726 Mr. & Mrs. Soddy and Mr. A. S. Russell on the 



Unfortunate arithmetical mistake in calculating the absorp- 

 tion coefficients caused him to assign values for these somewhat 

 less than one-half the real value. In consequence of this a 

 state of chaos exists in much of the literature of the subject. 

 Quite recently Tuomikoski has published absorption curves of 

 the radium 7-rays in lead from 0*4 to 18 cm. in which the 

 value of X decreased from 0'70 to 0*25. Still further support 

 of the heterogeneity of the 7-rays of radium is to be found in 

 papers by Kleeman (Phil. Mag. 1907, xiv. p. 643) and Madsen 

 (ibid. 1909, xvii. p. 447) who claim to have analysed the rays 

 into three and two homogeneous radiations respectively of 

 different penetrating power. On the other hand, Sir J. J. 

 Thomson has worked out the theory of absorption of the 

 7-rays, assuming that scattering takes place as in the case of 

 the cathode and /3-rays, and has shown that departures of the 

 absorption curves from the simple exponential law are to be 

 expected if scattering takes place even for a homogeneous 

 beam. (' Conduction of Electricity through Gases,' 1906, 

 2nd ed. p. 407). 



With regard to the 7-rays of uranium, Eve (Ph/s. Zelt. 

 1907, viii. p. 783), working with uranyi nitrate drew attention 

 to the relative poverty of uranium in 7-rays as compared to 

 /3-rays. He found the uranium 7-rays were absorbed 

 approximately exponentially in lead with a value for X 1'4, 

 which is about three times the value for the radium 7-rays, 

 In our previous paper (F. Soddy and A. S. Russell, Phil. 

 Mag. 1909, xvi. p. 620) the 7-radiation of uranium X pre- 

 parations from about 45 kilograms of uranyi nitrate was 

 exhaustively examined. The 7-rays w r ere naturally far more 

 powerful than those at Eve's disposal, and in addition the rays 

 had not first passed through a considerable thickness of 

 material as in Eve's experiments with uranyi nitrate. We 

 found the absorption did not proceed exponentially in any 

 substance until a thickness equivalent to 1 cm. of lead had 

 been penetrated and then the absorption coefficient remained 

 constant (and, except for lead, mercury, and very light sub- 

 stances, proportional to the density) at least until 5 cm. of 

 lead or the equivalent had been penetrated. The value of 

 X for substances of density between copper and aluminium 

 was only 1*18 times the corresponding value for radium 

 7-rays, while lead was somewhat anomalous with the ratio 

 1*465. The value of X found for lead for the radium 7-rays 

 was 0'495 from 1 cm. to 9 cm. The results for the initial 

 part of the absorption curves up to a thickness equivalent to 

 1 cm. of lead were reserved for the present communication. 

 It was obvious that the whole question of the absorption of the 



