THE 



AMERICANJOURNALOFSCIENCE 



[FO URTH SERIES.] 



Art. XXI. — On the Retardation of "Alpha Rays" by 

 Metal Foils, and its Variation with the Speed of the Alpha 

 Particles ; by T. Smith Taylor. 



[Contributions from the Sloane Laboratory of Yale College.] 



Introduction. 



In 1905, Bragg and Kleeman* observed that the a-ray 

 ionization curve, obtained with a metal sheet over the sonrce 

 of the rays (radium), did not sutler the same drop from the 

 normal curve at all points. The portion of the curve corre- 

 sponding to radium C suffered a greater drop than that due to 

 radium itself. The loss of range of the a particle of radium C 

 in passing through the sheet of metal was thus greater than the 

 loss of ran^e of the a particle from radium in passing through 

 the same sheet. This seemed to be evidence that the slower 

 a particles from radium were a little less affected by their 

 passage through the sheet than the swifter a particles from 

 radium C. 



Kucera and Masekf measured the amount by which the 

 range of the a particles from radiotellurium (polonium) was 

 cut down by their passage through a sheet of aluminium 

 and a sheet of platinum. They measured this diminution in 

 range produced by the aluminium and the -platinum when the 

 sheet was directly over the radiotellurium and when the sheet 

 was 1*9 CIU from it. They found the diminution produced in 

 the latter case to be less, by 10 per cent for the aluminium 

 and 12*5 per cent for the platinum, than in the former case. 

 They also describe experiments from which one can conclude 

 that the range lost by an a particle in going through two sheets 

 of dissimilar metals is independent of the order in which it 

 passes through them. This appears to be inconsistent with the 

 previous statement. 



Experiments made by McClung4 Rutherford§ and Levin || 

 are also inconsistent with Bragg's results as well as with the 



* Phil. Mag., Sept. 1905. t Pnys. Zeitschr., xix, pp. 630-40, 1906. 



JPhil. Mag., Jan. 1906. § Phil. Mag., Aug. 1906. 



||Phys. Zeitschr., xv, 519-521, 1906. 

 Am. Jour. Sci.— Fourth Series, Vol. XXVI, No. 153.— September, 1908. 

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