JoLY — Radiant Matter. 



83 



around a radioactive body in equilibrium, may be studied at our leisure, 

 visibly inscribed in the ancient rooks. 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 

 disk-shaped marks. In the centre of each is a small crystal. This in most 

 cases is tlie 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 " pleochroio 

 lialoes," as they are called, lias been known to more than one generation of 

 petrologists, and has always excited interest. Their origin lias till lately 

 been unexplained. SoUas, 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 radioactive bodies, 

 it was found that zircons were intensely radioactive — a concentration of 

 uranium having in some manner taken place in these early formed bodies. 

 The minerals apatite and allenite are also sometimes conspicuously radioactive, 

 and around these, also, haloes often exist. 



Let us then suppose that the halo is due to the radioactivity of the minute 

 crystal around which it extends. We know tliat the radioactive 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 ionization, just as they colour 

 glass or affect a photographic plate, then there will be a certain region affected 

 extending just so far as the rays can penetrate and no farther. 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. 



o2 



