84 



Scientific Proceedings^ Royal Dublin Society, 



We see, as might have been expected, and as indeed T showed you at the 

 beginning of this lecture, tliat the mica is mucli more effective in stopping 

 the rays than is tlie air. The extreme penetration of tlie rays from Ra 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 Th C, that 

 is about forty-thousandths of a millimetre. Now these are just the dimen- 

 sions we find in the rocks when, bj^ suitable ajopliances, we measure the sizes 

 of haloes. Some have a radial dimension of 0-033 mms., and are then easily 

 identified as due to the uranium series, and some scale 0'040 mms. : these are 

 thorium haloes. Many scores of measurements confirm these results. Aclinium 

 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. 



fe. 



capillary 



It is of much interest to note that Rutherford has geuerated the equivalent 

 of a halo in glass. In the course of experiments in which he had radium 

 emanation contained in a capillary tube, the halo developed as a coloured 

 border around the capillary, the radial dimensions being just such as corre- 

 sponded with the iDenetration of a rays in glass. In the figure, which I owe 

 to the kindness of Professor Rutherford, tlie central dark band is the capillary, 

 the bordering narrow shaded area, the halo. 



It may also be mentioned tliat the experimental application of radium to 



