The Eccentric Engineer: Bohr, de Hevesey and the hidden treasures of ‘Jewish Science’

Having a Nobel Prize isn’t usually dangerous, but in Copenhagen in the spring of 1940 it definitely was. Yet Niels Bohr found a solution to the problem using a bit of chemical engineering.

Bohr’s specific problem was the Nobel medal that is presented with the prize. Until 1980, this 200g medal was made from solid 23-carat gold (they are now made from 18-carat recycled gold). Bohr had two of them.

Now this might seem like a nice problem to have, but it actually put his and other lives at risk. These weren’t his medals; he auctioned his 1922 prize in March 1940 for the benefit of the Fund for Finnish Relief. These were medals he had been given for safe keeping by physicists James Franck, who had fled Germany shortly after Hitler’s rise to power, and Max von Laue, who even then was still in Germany and teaching the proscribed ‘Jewish physics’ of Albert Einstein.

In April 1940, the Nazis had just invaded Bohr’s home country of Denmark. Any day now, his Institute of Theoretical Physics in Copenhagen was likely to be searched, certainly in the hope of uncovering his work on nuclear theory, but also to discover who was working there and where their sympathies lay.

Since 1933, the Institute had been a refuge for German Jewish physicists and the discovery of the medals put them all at risk. Firstly, Hitler had a hatred of the Nobel Prizes, which he thought rewarded ‘Jewish science’ rather than his ‘Deutsche Physik’ and hence banned German nationals from receiving them. More prosaically, Germany had strict currency controls in place and the export of gold was forbidden. Gold medals from German laureates accumulated in Denmark would look highly suspicious.

Clearly, Bohr had to get rid of the medals, but what could he do with them? He consulted a colleague at the Institute, George de Hevesy, a chemical engineer and radio-chemist who would himself win a Nobel in 1943 for his invention of radioactive tracers. He suggested burying the medals, but this bothered Bohr. What if they were found? The medals had the names of recipients engraved on them so it would be clear that they were concealing ‘German’ gold. Wasn’t there some other way?

The answer that sprang to de Hevesy’s mind was alchemy. There was a substance that might help. Alchemists had called it ‘aqua regia’ – ‘royal water’ – and it consisted of a highly caustic combination of three parts hydrochloric acid to one part nitric acid. It had gained its regal name because it has one unique property – it dissolves gold (and platinum). De Hevesy suggested they dissolve the medals and recover the gold later. Who would look for two Nobel prizes in solution?

So de Hevesy set to work. As he wrote to von Laue after the war, dissolving the medals was in itself no mean feat as gold is “exceedingly unreactive and difficult to dissolve”. Firstly, aqua regia decomposes very quickly so its components can only be mixed just before use. Even then, the reaction is tricky. The nitric acid is a strong oxidiser which dissolves a tiny amount of gold, forming gold ions. The hydrochloric acid provides a supply of chlorine ions that react with the gold ions, forming chloroauric acid and removing the gold from solution. This in turn allows the nitric acid to dissolve a further tiny quantity of gold and so on.

Having presumably milled the medals first – de Hevesy doesn’t mention this step – he very slowly managed to dissolve both medals completely, his memoir ‘Adventures in Radioisotope Research’ noting: “While the invading forces marched in the streets of Copenhagen, I was busy dissolving Laue’s and also James Frank’s medals.”

Having finished the work, he placed the deep orange liquid in two unmarked jars on a shelf. The Institute was searched as predicted, but the innocuous-looking jars were left untouched.

By 1943, Copenhagen was becoming too dangerous for many scientists. De Hevesy was Jewish and Bohr’s mother had Jewish heritage. Both men fled to Sweden, with Bohr then travelling on to England.

After the war de Hevesy stayed in Sweden until 1961, but he did make a return visit to the Institute and there he found the two bottles still unmolested. He took the opportunity to precipitate out the gold from the aqua regia and return it to Bohr. A letter from Bohr dated 24 January 1950 shows that he then delivered the gold back to the Royal Swedish Academy of Science where the medals were recoined.

James Frank and Max von Laue were finally reunited with their medals in 1952.