182 Mr. D. C. H. Florance : A Study of the Ionization 



from Al to Pb. Hence we conclude that the ft rays from 

 the walls of the vessel are similar to the ft rays from 

 Ur X. With 7 rays we have not only this " wall effect," 

 but we have also the " gas effect" ; and this shows itself in 

 the final slope of the curve, which at 80 atmospheres is still 

 increasing, and at a rate apparently dependent on the material 

 of the plates. 



Taking the simplest case, that of two plates of aluminium 

 1 cm. apart, curve B (fig. 7), between and 20 atmospheres, 

 is similar to that obtained by Laby and Kaye (Phil. Mag. 

 Dec. 1908) when the sides of their ionization-chamber were 

 aluminium and at a distance 1*1 cm. apart. Assume that 

 the average coefficient of absorption of the ft rays proceeding 

 from the walls is equal to 0*04, which was the experimental 

 value found for the ft rays of Ur X. The ionization-pressure 

 curve for these ft rays reaches a maximum at about 80 atmo- 

 spheres. For y rays, when the plates are 1 cm. apart, the 

 ionization due to the ft rays set up in the gas should be 

 small at 5 atmospheres. If we assume, then, that most of the 

 ionization in the gas at this pressure is due to the ft rays 

 from the walls, the ionization at the other pressures can be 

 represented by the curve a of fig. 9. The difference between 

 the curves B and x is the curve y, which consequently repre- 

 sents the value of the ionization due to the y rays in the gas 

 at the different pressures. It seems improbable that the 

 effect duo to the y rays in the gas is smaller than that repre- 

 sented by the curve y. 



The " gas effect " is given by the expression . 



k'm-k f (i-e-^)i\', 



where Jc' and \' are the mass absorption coefficients for y and 

 ft rays respectively. A correction has to be made for the 

 ionization due to the ft rays that are reflected from the Al 

 plates. It is difficult to decide what values ought in this 

 case to be given to the absorption coefficients of the ft rays 

 before and after reflexion, and also the value to be assigned 

 to the constant of reflexion , By an arbitrary choice of these 

 values the curve y can be obtained. 



To determine the maximum limit of the y ray effect, a 

 tangent is drawn to the curve ft at 80 atmospheres and the 

 line Oz is drawn parallel to it. The straight line Oz would 

 represent the ionization due to the y rays in the gas if the 

 bottom plate had been the same density as air. Since the 

 bottom plate has cut out the ft rays that would have come 



