2o6 



NA TURE 



[December 17, 1908 



V..\mO-.\Cl-lVE CHANGES IN THE EARTH.' 

 T WISH particularly to refer to manifestations of radio- 

 -'■ activity which are observed, not in artificially pre- 

 pared materials like radium, but in the rocks and minerals 

 of th'J earth's crust, as we find them in nature. Let us 

 consiiJer, in the first place, the most conspicuous cases of 

 this kind. The source from which radium is obtained is 

 the mineral pitchblende. This mineral occurs in veins, 

 like the majority of the useful metals ; I may refer par- 

 ticularly to the mineral veins of Cornwall, so long famous 

 as a source of tin. These veins are of the nature of 

 cracks, running through the granite and through the slate 

 which adjoins it. The cracks have been filled up by the 

 various metallic ores which have been introduced by pre- 

 cipitation or sublimation, the e.xact nature of the process 

 being somewhat obscure. 



I will now show you an experiment, due to Sir W. 

 Crookes, which illustrates the radio-activity of pitchblende 

 in a very beautiful manner. A flat polished slab of pitch- 

 blende intergrown with a variety of other material which 

 is not radio-active was laid face to face with a photo- 

 graphic plate, which was developed after the lapse of 

 about a week of contact. The radium and other radio- 

 active substances contained in the pitchblende have acted 

 photographically upon the plate, while, of course, those 

 portions of the materia! which are not radio-active have 

 exerted no such action. Thus pitchblende has, as it were, 

 taken its own portrait, which I now show you on tne 

 screen. 



Pitchblende, the principal radium ore, contains, as you 

 know, only an infinitesimal percentage of radium, the 

 bulk of the substance being made up of oxide of uranium. 

 Uranium is commonly spoken of as a rare metal ; but 

 terms of this kind are comparative only, and in contrast 

 with radium, which is more than a million times scarcer, 

 it seems common enough. Now I wish to speak for a 

 little about this association of uranium and radium in 

 pitchblende. Is it accidental, or has it some special signifi- 

 cance? I hope to be able to convince you that it has. 



In the early days of radium it was common to hear 

 the difficulty emphasised that while there was no reason 

 for doubting that the radium which was found in the 

 earth had been there as long as other metals, a substance 

 that was continually giving out energy in this way was 

 obviously defying the greatest physical generalisation of 

 the nineteenth century — the law of the conservation of 

 energy. We cannot, however, afford to sacrifice this law 

 so easily, and a ready mode of escape offers itself if we 

 suppose that a continual waste of radium is occurring. 

 In that case it becomes necessary to suppose, also, that 

 the supply is in some way replenished, for otherwise all 

 the radium would have wasted long ago. From what 

 material are the fresh supplies of radium derived? They 

 must be derived from some other substance contained in 

 the mineral where the radium is found, and there is now 

 reason to feel sure that uranium is the substance in 

 question. 



We have convincing proof of this in the fact that the 

 amount of radium found in the mineral is always in direct 

 proportion to the quantity of uranium which it contains. 

 I should perhaps say, to avoid misconception, that there 

 is good reason for Ijelieving that several transitional stages 

 exist through which uranium passes on its road to become 

 radium. It is not necessary, however, to take into account 

 the existence of these intermediate products in order to 

 form a clear idea of the process by which the supply of 

 radium is kept up. Uranium changes spontaneouslv, 

 though very slowly, into radium, and the amount of 

 radium produced per annum, for example, will be pro- 

 portionate to the amount of uranium present. On the 

 other hand, a certain fraction of the total amount of 

 radium present decays per annum, and the balance of this 

 account of profit and loss will represent the amount of 

 radium found in the mineral at any time that we examine 

 it. There will be no difficulty in seeing that on this theory 

 the amount of the radium in the mineral should be pro- 

 portionate to the amount of uranium, and exoeriment fully 

 confirms the theory by showing that such is in fact the 



' Discourse deliveretl at the Royal Institution by the Hon. R. J. Strutt. 

 F R.S. 



