JoLY — Radiant Matter. 85 



biotite produces just such a darkening of fclie mica after some months as we 

 see in the natural halo. 



The circular or disk-like appearance of the halo is due to the fact that it 

 is presented to us as the cross-section of a spliere. The true form is spherical. 

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

 the cleavage, the form is still circular. (Plate IIIa, Fig. 2). 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. Tlie effects of the former must He inside those due to the latter. 

 The obliteration of the inner shells or spheres of ionization 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 blackeniug ultimately resulting. If the radiant matter has been 

 acting intensely on the mica for a very long time, the several shells of 

 ionization are merged in the accumulation of tlie feebler effects which are 

 always progressing at all points along the path of the ray as shown in tlie 

 Bragg em-ve. 



We should expect, however, to meet cases where, either from the 

 smalluess of the quantity of radioactive material, or from the recentness 

 of the formation of the rock, there is a proper or correct exposure, so that 

 the successive shells of ionization, which we may picture to ourselves as 

 surrounding a particle of pitchblende in air, would, as developed in the mica, 

 be made visible to the eye. In this anticipation we assume that Bragg's 

 laws apply to the ionization of a solid. 



Now, we do, indeed, find the several spheres of ionization — or at least 

 many of them — beautifully depicted in certain minerals ; and thus we, at 

 one and the same time, find additional, indeed overwhelming, evidence 

 that the haloes are due to a rays, and also, what would be hard to 

 establish experimentally, that Eragg's laws govern the effects in the solid 

 medium. 



Here is a group of well-exposed haloes in the biotite of Co. Carlow. 

 (Plate IIIa.) Tou see the outer ring due to Ea C, and the gap of feebler 

 ionization between it and the shell due to Ea A. We even find some which 

 are actually ' under-exposed.' These often have got no further than the 

 record made by the intense triple effect due to uranium and ionium. I 

 show you this photograph again, but this time with an engraved scale of 



