CHEMICAL, ELEMENTS AND ATOMS UEBAIN 211 



It seems to me that to Thalen belonged only the credit for better 

 delineating the group of the rare earths already discovered. Lecoq 

 de Boisbaudran, an intrepid worker and extraordinary observer, 

 discovered gallium. 



This name, my dear Brauner, should recall the triumph of the 

 cause which you defended, for gallium seemed to be the eka- 

 aluminum which Mendeleeff had so exactly predicted. I was a friend 

 of Lecoq de Boisbaudran. He gave me great encouragement in my 

 researches on the rare earths at a time when I greatly needed it, for 

 you know from your oAvn experience how many such researches are 

 deceptive and what perseverance they require. Lecoq de Boisbaudran 

 had genius and a great soul. I have alwaj^s been surprised that he 

 opposed to the great foresight of Mendeleeff the very interesting, 

 but very restricted, personal considerations upon the constitution of 

 spectra which he asserted had been his only giiide in his researches 

 on gallium. His method of the uncondensed spark allowed him to 

 place in their proper order gadolinium and ytterbium, which Marig- 

 nac, the last representative of a brilliant group of chemists, had 

 discovered by the older methods of pure chemistry. 



Marignac would have closed the chapter of the discovery of ele- 

 ments by purely chemical methods had not Winlder had the good 

 luck to analyze argyrodite and find in that very rare mineral, ger- 

 manium in large quantities. Henceforth we will expect to find all 

 the new elements discovered by the methods of physics, more espe- 

 cially through spectroscopy, the technique of which has been adopted 

 in several chemical laboratories. 



No one doubts that spectra are well adapted as an elementary 

 character, presenting characteristics identical in all the combina- 

 tions and mixtures of an element. There have indeed been attrib- 

 uted to spectra more value, more constancy than they really have. 

 All kinds of spectra — those observed in solutions, in phosphorescent 

 cathode spectra, those in the electric spark between solids in vacuo — 

 are considered of the same worth as- elementary characters. 



You, my dear Brauner, inaugurated with the rare earths that 

 remarkable epoch in which absorption spectra played the principal 

 part. Then, afterwards, Lecoq de Boisbaudran discovered sama- 

 rium and Marignac gadolinium while you established the com- 

 pound nature of didyraium. 



In 1882 you decided very tardily to publish the results of j^our 

 important researches. Certain variations in the absorption spec- 

 trum of didymium had been noted by such men as Delafontaine, 

 J. L. Smith, Lecoq de Boisbaudran, and then by Cleve; but they 

 were inclined to attribute the variations to differences in acidity of 

 the solutes. • Having eliminated that source of error, vou deter- 



