"68 Prof. J. 0. McLennan on the 



the reductions were made, we have 



I lp +1 ls = 90-05 (iv.) 



I/p-Ma.s-49-5 (v.) 



and 100 T , . x 



From which we have 



h p = 33-Oc 



I/, = 57-00 



I« = 16-3, 

 or Ii s = 1-735 I lp , 



and I M = -493 I lp . 



Similar calculations were made on the readings obtained 

 with cylinders Nos. 2 and 3, and the results of the three are 

 recorded in columns 4 and 5 of Table V. From these it 

 will be seen that the ionization produced in the air in a lead 

 cylinder by the gamma rays from radium is only one-half 

 that produced in it by the secondary rays excited in the lead 

 walls by these same rays. On the other hand, with gamma 

 rays of the same intensity entering an aluminium cylinder 

 of the same size as the lead one, the results show that the 

 ionization produced by the penetrating gamma rays is ap- 

 proximately twice that produced by the secondary rays 

 excited by these gamma rays. 



It will also be seen from the numbers given in the above 

 table, that we have sufficient data to calculate the ionization 

 produced by the radium in a cylinder of any material of the 

 same dimensions as those used in this investigation, provided 

 it was placed in the standard position indicated above. 



For example, Column 6 of Table V. gives the reduced 

 readings corresponding to the gamma rays alone which 

 entered the respective cylinders. From Table III. the ab- 

 sorption powers of these cylinders are known in percentages; 

 and by means of these numbers values can be calculated for 

 the ionization which would be produced in the same volume 

 of free air by the gamma rays from the radium. Column 7 

 of Table V. contains the values of J Ip corrected in this way, 

 and the mean of the results is 36-3. This number, it will be 

 seen, represents the ionization which would be produced by 

 the gamma rays from the radium, used in these experiments 

 in a cylinder of any metal 60 cms. high, and 24 cms. in 

 diameter, situated in relation to this radium exactly as the 

 cylinders were in the experiments described above on the 



