The halo has, so far, told us that the additive law is obeyed in solid media, and that the increased ionisation attending the slowing down of the ray obtaining in gases, also obtains in solids; for, otherwise, the halo would not commence its development as a spherical sh.e.l.l or envelope. But here we learn that there is probably a certain difference in the course of events attending the immediate pa.s.sage of the ray in the gas and in the solid. In the former, initial recombination may obscure the intense ionisation near the end of the range. We can only detect the true end-effects by artificially separating the ions by a strong electric force. If this recombination happened in the mineral we should not have the concentric spheres so well defined as we see them to be. What, then, hinders the initial recombination in the solid? The answer probably is that the newly formed

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ion is instantly used up in a fresh chemical combination. Nor is it free to change its place as in the gas. There is simply a new equilibrium brought about by its sudden production. In this manner the conditions in the complex molecule of biot.i.te, tourmaline, etc., may be quite as effective in preventing initial recombination as the most effective electric force we could apply. The final result is that we find the Bragg curve reproduced most accurately in the delicate shading of the rings making up the perfectly exposed halo.

That the shading of the rings reproduces the form of the Bragg curve, projected, as it were, upon the line of advance of the ray and reproduced in depth of shading, shows that in yet another particular the alpha ray behaves much the same in the solid as in the gas. A careful examination of the outer edge of the circles always reveals a steep but not abrupt cessation of the action of the ray. Now Geiger has investigated and proved the existence of scattering of the alpha ray by solids. We may, therefore, suppose with much probability that there is the same scattering within the mineral near the end of the range. The heavy iron atom of the biot.i.te is, doubtless, chiefly responsible for this in biot.i.te haloes. I may observe that this shading of the outer bounding surface of the sphere of action is found however minute the central nucleus. In the case of a nucleus of considerable size another effect comes in which tends to produce an enhanced shading. This will

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result if rays proceed from different depths in the nucleus. If the nucleus were of the same density and atomic weight as the surrounding mica, there would be little effect. But its density and molecular weight are generally greater, hence the r.e.t.a.r.dation is greater, and rays proceeding from deep in the nucleus experience more r.e.t.a.r.dation than those which proceed from points near to the surface. The distances reached by the rays in the mica will vary accordingly, and so there will be a gradual cessation of the effects of the rays.

The result of our study of the halo may be summed up in the statement that in nearly every particular we have the phenomena, which have been measured and observed in the gas, reproduced on a minute scale in the halo. Initial recombination seems, however, to be absent or diminished in effectiveness; probably because of the new stability instantly a.s.sumed by the ionised atoms.

One of the most interesting points about the halo remains to be referred to. The halo is always uniformly darkened all round its circ.u.mference and is perfectly spherical. Sections, whether taken in the plane of cleavage of the mica or across it, show the same exactly circular form, and the same radius. Of course, if there was any appreciable increase of range along or across the cleavage the form of the halo on the section across the cleavage should be elliptical. The fact that there is no measurable ellipticity is, I think, one which on first consideration would not be expected.

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For what are the conditions attending the pa.s.sage of the ray in a medium such as mica? According to crystallographic conceptions we have here an orderly arrangement of molecules, the units composing the crystal being alike in ma.s.s, geometrically s.p.a.ced, and polarised as regards the attractions they exert one upon another. Mica, more especially, has the cleavage phenomenon developed to a degree which transcends its development in any other known substance. We can cleave it and again cleave it till its flakes float in the air, and we may yet go on cleaving it by special means till the flakes no longer reflect visible light.

And not less remarkable is the uniplanar nature of its cleavage.

There is little cleavage in any plane but the one, although it is easy to show that the molecules in the plane of the flake are in orderly arrangement and are more easily parted in some directions than in others. In such a medium beyond all others we must look with surprise upon the perfect sphere struck out by the alpha rays, because it seems certain that the cleavage is due to lesser attraction, and, probably, further s.p.a.cing of the molecules, in a direction perpendicular to the cleavage.

It may turn out that the s.p.a.cing of the molecules will influence but little the average number per unit distance encountered by rays moving in divergent paths. If this is so, we seem left to conclude that, in spite of its unequal and polarised attractions, there is equal r.e.t.a.r.dation and equal ionisation in the molecule in whatever

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direction it is approached. Or, again, if the encounters indeed differ in number, then some compensating effect must exist whereby a direction of lesser linear density involves greater stopping power in the molecule encountered, and vice versa.

The nature of the change produced by the alpha rays is unknown.

