestations of energy which proceed in absolute indifference to and completely unaffected by any of the pygmy second-order influences of the world external to themselves, the old world of chemistry and physics.

These radioactive disintegrations of the atom proceed in a long sequence of successive changes at characteristic rates. The primary parent-elements, uranium and thorium, each stand, as it were, at the head of a long genealogical table, comprising some fourteen members in the first, and twelve members in the second case, before the processes come to an end and the outflow of energy accompanying them ceases.

Each of the changes proceeds at definite rates which, so far as has been ascertained, are absolutely independent of every known consideration, and so it comes about that each of these successive products has a characteristic average period of life. Its atom remains in existence for a period of time which is, on the average, definite, and which varies among the various successive members between the extremes, estimated indirectly in a variety of ways, of a hundred-thousand-millionth of a second on the one hand and twenty thousand million years on the other. The two parent-elements are the longest lived, and preserve the strain of their less enduring children throughout the ages, over periods which exceed those covered even by the utmost estimates of the duration of geological time.

Radium is but one of the products of the uranium series, and its special interest is chiefly to be ascribed to the fact that the rate at which it changes, estimated as one-two thousand five hundredth part per

THE RATE OF CHANGE OF RADIUM

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annum, is so slow that over ordinary periods of time it is imperceptible, and yet so rapid that the amount of energy continuously evolved is, considering the excessively minute quantity of matter, truly astonishing. The other members of the series which change more rapidly possess a radioactivity which, though it is more intense, is more ephemeral. Moreover, the quantity of each member of the series, coexisting with its parent, is proportional to its period of life. For it is a balanced or equilibrium quantity, when the rate of formation equals the rate of change. The more quickly changing members never accumulate in ponderable quantity and, for them, it is impossible to prove, by the older methods of science, that they are, indeed, new elementary substances, possessing distinct chemical character, atomic weight and spectrum. For radium, though the proportion in which it exists in the richest uranium mineral is exceedingly minute, it is just possible to obtain enough to weigh and to prepare in a pure condition for chemical examination.

The most slowly changing members, on the other hand, are the parent-elements, uranium and thorium, which were well studied by chemists for a century, so feeble is their radioactivity and so slow their rate of disintegration, without a suspicion that, in them, the oft-suspected process of the evolution of the elements was still in progress before their eyes.

There is a certain quality of permanence about experimental scientific discovery which is not always believed. An important addition to experimental knowledge, whether made in the time of Robert Boyle or yesterday, is never displaced. Points of view may change, theories interpreting and explaining experimental knowledge may have their periods of adolescence, maturity and decline, but the framework of the structure, the experimental fact round

which ideas are arranged, is too well and truly laid to fear demolition. Even when, as in the present day, the foundations of science are shifted to an ever deeper and more fundamental plane the experimental basis of fact is unthreatened. The idea that the whole edifice of chemical science was tottering to its fall as the result of the discovery of the intra-atomic changes of the radio-elements, is one that has always been too absurd to call for reply. But for that science and its clear-cut conception of the chemical elements, the result of more than three centuries of continuous experimental labour, the facts of radioactivity might still have been as arresting and magnificent as any discoveries ever were. But without the older knowledge, exquisitely and finely wrought, for the newer knowledge to dovetail into and complete, the chief human significance of the new science and its power to interpret the physical side of the drama of life could scarcely have been so early perceived.

Radium, no longer a mystery, one of the chemical elements doing what more than a score are doing at their own characteristic rates, owes its peculiar position to the fact that it is changing neither too slowly nor too quickly in reference to the allotted span of threescore years and ten. Energy of the order of a million times that evolved in the combustion of the same weight of coal, instead of being, as in the case of radium, evolved in the average period of 2500 years, is in the case of uranium and thorium spread over a term of thousands of millions of years. The effects are small but enduring, and, almost imperceptible in themselves, come to maturity in due time in the cosmical calendar. Small as is the proportion of uranium and thorium in the rocks of the earth, the energy they evolve is estimated to be far more than the earth loses to outer space,

A UNIVERSE OF PERMANENT RÉGIME 21

if the surface composition of the rocks is maintained uniformly throughout the core. Unless this is not the case, or unless the energy they evolve is being utilised in unknown ways, the conclusion follows that the interior of the globe must be getting hotter instead of colder. The uncomfortable prediction of the ultimate destruction of the world by fire, is now at least as probable as the former fate pictured by science, that the world must be steadily cooling, and that it was only a matter of time before it became lifeless and dead.

The clock wound up in the beginning to run for a certain time, a universe provided at its creation with a certain store of available energy to dissipate and live by at an ever decreasing rate, until it arrived ultimately and inevitably at complete physical stagnation and death, is being displaced by a less arbitrary view as science advances and invades more and more the vast territory still beyond its ken. It is at least legitimate to conceive a universe of permanent régime, carrying in its smallest ultimate particles the seeds of its own regeneration. But the linking of the ends of the process together into such a closed cycle still involves the assumption of events that remain unknown and a reversal of the known continuous direction of energy transformations. Such a reversal may well occur under conditions still, and possibly for ever, beyond the power of experiment to reproduce in the laboratory.

The dream of the alchemist, the transmutation of the elements, so far from being a chimerical idea, or a process to be sought for possibly in the transcendental chemistry of glowing suns, is in continuous natural operation on the earth amongst the most complex sorts of atoms known to the chemist. All heavy elements, presumably, if they could be transmuted artificially into lighter ones, would evolve

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