40 



ROYAL SOCIETY OF CANADA 



secondary radiation from the plate. Further increase of thickness 

 causes a very gradual decrease in ionization owing to the slow ab- 

 sorption of the ;- rays. In fact the curve has an equation involving 

 approximately the difference between two simple exponentials. 

 Provided, therefore, that the plates at A and B are more than one 

 or two millimetres thick, it makes but a minute difference, in the 

 reversal method, if they are several millimetres thick, so gradual 

 is the absorption of the y rays. The changes produced in the elec- 

 troscope on reversing A and B are, then, mainly dependent not 

 on the thickness but on the atomic weight of the plate in position B. 

 This conclusion was confirmed experimentally. An exception 

 must be made in the case of lead, for an increase of thickness of the 

 lead plates hardens the ;- rays rapidly unless they have previously 

 passed through screens of lead. 



The method of procedure adopted was as follows. Tin and 

 lead plates were placed at A and B and a measurement taken; the 

 plates were reversed, and again a reading made. The readings in 

 the electroscopes were in the ratio of 100 to 92. Lead was similarly 

 compared with all the other substances as shown in the first column 

 of Table II. Then tin, assumed 92, was compared with the remain- 

 ing substances as given in the second column, and so forth. 



Ra Bfj 14 mg. in lead 0.6 cm. thick. Distance 15 cm. 



The corresponding curve II., fig. 2, shows the effect of the atomic 

 weight of plate B on the electroscope'^reading. Similar curves were 

 obtained by Bragg/ who used 11 elements. It appears difficult 

 or impossible to'estimate the proper deduction to be made from the 

 mean values in the right hand column so as to arrive at the absolute 

 values of the emergent secondary radiations. The readings given 

 in Table II. were obtained with a vertical cylindrical zinc electro- 

 scope 21 cm. high, 16 cm. diameter, having a thin aluminium base 

 with the radium placed below it at 15 cm. distance. The plates 

 A and B were clamped close to the aluminium. 



' Trans. Roy. Soc. S. Aust. 7, 32, 1908. 