But the formation of the halo is not, at least in its earlier stages, attended by destruction of the crystallographic and optical properties of the medium. The optical properties are unaltered in nature but are increased in intensity. This applies till the halo has become so darkened that light is no longer transmitted under the conditions of thickness obtaining in rock sections. It is well known that there is in biot.i.te a maximum absorption of a plane-polarised light ray, when the plane of vibration coincides with the plane of cleavage. A section across the cleavage then shows a maximum amount of absorption. A halo seen on this section simply produces this effect in a more intense degree. This is well shown in Plate XXIII (lower figure), on a portion of the halo-sphere. The descriptive name "Pleochroic Halo" has originated from this fact. We must conclude that the effect of the ionisation due to the alpha ray has not been to alter fundamentally the conditions which give rise to the optical properties of the medium. The increased absorption is probably a.s.sociated with some change in the chemical state of the iron present. Haloes are, I believe, not found in minerals from which this

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element is absent. One thing is quite certain. The colouration is not due to an acc.u.mulation of helium atoms, _i.e._ of spent alpha rays. The evidence for this is conclusive. If helium was responsible we should have haloes produced in all sorts of colourless minerals. Now we sometimes see zircons in felspars and in quartz, etc., but in no such case is a halo produced. And halo-spheres formed within and sufficiently close to the edge of a crystal of mica are abruptly truncated by neighbouring areas of fclspar or quartz, although we know that the rays must pa.s.s freely across the boundary. Again it is easy to show that even in the oldest haloes the quant.i.ty of helium involved is so small that one might say the halo-sphere was a tolerably good vacuum as regards helium. There is, finally, no reason to suppose that the imprisoned helium would exhibit such a colouration, or, indeed, any at all.

I have already referred to the great age of the halo. Haloes are not found in the younger igneous rocks. It is probable that a halo less than a million years old has never been seen. This, prima facie, indicates an extremely slow rate of formation. And our calculations quite support the conclusions that the growth of a halo, if this has been uniform, proceeds at a rate of almost unimaginable slowness.

Let us calculate the number of alpha rays which may have gone to form a halo in the Devonian granite of Leinster.

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It is common to find haloes developed perfectly in this granite, and having a nucleus of zircon less than 5 x 10-4 cms. in diameter. The volume of zircon is 65 x 10-12 c.cs. and the ma.s.s 3 x 10-10 grm.; and if there was in this zircon 10-8 grm. radium per gram (a quant.i.ty about five times the greatest amount measured by Strutt), the ma.s.s of radium involved is 3 x 10-18 grm. From this and from the fact ascertained by Rutherford that the number of alpha rays expelled by a gram of radium in one second is 3.4 x 1010, we find that three rays are shot from the nucleus in a year. If, now, geological time since the Devonian is 50 millions of years, then 150 millions of rays built up the halo. If geological time since the Devonian is 400 millions of years, then 1,200 millions of alpha rays are concerned in its genesis. The number of ions involved, of course, greatly exceeds these numbers. A single alpha ray fired from radium C will produce 2.37 x 105 ions in air.

But haloes may be found quite clearly defined and fairly dark out to the range of the emanation ray and derived from much less quant.i.ties of radioactive materials. Thus a zircon nucleus with a diameter of but 3.4 x 10-4 cms. formed a halo strongly darkened within, and showing radium A and radium C as clear smoky rings.

Such a nucleus, on the a.s.sumption made above as to its radium content, expels one ray in a year. But, again, haloes are observed with less blackened pupils and with faint ring due to radium C, formed round nuclei

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of rather less than 2 x 10-4 cms. diameter. Such nuclei would expel one ray in five years. And even lesser nuclei will generate in these old rocks haloes with their earlier characteristic features clearly developed. In the case of the most minute nuclei, if my a.s.sumption as to the uranium content is correct, an alpha ray is expelled, probably, no oftener than once in a century; and possibly at still longer intervals.

The equilibrium amount of radium contained in some nuclei may amount to only a few atoms. Even in the case of the larger nuclei and more perfectly developed haloes the quant.i.ty of radium involved is many millions of times less than the least amount we can recognise by any other means. But the delicacy of the observation is not adequately set forth in this statement. We can not only tell the nature of the radioactive family with which we are dealing; but we can recognise the presence of some of its const.i.tuent members. I may say that it is not probable the zircons are richer in radium than I have a.s.sumed. My a.s.sumption involves about 3 per cent. of uranium. I know of no a.n.a.lyses ascribing so great an amount of uranium to zircon. The variety cyrtolite has been found to contain half this amount, about. But even if we doubled our estimate of radium content, the remarkable nature of our conclusions is hardly lessened.

It may appear strange that the ever-interesting question of the Earth's age should find elucidation from the

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study of haloes. Nevertheless the subjects are closely connected.

The circ.u.mstances are as follows. Geologists have estimated the age of the Earth since denudation began, by measurements of the integral effects of denudation. These methods agree in showing an age of about rob years. On the other hand, measurements have been made of the acc.u.mulation in minerals of radioactive _debris_--the helium and lead--and results obtained which, although they do not agree very well among themselves, are concordant in a.s.signing a very much greater age to the rocks. If the radioactive estimate is correct, then we are now living in a time when the denudative forces of the Earth are about eight or nine times as active as they have been on the average over the past. Such a state of things is absolutely unaccountable. And all the more unaccountable because from all we know we would expect a somewhat _lesser_ rate of solvent denudation as the world gets older and the land gets more and more loaded with the washed-out materials of the rocks.

