A SCIENTIFIC CAKEEll 17 



the X-ray spectrum of the elements 68 through 12. 1 was ah-eady in 

 Holland on the way to Oxford when the first World War broke out, soon 

 followed by the tragic death of Moseley. While talking on a field telephone 

 at Gallipoli, a bullet struck the head of this ingenious and most remarkable 

 man. By a curious coincidence, I was to occupy myself extensively with 

 the X-ray spectrum of the element 72 eight years later. 



Measurement of Self -diffusion 



As I was at that time a Hungarian subject (1 am now a Swedish citizen) 

 1 was drafted into the Austro — Hungarian army. I spent much of 

 1hat time as technical supervisor of electrolytic copper works. While 

 located in Carpathian plants, I fitted up a laboratory on a very modest 

 scale and studied the difference in the chemical behaviour of the active 

 deposit of thorium when present in ionic and colloidal state. For several 

 months after the end of the war it was not possible to leave Hungary. 

 During these months I started with my friend Groh to study self-diffusion 

 in molten and in solid lead, using radium D as an indicator. We fused 

 a radiolead-rod on to the top of an inactive lead-rod, heated this solid 

 system to 200° — 300°, and determined the dislocation of the radium D 

 atoms. From the extent of dislocation, the rate of self-diffusion of lead 

 was calculated. This early, rough method was improved later during 

 my stay in Copenhagen. Together with the Russian scientist Mrs. Ob- 

 rut she va, we condensed the lead isotope thorium B on top of a lead foil 

 and counted the number of scintillations produced by the a-rays emitted 

 by the disintegration products of thorium B. Upon heating of the sample, 

 thorium B diffused into the lead foil, resulting in a reduction of the 

 number of scintillations observed. From this reduction, the diffusion 

 rate of lead in lead could be calculated. Heisenberg, then lecturer in 

 Copenhagen, very kindly at that time helped us with these calculations. 

 Later on in Freiburg, Seith and myself made use of the recoil pheno- 

 menon to measure self-diffusion in lead. This is an exceedingly sensitive 

 method, which permitted measurement of diffusion rates as slow as 

 10-14 cm2/day. 



The Rockefeller Foundation started to support my investigations in 

 1930 and continued most generously to do so for the following twenty- 

 five years. 



Niels Bohr's Institute 



In the first days of May, 1919, I left for Copenhagen to spend some 

 time with Niels Bohr at the charming summer house in Tibirke. At that 

 time his premises were at the Technological Institute of Copenhagen, 

 from which he directed the construction of his new institute. When he 



2 Heresy 



