A SCIENTIFIC CAKEEK 1 9 



of the distribution of labelled lead and bismuth in healthy rabbits was 

 carried out earlier, in 1924, in Copenhagen. I approached the great 

 pathologist Aschoff to delegate one of his collaborators to help us in 

 our work. He first delegated the director of a hospital on the island of 

 Formosa and later, to help him, his chief chemist, Rudolf Schoenheimer. 

 This was Schoenheimer's first experience with tracer work, a field to 

 which he later, jointly with his eminent colleague Rittenberg, made 

 unsurpassed contributions. Schoenheimer was already at that date a 

 very nervous man. He moved his limbs incessantly, smoked cigarettes, 

 and consumed coffee on a much too liberal scale. When our work was 

 finished, he left Freiburg and I never saw this most merited man again. 



Separation of Isotopes 



When I went to Copenhagen in the spring of 1920, Bohr's institute 

 was not yet ready. I associated with the eminent physicochemist Br0n- 

 sted to investigate a problem of great interest to both of us, namely, the 

 partial separation of isotopes on a preparative scale. We based our pro- 

 cedure on the more rapid rate of evaporation of the lighter isotope from 

 a liquid. We distilled mercury in high vacuum at 40° and prevented the 

 more rapidly evaporating lighter isotopes from being reflected back into 

 the liquid mercury by freezing them on a glass surface cooled with 

 liquid air. By repeating this process some hundred times, we obtained a 

 light and a heavy mercury fraction. The results were controlled by both 

 density measurements and atomic weight determinations, the latter 

 being carried out by Honigschmid in Munich. 



When partially separating the isotopes of chlorine, we made use of the 

 above-mentioned method again. We distilled concentrated solutions 

 of hydrochloric acid in water and obtained several liters of water con- 

 taining hydrochloric acid with different isotopic chloride composition. 

 I suggested to Br0nsted that he have a look at the density of the water ob- 

 tained. He objected to my suggestion, as shortly before two distinguished 

 German chemists, Vollmer and Stern, had searched without success for 

 other isotopes of hydrogen and oxygen than ^H and ^^0. These workers 

 carried out diffusion experiments through porous membranes. When I, 

 after Urey's discovery of deuterium, reminded Br0nsted of my suggestion, 

 he answered : "A discovery like this should not be made fortuitously ; 

 it should be based on careful considerations like Urey's." 



Bohr was highly interested in our separation experiments and keenly 

 followed our progress. Bohr's greatness is due not only to his ingenuity 

 but to the unique catholicity of his interests, his sagacity, and his immense 

 conscientiousness. When as a young man he intended to publish his 

 first "letter" to the editor of Nature, he wrote the note ov^r and over 

 again. Finally his brother, who later achieved fame as a great mathema- 



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