476 



NATURE 



[August 9, 191 7 



A candidate desiring to offer any of the subjects 

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 and (58) agriculture, other equivalent training will be 

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Extra Numerutn. 



Candidates may take, in addition to the abpve, one 

 of the translation papers of Section A in a language 

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 than one of the Scandinavian languages, nor more 

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RADIO-ACTIVE HALOS.' 



' . II- 



WE shall now see that the thorium halo follows 

 faithfully the same laws of development as the 

 uranium halo, whatever we may assume as to the 

 nature of these laws. 



By plotting the seven o-ray curves of ionisation 

 which must contribute to the formation of a halo in the 

 medium surrounding a particle containing the parent 

 element thorium, and then, as before, adding up the 

 ordinates, we get for the total ionisation responsible 

 for the thorium halo the next curve (Fig. 6). 

 ,i Note that the 



?. 81 1 1 1 1 1 1 1 1 single conspicuous 



maximum d i s- 

 played by the cor- 

 responding curve 

 for the uranium 

 halo is now re- 

 placed by two 

 maxima, the one 

 which is nearer 

 the centre being a 

 little the lower. 

 Beyond these two 

 maxima the curve 

 9 descends steeply 

 cm. with two excre- 

 scences before the 

 minimum of ion- 



5 6 7 



Fig. 6. — Integral curve for tborium halo. 



isation is reached. Then the curve reascends to the 

 Jow maximum due to ThCi. 



Now, the first beginning of the thorium halo shows 

 two rings, and the radial dimensions of these rings 

 are in good agreement with the positions of the two 

 maxima of the curve. The inner ring has not been 

 found alone. Next we find the space within and 

 around these rings growing darker, accompanied by 

 the early appearance of the outer ring due to ThCi, just 

 as in the case of the uranium halo we observe the 

 early appearance of the ring due to RaCj. The next 

 stage, so far certainly observed, shows the loss of the 

 internal features, the resulting halo exhibiting much 

 the same appearance as the uranium halo in the final 

 stage of development. 



Above the ionisation rurve for the thcwium halo I 

 have marked the several features of the halo. The 

 agreement of the observed \\ ith the theoretical features 

 is even closer than in the case of the uranium halo. 



When we consider the successive steps in the genesis 

 of the radio-active halo, wliich I have now laid before 

 you, we can only come to the conclusion that some 



r Discourse delivered at the Roj^l Institution on Friday, ^fay ii, by 

 Prof. J. Joly, F.R.S. Continued from p. 458. 



cause exists which tends to accentuate the effects 

 going on in the outer regions of the halo. Could 

 we assign a cause for the strengthening of the outer 

 effects of ionisation, or, what comes to the same 

 thing, for the weakening of the inner effects, every 

 feature of the halo becomes explained by the curve of 

 integral ionisation — that is, by the curve which simply 

 sums the effects of the several Bragg curves. We 

 would then find an explanation of the appearance of 

 successive rings and of the appearance of the effects 

 of the extreme or limiting ray at such an early stage 

 of development. 



If we assume that the process which results in the 

 formation of a halo under the influence of the a ray 

 is essentially similar in nature to that which is re- 

 sponsible for the photographic image under the stimu- 

 lus of light, the desired explanation of the weakening 

 of the inner features is forthcoming. For the 

 phenomenon of reversal or of solarisation, well known 

 to photographers, would assuredly lead to the weaken-* 

 ing of the inner parts of the image. The repetition 

 of stimuli at or near the same spot is necessarily more 

 marked in the inner than in the outer parts of the halo, 

 and the ionisation accumulating in the region 

 traversed by the external limiting o rays is to a large 

 extent exempt from the effects of repetition. 



Now there are features in common between the 

 halo image and the photographic image. Both are 

 brought about by ionisation in a sensitive medium. 

 There is so much indirect evidence for this view that 

 we can scarcely doubt its truth. The salts of iron in 

 many forms have been found to be photographically 

 sensitive. In the photochemistry of chlorophyll they 

 appear to play a fundamental part in Nature. Again, 

 we may interpret the fact that the halo may be obliter- 

 ated by heat, as proof of instability. Finally, the 

 photographic plate is affected by the a ray in a manner 

 not readily distinguished from that due to light. 



Halos have been found which show all the appear- 

 ance of reversal. In them we find the penumbra re- 

 placed by a band which is darker than the region 

 lying within. Normal halos in its neighbourhood, by 

 contrast, well show the peculiar change which .affects 

 the reversed halo. It is the negative of a halo. What 

 is this appearance due to, if not to reversal? The effect 

 must arise from very intense ionisation. The reversal 

 has cleared the inner pupil more or less, but the repe- 

 tition of stimuli has not been sufficient to affect the 

 penumbra in the same manner. If these views are 

 correct we may claim to know something of the 

 nature of the phenomena which lead to the building 

 up of the halo. We mav regard the radio-active 

 nucleus as emitting, for countless ages, radiations 

 which slowly act, according to the laws affecting the 

 latent photographic image, upon the surrounding 

 medium. We mast suppose the electric charge upon 

 the o ray to affect the stability of the sensitive mineral, 

 ionising the constituent atomic systems, and, finally, 

 producing stresses and, possibly, displacements, which 

 are revealed in the increased colour absorption. 



Hitherto I have more especially dwelt upon the 

 points of agreement between the observed and the 

 theoretical halo. I venture to think that the agree- 

 ment sets beyond any doubt not only the radio-active 

 origin, but also the general mode of development of 

 halos. I shall now refer to some details iri which 

 the observed halo is not in perfect agreement with the 

 curve of ionisation. 



In the case of the thorium halo the measured dimen- ■ 

 sions of the halo are in very perfect accord with the 

 ionisation curve. The agreement seems generaHy as 

 perfect as we could expect. There is, "however, a very '; 

 small appearance of misfit in the location of the first ^ 

 ring. The estimates I have made of the radius of 'A 



NO. 2493, VOL. 99] 



