880 Dr. J. W. Nicholson on a Structural 



considerable reason for the belief that this gas is the actinium 

 emanation. 



Thorium Emanation. — If the thorium emanation is derived 

 from thorium by the expulsion of two a particles, both being 

 helium atoms, the atomic weight of this emanation is about 

 224*4, or very close to the value for niton. The decimal in 

 the atomic weight of thorium is not definitely settled, but it 

 is evident that the atomic weights of the two emanations are 

 within a few units of each other, and this fact renders it 

 possible to take up a definite position with regard to them. 



If the Periodic Table is to be accepted at all, it is evident 

 that it contains no places for two inert gaseous elements so 

 close together. Thorium emanation, though more unstable, 

 is as much an element as niton. The suggestion may be 

 made that the atomic weights of these emanations are 

 identical, but that they consist of the same groupings in the 

 atom arranged differently, in a more stable manner in the 

 case of niton. Examples of elements with identical atomic 

 weights are familiar in radioactive transformations. The 

 case must occur whenever one element is derived from 

 another by the expulsion of /3 particles and 7 rajs only. 

 Two such emanations could break down in different ways. 



This seems to be the only possible solution of the difficulty, 

 and it is a matter which could perhaps be tested by spectro- 

 scopic work. For it involves the conclusion that the a 

 particles from thorium and radium are not of the same 

 character, if thorium is really the active constituent in its 

 preparations. 



Actinium Emanation. — A probable constitution of this 

 emanation has been suggested, which leads to an atomic 

 weight of 152*49. It is possible to obtain some degree of 

 experimental verification of this value. Of the various 

 attempts which have been made to find this atomic weight, 

 that of Russ * appears to be the most satisfactory. The 

 method of diffusion was used, and special precautions taken 

 against unusual behaviour of small quantities of gas. Russ 

 also performed experiments confirmatory of the method, and 

 his final conclusion was that the thorium emanation is 

 1*42 times as heavy as that from actinium. The thorium 

 emanation, in any case, has an atomic weight of about 222, 

 and this leads to a value about 155 for actinium emanation. 



This is sufficiently close to the value we have suggested, 

 and decides the place of the emanation in the table. The 

 atomic weight of actinium itself is therefore about 160. A 



* Phil. Mag., March 1909= 



