PLEOCHROIC HALOES 141 



pany uranium and radium, but in very subordinate 

 amount. The absence of actinium haloes clearly 

 supports this view. For if actinium was an inde- 

 pendent element we would be sure to find 

 actinium haloes. The difference in radius should 

 be noticeable. If, on the other hand, actinium 

 was always associated with uranium and radium, 

 then its effects would be submerged in those of 

 the much more potent effects of the uranium 

 series of elements. 



It will have occurred to you already that if the 

 radioactive origin of the halo is assured, the shape 

 of a halo is not really circular, but spherical. 

 This is so. There is no such thing as a disc- 

 shaped halo. The halo is a spherical volume 

 containing the radioactive nucleus at its centre. 

 The true radius of the halo may, therefore, only 

 be measured on sections passing through the 

 nucleus. 



In order to understand the mode of formation 

 of a halo we may profitably study on a diagram 

 the events which go on within the halo-sphere. 

 Such a diagram is seen in Fig. 4. It shows to 

 relatively correct scale the limiting range of all 

 the alpha-ray producing members of the uranium 

 and thorium families. We know that each mem- 

 ber of a family will exist in equilibrium amount 

 within the nucleus possessing the parent element. 

 Each alpha ray leaving the nucleus will just 

 attain its range and then cease to affect the mica. 

 Within the halo-sphere, there must be, therefore, 



