Not rosa crux, rose cross, but ros crux, dew cross. Dew was considered by the ancient chemists as the most powerful solvent of gold: and cross in alchemy is the synonym of light, because any figure of a cross contains the three letters L V X (light). “Lux” is the menstruum of the red dragon (i.e. corporeal light), and this gross light properly digested produces gold, and dew is the digester. Hence the Rosicrucians are those who used dew for digesting lux or light, with the object of finding the philosopher's stone.
As for the Rosycross philosophers, Whom you will have to be but sorcerers, What they pretend to is no more Than Trismegistus did before, Pythagoras, old Zoroaster, And Apollonius their master.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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