Plutarch, De Pythiae oraculis
Frank Cole Babbitt, Ed.

Since, therefore, we urged him on and gave him his opportunity, Theon said that the air in Delphi is dense and compact, possessing a certain vigour because of the repulsion and resistance that it encounters from the lofty hills ; and it is also tenuous and keen, as the facts about the digestion of food bear witness. So the air, by reason of its tenuity, works its way into the bronze and cuts it, disengaging from it a great quantity of rust like dust, but this it retains and holds fast, inasmuch as its density does not allow a passage for this. The rust gathers and, because of its great abundance, it effloresces and acquires a brilliance and lustre on its surface.

When we had accepted this explanation, the foreign visitor said that the one hypothesis alone was sufficient for the argument. ‘The tenuity,’ said he, ‘will seem to be in contravention to the reputed density of the air, but there is no need to bring it in. As a matter of fact the bronze of itself, as it grows old, exudes and releases the rust which the density of the air confines and solidifies and thus makes it visible because of its great abundance.’

Theon, taking this up, said, ‘My friend, what is there to prevent the same thing from being both [p. 269] tenuous and dense, like the silken and linen varieties of cloth, touching which Homer¹ has said Streams of the liquid oil flow off from the close-woven linen, showing the exactitude and fineness of the weaving by the statement that the oil does not remain on the cloth, but runs off over the surface, since the fineness and closeness of the texture does not let it through ? In fact the tenuity of the air can be brought forward, not only as an argument regarding the disengaging of the rust, but, very likely, it also makes the colour itself more agreeable and brilliant by blending light and lustre with the blue.’