Radiation from Substances of Low Atomic Weight. 141 



of the tertiary to approximately the same figure in each 

 case. 



The tertiary ionization was not sufficiently intense to 

 enable the analysis to be pushed to extreme limits. 



A glance at column 3, however, shows that the signs of 

 heterogeneity are quite pronounced, while a comparison 

 between columns 3 and 4 indicates sufficiently the degree of 

 softening which has taken place. 



The observed effects might be reasonably explained by any 

 of the following hypotheses : — 



(1) That there were impurities present in the carbon, 

 presumably of a kind which would furnish secondary radia- 

 tions of a softer type than that of the secondary exciting 

 beam. The presence of such impurities would account for 

 both the heterogeneity and the softer character of the scat- 

 tered radiation as a whole. It was evident that the sub- 

 stances to be looked for were traces of metals of higher 

 atomic weight than carbon or of their salts. 



(2) That the exciting secondary radiation was imperfectly 

 homogeneous and that a preferential scattering by the carbon 

 of a soft constituent, which might be supposed to be present, 

 took place. 



(3) That carbon itself emits a homogeneous radiation which 

 is feeble in character but very much softer than any of the 

 exciting beams used. 



These hypotheses were examined in detail. 



Presence of Impurities. 



A piece of the carbon used as radiator was carefully 

 analysed, and was found to contain traces of iron but no 

 perceptible traces of any other metallic substances. The 

 amount of iron present was extremely small. To test how 

 far this impurity and that of any other metallic compounds 

 might explain the observed results, two similar carbon blocks 

 were taken, one preserved intact and the other ground to a 

 powder, and digested in a mixture of hot strong nitric and 

 hydrochloric acids for several hours. The mixture was 

 allowed to settle and the clear solution poured off. The 

 residue was washed and further digested with strong nitric 

 acid. Water was added, and the liquid was then filtered, 

 and this residue carefully washed with distilled water and 

 allowed to dry. The powder was made up into a radiator 

 on a frame (this frame was carefully shielded from the 

 secondary exciting beam) between two layers of tissue-paper. 

 The two carbon radiators were now compared in their 

 behaviour when subjected to the same secondary beam. 



