Galileo studied the “Stone of Bologna” or “solar sponge,” produced by alchemists from calcining spar (barium sulfide), which glows in the dark. Galileo inferred from its cool luminescence that light is not the same as heat, but a distinct entity, contra Aristotle. In this book the philosopher Liceti responded, adapting Aristotelian theory to accommodate the Bologna Stone. He sent a copy to Galileo which prompted a lengthy correspondence.