Plutarch, De Pythiae oraculis
Goodwin, Ed.

When we had made our acknowledgment that he had spoken truth, and besought him to proceed, he told us that the air of the city of Delphi is heavy, compacted, thick, and forcible, by reason of the reflection and resistency of the adjacent mountains, and besides that, is sharp and cutting (as appears by the eager stomachs and swift digestion of the inhabitants); and that this air, entering and penetrating the brass by its keenness, fetches forth from the body of the brass much rust and earthy matter, which afterwards it stops and coagulates by its own density, ere it can get forth; by which means the rust abounding in quantity gives that peculiar grain and lustre to the superficies. When we approved this argument, the stranger declared his opinion, that it needed no more than one of those suppositions to clear the doubt; for, said he, that tenuity or subtilty seems to be in some measure contrary to that thickness supposed to be in the air, and therefore there is no reason to suppose it; for the brass, as it grows old, of itself exhales and sends forth that rust, which afterwards, being stopped and fixed by the thickness of the air, becomes apparent by reason of its quantity. Then Theo replied: and what hinders but that the same thing may be thick and thin both together, like the woofs of silk or fine linen?—of which Homer says:

Thin was the stuff,
Yet liquid oil ran o'er the tissued woof,
¹

intimating the extreme fineness of the texture, yet so close woven that it could not suffer oil to pass through it. In like manner may we make use of the subtilty of the air, not only to scour the brass and fetch the rust out of it, but also to render the color more pleasing and more azure-like, by intermixing light and splendor amidst the blue.

[p. 73]