Both the methods referred to of finding the age a.s.sume the principle of uniformity. The geologist contends for uniformity throughout the past physical history of the Earth. The physicist claims the like for the change-rates of the radioactive elements.

Now the study of the rocks enables us to infer something as to the past history of our Globe. Nothing is, on the other hand, known respecting the origin of uranium or thorium--the parent radioactive bodies. And while not questioning the law

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and regularity which undoubtedly prevail in the periods of the members of the radioactive families, it appears to me that it is allowable to ask if the change rate of uranium has been always what we now believe it to be. This comes to much the same thing as supposing that atoms possessing a faster change rate once were a.s.sociated with it which were capable of yielding both helium and lead to the rocks. Such atoms might have been collateral in origin with uranium from some antecedent element. Like helium, lead may be a derivative from more than one sequence of radioactive changes. In the present state of our knowledge the possibilities are many. The rate of change is known to be connected with the range of the alpha ray expelled by the transforming element; and the conformity of the halo with our existing knowledge of the ranges is reason for a.s.suming that, whatever the origin of the more active a.s.sociate of uranium, this pa.s.sed through similar elemental changes in the progress of its disintegration. There may, however, have been differences in the ranges which the halo would not reveal. It is remarkable that uranium at the present time is apparently responsible for two alpha rays of very different ranges. If these proceed from different elements, one should be faster in its change rate than the other. Some guidance may yet be forthcoming from the study of the more obscure problems of radioactivity.

Now it is not improbable that the halo may contribute directly to this discussion. We can evidently attack

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the biot.i.te with a known number of alpha rays and determine how many are required to produce a certain intensity of darkening, corresponding to that of a halo with a nucleus of measurable dimensions. On certain a.s.sumptions, which are correct within defined limits, we can calculate, as I have done above, the number of rays concerned in forming the halo. In doing so we a.s.sume some value for the age of the halo. Let us take the maximum radioactive value. A halo originating in Devonian times may attain a certain central blackening from the effects of, say, rob rays. But now suppose we find that we cannot produce the same degree of blackening with this number of rays applied in the laboratory. What are we to conclude? I think there is only the one conclusion open to us; that some other source of alpha rays, or a faster rate of supply, existed in the past. And this conclusion would explain the absence of haloes from the younger rocks; which, in view of the vast range of effects possible in the development of haloes, is, otherwise, not easy to account for. It is apparent that the experiment on the biot.i.te has a direct bearing on the validity of the radioactive method of estimating the age of the rocks. It is now being carried out by Professor Rutherford under reliable conditions.

Finally, there is one very certain and valuable fact to be learned from the halo. The halo has established the extreme rarity of radioactivity as an atomic phenomenon. One and all of the speculations as to

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the slow breakdown of the commoner elements may be dismissed. The halo shows that the mica of the rocks is radioactively sensitive.

The fundamental criterion of radioactive change is the expulsion of the alpha ray. The molecular system of the mica and of many other minerals is unstable in presence of these rays, just as a photographic plate is unstable in presence of light. Moreover, the mineral integrates the radioactive effects in the same way as a photographic salt integrates the effects of light. In both cases the feeblest activities become ultimately apparent to our inspection. We have seen that one ray in each year since the Devonian period will build the fully formed halo: an object unlike any other appearance in the rocks. And we have been able to allocate all the haloes so far investigated to one or the other of the known radioactive families. We are evidently justified in the belief that had other elements been radioactive we must either find characteristic haloes produced by them, or else find a complete darkening of the mica. The feeblest alpha rays emitted by the relatively enormous quant.i.ties of the prevailing elements, acting over the whole duration of geological time--and it must be remembered that the haloes we have been studying are comparatively young--must have registered their effects on the sensitive minerals. And thus we are safe in concluding that the common elements, and, indeed, many which would be called rare, are possessed of a degree of stability which has preserved them un

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

THE USE OF RADIUM IN MEDICINE [1]

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

[1] A Lecture to Postgraduate Students of Medicine in connection with the founding of the Dublin Radium Inst.i.tute, 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 quant.i.ty of radium a certain very small percentage of the total number of atoms present break up per second. By "breaking up" we mean their trans.m.u.tation to another element. Radium, which is a solid element under ordinary conditions, gives rise by trans.m.u.tation 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 trans.m.u.tation 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 p.r.o.nounced. A relatively large fraction of its atoms trans.m.u.te 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 trans.m.u.tations, till finally a stable substance is formed, identical with ordinary lead in chemical and physical properties, but possessing a slightly lower atomic weight.