T. S. Taylor— Retardation of " Alpha Rays." 179 



Since the air equivalent does decrease with the range of the 

 a particle entering it, the ratio of the air equivalent to the 

 thickness of a given sheet of metal should be less than the 

 same ratio for a thinner sheet of the same metal. This is 

 shown to be true by the last column of Table II. From the 

 curves in figure 4, we see that the change is more pronounced 

 the thicker the metal foil and the heavier the metal. 



The possibility that the observed variations in the ionization 

 may be due to secondary rays is precluded by the fact that 

 numerous direct determinations of the Bragg ionization curves 

 with the metal sheets near the polonium and again near the. 

 ionization chamber showed no irregularities in the curves, as 

 would be expected were secondary rays present in any appre- 

 ciable amount. 



Thus far, from a comparison of the data given in Table III 

 and represented graphically in figure 4, I have been unable 

 to deduce any definite statement relative to the rate of change 

 in the air equivalents, except that, for the same metal, the 

 thicker the foil the more marked is the change and that, for 

 sheets of different metals of about the same air equivalents, 

 the rate of change is in the order of their atomic weights ; 

 (i. e., Al, Ag, Sn, Au, Pb). Further experiments are in pro- 

 gress and it is hoped that they will furnish the desired infor- 

 mation, when completed. 



• In conclusion, I wish to express my gratitude to Prof. 

 Bumstead, at whose suggestion these experiments were under- 

 taken, for his valuable suggestions, and interest in the work ; 

 also to Prof. Boltwood, who kindly prepared the polonium for 

 me and gave me many helpful suggestions. 



Results. 



The air equivalents of metal foils decrease with the range 

 (i. e., with the speed) of the a particles entering them. The 

 change is very small for thin foils of the lighter metals when 

 the range of the a particles is high ; but, when the range is 

 low for thin sheets, or when the sheets are thicker, the change 

 becomes quite marked. In comparing the change for sheets 

 of different metals of nearly equal air equivalents, the rate of 

 change is seen to be in the order of the atomic weights of the 

 metals. 



New Haven, Conn., June 15, 1908. 



