y-Rays of Thorium and Actinium. 131 



by the short-lived /3-ray product between mesothorium 1 and 

 radiothorium, which we shall refer to as mesothorium 2 

 (Halm, Phys. ZeiU 1908, ix. pp. 245 & 246), and that given 

 by the last known product of the disinte oration series, 

 thorium D (Hahn and Meitner, Phys. Zeit. 1908, ix. p. 649). 

 The paper conveniently divides itself into three sections. The 

 first deals with the relative intensity of the 7- and /3-rays of 

 these substances. In the second section a number of so far 

 unexplained effects in the measurement of the absorption 

 coefficients of the 7-rays are described in detail. In the last 

 section the penetrating power of the actinium and thorium 

 types of 7-rays are compared with that of radium (J. A brief 

 indication of the general character of the results may con- 

 veniently precede their detailed consideration. 



The two thorium products resemble radium C remarkably 

 closely both in their <y//3 ratio, and in the penetrating power 

 of their 7-rays, and, although interesting differences exist, 

 these are comparatively small. The most penetrating 7-ray 

 known is that given by thorium D, that of mesothorium 2, 

 speaking in a general sense, being about as much less pene- 

 trating than that of radium C as that of thorium D is more 

 penetrating *. These three bodies are sharply distinguished 

 from all the other /3- and 7-ray products by their high and 

 similar y/j3 ratio. At the other extreme are radium B and 

 radium E, which we have not examined, the 7-rays from 

 which are, either not at all, or only barely detectable f. It is 

 perhaps still natural to leave radium B out of the compa- 

 rison, as its /3-rays are excessively feebly penetrating; but the 

 established very low value of the 7-rays of radium E, taken in 

 conjunction with the character of its /3-rays, which, although 

 feebly penetrating, are of the same order as those of 

 actinium C and mesothorium 2, furnishes another example 

 of the lack of connexion between the two types of rays. 

 We have to consider actinium C and uranium X, each of 

 which differs in this respect from all the other types. For 

 uranium X, as we have previously found, both /3- and 



* Apparently nothing lias been previously published with reference 

 to the y-rays of mesothorium 2 ; but it should be mentioned that Eve 

 {Phys. Zeit. 1907, viii. p. 185), in comparing- the y-rays of a preparation 

 of radiothorium (thorium D) with those of radium, found that they were 

 almost identical in penetrating- power, although the measurements indi- 

 cated that the radiothorium y-rays were a little the more penetrating - . 

 The difference, however, Eve considered to be within the error of 

 the experiments. 



t II. W. Schmidt, Phys. Zeit. 1906, vii. p. 764 ; Ann. Physik, 1906 

 [4] xxi. p. 609. Meyer and von Schweidler. Wien. Ber. 1906, ex v. It a. 

 p. 697. II. W. Schmidt, Phys. Zeit. 1907, viii. p. 361. 



K 2 



