What then, said Diogenianus, do you believe to be the cause of this extraordinary color in the brass? And Theo replied: Seeing that of those first and most natural elements, which are and ever will be,—that is to say, fire, air, earth, and water,—there is none that approaches so near to brass or that so closely environs it as air alone, we have most reason to believe that the air occasions it, and that from thence proceeds the difference which brass displays from other metals Or did you know this even before Theognis was born, as the comic poet intimates; but would you know by what natural quality or by what virtual power this same air thus colors the brass, being touched and surrounded by it? Yes, said Diogenianus; and so would I, dear son, replied the worthy Theo. First then let us endeavor, altogether with submission to your good pleasure, said the first propounder, to find out the reason wherefore of all moistures oil covers brass with rust. For it cannot be imagined that oil of itself causes that defilement, if when first laid on it is clean and pure. By no means, said the young gentleman, in regard the effect seems to proceed from another cause; for the rust appears through the oil, which is thin, pure, and transparent, whereas it is clouded by other more thick and muddy liquors, and so is not able to show itself. It is well said, son, replied the other, and truly; but hear, however, and then consider the reason which Aristotle produces. I am ready, returned the young gentleman. He says then, answered the other, that the rust insensibly penetrates and dilates itself through other liquids, as being of parts unequal, and of a thin substance; but that it grows to a consistency, and is, as it were, incorporated by the more dense substance of the oil. Now if we could but suppose how this might be done, we should not want a charm to lull this doubt asleep.