T. S. Taylor — Retardation of '" Alpha Rays" 177 



tities in these columns shows that possible errors in plotting 

 the Bragg carves (e. g., I and II in figure 3) produced no errors 

 of importance in the reduction. 



In this manner, the air equivalent of the metal sheets given 

 in Table II were determined for the various distances of the 

 sheets from the polonium. The values obtained are found in 

 Table III. 



Table III. 



Kange in 





















cms. of 

 entering 

 particle. 



A An 



B An 



A Pb 



B Pb 



A Sn 



BSn 



A Al 



B Al 



AAg 





















3-17 



0-635 



0-857 



0-923 



1-231 



0-886 



1-800 



0-574 



1-040 



0-676 



2-97 



0-628 



0-849 



0-914 



1-220 



0-879 



1-787 



0-574 



1-029 



0-673 



2-77 



0-621 



0-839 



0-903 



1-207 



0-871 



1-768 



0-573 



1-017 



0-669 



2-57 



0*614 



0*828 



0-891 



1-193 



0-862 



1-744 



0572 



1 -005 



0-664 



2-37 



0-607 



0-816 



0-878 



1-178 



0-852 



1-718 



0-571 



0-990 



0-659 



2-17 



0-599 



0*801 



0-861 



1-160 



0-840 





0-568 



0-970 



0-652 



1-97 



0-591 



0-783 



0-843 



1-138 



0-827 





0-563 



0-949 



0-644 



1-77 



0580 



0-761 



0-819 





0-810 





0-555 





0-635 



1-57 



0-569 



0-735 







0-797 





0-544 





0-625 



1-37 



0-553 













0-532 





0-613 



The values of the air equivalents for each of the metal 

 sheets recorded in Table III represent the results obtained 

 from a series of determinations similar to that given for A Au 

 in Table I. The agreement in the values, obtained for the 

 different positions of the polonium, was equally as good as that 

 for the A Au. These separate tables for each metal sheet 

 have been omitted for the sake of brevity and only the average 

 results given in Table III. 



The curves in figure 4 represent the results as recorded in 

 Table III. By noting the slope of these curves, one can 

 obtain some comparison of the rate at which the air equiva- 

 lents of the various sheets are changing. For the thinner 

 sheet of Al, the air equivalent is almost constant for the higher 

 ranges, but, as the range of the entering a particle decreases, 

 the air equivalent decreases slowly and, in the lower ranges 

 the decrease becomes quite marked. The thicker Al, however, 

 shows a marked change in its air equivalent for even the high 

 ranges. The statement of McClung, Levin and Rutherford 

 that equal successive layers of aluminium foil diminish the 

 range by equal amounts seems to hold true for thin sheets of 

 foil when the range is high ; but when the metal sheet is 

 thicker, or for thin sheets when the range is low, it does not 

 hold. The slight difference however in the air equivalent of 



