Part 38

changed since the beginning of geological time. Each unaffected flake of mica is, thus, unassailable proof of a fact which but for the halo would, probably, have been for ever beyond our cognisance.

THE USE OF RADIUM IN MEDICINE

IT has been unfortunate for the progress of the radioactive treatment of disease that its methods and claims involve much of the marvellous. Up till recently, indeed, a large part of radioactive therapeutics could only be described as bordering on the occult. It is not surprising that when, in addition to its occult and marvellous characters, claims were made on its behalf which in many cases could not be supported, many medical men came to regard it with a certain amount of suspicion.

Today, I believe, we are in a better position. I think it is possible to ascribe a rational scientific basis to its legitimate claims, and to show, in fact, that in radioactive treatment we are pursuing methods which have been already tried extensively and found to be of definite value; and that new methods differ from the old mainly in their power and availability, and little, or not at all, in kind.

Let us briefly review the basis of the science. Radium is a metallic element chemically resembling barium. It

A Lecture to Postgraduate Students of Medicine in connection with the founding of the Dublin Radium Institute, delivered in the School of Physic in Ireland, Trinity College, on October 2nd, 1914

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possesses, however, a remarkable property which barium does not. Its atoms are not equally stable. In a given quantity of radium a certain very small percentage of the total number of atoms present break up per second. By "breaking up" we mean their transmutation to another element. Radium, which is a solid element under ordinary conditions, gives rise by transmutation to a gaseous element--the emanation of radium. The new element is a heavy gas at ordinary temperatures and, like other gases, can be liquified by extreme cold. The extraordinary property of transmutation is entirely automatic. No influence which chemist or physicist can apply can affect the rate of transformation.

The emanation inherits the property of instability, but in its case the instability is more pronounced. A relatively large fraction of its atoms transmute per second to a solid element designated Radium A. In turn this new generation of atoms breaks up--even faster than the emanation--becoming yet another element with specific chemical properties. And so on for a whole sequence of transmutations, till finally a stable substance is formed, identical with ordinary lead in chemical and physical properties, but possessing a slightly lower atomic weight.

The genealogy of the radium series of elements shows that radium is not the starting point. It possesses ancestors which have been traced back to the element uranium.

Now what bearing has this series of transmutations

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upon medical science? Radium or emanation, &c., are not in the Pharmacopoeia as are, say, arsenic or bismuth. The whole medicinal value of these elements resides in the very wonderful phenomena of their radiations. They radiate in the process of transmuting.

The changing atom may radiate a part of its own mass. The "alpha"-ray (a-ray) is such a material ray. It is an electrified helium atom cast out of the parent atom with enormous velocity--such a velocity as would carry it, if not impeded, all round the earth in two seconds. All alpha-rays are positively electrified atoms of the element helium, which thereby is shown to be an integral constituent of many elements. The alpha-ray is not of much value to medical science, for, in spite of its great velocity, it is soon stopped by encounter with other atoms. It can penetrate only a minute fraction of a millimetre into ordinary soft tissues. We shall not further consider it.

Transmuting atoms give out also material rays of another kind: the ss-rays. The ss-ray is in mass but a very small fraction of, even, a hydrogen atom. Its speed may approach that of light. As cast out by radioactive elements it starts with speeds which vary with the element, and may be from one-third to nine-tenths the velocity of light. The ss-ray is negatively electrified. It has long been known to science as the electron. It is also identical with the cathode ray of the vacuum tube.

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Another and quite different kind of radiation is given out by many of the transmuting elements:--the y-ray. This is not material, it is ethereal. It is known now with certainty that the y-ray is in kind identical with light, but of very much shorter wave length than even the extreme ultraviolet light of the solar spectrum. The y-ray is flashed from the transmuting atom along with the ss-ray. It is identical in character with the x-ray but of even shorter wave length.

There is a very interesting connection between the y-ray and the ss-ray which it is important for the medical man to understand--as far as it is practicable on our present knowledge.

When y-rays or x-rays fall on matter they give rise to ss-rays. The mechanism involved is not known but it is possibly a result of the resonance of the atom, or of parts of it, to the short light waves. And it is remarkable that the y-rays which, as we have seen, are shorter and more penetrating waves than the x-rays, give rise to ss-rays possessed of greater velocity and penetration than ss-rays excited by the x-rays. Indeed the ss-rays originated by y-rays may attain a velocity nearly approaching that of light and as great as that of any ss-rays emitted by transmuting atoms. Again there is demonstrable evidence that ss-rays impinging on matter may give rise to y-rays. The most remarkable demonstration of this is seen in the x-ray tube. Here the x-rays originate where the stream of ss- or cathode-rays

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are arrested on the anode. But the first relation is at present of most importance to us--i.e. that the y-or x-rays give rise to ss-rays.

This relation gives us additional evidence of the identity of the physical effects of y-, x-, and light-rays --using the term light rays in the usual sense of spectral rays. For it has long been known that light waves liberate electrons from atoms. It has been found that these electrons possess a certain initial velocity which is the greater the shorter the wave length of the light concerned in their liberation. The whole science of "photo-electricity" centres round this phenomenon. The action of light on the photographic plate, as well as many other physical and chemical phenomena, find an explanation in this liberation of the electron by the light wave.

Here, then, we have spectral light waves liberating electrons--i.e. very minute negatively-charged particles, and we find that, as we use shorter light waves, the initial velocity of these particles increases. Again, we have x-rays which are far smaller in wave length than spectral light, liberating much faster negatively electrified particles. Finally, we have y-rays--the shortest nether waves of all-liberating negative particles of the highest velocity known. Plainly the whole series of phenomena is continuous.

We can now look closer at the actions involved in the therapeutic influence of the several rays and in

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this way, also, see further the correlation between what may be called photo-therapeutics and radioactive therapeutics.

The ss-ray, whether we obtain it directly from the transforming radioactive atom or whether we obtain it as a result of the effects of the y- or x-rays upon the atom, is an ionising agent of wonderful power. What is meant by this? In its physical aspect this means that the atoms through which it passes acquire free electric charges; some becoming positive, some negative. This can only be due to the loss of an electron by the affected atom. The loss of the small negative charge carried in the electron leaves the atom positively electrified or creates a positive ion. The fixing of the wandering electron to a neutral atom creates a negative ion. Before further consideration of the importance of the phenomenon of ionisation we must fix in our minds that the agent, which brings this about, is the ss-ray. There is little evidence that the y-ray can directly create ions to any large extent. But the action of liberating high-speed ss-rays results in the creation of many thousands of ions by each ss-ray liberated. As an agent in the hands of the medical man we must regard the y-ray as a light wave of extremely penetrating character, which creates high-speed ss-rays in the tissues which it penetrates, these ss-rays being most potent ionising agents. The ss-rays directly obtained from radioactive atoms assist in the work of ionisation. ss-rays do not

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