Taylor — Retardation of Alpha Bays by Metals. 367 



which the hydrogen-equivalent of the celloidin sheet decreases 

 with the speed of the alpha particle, is the same as the rate at 

 which the air-equivalent of the B hydrogen increases as the 

 speed of the entering alpha particle decreases. 



The possibility that the observed variations in the ionization, 

 which have been taken to be the measures of the changes in 

 the air-equivalents, may be due to secondary rays is precluded 

 by the fact that numerous direct determinations of the Bragg 

 ionization curves with and without the metal sheets near the 

 polonium and again near the ionization chamber, showed no 

 irregularities in the curves, as would be expected were second- 

 ary rays present in any appreciable amount. The behavior of 

 the air-equivalents of the hydrogen sheets in no way conforms 

 to what might be expected to be produced by secondary rays. 



The increasing of the air-equivalents of the hydrogen sheets 

 and the decreasing of the hydrogen-equivalents of the celloidin 

 sheets when moved away from the source of rays gave occasion 

 for suspecting that some differences might be found to exist 

 between the Bragg ionization curves obtained in atmospheres 

 of air and hydrogen respectively. To determine these curves 

 use was made of an apparatus, constructed for Mr. F. E. 

 Wheelock of this Laboratory, which was similar to the one 

 used thus far in the work except that the vessel enclosing the 

 main part of the apparatus could be completely exhausted. 

 To make any comparison of the two ionization curves it was 

 necessary to determine them under similar conditions, i. e. the 

 same source of rays was used in the two cases and the pressure 

 of the air was so reduced as to make the range of the a-particles 

 in air equal to their range in hydrogen at normal pressure. 

 Polonium was used as the source of rays and several Bragg 

 curves were obtained in hydrogen at normal pressure and in 

 air at a reduced pressure of about I7 cms of mercury. Two of 

 the curves are shown in figure 2. The dotted portion of each 

 curve is assumed to be the form it would take were it possible 

 to move the polonium entirely up to the ionization chamber. 

 At all events, these assumed portions of the curves can differ 

 but little from what the actual curves would be. 



It is to be observed that the two curves in figure 2 present 

 slight differences in form. The probable interpretation 

 of these differences will now be considered. Any given 

 abscissa of either curve is a measure of the ionization produced 

 by the particles in the gas in the chamber when the polonium 

 was at a distance from the chamber represented by the ordinate 

 corresponding to the given abscissa. Consequently the total 

 area enclosed by the two axes of reference and either curve is 

 proportional to the total ionization produced in the gas in 

 which the curve was determined. By measuring these areas 



