I pose an interesting question: did Fulcanelli & other alchemists understand the mechanics of radioactivity? Fulcanelli's Wikipedia article mentions a number of interesting things regarding this subject. Here's a quote form the article in question:

"The meeting between Jacques Bergier and Fulcanelli took place in June 1937 in a laboratory of the Gas Board in Paris. According to Neil Powell, the following is a translation of the original verbatim transcript of the rendezvous. Fulcanelli told Bergier:

'You're on the brink of success, as indeed are several other of our scientists today. Please, allow me, be very very careful. I warn you... The liberation of nuclear power is easier than you think and the radioactivity artificially produced can poison the atmosphere of our planet in a very short time, a few years. Moreover, atomic explosives can be produced from a few grains of metal powerful enough to destroy whole cities. I'm telling you this for a fact: the alchemists have known it for a very long time... "I shall not attempt to prove to you what I'm now going to say but I ask you to repeat it to M. Hellbronner: certain geometrical arrangements of highly purified materials are enough to release atomic forces without having recourse to either electricity or vacuum techniques... The secret of alchemy is this: there is a way of manipulating matter and energy so as to produce what modern scientists call 'a field of force'. The field acts on the observer and puts him in a privileged position vis-à-vis the Universe. From this position he has access to the realities which are ordinarily hidden from us by time and space, matter and energy. This is what we call the Great Work.'[13]

When Bergier asked Fulcanelli about the Philosopher's Stone, the alchemist answered: '...the vital thing is not the transmutation of metals but that of the experimenter himself. It is an ancient secret that a few people rediscover each century. Unfortunately, only a handful are successful...'[14]" - http://en.wikipedia.org/wiki/Fulcanelli

Geometric arrangements of highly purified materials: what materials, and how are they to be "geometrically" arranged? A field of force, and a privileged position? How deep does this secret go? It seems that Fulcanelli has a very extensive understanding of nuclear mechanics, and his understanding was so advanced that even today it seems that nuclear physicists are still left in the dust.