Damis narrates further that Apollonius met with a woman who was white from feet to breasts but black in the upper region. His disciples were alarmed at the prodigy, but the master gave her his hand, for he knew her. He told them that she was the Indian Venus, whose colours are those of the bull Apis, adored by the Egyptians. This harlequin female is magical science, the white limbs—or created forms—of which reveal the black head, or that supreme cause which is unknown to man at large. But Philostratus and Damis knew, and it was under emblems like these that they gave expression in concealment to the doctrine of Apollonius. The secret of the Great Work is contained in the fifth to the tenth chapters of this third book, and the form of symbolism adopted is that of dragons defending the entrance to a palace of the wise.[130] There are three species of dragons —dwellers respectively of marshes, plains and mountain. The mountain is Sulphur, the marsh Mercury and the plain is the Salt of the Philosophers. The dragons of the plain are pointed on the back, like a saw-fish, referring to the acid potency of salt. Those of the mountains have golden scales and a golden beard, while the sound of their creeping movement is like the tinkling of copper. In their head is a stone by which all miracles can be worked. They bask on the shores of the Red Sea and they are caught by the help of a red cloth embroidered with golden letters; on these enchanted letters they lay their head and fall asleep, and they are then decapitated with an axe. Who does not recognise here the Stone of the Philosophers, the Magistery at the Red and the famous regimen of fire, represented by golden letters? Under the name of Citadel of the Wise, Philostratus goes on to describe the Athanor as a hill surrounded by a mist but open on the southern side. It has a well four paces in breadth, from which an azure vapour rises, drawn up by the warmth of the sun and displaying all colours of the rainbow. The bottom of the well is sanded with red arsenic. In its vicinity is a basin filled with fire and thence rises a livid flame, odourless and smokeless, and never higher or lower than the basin-edge. There are also two reservoirs of black stone, in one of which rain is stored and in the other wind. The rain-cistern is opened when there is excessive drought and then clouds come forth which water the whole country. It would be difficult to describe more exactly the Secret Fire of the Philosophers and that which they term their _Balneum Mariæ_. It follows from this account that the ancient alchemists employed electricity, magnetism and steam in their Great Work.