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THE SOURCE OF COSMICAL ENERGY

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that of a struggle, still largely internecine rather than co-operative, for a miserably inadequate allowance of energy. He looks back across the gulf of time from the day of the nameless and forgotten savage, who first discovered the art of kindling a fire, to himself, his logical descendant, master of a world largely nourished by the energy of fuel, and humming with the music of inanimate machinery. He turns his thoughts downward into the earth and wonders how long the source of his new life will hold out. He looks up once more at the unchanging stars and realises, as one who before has been but blind, that the immeasurable interval that separates him from the hidden sources which bear the universe along, is no immeasurable interval of space, whatever it may be of future time. The main stream sweeps past his doors, and the great gulf that yawns between him and the consummation of his emancipation looks small enough compared with the gulf that yawns behind.

No better illustration could be chosen of the spirit of absolute detachment from practical affairs, in which the highest and most practical knowledge is won, than the familiar history of the march of events which, in the closing decade of the nineteenth century and the opening years of the twentieth, revealed the immanence and accessibility of cosmical energy. To spend a feverish life in the attempt to transmute base-metal into gold, or to discover the secret of perpetual motion, would be to tread a well-trodden highway leading nowhere. But to exhibit a divine curiosity in an abstruse phenomenon, such as the rays given out by fluorescent substances, and whether any of them, like the X-rays, are able to penetrate an opaque material, to follow nature rather than to lead, and to win a grain of knowledge for the communism of science, is to stumble upon secrets such as these unawares.

1898

So radium was discovered, and it has been remarked that the future race will date the coming in of its kingdom from this discovery, mainly due to a woman.

Through the wealth of new discovery that followed the recognition and investigation of natural radioactivity, we have but to pursue still the single connected thread which science has shot through the whole fabric of human history, Rays of a fundamentally new character are given out by radium, of various kinds and intense interest, and a thousand new phenomena make themselves manifest, but like galley slaves and fertilisers, waterfalls and food, they must here be brought into line from the single view point. Their energy is of the same category and obeys the same laws as the forms which before have nourished and embellished life. Not yet, at least, has science got outside the jurisdiction of that universal legislation, whatever may be its ultimate aspirations.

The energy evolved by radium spontaneously, however new and wonderful it may be, is yet measurable in current coin. In a single day it approaches in magnitude the energy evolved from a similar weight of any materials undergoing the most energetic chemical reactions known. In a year it evolves about 150 times as much energy as would be evolved in the complete combustion of the same weight of coal. Yet in the fifteen years that have elapsed since the discovery no measurable diminution of this rate of emission has been observed. If science is right, this emission will steadily decrease as the centuries roll by. But it is still possible to put a value on the total amount of energy a given weight of radium will furnish before the outflow comes to an end. It is about a third of a million times as great as would be evolved in the

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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. Mureover, 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

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