In 1733, Voltaire met Émilie du Châtelet , a married mother of three who was 12 years his junior and with whom he was to have an affair for 16 years.  To avoid arrest after the publication of Letters , Voltaire took refuge at her husband's château at Cirey-sur-Blaise , on the borders of Champagne and Lorraine .  Voltaire paid for the building's renovation,  and Émilie's husband, the Marquis du Châtelet, sometimes stayed at the château with his wife and her lover.  The relationship had a significant intellectual element. Voltaire and the Marquise collected over 21,000 books, an enormous number for the time. [ citation needed ] Together, they studied these books and performed experiments in the natural sciences at Cirey, which included an attempt to determine the nature of fire.