Perpetual fires, luminous substances and Phosphorus
Alchemists often used light as a symbol of the spirit, therefore they were especially interested in light that seemed to be trapped in matter. This was also coupled with the idea of a "perpetual fire".
There were some early ideas about the existence of "ever-burning" lights that were supposed to have been found in tombs or subterranean vaults - an example of this is found in the Rosicrucian Fama fraternitatis, 1614.
Alchemists also found in Nature certain glowing materials in animals or vegetable matter. It proved possible to extract the glow from glow worms tails.
In the 17th century there was a great interest in luminous or phosphorescent substances, and this culminated in the discovery of the element phosphorus by Hennig Brandt in 1669.
It appears that there were at least three phosphorescent substances given the name "phosphorus" in the 17th century. The first was the 'Bologna phosphorus' (actually a native ore found at Bologna), Balduin's 'Phosphorus hermeticus', and true elementary phosphorus.
The ever burning lights of Trithemius