For centuries, the ancient alchemists spoke in symbols and metaphors. The language they used was not necessarily just to serve the purpose of secrecy, but in order to not dillute the enormity or profundity of the ideas they were attempting to distill and communicate.
Alchemy is the science of change in the world. It is internal, and it is external, but not necessarily at the same time. It is the study of and the implementation of the principles that bring about change.
When the sidhe were scattered and lost, the people forgot the profound lessons at the heart of alchemy, a plague overtook the people. They forgot what true alchemy was.
There is a tradition amongst the alchemists of old. It is told of an emerald tablet that upon which was inscribed all the secrets and solutions to the mysteries of alchemy...