A Calculus of Angels — Greg Keyes

In an alternate eighteenth-century Europe devastated by alchemical disaster, Sir Isaac Newton and his able assistant, Benjamin Franklin, confront enemies who seek humankind's destruction

Sir Isaac Newton's discovery of philosopher's mercury in 1681 gave rise to a remarkable new branch of alchemical science. Forty years later, the world stands poised on the brink of a new dark age . . .
 
England is in ruins, crushed by an asteroid called to Earth by the very alchemy Newton unleashed. France is in chaos following the long-delayed death of Louis XIV. Cotton Mather, Blackbeard, and the Choctaw shaman Red Shoes set sail from the American colonies to investigate the silence lying over the Old World. And in Russia, Tsar Peter the Great, now host to the evil entity that kept the Sun King alive, seizes a golden opportunity for conquest as he marches his unstoppable army across a devastated continent.
 
Meanwhile Newton and his young apprentice, Ben Franklin, hide out in Prague, awaiting the inevitable violent collision of all these disparate elements -- human and demonic alike -- while a fugitive Adrienne de Mornay de Montchevreuil pursues the secrets of the malakim and her own role in their conspiracy to obliterate humankind.
 
The second volume of the Age of Unreason series, Greg Keyes's masterwork of alternate history, A Calculus of Angels brilliantly expands the scope of the world he introduced in Newton's Cannon as an unforgettable cast of historical heavyweights collide on a different Earth where magic and science coexist.