and Nomenclature of the Radioactive Emanations. 627 



emits a particles o£ characteristic range. It has, therefore, 

 been generally assumed that two successive products are 

 present, one of period of 55 minutes emitting a particles of 

 range 5*0 cm., followed by a product of probably rapid trans- 

 formation emitting « particles of range 8' 6 cm. Although a 

 number of attempts have been made by various observers, 

 it has not so far been found possible to separate the two 

 products; for they always appear together and decay together 

 with the same period, and each product seems to emit about 

 the same number of a. particles per second. 



Hahn has also shown that from a plate coated with the 

 active deposit of thorium, a product emitting ft and y rays 

 of period three minutes can be obtained by recoil. We thus 

 see that the analogy apparently breaks down at this point ; 

 for radium C or actinium C emits only one type of a. rays. 

 In addition, in the case of actinium and thorium, the peue- 

 trating ft and <y rays arise from a distinct product following 

 the a ray transformation. Recently, however, Hahn and 

 Meitner * drew attention to the existence of a new product in 

 radium C. This has been confirmed and examined in detail 

 by Fajans t, who found that the product has a period of 1*4 

 minutes, and emits only ft and probably 7 rays. The absorption 

 of these rays is about the same as for those ascribed to ordinary 

 radium C. This new product is obtained by recoil from pure 

 radium C, but in exceedingly small relative quantity, about 

 1/20000, measured by the ft rays. In this respect, it differs 

 markedly from the ft ray products obtained by recoil from 

 the corresponding products of thorium and actinium, where 

 the relative quantities obtained by recoil are about 10,000 

 times greater. Fajans has discussed the question of the 

 position of this new ft ray product in the radium series, and 

 concludes that radium breaks up in two distinct ways, and 

 that the new product is to be regarded as a lateral branch of 

 the main radium series. 



At the Radiology Congress in Brussels, the question of 

 nomenclature was discussed, and it was generally agreed that 

 if a product considered simple was shown to be complex, the 

 original name should, if possible, be retained to signify the 

 group, and that the individual components should be dis- 

 tinguished by numbers. For example, there are many 

 practical advantages in retaining the name radium C to 

 include the two or more components that may be present. 

 So far only two components have been definitely dis- 

 tinguished, and these will be called radium C x and radium C 2 . 



* Phys. Zeil. x. p. 697 (1909). 

 f Phys. Zeit. xii. p. 369 (1911). 



