Question of the Homogeneity ofy-JRays. 729 



electroscope, the uranium X preparations being 14*63 cm. 

 below the upper surface of the base. The curves are shown 

 in fig. 1 (PI. XII.). The ordinates represent logarithms of the 

 ionization and the abscissae the thickness multiplied by density 

 of material, i. e. what may be termed equivalent thickness. 

 The ordinates have been corrected for the decay of the activity 

 of the preparation during measurements and so the curves show 

 the approximate relative magnitude of the ionization when 

 equivalent thicknesses of various materials form the base of 

 the electroscope. The actual results are plotted and no 

 attempt has been made to smooth the curves. In the case of 

 lead part of the irregularities are doubtless due to irregu- 

 larities in the thickness of the only foil then available. It 

 will be seen that the curves arrange themselves one above the 

 other in oider of the density of the material, the lighter 

 substances giving for equivalent thicknesses far more 

 ionization than the denser. The secondary radiation pro- 

 duced by the primary y-rays of: uranium X from the emergent 

 surface of the absorbing plates obviously increases as the 

 density of the material diminishes. According to the neutral- 

 pair theory of y-rays the emergent radiations should all be 

 equal if the density law of absorption holds true lor both {3- 

 and y-rays, and if the latter are homogeneous (Bragg and 

 Madsen, Phil. Mag. 1908, xv. p. 670). The dotted lines 

 in the curve represent the prolongation back to the zero axis 

 of the straight part of the absorption curves over the range 

 at which absorption is exponential. The lead and iron dotted 

 lines are from actual observations taken at the same time as 

 the others with greater thicknesses of metal than can be 

 shown on the curve. Unfortunately in other cases it was not 

 possible to do this. The zinc dotted line is drawn with an 

 arbitrary ordinate, from the value of \, 0*37. before found. It 

 will be seen that the curves differ from each other progressively 

 as the density of the material decreases. For zinc the curve 

 is not very different from what would be the case if it 

 consisted of two superimposed curves, one due to the hard 

 y-ray? and a second due to a solter type with a coefficient of 

 absorption about 14 times greater. But for the lighter materials, 

 for example cardboard and aluminium, this is not the case. 

 With diminishing thickness the slope of the curve at first is 

 greater than that of the hard y-rays, then diminishes and runs 

 for a little practically parallel to it, and then increases again. 

 But just before the /9-rays begin to enter the electroscope a 

 well-marked diminution of slope is again noticeable in the 

 card curve. 



Attention may first he directed to the dotted lines. These 



