July 20, 191 1] 



NATURE 



99 



of between 6 and 7 centimetres from it. This is entirely 

 due to radium C. Within this shell we have a spherical 

 shell due to radium A. It is the next we meet as we go 

 inwards. It has an extreme diameter of 4-8 cm. The 

 next shell is created by emanation. Its radius is 42 cm. 

 The shell due to radium F succeeds at 3-8 cm. ; then comes 

 that made by radium, and, lastly, a very intense one due 

 to the nearly coincident effects of three rays, two due to 

 uranium and one to ionium. The weight of this particle 

 of pitchblende is about one-tenth of a gram. If all its 

 rays escaped freely at its surface, some 9600 a rays would 

 leave it per second, and the number of ions created in 

 I the air per second would be about 960 millions. The 

 diagram (Fig. 2) shows the successive shells, as they could 

 be formed in air, to half scale. 



We shall now pursue the study of radiant matter within 

 the confines of another branch of science — that which deals 

 with the nature, origin, and structure of the rocks. We 

 gain this much by the transfer, that the invisible effects 

 we have just been endeavouring to picture to ourselves as 

 taking place around a radio-active body in equilibrium may 

 be studied at our leisure, visibly inscribed in the ancient 

 rocks. We require the microscope, however, in order to 

 carry on our observations. 



If we extract a flake of brown mica from the granite 

 near Dublin and look at it through the microscope, we 



find here and there dark circular or disc-shaped marks. 

 In the centre of each is a small crystal. This in most 

 cases is the mineral zircon, which became enclosed in the 

 mica at an early stage in the formation of that mineral. 

 The dark area extends around the zircon like a darkened 

 border, and, if the crystal is small enough, takes on the 

 form of a perfectly true circle. 



The remarkable occurrence of these dark circular spots, 

 or " pleochroic haloes," as they are called, has been known 

 to more than one generation of petrologists, and has 

 always excited interest. Their origin has until lately been 

 unexplained. Sollas, some years ago, prophetically stated 

 his belief that they were to be ascribed to the presence of 

 some rare earth in the zircon. When the minerals of the 

 rocks were searched by Strutt for radio-active bodies, it 

 was found that zircons were intensely radio-active — a con- 

 centration of uranium having in some manner taken place 

 in these early formed bodies. The minerals apatite and 

 allenite are also sometimes conspicuously radio-active, and 

 around these, also, haloes often exist. 



Let us then suppose that the halo is due to the radio- 

 activity of the minute crystal around which it extends. We 

 know, that the radio-active elements in the zircon discharge 

 helium atoms at high velocity into the surrounding mica. 

 If these a rays have power to affect the mica by ionisa- 



tion, just as they colour glass or affect a photographic 

 plate, then there will be a certain region affected extend- 

 ing just so far as the rays can penetrate and no further. 

 It will be a test of this explanation if the radius of the 

 circular marks is found to be just the correct distance to 

 which the rays could travel in mica. 



Now Bragg and Kleeman have determined the principles 

 upon which we may estimate from the observed ranges in 

 air the range of a rays in any substance the chemical 

 nature and density of which are known. Accordingly, we 

 may calculate the ranges of several a rays in biotite. The 

 table below gives the results. 



Range 



Biotite. 



Radium C ... 



Radium A ... 



Emanation ... 



Radium F ... 

 Radium 



Ionium 



Uranium 



0033 Thorium C 0-040 



0023 Thorium X 0026 



0-020 Thorium emana- 



0018 tion 0-025 



0017 Thorium B 0-023 



0013 Radiothorium ... 0-018 



0013 Thorium 0-016 



NO. 2177, VOL. 87] 



We see, as might have been expected, and as, indeed, 

 was shown to you at the beginning of this lecture, that the 

 mica is much more effective in stopping the rays than is 

 the air. The extreme penetration of the rays from radium 

 C is only thirty-three thousandths of a millimetre — a 

 distance invisible to the unaided eye. This should be the 

 limiting radius of a halo formed from the elements derived 

 from uranium. If the thorium series was responsible, 

 then we might expect haloes having a radius extending 

 to the range of thorium C, that is, about forty thousandths 

 of a millimetre. Now these are just the dimensions we 

 find in the rocks when, by suitable appliances, we measure 

 the sizes of haloes. Some have a radial dimension of 

 0-033 rnm., and are then easily identified as due to the 

 uranium series, and some scale 0-040 mm. ; these are 

 thorium haloes. Many scores of measurements confirm 

 these results. Actinium haloes are not found ; and this 

 fact supports the inference already alluded to, that this 

 element is derived from uranium as a very subordinate 

 derivative, its effects being masked by the much greater 

 vigour of the radiations from the radium series of 

 elements. There is, then, no doubt, from the foregoing 

 evidence alone, that haloes are the result of radiant 

 matter. 



It is of much interest to note that Rutherford has 

 generated the equivalent of -a halo in glass. In the course 

 of experiments in which he had radium emanation con- 

 tained in a capillary tube, the halo developed as a coloured 

 border around the capillary, the radial dimensions being 

 just such as corresponded with the penetration of a rays 

 in glass. In the figure (Fig. 3), which I owe to the kind- 

 ness of Prof. Rutherford, the central dark band is the 

 capillary, the bordering narrow shaded area the halo. 



It may also be mentioned that the experimental applica- 

 tion of radium to biotite produces just such a darkening 

 of the mica after some months as we see in the natural 

 halo. 



The circular or disc-like appearance of the halo is due 

 to the fact that it is presented to us as the cross-section 

 of a sphere. The true form is spherical. This is proved 

 by the fact that when a crystal of mica is cut across the 

 cleavage, the form is still circular (Fig. 5). This shows 

 that the a rays are projected equal distances, or at least 

 produce equal effects, along and across the cleavage — a fact 

 not without considerable interest in itself, for it would 

 hardly be expected on first consideration. 



In the haloes which we have seen upon the screen there 

 is no differentiation between the effects of the slower 

 moving rays and those which move faster. The effects 

 of the former must lie inside those due to the latter. The 

 obliteration of the inner shells or spheres of ionisation is 

 explained on the same principles as account for the loss 

 of detail upon an over-exposed photographic plate. In 

 the case of over-exposure the contrast is lost, because the 

 effects of the lower lights have overtaken those of the 

 higher lights, a uniform blackening ultimately resulting. 

 If the radiant matter has been acting intensely on the 

 mica for a very long time, the several shells of ionisation 

 are merged in the accumulation of the feebler effects 



