604 Mr. R. T. Beatty on Secondary 



remains finite except when sins(£ = 0. Thus i£ Bz be con- 

 strained to remain zero all round the circumference of the 

 bottom, no vibration of this kind is possible ; but the bottom 

 may be supported at the places defined by sin s<p = 0, which 

 are situated under the nodes N (fig. 1) of the radial motion 

 at the upper rim. 



If, as in the above experiments, we limit ourselves to the 

 principal vibration for which s = 2, and if the height be /, we 

 have for the maximum amplitudes 



Br z=t = 21, ah<f) z= i = I, Bz =s \a. . . . (2) 



These relations were verified by observation under the 

 microscope. The ratio of the maximum radial to the maxi- 

 mum circumferential motion at the upper rim was found to 

 be almost exactly 2 : 1. The accurate observation of Bz, as 

 found at the bottom, was more difficult on account of its 

 relative smallness. In the actual case (85 per second) / = 8 ins., 

 2a = 4 -J ins., so that 8£/2a = 14. This is the theoretical ratio of 

 the maximum radial motion at the rim to the maximum 

 axial motion. By observation the mean number was 15, as 

 close to theory as could be expected. 



Aug. 15, 1907. 



LX. Secondary Rontgen Radiation in Air. By R. T. 

 Beatty, M.A., B.E. (R.U.I.), Queens College, Belfast, 

 and University of Liverpool*. 



FROM experiments made by Barklaf, it seems that the 

 constituents of a Rontgen beam are scattered approxi- 

 mately in equal proportions by certain elementary substances, 

 while others transform the incident radiation into a more 

 homogeneous beam which is more easily absorbed by matter 

 than the primary radiation. Further, the nature of the 

 radiation from all the elements examined is found to vary 

 in a periodic manner with the atomic weight of the radiator. 



In observing the behaviour of those solids which scatter 

 radiations, the results are complicated by the fact that the 

 different constituents of the secondary beam come from dif- 

 ferent thicknesses of the solid. Thus the radiations coming 

 from deeper layers of the solid are robbed of their more 

 absorbable constituents in emerging, and so perfect scattering 

 of the primary beam is not to be expected. 



Now in air the absorption of Rontgen radiation is very 

 small, and as air is included in the category of scatterers 



* Communicated by Professor Wilberforce. 

 t Barkla, Phil. Mag. June 1906. 



