There are two basic types of Alchemy: The Laboratory Alchemy involving actual chemical experiments in the lab which may lead to creating the Philosophers Stone; and the Psychological Alchemy which may lead to the achievement of a higher conscious state in the individual human being.
‘The Green Lion Devouring The Sun' is a popular alchemical symbol. On a chemical level this is a metaphor for when a green, liquid sulfate called “vitriol” purifies matter, leaving behind the gold within the matter. Very pure vitriol is an acid that eats through practically anything, except gold.
To naturalist alchemists, the Green Lyon metaphor described the process whereby plants used the rays of the sun to facilitate their own growth - a process we call photosynthesis.
On a human level, the green lion eating the sun is a metaphor for when a person’s “consciousness [is] overwhelmed by violent, frustrated desires” (Fabricus). In other words, inside a person, when the metaphor of the green lion eating the sun takes place, the person is compelled to act out destructively.
In contrast, when the green lion eats the sun in a person on a higher spiritual path (for example, an alchemist), the person allows the green lion to eat them in order to be purified — to become spiritually golden.
“There is this one green lion, which closes and opens the seven indissoluble seals of the seven metallic spirits which torments the bodies, until it has perfected them, by means of the artist’s long and resolute patience.” — “The Cosmopolite,” (16th century)