144 Dr. C. A. Sadler and Mr. P. Mesliam onRontgen 



It will be seen from columns 5 and 6 that while the 

 secondary beam is cut down about 25 per cent, by a piece of 

 aluminium "02 cm. in thickness, the tertiary radiation is not 

 perceptibly reduced, while three thicknesses of *01 Al 

 reduces the tertiary radiation by less than 5 per cent. But 

 the soft radiation is practically all absorbed by the first two 

 pieces of aluminium, for the first sheet absorbs 20 per cent., 

 the second 7 per cent., while the third only 4 per cent, of 

 the beam, which is about the amount by which a beam con- 

 sisting solely of the hard constituent would be reduced by 

 such a thickness of aluminium. But the ratio of the original 

 ionizations produced by the soft and hard components re- 

 spectively is at least as high as 1*5. Jf this relatively high 

 proportion of soft radiation takes so small a share in the 

 excitation of radiation from carbon, we are probably justified 

 in concluding that the softened character of the carbon 

 radiation cannot be explained by reference to any lack 

 of homogeneity in the secondary beams used in these 

 experiments. 



Characteristic Radiation from Carbon. 



That the effect was not due to corpuscular radiation from 

 the carbon reaching the tertiary ionization chamber can be 

 easily demonstrated, for the shortest distance between these 

 two was at least 3 cm. But it was shown by one of us* 

 that the most penetrating type of corpuscular radiation 

 excited by a Rontgen beam failed to penetrate a distance of 

 more than 2 cm. in air at atmospheric pressure. The ex- 

 periment with tin as secondary suggested that hard rays 

 were most effective in exciting radiation in carbon. It now 

 became necessary to consider our third hypothesis — the pos- 

 sibility of there being a homogeneous radiation from carbon 

 superposed upon the scattered. 



It was evident that if we had a mixture of two homo- 

 geneous radiations— -the scattered homogeneous secondary 

 and the homogeneous tertiary characteristic of carbon — we 

 could by suitable absorptions by sheets of aluminium find the 

 absorption coefficient in Al of this latter component. 



For let l l and I 2 be the relative intensities of the two 

 homogeneous radiations. If a be the absorption by 1 cm. of 

 air of the radiation I l5 and /3 that of I 2 , the absorption in the 

 electroscope of these two radiations will be proportional to 



I^l-*-^) and J 2 (l- e - l3k ), 

 * Sadler. Phil. Mag. March 19] 0, pp. 338-356, 



