different Metals by Rontgen Rays. 295 



primary rays. The fact that in the case of lead and copper the 

 curves cross each other, and that in the case of zinc and nickel 

 the harder lies below the softer, would seem to show that for 

 these metals the harder primary rays produced less penetrating 

 secondary rays. But there are several reasons to account for 

 this. In the first place, for zinc, nickel, and copper a compar- 

 atively large proportion of the secondary rays consists of those 

 of the Rontgen type which are much more penetrating than the 

 corpuscular rays, and in the case of lead there is an appreciable 

 quantity of these rays present. Owing to the shortness of the 

 ionization chamber, U A," a considerable proportion of the 

 secondary Rontgen rays doubtless passed entirely through the 

 chamber, producing very few ions. If now the hard primary 

 rays produce secondary rays of the Rontgen type more penetra- 

 ting than those produced by the softer primary rays, a larger 

 proportion of these penetrating secondary rays would pass 

 through the chamber, without producing ions, than of the less 

 penetrating ones. Thus the less penetrating rays would produce 

 more ionization, and the curve corresponding to them would lie 

 higher than the curve corresponding to the more penetrating 

 secondary rays. This reasoning would not apply to the corpus- 

 cular secondary rays, since the most penetrating of these are 

 absorbed by a few millimeters of air. But in all but two cases 

 the part of the curve corresponding to the corpuscular rays 

 shows greater penetration when the corpuscles are produced 

 by hard primary rays than by soft. In the two exceptions, 

 namely zinc and nickel, there are so many of the Rontgen type 

 of secondary rays present that they would obscure the effect 

 of the corpuscular type, and the above reasoning would again 

 apply. 



In order to show more clearly the variation of penetrating 

 power of the corpuscular secondary rays for different hardnesses 

 of the primary, I have subtracted from the whole curve the 

 amount, shown by the broken straight lines, which seems to be 

 about the proportion of secondary Rontgen rays present ; and 

 again plotted the curves after reducing the initial intensity to 

 unity. These are shown by the broken curved lines. The 

 intensities are given in columns III and III' of tables 1 to 7. 

 In every case the curve corresponding to hard primary rays 

 lies above that for the soft, except in nickel and copper, where 

 they cross very near the last point. This is probably due to 

 not allowing enough for the secondary Rontgen rays. 



If we consider the amount of the rays left unabsorbed after 

 passing through -002 mm of aluminium, as shown by these latter 

 curves, it is found to be between 0*10 and 0*20 of the total 

 amount of corpuscular rays coming off. This agrees very well 

 with the absorptions found by Seitz for cathode rays of high 



