142 Dr. C. A. Sadler and Mr. P. Mesham on Rontgen 



Identical results were obtained in each case within the limits 

 o£ experimental error (a radiator made up of several sheets of 

 tissue-paper behaved in a similar manner to a block of carbon). 



Other substances besides carbon were examined, but not 

 in such detail, e. g. a block of paraffin wax gave a softening 

 effect, though not quite to the same extent as a similar block 

 of carbon . 



We were forced to conclude that this first hypothesis was 

 no longer tenable. We had satisfied ourselves that in the 

 powdered carbon no appreciable amount of metallic sub- 

 stance remained after treatment. The behaviour of paraffin 

 and of paper appeared to negative the possibility of the 

 softening in carbon being due to organic impurities. For 

 had this been the case, we would have expected the softening 

 to have been much more pronounced in paraffin than in carbon. 



Heterogeneity of the Secondary Beams. 



It has been shown in previons papers that the secondary 

 radiation from substances like copper is nearly all of the 

 characteristic type. It was pointed out that the radiation 

 was remarkably homogeneous. This has been demonstrated 

 in two ways. The first, the absorption method, showed that 

 a thin sheet oi aluminium, say, absorbed as nearly as possible 

 the same percentage of a parallel secondary beam from 

 copper, when the beam was reduced to only 5 per cent, of 

 its original components by a suitable thickness of aluminium, 

 as of the original beam. This method of analysis is not so 

 sensitive as it might appear to be at first sight. For a 

 nearly pure homogeneous beam with a slight admixture of 

 other constituents differing slightly in hardness from the 

 main beam would behave in much the same way when the 

 absorption was pushed to this extent only. 



The second method* depends upon the fact that the 

 fluorescent radiation is only excited when the exciting radiation 

 is of a more penetrating type than itself. Now a sheet of pure 

 iron subjected to the radiation from a similar sheet of iron 

 emits an extremely feeble radiation compared with that pro- 

 duced when it is subjected to a more penetrating radiation. 

 But if the secondary radiation from iron had contained an 

 appreciable amount of a soft constituent, this soft constituent 

 would have been excited by the normal fluorescent radiation 

 from iron. On the other hand, if the iron radiation possesses 

 a constituent harder than the normal, it would excite the 

 normal constituent from the second plate. The radiation so 

 excited when iron radiation falls upon iron is as stated above 

 very feeble in amount, and is on the whole slightly softer 

 * Sadler, Phil. Mag. July 1909, pp. 107-132. 



