[eve] 



PRIMARY AND SECONDARY GAMMA RAYS 



37 



in the experiments described the foil produced a negligible effect 

 on the secondary rays investigated, when proceeding from the sur- 

 faces of plates close to the aluminium. The radium, 14. mg. pure Ra 

 Br^, was placed sometimes entirely inside a closed hollow cylinder 

 of iron or lead of the desired thickness, sometimes in a cylinder 

 such as H, with an open end so that a cone of rays was produced 

 passing through the electroscope, 12 cm. long. The results obtained 

 by these two methods'Jwere not different when suitable metal screens 

 were placed at S. Plates of various substances were placed at A, 

 B, C, D; these will always be referred to as plates and in the order 

 A, B; C, D, and the electroscope will always be understood as between 

 B and C. It must further be noted that the y rays from the radium 

 C traversed the electroscope and produced ionization, and that the 

 secondary rays from the top, bottom and two walls of the elec- 

 troscope further contributed to the total ionization, and no attempt 

 has been made to make deductions for these. It was merely desired 

 to ascertain the effects of reversing the plates at A and B, and those 

 at C and D. 



An example will make this clear. 



The last column gives the four readings obtained for the stated 

 four arrangements of the plates. The order given was always fol- 

 lowed and in future the results may be given as percentages, 

 thus, 



(Al, Pb) 100, 99, 83.5 81. 

 Substituting carbon for aluminium the readings gave, 



(C, Pb) 100. 105, 80, 84. 

 Bragg has published about a dozen readings of this character. I 

 find that, for reasons subsequently stated, the product of the ex- 

 trem.e is nearly equal to the product of the mean readings. 



