132 Messrs. A. S. Russell and F. Soddy on the 



7-rays are similar in penetrating power to those o£ thorium 

 and radium C, but the y/j3 ratio is of an entirely dif- 

 ferent and lower order of magnitude. For actinium C, 

 both /3- and 7-rays are less penetrating, and the y//3 ratio, 

 although of the same order, is distinctly greater than for 

 uranium X. Indeed there is no rule about the matter. 

 From the fact that hard 7-rays usually accompany hard 

 /3-rays, it might be supposed that thorium D, which gives 

 the most penetrating 7-ray known, would have also the 

 highest y/ft ratio. Whereas it is distinctly lower, both than 

 that of radium C and that of mesothorium 2. 



Owing to the importance of the 7-ray method as a standard 

 means of comparison of radioictive substances, we have 

 thought it advisable in the second section to collect a number 

 of observations showing how the most curious changes of 

 penetrating power of the 7-rays are brought about by the 

 slightest change in the experimental disposition, although 

 we have no general explanation of these effects to offer. 



Section I. — The Ratio of the 7- to the ft-rays of 

 Mesothorium 2, Thorium D, Actinium 0, and Radium C. 



In a previous paper (I. p. 629) the ratio of the 7- to the 

 /3-rays of uranium X has been accurately compared under 

 the same conditions with the same ratio for radium C. It 

 was found, assuming the rays to be homogeneous, by mea- 

 suring the 7-rays through 1 cm. of lead and calculating the 

 initial intensities from the known absorption coefficients, 

 that the y/@ ratio for uranium X was about 50 times smaller 

 than that for radium 0. When the 7-rays were measured 

 through CH) cm. of aluminium, the ratio, uncorrected for 

 absorption, was 18 times less. Similar measurements have 

 now been extended to actinium and thorium. It may 

 be pointed out that we are measuring a highly complicated 

 effect in this y//3 ratio, and that before an} T absolute com- 

 parison is possible, it would be necessary to have a good deal 

 more data than are at present available. We have been 

 concerned only with the relative order of magnitude of the 

 ratio sought, and have attempted to get at least a rough idea 

 of this order by carrying out the 7- and /3-ray measurements 

 respectively for both substances under exactly the same 

 conditions. Considerable differences will be found in the 

 ratio according to the method of measurement employed, as 

 is only to be expected; but the general order of the effects 

 measured is sufficiently correctly indicated by the mea- 

 surements here oiyen. 



