17'2 HiOFKSSOR E. RUTHERFORD ON II 1 1. 



special name. At the same time, it is Incoming more and more necessary that each 

 product should be labelled in such a way as to indicate its place in the succession of 

 changes. This difficulty is especially felt in discussing the numerous changes in the 

 active deposits from the different emanations. Many of the names attached to the 

 products were given at the time of their discovery, before their position in the scheme 

 of changes was understood. In this way the names UrX, ThX were applied to the 

 active residues obtained by chemical treatment of uranium and thorium. Since, in 

 all probability, these products are the first products of the two elements, it may be 

 advisable to retain these names, which certainly have the advantage of brevity. The 

 name ' emanation ' was originally given to the radioactive gas from thorium, and has 

 since been applied to the similar gaseous products of radium and actinium. 



Finding the name ' radium emanation ' somewhat long and clumsy, Sir WILLIAM 

 RAMSAY* has recently suggested ' ex-radio ' as an equivalent. This name is certainly 

 brief and is also suggestive of its origin ; but at least four other ex-radios, whose 

 parentage is as certain as that of the emanation, remain unnamed. A difficulty 

 arises in applying the corresponding names ex-thorio, ex-actinio to the other gaseous 

 products, for, unlike radium, the emanations of thorium and actinium are probably 

 the second and not the first disintegration product of the radio-elements in question. 

 Another name thus has to be applied to the first product in these cases. It may be 

 advisable to give a special name to the emanation, as it so far has been the product 

 most investigated and the first to be chemically isolated ; but, on the other hand, 

 the name ' radium emanation ' is historically interesting, and suggests a type of 

 volatile or gaseous matter. Since the term ' excited ' or ' induced ' activity refers 

 only to the radiations from the active body, a name is required for the radiating 

 matter itself. The writer some time ago suggested the name ' emanation X.' t This 

 title was given from analogy to the names UrX and ThX, to indicate that the active 

 matter was product of the emanation. The name, however, is not very suitable, and, 

 in addition, can only be applied to the initial product deposited, and not to the 

 further products of its decomposition. It is very convenient in discussing mathe- 

 matically the theory of successive changes to suppose that the deposited matter 

 called A is changed into B, B into C, C into D, and so on. I have therefore 

 discarded the name emanation X, and have used the terms radium A, radium B, and 

 so on, to signify the successive products of the decomposition of the emanation of 

 radium. A similar nomenclature is applied to thorium and actinium. This system 

 of notation is elastic and simple, and I have found it of great convenience in the 

 discussion of successive products. In speaking generally of the active matter, which 

 causes excited activity, without regard to its constituents, I have used the term 

 ' active deposit." The scheme of nomenclature employed in the paper is clearly shown 

 below : 



* ' IW. Roy. Soc.,' p. 470, June, 1904 ; '('..nipt.-; Rctuins,' 138, June 6, 190-1. 

 t '1'hil. MM-;.,' February, 1904. 



