Plutarch Quaestiones Convivales 6.9.2

[2] First of all, its smoothness shows the evenness of its parts; for touch it where you please, it is all alike. Besides, you may see your face in it as perfectly as in a mirror; for there is nothing rough in it to hinder the reflection, but by reason of its humidity it reflects to the eyes the least particle of light from every part of it. As, on the contrary, milk, of all other liquids, does not return our images, because it hath too many terrene and gross parts mixed with it; again, oil of all liquids makes the least noise when moved, for it is perfectly humid. When other liquids are moved or poured out, their hard and grosser parts fall and dash one against another, and so make a noise by reason of their roughness. Moreover, oil only is pure and unmixed; for it is of all other liquids most compact, nor has it any empty spaces and pores between the dry and earthy parts, to receive what chances to fall upon it. Besides, because of the similitude of parts, it is closely joined together, and unfit to be joined to any thing else. When oil froths, it does not let any wind in, by reason of the contiguity and subtility of its parts; and this is also the cause why fire is nourished by it. For fire feeds upon nothing but what is moist, for nothing is combustible but what is so; for when the fire is kindled, the air turns to smoke, and the terrene and grosser parts remain in the ashes. Fire preys only upon the moisture, which is its natural nourishment. Indeed water, wine, and other liquors, having abundance of earthy and heavy parts in them, by falling into fire part it, and by their roughness and weight smother and extinguish it. But oil, because purely liquid, by reason of its subtility, is overcome by the fire, and so changed into flame.