"The ancients had a striking mythical image called the Ourobouros that represented the totality of the alchemical process—a dragon or snake swallowing its own tail. It is a symbol of the great circulation of Nature: what was above as the very subtle spirit descends and becomes—by adaptation through a process of heat and digestion—the below, or earthy and fixed. Then by reversing the process, what is below—fixed, heavy and earthy—ascends and gradually becomes volatile and subtle as the above. By this great circulation of Nature, from above to below and from below to above, the circle is closed. Homer, the Greek philosopher, called it the golden chain because Nature is in reality a chain, in itself like a circle, where each link supports the other, thus making a chain from Heaven to Earth.
In alchemy, this chain is the series of substances and chemical states that appear in the course of the alchemical process. It also refers to the chain of alchemical adepts, who link Heaven with Earth, beginning with the great Egyptian alchemist Hermes Trismegistus, who wrote on his Emerald Tablet, “Make that what is above become like what is below, then what is below returns to what is above, thus creating the miracles of one thing.” In these few simple words he concealed and summarized all of the knowledge, practice and mysteries of the Hermetic (alchemical) science."