456 



NATURE 



[August 2, 19 17 



'*^- 



RADIO-ACTIVE HA LOS.' 



THIS discourse is concerned with certain very minute 

 objects of the rocks — so minute as to be visible 

 only with the aid of the microscope — known to 

 petrologists by the cumbersome name of " pleochroic 

 halos." Although we shall be occupied mainly in con- 

 sidering quite recent additions to our knowledge of 

 halos, yet, in view of the fact that many of this audi- 

 ence will probably hear of them now for the first time, 

 it is necessary to hegin with some elementary remarks. 



The halos of the rocks have been known since the 

 application of the microscope to rock study ; but until 

 recently their origin and nature were quite unknown. 

 Nor could it hav^e been otherwise, for they find their 

 explanation in the facts of radio-active science only — 

 a science the origin of which dates back but little 

 before the bsglnnlng of the present century. The 

 student of the rocks in past times seems to have re- 

 garded these objects with but little more than passing 

 interest. Had he paid more attention to them a case 

 replete with extraordinary mystery could 'have been 

 made out, and one which at the time must have re- 

 mained absolutely inexplicable. The lack of attention 

 to the detail displayed by halos, and the failure of the 

 earlier observers to notice the mathematical regularity 

 of their dimensions, well illustrate how advance in 

 one domain of science may influence our recognition 

 of facts in another. 



The most familiar type of halo consists simply of a 

 darkened border surrounding some minute mineral par- 

 ticle within the rock. The formation of the coloured 

 border Indicates some alteration In the medium in 

 which it is formed, and this alteration is evidently 

 conditioned by the presence of the central mineral. 

 If the latter is very small and about equally developed 

 In all directions, the halo takes on the form of a sphere 

 having at its centre the mineral which has originated 

 it. In a section of the medium containing the halo 

 this sphere appears as a coloured disc ; but as we find 

 the same appearance, no matter In what direction we 

 section the halo, Its spherical form is 'beyond doubt. 



Certain facts respecting the formation of halos have 

 for long been available. Only quite a few substances 

 can originate a ihalo. Of these the minerals zircon 

 and orthite are the commonest, and the first much 

 more so than the second. Again, only in certain media 

 surrounding such minerals can a halo be developed. 

 Of such media the several varieties of brown mica 

 are the most abundant and the most valuable. It 

 would appear that all media sensitive to the forma- 

 tion of halos contain iron as a constituent. 



While these facts have long been available, the next 

 I have to mention Is a recently discovered one. All 

 those minerals which give rise to halos are found, 

 when examined in large crystals, to contain radio- 

 active substances. 



Now, such substances, we well know, are continu- 

 ally radiating. They give out various sorts of rays. 

 This leads us_to suspect that the halo may, in fact, be 

 generated In some way by these radiations. 

 There are three sorts of radiations — the a, ^i and y 

 rays. The last two cannot possibly be responsible. 

 They are far too penetrating to account for these 

 microscopic effects. The o rays can alone be con- 

 cerned. 



Before pursuing further our inquiry in this direction, 

 let us examine the nature of the halo itself. It is not 

 merely a stain or lodgment of colouring matter in the 

 medium. If we apply optical examination with polar- 

 ised light to a halo in brown mica we find that the 

 peculiar optical properties of the mica, whicii are, of 



1 Discourse delivered at the Royal Institution on Friday, May ii, by 

 Prof. J. Joly. F.R.S. 



NO. 2492, VOL. 99] 



course, referable to the orderly arrangement prevailing 

 among its molecules, exist within the halo just as 

 elsewhere. In fact, we might say they are accen- 

 tuated. The remarkable absorption of the ray polarised 

 in the plane of cleavage of the mica is more complete 

 in the halo. We occasionally see a halo which ex- 

 tends across the edges of two distinct flakes of mica. 

 If the light is polarised and the plane of polarisation 

 is in the plane of cleavage of one of the flakes, and 

 is inclined to the cleavage of the other flake, that part 

 of the halo which is contained in the first crystal of 

 mica is Intensely black. In the other crystal of mica 

 the halo is much lighter in colour. Plainly the effect 

 upon the mica, however exerted, has been such as to 

 increase. the absorption of a ray vibrating in the plane 

 of cleavage. The crystallographic structure has not 

 been disturbed. If iron is not a constituent of the 

 medium no visible effect is produced. 



Of the last statement we sometimes find very beau- 

 tiful evidence in the case of halos which originate 

 from a nucleus located outside the sensitive medium, 

 but within a certain distance of it. Fig. i will ex- 

 plain. The originating crystal is In quartz, a sub- 

 stance which never contains halos. There is no iron 

 in its constitution. But the halo-forming influence ex- 

 tends to a neighbouring crystal of mica. This influ- 

 ence, which develops an outlying part of the halo- 

 sphere in the mica, must have traversed the quartz. 

 It has left no 

 record therein. 

 If the halo was 

 something o f 

 the nature of a 

 stain diffused 

 outwards from 

 the central sub- 

 stance — as some 

 of the earlier 

 observers main- 

 tained—the ab- 

 sence of the 

 colouring mate- 

 rial from the 

 quartz is not 

 easily e x- 

 plained. But if 



the halo is — as we have hinted — due to radiations pro- 

 ceeding from the zircon — radiations which only aflect 

 certain unstable atoms — the appearance at once finds 

 simple explanation. The quartz Is not sensitive to the 

 ra)'s ; the mica is. 



But the primary evidence for the radio-active origin 

 of the halo is to be found in its dimensions. The fully 

 developed halo has been found in two sizes. One of 

 these shows a radial dimension of 00333 mm. ; the 

 other scales, radially, 00408 mm. There are two 

 primary radio-active elements, as everyone knows — 

 uranium and thorium. If the central or originating 

 substance contains uranium it will of necessity contain 

 all the eight a-ray-emitting substances which' the 

 uranium-radium series embraces. Similarly, If thfe 

 central substance contains thorium, there are seven 

 a-ray-producing substances which must be present. 

 Now each of these various radiating substances emits 

 o rays which fwssess a certain specific velocity of 

 emission, and, consequently, a specific power of pene- 

 tration. The most penetrating ray of the uranium 

 series is that of radium-C. In air this ray will travel 

 694 cm. 'Before it comes to rest. The most penetrating 

 ray of the thorium series is that of thorium-Ci. This 

 will penetrate 860 cm. before coming to rest. A very 

 few rays travel further, but this does not affect thf- 

 matter. Now Bragg has shown how we may calcu- 

 : late the range in any medium if we know its chemical 



Fig. I. — Nucleus of halo located outside a sensitive 

 medium. 