NO. 2042, VOL. 7(/| 



case. We have here a clear and . distinct case-of. the 

 transmutation of metals, so long unsuccessfully, searched 

 for. 



Let us now come back to the pitchblende. 



What was the source of , metalliferous- ores found in 

 mineral veins is a very much ve.xed question, and no -solu- 

 tion of it which has yet been , proposed can be said- to 

 be altogether free from difficulty. One of the most 

 plausible theories, however, supposes that the metals. have 

 been derived from the rocks by which the veins .are 

 traversed. We are not here concerned with iitetalliferous 

 ores in general, but only with those which carry radio- 

 active material. In deciding whether the granite of Corn- 

 wall can be supposed to furnish the uranium • of pitch- 

 blende, it is, of course, fundamental to know whether 

 any uranium is present in the rock. It should ■ be said, 

 by way of preface, that the quantity mustj at best, be 

 very small, and certainly too small for detection by the 

 methods of chemical analysis- as ordinarily applied. We 

 have seen that uranium in nature is invariably accom- 

 panied by a proportionate quantity of radium, and as it 

 is in practice much easier to detect minute quantities of 

 radium than to detect the corresponding quantities of 

 uranium, it is best to look for the former only, and to 

 be content to infer the presence of the latter. 



I have made a large number of e.xperiments to find out 

 how much radium there may be, not only in Cornish 

 granite, but in a large variety of other rocks. In every 

 case the presence of radium has been established, though 

 only to the extent of about one-millionth part of what 

 is found in pitchblende, and even that, it will be remem- 

 bered, is not much. If we take into account the very 

 large bulk of the granite and the very small bulk of the 

 pitchblende veins running through it, there is no difiiculty 

 in admitting that the granite was capable of supplying 

 the radio-active material of the pitchblende. 



Granite, of course, consists of a variety of different 

 minerals, which give it its mottled appearance. These 

 minerals, there is no reason to doubt, have been formed 

 in the successive stages of crystallisation of an originally 

 molten mass. There is a mineral called zircon, of which 

 the jacinths sometimes set by jewellers are a variety, 

 which is present in very minute crystals in granite. These 

 minute crystals of zircon have a very characteristic 

 geometrical shape ; a square prism terminated at each end 

 by a pyramid. The fact that they have this perfect shape 

 is a proof th.'it Ihey have been perfectly free to assume 

 their natural form, and have not been hampered for want 

 of space by other minerals surrounding them. The in- 

 ference is plain that zircon has been one of the first 

 minerals to crystallise in the consolidation of granite. 



I have found that this zircon is very much richer in 

 radium than the granite generally, though, on the other 

 hand, it is poor compared with pitchblende. It seems 

 clear that the minerals which crystallise first take an 

 unfair share of the radio-active elements, leaving the rest 

 of the magma impoverished. 



In the light of this observation. Prof. Joly, of Dublin, 

 has been enabled to explain a curious appearance which 

 is seen when a section of the granite thin enough to be 

 transparent is examined under the microscope. This 

 appearance is seen in one of Prof. Joly's photographs of 

 a minute crystal of zircon, which is embedded in a large 

 crystal of mica. '\'ou will observe that the material 

 surrounding the zircon for a definite distance outwards 

 has become d.irkened in colour. The altered region round 

 the speck of zircon is practically circular, and is remin- 

 iscent of a spot of grease on cloth. 



Prof. Joly has pointed out that this alteration in the 

 surrounding materials inust be due to the radio-activity 

 of the zircon. That radio-active materials are capable of 

 producing such colorations has been known from the early 

 days of radium. You see, for instance, projected on the 

 screen, the image of a glass bottle, in which a radium 

 preparation has been kept. Though originally of colour- 

 less glass, it has been stained a deep purple by long- 

 continued action of radium. 



It may, perhaps, be thought that this idea, though 

 plausible, is no more than a guess. ' It is. however, much 

 more than that. We know, from the investigations of 

 Prof. Bragg and Mr. Kleeman, that the a particles of 



