LSJ s.v. φακός II.
'anything shaped like lentils: 1. hot-water bottle, 1088.46 (i A. D.); φ. ὀστράκινος Nat.Mul.34; πυρίη φακῶν τῶν κεραμήων CA2.5; φ. τοῦ ἐλαίου oil-flask, 1 Ki.10.1 ; τοῦ ὕδατος ib.26.11.'
Sure enough, such a hot-water bottle does feature in P.Oxy. VIII 1088, which describes various remedies. The translation of the ed.pr. of the relevant section (col. ii, lines 38-47) is as follows:
'For quartan fever : juice of silphium 1 ob., myrrh. 1 ob.,
Another dose: hemlock 3 dr., henbane 3 dr. (ὑοσκυάμο(υ) (δραχμαὶ) γ), opium 2dr. (ὀπίου (δραχμαὶ) β), castor 1 dr. (καστορήου (δραχμὴ) α), black hellebore 1 dr.;
pound and work them up separately with water and pastilles the size of an Egyptian bean, then dry in the shade and give them to the patient to drink fasting (νήστηι), rubbing them in half a cotyle of raisin wine, having previously given him a bath two hours before taking; apply a warm bottle to the feet, and cover him with blankets (προλούσας πρὸ τῆς λήμψεω(ς) ὁρῶν (= ὡρῶν) β καὶ φακὸν πρὸς τοὺς πόδας προτιθείς, καὶ σκεπάζειν ἱματίοις).'
43 'The Egyptian bean was a common measure of magnitude;...'.
44 'νήστηι : cf. Moeris... νῆστις Ἀττικοί, νήστης Ἕλληνες....'
46 φακόν : Hp. Περὶ γυν. φύσ. is cited, as in LSJ,... καὶ τοῖς ὀστρακίνοις τὸ ὕδωρ ἐγχέων ξεστόν. 'They were so called on account of their shape.'