170 T. 8. Taylor— Retardation of " Alpha Bays." 



first statement of Kucera and Masek above. Rutherford 

 found that the a particle loses energy at a uniform rate dur- 

 ing its passage through aluminium. McClung; Levin, and 

 Kucera and Masek* obtained results indicating that each suc- 

 cessive layer of aluminium foil which is laid upon a radio- 

 active substance diminishes the range of the a particle by an 

 equal amount. 



On the other hand, Braggf observed that the stopping 

 power is not independent of the speed as stated by Rutherford, 

 McClung and Levin ; Bragg found that the ionization curve, 

 obtained when a sheet of gold foil was directly above the 

 radium C, suffered a greater drop than when the sheet was at 

 a distance of l - 5 cm from it. The range of the a particle when 

 two metals, Al and Au, were placed over the radium, was not 

 independent of the order in which they were arranged. The 

 range was diminished by a greater amount when the gold foil 

 was next to the radium and the aluminium over it than when 

 the order was reversed. Similar results were obtained for 

 aluminium and tin when their order was reversed as in the 

 above case. MeyerJ also observed this same effect. 



Kucera and Masek,§ and Meitner,| ascribe these latter 

 effects, as they observed them, to a difference in the amount 

 of the scattering of the rays by the two metals. Their argu- 

 ment as presented does not seem to be conclusive ; for the 

 scattering, if it exists, is a differential effect. 



In all the experiments cited, the method of obtaining the air 

 equivalents^ corresponding to different positions of the screen 

 has been to measure the range with the source uncovered and 

 then with the screen in place. The difference of these values 

 gives the air equivalent of the sheet. 



As, however, the variations in the air equivalent are small, 

 this is not a very accurate or sensitive method ; since the air 

 equivalent itself (whose small variations are to be observed) is 

 obtained as the difference between two much larger quantities 

 (the ranges) neither of which can be determined with great 

 accuracy. A method of observing the variations of the air 

 equivalent w T as sought which should be free from these dis- 

 advantages. 



Description of Apparatus. 



The apparatus, as shown in figures 1 and 2, w r as similar to 

 that used by Bragg. The ionization chamber, which was 5 cm 

 in diameter and 2 mm in depth, was formed by the wire gauze, 



*Loc. cit. f Phil. Mag., April, 1907. 



X Phys. Zeitschr., xiii, 425, July, 1907. §Ibid. vii, 19, 1906. 



|| Ibid, viii, 489, 190'7. 



"f[ By air-equivalent is meant the diminution in the range of the a particle 

 produced by the foil when placed over the source of rays, or the amount by 

 which the range of the a particles in air is cut down by their passage 

 through the foil. 



