194 Action of the a. Rays on Glass. 



from the walls of the capillary is the same at all points along 

 the tube, and even follows a slight bulge in the capillary 

 tube. 



The width of the coloured area was measured from the 

 photograph and also from longitudinal sections of a part of 

 the tube. The mean depth of the coloured area was *039 

 millimetre. From the law of absorption of the a rays 

 found by Bragg, viz., that the stopping power of an atom 

 for an a particle is proportional to the square root of its 

 atomic weight, it can be calculated from a knowledge of the 

 chemical composition of the glass, that the range of the 

 a particle from radium C in the glass should be about *041 

 millimetre. The agreement of theory and experiment is 

 sufficiently close to show clearly that the coloured area is 

 due to the a rays, and that the edge of the colouration marks 

 the extreme points that the swiftest a particles were able to 

 penetrate. 



The sharpness of the edge of the coloured area is a result 

 of a difference of refractive index between the altered and 

 unaltered glass. Dr. Stansfield kindly made an examination 

 of this point for me, and found that the altered glass had a 

 refractive index several parts in ten thousand higher than 

 the unaltered part. Whether this change in refractive 

 index results from chemical changes in the composition of 

 the glass due to the action of the a rays, or is due to the 

 helium fired into the glass, is difficult to fix with certainty. 

 It can readily be calculated from the magnitudes involved 

 in the experiments that if the helium fired into the glass in 

 the form of a particles remained in the glass, its presence 

 would increase the refractive index of the glass several parts 

 in ten thousand. 



The darkening observed in the glass tube is not to be 

 ascribed to the result that one end was kept for several days 

 in liquid air. A similar effect was noted in the part not 

 exposed in the liquid air, and also in other glass tubes which 

 had contained sufficient radium emanation to strongly colour 

 the glass. There can be no doubt that the effect artificially 

 produced in glass corresponds to the halos observed in mica, 

 and confirms the correctness of the interpretation of Professor 

 Joly. It is intended to continue experiments of a similar 

 character, using other materials beside glass to contain the 

 emanation. 



I desire to thank Dr. Stansfield for^ his assistance in the 

 photographs, and Mr. Hickling for his kindness in cutting- 

 sections of the glass tubes. 





