Secondary Rontgeri Radiation from Carbon. 781 



filter-paper as radiator. Filter-paper was used as thin sheets 

 of carbon are not readily obtained. The fact that filter-paper 

 contains oxygen and hydrogen as well as carbon does not 

 affect the argument. 



The air effect for the secondary beam was about 26 per cent, 

 of the total. Table VI. (a) indicates that the secondary beam 

 is 2 per cent, softer than the primary beam. We may, 

 therefore, assume that the obliquity, &c. correction for the 

 secondary beam amounts to 2 per cent. When the primary 

 beam was harder the results given in Table VI. (b) were 

 obtained. The secondary beam was then appreciably harder 

 than the primary, indicating that a hard fluorescent radiation 

 was given off by the filter-paper. The probability is strong 

 that this is due to the carbon contained in it. When a thick 

 piece of carbon was used as radiator and the hard portions 

 of a hard beam allowed to fall on it, the results could not be 

 so simply deduced as it absorbed about 50 per cent, of the 

 beam incident on it. As, however, the beams used were not 

 very heterogeneous, the correction due to the absorption of the 

 secondary and primary beam was not great. It was found that 

 when a moderately hard beam was used, the hard portions of 

 it produced a radiation in the carbon which after absorption in 

 the radiator itself was approximately of the same penetrating 

 power as the primary. The thickness of aluminium succes- 

 sively used as test pieces were 0*15, 0*8, and 1*4 mm. The 

 corrections due to the absorption of the primary and secondary 

 beam by the radiator would make the secondary radiation 

 softer than the primary. As, however, there will be a con- 

 siderable amount of extra radiation emitted which is softer 

 than the primary it is impossible to say at present whether 

 this softening is due to the extra radiation or to the fluores- 

 cent radiation, or to both jointly. Further experiments 

 need to be made. 



The experiments on the absorption of the carbon radiation 

 have so far been confined to one particular direction, viz., 

 about 90° to the direction of the beam. A number of ex- 

 periments were made using the apparatus shown diagram- 

 matically in fig. 1. It was found that the radiation in the 

 direction «=90° was softer to a first sheet than the radiation 

 in the direction a = 150°. The results of a number of values 

 were averaged, and are given in Table VII. (p. 782). 



These results, though apparently anomalous, simply mean 

 that the softer portions of the beam have a low value of 



j^ , as also have the very hard portions, which we have 

 explained as being due to the presence of iron and carbon 



