220 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



Subsequently, making perpendicular the two fields which J. J. 

 Thomson used for the deviation of the positive rays, Aston has shown 

 that at least a third of the more common elements must be looked 

 upon as mixtures of isotopes. The fractionation to wdiich Ilevesy 

 subjected mercury vapor showed clearly the multiple isotopy of that 

 element. I am sure you have been glad that isotopes are not 

 absolutely rebellious to the methods of analysis which are generally 

 considered chemical. It remains to be said that the spectra of 

 isotopes are not strictly the same, which doubtless you hoped would 

 be the case. We now have abundant proof. 



If the isotopes can be separated chemically (time and the neces- 

 sary efforts are negligible conditions from a theoretical point of 

 view), and if they have different spectra (it will be necessary only 

 to have recourse to the sixth order in the echelon spectroscope 

 of Michelson or the interferometer of Fabry), we do not see w-hy 

 they should not be called elements. It is clear that they are. Be- 

 tween them and the individuals of a group of elements like the rare 

 earths there is, to you and me, only a difference of degree in the 

 analogy. 



In this it seems that we misconceive the rights of the electrons 

 which give the primary significance to the atomic number, 

 which is all important in the defining of a chemical element. Each 

 element of the group of rare earths has its atomic number, and there 

 is only one atomic number for all the elements of an isotopic group. 

 Have we a sufficient reason for defining an element by its atomic 

 number? Is it right to define the group rather than the nature? 

 No agreement has yet been reached. But if we are now beaten in 

 the search for the proper definition some day we will certainly 

 find it. 



The International Commission on Chemical Elements, it is true, 

 has defined the element through its atomic number. That is a 

 victory for the periodic classification which should somewdiat satisfy 

 you. The definition has been given only a provisory right, a con- 

 cession which should please you. 



It is time I should close. I could not pretend to exhaust the 

 subject which I had the audacity to commence. I have tried to 

 recall certain memories. Perhaps I have been too personal. A 

 historian should avoid all sentiment, which a contemporary does 

 with difficulty. A reticence is required Avhicli I lack. 



I could not speak as a professor and take a dogmatic attitude in 

 the presence of a master like you ; indeed, such an attitude is never 

 mine. Wherefore I have spoken as a narrator and friend. If 

 aught lasts of this talk I wish that it might be the meuiory of the 

 great friendship wilii which you have honored me, my dear 

 Brauner. 



