Select any day in the life of Étienne-Gaspard Robertson (1763-1837) and you might find the Liège-born artist and scientist conducting experiments in electricity, floating through the sky in a hot air balloon, or conjuring demons and the ghosts of French luminaries such as Marat and Rousseau. But it’s these necromantic arts that most distinguish his towering tomb in Cimitière du Père Lachaise. Robertson spent his young adulthood in Paris during the height of the Terror. He was a talented painter from an early age, and made a living through small painting…