and Nomenclature of the Radioactive Emanations. 625 



nomenclature desirable, in order to give a definite name to 

 each of these products, and to specify its position in the 

 radioactive series. There seems to be no doubt that these 

 short-lived products do not exist in the gaseous state, but 

 behave as solids. This is borne out by the experimental 

 results included in this paper, for there is no evidence that 

 the products considered escape from the negatively charged 

 wire except by the well-known process of radioactive recoil. 



It seems fairly certain that the new products ought to be 

 included as members of the " active deposits " derived from 

 the thorium and actinium emanations. They resemble the 

 first member of the active deposit of radium, namely radium 

 A, in carrying ;i positive charge, and in their concentration on 

 the cathode in a strong electric field. If they are considered 

 to be members of the group of products now generally 

 included under the term "active deposit," there is seen to be 

 a remarkablv close analogy in the successive transformations 

 of the three emanations. It is seen that the new products 

 are very analogous, both as regards physical properties and 

 nature of radiation emitted, to the first product of the radium 

 emanation, namely radium A. It is, therefore, suggested 

 that the new product in thorium should be called thorium A, 

 and in actinium, actinium A. In consequence it is necessary 

 to denote the products previously called thorium A and 

 actinium A, thorium B and actinium B respectively, and 

 similarly for the later products. 



The scheme of nomenclature proposed, starting from the 

 emanations, is shown in the following table (p. 626), where the 

 nature of the radiation emitted and the half-value period of 

 transformation are added. 



In this scheme, it is seen that not only are the A products 

 similar in general character, but also the B products, for each 

 of the latter emits easily absorbed {3 rays. 



There is an apparent divergence in the mode of trans- 

 formation of the third product of the three series. In the 

 radium series, the product ordinarily called radium C emits 

 not only a rays but also ft and y rays. The third product of 

 the actinium series, now named actinium C, emits only one 

 type of a. rays, and is followed by another product to be called 

 actinium D, emitting ft and <y rays. This latter product was 

 isolated by Hahn usino- the recoil method. In the case of 

 thorium, the transformation of the second product, thorium B, 

 is followed by a product of half-value period 55 minutes, 

 which Hahn show T ed emitted two distinct types of a ravs, 

 w T hose ranges in air are 5*0 and 8*6 cm. In all other radio- 

 active transformations, it has been observed that each product 



