G6S Softening of llontgen Rays in Transmission through Matter. 



composite beam, absorbs 38'G per cent, after cutting down by 

 iron, and 38*2 per cent, after cutting down by nickel. 



Other experiments have been carried out on similar lines, 

 using anticathodes of different metals. In general, the phe- 

 nomena observed are of the same kind as those already 

 described. There are one or two outstanding features which 

 are being further investigated, These include : — 1. An ap- 

 parent considerable softening of a primary beam from certain 

 anticathodes when tested bv aluminium after having been 

 cut down by aluminium. 2. A variation in the components 

 of primary beams of the same average hardness produced 

 under different conditions. 



In conclusion, we wish to point out the importance of 

 using as a test-substance, when comparing the penetrating 

 powers of different beams, one in which the fraction of the 

 total absorption due to the emission of secondary characteristic 

 radiation is small, e.g. Al, C, &c. 



Attention may also be directed to the use of the dis- 

 tinguishing properties of the characteristic radiations of 

 various elements as affording an effective means for the 

 analysis of heterogeneous beams. 



Summary. 



An apparent softening of a heterogeneous primary beam 

 in the process of transmission through matter has been 

 observed. 



This effect is shown to be connected with the selective 

 absorption by a substance of those constituents of the beam, 

 which can readily excite its characteristic homogeneous 

 radiation. 



Confirmation of this view has been obtained by an analysis 

 of the primary beam. 



We wish to place on record our appreciation of the kindly 

 interest Professor Wilberforce has shown and the encourage- 

 ment we have received from him throughout the course of 

 these experiments. 



Georsre Holt Physics Laboratory, 



University of Liverpool. 



March 27th, 1911. 



