“In fact, someone recently asked me why we urinate cold urine in the baths, but hot outside them. He did not understand that urine itself is equally warm both in the baths and out of them, and that it is the surface of one's body that is not in the same state while bathing and before. For when we are bathing, our body's surface is warmer than the urine, but outside it is colder, so it's reasonable that the urine seems hot to it when outside the bath, but cold inside.”

καί τις ἔναγχος ἡμῖν προὔβαλε διὰ τί ψυχρὸν μὲν ἐν τοῖς βαλανείοις οὐροῦμεν, ἔξω δὲ θερμὸν, οὐ παρακολουθῶν ὅτι τὸ μὲν οὖρον αὐτὸ χλιαρὸν ὁμοίως ἐστὶν ἔν τε τοῖς βαλανείοις κᾀκτὸς, ἡμεῖς δὲ οὐχ ὡσαύτως διακείμεθα τὴν ἐπιφάνειαν τοῦ σώματος ἔν τε τῷ λούεσθαι καὶ πρὸ τούτου. λουόμενοι μὲν γὰρ θερμοτέραν ἔχομεν αὐτὴν ἢ κατὰ τὸ οὖρον, ἔξω δὲ ψυχροτέραν· ὥστε εὐλόγως αὐτῇ καὶ τὸ οὖρον ἔξω μὲν τοῦ βαλανείου θερμὸν, ἔνδον δὲ ψυχρὸν φαίνεται.

Galen, Simple Drugs 3.8, 11.554–555 Kühn