The 18th century witnessed a remarkable reevaluation of the musical medium: early enlightenment thinkers argued that an individual musical sound—a single note played on an oboe, for example—was meaningless. Throughout the century, however, composers, musicians, theorists, philosophers, and scientists became increasingly fascinated with the qualities—both acoustical and emotional—inherent in tones and timbre. My project explores this profound change by drawing on philosophical discussions of sensation and cognition, musical works, instruments and musical machines, orchestration treatises, and 18th-century acoustical science. In particular, I examine what it means to think about musical sonority, how the concept of timbre alters ideas of musical meaning, and the changes in music`al culture that made timbre the focus of attention for composers, philosophers, music critics, and scientists.