The Refutation of All Heresies

But putting a skull on the ground, they make it speak in this manner. The skull itself is made out of the caul of an ox; and when fashioned into the requisite figure, by means of Etruscan wax and prepared gum, (and) when this membrane is placed around, it presents the appearance of a skull, which seems to all to speak when the contrivance operates; in the same manner as we have explained in the case of the (attendant) youths, when, having procured the windpipe of a crane, or some such long-necked animal, and attaching it covertly to the skull, the accomplice utters what he wishes. And when he desires (the skull) to become invisible, he appears as if burning incense, placing around, (for this purpose,) a quantity of coals; and when the wax catches the heat of these, it melts, and in this way the skull is supposed to become invisible.