366 Taylor — Retardation of Alpha Rays oy Metals. 



be detected by measuring directly the air-equivalent in the 

 two positions. This is probably the explanation of the above 

 statements by McClung, Levin and Rutherford. 



Since the air-equivalent of a metal sheet decreases with the 

 speed of the alpha particle entering it, the ratio of the air-equiva- 

 lent 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 I. For 

 the hydrogen sheets, on the contrary, the same ratio should 

 increase as the thickness of the cell or sheet of hydrogen 

 increases. This is also confirmed by the last column of 

 Table I. 



While the air-equivalent of the sheet of celloidin remains 

 constant the hydrogen-equivalent of the same does not remain 

 constant but decreases as the range of the alpha particle in 

 hydrogen decreases. The curve " Celloidin in Hydrogen," 

 figure 1, which was plotted from the results recorded in Table 

 IY, illustrates this point. It is to be noted also from the curve 

 " A Gold in Hydrogen," figure 1, that the rate at which the 

 hydrogen-equivalent of the A gold decreases is much greater 

 than the rate at which its air-equivalent decreases. The curve 

 designated "A Gold in Air," figure 1, is the portion of the 

 " A Gold " curve in the same figure that lies to the left of the 

 abscissa, 3*0. The coordinates of that portion of the curve 

 are magnified about 4 2/3 times so as to be plotted on the 

 same scale as the curves obtained in the hydrogen atmosphere. 

 4 2/3 is the ratio of the thickness of a hydrogen sheet to its 

 air-equivalent when near the radium. The slope of the curve 

 " A Gold in Air " is practically the same as that of " Cel- 

 loidin in Hydrogen," as can be seen from the figure. The 

 angle which the curve " A Gold in Hydrogen " makes with 

 the curve " A Gold in Air " is about the same as the angle 

 which the curve " Celloidin in Hydrogen " makes with the 

 axis of abscissas. The slope of the curve "A Gold in Hydrogen" 

 is nearly 3 3/4 times the slope of the curve " Celloidin in 

 Hydrogen." But 3 3/4 is the ratio of the square root of the 



atomic weight of gold to that of air y 1 = 3 -75 + 



Hence the rates, at which the hydrogen-equivalents of the 

 gold and celloidin sheets decrease with the speed of the alpha 

 particle entering the sheets, are proportional to the square 

 roots of their respective atomic weights. Moreover the slope 

 of the curve "Celloidin in Hydrogen" is numerically equal 

 (but of opposite sign) to the slope of the curve " B Hydrogen " 

 in air. The hydrogen-equivalent of the celloidin sheet was 

 somewhat larger than the thickness of the "B Hydrogen" 

 cell, but it seems entirely proper to conclude that the rate at 



